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Cenozoic deep-sea temperatures and polar glaciation: The oxygen isotope record
... Several other potential drivers of Cenozoic cooling not involving the carbon cycle have also been proposed. It has been suggested that the opening of the Drake Passage and Tasman Gateway in the Southern Ocean allowed the formation and strengthening of the Antarctic Circumpolar Current, which in turn thermally isolated Antarctica enough to initiate regional cooling and ice sheet growth [Frakes and Kemp, 1972;Kennett and Shackleton, 1976;Kennett, 1977;Flower, 1999;Exon et al., 2000;Smith and Pickering, 2003;Sijp et al., 2014]. The changing albedo of the Earth's surface would have also had a significant impact on global climate. ...
... Icehouse conditions temporarily abated during the Miocene Climatic Optimum (MCO) from ~17.0 to ~14.6 Ma [Vincent and Berger, 1985;Wright et al., 1992;Kennett and Flower, 1993;Holbourn et al., 2005Holbourn et al., , 2007. Global temperatures were 2 to 4 o C warmer than today and the Antarctic ice sheet retreated significantly, while the Monterey Carbon Isotope Excursion (MCIE), CCD shoaling, and extensive organic carbon burial indicate a contemporaneous carbon cycle perturbation potentially linked to the Columbia River Basalt eruptions [Flower, 1999;You et al., 2009;You, 2010;Passchier et al., 2011;Foster et al., 2012]. This partial reversion to greenhouse conditions was short-lived though, and icehouse conditions were restored during the mid-Miocene Climate Transition (MMCT) from ~14.6 to ~12.5 Ma with an abrupt benthic δ 18 O and δ 13 C shift at ~13.9 Ma [Miller et al., 1991;Flower andKennett, 1993, 1994;Holbourn et al., 2005Holbourn et al., , 2007. ...
... Global temperatures were 2 to 4 o C warmer than today and the Antarctic ice sheet retreated significantly, while the Monterey Carbon Isotope Excursion (MCIE), CCD shoaling, and extensive organic carbon burial indicate a contemporaneous carbon cycle perturbation potentially linked to the Columbia River Basalt eruptions [Flower, 1999;You et al., 2009;You, 2010;Passchier et al., 2011;Foster et al., 2012]. This partial reversion to greenhouse conditions was short-lived though, and icehouse conditions were restored during the mid-Miocene Climate Transition (MMCT) from ~14.6 to ~12.5 Ma with an abrupt benthic δ 18 O and δ 13 C shift at ~13.9 Ma [Miller et al., 1991;Flower andKennett, 1993, 1994;Holbourn et al., 2005Holbourn et al., , 2007. ...
Over the course of the Cenozoic the Earth system has shifted from a CO2-rich ‘Greenhouse’ climate state to a CO2-poor ‘Icehouse’ climate state. This trend is punctuated by numerous perturbations to the carbon-climate system, but the extent of the coupling between the carbon cycle and climate system, the drivers of these perturbations, and their relationship to the longer-term Cenozoic trend is still debated. In this thesis, I use biogeochemical modelling and numerical analysis to explore the key research question: ‘What were the drivers of carbon-climate system perturbations during the Cenozoic?’, with a focus on perturbations during the Eocene-Oligocene Transition and the mid-Miocene, the role of tipping points during these periods, and the long-term evolution of the ocean carbonate system.
The potential impact of the Columbia River Basalt large igneous province on the mid-Miocene Earth system is investigated using two biogeochemical box models. This modelling indicates that ‘cryptic degassing’ from intrusive and/or crust-contaminated magma of a magnitude within the estimated possible range can drive the observed carbon cycle perturbation and warming around 16.0 Ma, but cannot by itself explain other features of the mid-Miocene palaeorecords.
The hypothesised drivers of the Eocene-Oligocene Transition (EOT) carbon cycle perturbation are explored using a biogeochemical box model. The results suggest that the glacioeustatic fractionation of carbonate burial from shelf to basin can explain most of the deepening of the carbonate compensation depth (CCD) at the EOT, but that the benthic carbon isotope excursion most likely requires additional drivers.
The shelf-basin carbonate burial fractionation hypothesis is examined further in order to quantify the relationship between shelf carbonate burial extent, the CCD, and changing sea-level during the Cenozoic. This analysis confirms that carbonate burial fractionation can drive most of the CCD deepening at the EOT but is less important either before or since then, and also indicates that the sensitivity of the CCD to sea level change has significantly declined during the Cenozoic.
Palaeorecords of a number of perturbations to the carbon-climate system during the Cenozoic are analysed in search for ‘early warning signals’ (EWS) indicative of systemic instability and impending critical transitions, and the reliability of this method when applied to palaeorecords is critically explored. EWS are found prior to some (e.g. the EOT and Palaeocene-Eocene Thermal Maximum) but not all of the events, and the results and technique are judged to be moderately reliable.
... This mid-Miocene global cooling induced the establishment of a major ice-sheet on East Antarctica (Kennett 1985;Zachos et al. 2001), as indicated by successive ␦ 18 O shifts beginning at 14.5 Ma (Mi-events Mi3a, 3b, 4;Miller et al. 1987). The establishment of a major ice field in the Southern Hemisphere probably contributed to strengthening the poleto-equator temperature gradient (Kennett 1985;Woodruff and Savin 1989;Flower and Kennett 1994;Flower 1999), thus intensifying ocean circulation patterns and coastal upwelling, which in turn impacted productivity and sedimentation patterns on the shelf (Kennett 1985;Vincent and Berger 1985;John et al. 2002). ...
... The modern current-swept Marion Plateau is an ocean-facing area with surface waters dominated by the southern edge of the westward-flowing South Equatorial Current (Fig. 11, Pickard et al. 1977;Tomczak and Godfrey 1994). Because the permanent establishment of the East Antarctic ice sheet around 13.6 Ma induced major changes in worldwide oceanic circulation and climatic pattern (Kennett 1985;Miller et al. 1991b;Flower and Kennett 1994;Flower 1999), and because no barrier existed to shelter the Marion Plateau from the open ocean, it is reasonable to assume that current patterns on the plateau shifted and/or strengthened in response to global paleoceanographic changes taking place at that time (Fig. 16). Recently Anselmetti et al. (2000) have shown how the effect of changes in the strength of the Florida Current can be tied to sea-level changes and affect the interfingering of proximal and distal slope sediment adjacent to the Great Bahama Bank. ...
In this paper we explore the relative control of paleocean-ography, eustasy, and water temperature over the evolution of a car-bonate slope system deposited on the Marion Plateau (Northeastern Australia). Growth of several carbonate platforms started in the early Miocene on this plateau, and although they occurred in low-latitude subtropical waters they are composed mainly of heterozoan organisms. We investigated an upper to distal slope transect drilled during ODP Leg 194 and located close to the Northern Marion Platform. We reconstructed mass accumulation rates of carbonate as well as the evolution in the ratios of carbon and oxygen stable isotopes. Power spectrum analysis of the carbon isotope record revealed the existence of cycles with main frequencies centered around 409 Kyr and 1800 Kyr. We interpret the 409 Kyr cycle as being paced by changes in the eccentricity of the Earth orbit, and we suggest that the 1800 Kyr cycle could be linked to long-term eustatic changes. Finally, on the basis of the timing of changes in mass accumulation rates of carbonate we infer that the strength and direction of oceanic currents affected sedimen-tation on the Marion Plateau by shifting depocenters of slope sedi-mentation, a process probably further modulated by sea-level changes. We argue that the evolution and demise of the heterozoan carbonate systems present on the Marion Plateau were controlled mainly by the evolution of strong benthic currents, and that eustasy and water temperature alone did not account for the drowning of the platforms in the mid Miocene.
... The Eocene -Oligocene is a critical period in Cenozoic climate evolution that coincided with the development of the first East Antarctic ice-sheet, close to the Eocene/Oligocene (E/O) boundary (Barrett, 1996;Flower, 1999). The timing and causes of the inception of the glacial history of Antarctica are still being debated (DeConto and Polland, 2003;Barrett, 2003). ...
... During the past three decades, numerous investigations carried out on sedimentary sequences drilled in the Southern Ocean and along the Antarctic continental margin (Ocean Drilling Program-ODP, CIROS and Cape Roberts Project) have provided major contributions to the advancement of our understanding of the climatic, oceanographic and biological evolution of Antarctica and the Southern Ocean (Barker et al., 1999;O'Brien et al., 2001;Exon et al., 2001;Cape Roberts Science Team, 1998, 1999Raine and Askin, 2001;Barrett, 2003). ...
The evolution of the Southern Ocean climate during the late Eocene–late Oligocene interval is examined through high-resolution, quantitative calcareous nannofossil analyses on samples from the Southern Ocean sections on Maud Rise and Kerguelen Plateau. We determined the abundance patterns of the counted species to clarify the biostratigraphy, which we correlated with high-resolution magnetostratigraphy [Roberts, A.P., Bicknell, S.J., Byatt, J., Bohaty, S.M., Florindo, F., Harwood, D.M., 2003a. Magnetostratigraphic calibration of Southern Ocean diatom datums from the Eocene–Oligocene of Kerguelen Plateau (Ocean Drilling Program Sites 744 and 748). In: Florindo, F., Cooper, A.K., O'Brien, P.A. (Eds.), Antarctic Cenozoic Palaeoenvironments: Geologic Record and Models. Palaeogeogr., Palaeoclimatol., Palaeoecol. 198 145–168; Florindo, F., Roberts, A.P., in press. Eocene–Oligocene magnetobiochronology of ODP Sites 689 and 690, Maud Rise, Weddell Sea, Antarctica. Geol. Soc. Am. Bull.], and used this data to interpret paleoceanographic changes through the late Eocene to late Oligocene. Percentage plots of the individual species, compared with R-mode principal component and cluster analysis results, allowed us to divide the assemblages into three groups: temperate-water taxa, cool-water taxa, and no temperature-affinity taxa. We attempt correlations between these paleoecological groups and the major sea-surface temperature (SST) variations with tectonic and paleoceanographic changes in the Southern Ocean. During the late Eocene, the nannofossil assemblage data reveal that there were several minor SST decreases (coolings) from 36 to 34 Ma, before the Eocene/Oligocene (E/O) boundary. A sharp cooling event, dated at 33.54 Ma (earliest Oligocene), occurred about 160 kyr after the E/O boundary, which is dated at 33.7 Ma. Relatively stable, cool conditions are interpreted to persist until the latest Oligocene, when an increase in abundance of temperate-water taxa, which corresponds to an antithetical decrease in abundance of cool-water indicators, is recorded.On the basis of our dating, the opening of the Drake Passage, allowing shallow-water circulation, began by 33.54 Ma at the latest, while the establishment of deep-water connections through the Tasmanian Gateway occurred at 33 Ma, as suggested by Exon et al. [Proc. ODP, Init. Rep. 189 (2001) 1].
