Article

Variability in diatom and silicoflagellate assemblages during mid-Pliocene glacial-interglacial cycles determined in Hole U1361A of IODP Expedition 318, Antarctic Wilkes Land Margin

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Abstract

The Earth is currently experiencing climatic changes that will result in similar environmental conditions to those experienced during the mid-Pliocene (5.3–3.6 million years ago [Ma]), such as similar atmospheric CO2 concentrations, elevated sea surface temperature, and higher sea-levels due to polar ice melt. Studying the temporal distribution of Antarctic diatoms and silicoflagellates from this epoch provides insights into environmental conditions and sea-ice configurations during that time, and remains the only way to realistically estimate future phytoplankton community responses and Southern Ocean sea-ice extent during large-scale transient changes of similar magnitude to that anticipated by anthropogenic causes. In this study, we identified and quantified diatoms and silicoflagellates in a sediment core section obtained from the Antarctic Wilkes Land margin (IODP Expedition 318, Hole U1361A), spanning four glacial and interglacial cycles around the Gauss/Gilbert geochron boundary (3.6 Ma) between about 3.69 and 3.56 Ma. Two major abundance peaks (around 79.66 and 76.86 mbsf; representing time intervals around 3.65 Ma and 3.58 Ma, respectively) were identified. Both peaks temporally match previously determined high productivity warm intervals (interglacials). Diatom and silicoflagellate assemblages in these two interglacial periods differed: the abundance peak in the older sediments is dominated by the pennate diatoms Fragilariopsis barronii, Rouxia naviculoides and Rouxia antarctica; indicative of seasonal, meltwater associated stratification. The abundance peak in the younger sediments is composed principally of Chaetoceros resting spores, suggesting higher productivity, strong stratification and prolonged ice-free, open water conditions. Analysing highly abundant diatoms and environmental indicator species from this IODP Site in relation to the previously determined palaeo-productivity proxy Ba/Al allowed us to refine the age model. Our study provides orbital-scale insights into the variability of Antarctic diatom and silicoflagellate assemblages during the mid-Pliocene, thereby offering a reference for future predictions of extant diatom responses to ongoing climate change.

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... Previous studies within the vicinity of the Wilkes Subglacial Basin have utilized detrital sediment provenance, grain size, icerafted debris accumulation, grounding line advances and retreats, and sea ice and temperature reconstructions to reveal a dynamic picture of the early to middle Pliocene and middle Miocene ice margin, with substantial ice retreat into the basin during warmer times, and ice advance onto the outer shelf at colder times (Armbrecht et al., 2018;Cook et al., 2013Cook et al., , 2017Orejola et al., 2014;Patterson et al., 2014;Pierce et al., 2017;Reinardy et al., 2015;Sangiorgi et al., 2018). Substantial ice retreat into the Wilkes Subglacial Basin during the Pliocene and Miocene is also indicated by models that incorporate complex flow regimes of ice streams, capture marine ice sheet instability, and include parameterizations for cliff failure and hydrofracturing of buttressing ice shelves (de Boer et al., 2015;Pollard et al., 2015;Gasson et al., 2016). ...
... The material utilized is from IODP Site U1361A (64 • 24 S, 143 • 53 E; 3454 m water depth), which recovered a near-continuous Pliocene record (Escutia et al., 2011), comprised of alternating diatom-poor laminated muds and diatom-rich/bearing silty-mud units (Fig. 2), representing repeated glacial and interglacial cycles. Interglacial periods are associated with peak diatom abundance counts (Armbrecht et al., 2018;Taylor-Silva and Riesselman, 2018) and higher productivity . ...
... Large-scale ice retreat is therefore more likely to cause the observed provenance shifts than a switch in sediment delivery between ice streams. Marine productivity in the local Southern Ocean would subsequently increase as a result of the accompanying decrease in sea ice extent (Armbrecht et al., 2018;Taylor-Silva and Riesselman, 2018) and associated changes in nutrient delivery, allowing marine productivity to increase, as documented in the Ba/Al and biogenic silica records (Fig. 2). ...
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... Global cooling and the onset of Northern Hemisphere glaciation at B2.7 Ma corresponded with a continued cooling trend in Antarctica and increasing seasonal sea ice persistence/extent around the continental margin (Armbrecht et al., 2018;Cortese and Gersonde, 2008;Escutia et al., 2009;Kennett and Barker, 1990;McKay et al., 2012a;Riesselman and Dunbar, 2013;Taylor-Silva and Riesselman, 2018). Cosmogenic isotope dating studies around the EAIS margin suggest a thicker ice sheet existed during glacials in the late Pliocene and early Pleistocene, compared to late Pleistocene glacials. ...
