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Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies
Abstract
Earth's mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth's present and past climate system, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent-ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian-Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time.
... In recent decades, proxy techniques and model experiments have been applied to explore the evolution of Southern Ocean circulation from the late Eocene pattern of two gyres (the subtropical and subpolar gyres) to the modern Antarctic Circumpolar Current (ACC) (D. J. Hill et al., 2013;Sauermilch et al., 2021;Scher et al., 2015;Sijp & England, 2004;Stickley et al., 2004). The modern ACC is characterized by a circumpolar pathway penetrating both Tasman Gateway (TG) and Drake Passage (DP), with a volume transport through the DP of 137 Sv (Meredith et al., 2011, 1 Sv = 10 6 m 3 s −1 ), or around 173 Sv if the near-bottom flow is included (Donohue et al., 2016). ...
... In previous studies, the opening and the deepening of ocean gateways (Baatsen et al., 2020;D. J. Hill et al., 2013;Sijp et al., 2011), the change in strength and location of wind stress (Sauermilch et al., 2021;Scher et al., 2015;Xing et al., 2022), and the declining of atmospheric CO 2 (Goldner et al., 2014;Ladant et al., 2014;Lefebvre et al., 2012) have been shown to have critical roles in the development of the proto-ACC and the transition of the early Southern Ocean toward its modern circulation. Nevertheless, all the studies obtain a DP transport of the proto-ACC not exceeding 90 Sv, even with a deep TG (1,500 m; DP transport of 12.5 Sv) (Sauermilch et al., 2021) and/or strong wind stress (maximum wind stress of 0.2 N m −2 ; DP transport of 44.8 Sv) (Xing et al., 2022), or modern pCO 2 (280 ppm; DP transport of 89 Sv) (Lefebvre et al., 2012). ...
... It has previously been proposed that the deepening of the Tasman Gateway (TG), and the alignment of the westerly winds with the TG, are vital prerequisites for the inception of a proto-ACC during the Eocene-Oligocene Transition (EOT) and its evolution to a modern-strength ACC (Sauermilch et al., 2021;Scher et al., 2015;Sijp et al., 2011;Xing et al., 2022). Nevertheless, model simulations taking into account these changes in bathymetry and winds do not produce an ACC with a transport comparable to its modern values (D. ...
Plain Language Summary
The evolution of ocean circulation from the early Southern Ocean around 50 million years ago to today has seen much debate over the past decades. The main characteristic of the modern Southern Ocean is the prevalence of the Antarctic Circumpolar Current (ACC), the world's strongest current. In the past it has been thought that the deepening of ocean gateways, and changes in the strength and location of winds, led to an ACC of similar strength to its modern equivalent. Nevertheless, ocean models simulating these changes typically reproduce an ACC with less than a third of the modern ACC's strength. Here we show that the missing ingredient in the transition to a modern ACC is deep convection around the Antarctic continent. Deep convection is due to a combination of cooling and increase in salinity by sea ice formation which allows for water at the ocean surface to become denser than the water below, leading to the mixing of the water column to great depths. This deep convection allows for the ocean to be energized, leading to a modern‐strength Antarctic Circumpolar Current. A cool climate around the Antarctic continent is therefore crucial for the development of a modern‐strength Antarctic Circumpolar Current.
... The Cenozoic origins of AMOC and its possible relations with Southern Ocean gateway opening and Antarctic glaciation are far from well understood [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . Proto-North Atlantic Deep Water, ...
... Most work on the early history of AMOC has focused on tracing water-mass properties and flow using physicochemical approac hes [3][4][5][6]8,9,12,15 . In this Article, we take a new approach, employing microbial source indicators, based on distributions of glycerol dialkyl glycerol tetraethers (GDGTs) (Extended Data Fig. 1), to infer water-mass oxygenation history. ...
... This observation contradicts suggestions 5,8,9 that a modern-like AMOC was initiated within this pre-EOT interval (Fig. 3). Our GDGT records indicate a later date for AMOC onset, consistent with interpretations of neodymium isotope (ɛ Nd ) records 6,12 . While there is some uncertainty over its precise timing, the long-term trend in oxygenation of AMOC-feed waters appears to have reversed at the EOT (Fig. 3d,e). ...
The Atlantic meridional overturning circulation (AMOC) exerts a major control on the global distribution of heat, dissolved oxygen and carbon in the ocean. Yet the timing and cause of the inception of this system and its evolution since the start of the Cenozoic Era 65 million years ago (Ma) remain highly uncertain. Here we present records of microbial source indicators based on glycerol dialkyl glycerol tetraether distributions from the Cenozoic Northwest Atlantic Ocean (~43‒18 Ma) and use them to infer changes in AMOC-driven deep-ocean oxygenation. At this location, oxygenation is strongly controlled by southwestward Deep Western Boundary Current transport of newly formed deep waters that feed AMOC. Our Eocene data show short-term high-amplitude variability and an overall decrease in oxygenation of AMOC-feed waters culminating in especially poor ventilation between ~36.5 and ~34 Ma. AMOC-feed waters became better oxygenated upon initiation of Antarctic glaciation at the Eocene/Oligocene transition, ~34 Ma, and were consistently well ventilated from ~30 Ma. Our findings indicate a close association between the inception of Antarctic glaciation and AMOC and suggest that both vertical mixing and wind-driven upwelling in the Southern Ocean were key to fully establishing AMOC as an agent of deep-ocean ventilation.
... Australia did not rapidly move away, with slow spreading until ~45 Ma [Williams et al., 2019], and the 1551 Tasman Gateway was not opened until 33 Ma [Scher et al., 2015]. Spreading on this margin west of the 1552 George V fracture zone between 57-50 Ma may have been accommodated by sinistral transtension in East 1553 ...
... Antarctica remained important as the Drake Passage allowed throughflow by 42 Ma [Scher andMartin, 1591 2006] and the Tasman gateway by 33 Ma [Scher et al., 2015]. Through the Oligocene these gateways 1592 developed more fully [van de Lagemaat et al., 2021], allowing by the Miocene a fully developed Antarctic 1593 Circumpolar Current. ...
... Antarctica remained important as the Drake Passage allowed throughflow by 42 Ma [Scher andMartin, 1591 2006] and the Tasman gateway by 33 Ma [Scher et al., 2015]. Through the Oligocene these gateways 1592 developed more fully [van de Lagemaat et al., 2021], allowing by the Miocene a fully developed Antarctic 1593 Circumpolar Current. ...
... 一是深 部过程通过调节大气CO 2 浓度影响温度, 包括岩浆-变 质作用的碳释放 [24] 、岩浆岩化学风化的碳消耗 [24] 、板 块运动改变海陆分布调节化学风化碳吸收速率 [11] 、火 山喷发及岩浆岩化学风化增加营养物质的输送促进大 洋初级生产力导致的碳埋藏 [25] . 二是深部过程通过板 块运动改变海陆分布调整大洋环流, 影响热量传输, 进 而调节地表温度 [26] . 三是火山活动可将SO 2 送入平流 层, 形成硫酸盐气溶胶, 对太阳辐射产生遮挡作用, 引 发显著降温 [27] . ...
... 板块位置变动还可改变大洋环流, 进而影响热量 传输. 在偏冷的气候背景下, 如果高、低纬间洋流受 阻, 低纬热量无法传递至高纬地区, 可导致高纬大幅降 温, 甚至发育冰盖 [26] . 全球板块聚合形成超大陆, 导致大陆外围俯冲带 岩浆活动占据主导, 大陆内部地幔柱岩浆活动减弱, 大 火成岩省稀少 [8] . ...
... 在变冷过程中, 系列构造事件加速了冰盖形成与 扩展(图3). 始新世末, 塔斯马尼亚海道及德雷克海道 打开促使绕南极流形成, 限制了南极大陆与低纬的热 量交换, 导致南极冰盖形成 [26,116] ; 晚中新世, 印尼海道 关闭阻碍热量由低纬向高纬传递, 引发北半球变冷 [117] ; ...
... The colonisers that arrived subsequently thus had access to a vast and varied ecospace that enabled them to diversify widely, and generally continuously, at least until the last few million years (e.g. Samonds et al., 2013;Burbrink et al., 2019;Belluardo et al., 2022). ...
... This was when the ancestors of almost all of the vertebrate assemblage components on Madagascar are believed to have arrived (e.g. Samonds et al., 2013). As a consequence, for many years opinions were based upon anatomical comparisons of the taxa with their off-island relatives. ...
... His second point (sentences #2 and #3), disregards the fact that ocean circulation paths are not fixed, but instead change as the tectonic plates continually reconfigure [see below in this section, specifically the reference to Ali & Huber (2010) and the dramatic modification in surface-water flow in the SW Indian Ocean that occurred 15-20 Mya]. In some cases, the oceanographical responses have been profound and geologically rapid, for instance with the formation of the Panama Isthmus (Schneider & Schmittner, 2006), and the openings of both the Tasmanian Gateway (Scher et al., 2015) and the Drake Passage (Toumoulin et al., 2020). McCall then marshalled the geological evidence, concluding that the Davie Ridge was exposed between 45 and 26 Mya. ...
Despite discussions extending back almost 160 years, the means by which Madagascar's iconic land vertebrates arrived on the island remains the focus of active debate. Three options have been considered: vicariance, range expansion across land bridges, and dispersal over water. The first assumes that a group (clade/lineage) occupied the island when it was connected with the other Gondwana landmasses in the Mesozoic. Causeways to Africa do not exist today, but have been proposed by some researchers for various times in the Cenozoic. Over-water dispersal could be from rafting on floating vegetation (flotsam) or by swimming/drifting. A recent appraisal of the geological data supported the idea of vicariance, but found nothing to justify the notion of past causeways. Here we review the biological evidence for the mechanisms that explain the origins of 28 of Madagascar's land vertebrate clades [two other lineages (the geckos Geckolepis and Paragehyra) could not be included in the analysis due to phylogenetic uncertainties]. The podocnemid turtles and typhlopoid snakes are conspicuous for they appear to have arisen through a deep-time vicariance event. The two options for the remaining 26 (16 reptile, five land-bound-mammal, and five amphibian), which arrived between the latest Cretaceous and the present, are dispersal across land bridges or over water. As these would produce very different temporal influx patterns, we assembled and analysed published arrival times for each of the groups. For all, a 'colonisation interval' was generated that was bracketed by its 'stem-old' and 'crown-young' tree-node ages; in two instances, the ranges were refined using palaeontological data. The synthesis of these intervals for all clades, which we term a colonisation profile, has a distinctive shape that can be compared, statistically, to various models, including those that assume the arrivals were focused in time. The analysis leads us to reject the various land bridge models (which would show temporal concentrations) and instead supports the idea of dispersal over water (temporally random). Therefore, the biological evidence is now in agreement with the geological evidence, as well as the filtered taxonomic composition of the fauna, in supporting over-water dispersal as the mechanism that explains all but two of Madagascar's land-vertebrate groups.
... Australia did not rapidly move away, with slow spreading until ~45 Ma [Williams et al., 2019], and the 1551 Tasman Gateway was not opened until 33 Ma [Scher et al., 2015]. Spreading on this margin west of the 1552 George V fracture zone between 57-50 Ma may have been accommodated by sinistral transtension in East 1553 ...
... Antarctica remained important as the Drake Passage allowed throughflow by 42 Ma [Scher andMartin, 1591 2006] and the Tasman gateway by 33 Ma [Scher et al., 2015]. Through the Oligocene these gateways 1592 developed more fully [van de Lagemaat et al., 2021], allowing by the Miocene a fully developed Antarctic 1593 Circumpolar Current. ...
... Antarctica remained important as the Drake Passage allowed throughflow by 42 Ma [Scher andMartin, 1591 2006] and the Tasman gateway by 33 Ma [Scher et al., 2015]. Through the Oligocene these gateways 1592 developed more fully [van de Lagemaat et al., 2021], allowing by the Miocene a fully developed Antarctic 1593 Circumpolar Current. ...
... This gyral circulation broke down during the late Cenozoic, when a throughflow developed, mostly enabled by the opening and deepening of the Southern Ocean gateways (Kennett, 1977;Sauermilch et al., 2021). While the Tasmanian Gateway deepened between 35.5 and 33.5 Ma and widened between 33 and 30 Ma (Scher et al., 2015), the timing and nature of the opening, widening, and deepening of the Drake Passage remains elusive with age estimates of a deep opening ranging from the Eocene to earliest Miocene (Barker and Burrell, 1977;Barker et al., 2007;Livermore et al., 2007;Lagabrielle et al., 2009;Eagles and Jokat, 2014;Maldonado et al., 2014;van de Lagemaat et al., 2021). The most recent research (Evangelinos et al., 2024) estimates that the modern, vigorous, and deep-reaching ACC developed some time in the late Miocene and reduced the size of the once large subpolar gyres into comparatively small gyres in the Weddell Sea and the Ross Sea today. ...
... In the absence of calcareous (and often even siliceous) microfossils in Cenozoic Southern Ocean sediments, studies from the southern Pacific and southern Indian oceans have successfully utilized organic components preserved in these sedimentary archives. Notably, investigations of the remains of dinoflagellates (their organic cysts) and molecular organic geochemical analyses are employed to reconstruct the Oligocene-Miocene evolution of oceanic conditions, temperature gradients, and paleo-positions of frontal systems in the Southern Ocean (Hannah, 2006;Lyle et al., 2007;Guerstein et al., 2010;Houben et al., 2013;Prebble et al., 2013;Scher et al., 2015;Warny et al., 2016;Hartman et al., 2018;Sangiorgi et al., 2018;Bijl et al., 2018a;Parras et al., 2020;Hoem et al., 2021a, b;Amenábar et al., 2022;Duncan et al., 2022;. The Oligocene record of Integrated Ocean Drilling Program (IODP) Site U1356 offshore Wilkes Land (Bijl et al., 2018a;Hartman et al., 2018) and Deep Sea Drilling Project (DSDP) Site 274 offshore from the Ross Sea embayment (Hoem et al., 2021a) show surprisingly warm (10-21°C) sea surface temperatures (SSTs) and dinoflagellate cyst (dinocyst) assemblages, which indicate oligotrophic, fully marine, and temperate waters in areas proximal to the East Antarctic Ice Sheet. ...