... However, Flower (1999) has since suggested that 'perhaps the largest step in the transition from the "greenhouse world" to the "icehouse world" occurred near the Eocene/Oligocene boundary'. This transition was dated by Flower at ca 33.6 Ma, significantly younger than the 40 Ma of Robin (1988), and approximately 10 Myr younger than our interpretation of the age of the eoc unconformity. ...
... This transition was dated by Flower at ca 33.6 Ma, significantly younger than the 40 Ma of Robin (1988), and approximately 10 Myr younger than our interpretation of the age of the eoc unconformity. In a similar time-frame to the major oceanic cooling interpreted by Flower (1999), the gateway between the South Tasman Rise and Antarctica opened, leading to a major change in oceanic circulation (Exon et al., 2001(Exon et al., , 2002. ...
Detailed seismic stratigraphic analysis of 2D reflection seismic data from the East Antarctic rifted continental margin provides an ancient analogue for turbidity fan systems at high-latitude margins. Reflector sequences characteristic of turbidity flow channel migration and aggradation are identified, along with other depositional elements integral to the understanding of deep-water deposition, such as channel levees, splays and wavy bedforms. Wavy bedforms are contrasted with slump-related features, which although present do not explain the morphological features interpreted here as sediment waves. Slump-related features suggest, qualitatively, a relationship between a basal detachment layer and a widespread bottom-simulating reflector.The influence of the initial morphology on slope deposition and distributary patterns for turbidite deposits is clear in these data. Healed-slope accommodation on an above-grade slope is evident in seismic profiles. Ponded and/or healed-slope accommodation is associated with pre-existing bathymetric highs, interpreted as rift-related, serpentinised, peridotite ridges.
... In another assessment of the initiation of Antarctic glaciation at the Eocene-Oligocene boundary, the second paper in the volume (Hay et al., 2004) takes a Table 1 Geologic time intervals that are discussed in the contributions of this special issue Preface speculative look at the mechanism for the major isotopic excursion at the Eocene-Oligocene boundary. Many workers now contend that this excursion resulted from continent-wide glaciation of Antarctica, and they justify the ice volume explanation above temperature because, if the isotopic excursion were due solely to temperature, it would require freezing of the ocean to a significant depth (Kennett, 1977;Flower, 1999). Hay et al. (2004), however, contend that higher salinity oceans in the Palaeogene would lower the freezing point to the degree that such a claim no longer holds and only a secondary part of the D 18 O change is related to the formation of permanent sea ice on the Arctic and perhaps in areas around Antarctica. ...
... Recently, a series of three holes was drilled in McMurdo Sound, Ross Sea as part of the Cape Roberts Project (CRP) (Cape Roberts Science Team, 1998, 1999, 2000Hambrey et al., 1998;Barrett et al., 2000Barrett et al., , 2001. This project was a cooperative venture between scientists from seven countries, i.e., Australia, Britain, Germany, Italy, The Netherlands, New Zealand, and the USA, and it was set-up to investigate the early history of the EAIS and the West Antarctic Rift System by coring approximately 1500 m of Cenozoic strata and 100 m of Palaeozoic-age Beacon Supergroup rocks at the edge of the present ice sheet and close to the TAM. ...
... In another assessment of the initiation of Antarctic glaciation at the Eocene-Oligocene boundary, the second paper in the volume (Hay et al., 2004) takes a Table 1 Geologic time intervals that are discussed in the contributions of this special issue Preface speculative look at the mechanism for the major isotopic excursion at the Eocene-Oligocene boundary. Many workers now contend that this excursion resulted from continent-wide glaciation of Antarctica, and they justify the ice volume explanation above temperature because, if the isotopic excursion were due solely to temperature, it would require freezing of the ocean to a significant depth (Kennett, 1977;Flower, 1999). Hay et al. (2004), however, contend that higher salinity oceans in the Palaeogene would lower the freezing point to the degree that such a claim no longer holds and only a secondary part of the D 18 O change is related to the formation of permanent sea ice on the Arctic and perhaps in areas around Antarctica. ...
... Recently, a series of three holes was drilled in McMurdo Sound, Ross Sea as part of the Cape Roberts Project (CRP) (Cape Roberts Science Team, 1998, 1999, 2000Hambrey et al., 1998;Barrett et al., 2000Barrett et al., , 2001. This project was a cooperative venture between scientists from seven countries, i.e., Australia, Britain, Germany, Italy, The Netherlands, New Zealand, and the USA, and it was set-up to investigate the early history of the EAIS and the West Antarctic Rift System by coring approximately 1500 m of Cenozoic strata and 100 m of Palaeozoic-age Beacon Supergroup rocks at the edge of the present ice sheet and close to the TAM. ...
The Cenozoic evolution of the Antarctic cryosphere and fluctuations in its ice sheet cover are considered to be one of the major influences on low- and mid-latitude deep-sea sedimentary records. Long-term Cenozoic trends and short-term climate fluctuations (≤40 ka) alike are inferred to have been driven or modulated by changes in Antarctic ice sheet volume (Kennett, 1977; Imbrie and Imbrie, 1980; Zachos et al., 1997, 2001; Shackleton et al., 1999; Lear et al., 2000; Naish et al., 2001). Similarly, changes in sea level elevations at continental margins are also inferred to result from growth and decay in Antarctic ice sheet volume throughout the Cenozoic (Barrett et al., 1987; Haq et al., 1987). Yet, direct records of the Antarctic cryosphere and its ice sheets are sparse at best, and much of the inference remains untested. Recent efforts have begun to change this, and the last decade has seen several expeditions to the Antarctic and Southern Oceans, which have recovered new high-quality sedimentary core and seismic reflection records of Southern high-latitude Cenozoic ice sheets and climate. These include the Cape Roberts Project (CRP) (Cape Roberts Science Team, 1998; Hambrey et al., 1998; Cape Roberts Science Team, 1999; Barrett et al., 2000; Cape Roberts Science Team, 2000; Barrett et al., 2001; Davey et al., 2001), ODP Leg 177 (Gersonde et al., 1999, 2003), Leg 178 (Barker et al., 1999, 2002), Leg 182 (Feary et al., 2000; Hine et al., 2004), Leg 188 (O’Brien et al., 2001; Cooper et al., 2004), and Leg 189 (Exon et al., 2001, in press), and various RVIB NB Palmer and Polarstern cruises. Recent results from these expeditions were presented at a special session of the EGS–AGU Joint assembly held in Nice, France, in April 2003. The focus of the session was the many orders and scales of variation of Antarctic ice sheets and climate from Antarctic and sub-Antarctic records derived from outcrop studies, deep sea and continental margin drilling, and seismic reflection investigations. The session also included new modelling results utilizing new data from these recent expeditions and preliminary results of geophysical surveys defining sub-ice shelf and sea ice sedimentary basins identified as drilling targets in the near future under the ANDRILL program (Harwood et al., 2002; Florindo et al., 2003a). Published 1-7 3.8. Geofisica per l'ambiente JCR Journal
... The first and most major of these, Mi-1 (Miller et al. 1991), is at the Oligocene-Miocene boundary, at ~23 Ma. This is followed by at least five further δ 18 O stages, Mi-2 to Mi-6 (and substages), at ~16 Ma, ~15 Ma, ~13 Ma, ~12 Ma and ~10 Ma (Miller et al. 1991;Flower 1999). A significant overall positive step in δ 18 O occurred between Mi-2 and Mi-4, between ~16 Ma and ~12 Ma (e.g. ...
The Phanerozoic is comprised of over 540 million years and, with its defining accompaniment of abundant complex life, provides us with a unique perspective on the extremes of climate change. Understanding these extremes is particularly important if we are to anticipate the possible effects of global warming. The broad sweep of climate change through the Phanerozoic began with relatively cool global temperatures and recovery from late Proterozoic glaciation. This was followed by a mid-Cambrian to Ordovician episode of relatively warm global climate, after which global climate cooled, culminating in the major glaciations of the Carboniferous and Permian Periods. The Triassic and Early Jurassic were warm. The Late Jurassic-Early Cretaceous Period was cool, although without full global glaciation. Global temperatures peaked in the mid-Cretaceous. Since then, global climates have cooled, culminating in Neogene glaciation. These c. 100-million-year trends in overall climate show short intense excursions of contrasting climate, many of which have been associated with the mass extinction of life, and with major volcanic and tectonic events. This paper argues that, through the Phanerozoic, two overlapping stable climate regimes appear to have dominated: a bigh-CO2 (> 1000 ppmv), largely warm climate regime, punctuated by many short-lived episodes of glaciation; and a low-CO2 (< 1000 ppmv), largely cool regime, marked by protracted episodes of superglaciation.