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... These references largely follow a constrained optimization (CONOP)-based synthesis of Southern Hemisphere biostratigraphy (Cody et al., 2008). Additional biostratigraphic data for diatoms were taken from , Censarek and Gersonde (2002), , Winter and Iwai (2002), Zielinski and Gersonde (2002), Ol-ney et al. (2007), Tauxe et al. (2012), Cody et al. (2012, Winter et al. (2012), andArmbrecht et al. (2018). Radiolarian biostratigraphy for the middle Miocene to Pleistocene additionally follows and Florindo et al. (2013). ...
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This section provides an overview of operations, depth conven- tions, core handling, curatorial procedures, and analyses performed on the R/V JOIDES Resolution during International Ocean Discov- ery Program (IODP) Expedition 382. This information applies only to shipboard work described in the Expedition reports section of the Expedition 382 Proceedings of the International Ocean Discov- ery Program volume. Methods used by investigators for shore-based analyses of Expedition 382 data will be described in separate indi- vidual postcruise research publications.
... Phytoplankton paleoecology and Southern Ocean paleoclimatology are of particular interest during time periods that are used as analogues of modern warming, e.g. the warm Pliocene (e.g. Cook et al., 2013;Armbrecht et al., 2018). The Antarctic Drilling (ANDRILL) Program recovered a~1300 m drill core (AND-1B) that records orbitally-paced glacial/interglacial variability from the Ross Sea . ...
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... However, to date, it has not been possible to achieve a detailed picture of all living organisms that have occupied global oceans in the past, restricting estimates of past environmental conditions and climate. The techniques that have traditionally been applied to reconstruct marine palaeocommunities are limited, such as microscopy to investigate the microfossil record (e.g., Winter et al., 2010;Armbrecht et al., 2018). Due to dissolution and degradation of phytoplankton and microzooplankton while sinking to the seafloor post-mortem, only the most robust skeletons and shells are preserved within a complex geological record (Loucaides et al., 2011). ...
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Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.
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Models predict considerable spatial variability in the magnitude of future climate change around Antarctica, suggesting that some sectors of the continent may be more affected by these changes than others. Furthermore, the geometry of the bedrock topography underlying the East and West Antarctic ice sheets, together with regional differences in ice thickness, mean that certain ice drainage basins may respond more or less sensitively to environmental forcings. Here we use an ensemble of idealized climates to drive ice-sheet simulations that explore regional and continental-scale thresholds, allowing us to identify a hierarchy of catchment vulnerabilities based on differences in long-term catchment-averaged ice loss. Considering this hierarchy in the context of recent observations and climate scenarios forecast for 2100 CE we conclude that the majority of future ice loss from East Antarctica, both this century and over subsequent millennia, will likely come from the Recovery subglacial basin in the eastern Weddell Sea.
Article
Over the past 37 years, satellite records show an increase in Antarctic sea ice cover that is most pronounced in the period of sea ice growth. This trend is dominated by increased sea ice coverage in the western Ross Sea, and is mitigated by a strong decrease in the Bellingshausen and Amundsen seas. The trends in sea ice areal coverage are accompanied by related trends in yearly duration. These changes have implications for ecosystems, as well as global and regional climate. In this review, we summarise the research to date on observing these trends, identifying their drivers, and assessing the role of anthropogenic climate change. Whilst the atmosphere is thought to be the primary driver, the ocean is also essential in explaining the seasonality of the trend patterns. Detecting an anthropogenic signal in Antarctic sea ice is particularly challenging for a number of reasons: the expected response is small compared to the very high natural variability of the system; the observational record is relatively short; and the ability of global coupled climate models to faithfully represent the complex Antarctic climate system is in doubt.
Article
Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6-9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics - including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs - that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.
Chapter
Palaeo sea-ice estimation, first addressed in the mid-1970s, has advanced greatly through the development of additional proxies, increasingly robust statistical methods, and the acquisition of more highly resolved records in both the Antarctic and Arctic. The use of multiple proxies provides the most reliable records. An essential part of multi-proxy studies is the distribution of dinocysts in the Arctic, and diatoms in the Antarctic. These proxies combined with geochemical and sedimentological tracers, as well as new work focused on the use of biomarkers specific to organisms that live in sea ice, such as IP25, are allowing far greater understanding of sea ice type, persistence and extent. Through the ensuing proxy assessments, the history of sea ice development and variability throughout the polar regions is addressed, highlighting palaeo sea-ice reconstructions for specific time-slices, including the initiation of Arctic sea ice, and sea-ice variability since the Last Glacial Maximum at both poles.