Through the Cenozoic (66–0 Ma), the dominant mode of ocean surface circulation in the Southern Ocean transitioned from two large subpolar gyres to circumpolar circulation with a strong Antarctic Circumpolar Current (ACC) and complex ocean frontal system. Recent investigations in the southern Indian and Pacific oceans show warm Oligocene surface water conditions with weak frontal systems that started to strengthen and migrate northwards during the late Oligocene. However, due to the paucity of sedimentary records and regional challenges with traditional proxy methods, questions remain about the southern Atlantic oceanographic transition from gyral to circumpolar circulation, with associated development of frontal systems and sea ice cover in the Weddell Sea. Our ability to reconstruct past Southern Ocean surface circulation and the dynamic latitudinal positions of the frontal systems has improved over the past decade. Specifically, increased understanding of the modern ecologic affinity of organic-walled dinoflagellate cyst (dinocyst) assemblages from the Southern Ocean has improved reconstructions of distinct past oceanographic conditions (sea surface temperature, salinity, nutrients, and sea ice) using downcore assemblages from marine sediment records. Here we present new late Oligocene to latest Miocene (∼ 26–5 Ma) dinocyst assemblage data from marine sediment cores in the southwestern Atlantic Ocean (International Ocean Discovery Program (IODP) Site U1536, Ocean Drilling Program (ODP) Site 696 and piston cores from Maurice Ewing Bank). We compare these to previously published latest Eocene–latest Miocene (∼ 37–5 Ma) dinocyst assemblage records and sea surface temperature (SST) reconstructions available from the SW Atlantic Ocean in order to reveal oceanographic changes as the Southern Ocean gateways widen and deepen. The observed dinocyst assemblage changes across the latitudes suggest a progressive retraction of the subpolar gyre and southward migration of the subtropical gyre in the Oligocene–early Miocene, with strengthening of frontal systems and progressive cooling since the middle Miocene (∼ 14 Ma). Our data are in line with the timing of the removal of bathymetric and geographic obstructions in the Drake Passage and Tasmanian Gateway regions, which enhanced deep-water throughflow that broke down gyral circulation into the Antarctic circumpolar flow. Although the geographic and temporal coverage of the data is relatively limited, they provide a first insight into the surface oceanographic evolution of the late Cenozoic southern Atlantic Ocean.
... The moderate abundance (rather than dominance) of coldwater taxa (Chiasmolithus, R. daviesii) indicate that Site U1553 was not strongly influenced by cold waters originating in the subantarctic and Antarctic regions, in comparison to records from Southern Ocean sites Kerguelen Plateau and Maud Rise where these taxa are more abundant (Persico & Villa, 2004;Villa et al., 2008Villa et al., , 2014. Overall, our assemblage record suggests that Site U1553 was mostly situated north of the proto-SAF during our record, in agreement with paleoceanographic reconstructions of the region based on a range of biogenic and geochemical proxies (Hoem et al., 2021;Kamp et al., 1990;Nelson & Cooke, 2001;Pascher et al., 2015;Sarkar et al., 2019;Scher et al., 2015). Assemblage composition at Site U1553 is similar to the midlatitude Tasman Sea (IODP Site U1509), which was also dominated by Reticulofenestra, Cyclicargolithus and Coccolithus and was likely to have been situated north of the proto-STF during this time (Viganò et al., 2024). ...
... There is general consensus that surface nutrient availability increased and temperatures cooled across the southern high latitudes during the Late Eocene and into the early Oligocene (Bordiga et al., 2015;Diester-Haass & Zahn, 2001;Dunkley Jones et al., 2008;Fioroni et al., 2012;Jones et al., 2019;Persico & Villa, 2004;Villa et al., 2008Villa et al., , 2014Villa et al., , 2021, leading to enhanced high latitude primary production (Anderson & Delaney, 2005;Diester-Haass & Zahn, 1996;Latimer & Filippelli, 2002;Rodrigues de Faria et al., 2024;Salamy & Zachos, 1999). In the Australian-New Zealand subantarctic region, nutrient supply is likely to have responded to Tasman Gateway opening and the progressive development of a weak, proto-Antarctic circumpolar current during the late Eocene and Oligocene (Kennett et al., 1975;Sarkar et al., 2019;Sauermilch et al., 2021;Scher et al., 2015). This is proposed to have altered the strength and location of the proto-Subtropical Front (STF) and prevailing ocean currents in the region (Hodel et al., 2022;Hoem et al., 2021Hoem et al., , 2022Pascher et al., 2015), thus enhancing the upwelling of more nutrient-enriched water masses in the middle and high latitudes of the South Pacific in the Early Oligocene (Viganò et al., 2024). ...
Marine phytoplankton community composition influences the production and export of biomass and inorganic minerals (such as calcite), contributing to core marine ecosystem processes that drive biogeochemical cycles and support marine life. Here we use morphological and assemblage data sets within a size‐trait model to investigate the mix of cellular biogeochemical traits (size, biomass, calcite) present in high latitude calcareous nannoplankton communities through the Oligocene (ca. 34–26 Ma) to better understand the biogeochemical consequences of past climate variability on this major calcifying phytoplankton group. Our record from IODP Site U1553 in the southwest Pacific reveals that nannoplankton communities were most size diverse during the earliest Oligocene, which we propose is linked to evidence for increased nutrient availability in the region across the Eocene‐Oligocene transition. In addition to driving changes in community size structure, early Oligocene extinctions of the largest Reticulofenestra species combined with an increasing dominance of heavily calcified, small‐medium‐sized cells through time also led to an overall increase in community inorganic to organic carbon ratios (PIC:POC) throughout the Oligocene. Crucially, genus‐level cellular PIC:POC diversity meant that abundance was not always the best indicator of which species were the major contributors to community biomass and calcite. As shifts in plankton size structure and calcareous nannoplankton PIC:POC have previously been highlighted as important in biological carbon pump dynamics, our results suggest that changes in community composition that are coupled to changes in community biogeochemical trait diversity have the potential to significantly alter the role of calcareous nannoplankton in marine biogeochemical processes.
... The EOT is associated with significant changes in fundamental components of the Earth's system, including sea level, atmospheric CO 2 , orbital configuration, land to sea ratio, continental weathering rates, ocean chemistry, circulation and productivity, as well as changes in the role of many positive climate feedbacks, which triggered transient and permanent modifications in marine and terrestrial fauna and Two groups of hypotheses exist to explain the onset and development of Antarctic ice sheets: (a) "the oceangateway hypothesis" and (b) "the CO 2 hypothesis". The "ocean-gateway hypothesis", first proposed in the late 1970's, suggests that the opening of Southern Ocean gateways (Exon, Kennet, Malone et al., 2001;Hodel et al., 2022;Livermore et al., 2007;Scher et al., 2015) allowed for a rapid intensification of the Antarctic Circumpolar Current (ACC), which reduced poleward heat flux and thermally isolated Antarctica (Kennett, 1977;Sijp et al., 2004). Subsequent modeling and proxy studies suggest that a global decline in atmospheric CO 2 drove global cooling and ice sheet growth at the EOT (Anagnostou et al., 2016;DeConto & Pollard, 2003;Pagani et al., 2011;Pearson et al., 2009;Zachos & Kump, 2005), with ice-sheet feedbacks and changes in paleoceanography playing a secondary role (Hutchinson et al., 2021;Sauermilch et al., 2021). ...
... The observed early Oligocene enhanced eutrophication at Site U1509 may be linked to a crucial aspect of the late Eocene-early Oligocene paleoceanographic reorganization: the development of a proto-Antarctic Circumpolar Current (pACC) (e.g., Kennett et al., 1975;Sarkar et al., 2019;Sauermilch et al., 2021;Scher et al., 2015). This evolution likely intensified nutrient transport by southern water masses, leading to heightened upwelling systems at middle-high latitudes (Kennett et al., 1975). ...
The Eocene‐Oligocene transition (EOT; ∼34 Ma) was one of the most prominent global cooling events of the Cenozoic, coincident with the emergence of continental‐scale ice‐sheets on Antarctica. Calcareous nannoplankton experienced significant assemblage turnover at a time of long‐term surface ocean cooling and trophic conditions, suggesting cause‐effect relationships between Antarctic glaciation, broader climate changes, and the response of phytoplankton communities. To better evaluate the timing and nature of these relationships, we generated calcareous nannofossil and geochemical data sets (δ¹⁸O, δ¹³C and %CaCO3) over a ∼5 Myr stratigraphic interval recovered across the EOT from IODP Site U1509 in the Tasman Sea, South Pacific Ocean. Based on trends observed in the calcareous nannofossil assemblages, there was an overall decline of warm‐oligotrophic communities, with a shift toward taxa better adapted to cooler more eutrophic conditions. Assemblage changes indicate four distinct phases caused by temperature decrease and variations in paleocurrents: late Eocene warm‐oligotrophic phase, precursor diversity‐decrease phase, early Oligocene cold‐eutrophic phase, and a steady‐state cosmopolitan phase. The most prominent shift in the assemblages occurred during the ∼550 kyr‐long precursor diversity‐decrease phase, which has relatively high bulk δ¹⁸O and %CaCO3 values, and predates the phase of maximum glacial expansion (Earliest Oligocene Glacial Maximum–EOGM).
... In contrast, oceanic biotas in the Southern Hemisphere have been connected since Antarctica separated from South America with the formation of the Drake Passage (30-40 million years BP), creating the Antarctic Circumpolar Current (ACC) that flows from the west, connecting the Pacific, Atlantic, Indian, and Southern, and Antarctic oceans [3,47,86]. Thus, marine biotas have maintained genetic fluxes between the edges of South America and Antarctica [69]. ...
... Their impact on native fauna has been estimated through diet studies and population censuses of ground-nesting birds. In the Cape Horn Biosphere Reserve, mink diet includes similar proportions (number of food items) of native and exotic mammals, birds, and fish [20,[85][86][87][88][89]. Mink represents a critical threat to the biodiversity of terrestrial, freshwater, and coastalmarine ecosystems, including functionally key avifauna at the marine-terrestrial interface [21]. ...
Chilean Patagonia encompasses the two southernmost terrestrial ecoregions of the temperate forest biome of South America (North-Patagonian and Sub-Antarctic Magellanic) and the two western marine ecoregions of the Magallanes Province (Chiloense, and Channels and Fjords of Southern Chile). These ecoregions are immersed in a complex mosaic of terrestrial (with marked altitudinal gradients), freshwater (including wetlands, rivers, lakes, and lagoons) and marine ecosystems (with myriad islands, channels, and fjords). With more than 100,000 km of coastline, most environments in the region exhibit strong land-sea interdependency in energy and nutrient flows. The goals of the chapter are to: (i) describe the main ecological features of the marine-terrestrial interface in the channels, fjords, and archipelagoes; (ii) identify major anthropogenic impacts on marine-terrestrial connectivity; (iii) describe the most important matter and energy flows across aquatic and terrestrial ecosystem; (iv) discuss the conservation status of species that are dependent on this interface; (v) identify those public protected areas that have extensive areas of marine-terrestrial interface. The major nutrient exchanges in the marine-terrestrial interface include carbon and nitrogen-rich sediment flows transported to the ocean by the rivers and streams, and abundant debris of siliceous rocks from land to ocean carried by rivers draining glaciers and ice fields. The most important vectors of biological transport of materials between the ocean and land are large marine mammals and seabirds. This includes historical records of whale landings that mobilize nutrients from ocean bottoms to the coastal zones and large populations of seabirds that nest in the archipelagos. Major threats to the marine-terrestrial interface include the massive populations of naturalized salmon that circulate in the fjords, streams, and channels. Salmon proliferation has altered the nutrient transport from the ocean to the continental rivers. Three species of exotic mammals have increased in numbers and impact at the interface between oceans, land, and freshwater systems—the beaver (Castor canadensis), the North American mink (Neovison vison), and the muskrat (Ondatra zibethicus). In contrast to traditional views on conservation and management that segregated land–ocean interfaces, our analysis in this chapter suggests that in order to understand ecosystem functioning in Chilean Patagonia as well as to establish comprehensive conservation programs, it will be essential to address the interrelationships of biophysical processes at the marine-terrestrial interface.
... The ACC is a key component of the 'ocean conveyor belt', playing a role in the global transport of heat (Katz, et al. 2011). Moreover, this circumpolar current influences the strength of meridional overturning circulation and several authors have proposed that this current is one of the main drivers of the Atlantic meridional overturning circulation (AMOC) (Toggweiler and Samuels, 1995;Toggweiler and Bjornsson, 2000;Scher and Martin, 2006, Kuhlbrodt et al., 2007, Scher et al., 2015, Sarkar et al., 2019. ...
... Tectonic reconstructions for the Drake Passage timing opening remain controversial, ranging from the late Eocene (ca 41 Ma; Scher andMartin, 2004, 2006) Barker 2001). Even if the timing of the deepening of the Drake Passage is less well constrained, a "proto-ACC" has been proposed as an earlier expression of the ACC and it is defined as a shallow-depth circumpolar current (Scher et al., 2015, Sarkar, et al., 2019. Cramer et al. 2009 suggested that "proto-ACC" would have played an important role in the ocean circulation changes that occurred in the Eocene. ...
The Eocene-Oligocene transition (ca 40-33 Ma) marks a transformation from an ice free to an ice-house climate mode that is well recorded by oxygen stable isotopes and sea surface temperature proxies. Opening of the Southern Ocean gateways and decline in atmospheric carbon dioxide have been hypothesised as possible triggers of the major climate shift during the Cenozoic. However, the identification of the driving mechanisms remains controversial and it depends on a better understanding of how the different environmental changes correlate to each other. In this study, we investigate the spatio-temporal variation in export productivity using biogenic Ba (bio-Ba) from different Ocean Drilling Program (ODP) Sites in the Southern Ocean, focusing on possible mechanisms that controlled them as well as correlation of export productivity changes to changes in the global carbon cycle. We document two significant SO region high export productivity late-Eocene events (ca. 37 and 33.5 Ma) that are correlated to pronounced changes in global atmospheric pCO2. We propose that paleoceanographic changes that followed Southern Ocean gateway openings, along with more variable increases in circulation driven by episodic expansion and decline of the Antarctic ice sheet, drove enhanced SO export production in the late Eocene through basal Oligocene. These factors may have driven the episodic reduction of atmospheric carbon dioxide and contributed to Antarctic glaciation during the Eocene-Oligocene transition.