... Proxy records and climate model simulations suggest that the warmth of the MCO (2 to 4 • C warmer than today) can be largely explained by atmospheric CO 2 concentrations increasing to >400 ppmv (Flower, 1999;Foster et al., 2012;Henrot et al., 2010;Herold et al., 2011;Knorr et al., 2011;Kürschner et al., 2008;You, 2010;You et al., 2009). Our simulations show a ∼90 ppmv increase by 15.9 Ma, suggesting that at least some of this increase could have been derived from the CRB. ...
Large Igneous Provinces (LIPs) have been emplaced throughout Earth's history, erupting great quantities (>104 km3>104 km3) of lava in long-lived (>105 y>105 y) events that have been linked to major environmental disruptions. The largest LIP eruptions (e.g. Siberian Traps) are widely considered to have had an impact on global climate through basalt CO2 degassing but the impact of the more numerous smaller LIPs is debated. Here we test the hypothesis that LIPs had a greater impact on Earth's climate history than previously estimated because of the ‘cryptic degassing’ of intruded and crust-contaminated magma, injecting extra CO2 over and above that coming from sub-aerial basalts. We use biogeochemical box models to investigate the potential impact of the Columbia River Basalts (CRB) during the mid-Miocene where multiple palaeorecords for this geologically relatively recent event enable more rigorous data-model comparison. We find that the effect on the long-term carbon cycle of basalt degassing from the CRB alone is negligible, but that a total CRB emission of 4090–5670 Pg of carbon with 3000–4000 Pg of this carbon emitted during the Grande Ronde Basalt eruptions, a flux within the acceptable estimated range when cryptic degassing is included, does well in reproducing the record of benthic δ 13C and atmospheric CO2 change during the core of the Miocene Climatic Optimum. Nevertheless, mechanisms other than degassing are required to drive observed warmth before 16.3 Ma and to match observed calcite compensation depth behaviour after ∼15.4 Ma∼15.4 Ma. Hence, our findings rule out the possibility that CRB emplacement alone can fully explain the mid-Miocene record but they demonstrate the enhanced climate impact that occurs when substantial cryptic degassing accompanies LIP emplacement.
... The presented materials provide grounds for out lining successive stages in the development of glacia tion in Antarctica and its influence on sedimentation in the Southern Ocean. It should be noted that such attempts to define development stages in the glacia tion of Antarctica were undertaken long ago using the data on oxygen isotope ratios in foraminiferal tests (Flower, 1999;Kennett, 1987). ...
Literature pertaining to history of the Cenozoic glaciation in Antarctica and its influence on sedimentation in marginal areas of the continent and deep-water domains of the Southern Ocean are reviewed. Original data obtained by G.L. Leichenkov on seismostratigraphy of the East Antarctica margin are also used. Particular attention is paid to the history of silica accumulation, ice-rafted material deposition, and formation of contourites. The results make it possible to define the following main stages of Antarctic glaciation in the Cenozoic: (1) middle-late Eocene; (2) Oligocene; (3) early Miocene; (4) middle Miocene; (5) late Miocene-early Pliocene; and (6) late Pliocene-Holocene.
... Much of the supposed history of glaciation of Antarctica has come from interpretation of oxygen isotope records (Flower, 1999). A more direct line of evidence comes from records of sea-level change. ...
Today the ocean is characterized by pools of warm tropical-subtropical
water bounded poleward and at depth by cold water. In the tropics and
subtropics the warm waters are bounded at depth by the thermocline,
which outcrops on the ocean surface between the subtropical and polar
frontal systems that form the poleward boundary. It is along and between
the frontal systems that the thermocline waters enter the ocean
interior. These frontal systems from beneath the maxima of the zonal
component of the westerly winds. Today the location of the westerly
winds is stabilized by the persistent high pressure systems at the polar
regions produced by the ice cover of the Antarctic and sea-ice cover of
the Arctic. Evidence from the paleobiogeographic distribution of
plankton fossils indicates that during the Eocene and earlier times the
subtropical and polar frontal systems were not persistent features.
Recent climate model experiments show that without perennial ice cover
in the polar regions a seasonal alternation between high and low
atmospheric pressure systems can occur. These seasonal alternation would
force major changes in the location and strength of the westerly winds,
preventing the development of the well-defined frontal systems that
characterize the earth today. Without the subtropical and polar frontal
systems the thermocline would be less well developed and the pycnocline
might even be dominated by salinity differences. Evidence from ocean
drilling suggests that the glaciation of East Antarctica began at the
Eocene-Oligocene boundary, but took time to spread over the entire
continent. The presence of calcareous nannoplankton in the Arctic basin
during the Eocene and earlier, and their absence in Oligocene and
younger strata suggest that the ice cover of the Arctic Ocean also
developed at the Eocene-Oligocene boundary. Both of these events appear
to be related to the development of the modern oceanic structure, but it
remains uncertain whether the ocean changed in response to the
development of ice covered polar regions or vice versa.
... Based on a recent global compilation of deep-sea benthic foraminifer oxygen isotope records ( Zachos et al., 2001), the long-term increase in Cenozoic δ 18 O of ~3.2‰ must reflect some combination of deep-sea cooling and polar ice sheet growth. Because present-day ice sheets account for ~1‰ equivalent δ 18 O, the remaining 2.2‰ is consistent with cooling of 9°11°C over the course of the Cenozoic ( Miller et al., 1987;Flower, 1999;Zachos et al., 2001). Three rapid increases in benthic δ 18 O are generally inferred to include significant ice sheet growth, including increases near the Eocene/Oligocene boundary at ~33.5 Ma, during the middle Miocene at ~14 Ma, and during the late Pliocene at ~2.75 Ma. ...
Magnetic polarity stratigraphy and stable isotope data from Ocean Drilling Program Leg 202 Core 202-1237B-31H allow calibration of the late Oligocene climate transition to the geomagnetic polarity timescale (GTS 2004). Oxygen isotope (δ18O) values in the benthic foraminifer Cibicidoides mundulus decrease from ~2.20‰ in upper Chron 9n (Oi2b) to ~1.90‰ in upper Chron 8r. The mean decrease is smaller than sug- gested by a recent compilation of benthic foraminifer δ18O records. The decrease occurs in three steps from 323 to 321 meters composite depth (mcd) within Chron 8r (27.027-26.554 Ma), and is here termed the "Chron 8r δ18O shift." Linear interpolation within Chron 8r estimates an age of 26.35 Ma for the final decrease to lower values marking initia- tion of the late Oligocene climatic optimum. The initial decrease at ~26.7 Ma is closely associated with the last occurrence of Paragloborota- lia opima (P21/P22 planktonic foraminifer zonal boundary) between 322.51 and 322.76 mcd in lowermost Chron 8r. This study is a modest step toward an integrated magnetic polarity and astronomical chrono- stratigraphy for the late Oligocene based on Site 1237.
... Miller et al. Zachos et al., 1992; Flower, 1999). The development of the Circum-Antarctic Current led to the thermal isolation of Antarctica , the expansion of continental ice sheets, and a general global cooling , resulting in prominent positive shifts in both carbon and oxygen isotopic curves. ...
During Ocean Drilling Program (ODP) Leg 182, nine sites were drilled across the southern Australian margin in the Great Australian Bight, with the objective of obtaining a more detailed understanding of cool-water carbonate depositional processes and global environmental change in mid-latitude settings. Drilling results provide insights into the temporal and spatial aspects of cool-water carbonate deposition in shelf edge and slope environments. Drilling showed that the spectacu-lar prograding clinoform sequence forming the upper slope and outer-most shelf of the Great Australian Bight margin was rapidly deposited during the Pleistocene. Meter-scale lithologic cycles rapidly accumu-lated in response to orbitally forced sea level fluctuations. Skeletal ele-ments within these wackestone to packstone, coarsening-upward cycles consist of tunicate spicules, brown bioclasts, bryozoan fragments, and red coralline algal debris—a heterozoan assemblage typical of cool-water carbonates. The high accumulation rates, comparable to rates in warm-water carbonate environments, reflect partitioning of sedimenta-tion between the shelf and slope as high wave energy from the South-ern Ocean interacted with sea level fluctuations to generate vigorous off-shelf transport. Mound features visible on seismic reflection data on and underlying the uppermost slope throughout the Pleistocene of the central and western Great Australian Bight are in situ bryozoan reef mounds. These mounds consist of diverse suites of bryozoans, together with coralline algae, echinoid spines, and benthic foraminifers, D.A. FEARY ET AL. LEG 182 SYNTHESIS: EXPOSED SECRETS OF THE GAB 2 packstone matrix. Seismic and isotopic data indicate that mounds de-veloped cyclically in response to glacial–interglacial productivity cycles. Increased upwelling during sea level lowstands promoted active mound growth, in contrast to the thin mud accumulations that draped inactive mounds during highstands. Such mounds have not previously been de-scribed from the "modern" ocean and provide unlithified analogs for similar features that occur in the mid-to late Paleozoic rock record. The interaction of high-salinity (up to 106‰) interstitial brines with abundant organic matter within the upper parts of the sedimentary suc-cession produced high concentrations of methane (up to 50%) and hydrogen sulfide (up to 15%). This unusual chemical environment lead to extensive carbonate recrystallization and dissolution of high-magnesium calcite and the precipitation of low-magnesium calcite and dolomite. In certain environments, therefore, cool-water carbonates may be at least as diagenetically active as their warm-water counter-parts. The brines probably formed in shallow evaporative pools and lagoons on the shelf during sea level lowstands, seeped into the under-lying sediments, and flowed toward the upper slope. Thermodynamic considerations suggest that H 2 S and CH 4 disseminated gas hydrates might be present within the Great Australian Bight succession.