Article
In the first season of drilling, the Cape Roberts Project (CRP) recovered one drillcore (CRP-1) from Roberts Ridge in western McMurdo Sound, Ross Sea, Antarctica. Diatom biostratigraphy places the upper six lithostratigraphic units (Units 1.1,2.1,2.2,2.3, 3.1, and 4.1) of CRP-1 (0.0 to 43.15 mbsf) within the Quaternary. Both non-marine and marine Quaternary diatoms occur in variable abundance in the Quaternary interval of CRP-1. Biostratigraphic data resolve two Quaternary time slices or events within CRP-1. Marine diatom assemblages in Units 4.1 and 3.1 represent sedimentation within the diatom Actinocyclus ingens Zone (1.35 to 0.66 Ma). Further refinement of the age of Unit 3.1 places deposition in the interval 1.15 to 0.75 Ma based on the common occurrence of Thalassiosira elliptipora and correlation to the Southern Ocean acme of this taxon. The absence of Actinocyclus ingens and the presence of Thalassiosira antarctica in Unit 2.2 require a younger zonal assignment for this interval, within the diatom Thalassiosira lentiginosa Zone (0.66 to 0.0 Ma). A new diatom species, Rouxia leventerae, is described from marine assemblages of Units 2.2, 2.3, 3.1, and 4.1. Lithostratigraphic Unit 3.1 (33.82 to 31.89 mbsf) is abryozoan-dominated skeletal-carbonate facies. Low abundance of Fragilariopsis curta and Fragilariopsis cylindrus within this unit combined with the relatively high abundance of species associated with open water indicates deposition in waters that remained ice free for much or all of the year. Diatom assemblages suggest carbonate deposition in Unit 3.1 is linked to a significant early Pleistocene event in McMurdo Sound, when elevated surface-water temperatures inhibited the formation of sea ice.
Article
[1] We present a 5.3- Myr stack ( the " LR04'' stack) of benthic delta(18)O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic delta(18)O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene- Pleistocene derived from tuning the delta(18)O stack to a simple ice model based on 21 June insolation at 65degreesN. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic delta(18) O than previously published stacks of the late Pleistocene as the result of higher-resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41- and 23- kyr components suggest that the precession component of delta(18)O from 2.7 - 1.6 Ma is primarily a deep- water temperature signal and that the phase of delta(18)O precession response changed suddenly at 1.6 Ma.
Article
Stephanocha nom. nov. (Silicoflagellata, Dictyochophyceae) is herein proposed as a replacement name for the illegitimate genus name Distephanus, which is a later homonym of Distephanus, a flowering plant. The old generic description is emended, with S. rotunda as the designated type, and new combinations are made for all bona fide silicoflagellate taxa previously assigned (including those tentatively assigned) to Distephanus, except most already transferred to Distephanopsis.
Article
Diatom analysis of a sediment core recovered at IODP Site U1358 on the continental shelf off the Adélie Coast indicated that the lower section of the core contained an assemblage dating back to the Thalassiosira innura Zone of the lower Pliocene that ranges from 4.2 to 5.12 Ma. Based on lithological descriptions at both a macro- and micro-scale of this early Pliocene part of the core, four facies were interpreted from the diamictons representing the progressive advance and retreat of the grounding line over the site. Facies 1a and 1b contain a distinct directional signal from the orientation of the a-axis of clasts with several phases of fabric development along with both brittle and ductile deformation features that are common in sediments that have been subglacially deformed. Facies 1c and 1d are finely laminated and were deposited in open marine conditions. The four facies within the depositional model provide for the first time direct evidence for ice advancing across the shelf adjacent to the Wilkes Subglacial Basin on at least four occasions separated by three periods of open marine conditions indicating retreat of grounded ice inland of the site during a warmer than present early Pliocene. The times of open marine conditions are correlated with previous findings from the neighbouring rise sites that also indicated an oscillating ice margin. This has significant implications because firstly it suggests a dynamic East Antarctic Ice Sheet (EAIS) that is probably far more sensitive to climatic and oceanic forcing even during relatively short time periods than had previously been thought. Secondly it suggests that proxies used to interpret the advance and retreat of the grounding line from the rise can be linked with direct evidence of grounding line migration from the shelf. It also has important implications for the future behaviour and sensitivity of the EAIS under present continuing warming conditions. Together with results from the rise, this paper provides a crucial ice extent target for a new ice sheet model of this region during the Pliocene.