... The ACC plays a crucial role in global ocean circulation, which is intimately linked to the meridional overturning circulation cells of the adjoining Atlantic, Indian, and Pacific Oceans (Marshall and Speer, 2012), but its evolution on the tectonic time scale is still a topic of debate (Scher and Martin, 2004;Heinrich et al., 2011;Hill et al., 2013;Scher et al., 2015;Bijl et al., 2018;Sarkar et al., 2019). The weaker-thanpresent proto-ACC was initiated in the Late Eocene due to the opening of the Drake Passage at approximately 36 Ma (Sarkar et al., 2019). ...
... There are inter-proxy uncertainties about the exact time when the ACC increased to its modern intensity. Some studies (Scher and Martin, 2004;Hill et al., 2013;Scher et al., 2015) suggest that it ranged from the Middle Eocene (41 Ma) to the Early Miocene (23 Ma), while some studies (Heinrich., 2011;Bijl et al., 2018) suggest that it occurred only after the Middle Miocene Climatic Transition (14 Ma). The stronger ACC in MMCO simulated by FGOALS-g3 may support the former conclusions. ...
The warmer-than-present (5-10 • C) climate during the Miocene Climate Optimum (MMCO, approximately 16.9-14.7 Ma) is likely to serve as a reference for future pessimistic warming scenarios. Forced with MMCO boundary conditions, the warming and ocean circulation changes are simulated by the fully coupled climate model FGOALS-g3 with a nominal 1 • horizontal resolution ocean component model. Under a 400 ppmv CO 2 concentration, the model generally simulates the MMCO temperature well with small biases at mid and low latitudes compared to the proxy data. Large biases at high latitudes show that FGOALS-g3 fails to reproduce the weak meridional gradient indicated by the proxy record. MMCO surface albedo decreases significantly owing to changes in worldwide forest cover in the boundary condition and the amount of sea ice melt due to the warming climate compared with the PI run. Based on the Energy Balance Model decomposition, warming by the lower surface albedo reaches 1.4-2.7 • C, which is comparable to greenhouse effect warming (~2.7 • C). Accompanied by MMCO global ocean warming and land-sea distribution changes, both oceanic wind-driven and thermohaline circulations strengthen. The Antarctic Circumpolar Current in the MMCO is stronger due to the enhanced westerly wind stress and the reduced sea ice extent. The intensified MMCO Atlantic Meridional Overturning Cell (AMOC) relative to PI is likely linked to the altered ocean-gateway configuration, particularly at low and middle latitudes. When the MMCO Panama Seaway and Tethys Seaway open, waters from the Pacific and the Indian Ocean converge and mix in the western North Atlantic. Combined with the supplemental water (~30 Sv), the Gulf Stream is enhanced and flows more poleward, causing the sea ice to retreat, leading to a deeper mixed layer. Consequently, the Subpolar North Atlantic salinification causes stronger convective motion and the appearance of the Atlantic Meridional Overturning Cell.
... The initiation and subsequent evolution of the ACC is widely thought to have been dominated by the progressive tectonic opening of two Southern Ocean (SO) seaways, that is, the Tasman Gateway (TG) between Australia and East Antarctica and the Drake Passage (DP) between South America and the Antarctic Peninsula (Kennett, 1977) (Figure 1, inset). The deepening of the TG began at around 35. 5 Ma and ended at about 30.2 Ma, while the tectonic evolution of the DP was more complex (Scher et al., 2015;Stickley et al., 2004). Evidence for its initial opening was found at approximately 41 Ma allowing the first shallow Pacific seawater inflow into the Atlantic sector of the SO (Scher & Martin, 2006). ...
The Antarctic Circumpolar Current (ACC) is Earth's largest current flowing around Antarctica at all depths and connecting major ocean basins, thus representing an important component of Earth's climate. However, the timing and key controls determining ACC flow path and its strength as a function of past climatic boundary conditions that ultimately resulted in its modern configuration remain unclear due to major uncertainties in paleoceanographic and tectonic reconstructions. Here we present a unique high‐resolution laser ablation‐derived late Cenozoic seawater lead isotope record obtained from a hydrogenetic ferromanganese crust from the Pacific sector of the Southern Ocean. Our Pb isotope data reveal that the ACC has experienced five stable circulation states since the early Miocene which were separated by four major transitions observed at 17.5‐14.6, 12, 10 and 5 Ma. We suggest that the relatively abrupt transitions between ACC circulation state were mainly induced by tectonic changes, whereas the impact of climatic changes was of secondary importance. According to our data the modern ACC configuration formed 5 million years ago, likely in response to the closure of the Panama Seaway. Since the Drake Passage (DP) has already been an open seaway since at least the late Miocene, our results demonstrate that DP opening was not the only factor affecting past ACC circulation. Our data also show that changes in the latitudinal position of the ACC were linked to the middle Miocene waxing and waning of the Antarctic ice sheets, which emphasizes the ACC's critical role as a key control of Antarctic glaciation.
... While these are occasionally captured by the clockwise Weddell Sea Gyre, the majority either melt or disintegrate before reaching the Weddell Sea, or drift sufficiently offshore while traversing the narrow Eastern Antarctic continental shelf to be captured by the clockwise Antarctic circumpolar current (Budge and Long, 2018). The Antarctic circumpolar current is likely to have existed since at least ~30 Ma (Scher et al., 2015) and thus pre-dates deposition of our samples, although its modern strength may have been attained as late as ~12 Ma (Evangelinos et al., 2024). While the development of Weddell Sea circulation is less well understood, a regional unconformity in the Scotia Sea dated to ~8 Ma records contourite reorientation when bottom water flow driven by the Weddell gyre locally overcame the Antarctic Circumpolar Current (Pérez et al., 2021). ...
... Tasmanian Gateway (Scher et al., 2015) and the resulting onset of the eastward transport of the ACC in the early Oligocene Hochmuth et al., 2020) and the inception of a modern ACC in the late Miocene (Evangelinos et al., 2024). Figure S4 in Supporting Information S1. Figure S5 in Supporting Information S1. ...
The deep glacial trough and hinterland of the Denman Glacier (East Antarctica) makes the area around the Shackleton ice shelf sensitive to ice loss due to warmer deep water intruding onto the continental shelf in the near future. In addition, the configuration of the ocean currents offshore is an important factor in priming the local and regional vulnerability to warm water intrusions. Here, we use reflection seismic data sets from the Bruce Rise offshore the Denman‐Shackleton region to investigate the Cenozoic history of the ocean bottom current configurations offshore and their influence on the Cenozoic sedimentation patterns of the Denman‐Shackleton region. On the Bruce Rise, sediment drift building, and erosional features indicate three distinct ocean current configurations, (a) the production of dense shelf waters in times of a smaller East Antarctic Ice Sheet (EAIS), (b) periods dominated by a strong Antarctic Slope Current (ASC) and (c) periods with a weak ASC. During the early establishment of the EAIS, the Denman‐Shackleton area contributed to the production of Antarctic Bottom Water, a process which stabilizes the regional ice sheet. With a growing icesheet, the ASC strengthened representing an effective barrier between the continental shelf and the warmer water masses of the deeper ocean during most of the times of an extended EAIS. The transition in the paleoceanographic setting from a strong, erosive ASC, toward a weak ASC increases the vulnerability of the Denman‐Shackleton continental shelf to deep water intrusions as we are observing today.
... The same paleogeographic changes might be the main reason for the presence of floristic migration routes across Eurasia in the early Oligocene (e.g., Akhmetiev et al., 2009). The decline of global atmospheric CO 2 (Beerling and Royer, 2011;Sheldon et al., 2012;Anagnostou et al., 2016;Elsworth et al., 2017), strengthening of changes in the Antarctic circumpolar current, opening of the Southern Ocean gateways (Tasman Seaway and Drake Passage) and orbital variations (Egan et al., 2013;Scher et al., 2015;Coxall et al., 2018;Tardif et al., 2021) may be driving factors of that climate transition (Coxall and Pearson, 2007). The EOT has been extensively studied and described from marine and terrestrial sediments (Zachos et al., 1996;Liu et al., 2009;Pearson et al., 2009;Coxall et al., 2018;Wade et al., 2020;Toumoulin et al., 2022). ...
The Eocene-Oligocene transition (EOT) marked a rapid global cooling event, often considered as the beginning of the modern icehouse world. Influenced by various factors, including tectonic activity and paleogeographic settings, the terrestrial records indicate a diverse response of fauna and vegetation to this global event. We examined nine macrofossil assemblages from seven fossil localities on the southeastern margin of the Tibetan Plateau and from the mid-latitudinal Europe ranging from the latest Bartonian and Priabonian (37.71–33.9 Ma) to the Rupelian (33.9–27.82 Ma). Our aims were to trace and compare the vegetation history of both regions in the late Eocene and early Oligocene. The results show that both regions experienced changes in vegetation composition in response to climate change, characterized by a decrease in the percentages of broad-leaved evergreen elements and distinctive changes in general vegetation types. A general change in the overall vegetation type from subtropical broad-leaved evergreen forests in the late Eocene to temperate broad-leaved mixed deciduous evergreen forests, or mixed mesophytic forests, in the early Oligocene is recognized in both regions. The results indicate a clear change in leaf architecture, leaf margin states, and secondary venation types in the mid-latitudinal Europe, while the results from the south-eastern margin of the Tibetan Plateau show a distinct reduction in leaf size. Our data suggest that both global and regional factors played key roles in shaping the vegetation in the two regions.
... The effects of both mechanisms on the Eocene-Oligocene cooling event have been investigated thoroughly, with a dramatic cooling of the surface waters recorded once the Antarctic Circumpolar Current formed deep-water currents (>300 m), demonstrating that tectonic changes, which allowed the Antarctic Circumpolar Current to form, are of major importance for the Eocene-Oligocene cooling event (Sauermilch et al., 2021). These tectonic changes involved the complex continental break-up of Southern Gondwanaland during the Cenozoic, which led to the complete separation of the Antarctic continent and marked the onset of the Antarctic Circumpolar Current (Barker et al., 2007;Eagles et al., 2006;Eagles and Jokat, 2014;Scher et al., 2015). The tectonic opening of both the Tasman Seaway, between the South Tasman Rise south of Tasmania and Antarctica, and Drake Passage (Figure 1), between South America and Antarctica, allowed the Antarctic Circumpolar Current to develop. ...
The interplay between regional tectonics and the development of a major ocean gateway between the Pacific and the Atlantic Ocean has resulted in numerous paleogeographic reconstruction studies that describe the Cenozoic tectonic history of the Scotia Sea region. Despite the multitude of published tectonic reconstructions and the variety of geological and geophysical data available from the Scotia Sea, the geological history remains ambiguous. We present a comparative paleogeographic analysis of previously published tectonic reconstructions to identify agreements and conflicts between these reconstructions. We propose an alternative model to explain the Cenozoic evolution of the Scotia Sea region. The paleogeographic comparison shows that most reconstructions agree on the tectonic evolution of the South Scotia Ridge and the East Scotia Ridge. Major differences between the reconstructions are the role of the westward subducting plate below the South Sandwich plate, and the age and origin of the Central Scotia Sea. Tectonic reconstructions assume that the Central Scotia Sea is either part of a Cenozoic back‐arc basin, or a captured piece of Cretaceous oceanic crust. We propose a new alternative tectonic reconstruction that brings these two prevailing hypotheses elegantly together. Here, we identified new geographical blocks consisting of thinned continental or Cretaceous oceanic fragments that originate from the Paleo‐Pacific Weddell Sea gateway from high‐resolution bathymetry. These fragments are now part of the Central Scotia Sea and have been affected by early back‐arc tectonic activity of the South Sandwich subduction zone, leading locally to the formation of Cenozoic‐aged crust in the Central Scotia Sea.
... In fact, the Antarctic region is considered to be a centre of origin of some deep-water fauna, including megaleledonid octopuses (Collins & Rodhouse, 2006;Strugnell et al., 2008), with high levels of endemism (Dayton et al., 1994;Rosa et al., 2019). The Antarctic fauna was isolated by the separation of Antarctica from South America and Australia, and the subsequent formation of the Antarctic Circumpolar Current (Livermore et al., 2005;Scher et al., 2015). Also, high levels of octopod endemism are associated with evolutionary forces that favour holobenthic development (and larger eggs; the socalled 'Thorson's rule') and enhanced in situ speciation (Ibáñez et al., 2018). ...
... Previously reported seawater ε Nd records in the Pacific Ocean (>1500 m water depth) have been used to characterize changes in deep-water circulation over the Oligocene-Miocene (Table S2; Fig. S3) (Ling et al., 1997;van de Flierdt et al., 2004;Scher et al., 2015;Le Houedec et al., 2016;McKinley et al., 2019). High-resolution ε Nd records from ODP Site 807 (2800 m, average -4.1) allow us to constrain the influence of Upper Circumpolar Deep Water (UCDW) on the SCS (Le Houedec et al., 2016). ...