... " Based on a recent global compilation of deep-sea benthic foraminifer oxygen isotope records (Zachos et al., 2001), the long-term increase in Cenozoic δ 18 O of ~3.2‰ must reflect some combination of deep-sea cooling and polar ice sheet growth. Because present-day ice sheets account for ~1‰ equivalent δ 18 O, the remaining 2.2‰ is consistent with cooling of 9°– 11°C over the course of the Cenozoic (Miller et al., 1987; Flower, 1999; Zachos et al., 2001). Three rapid increases in benthic δ 18 O are generally inferred to include significant ice sheet growth, including increases near the Eocene/Oligocene boundary at ~33.5 Ma, during the middle Miocene at ~14 Ma, and during the late Pliocene at ~2.75 Ma. ...
Magnetic polarity stratigraphy and stable isotope data from Ocean Drilling Program Leg 202 Core 202-1237B-31H allow calibration of the late Oligocene climate transition to the geomagnetic polarity timescale (GTS 2004). Oxygen isotope (δ 18 O) values in the benthic foraminifer Cibicidoides mundulus decrease from ~2.20‰ in upper Chron 9n (Oi2b) to ~1.90‰ in upper Chron 8r. The mean decrease is smaller than sug-gested by a recent compilation of benthic foraminifer δ 18 O records. The decrease occurs in three steps from 323 to 321 meters composite depth (mcd) within Chron 8r (27.027–26.554 Ma), and is here termed the "Chron 8r δ 18 O shift." Linear interpolation within Chron 8r estimates an age of 26.35 Ma for the final decrease to lower values marking initia-tion of the late Oligocene climatic optimum. The initial decrease at ~26.7 Ma is closely associated with the last occurrence of Paragloborota-lia opima (P21/P22 planktonic foraminifer zonal boundary) between 322.51 and 322.76 mcd in lowermost Chron 8r. This study is a modest step toward an integrated magnetic polarity and astronomical chrono-stratigraphy for the late Oligocene based on Site 1237.
... Much of the supposed history of glaciation of Antarctica has come from interpretation of oxygen isotope records (Flower, 1999). A more direct line of evidence comes from records of sea-level change. ...
Today, the ocean is characterized by pools of warm tropical–subtropical water bounded poleward and at depth by cold water. In the tropics and subtropics, the warm waters are floored at depth by the thermocline–pycnocline, which crops out on the ocean surface between the subtropical and polar frontal systems that form the poleward boundary. It is along and between the frontal systems that the thermocline waters enter the ocean interior. These frontal systems form beneath the maxima of the zonal component of the westerly winds. Today, the location of the westerly winds is stabilized by the persistent high-pressure systems at the polar regions produced by the ice cover of the Antarctic and sea-ice cover of the Arctic.
... A series of increases in benthic marine y 18 O provide the most commonly cited evidence for Palaeogene cooling and East Antarctic glaciation (e.g., Zachos et al., 2001). Oligocene isotope event 1 (Oi1) in the nomenclature of Miller et al. (1991) is the biggest (N 1x) of these increases and is generally believed to coincide with the sudden (b10 5 year) growth of the East Ant-arctic Ice Sheet (Flower, 1999;Zachos et al., 1996). The majority of palaeobotanical records of Antarctic margin vegetation growing during the late Palaeocene to early Miocene (bracketing the E/O boundary and of significance for assessing the proxy climate record across the Oi1 transition) are located within two distinct geographic regions: the Antarctic Peninsula and the Ross Sea. ...
Simulated climate for the Antarctic continent using the GENESIS (Version 2.1) Global Climate Model with 34 Ma boundary conditions is shown to be highly sensitive to polar vegetation type. Six experiments were run using different levels of atmospheric CO2, orbital configurations, ice sheet geometries and vegetation types to assess model sensitivity to Antarctica being covered with either a needle leaf evergreen forest or tundra. Simulations using 2× pre-industrial levels of CO2 (combined mean annual temperature (MAT) − 19 °C) are about 7 °C cooler than minimum estimates from the Antarctic Cenozoic plant record (MAT − 12 to 15 °C). However, simulations using 3× CO2 (MAT − 7 °C) are in good agreement with our empirical estimates of mean annual temperature. With ice sheets and orbits set up to represent early Oligocene interglacial conditions, the tundra climate is significantly cooler than the evergreen forest climate, with local, austral summer averages up to 6 °C cooler in non-glaciated areas and continental averages ∼2.5 °C cooler. In the model this is mainly due to higher albedo and decreases in net radiation and sensible and latent heat flux, especially during spring and summer. Feedbacks between coastal and continental cooling, marginal sea surface temperatures and sea ice also appear to be significant. A review of the late Palaeocene to earliest Miocene plant fossil record in the Antarctic Peninsula and Ross Sea regions shows that the vegetation was in transition ∼34 Ma, from a relatively diverse, mainly evergreen forest to a tundra vegetation. The modelled sensitivity of continental temperatures to a change from forest to tundra suggests vegetation-climate feedbacks during the Eocene–Oligocene transition played a significant role in the initial rapid glaciation of the continent.
... The early Oligocene marks a significant transition in global climate and provides the first strong evidence of permanent Cenozoic glaciation on Antarctica (Kennett, 1977; Miller et al., 1991; Zachos et al., 1992; Moss and McGowran, 1993; Flower, 1999; Barker et al., 1999 ). The progressive widening of the Australian –Antarctic Seaway (AAS) and subsidence of the Tasman Rise during the late Eocene contributed to the establishment of surface and deep water circulation, the thermal isolation and cooling of Antarctica, and provided the setting for the expansion of continental glaciation (Kennett, 1977). ...
Sediments from ODP Site 1128 in the Great Australian Bight record isotopic and mineralogic variations corresponding to orbital parameters and regional climate change during the early Oligocene climate transition and Oi1 glacial event. Bulk carbonate stable isotope analyses reveal prominent positive oxygen and carbon isotope shifts related to the inferred major increase in glaciation at approximately 33.6 to 33.48 Ma. The oxygen isotope excursion corresponds to a prolonged period of low eccentricity, suggesting ice-sheet growth during low seasonality conditions. The clay mineralogy is dominated by smectite throughout. The exclusive occurrence of highly crystalline smectite from 33.6 to 33.5 Ma suggests the occurrence of explosive volcanism that correlates with the positive oxygen isotope shift. The dominance of mixed-layer smectite from 33.5 to 33.4 Ma and an increase in illite following 33.4 Ma indicates a transition from cool, wet conditions to cool, dry conditions over Australia during the Oi1 glaciation. Clay mineralogy and carbonate percentages reveal precession-scale oscillations during the Oi1 event. Kaolinite varies inversely with smectite and percent carbonate. Variations in precipitation and runoff, and wind velocities during southern hemisphere summer perihelion and high eccentricity intervals may account for the precession-scale oscillations.
... 120 Ma) and became more isolated from other continents following the progressive fragmentation of Gondwana during the Cretaceous and Cenozoic (Lawver et al., 1992;DiVenere et al., 1994;Besse and Courtillot, 2002;Lawver and Gahagan, 2003). Despite its polar position, Antarctica is thought to have remained mostly ice-free, vegetated, and with mean annual temperatures above freezing until the initial growth of the East Antarctic Ice Sheet near the Eocene-Oligocene boundary (e.g., Barrett, 1996;Flower, 1999). ...
ODP sites 689 and 690 (Leg 113) were piston cored on Maud Rise, which is an isolated aseismic ridge in the eastern Weddell Sea that rises more than 2500 m above the surrounding abyssal plains. These sites form part of a depth transect for studies of vertical water-mass stratification around Antarctica during the latest Mesozoic and Cenozoic. A medium-resolution magnetostratigraphic investigation was previously carried out on these sequences and is a cornerstone for the Southern Ocean Paleogene and Neogene chronostratigraphic framework. Parts of the magnetostratigraphic interpretation have been called into question by biostratigraphers, and, recently, new paleomagnetic analyses in the upper Paleocene-middle Eocene portion of the records pointed out: (1) an unexplained cluster of ChRM declinations, and (2) stratigraphic intervals where the ChRM inclinations are noticeably lower than those predicted for a time-averaged geocentric axial dipole field. This evidence suggests that the paleomagnetic records are contaminated by either an inward-radial magnetization or a core-split overprint. The possibility of a pervasive declination cluster was also pointed out for younger stratigraphic levels, which raised doubts about the reliability of these important chronostratigraphic records. In order to clarify these concerns and to check the reliability of the records, a high-resolution reinvestigation was carried out on the Paleogene sediments recovered at sites 689 and 690. One hundred and sixty eight u-channel samples were collected from the Eocene and Oligocene portions of these records and were analyzed using a pass-through cryogenic magnetometer with measurements at 1-cm intervals (4.5-cm resolution). The new magnetostratigraphic results provide a superb record of geomagnetic field behaviour for c. 20 million years. A pervasive overprint appears to be present below the middle Eocene, which compromises the value of magnetobiostratigraphic interpretations for the lower part of the record.
... 120 Ma) and became more isolated from other continents following the progressive fragmentation of Gondwana during the Cretaceous and Cenozoic (Lawver et al., 1992;DiVenere et al., 1994;Besse and Courtillot, 2002;Lawver and Gahagan, 2003). Despite its polar position, Antarctica is thought to have remained mostly ice-free, vegetated, and with mean annual temperatures above freezing until the initial growth of the East Antarctic Ice Sheet near the Eocene-Oligocene boundary (e.g., Barrett, 1996;Flower, 1999). ...