Article
In the modern marine environment the silicoflagellate genus Dictyocha is rare, or absent, south of the Antarctic polar front (APF); the genus Distephanus, in contrast, is dominant. In sediments recovered from ODP Site 1165, 1600 km south of the front, however, three intervals where Dictyocha is abundant are interpreted to represent Pliocene warm events. Comparison of our data with 's [1974] modern core top silicoflagellate relationship with sea surface temperature (SST) indicates that at Site 1165 mean annual SST was approximately 5°C at 3.7 Ma (event I), and approximately 4°C at 4.3-4.4 Ma (event II) and 4.55-4.8 Ma (event III). Event I represents a 5.5°C warming, and events II and III represents a 4.5°C warming relative to modern mean annual SST. Dictyocha is absent from other Site 1165 Pliocene intervals, which suggests that cooler SST (<2°C) prevailed. The warm events detected at Site 1165 may represent times when North Atlantic Deep Water production and ocean heat transport into the Southern Ocean exerted maximum influence.
Article
A late Pliocene – early Pleistocene, 2.9–2.0 Ma, diatom record from the Antarctic Geological Drilling Program (ANDRILL) MIS drillcore AND-1B is presented. This core, recovered from beneath the Ross Ice Shelf south of Ross Island, comprises multiple diatomaceous-sediment units deposited during interglacial periods with open water over the core site. These represent interglacial phases of orbitally paced climate cycles and are punctuated by glacial advances. Extant diatom assemblages have limited presence in the late Pliocene record, which makes environmental interpretation less straight forward. We employ modern ecological data in combination with late Pliocene to present variation in diatom assemblages across the Southern Ocean oceanic fronts based on DSDP/ODP diatom biostratigraphic data to evaluate paleoenvironmental change for the 2.9–2.0 Ma interval of the AND-1B core. The diatom assemblages from AND-1B record a progressive environmental change through the late Pliocene–early Pleistocene. A relatively warm period with potential SST of up to 4 °C at ca. 2.9 Ma was succeeded by a reduction of warm water species and an increase of taxa associated with more southerly water masses until 2.58 Ma (at isotope stage G1). Younger, early Pleistocene, diatomaceous units are dominated by extinct Rouxia, Thalassiosira species and newly described Fragilariopsis species indicative of cold open water and drift ice. The last recorded cooling step occurs at the top of the interval studied (ca 2.0) Ma indicated by the trace abundance of Fragilariopsis and Actinocyclus species present in modern sea ice assemblages but absent in the late Pliocene–early Pleistocene AND-1B record, even though many of its species had their first occurrences during the Pliocene. The extant sea ice assemblage occurs with an abundance of 2–29% (average 10%) in the late Pliocene–early Pleistocene AND-1B record implying that the modern sea ice and ice shelf polar conditions were not established within the studied interval. The most frequently applied sea ice indicator, Fragilariopsis curta, is consistently present in low to moderate abundance (1–22%) together with Chaetoceros resting spores (2–30%) through the 2.9–2.0 Ma interval. The diatom assemblage shifts indicate a dynamic environment with an overall trend towards colder conditions after ca. 2.6 Ma but does not reach the Holocene configuration.
Article
It is essential to document how well the current generation of climate models performs in simulating past climates to have confidence in their ability to project future conditions. We present the first global, in-depth comparison of Pliocene sea surface temperature (SST) estimates from a coupled ocean–atmosphere climate model experiment and a SST reconstruction based on proxy data. This enables the identification of areas in which both the climate model and the proxy dataset require improvement.
Article
A new synthesis of diatom assemblage data from Ocean Drilling Program (ODP) Leg 178 suggests that the Pacific margin of the Antarctic Peninsula underwent a transition from a shallow shelf to an overdeepened shelf in the early Pliocene. This modification of the marine-scape was due to a relatively brief interval of erosion begun at 5.2Ma. The erosion was caused by high frequency advances of a super-inflated Antarctic Peninsula Ice Sheet (APIS). The frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the Polar Front migrated southward. By 5.12Ma, ice streams incised foredeepened glacial troughs into basement on the inner shelf. Sediment eroded from the inner shelf was transported through cross-shelf troughs and deposited in large trough-mouth-fan depocenters on the upper slope. Overdeepened shelf conditions became widespread as troughs widened and intra-trough banks beveled. By 4.25Ma, trough-mouth-fan construction ceased and subsequent advances of the APIS have been infrequent. We propose that the reduced frequency of grounding events signaled the transition to a modern foredeepened and overdeepened shelf. We hypothesize that a new glacial dynamic emerged in the early Pliocene because overdeepening led to accelerated heat exchange between the ocean and APIS in two ways. Firstly, the overdeepened shelf required that a larger area of the grounded ice sheet's marine terminus be in contact with the ocean. Secondly, erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region. The resultant accelerated melting at the APIS marine terminus, caused the super-inflated APIS to deflate on the mainland, which further decreased the possibility that grounded ice could advance on the overdeepened shelf.