A long-term neodymium isotope (εNd) record of fossil fish teeth was investigated to constrain the evolution of deep-water circulation in the abyssal South China Sea (SCS) during the late Oligocene−Miocene (27−10 Ma). Fish teeth samples were collected from the oceanic red beds at International Ocean Discovery Program Expedition 367 Site U1499 (water depth 3758 m). Seawater εNd values (from −7.1 to −4.8, average −6.1) prior to 15 Ma indicate that water masses in the abyssal SCS resulted from the mixing of more radiogenic Upper Circumpolar Deep Water (UCDW, average −4.5) and less radiogenic Lower Circumpolar Deep Water (LCDW, average −6.4). The general decrease in εNd values was attributed to an increasing influence of the unradiogenic LCDW at the studied site, consistent with the subsidence and the associated deepening of the SCS plain. After 15 Ma, seawater εNd dropped significantly to a range of −8.9 to −6.1 (average −7.5), indicating a slowdown in the hydrological connection between the deep-water masses in the SCS and the western Pacific Ocean. We argue that the formation of the Luzon Strait due to the uplift of the Luzon arc in the late Miocene led to the shallowing and narrowing of the SCS-Pacific channels. Consequently, penetration of LCDW was reduced and water masses in the abyssal SCS would have been less ventilated and strongly influenced by lithogenic input from the unradiogenic sediments of large Asian rivers draining the peri-Himalayan region.
... Similarly, the earliest dates for a shallow opening of the Tasman Gateway are also 50 Ma (Bijl et al., 2013) with a deep seaway between the Indian and Pacific basins in place by 35-32 Ma (Royer & Rollet, 1997) and the potential for a relatively rapid subsidence history (Stickley et al., 2004). Dredged sediment samples further suggest that rapid ACC flow through the Tasman Gateway might not have occurred for another 3-5 million years, when the gateway was aligned with the westerly wind jet (Scher et al., 2015). A hypothesis that was tested by shifting the zonal winds southwards in a model by Xing et al. (2022). ...
Ambiguity over the Eocene opening times of the Tasman Gateway and Drake Passage makes it difficult to determine the initiation time of the Antarctic Circumpolar Current. If the Tasman Gateway opened later than Drake Passage, then Australia may have prevented the proto‐ACC from forming. Recent modeling results have shown that only a relatively weak circumpolar transport results under Eocene surface forcing. This leads to warm and buoyant coastal water around Antarctica, which may impede the formation of deep waters and convective processes. This suggests that a change in deep water formation might be required to increase the density contrast across the Southern Ocean and increase circumpolar transport. Here we use a simple reduced gravity model with two basins, to represent the Atlantic and the Pacific. This fixes the density difference between surface and deep water and allows us to isolate the impact of deep water formation on circumpolar transport. With no obstacle on the southern boundary the circumpolar current increases its transport from 82.3 to 270.0 Sv with deep water formation. Placing an Antipodean landmass on the southern boundary reduces this transport as the landmass increases in size. However, circumpolar flow north of this landmass remains a possibility even without deep water formation. Weak circumpolar transport continues until the basin is completely blocked by the Antipodes. When the Antipodes is instead allowed to split from the southern boundary, circumpolar transport recovers to its unobstructed value. Flow rapidly switches to south of the Antipodes when the gateway is narrow.
... The long-term and gradual trends in our data appears inconsistent with a major influence exerted by faster-acting processes associated with onset of significant Antarctic glaciation. Tectonic forcing has been proposed as an underlying control on late Eocene to EOT ocean circulation changes in the South Atlantic (Borelli et al., 2014;Langton et al., 2016) and Southern Ocean (Scher, 2017;Scher & Martin, 2006;Scher et al., 2015). In the North Atlantic, the tectonically driven deepening and shoaling of the Greenland-Scotland Ridge (GSR) during the Cenozoic has been invoked as a major control on deep-water production and flow from the shallower Nordic Seas southward into the deeper North Atlantic (Abelson et al., 2008;Davies et al., 2001;Hutchinson et al., 2019;Via & Thomas, 2006). ...
The role played by ocean circulation in major transitions in Earth's climate is debated. Here, we investigate the physical evolution of the Deep Western Boundary Current (DWBC) in the western North Atlantic Ocean through the late Eocene‐to‐mid Oligocene (35−26 Ma) using terrigenous grain size and geochemistry records of marine sediment cores. Our records cover the most pivotal transition in Cenozoic climate history, the Eocene‐Oligocene Transition (EOT; ∼33.7 Ma), when Earth first became sufficiently cool to sustain large ice sheets on Antarctica. To assess changes in deep‐water circulation in the northwest Atlantic across the EOT, we assembled sortable silt (10–63 μm) grain‐size and Nd, Hf, and Pb radiogenic isotope records at two Integrated Ocean Drilling Program (IODP) drill sites on the Newfoundland ridges (Sites U1406 and U1411). These records reveal an overall gradual increase in sortable silt abundance (SS%) at both sites with no change in sediment provenance. We interpret a steady, long‐term invigoration of the DWBC, likely driven by deepening of the Greenland‐Scotland Ridge and resultant enhanced inflow of waters sourced from deep‐water production sites in the Nordic Seas to the North Atlantic Ocean. Our results do not support abrupt and widespread invigoration of bottom current activity in the North Atlantic synchronous with accelerated cooling and Antarctic ice growth at the EOT. Instead, our records suggest that the DWBC started to intensify before this pivotal event in Cenozoic climate history (at ∼35 Ma) and then further strengthened gradually across the EOT (∼34 Ma) and through the early‐to‐mid Oligocene (∼34‒26 Ma).
... This led to stronger density contrast and intensified Southern Westerly Winds across the Southern Ocean, establishing a vigorous deep-reaching circumpolar flow and an enhanced global overturning circulation, which amplified the late Cenozoic global cooling. the timing and processes that led to the development of the modern-like deep-reaching and homogeneous ACC (circumpolar flow extending from the surface to the seafloor), a topic that has been debated for more than four decades [3][4][5][6][7][8][9][10] . ...
The Antarctic Circumpolar Current plays a pivotal role in global climate through its strong influence on the global overturning circulation, ocean heat and CO2 uptake. However, when and how the Antarctic Circumpolar Current reached its modern-like characteristics remains disputed. Here we present neodymium isotope and sortable silt records from sediment cores in the Southwest Pacific and South Indian oceans spanning the past 31 million years. Our data indicate that a circumpolar current like that of today did not exist before the late Miocene cooling. These findings suggest that the emergence of a homogeneous and deep-reaching strong Antarctic Circumpolar Current was not linked solely to the opening and deepening of Southern Ocean Gateways triggering continental-scale Antarctic Ice Sheet expansion during the Eocene–Oligocene Transition (∼34 Ma). Instead, we find that besides tectonic pre-conditioning, the expansion of the Antarctic Ice Sheet and sea ice since the middle Miocene Climate Transition (∼14 Ma) played a crucial role. This led to stronger density contrast and intensified Southern Westerly Winds across the Southern Ocean, establishing a vigorous deep-reaching circumpolar flow and an enhanced global overturning circulation, which amplified the late Cenozoic global cooling.
... Notably, the closure of low-latitude seaways in regions like Indonesia and Panama has been linked to global cooling events (Cane and Molnar, 2001;Montes et al., 2015). The opening of high-latitude seaways and the closure of low-latitude ones exert similar influences on global climate, albeit operating on different principles (Scher et al., 2015). Furthermore, increasing land area at low latitudes can impact the temperature and salinity of equatorial oceans, potentially disrupting the significant temperature-salt difference between the poles and the equator. ...
Understanding changes in Earth's past can provide valuable insights into prediction of its future. An example is the interactions between the internal and external spheres of Earth. The cyclical northward breakup-drift of Gondwana, driven by the opening and closure of Proto-, Paleo-, and Neo-Tethyan oceans, facilitated the transfer of landmasses from the southern to the northern hemisphere, traversing the tropic region. We have observed a compelling correlation between episodic increases in landmass area within the tropic regions (those lying at less than 20°latitude) and a subsequent temperature decrease during the three major glacial periods in the last 500 million years. This phenomenon can be attributed to low latitude regions receiving more solar energy influx on Earth's surface than high latitude areas. In addition, an increase of landmass in tropic regions (low latitude) attenuates the net energy absorption by the Earth's surface, consequently impeding the conduction and convection of absorbed energy toward the poles. The result is a decrease in global surface temperature. The tropic regions, benefiting from abundant sunlight, create an ideal environment for the proliferation of marine plankton species. These species are important in the generation of organic-rich sediment. Massive biological debris is therefore deposited on continental margins when a continent drifts across the tropic region. This creates favorable conditions for future hydrocarbon and reservoir formation. Northward subduction of organic-rich sediments during the closure of the Tethyan oceans results in the generation of mafic arc magmas with low oxygen fugacity. This chemical environment helps the mineralization of reduced-type ore deposits such as tungsten, tin, and lithium. Subducted-driven plate tectonics in the Tethys realm changes the distribution of oceans and landmass, subsequently affecting the balance and distribution of solar energy across Earth's surface. These changes trigger consequential environmental shifts which in turn, impact the composition of rock and mineral along the Eurasian margin due to subduction. Consequently, the Tethyan realm and its history is an ideal natural laboratory for comprehending the processes and changes of the entire Earth's system.
... Movements of tectonic plates control the long-term circulation patterns of open oceans and marginal seas, thus exerting a significant influence on global climate on million-year timescales (Zachos et al., 2001;Knutz, 2008;Dummann et al., 2020;Straume et al., 2020) via the opening/closing and deepening/shallowing of oceanic gateways (Wunsch, 2002;Zhang et al., 2011;Sijp et al., 2014;Straume et al., 2020;Bahr et al., 2023). Considerable attention has therefore been lent to how tectonics may affect ocean circulation and associated climate change (Straume et al., 2020(Straume et al., , 2022, with examples including the Eocene-Oligocene opening of the Drake Passage and Tasman Gateway in the Southern Ocean (Zachos et al., 2001;Barker and Thomas, 2004;Scher et al., 2015), the Miocene-Pliocene closing of the Indonesian Seaway between the Pacific and Indian oceans (Gourlan et al., 2008;Smith et al., 2020), and the mid-Miocene-Pliocene closing of the Central American Seaway between the Pacific and Atlantic oceans (Brierley and Fedorov, 2016). ...
... Divergence time analyses suggest that the most recent common ancestor of the Nacellidae occurred around 30 Ma, close to the Eocene-Oligocene boundary, a period of marked worldwide tectonic, oceanographic and climatic changes, particularly in the Southern Ocean [34,35]. Since the Eocene, a global cooling trend is observed commonly explained by the combined effect of gradual northward movement of continents, the closure of the Tethys Sea, and the isolation of Antarctica and the initiation of the Antarctic Circumpolar Current [36,37]. Considering these settings, we propose that the cladogenetic process separating tropical/temperate Cellana from Antarctic/subantarctic Nacella coincides with the shift to glacial conditions in the Southern Ocean [38,39]. ...
Oceanic islands lacking connections to other land are extremely isolated from sources of potential colonists and have acquired their biota mainly through dispersal from geographically distant areas. Hence, isolated island biota constitutes interesting models to infer bio-geographical mechanisms of dispersal, colonization, differentiation, and speciation. Limpets of the genus Cellana (Nacellidae: Patellogastropoda) show limited dispersal capacity but are broadly distributed across the Indo-Pacific including many endemic species in isolated oceanic islands. Here, we examined main distributional patterns and geographic boundaries among Cellana lineages with special emphasis in the relationships of Southern Hemisphere oceanic islands species. Phylogenetic reconstructions based on mtDNA (COI) recognized three main clades in Cellana including taxa from different provinces of the Indo-Pacific. Clear genetic discontinuities characterize the biogeography of Cellana and several lineages are associated to particular areas of the Indo-Pacific supporting the low dispersal capacity of the genus across recognized biogeographical barriers in the region. However, evolutionary relationships within Cellana suggest that long-distance dispersal processes have been common in the history of the genus and probably associated to the origin of the species in Hawaii and Juan Ferná ndez Archipelago. Therefore, the presence of Cellana species in geographically distant Southern Hemisphere oceanic islands, such as the Juan Fernández Archipelago, suggests that long-distance dispersal mediated by rafting may have played an important role in the biogeography of the genus.
... Yet, the lack of a strong SST gradient, and absence of a prominent increase thereof across EOT, suggest a persistent wind driven gyral 330 circulation that connected all sites (Sauermilch et al., 2021). The position of the continents and the position of the winds had a strong effect on patterns of Southern Ocean circulation (Scher et al., 2015;Evangelinos et al., 2022), which during the EOT ...
At present, a strong latitudinal sea surface temperature (SST) gradient of ~16 °C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC-system, its development towards modern-day strength, and the consequences for e.g., the latitudinal SST gradient around the southern Atlantic Ocean, are still uncertain. Here we present new TEX86-biomarker records, calibrated to SST, from two sites located east of Drake Passage (southern South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: The Late Eocene–Early Oligocene (ODP Site 696) and Middle–Late Miocene (IODP Site U1536) transitions. Our results overall show rather temperate conditions (20–11 °C) during the Late Eocene to Early Oligocene interval, with a weaker latitudinal SST gradient (~8 °C) across the Atlantic sector of the Southern Ocean compared to present day (~16 °C). We ascribe the regional similarity in SSTs across the Late Eocene–Early Oligocene South Atlantic to a persistent, strong Subpolar Gyre circulation, connecting all sites, which can only exist in absence of a strong throughflow across the Drake Passage. Surprisingly, the southern South Atlantic records show comparable SSTs (~12–14 °C) during both the Earliest Oligocene Oxygen Isotope Step (EOIS, ~33.65 Ma) and the Miocene Climate Optimum (MCO, ~16.5 Ma). Apparently, maximum Oligocene Antarctic ice volume could coexist with warm ice-proximal surface ocean conditions, while at similar ocean temperatures, the Middle Miocene Antarctic ice sheet was strongly reduced. Southern South Atlantic SSTs cooled to ~5 °C at the onset of the Middle Miocene Climate Transition (MMCT, 14 Ma), making it the coldest oceanic region recorded around Antarctica and the likely main location for deep water formation. The already cold southern South Atlantic conditions at MMCT meant it experienced little cooling during the latter part of the Miocene, which contrasts the profound cooling due to northward expansion of the Southern Ocean frontal systems in the lower latitudes and other sectors of the Southern Ocean.
... TIBBETT ET AL. 10.1029/2022PA004496 2 of 21 the timing does not match the proposed mechanism. Deep water currents through the Tasman Gateway were first established around 30 Ma (Scher et al., 2015), that is, after the EOT. For the Drake Passage, full opening may have occurred even later, in the Miocene (Dalziel et al., 2013). ...