Magnetostratigraphic studies of Paleogene sediments piston-cored on Maud Rise, Weddell Sea (ODP Sites 689 and 690), are a cornerstone of Southern Ocean Paleogene and Neogene chronostratigraphy. However, parts of previous magnetostratigraphic interpretations have been called into question, and recent reinvestigation of the upper Paleocene–middle Eocene portion of Site 690 suggested that the records might be contaminated by spurious magnetizations, which raises doubts about the reliability of these important records. We undertook a high-resolution magnetostratigraphic study of Eocene-Oligocene u-channel samples from ODP Holes 689B, 689D, 690B, and 690C in order to address these concerns. A pervasive overprint appears to be present below the middle Eocene, which compromises magnetobiostratigraphic interpretations for the upper Cretaceous and lower Paleogene. Nevertheless, our new results provide a robust record of geomagnetic field behavior from 38.5 to 25 Ma and confirm the reliability of these sediments for calibration of biostratigraphic datum events during a crucial phase of earth history when major Antarctic ice sheets developed. Also, comparison of magnetozone thicknesses in multiple holes at the same site indicates that ~1.2–1.8 m of the stratigraphic record is missing at each core break, which corresponds to time breaks of 120–360 k.y. Lack of a continuous record within a single hole renders useless spectral analyses for investigating long geomagnetic and paleoclimatic time series. This observation reinforces the need for coring of multiple offset holes to obtain continuous paleoceanographic records. Sedimentary hiatuses have been identified only at the deeper of the two investigated sites (Site 690), which could mark a local response to the onset of the Antarctic Circumpolar Current. Published 46-66 2.2. Laboratorio di paleomagnetismo JCR Journal
The lithological-facies zoning of the Neo- and Eopleistocene sediments of the Caribbean Sea has been described for the first time based on deep-sea drilling data. Сorresponding maps and isopach schemes were processed using A.B. Ronov’s volumetric method to calculate the quantitative parameters of sedimentation for the distinguished types of Pleistocene sediments. It was revealed that the role of carbonate sediments increased from the east westward. The accumulation of lithogenic and carbonate planktic sediments was more intense in the Neopleistocene than in the Eopleistocene, which was related to the neotectonic orogeny on the Lesser Antilles.
High-resolution mass accumulation rates (MAR) were determined from lithologic logs based on downhole log and continuous core data for six sites at four continental margins around the Southern Ocean. Total MAR was calculated at Ocean Drilling Program (ODP) Site 1095 on the margin of the Antarctic Peninsula and ODP Site 1165 on the margin of Prydz Bay, Antarctica. Carbonate and noncarbonate (terrigenous) MAR were calculated for ODP Sites 1123 and 1124 east of New Zealand and Sites 1168 and 1172 west and southeast of Tasmania. Shifts in carbonate MAR are seen around Tasmania and New Zealand at 23 and 14 Ma, suggesting changes in deep water circulation and surface carbonate productivity at these times. Carbonate MAR dropped at Sites 1124 and 1168 at 23 Ma and increased at 14 Ma at Sites 1123, 1168, and 1172. The overall character of total MAR at Antarctic Sites 1095 and 1165 is a continual and relatively constant decrease in total MAR during the Neogene, with periods of stepwise decrease. One such decrease is seen at 14 Ma at Site 1165, coincident with increased carbonate MAR in New Zealand and Tasmania (Sites 1123, 1124, 1168 and 1172) and likely related to mid-Miocene expansion of the East Antarctic Ice Sheet. Another step drop in total MAR is seen at ∼9 Ma at Site 1165, possibly reflecting diversion of sediment to a newly formed Prydz Channel fan.
In this paper we explore the relative control of paleoceanography, eustasy, and water temperature over the evolution of a carbonate slope system deposited on the Marion Plateau (Northeastern Australia). Growth of several carbonate platforms started in the early Miocene on this plateau, and although they occurred in low-latitude subtropical waters they are composed mainly of heterozoan organisms. We investigated an upper to distal slope transect drilled during ODP Leg 194 and located close to the Northern Marion Platform. We reconstructed mass accumulation rates of carbonate as well as the evolution in the ratios of carbon and oxygen stable isotopes. Power spectrum analysis of the carbon isotope record revealed the existence of cycles with main frequencies centered round 409 Kyr and 1800 Kyr. We interpret the 409 Kyr cycle as being paced by changes in the eccentricity of the Earth orbit, and we suggest that the 1800 Kyr cycle could be linked to long-term eustatic changes. Finally, on the basis of the timing of changes in mass accumulation rates of carbonate we infer that the strength and direction of oceanic currents affected sedimentation on the Marion Plateau by shifting depocenters of slope sedimentation, a process probably further modulated by sea-level changes. We argue that the evolution and demise of the heterozoan carbonate systems present on the Marion Plateau were controlled mainly by the evolution of strong benthic currents, and that eustasy and water temperature alone did not account for the drowning of the platforms in the mid Miocene.
Paleoceanographic variability at southern high latitude Ocean Drilling Program (ODP) Site 747 was investigated in this study through the interval which spans the Middle Miocene Climate Transition (MMCT). Between 15.0 and 12.2 million years ago (Ma), foraminiferal δ18O records derived from both benthic (Cibicidoides spp.) and planktonic taxa (Globorotalia praescitula and Globigerina bulloides) reveal a history of changes in water column thermal and salinity structure and a strong imprint of seasonality. Prior to the MMCT, in the interval between 14.35 and 13.9 Ma, G. bulloides displays relatively high δ18O values similar to those of G. praescitula, interpreted to indicate weakening of the thermocline and/or increased seasonality with cooler early-spring and/or late-fall temperatures. Following this interval, G. bulloidesδ18O values diverge significantly from benthic and G. praescitula values, with G. bulloides values remaining relatively low for at least 600 kyr following the benthic foraminiferal δ18O shift during the MMCT at ~ 13.9 Ma. This divergence in δ18O records occurs in direct association with the Mi3 cooling and glaciation event and may suggest: (1) a strengthening of the vertical temperature gradient, with greater cooling of deep waters than surface waters, (2) changes in the depth habitat of G. bulloides, (3) changes in the dominant season of G. bulloides calcification, (4) modification of surface-water δ18O values in association with enhanced sea-ice formation, (5) increased surface-water carbonate ion concentration, and/or (6) a significant decrease in surface-water salinity across the MMCT. The first of these possible scenarios is not likely, particularly in light of recent Mg/Ca evidence for significant surface-water cooling in the Southern Ocean associated with the MMCT. Of the remaining possibilities, we favor a change in surface salinity to explain the observed trends in δ18O values and hypothesize that surface salinity may have decreased by up to 2 salinity units at ~ 13.9 Ma. In this scenario, the development of a lower-salinity Antarctic surface layer coincided with regional cooling of both surface and deep waters of the Southern Ocean during the Mi3 glaciation of East Antarctica, and contributed into the dominance of Neogloboquadrina spp. between 13.8 and 13.2 Ma. Additionally, the distinct patterns observed in planktonic foraminiferal δ18O records spanning the MMCT correspond with changes in the vertical δ13C gradient between planktonic and benthic foraminiferal records and major changes in planktonic foraminiferal assemblages at Site 747, providing further evidence of the environmental significance of this climatic transition.
Seismic reflection data show the existence of two major sedimentary basins along the continental margin of Wilkes Land and Terre Adélie, East Antarctica, that contain more than 5 s TWT (> 9 km) of sediments. Four seismic megasequences are identified (MS4 to MS1) that are bounded by: basement, unconformities of interpreted Turonian, Maastrichtian and early Middle Eocene age, and the seafloor. The 4–5 km thick rift and pre-rift sediments are concentrated in a margin-parallel basin (Sabrina Basin). On the basis of seismic correlation with the Australian margin, this basin is interpreted to be of Late Jurassic to mid-Cretaceous age. The post-rift sediments are generally thick along the margin and in the adjacent deep-ocean basin, but are particularly thick in a major depocentre off west Wilkes Land, named here the Budd Coast Basin (BCB). The BCB contains a maximum observed thickness of 5 s TWT (∼9 km) of post-rift sediments and its location suggests that the sediments were largely derived from a sub-glacial basin currently occupied by the Totten Glacier.
The sudden, widespread glaciation of Antarctica and the associated shift toward colder temperatures near the Eocene–Oligocene boundary (∼34 Ma) represents one of the most fundamental reorganizations of the global climate system recognized in the geologic record. This glacial inception and the subsequent evolution of the early East Antarctic Ice Sheet (EAIS) are simulated using a new, coupled global climate–dynamical ice sheet model accounting for the paleogeography, greenhouse gas concentrations, changing orbital parameters, and varying ocean heat transport. Suites of long (105 yr) climate–ice sheet simulations are used to investigate the effects of declining atmospheric CO2, compared to those of the tectonic opening of Southern Ocean gateways and the timing of mountain uplift in the Antarctic interior. In contrast to the established paradigm for the glaciation of Antarctica, which centers on the opening of the Southern Ocean gateways and the ‘thermal isolation’ of the continent, our results show that declining Cenozoic pCO2 may have played the dominant role. First, small isolated ice caps formed on the highest Antarctic plateaus. Then, as a CO2 threshold between ∼3× and 2× pre-industrial level (PAL) was crossed, height–mass balance feedbacks were initiated during orbital periods with cold austral summers, triggering much larger, highly dynamic terrestrial ice sheets. As CO2 continued to decline, these isolated ice caps eventually merged into a permanent continental-scale EAIS. In our model, neither the opening of the Southern Ocean gateways nor mountain uplift significantly affected the timing of the major ice sheet transition, given a scenario of gradually declining CO2 from 4× to 2× PAL over 10 million years around the Eocene–Oligocene boundary.