Article
Highly laminated, Holocene age, diatomaceous sediments are characteristic of the Palmer Deep, western Antarctic Peninsula (Ocean Drilling Program (ODP) Leg 178, Site 1098). From ~10,000 years B.P. to the present, laminations are comprised of several groups of diatoms. Chaetoceros resting spores, the dominant laminae former, result from intense spring blooms. Rhizosolenia, Proboscia, Thalassiothrix, and Corethron are common also, the consequence of summer production marked by a well-stratified water column. Each lamination represents a single productivity event, but laminations are not necessarily annual. High concentrations of a subpolar form of Eucampia antarctica, in laminations between ~9000 and 6700 years B.P., suggest early Holocene warmth, a consequence of southward intrusion of more subpolar waters. The glacial-interglacial transition is distinguished by pairs of laminae most likely deposited annually. Laminations with an overwhelming dominance of Chaetoceros resting spores alternate with more ``terrigenous'' laminae, representing alternation of intense spring blooms, with more mixed deposition during the summer. Proximity to retreating glacial ice results in the supply of silt and sand that provides a marker bed between successive blooms.
Article
One hundred and sixty-five surface sediment samples from the Southern Ocean were examined for distribution and relative abundance of Chaetoceros resting spores. The contribution of resting spores to the total diatom assemblage ranges from 0% in the Subantarctic Zone to 95% in the Antarctic Peninsula sector. On the basis of both absolute and relative abundances four ‘biogeographic’ zones are distinguished: (1) the Antarctic Peninsula sector, (2) the Embayment Systems (Ross Sea and Weddell Sea), (3) the Continental Shelf zone (water depth 2000 m).Chaetoceros resting spores abundance reaches up to 900 × 106 valves/g of dry sediment in the Gerlache Strait, southwest of the Antarctic Peninsula. The hydrology of this region is characterized by an intense stratification of the water column due to sea-ice meltwater inputs, continental glacial runoffs and thermal warming of the surface water layer. The availability of nutrients, the lack of vertical mixing in those surface waters having low salinity ( 2.4 °C) is thought to be the main pre-condition for development of large Chaetoceros species blooms. We propose that increased relative abundances of Chaetoceros resting spores in fossil diatom assemblages from the Southern Ocean can therefore be used as tracers of water-column stratification due to glacial melt water.
Article
The ANDRILL AND-1B drillcore in southern Victoria Land Basin, Antarctica recovered an upper Miocene to early Pleistocene continental shelf stratigraphic section, including a spectacular Pliocene and Pleistocene sequence of alternating diamictite and diatom-rich sediments. This punctuated sequence reflects variation between glacial sediments deposited by ice advance over the site and open-marine diatom productivity and sedimentation. An early Pliocene age for the base of diatom-rich sediment is constrained through integrated diatom biostratigraphy and magnetostratigraphy. The presence of marine diatom Shionodiscus tetraoestrupii from the top of the core to 583.64mbsf indicates the lowest diatom-bearing sediments are younger than 5Ma. This drillcore provides important new chronostratigraphic control for paleoenvironmental changes also recorded in three drillcores from the western coast of the McMurdo Sound area, DVDP-10 and -11 in Taylor Valley and CIROS-2 from the seaward edge of Ferrar Fjord. The refined continental shelf biostratigraphy developed from AND-1B provides a framework for regional correlation to understand better the timing and character of large paleoenvironmental changes in the western Ross Sea that involve the history of the West Antarctic Ice Sheet (WAIS). Multivariate analysis, along with traditional biostratigraphic approaches, enables the correlation and comparison of coeval intervals in these drillcores. A composite stratigraphic sequence from these four cores suggests the early and mid-Pliocene Ross Sea experienced extended intervals of open-marine conditions with minimal sea-ice cover and high diatom production and sedimentation. This new information provides important constraints on Antarctic paleoclimate and ice sheet history during an important interval when global climate was warmer than today. The history preserved in these four drillcores will be an important tool to guide and test future ice sheet and climate models.