The Eocene‐Oligocene transition (EOT) marks the shift from greenhouse to icehouse conditions at 34 Ma, when a permanent ice sheet developed on Antarctica. Climate modeling studies have recently assessed the drivers of the transition globally. Here we revisit those experiments for a detailed study of the southern high latitudes in comparison to the growing number of mean annual sea surface temperature (SST) and mean air temperature (MAT) proxy reconstructions, allowing us to assess proxy‐model temperature agreement and refine estimates for the magnitude of the pCO2 forcing of the EOT. We compile and update published proxy temperature records on and around Antarctica for the late Eocene (38–34 Ma) and early Oligocene (34–30 Ma). Compiled SST proxies cool by up to 3°C and MAT by up to 4°C between the timeslices. Proxy data were compared to previous climate model simulations representing pre‐ and post‐EOT, typically forced with a halving of pCO2. We scaled the model outputs to identify the magnitude of pCO2 change needed to drive a commensurate change in temperature to best fit the temperature proxies. The multi‐model ensemble needs a 30 or 33% decrease in pCO2, to best fit MAT or SST proxies respectively. These proxy‐model intercomparisons identify declining pCO2 as the primary forcing of EOT cooling, with a magnitude (200 or 243 ppmv) approaching that of the pCO2 proxies (150 ppmv). However individual model estimates span a decrease of 66–375 ppmv, thus proxy‐model uncertainties are dominated by model divergence.
... The southern-sourced bottom waters (>2000 m) in the northern Indian Ocean are strongly influenced by the AABW, with minor contributions from the North Atlantic Deep Water (NADW) [70]. The AABW is one of the most ancient water masses; it originates from dense shelf water around the Antarctic, supplies the lower limb of the global overturning circulation [71] and might have existed since approximately 40-32 Ma with the progressive opening of the Drake Passage and Tasmanian gateway [72,73]. Previous studies have assumed that the AABW has a significant influence on the hydrology of the Indian Ocean since the opening of the Drake Passage [54,74], and the e Nd of proto-AABW showed similar values compared with the modern e Nd value of the AABW [75]. ...
Enhanced silicate weathering induced by the uplift of the Himalayan-Tibetan Plateau (HTP) has been considered as the major cause of pCO2 decline and Cenozoic cooling. However, this hypothesis remains to be validated, largely due to the lack of a reliable reconstruction of the HTP weathering flux. Here, we present a 37-million-year record of the difference in the seawater radiogenic neodymium isotopic composition (DeNd) of Ocean Drilling Program (ODP) sites and Fe-Mn crusts between the northern and central Indian Ocean, which indicates the contribution of regional weathering input from the South Asian continent to the Indian Ocean. The results show a long-term increase in DeNd and thus provide the first critical evidence of enhanced South Asian weathering input since the late Eocene. The evolution coincided well with major pulses of surface uplift in the HTP and global climatic transitions. Our foraminiferal eNd record suggests that tectonic uplift and silicate weathering in South Asia, especially in the Himalayas, might have played a significant role in the late Cenozoic cooling.
This chapter provides an introduction into Chaps. 9 and 10, focussed on abiotic processes. Earth abiotic processes are influenced by interactions between terrestrial and extra-terrestrial forces inherited form the origins of the universe. Chaotic dynamics of abiotic processes are driving forces of biological evolution, providing energetic sources together with constraints for organic life with different factor combinations in marine and terrestrial habitats. Physical properties of water together with particular dynamic characteristics and giant dimensions of the three-dimensional oceanic habitats were preconditions for the evolution of animal life on the energetic basis of microorganisms. Physical properties of the atmosphere and lithosphere were primary preconditions for the evolution of terrestrial vegetation with trenchant consequences for the evolution of terrestrial animal life forms.
The Oligocene (33.9–23.03 Ma) had warm climates with flattened meridional temperature gradients, while Antarctica retained a significant cryosphere. These may pose imperfect analogues to distant future climate states with unipolar icehouse conditions. Although local and regional climate and environmental reconstructions of Oligocene conditions are available, the community lacks synthesis of regional reconstructions. To provide a comprehensive overview of marine and terrestrial climate and environmental conditions in the Oligocene, and a reconstruction of trends through time, we review marine and terrestrial proxy records and compare these to numerical climate model simulations of the Oligocene. Results, based on the present relatively sparse data, suggest temperatures around the Equator that are similar to modern temperatures. Sea surface temperatures (SSTs) show patterns similar to land temperatures, with warm conditions at mid- and high latitudes (∼60–90°), especially in the Southern Hemisphere (SH). Vegetation-based precipitation reconstructions of the Oligocene suggest regionally drier conditions compared to modern times around the Equator. When compared to proxy data, climate model simulations overestimate Oligocene precipitation in most areas, particularly the tropics. Temperatures around the mid- to high latitudes are generally underestimated in models compared to proxy data and tend to overestimate the warming in the tropics. In line with previous proxy-to-model comparisons, we find that models underestimate polar amplification and overestimate the Equator-to-pole temperature gradient suggested from the available proxy data. This further stresses the urgency of solving this widely recorded problem for past warm climates, such as the Oligocene.
The Eocene–Oligocene transition (EOT, ca. 40–33 Ma) marks a transformation from a largely ice-free to an icehouse climate mode that is well recorded by oxygen-stable isotopes and sea surface temperature proxies. Opening of the Southern Ocean gateways and decline in atmospheric carbon dioxide levels have been considered as factors in this global environmental transformation and the growth of ice sheets in Antarctica during the Cenozoic. A more comprehensive understanding is still needed of the interplay between forcing versus response, the correlation among environmental changes, and the involved feedback mechanisms. In this study, we investigate the spatio-temporal variation in export productivity using biogenic Ba (bio-Ba) from Ocean Drilling Program (ODP) sites in the Southern Ocean, focusing on possible mechanisms that controlled them as well as the correlation of export productivity changes to changes in the global carbon cycle. We document two high export productivity events in the Southern Ocean during the late Eocene (ca. 37 and 33.5 Ma) that correlate to proposed gateway-driven changes in regional circulation and to changes in global atmospheric pCO2 levels. Our findings suggest that paleoceanographic changes following Southern Ocean gateway openings, along with more variable increases in circulation driven by episodic Antarctic ice sheet expansion, enhanced export production in the Southern Ocean from the late Eocene through early Oligocene. These factors may have played a role in episodic atmospheric carbon dioxide reduction, contributing to Antarctic glaciation during the Eocene–Oligocene transition.
This chapter provides an updated overview of extant octopus species richness from the aspects of taxonomic rank, lifestyles, and habitat
and identifies global hotspots of coastal species richness (at both realm and ecoregion levels) for the ranks of family and genus. Also
assessed are global patterns of coastal endemicity and modality types of the latitudinal gradients of species richness. Most octopus species
are coastal (47% and 57% of genera and species, respectively), and the best-known shelf-associated species inhabit the realms of the
Central Indo-Pacific (CIP; 31%), Temperate Northern Pacific (TNP; 29%), and Temperate Australasia (TAUS; 15%). Yet, at increased
spatial resolution (eco-region level), the richness hotspot for coastal octopuses is located outside CIP, in the Central Kuroshio Current
eco-region (CKC; with 21 species) located within the TNP. The Caribbean and Cortezian ecoregions are hotspots for Octopus, the most
diverse genus. Shelf-associated octopuses show the highest rates of endemism in the Southern Ocean (87%), followed by Temperate
South America (67%) and TAUS (65%), and the lowest endemism in the Arctic (17%). On a global scale, the latitudinal gradients
of species richness (LGSR) of coastal octopuses peaks at 25°N, rather than near the equator. The existence of bimodality of LGSR
in coastal octopuses suggests that species may have evolved through thermal adaptation at the edges of the tropics, but analyses of historical
biogeography and species–area–energy hypotheses will be necessary to fully understand broad-scale variation in octopus biodiversity
and biogeography.
The Paleogene is a crucial period when terrestrial and marine ecosystems recovered from major disruptions and gradually approached their modern states. In the Qinghai-Tibetan Plateau and its surrounding regions, the Paleogene also represents a significant phase of tectonic evolution in the Qinghai-Tibetan Plateau-Himalaya orogeny, reorganization of Asian climates, and evolution of biodiversity. Due to limitations in research conditions and understanding, there are still many controversies regarding stratigraphic divisions in the Qinghai-Tibetan Plateau and its surrounding regions In recent years, extensive studies on sedimentary petrology, magnetostratigraphy, and isotope dating have been conducted in the region. Numerous fossils have been discovered and reported, contributing to a more systematic understanding of biostratigraphy. These studies have laid a solid foundation for the comprehensive investigation of the stratigraphy, biotas and paleogeographic evolution of the Qinghai-Tibetan Plateau and its surrounding regions during the Paleogene. In this paper, we integrate recent research on fossils, isotopic dating, magnetostratigraphy, and geochemistry to refine the stratigraphic divisions and correlation framework of different tectonic units in the region, building upon previous studies. Since the Second Tibetan Plateau Scientific Expedition and Research, the knowledge of Paleogene floras has gradually expanded. This paper discusses the biostratigraphic significance of extinct and newly appeared taxa based on the latest dating results of these plant species. The new understanding of fossil species such as the “Eucalyptus” and Arecaceae establishes connections between the Paleogene flora of the Qinghai-Tibetan region and the biotas of Gondwana, specifically Oceania and South America. The evolutionary history of key taxa near the Yarlung Zangbo suture zone indicates that the collision between the Indian and Eurasian plates occurred approximately 65–54 Ma. Paleoelevation reconstructions, based on plant fossils, suggest that the Hengduan Mountain had already formed their current topographic pattern prior to the Early Oligocene. The warm and humid lowlands adjacent to the main suture zones in the Paleogene Qinghai-Tibetan Plateau served as the primary pathway for biota exchanges. The relatively low elevation of the Himalaya during the Paleogene did not effectively block the moisture from the Indian Ocean.
The 87Sr / 86Sr of marine carbonates provides a key constraint on the balance of continental weathering and hydrothermal Sr fluxes to the ocean, and the mid-Oligocene to mid-Miocene period features the most rapid rates of increase in the 87Sr / 86Sr of the Cenozoic. Because previous records of the 87Sr / 86Sr increase with time were based on biostratigraphically defined age models in diverse locations, it was difficult to unambiguously distinguish million-year-scale variations in the rate of 87Sr / 86Sr change from variations in sedimentation rate. In this study, we produce the first 87Sr / 86Sr results from an Oligocene to early Miocene site with a precise age-model-derived orbital tuning of high-resolution benthic δ18O at Equatorial Pacific Ocean Drilling Program (ODP) Site 1218. Our new dataset resolves transient decreases in 87Sr / 86Sr, as well as periods of relative stasis. These changes can be directly compared with the high-resolution benthic δ18O at the same site. We find that slowing of the rate of 87Sr / 86Sr increase coincides with the onset of Antarctic ice expansion at the beginning of the mid-Oligocene glacial interval, and a rapid steeping in the 87Sr / 86Sr increase coincides with the benthic δ18O evidence for rapid ice retreat. This pattern may reflect either northward shifts in the Intertropical Convergence Zone precipitation to areas of nonradiogenic bedrock and/or lowered weathering fluxes from highly radiogenic glacial flours on Antarctica. We additionally generate the first 87Sr / 86Sr data from ODP Site 1168 on the Tasman Rise and Integrated Ocean Drilling Program (IODP) Site 1406 of the Newfoundland Margin during the Oligocene to early Miocene to improve the precision of age correlation of these Northern Hemisphere and Southern Hemisphere midlatitude sites and to better estimate the duration of early Miocene hiatus and condensed sedimentation.
Genetic variation is instrumental for adaptation to changing environments but it is unclear how it is structured and contributes to adaptation in pelagic species lacking clear barriers to gene flow. Here we applied comparative genomics to extensive transcriptome datasets from 20 krill species collected across the Atlantic, Indian, Pacific and Southern Oceans. We compared genetic variation both within and between species to elucidate their evolutionary history and genomic bases of adaptation. We resolved phylogenetic interrelationships and uncovered genomic evidence to elevate the cryptic Euphausia similis var. armata into species. Levels of genetic variation and rates of adaptive protein evolution vary widely. Species endemic to the cold Southern Ocean, such as the Antarctic krill E. superba, showed less genetic variation and lower evolutionary rates than other species. This could suggest a low adaptive potential to rapid climate change. We uncovered hundreds of candidate genes with signatures of adaptive evolution among Antarctic Euphausia but did not observe strong evidence of adaptive convergence with the predominantly Arctic Thysanoessa. We instead identified candidates for cold-adaptation that have also been detected in Antarctic fish, including genes that govern thermal reception such as TrpA1. Our results suggest parallel genetic responses to similar selection pressures across Antarctic taxa and provide new insights into the adaptive potential of important zooplankton already affected by climate change.
Knowledge of Antarctica's sedimentary basins builds our understanding of the coupled evolution of tectonics, ice, ocean, and climate. Sedimentary basins have properties distinct from basement‐dominated regions that impact ice‐sheet dynamics, potentially influencing future ice‐sheet change. Despite their importance, our knowledge of Antarctic sedimentary basins is restricted. Remoteness, the harsh environment, the overlying ice sheet, ice shelves, and sea ice all make fieldwork challenging. Nonetheless, in the past decade the geophysics community has made great progress in internationally coordinated data collection and compilation with parallel advances in data processing and analysis supporting a new insight into Antarctica's subglacial environment. Here, we summarize recent progress in understanding Antarctica's sedimentary basins. We review advances in the technical capability of radar, potential fields, seismic, and electromagnetic techniques to detect and characterize basins beneath ice and advances in integrated multi‐data interpretation including machine‐learning approaches. These new capabilities permit a continent‐wide mapping of Antarctica's sedimentary basins and their characteristics, aiding definition of the tectonic development of the continent. Crucially, Antarctica's sedimentary basins interact with the overlying ice sheet through dynamic feedbacks that have the potential to contribute to rapid ice‐sheet change. Looking ahead, future research directions include techniques to increase data coverage within logistical constraints, and resolving major knowledge gaps, including insufficient sampling of the ice‐sheet bed and poor definition of subglacial basin structure and stratigraphy. Translating the knowledge of sedimentary basin processes into ice‐sheet modeling studies is critical to underpin better capacity to predict future change.