Two recently drilled Caribbean sites contain expanded sedimentary records of the late Paleocene thermal maximum, a dramatic global warming event that occurred at ca. 55 Ma. The records document significant environmental changes, including deep-water oxygen deficiency and a mass extinction of deep-sea fauna, intertwined with evidence for a major episode of explosive volcanism. We postulate that this volcanism initiated a reordering of ocean circulation that resulted in rapid global warming and dramatic changes in the Earth's environment.
Early Miocene to Quaternary benthic foraminifers have been quantitatively studied (>63 mu m size fraction) in a southwest Pacific traverse of DSDP sites at depths from about 1300 to 3200 m down the Lord Howe Rise. Dominant species include Epistominella exigua, E. rotunda and Globocassidulina subglobosa, which prevail in the three sites, and Oridorsalis umbonatus, E. umbonifera, and Cassidulina carinata, which occur usually in frequencies of between 10 and 30%. Faunal changes in Neogene benthic foraminiferal assemblages are not similar in each of the three sites, but faunal successions are most similar between the two shallowest sites. The deepest site differs in composition and distribution of dominant species. There are three intervals during which the most important changes occur in benthic foraminiferal assemblages: the early middle Miocene (14 Ma); the late Miocene (6 Ma); and near the Pliocene/Pleistocene boundary at about 2 Ma. These major faunal changes are all associated with known major paleoceanographic events - the middle Miocene development of the Antarctic ice sheet; the latest Miocene global cooling and increased polar glaciation; and the onset of quasiperiodic glaciation of the Northern Hemisphere. These major paleoceanographic events undoubtedly had a profound effect on the intermediate and deep water mass structure of the Tasman Sea as recorded by changes in benthic foraminiferal assemblages. -from Authors
This is a summary of principal findings made by ODP Leg 113 investigators concerning the latest Cretaceous-Cenozoic climatic, cryospheric, and oceanographic history, and biogeographic developments of the Weddell Sea region, Antarctica. During Leg 113, 22 holes were drilled at 9 sites that sampled 4 contrasting environments: open-ocean pelagic sedimentation on Maud Rise (Sites 689 and 690), hemipelagic and terrigenous sediments on the East Antarctic Continental Margin (Sites 691-693), a turbiditic sequence in the deep Weddell Basin (Site 694), and hemipelagic and biogenic sediments on the South Orkney microcontinent (Sites 695-697). A wide range of sedimentologic, biotic, and isotopic evidence obtained in Leg 113 material indicates that sequential cooling and cryospheric development of Antarctica and the surrounding oceans during the Cenozoic profoundly affected the ocean/atmosphere circulation, sediments, and biota. Important cooling steps occurred during the latest Cretaceous, the Middle Eocene, near the Eocene/Oligocene boundary, in the Middle Oligocene, the Middle Miocene, the early late Miocene, the latest Miocene, and the late Pliocene. Distinct but temporary warming trends occurred during the late Paleocene and the latest Oligocene to early Miocene. -from Authors
The Antarctic continent has become more isolated through Cenozoic times, following the progressive fragmentation of the Gondwana supercontinent. This has led to cooling, the development of temporary ice sheets and ultimately to the present permanent ice cover, amounting today to 66 m of sea level equivalent. Glaciological modelling indicates the stages through which Antarctic ice sheets have formed, initially with ice caps on high mountains followed by coalescence of ice caps to cover much of East Antarctica, and culminating with a complete cover of East Antarctica and the grounding of ice over the West Antarctic archipelago. Short term (104-5 year) fluctuations in Antarctic ice volume have most likely been superimposed on this cooling history. The history of the continent and its ice cover is recorded in the ice itself (for the last 0.5 Ma), deposits (and landforms) both on land and beneath the surrounding seas, and from proxy records of isotopes and eustatic sea level changes in the oceans. From work to date, the history can be divided into 4 main periods (time scale of Berggren et al., 1985): - 65-36 Ma, preglacial times for the Antarctic region, - 36-15 Ma, the first phase of continental glaciation, with frequent ice sheets growing and decaying, in a few places reaching the margin of the continent, - 15-2 Ma, when an ice sheet persisted on the Antarctic continent, though its extent and variability is much in debate, - 2-0 Ma, an Antarctic glacial regime like the present. An ice sheet covered the whole continent for this period, but included sectors vulnerable to periodic collapse. Recent advances in Cenozoic global correlation, seismic stratigraphy of the Antarctic margin, glaciology and glacial sedimentology, ice sheet modelling, and in more accurate isotopic dating systems for marine and terrestrial deposits and surfaces on the continent, now make possible a much more detailed history of style and extent of Antarctic glaciation through Cenozoic times. This will require strategically planned and well located coring and outcrop studies for direct evidence of the glacial history of each sector of the continent in order to assemble a credible view of continent-wide ice sheet behaviour. Such studies, covering time scales ranging from thousands to millions of years, will be useful in providing real world tests for climate models being developed to foresee the consequences of future increases in CO2 levels. This work is especially timely in view of the current trend in global warming, and retreating ice shelves associated with regional warming in the Antarctic Peninsula.
Benthic oxygen and carbon isotopic results from a depth transect on Maud Rise, Antarctica, provide the first evidence for Warm Saline Deep Water (WSDW) in the Paleogene oceans. Isotopic reversals, well developed by at least 46Ma (middle middle Eocene), existed for much of the remaining Paleogene. We do not consider these reversals to be artifacts of differential diagenesis between the two sites or to have resulted from other potentially complicating factors. A progressive decrease and eventual reversal in benthic to planktonic δ18O gradients in Hole 690B, demonstrate that the deeper waters became warmer relative to Antarctic surface waters during the Eocene. The warmer deep waters of the Paleogene are inferred to have been produced at middle to low latitudes, probably in the Tethyan region which contained extensive shallow-water platforms, ideal sites for the formation of high salinity water through evaporative processes. The ocean during the Eocene, and perhaps the Paleocene, is inferred to have been two-layered, consisting of warm, saline deep waters formed at low latitudes and overlain by cooler waters formed at high latitudes. This thermospheric ocean, dominated by halothermal circulation we name Proteus. The Neogene and modern psychrospheric ocean Oceanus is dominated by thermohaline circulation of deep waters largely formed at high latitudes. An intermediate condition existed during the Oligocene, with a three-layered ocean that consisted of cold, dense deep waters formed in the Antarctic (Proto-AABW), overlain by warm, saline deep waters from low latitudes, and in turn overlain by cool waters formed in the polar regions. This we name Proto-oceanus which combined both halothermal and thermohaline processes. -from Authors
Cultured planktonic foraminifera, Orbulina universa (symbiotic) and Globigerina bulloides (nonsymbiotic), are used to reexamine temperature:δ18O relationships at 15°–25°C. Relationships for both species can be described by linear equations. Equations for O. universa grown under low light (LL) and high light (HL) share a slope of −4.80 (0.21‰ °C−1) with a HL-LL offset of −0.33‰ due to symbiont photosynthetic activity. The effect of [CO32−] on O. universa is −0.002‰ µmol−1 kg−1 and is insensitive to temperature. For G. bulloides, ontogenetic effects produce size-related trends in temperature:δ18O, whereby larger shells are enriched in 18O relative to smaller specimens. The O. universa temperature:δ18O relationships are more accurate than previously published equations for describing plankton tow data. Our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic Cibicidoides data below 10°C. Temperature:δ18O relationships for G. bulloides provide good agreement with field data for this species from the northeast Pacific.
A remarkable oxygen and carbon isotope excursion occurred in Antarctic
waters near the end of the Palaeocene (~57.33 Myr ago), indicating rapid
global warming and oceanographic changes that caused one of the largest
deep-sea benthic extinctions of the past 90 million years. In contrast,
the oceanic plankton were largely unaffected, implying a decoupling of
the deep and shallow ecosystems. The data suggest that for a few
thousand years, ocean circulation underwent fundamental changes
producing a transient state that, although brief, had long-term effects
on environmental and biotic evolution.
This paper describes the orogenic evolution of the southeast Anatolian orogenic belt based mostly on new geologic data collected from its constituent tectonic units in the course of new mapping programs of the past decade. The southeast Anatolian orogenic segment of the Alpides may be divided into three approximately east-west-trending zones. From south to north, they are the Arabian Platform, followed by a zone of imbrication, and then a zone of nappes. Southeastern Anatolia underwent two major episodes of Alpide deformation. The first occurred during the Late Cretaceous period, when ophiolite was emplaced on the Arabian Platform. The second episode of deformation formed the present orogenic segment leading to the amalgamation of different tectonic units. -from Author
We document nine lower-middle Eocene sequences on the New Jersey coastal
plain and compare them with global δ18O and Haq et al.
records. Early Eocene hiatuses do not match δ18O
changes, and it is unlikely that they are the result of glacioeustasy,
consistent with an ice-free early Eocene. Early-middle Eocene (49 43 Ma)
evidence for a link between sequences and δ18O is
equivocal, and the presence of large ice sheets is uncertain. Beginning
in the late-middle Eocene (43 42 Ma), concomitant increases in
planktonic and benthic δ18O records coincide with the
timing of hiatuses on the New Jersey coastal plain and a change from
carbonate-dominated to siliciclastic-dominated sedimentation. These
represent the development of the Antarctic ice cap and the beginning of
the “icehouse” world. Of the 14 sequences predicted by Haq
et al. for this interval, 9 are resolvable on the New Jersey margin, and
the other 5 appear to be combined with others. We conclude that although
ice-volume changes controlled sequences since at least 42 Ma, mechanisms
for sea-level change prior to then are still not fully understood.