Article
The LG.C.P. project 74/1/1 "a systems approach to accuracy in time", aims at a quantification of the refinement that may be attained with various methods of stratigraphic correlation. A considerable part of the pilot studies carried out by the "Dutch" working group deals with the application of numerical methods in biostratigraphy, in which one of the lines of research aims at a better understanding of counted numbers of specimens of individual taxa in suites of samples, as presented in so-called distribution charts or range charts. The biozonations constructed from such charts, from which in practice the more general zonation schemes are compiled, are commonly based on entries and exits of indivual taxa or groups of taxa. The corresponding datum levels based on single or multiple presence-absence criteria (as well as relatively high frequencies of taxa, so-called acmes) determine the zones recognized in the vertical successions. These methods and the actual counting on which they are - often unconsciously - based are being evaluated by the Utrecht team for several sections of the Mediterranean Neogene. The purpose of the present investigation was only to obtain a better documented insight in the reliability of different methods of collecting quantitative data.
Article
In the austral summer of 2006/7 the ANDRILL MIS (ANtarctic geological DRILLing-McMurdo Ice Shelf) project recovered a 1285†m sediment core from beneath the Ross Ice Shelf near Ross Island, Antarctica in a flexural moat associated with volcanic loading. The upper ~†600†m of this core contain sediments recording 38 glacial/interglacial cycles of Early Pliocene to Pleistocene time, including 13 discrete diatomite units (DUs). The longest of these, DU XI, is ~†76†m-thick, and has been assigned an Early to Mid-Pliocene age (5-3†Ma). A detailed record of the siliceous microfossil assemblages in DU XI is used in conjunction with geochemical and sedimentological data to subdivide DU XI into four discrete subunits of continuous sedimentation. Within each subunit, changes in diatom assemblages have been correlated with the [delta]18O record, providing a temporal resolution up to 600†yr, and allowing for the construction of a detailed age model and calculation of associated sediment accumulation rates within DU XI. Results indicate a productivity-dominated sedimentary record with greater proportions of hemipelagic mud accumulating during relatively cool periods. This implies that even during periods of substantial warmth, Milankovitch-paced changes in Antarctic ice volume can be linked to ecological changes recorded in diatom assemblages.
Article
The near-shore open-marine diatom record recovered in the ANtarctic geological DRILLing (ANDRILL) McMurdo Ice Shelf Project (MIS) AND-1B drillcore, McMurdo Sound, Antarctica, advances our understanding of the marine conditions present in the southern Ross Sea during the Pliocene and early Pleistocene. This diatom history is recorded within alternating diamictite and diatomite that reflect alternating glacial activity and high marine primary productivity. The diatomite units were deposited in a continental shelf open-marine setting during periods of reduced ice cover in West Antarctica. A new diatom biostratigraphic scheme spanning the last ca. 5 Ma is proposed for the Antarctic near-shore area, based on prior work from high latitude drillcores. Four new zones are proposed for the Pliocene/Pleistocene, with eight in total for the new zonal scheme, utilizing Actinocylus fasciculatus, Actinocyclus maccollumii, Fragilariopsis bohatyii, Rouxia antarctica, and Thalassiosira fasciculata as new zonal markers. The early Pliocene shares the most assemblage commonality with that of the Southern Ocean with greater numbers of endemic species observed in the late Pliocene and early Pleistocene; a group of related Fragilaripsis species characterizes much of this later part of the time column. Two new species are proposed, Fragilariopsis tigris sp. nov. Riesselman and Thalassiosira teres sp. nov. Winter; a formal name is also proposed for another species, Rhizosolenia harwoodii sp. nov. Winter. The new zonation is tied to a robust chronology utilizing diatom biostratigraphy, volcanic 40Ar/39Ar ages and magnetostratigraphy.
Article
Sediments from Ocean Drilling Program Site 1165 in the Indian Ocean sector of the Southern Ocean (off Prydz Bay) contain a series of layers that are rich in ice-rafted debris (IRD). Here we present evidence that IRD-rich layers at Site 1165 at 7, 4.8, and 3.5 Ma record short-lived, massive discharges of icebergs from Wilkes Land and Adélie Land, more than 1500 kilometers to the east of the depositional site. This distant source of icebergs is clearly defined by the presence of IRD hornblende grains with 40 Ar/ 39 Ar ages of 1200– 1100 Ma and 1550–1500 Ma, ages that are not found on the East Antarctic continent in locations closer to Site 1165. This observation requires enormous amounts of detritus-carrying drifting icebergs, most likely in the form of large icebergs. These events probably reflect destabilization, surge, and break-up of ice streams on the Wilkes Land and Adélie Land margins of the East Antarctic Ice Sheet, in the vicinity of the low-lying Aurora and Wilkes Basins. They occurred under warming conditions, but each coast seems to have produced ice-rafting events independently, at different times. The data presented here constitute the first evidence of far-traveled icebergs from specific source areas around the East Antarctic perimeter. Launch of these icebergs may have happened during quite dramatic events, perhaps analogous to "Heinrich Events" in the North Atlantic.