Although climate change has been implicated as a major catalyst of diversification, its effects are thought to be inconsistent and much less pervasive than localized climate or the accumulation of species with time. Focused analyses of highly speciose clades are needed in order to disentangle the consequences of climate change, geography, and time. Here, we show that global cooling shapes the biodiversity of terrestrial orchids. Using a phylogeny of 1,475 species of Orchidoideae, the largest terrestrial orchid subfamily, we find that speciation rate is dependent on historic global cooling, not time, tropical distributions, elevation, variation in chromosome number, or other types of historic climate change. Relative to the gradual accumulation of species with time, models specifying speciation driven by historic global cooling are over 700 times more likely. Evidence ratios estimated for 212 other plant and animal groups reveal that terrestrial orchids represent one of the best-supported cases of temperature-spurred speciation yet reported. Employing >2.5 million georeferenced records, we find that global cooling drove contemporaneous diversification in each of the seven major orchid bioregions of the Earth. With current emphasis on understanding and predicting the immediate impacts of global warming, our study provides a clear case study of the long-term impacts of global climate change on biodiversity.
From the Eocene (~50 million years ago) to today, Southern Ocean circulation has evolved from the existence of two ocean gyres to the dominance of the Antarctic Circumpolar Current (ACC). It has generally been thought that the opening of Southern Ocean gateways in the late Eocene, in addition to the alignment of westerly winds with these gateways or the presence of Antarctic ice sheet, was a sufficient requirement for the transition to an ACC of similar strength to its modern equivalent. Nevertheless, models representing these changes produce only a much weaker ACC. Here we show, using an eddying ocean model, that the missing ingredient in the transition to a modern ACC is deep convection around the Antarctic continent. This deep convection is caused by cold temperatures and high salinities due to sea-ice production around the Antarctic continent, leading to both the formation of Antarctic Bottom Water and a modern-strength ACC.
The miniature orb weaving spiders (symphytognathoids) are a group of small spiders (< 2 mm), including the smallest adult spider Patu digua (0.37 mm in body length), that have been classified into five families. The species of one of its constituent lineages, the family Anapidae, build a remarkable diversity of webs (ranging from orbs to sheet webs and irregular tangles) and even include a webless kleptoparasitic species. Anapids are also exceptional because of the extraordinary diversity of their respiratory systems. The phylogenetic relationships of symphytognathoid families have been recalcitrant with different classes of data, such as, monophyletic with morphology and its concatenation with Sanger-based six markers, paraphyletic (including a paraphyletic Anapidae) with solely Sanger-based six markers, and polyphyletic with transcriptomes. In this study, we capitalized on a large taxonomic sampling of symphytognathoids, focusing on Anapidae, and using de novo sequenced ultraconserved elements (UCEs) combined with UCEs recovered from available transcriptomes and genomes. We evaluated the conflicting relationships using a variety of support metrics and topology tests. We found support for the phylogenetic hypothesis proposed using morphology to obtain the "symphytognathoids'' clade, Anterior Tracheal System (ANTS) Clade and monophyly of the family Anapidae. Anapidae can be divided into three major lineages, the Vichitra Clade (including Teutoniella, Holarchaea, Sofanapis and Acrobleps), the subfamily Micropholcommatinae and the Orb-weaving anapids (Owa) Clade. Biogeographic analyses reconstructed a hypothesis of multiple long-distance transoceanic dispersal events, potentially influenced by the Antarctic Circumpolar Current and West Wind Drift. In symphytognathoids, the ancestral anterior tracheal system transformed to book lungs four times and reduced book lungs five times. The posterior tracheal system was lost six times. The orb web structure was lost four times independently and transformed into sheet web once.
The Asian monsoons are triggered by complex interactions between the atmosphere, Asian and African orography, and the surrounding oceans, resulting in highly seasonal climate and specific regional features. It was thought that the Asian monsoon was established during the Neogene, but recent evidence for monsoon-like precipitation seasonality occurring as early as the Paleogene greenhouse period challenges this paradigm. The possible occurrence of monsoons in a climatic and paleogeographic context very different from the present-day questions our understanding of the drivers underpinning this atmospheric phenomenon, in particular with regard to its dependence on geography. In this study, we first take advantage of the wealth of new studies to tentatively draw an up-to-date picture of Asian tectonic and paleoenvironmental evolution throughout the Cenozoic. We then analyse a set of 20 paleoclimate simulations spanning the late Eocene to latest Miocene (~40-8 Ma) in order to better understand the evolution of the distinct Asian monsoon subsystems. At odds with the traditional view of a monsoonal evolution driven mainly by Himalayan-Tibetan uplift, our work emphasizes the importance of peripheral mountain ranges in driving the evolution of Asian climate. In particular, the uplift of East African and Anatolian-Iranian mountain ranges, as well as the emergence of the Arabian Peninsula, contribute to shaping the modern South Asian summer monsoon. We also suggest that East Asian monsoon establishment and the aridification of inland Asia are driven by a combination of factors including increasing continentality, the orographic evolution of the Tibetan Plateau, Mongolia, Tian Shan and Pamir, and pCO2 decrease during the Cenozoic.
The Southern Ocean closes the global overturning circulation and is key to the regulation of carbon, heat, biological production, and sea level. However, the dynamics of the general circulation and upwelling pathways remain poorly understood. Here, a physics-informed unsupervised machine learning framework using principled constraints is used. A unifying framework is proposed invoking a semi-circumpolar supergyre south of the Antarctic circumpolar current: a massive series of leaking sub-gyres spanning the Weddell and Ross seas that are connected and maintained via rough topography that acts as scaffolding. The supergyre framework challenges the conventional view of having separate circulation structures in the Weddell and Ross seas and suggests that idealized models and zonally-averaged frameworks may be of limited utility for climate applications. Machine learning was used to reveal areas of coherent driving forces within a vorticity-based analysis. Predictions from the supergyre framework are supported by available observations and could aid observational and modelling efforts to study this climatologically key region undergoing rapid change.
In a recent paper, the author demonstrated that, in contrast with the prevailing view of eventual gradual regional differentiation from a hypothetical Cretaceous pantropical mangrove belt around the Tethys Sea, the Caribbean mangroves originated de novo in the Eocene after the evolutionary appearance of the first mangrove-forming tree species known for the region, the ancestor of the extant Pelliciera. This paper represents a second step in the analysis of the evolution of Caribbean mangroves dealing with the most important change experienced by these communities, occurring across the Eocenesingle bondOligocene transition (EOT), which is termed here the Caribbean mangrove revolution. This shift consisted of the disappearance of the primeval Pelliciera mangroves and their replacement by mangrove communities dominated by Rhizophora, a newly emerged mangrove tree that still dominates extant Caribbean mangroves. This paper first reviews the available literature on the EOT global disruption (tectonic and paleogeographic reorganizations, ocean circulation, cooling, Antarctic glaciation, sea-level fall) and its regional manifestations in the study area, along with the corresponding biotic responses. This provides the paleoenvironmental framework with which to analyze the EOT mangrove revolution using the >80 pollen records available for the region. In the circum-Caribbean region, cooling of 3–6 °C and a sea-level fall of 67 m were recorded between 33.8 and 33.5 Ma, which led to significant shifts in dispersal pathways and barriers, as well as in marine paleocurrents. Late Eocene mangroves were dominated by the autochthonous Pelliciera (up to 60% of pollen assemblages), while Rhizophora, which likely arrived from the Indo-Pacific region by long-distance dispersal, was absent or very scarce. After the EOT, the situation was radically different, as the mangroves were widely dominated by Rhizophora, and Pelliciera, when present, was a subordinate mangrove element (<10%). At the same time, Pelliciera, which had been restricted to a small patch (Central America and NW South America or CA/NWSA) during the Eocene, expanded its range across the Caribbean and beyond, always as a minor component of Rhizophora mangroves. The dominance shift could have been due to the EOT cooling, by favoring the expansion of the euryclimatic and vagile Rhizophora over the stenoclimatic Pelliciera, of limited dispersal ability. This is considered a case of competitor coexistence by niche segregation. In addition, Rhizophora could have facilitated the expansion of Pelliciera by providing refuge against environmental and biotic stressors, notably light intensity and salinity. The Eocene Pelliciera mangroves never returned, but this species survived to the present as a minor element and experienced significant range shifts along three main phases, namely, EOT–Miocene expansion to the whole Neotropics, Mio-Pliocene contraction to the southern Caribbean margin and Pliocene to recent reorganization to the original Eocene CA/NWSA location. The potential role of Neogene and Pleistocene climatic shifts and human activities in these biogeographical loops (taxon cycles) is discussed, with an emphasis on precipitation. The paper ends by suggesting some prospects for future research.
The Neogene (23.04–2.58 Ma) is characterised by progressive buildup of ice volume and climate cooling in the Antarctic and the Northern Hemisphere. Heat and moisture delivery to Antarctica is, to a large extent, regulated by the strength of meridional temperature gradients. However, the evolution of the Southern Ocean frontal systems remains scarcely studied in the Neogene. Here, we present the first long-term continuous sea surface temperature (SST) record of the subtropical front area in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 off western Tasmania. This site is, at present, located near the subtropical front (STF), as it was during the Neogene, despite a 10∘ northward tectonic drift of Tasmania. We analysed glycerol dialkyl glycerol tetraethers (GDGTs – on 433 samples) and alkenones (on 163 samples) and reconstructed the paleotemperature evolution using TEX86 and U37k′ as two independent quantitative proxies. Both proxies indicate that Site 1168 experienced a temperate ∼ 25 ∘C during the early Miocene (23–17 Ma), reaching ∼ 29 ∘C during the mid-Miocene climatic optimum. The stepwise ∼ 10 ∘C cooling (20–10 ∘C) in the mid-to-late Miocene (12.5–5.0 Ma) is larger than that observed in records from lower and higher latitudes. From the Pliocene to modern (5.3–0 Ma), STF SST first plateaus at ∼ 15 ∘C (3 Ma), then decreases to ∼ 6 ∘C (1.3 Ma), and eventually increases to the modern levels around ∼ 16 ∘C (0 Ma), with a higher variability of 5∘ compared to the Miocene. Our results imply that the latitudinal temperature gradient between the Pacific Equator and the STF during late Miocene cooling increased from 4 to 14 ∘C. Meanwhile, the SST gradient between the STF and the Antarctic margin decreased due to amplified STF cooling compared to the Antarctic margin. This implies a narrowing SST gradient in the Neogene, with contraction of warm SSTs and northward expansion of subpolar conditions.
The Southern Ocean closes the global overturning circulation and is key to the regulation of carbon and heat, biological production, and sea level. However, the dynamics of the general circulation and upwelling pathways remain poorly understood. Here, a unifying framework is proposed invoking a semi-circumpolar `supergyre' south of the Antarctic circumpolar current: a massive series of ‘leaking’ sub-gyres spanning the Weddell and Ross seas that are connected and maintained via rough topography that acts as scaffolding. The supergyre framework challenges the conventional view of having separate circulation structures in the Weddell and Ross seas and suggests a limited utility for climate applications of idealized models and conventional zonal averaged frameworks. Machine learning was used to reveal areas of coherent driving forces within a vorticity-based analysis. Predictions from the supergyre framework are supported by available observations and could aid observational and modelling efforts of the climatically key region undergoing rapid change.
The interaction between bottom currents, platform-derived particles, and the bathymetric framework of isolated carbonate platforms can result in complex current-controlled depositional patterns, which are not entirely understood. The continuous supply and usually local deposition of carbonate particles around platforms, combined with the natural variations in the hydrodynamic regime at various depths, contribute significantly to the heterogeneity of carbonate contourite drifts. This thesis sheds light on the interplay between bottom currents and isolated carbonate platforms in two representative study areas. Moreover, this study presents a classification of carbonate contourite (sediment) drifts related to the architecture and sediment distribution of carbonate platforms based on seismic datasets, as well as the identification of the main driving mechanisms and depositional architecture. Two realms of isolated carbonate platforms were studied, which are located in the Bahamian and Maldivian archipelagos. Both are influenced by major ocean current systems, which were investigated to understand their effects on the depositional patterns. The study areas were targeted by geophysical methods during two scientific cruises, which produced dense grids of high-resolution seismic datasets, allowing new insights to be gained into the sedimentary dynamics of both archipelagos. To achieve the main objectives seismic and hydroacoustic datasets around isolated carbonate platforms were compiled and analyzed, combined with lithostratigraphic and chronological borehole data from multiple sites of the IODP expedition 359 and the ODP Site 1006. (Re-)Interpretation of multichannel seismic cross sections, subordinated echosounder sub-bottom profiling and bathymetric data as well as former inventory data revealed the history of oceanographic processes resulting in various geomorphologies of contourite drifts. The seismic data emphasize the diversity of the sedimentation pattern developed under the influence of long-term bottom current activity.
Ocean circulation and hence global climate are nowhere more strongly
changed than through the opening or closing of gateways or seaways that
link major oceans. The break-up of Gondwana, and the northward flight of
its continental fragments from Antarctica, is a case in point. Profound
climatic consequences resulted from shifts in ocean and atmospheric
circulation due to drastic changes in global geography.