Oligocene to middle Miocene sequence boundaries on the New Jersey coastal plain (Ocean Drilling Project Leg 150X) and continental slope (Ocean Drilling Project Leg 150) were dated by integrating strontium isotopic stratigraphy, magnetostratigraphy, and biostratigraphy (planktonic foraminifera, nannofossils, dinocysts, and diatoms). The ages of coastal plain unconformities and slope seismic reflectors (unconformities or stratal breaks with no discernible hiatuses) match the ages of global δ18O increases (inferred glacioeustatic lowerings) measured in deep-sea sites. These correlations confirm a causal link between coastal plain and slope sequence boundaries: both formed during global sea-level lowerings. The ages of New Jersey sequence boundaries and global δ18O increases also correlate well with the Exxon Production Research sea-level records of Haq et al. and Vail et al., validating and refining their compilations.
Ocean Drilling Program (ODP) Site 677 provided excellent material for high resolution stable isotope analysis of both benthonic and planktonic foraminifera through the entire Pleistocene and upper Pliocene. The oxygen isotope record is readily correlated with the SPECMAP stack (Imbrie et al. 1984) and with the record from DSDP 607 (Ruddiman et al. 1986) but a significantly better match with orbital models is obtained by departing from the timescale proposed by these authors below Stage 16 (620 000 years). It is the stronger contribution from the precession signal in the record from ODP Site 677 that provides the basis for the revised timescale. Our proposed modification to the timescale would imply that the currently adopted radiometric dates for the Matuyama–Brunhes boundary, the Jaramillo and Olduvai Subchrons and the Gauss–Matuyama boundary underestimate their true astronomical ages by between 5 and 7%.
The climate transition from the warm mid-Pliocene (around 3.2 Ma) to the onset of northern hemisphere ice ages around 2.4 Ma is examined. Evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe. While the cause of individual glacial-interglacial oscillations is tied to Milankovitch variations in the Earth's orbit around the sun, these insolation changes cannot account for the longterm cooling trend which culminated in Northern Hemisphere glaciation. In the final section of this paper, mechanisms of long-term climate change are examined with special emphasis on those that have been proposed to explain the late Neogene cooling of the northern hemisphere. -from Author
Sedimentological and stable isotope data from a pelagic sequence recovered from the southern Indian Ocean provide the most convincing evidence to date for short-term expansion of a large ice sheet on Antarctica during the earliest Oligocene (˜36 Ma). Terrigenous debris identified as ice-rafted in origin on the basis of textural, compositional, and size criteria is present at the same stratigraphic level as the ubiquitous early Oligocene oxygen isotope shift. The highest benthic foraminiferal delta18O values (>3.00/00) of the Paleogene occur in samples from within the ice-rafted debris interval. These values are similar to those recorded by Holocene benthic foraminifera, implying that the ice sheet may have attained a volume similar to that of the present-day ice sheet on Antarctica. The stratigraphic distribution of ice-rafted debris and high oxygen isotope values indicate that these conditions persisted for roughly 100 ka.
The New Jersey Sea Level Transect was designed to evaluate the relationships among global sea level (eustatic) change, unconformity-bounded sequences, and variations in subsidence, sediment supply, and climate on a passive continental margin. By sampling and dating Cenozoic strata from coastal plain and continental slope locations, we show that sequence boundaries correlate (within +/-0.5 myr) regionally (onshore-offshore) and interregionally (New Jersey-Alabama-Bahamas), implicating a global cause. Sequence boundaries correlate with delta18O increases for at least the past 42 myr, consistent with an ice volume (glacioeustatic) control, although a causal relationship is not required because of uncertainties in ages and correlations. Evidence for a causal connection is provided by preliminary Miocene data from slope Site 904 that directly link delta18O increases with sequence boundaries. We conclude that variation in the size of ice sheets has been a primary control on the formation of sequence boundaries since ~42 Ma. We speculate that prior to this, the growth and decay of small ice sheets caused small-amplitude sea level changes (
Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind‐driven and density‐driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000‐ and 41,000‐year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end‐member states needed to describe the system's evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems.
“Solutions to problems involving systems of such complexity are not born full grown like Athena from the head of Zeus. Rather they evolve slowly, in stages, each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step.”
—Victor P. Starr
In a steady-state ocean, input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes. For the elements strontium, calcium, and carbon, rivers provide the primary input, whereas marine biogenic sedimentation dominates removal. Dissolved fluxes in rivers are related to rates of continental weathering, which in turn are strongly dependent on rates of uplift. The largest dissolved fluxes today arise in the Himalayan and Andean mountain ranges and the Tibetan Plateau. During the past 5 m.y., uplift rates in these areas have increased significantly; this suggests that weathering rates and river fluxes may have increased also. The oceanic records of carbonate sedimentation, level of the calcite compensation depth, and delta¹³C and deltaâ¸â·Sr in biogenic sediments are consistent with a global increase in river fluxes since the late Miocene. The cooling of global climate over the past few million years may be linked to a decrease in atmospheric COâ driven by enhanced continental weathering in these tectonically active regions.
Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000‐year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000‐year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000‐year cycle requires a source of climatic inertia having a time constant (∼15,000 years) much larger than the other cycles (∼5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity‐driven modulations in the 23,000‐year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher‐frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000‐year oscillations.
Whether a glacier or ice sheet influences the climate depends very much on the scale…. The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence.
Johannes Oerlemans [1991, p. 155]
Evaluates compiled d13C data from benthic foraminifera in order to define deepwater production for Miocene. Defines two modes of bottom water production: (1) Southern Component Water during earliest Miocene (24-20 Ma) and early middle Miocene (15-12.5 Ma) and (2) Northern Component Water, Tethyan water, and SCW (20-16 Ma) and NCW, SWC (12.5-10 Ma). Re-evaluates silica switch hypothesis and concludes that aging water in Pacific supports deepwater production model. Argues that data used for origninal hypothesis contained siliceous forms susceptable to dissolution. Diatoms outside upwelling zones represent a better control. Defines a method from tight stratigraphic control.
Isotopic records across the "Latest Paleocene Thermal Maximum' (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<104 years) of the latest Paleocene (~55.6 Ma). There also was a coeval -2 to -3‰ excursion in the δ13C of the ocean/atmosphere inorganic carbon reservoir. A rapid δ13C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate water depths. In terms of the present-day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 1018 g of carbon with a δ13C of approximately -60‰. -from Authors
We report here that DSDP Site 552A, cored with the hydraulic piston corer on the west flank of Rockall Bank, recovered an undisturbed sequence of alternating white deep-sea carbonate oozes and dark-coloured layers that are rich in glacial debris. Oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice-rafting occurred at about 2.4 Myr, and was preceded by a minor pulse of ice-rafting at about 2.5 Myr. The carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present-day North Atlantic deep water at least since 3.5 Myr.
The oxygen and carbon isotopic record of G. sacculifer in DSDP Holes 502 and 502B shows that the Quaternary interval is almost complete. Two modes of delta18O variation reflect the changing ice budget on the continents and imply different envelopes of sea level variation during the early and late Quaternary. The division is at about 900 ka BP. -after Author
A global synthesis of Miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that Miocene thermohaline circulation evolved through three regimes corresponding approximately to early, middle, and late Miocene times. There is evidence for major qualitative differences between the circulation of the modern ocean and the Miocene ocean prior to 11 Ma. The distribution pattern of siliceous oozes in Miocene sediments is consistent with our proposed reconstruction of thermohaline circulation. Major changes which occurred in circulation during the middle Miocene were probably related to the closing of the Tethys and may have contributed to rapid middle Miocene growth of the Antarctic ice cap. -from Authors
At Deep Sea Drilling Project sites 289 and 574, the mid-Miocene carbon isotope records are characterized by an interval of high δ13C values between 17 and 13.5 Ma upon which are superimposed a series of periodic or quasi-periodic fluctuations in δ13C values. These fluctuations have a period of approximately 440 kyr, suggestive of the 413 kyr cycle predicted by Milankovitch theory. We have assigned an absolute chronology, based on biostratigraphic and magneto-biostratigraphic datum levels, to the isotope stratigraphy and have used that chronology to correlate unconformities, seismic reflectors, carbonate minima, and dissolution intervals. The chronology indicates that the major mid-Miocene deepwater cooling/ice volume expansion took 2 m.y. and was not abrupt as had been reported previously. The cooling was not monotonic, and the 2 m.y. interval included an episode of especially rapid cooling as well as a brief return to warmer conditions before the final phase of the cooling period. -from Authors
Core Vema 28-238 preserves an excellent oxygen isotope and magnetic stratigraphy and is shown to contain undisturbed sediments deposited continuously through the past 870,000 yr. Detailed correlation with sequences described by Emiliani in the Caribbean and Atlantic Ocean is demonstrated. The boundaries of 22 stages representing alternating times of high and low Northern Hemisphere ice volume are recognized and dated. The record is interpreted in terms of Northern Hemisphere ice accumulation, and is used to estimate the range of temperature variation in the Caribbean.
Appreciable lower Oligocene clastic detritus interpreted to be ice-rafted debris (IRD) was recovered at Ocean Drilling Program (ODP) Site 748. The physiologic and tectonic setting of the site and the coarse size of the material rule out transport of the clastics by turbidity currents, nepheloid layers, or wind. The co-occurrence of the IRD, biosiliceous-rich nannofossil ooze, fish skeletal debris, and glauconite probably denotes changes in patterns and/or intensity of oceanic circulation associated with a profound climatic cooling. These changes apparently enhanced upwelling and surface-water productivity and the development of a trophic structure that supported abundant vertebrate marine life, perhaps an important step in the evolution of vertebrate marine faunas. -from Authors
High-resolution (ca. 5 k.y.) δ 18 O and δ 13 C data from Ocean Drilling Program Hole 929A near the Oligocene/Miocene boundary document Milankovitch-scale climate variability from ca. 24.8 to 23.1 Ma. Stable isotopic data based on Cibicidoides mundulus better deÞne the timing and magnitude of a δ 18 O maximum at ca. 23.9 Ma (event Mi1), and its association with the δ 13 C maximum near the Oligocene/Miocene boundary. Mi1 is marked by a gradual increase of 1.2ä over about 250 k.y. from 23.9 to 23.6 Ma to a maximum of 2.15ä, followed by two rapid (63 μ m/g), with a domi- nant period of about 41 k.y., especially during the 24.0 - 24.8 Ma interval. Concentration of variance at this period strongly sug- gests a high-latitude control, probably East Antarctic ice sheet variability.