Article
Comparison of diatom data from modern surface sediments in Prydz Bay and the Kerguelen Plateau with diatom assemblages from the Sørsdal Formation, Vestfold Hills, indicates that the climate was warmer than present during the early Pliocene (4.5–4.1 Ma). Extant, sea-ice associated diatoms are significantly less abundant throughout the Sørsdal Formation than in the modern Antarctic coastal zone. Extant diatoms in the Sørsdal Formation, including Stellarima stellaris, Thalassiosira oliverana, Fragilariopsis sublinearis, Pseudo-nitzschia turgiduloides and Eucampia antarctica var. recta, are consistent with annual sea-surface temperatures (SST) of between −1.8 and 5.0°C. The presence of S. stellaris indicates that the summer SSTs were >3°C during some intervals. The absence of calcareous coccoliths and the silicoflagellate Dictyocha suggests that the upper limit for summer SST was <5°C. These data indicate that early Pliocene summer SST were between 1.6 and 3°C warmer than today. Abundant Chaetoceros cysts infer that stratified, open-water conditions were present during summer/spring. Ice sheet models suggest that warming of the magnitude evident in the Sørsdal Formation (≤3°C) should have resulted initially in increased snow accumulation and ice sheet growth. However, ice sheet growth was probably short-lived, as the long-term response to this warming in the early Pliocene resulted in a significant decrease in ice volume and deposition of the Sørsdal Formation. Other factors, such as increased basal-ice sliding and higher discharge (icebergs and melt-water), probably led to significantly elevated ablation rates from the Pliocene ice sheet, resulting in ice sheet retreat.
Article
Drift sediments recovered from the East Antarctic continental rise at Ocean Drilling Program (ODP) Site 1165 are used to infer variations in East Antarctic Ice Sheet (EAIS) stability and sea ice coverage during the late Miocene and early Pliocene. A significant increase in the deposition of biogenic opal from ∼5.8 to 5.2 Ma points to an early Pliocene reduction in sea ice and a subsequent increase in biological productivity. Time intervals at ∼7.2 to 6.6 Ma and ∼5.2 to 4.8 Ma are characterized by pronounced maxima in the long-term trend of terrigenous matter accumulation (MARter) indicating high continental erosion rates potentially caused by ice sheet growth. A Southern Ocean wide impact of these events is suggested by similar evidence found at ODP Site 1095 (Antarctic Peninsula). Superimposed on the MARter maxima we observe enhanced orbital variability in iron accumulation at Site 1165 pointing to a dynamic behavior of the EAIS with waxing and waning ice masses. From the concurrence of these high amplitude ice sheet fluctuations with maximum variance in Earth's obliquity, we propose that the insolation gradient between high and low latitudes affected the delivery of moisture to Antarctica and thus controlled ice volume variations.
Article
Gravity cores from the South Kerguelen Plateau contain sections representing two Quaternary intervals: (1) <0.62 Ma and (2) 1.6–1.25 Ma; three Pliocene intervals: (1) 2.2–1.6 Ma, (2) 3.1/3.0–2.64 Ma, (3) 3.2–3.1/3.0 Ma, and at least three disconformities: (1) 0.62 Ma until possibly the Last Glacial Maximum, (2) 2.2–1.6 Ma, and (3) 3.1–2.2 Ma. The disconformities probably formed during intense glacial intervals when the velocity of the Antarctic Circumpolar Current increased. Increased current velocity winnowed away finer biogenic sediment, to create ice rafted debris lags that protected the underlying sediment. Sediment deposition occurred during the less cold climatic intervals. In these deposits, three diatom assemblages were identified by comparison with extant diatoms. The diatom data indicate fluctuating Quaternary open pack-ice and open water conditions. The open pack-ice assemblage differs from modern sea-ice assemblages, and may reflect differences in the sea-ice setting during past colder intervals at latitudes north of the Antarctic Divergence. A silicoflagellate ratio was used to reconstruct Pliocene summer sea-surface temperatures. Two intervals with surface water temperatures warmer than today were identified, during which conditions were 3.0–4.5°C warmer (3.1–2.64 Ma), and 4.5°C warmer (3.2–3.1 Ma) as far south as 62°S. This would have occurred if either the Antarctic Polar Front Zone was ∼1200 km further south, or if the temperature gradient across the associated oceanic fronts would have been significantly lower. The Pliocene intervals contain a large amount of ice rafted debris that may reflect an environment heavily influenced by icebergs due to warmer glacial conditions in Antarctica, with increased sediment discharge and iceberg calving.