During the Cenozoic, the northward flight of southern continents led to
the opening of gateways at southern high latitudes while progressively
restricting and closing gateways in the low latitudes. Considerable
previous research has dealt with the opening and expansion of the two
Cenozoic gateways—the Tasmanian Gateway south of Australia and the
Drake Passage south of America—which allowed the Antarctic
Circumpolar Current (ACC) to develop and progressively isolate
Antarctica thermally. It is generally accepted that full opening of the
Tasmanian Gateway occurred earlier than that of Drake Passage, although
the time of opening of Drake Passage remains controversial. It has long
been proposed that a climatic threshold leading to major initial
Antarctic ice sheet accumulation occurred during the Eocene-Oligocene
transition as the Tasmanian Gateway opened, triggering ACC formation and
resultant thermal isolation of the Antarctic continent (Gateway
Hypothesis). South of Australia, Paleogene rifting slowly opened the
Australo-Antarctic Gulf, but the Indian and Pacific Oceans remained
separated by the Tasmanian land bridge until the latest Eocene,
preventing earlier development of the ACC; waters derived from low
latitudes efficiently transported heat towards the Antarctic continent,
contributing to the maintenance of global greenhouse conditions. Early
ocean drilling in the Tasmanian Gateway between Australia and Antarctica
provided a basic framework of paleoenvironmental changes associated with
the opening, but stratigraphic resolution was too limited to fully test
potential interrelationships of plate tectonics, circum-polar
circulation and global climate. So, until recently, the timing of events
has been inadequately constrained.
Development of the Antarctic Circumpolar Current (ACC) during the Cenozoic is controversial in terms of timing and its role in major climate transitions. Some propose that the development of the ACC was instrumental in the continental scale glaciation of Antarctica and climate cooling at the Eocene/Oligocene boundary. Here we present climate model results that show that a coherent ACC was not possible during the Oligocene due to Australasian paleogeography, despite deep water connections through the Drake Passage and Tasman Gateway and the initiation of Antarctic glaciation. The simulations of ocean currents compare well to paleoenvironmental records relating to the physical oceanography of the Oligocene and provide a framework for understanding apparently contradictory dating of the initiation of the ACC. We conclude that the northward motion of the Australasian land masses and the reconfiguration of the Tasman Seaway and Drake Passage are necessary preconditions for the formation of a strong, coherent ACC.
Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine mean flow and eddy fluxes at 900-m depth in the Southern Ocean. Mean temperature and dynamic topography from float data are consistent with earlier estimates from hydrographic surveys, although floats imply warmer temperatures and narrower frontal structures than do atlas data. Differences between hydrographic and ALACE dynamic topography suggest the presence of eastward bottom velocities of about 2 cm s-1 below the eastward-flowing jets of the Antarctic Circumpolar Current. Flow is steered by bathymetry and can be represented as an equivalent barotropic system with an e-folding depth of about 700 m.
Six dredgings, five taken during fishing operations, have recovered sedimentary rocks from the volcanic framework of Cascade Seamount, which is built on thinned continental crust on the East Tasman Plateau off eastern Tasmania. The top of the original volcano is now about 600 m below sea‐level. Planktonic foraminifers and supplementary calcareous nannofossils yielded Late Eocene‐Early Oligocene, mid‐Late Oligocene, Early/Middle Miocene, approximately Late Miocene and Quaternary ages. The history of the seamount is more complex than for most seamounts built on oceanic crust and may involve several intervals of volcanism, burrowing or dissolution and cavity infilling. Some of the volcanism was subaerial and some submarine. Older rocks include conglomerate, volcaniclastic sandstone, and a variety of interstitial sparry calcite, formed under a shallow‐water, high‐energy regime, whereas later sediments are mainly oozes. The oldest sedimentary rocks are Late Eocene, shallow water, fully marine with calcareous algae and abundant echinoid debris. Two phases, Chiloguembelina‐dominated and Globigerinatheka‐dominated, can be differentiated. Warm waters prevailed in the Late Eocene and in the Middle Miocene, cooler conditions between and since, consistent with other indications around the southern Australian margin. Post‐depositional alteration consists of phosphatisation and development of ferromanganese crusts.
Argo float profiles of temperature, salinity, and pressure are used to derive the mixed-layer depth (MLD) in the Southern Ocean. MLD is determined from individual profiles using both potential density and potential temperature criteria, and a monthly climatology is derived from individual MLDs using an objective mapping method. Quantitative data are available in the auxiliary material. The spatial structures of MLDs are similar in each month, with deep mixed layers within and just north of the Antarctic Circumpolar Current (ACC) in the Pacific and Indian oceans. The deepest mixed layers are found from June to October and are located just north of the ACC where Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are formed. Examination of individual MLDs indicates that deep mixed layers (MLD >= 400 m) from both the density and temperature criteria are concentrated in a narrow surface density band which is within the density range of SAMW. The surface salinity for these deep mixed layers associated with the SAMW formation are slightly fresher compared to historical estimates. Differences in air-sea heat exchanges, wind stress, and wind stress curl in the Pacific and Indian oceans suggest that the mode water formation in each ocean basin may be preconditioned by different processes. Wind mixing and Ekman transport of cold water from the south may assist the SAMW formation in the Indian Ocean. In the eastern Pacific, the formation of mode water is potentially preconditioned by the relative strong cooling and weak stratification from upwelling.
The warmest global temperatures of the past 85 million years occurred during a prolonged greenhouse episode known as the Early Eocene Climatic Optimum (52-50 Ma). The Early Eocene Climatic Optimum terminated with a long-term cooling trend that culminated in continental-scale glaciation of Antarctica from 34 Ma onward. Whereas early studies attributed the Eocene transition from greenhouse to icehouse climates to the tectonic opening of Southern Ocean gateways, more recent investigations invoked a dominant role of declining atmospheric greenhouse gas concentrations (e.g., CO2). However, the scarcity of field data has prevented empirical evaluation of these hypotheses. We present marine microfossil and organic geochemical records spanning the early-to-middle Eocene transition from the Wilkes Land Margin, East Antarctica. Dinoflagellate biogeography and sea surface temperature paleothermometry reveal that the earliest throughflow of a westbound Antarctic Counter Current began similar to 49-50 Ma through a southern opening of the Tasmanian Gateway. This early opening occurs in conjunction with the simultaneous onset of regional surface water and continental cooling (2-4 degrees C), evidenced by biomarker- and pollen-based paleothermometry. We interpret that the westbound flowing current flow across the Tasmanian Gateway resulted in cooling of Antarctic surface waters and coasts, which was conveyed to global intermediate waters through invigorated deep convection in southern high latitudes. Although atmospheric CO2 forcing alone would provide a more uniform middle Eocene cooling, the opening of the Tasmanian Gateway better explains Southern Ocean surface water and global deep ocean cooling in the apparent absence of (sub-) equatorial cooling.
It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic and Pacific hot spots since the Late Cretaceous. At the same time, a fixed Indo-Atlantic hot spot reference frame has been argued for and used since the advent of plate tectonics, implying relatively little motion between the hot spots in this domain since about 130 Ma. Most plumes purported to have caused these hot spots, while being advected in the global-scale mantle flow field, are assumed to move an order of magnitude more slowly than plates. However, the lifetime of a plume may be over ~100 Myr, and the integrated motion of a plume is expected to be significant over these times. The uncertainties inherent in hot spot reconstructions are of a magnitude similar to the expected plume motion, and so any differences between a fixed and moving frame of reference must be discernible beyond the level of these uncertainties. We present a method for constraining hot spot reconstruction uncertainties, similar to that in use for relative plate motion. We use a modified Hellinger criterion of fit for the hot spot problem, using track geometries and radiometric dating, and derive covariance matrices for our Indo-Atlantic rotations for the last 120 Myr. However, any given mantle convection model introduces additional uncertainties into such models, based on its model parameters and starting conditions (e.g., choice of global tomography model, viscosity profile, nature of mantle phase transitions). We use an interactive evolutionary approach, where we constrain the hot spot motion resulting from convection models to fit paleomagnetic constraints, and converge on an acceptable motion solution by varying unknowns over several generations of simulations. Our hot spot motion model shows large motion (5-10°) of the Indo-Atlantic hot spots for times >80 Ma, consistent with available paleomagnetic constraints. The differences between the fixed and moving hot spot reference frames are not discernible over the level of uncertainty in such rotations for times
1] We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems. Components: 10,219 words, 12 figures.
Magnetic anomaly and fracture zone data on the Southeast Indian Ridge
(SEIR) are analysed in order to constrain the kinematic history of the
Macquarie Plate, the region of the Australian Plate roughly east of
145°E and south of 52°S. Finite rotations for
Australia-Antarctic motion are determined for nine chrons (2Ay, 3Ay, 5o,
6o, 8o, 10o, 12o, 13o and 17o) using data limited to the region between
88°E and 139°E. These rotations are used to generate synthetic
flowlines which are compared with the observed trends of the easternmost
fracture zones on the SEIR. An analysis of the synthetic flowlines shows
that the Macquarie Plate region has behaved as an independent rigid
plate for roughly the last 6 Myr. Finite rotations for
Macquarie-Antarctic motion are determined for chrons 2Ay and 3Ay. These
rotations are summed with Australia-Antarctic rotations to determine
Macquarie-Australia rotations. We find that the best-fit
Macquarie-Australia rotation poles lie within the zone of diffuse
intraplate seismicity in the South Tasman Sea separating the Macquarie
Plate from the main part of the Australian Plate. Motion of the
Macquarie Plate relative to the Pacific Plate for chrons 2Ay and 3Ay is
determined by summing Macquarie-Antarctic and Antarctic-Pacific
rotations. The Pacific-Macquarie rotations predict a smaller rate of
convergence perpendicular to the Hjort Trench than the Pacific-Australia
rotations. The onset of the deformation of the South Tasman Sea and the
development of the Macquarie Plate appears to have been triggered by the
subduction of young, buoyant oceanic crust near the Hjort Trench and
coincided with a clockwise change in Pacific-Australia motion around 6
Ma. The revised Pacific-Australia rotations also have implications for
the tectonics of the Alpine Fault Zone of New Zealand. We find that
changes in relative displacement along the Alpine Fault have been small
over the last 20 Myr. The average rate of convergence over the last 6
Myr is about 40 per cent smaller than in previous models.
Despite decades of study the prerift configuration and early rifting history between Australia and Antarctica is not well established. The plate boundary system during the Cretaceous includes the evolving Kerguelen–Broken Ridge Large Igneous Province in the west as well as the conjugate passive and transform margin segments of the Australian and Antarctic continents. Previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history. We investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair. Potential field data are used to define the boundaries of stretched continental crust on a regional scale. Integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary. We then use the prerift plate boundary positions, along with additional constraints from geological structures and large igneous provinces within the same Australian and Antarctic plate system, to compute "full‐fit" poles of rotation for Australia relative to Antarctica. Our preferred model implies that the Leeuwin and Vincennes Fracture Zones are conjugate features within Gondwana, but that the direction of initial opening between Australia and Antarctica does not follow the orientation of these features; rather, the geometry of these features is likely related to the earlier rifting of India away from Australia‐Antarctica. Previous full‐fit reconstructions, based on qualitative estimates of continental margin overlaps, generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration.
The Ekman divergence around Antarctica raises a large amount of deep water to the ocean's surface. The regional Ekman transport moves the upwelled deep water northward out of the circumpolar zone. The divergence and northward surface drift combine, in effect, to remove deep water from the interior of the ocean. This wind-driven removal process is facilitated by a unique dynamic constraint operating in the latitude band containing Drake Passage. Through a simple model sensitivity experiment we show that the upwelling and removal of deep water in the circumpolar belt may be quantitatively related to the formation of new deep water in the northern North Atlantic. These results show that stronger winds in the south can induce more deep water formation in the north and more deep outflow through the South Atlantic. The fact that winds in the southern hemisphere might influence the formation of deep water in the North Atlantic brings into question long-standing notions about the forces that drive the ocean's thermohaline circulation.
1] An idealized general circulation model is constructed of the ocean's deep circulation and CO 2 system that explains some of the more puzzling features of glacial-interglacial CO 2 cycles, including the tight correlation between atmospheric CO 2 and Antarctic temperatures, the lead of Antarctic temperatures over CO 2 at terminations, and the shift of the ocean's d 13 C minimum from the North Pacific to the Atlantic sector of the Southern Ocean. These changes occur in the model during transitions between on and off states of the southern overturning circulation. We hypothesize that these transitions occur in nature through a positive feedback that involves the midlatitude westerly winds, the mean temperature of the atmosphere, and the overturning of southern deep water. Cold glacial climates seem to have equatorward shifted westerlies, which allow more respired CO 2 to accumulate in the deep ocean. Warm climates like the present have poleward shifted westerlies that flush respired CO 2 out of the deep ocean., Midlatitude westerlies, atmospheric CO 2 , and climate change during the ice ages, Paleoceanography, 21, PA2005, doi:10.1029/2005PA001154.
We analyzed 87Sr/86Sr ratios in foraminifera, pore fluids, and fish teeth for samples ranging in age from Eocene to Pleistocene from four Ocean Drilling Program sites distributed around the globe: Site 1090 in the Cape Basin of the Southern Ocean, Site 757 on the Ninetyeast Ridge in the Indian Ocean, Site 807 on the Ontong-Java Plateau in the western equatorial Pacific, and Site 689 on the Maud Rise in the Southern Ocean. Sr isotopic ratios for dated foraminifera consistently plot on the global seawater Sr isotope curve. For Sites 1090, 757, and 807 Sr isotopic values of the pore fluids are generally less radiogenic than contemporaneous seawater values, as are values for fossil fish teeth. In contrast, pore fluid 87Sr/86Sr values at Site 689 are more radiogenic than contemporaneous seawater, and the corresponding fish teeth also record more radiogenic values. Thus, Sr isotopic values preserved in fossil fish teeth are consistently altered in the direction of the pore fluid values; furthermore, there is a correlation between the magnitude of the offset between the pore fluids and the seawater curve, and the associated offset between the fish teeth and the seawater curve. These data suggest that the hydroxyfluorapatite of the fossil fish teeth continues to recrystallize and exchange Sr with its surroundings during burial and diagenesis. Therefore, Sr chemostratigraphy can be used to determine rough ages for fossil fish teeth in these cores, but cannot be used to fine-tune age models. In contrast to the Sr isotopic system, our Nd concentration data, combined with published isotopic and rare earth element data, suggest that fish teeth acquire Nd during early diagenesis while they are still in direct contact with seawater. The concentrations of Nd acquired at this stage are extremely high relative to the concentrations in surrounding pore fluids. As a result, Nd isotopes are not altered during burial and later diagenesis. Therefore, fossil fish teeth from a variety of marine environments preserve a reliable and robust record of deep seawater Nd isotopic compositions from the time of deposition.
Long-term records of neodymium (Nd) isotopes from sedimentary archives can be influenced by both changes in water mass mixing and continental weathering. Results of Nd isotopic analyses of fossil fish teeth from ODP Site 689 (Maud Rise, Southern Ocean) provide a long, continuous, high-resolution marine sediment Nd isotope record (expressed in ɛNd units). Correlation of down core secular variations between the ɛNd record, δ¹³C values from benthic foraminifera, and clay mineral assemblages demonstrates that long-term variability of Nd isotope ratios reflect changes in ocean circulation, and that only minor fluctuations in ɛNd values are associated with changes in continental weathering on Antarctica.
Ten international laboratories participated in an inter-laboratory comparison of a fossil bone composite with the objective of producing a matrix and structure-matched reference material for studies of the bio-mineralization of ancient fossil bone. We report the major and trace element compositions of the fossil bone composite, using in-situ method as well as various wet chemical digestion techniques. For major element concentrations, the intra-laboratory analytical precision (%RSDr) ranges from 7 to 18%, with higher percentages for Ti and K. The %RSDr are smaller than the inter-laboratory analytical precision (%RSDR; <15–30%). Trace element concentrations vary by 5 orders of magnitude (0.1 mg kg?1 for Th to 10,000 mg kg?1 for Ba). The intra-laboratory analytical precision %RSDr varies between 8 and 45%. The reproducibility values (%RSDR) range from 13 to <50%, although extreme value >100% was found for the high field strength elements (Hf, Th, Zr, Nb). The rare earth element (REE) concentrations, which vary over 3 orders of magnitude, have %RSDr and %RSDR values at 8–15% and 20–32%, respectively. However, the REE patterns (which are very important for paleo-environmental, taphonomic and paleo-oceanographic analyses) are much more consistent. These data suggest that the complex and unpredictable nature of the mineralogical and chemical composition of fossil bone makes it difficult to set-up and calibrate analytical instruments using conventional standards, and may result in non-spectral matrix effects. We propose an analytical protocol that can be employed in future inter-laboratory studies to produce a certified fossil bone geochemical standard.
The New Zealand sector of the Southern Ocean (NZSSO) has opened about the Indian-Pacific spreading ridge throughout the Cenozoic. Today the NZSSO is characterised by broad zonal belts of antarctic (cold), subantarctic (cool), and subtropical (warm) surface-water masses separated by prominent oceanic fronts: the Subtropical Front (STF) c. 43°S, Subantarctic Front (SAF) c. 50°S, and Antarctic Polar Front (AAPF) c. 60°S. Despite a meagre database, the broad pattern of Cenozoic evolution of these fronts is reviewed from the results of Deep Sea Drilling Project-based studies of sediment facies, microfossil assemblages and diversity, and stable isotope records, as well as from evidence in onland New Zealand Cenozoic sequences. Results are depicted schematically on seven paleogeographic maps covering the NZSSO at 10 m.y. intervals through the Cenozoic. During the Paleocene and most of the Eocene (65-35 Ma), the entire NZSSO was under the influence of warm to cool subtropical waters, with no detectable oceanic fronts. In the latest Eocene (c. 35 Ma), a proto-STF is shown separating subantarctic and subtropical waters offshore from Antarctica, near 65°S paleolatitude. During the earliest Oligocene, this front was displaced northwards by development of an AAPF following major global cooling and biotic turnover associated with ice sheet expansion to sea level on East Antarctica. Early Oligocene full opening (c. 31 Ma) of the Tasmanian gateway initiated vigorous proto-circum-Antarctic flow of cold/cool waters, possibly through a West Antarctic seaway linking the southern Pacific and Atlantic Oceans, including detached northwards "jetting" onto the New Zealand plateau where condensation and unconformity development was widespread in coolwater carbonate facies. Since this time, a broad tripartite division of antarctic, subantarctic, and subtropical waters has existed in the NZSSO, including possible development of a proto-SAF within the subantarctic belt. In the Early- early Middle Miocene (25-15 Ma), warm subtropical waters expanded southwards into the northern NZSSO, possibly associated with reduced ice volume on East Antarctica but particularly with restriction of the Indonesian gateway and redirection of intensified warm surface flows southwards into the Tasman Sea, as well as complete opening of the Drake gateway by 23 Ma allowing more complete decoupling of cool circum-Antarctic flow from the subtropical waters. During the late Middle-Late Miocene (15-5 Ma), both the STF and SAF proper were established in their present relative positions across and about the Campbell Plateau, respectively, accompanying renewed ice buildup on East Antarctica and formation of a permanent ice sheet on West Antarctica, as well as generally more expansive and intensified circum-Antarctic flow. The ultimate control on the history of oceanic front development in the NZSSO has been plate tectonics through its influence on the paleogeographic changes of the Australian-New Zealand-Antarctic continents and their intervening oceanic basins, the timing of opening and closing of critical seaways, the potential for submarine ridges and plateaus to exert some bathymetric control on the location of fronts, and the evolving ice budget on the Antarctic continent. The broad trends of the Cenozoic climate curve for New Zealand deduced from fossil evidence in the uplifted marine sedimentary record correspond well to the principal paleoceanographic events controlling the evolution and migration of the oceanic fronts in the NZSSO.
Since the inception of the international GEOTRACES program, studies investigating the distribution of trace elements and their isotopes in the global ocean have significantly increased. In spite of this large-scale effort, the distribution of neodymium isotopes (143Nd/144Nd, ) and concentrations ([Nd]) in the high latitude South Pacific is still understudied, specifically north of the Antarctic Polar Front (APF). Here we report dissolved Nd isotopes and concentrations from 11 vertical water column profiles from the South Pacific between South America and New Zealand and across the Antarctic frontal system. Results confirm that Ross Sea Bottom Water (RSBW) is represented by an value of , and for the first time show that these Nd characteristics can be traced into the Southeast Pacific until progressive mixing with ambient Lower Circumpolar Deep Water (LCDW) dilutes this signal north of the APF. That is, behaves conservatively in RSBW, opening a path for studies of past RSBW behavior.
Ephemeral polar glaciations during the middle-to-late Eocene (48–34 Ma) have been proposed based on far-field ice volume proxy records and near-field glacigenic sediments, though the scale, timing, and duration of these events are poorly constrained. Here we confirm the existence of a transient cool event within a new high-resolution benthic foraminiferal δ18O record at Ocean Drilling Program (ODP) Site 738 (Kerguelen Plateau; Southern Ocean). This event, named the PrOM - Priabonian Oxygen isotope Maximum Event, lasted ~140-kyr and is tentatively placed within magnetochron C17n.1n (~37.3 Ma) based on correlation to ODP Site 689 (Maud Rise, Southern Ocean). A contemporaneous change in the provenance of sediments delivered to the Kerguelen Plateau occurs at the study site, determined from the <63 µm fraction of decarbonated and reductively leached sediment samples. Changes in the mixture of bottom waters, based on fossil fish tooth ϵNd, were less pronounced and slower relative to the benthic δ18O and terrigenous ϵNd changes. Terrigenous sediment ϵNd values rapidly shifted to less radiogenic signatures at the onset of the PrOM Event, indicating an abrupt change in provenance favoring ancient sources such as the Paleoproterozoic East Antarctic craton. Bottom water ϵNd reached a minimum value during the PrOM Event, though the shift begins much earlier than the terrigenous ϵNd excursion. The origin of the abrupt change in terrigenous sediment provenance is compatible with a change in Antarctic terrigenous sediment flux and/or source as opposed to a reorganization of ocean currents. A change in terrigenous flux and/or source of Antarctic sediments during the oxygen isotope maximum suggests a combination of cooling and ice growth in East Antarctica during the early late Eocene.
ODP Leg 189 Site 1172, drilled in the East Tasman Plateau, has allowed
documentation of the Tasman Gateway during pre-rifting and rifting of
Australia from Antarctica in the Late Cretaceous and early Paleogene,
and of subsequent paleoceanographic events. The new 2G 750R magnetometer
on the JOIDES Resolution brought much of the weakly magnetized carbonate
Neogene section into a measurable range, and the siltstones provided
reliable magnetostratigraphy for much of the earlier time interval.
Magnetostratigraphic highlights include (1) demonstration of at least an
0.8 m.y. hiatus encompassing the Cretaceous-Paleogene boundary (2)
assistance with establishing the Paleocene-Eocene boundary and its
associated thermal event, with astronomic tuning of the Eocene
magnetostratigraphy and with dinocyst-diatom-magneto-stratigraphy
documenting the Eocene-Oligocene transition and the deep opening of the
Tasmanian gateway at 35.5 Ma and (3) a reliable Pliocene-Pleistocene
record suggesting a strongly condensed interval between approximately
0.78 and 1.7 Ma within an otherwise complete section. The record
established overall average sedimentation rates of 10m/m.y. for the
Maastrichtian to late Eocene siliciclastic sediments laid down as the
South Tasman Rise rifted from Antarctica, and 20m/m.y. for pelagic
carbonates as the East Tasman Plate moved northward away from Antarctica
from the Oligocene to present.
A primitive equation, three-dimensional numerical model of the ocean, employing idealized versions of the real topography and surface boundary conditions, is used to study the water mass structure of the World Ocean. In particular, the response of the model to three fundamental changes in boundary conditions is investigated in an attempt to identify the mechanisms in the model which are responsible for the establishment of the largest scale features of the global water-mass structure. With the Drake Passage closed, thermohaline driving alone, and a fresh North Atlantic surface salinity specified, only the coarsest aspects of the observed T and S structure are reproduced and the entire World Ocean below the thermocline is dominated by water formed at the southern boundary. The salinity configuration in particular, lacks much of its observed structure in this case. When the Drake Passage is opened, the resulting circumpolar flow serves to isolate the extreme southern ocean. This allows waters of northern and midlatitude origin to invade the subthermocline zones, producing the familiar tongues of fresh water at intermediate depths. Wind driving further isolates the extreme Southern Ocean and improves the shape and positioning of the fresh water lenses, particularly in the Southern Ocean. Finally, increasing the salinity of water formed at the surface of the northern Atlantic produces distinct salinity maxima in the deep water throughout the World Ocean, bringing the overall salinity structure into broad agreement with observations. Passive tracers are used to establish water mass origins.
A primitive equation, three-dimensional numerical model of the ocean, employing idealized versions of the real topography and surface boundary conditions, is used to study the water mass structure of the World Ocean. In particular, the response of the model to three fundamental changes in boundary conditions is investigated in an attempt to identify the mechanisms in the model which are responsible for the establishment of the largest scale features of the global water-mass structure. With the Drake Passage closed, thermohaline driving alone, and a fresh North Atlantic surface salinity specified, only the coarsest aspects of the observed T and S structure are reproduced and the entire World Ocean below the thermocline is dominated by water formed at the southern boundary. The salinity configuration in particular, lacks much of its observed structure in this case. When the Drake Passage is opened, the resulting circumpolar flow serves to isolate the extreme southern ocean. This allows waters of northern and midlatitude origin to invade the subthermocline zones, producing the familiar tongues of fresh water at intermediate depths. Wind driving further isolates the extreme Southern Ocean and improves the shape and positioning of the fresh water lenses, particularly in the Southern Ocean. Finally, increasing the salinity of water formed at the surface of the northern Atlantic produces distinct salinity maxima in the deep water throughout the World Ocean, bringing the overall salinity structure into broad agreement with observations. Passive tracers are used to establish water mass origins.
A seamount chain extending from the Balleny Islands to the East Tasman Plateau records the passage of the Australian and Antarctic plates over the Balleny plume. A poorly known seamount chain trending northeast from the East Tasman Plateau across the Tasman Sea to the western edge off the Lord Howe Rise represents a possible older trace of the plume. Late Cretaceous inception of this plume, and of another beneath Marie Byrd Land on the stationary Antarctic plate, may have been involved in the initiation of spreading at ˜80 Ma in the Tasman Sea and southwest Pacific Ocean. The Balleny plume isotopic and trace element signature, indicative of a high U/Pb mantle source, is recorded in Cenozoic Tasmanian basalts but is not present in the adjacent Victorian mafic lava-field province, located farther from the plume trace.
The application of radiogenic isotopes to the study of Cenozoic circulation patterns in the South Pacific Ocean has been hampered by the fact that records from only equatorial Pacific deep water have been available. We present new Pb and Nd isotope time series for two ferromanganese crusts that grew from equatorial Pacific bottom water (D137-01, "Nova," 7219 m water depth) and southwest Pacific deep water (63KD, "Tasman," 1700 m water depth). The crusts were dated using (10)Be/(9) Be ratios combined with constant Co-flux dating and yield time series for the past 38 and 23 Myr, respectively. The surface Nd and Pb isotope distributions are consistent with the present-day circulation pattern, and therefore the new records are considered suitable to reconstruct Eocene through Miocene paleoceanography for the South Pacific. The isotope time series of crusts Nova and Tasman suggest that equatorial Pacific deep water and waters from the Southern Ocean supplied the dissolved trace metals to both sites over the past 38 Myr. Changes in the isotopic composition of crust Nova are interpreted to reflect development of the Antarctic Circumpolar Current and changes in Pacific deep water circulation caused by the build up of the East Antarctic Ice Sheet. The Nd isotopic composition of the shallower water site in the southwest Pacific appears to have been more sensitive to circulation changes resulting from closure of the Indonesian seaway.
This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.
Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANTXXIV/3 and ANTXXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200 km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (<0.12 pmol/kg and 10 pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labeled by terrigenous inputs. Near South Africa Nd isotope values as low as εNd = −18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to εHf = 6.1 and εNd = −4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (εNd ∼ −8 to −8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Passage. The Hf isotope compositions in the entire study area only show a small range between εHf = + 6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from εNd = −18.9 to −4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.