Article
The West Antarctic ice sheet (WAIS), with ice volume equivalent to ~5 m of sea level, has long been considered capable of past and future catastrophic collapse. Today, the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams. Grounding lines are several hundred meters below sea level and the bed deepens upstream, raising the prospect of runaway retreat. Projections of future WAIS behavior have been hampered by limited understanding of past variations and their underlying forcing mechanisms. Its variation since the Last Glacial Maximum is best known, with grounding lines advancing to the continental-shelf edges around ~15 kyr ago before retreating to near-modern locations by ~3 kyr ago. Prior collapses during the warmth of the early Pliocene epoch and some Pleistocene interglacials have been suggested indirectly from records of sea level and deep-sea-core isotopes, and by the discovery of open-ocean diatoms in subglacial sediments. Until now, however, little direct evidence of such behavior has been available. Here we use a combined ice sheet/ice shelf model capable of high-resolution nesting with a new treatment of grounding-line dynamics and ice-shelf buttressing to simulate Antarctic ice sheet variations over the past five million years. Modeled WAIS variations range from full glacial extents with grounding lines near the continental shelf break, intermediate states similar to modern, and brief but dramatic retreats, leaving only small, isolated ice caps on West Antarctic islands. Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years. Our simulation is in good agreement with a new sediment record (ANDRILL AND-1B) recovered from the western Ross Sea, indicating a long-term trend from more frequently collapsed to more glaciated states, dominant 40-kyr cyclicity in the Pliocene, and major retreats at marine isotope stage 31 (~1.07 Myr ago) and other super-interglacials. Supplementary Materials are included with the file. Four video files are attached below.
Palaeo sea-ice distribution and reconstruction derived from the geological records
  • L K Armand
  • A Leventer
Armand, L.K., Leventer, A., 2010. Palaeo sea-ice distribution and reconstruction derived from the geological records. In: Thomas, D.N., Dieckmann, G.S. (Eds.), Sea Ice. John Wiley & Sons, Ltd., Chichester, West Sussex, UK, pp. 469-530.
Expedition 318 summary
  • C Escutia
  • H Brinkhuis
  • A Klaus
  • Exp The
Escutia, C., Brinkhuis, H., Klaus, A., the Exp. 318 Scientists, 2011. Expedition 318 summary. In: Proceedings of the Integrated Ocean Drilling Program. vol. 318. pp. 1-59.
Sea Ice Index, Version 2. [median sea-ice extent
  • F Fetterer
  • K Knowles
  • W Meier
  • M Savoie
  • A K Windnagel
Fetterer, F., Knowles, K., Meier, W., Savoie, M., Windnagel, A.K., 2016. Sea Ice Index, Version 2. [median sea-ice extent]. NSIDC: National Snow and Ice Data Center, Boulder, Colorado USA updated daily. https://doi.org/10.7265/N5736NV7 (Date Accessed: Nov. 2015).
Antarctic contribution to global sea level in a high CO 2 world
  • N R Golledge
  • R H Levy
  • T R Naish
  • R M Mckay
  • G W Edward
  • D E Kowalewski
  • C J Fogwill
Golledge, N.R., Levy, R.H., Naish, T.R., McKay, R.M., Edward, G.W., Kowalewski, D.E., Fogwill, C.J., 2016. Antarctic contribution to global sea level in a high CO 2 world. Geophys. Res. Abstr. 18, 2016 (EGU2016-1407, EGU General Assembly).
Assessing recent trends in high-latitude Southern Hemisphere surface climate
  • J Jones
  • S Gille
  • H Goosse
  • N Abram
  • P Canziani
  • D Charman
  • K Clem
  • X Crosta
  • C De Lavergne
  • I Eisenman
  • M England
  • R Fogt
  • L M Frankcombe
  • G Marshall
  • V Masson-Delmotte
  • A Morrison
  • A Orsi
  • M Raphael
  • J Renwick
  • D Schneider
  • G Simpkins
  • E Steig
  • B Stenni
  • D Swingedouw
  • T Vance
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