A new model has been constructed for calculating the level of atmospheric CO{sub 2} over Phanerozoic time which is much simpler mathematically than the BLAG model, but more complex geologically and biologically. Results suggest that there has been a notable pattern of varying atmospheric CO{sub 2} level over the past 570 my with high levels during the Mesozoic and early Paleozoic and low levels during the Permo-Carboniferous and late Cenozoic. Sensitivity analysis shows that, within reasonable limits of the principal governing parameters, this qualitative trend of varying CO{sub 2} is relatively insensitive to the actual values chosen for these parameters. Causes of the CO{sub 2} variation are multiple, and no single geological or biological process can be called upon to explain all CO{sub 2} variation for all time. The calculated trend of CO{sub 2} over time agrees well with independent estimations of paleoclimates. Thus, the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study.
Geologic evidence indicates that net vertical uplift occurred on a large (kilometer) scale and at accelerating rates during the middle and late Cenozoic in plateaus of southern Asia and the American west. Based on this evidence, General Circulation Model sensitivity tests were run to isolate the unique effects of plateau uplift on climate. The basic direction of most of these simulated responses to progressive uplift is borne out by changes found in the geologic record. The modeled changes result from increased orographic diversion of westerly winds, from cyclonic and anticyclonic surface flow induced by summer heating and winter cooling of the uplifted plateaus, and from the intensification of vertical circulation cells in the atmosphere caused by exchanges of mass between the summer-heated (and winter-cooled) plateaus and the mid-latitude oceans. -from Authors
High-resolution stable isotopic records are presented for the epi-benthic foraminifer Cibicidoides, the inferred shallow-dwelling planktonic Globigerinoides quadrilobatus, and the inferred deep-dwelling planktonic Globoquadrina dehiscens from the middle Miocene (~16-12 Ma) of Deep Sea Drilling Project site 588A, Lord Howe Rise, southwest Pacific. High-resolution, multiple species oxygen and carbon isotopic data define the timing and character of the well-known middle Miocene climatic-oceanographic transition with a resolution comparable to Quaternary records. The benthic foraminiferal delta18O record is marked by several large fluctuations from ~16 to 14.8 Ma, followed by a series of rapid (
We observe strong negative correlation between Tertiary low- to mid-latitude planktonic foarminiferal delta18O and the difference between the data and coeval benthic foraminiferal delta18. Late Quaternary data do not show this correlation. Coupling statisical model/delta18O comparisons and evidence for Antarctic ice and ocean temperature variation, we infer that Tertiary ice volume, recorded by tropical planktonic delta18O, increased as the deep ocean warmed. Because the isotopic signatures of deepwater temperature variation and ice volume change were of opposite sign, the sum of these signals in Tertiary benthic delta18O became lost in the noise. This renders low correlation between Tertiary planktonic and benthic delta18O time series compared to late Quaternary data. We contend that Tertiary ice sheet growth was commonly driven by warming of deep water from low-to mid-latitude marginal seas (snow gun hypothesis). In contrast, late Quaternary ice sheets grew as deep water, formed at high latitude, cooled. Because tectonic forcing and orbital forcing at low-latitude primarily controlled production and temperature variations of this Warm Saline Deep Water, these influences largely dictated Tertiary ice volume fluctuations. Through the Tertiary, we infer ice volume fluctuations to be an important component of sea level history on timescales between 103 and 107 years.
Oxygen and carbon isotope ratio measurements are presented for Globigerinoides ruber and for benthic species (mainly Uvigerina spp.) in the Pleistocene and uppermost Pliocene section. This provides the best available continuous Pleistocene stable isotope records from any location. Oxygen isotope stage 22 (age about 0.85 Ma) was of similar magnitude to the most extensive glacials of the Brunhes and constitutes a logical base for the middle Pleistocene. Oxygen isotope stages as defined by Ruddiman et al (1986) and by Raymo et al (in press) back to stage 104 are recognized. Although the internationally agreed base of the Quaternary at or near stage 62 (about 1.6 Ma) is not marked by a major isotopic event, it does approximate the base of a regime characterized by highly regular 41 000-yr climate cycles. The records will permit a new attempt to develop a chronology for the early Pleistocene based on tuning to the orbital frequencies. The carbon isotope records also appear to contain considerable variance at orbital frequencies throughout the sequence analyzed. -from Authors
Ocean Drilling Program (ODP) Site 677 provided excellent material for high resolution stable isotope analysis of both benthonic and planktonic foraminifera through the entire Pleistocene and upper Pliocene. The oxygen isotope record is readily correlated with the SPECMAP stack and with the record from DSDP 607 but a significantly better match with orbital models is obtained by departing from the timescale proposed by these authors below Stage 16 (620 000 years). Our proposed modification to the timescale would imply that the currently adopted radiometric dates for the Matuyama-Brunhes boundary, the Jaramillo and Olduvai Subchrons and the Gauss-Matuyama boundary underestimate their true astronomical ages by between 5 and 7%. -from Authors
Two recently drilled Caribbean sites contain expanded sedimentary records of the late Paleocene thermal maximum, a dramatic global warming event that occurred at ca. 55 Ma. The records document significant environmental changes, including deep-water oxygen deficiency and a mass extinction of deep-sea fauna, intertwined with evidence for a major episode of explosive volcanism. We postulate that this volcanism initiated a reordering of ocean circulation that resulted in rapid global warming and dramatic changes in the Earth's environment.
High-resolution records of δ18O, δ13O, and percent CaCO3 from the late Pliocene North Atlantic (Deep Sea Drilling Project sites 607 and 609) are presented and oxygen isotope stages are formalized back to stage 116 at 2.73 Ma. From 2.8 to 1.6 Ma, the interval studied, variations in these records were dominated by the 41-kyr component of orbital obliquity. Significant variation at the orbital frequencies of eccentricity (96-kyr) and precession (23-kyr) are observed in the δ18O record between 1.6 and 2.1 Ma, but not before. Prior to 2.4 Ma (stage 100), δ18O variations suggest ice sheet growth 1/4 to 1/2 as large as late Pleistocene ice volumes; however, these events are below the threshold needed to result in extensive ice-rafting to the open North Atlantic Ocean. After 2.4 Ma, ice sheets appear to be, on average, 1/2 as large as those of the late Pleistocene. The δ18O record indicates that some glacial suppression of North Atlantic Deep Water occurred both before and after 2.4 Ma and that glacial-interglacial transfers of 12C between the continents and oceans appear to have been larger in the late Pliocene relative to the late Pleistocene. In addition, the strong 23-kyr power observed in δ18O between 2.75 and 2.10 Ma suggests that deep-sea circulation (or changes in biomass) is controlled, in part, by climatic variations unrelated to ice sheets.
We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. The new time scale has several significant differences from previous time scales. For example, chron C5n is ~0.5 m.y. older and chrons C9 through C24 are 2-3 m.y. younger than in the chronologies of Berggren et al. (1985b) and Harland et al. (1990). Additional small-scale anomalies (tiny wiggles) that represent either very short polarity intervals or intensity fluctuations of the dipole field have been identified from several intervals in the Cenozoic. Spreading rates on several ridges were analyzed in order to evaluate the accuracy of the new time scale. Globally synchronous variations in spreading rate that were previously observed around anomalies 20, 6C, and in the late Neogene have been eliminated. The new time scale helps to resolve events at the times of major plate reorganizations. -from Authors
Oxygen isotope analyses of foraminiferal tests in deep-sea cores have been interpreted as showing that the temperature of seawater in the Caribbean and equatorial Atlantic varied by as much as 6° C during glacial cycles. This evidence has now been reinterpreted, and changes in oxygen isotope composition are now said to correspond with the extraction of large amounts of water from the oceans during glacial periods and the recirculation of this water during periods when glaciers were at their present levels.
We have tested the sensitivity of an ocean general circulation model to
changes in solid boundary conditions by opening the central American
isthmus in order to simulate possible circulation patterns existing
prior to about 3.0-4.0 Ma. The model was driven by present winds,
atmospheric temperatures, and moisture fluxes, with the only difference
between control and perturbed runs being the open isthmus. Significant
changes result for the perturbed case, almost all of which can be traced
to changes in surface salinity distribution. With an open isthmus,
lower-salinity waters from the Pacific dilute North Atlantic surface
waters by >1.0‰. The lower surface salinities cause the
thermohaline cell in the North Atlantic to collapse, with North Atlantic
Deep Water (NADW) production reduced to near zero. This response greatly
weakens poleward ocean heat transport in the North Atlantic. The
"no-NADW" result appears to be insensitive to initial conditions and
represents a circulation mode different than a "low-NADW" mode found in
the same ocean model using present geographic forcing. There is some
agreement between model predictions and geologic data, which indicate a
significantly different deep-water circulation pattern prior to
˜2.4 Ma. However, observations suggests that the time of major
transition in circulation regimes occurs earlier than the final closure
of the isthmus. This discrepancy may reflect the fact that near-closure
of the isthmus by 7-10 Ma could have been sufficient to alter the
circulation. As ocean heat transport is a key source of warmth to the
high latitudes of the North Atlantic, some compensating factor
(CO2?) may have been required to restore warmth to that
region prior to ˜3.0-4.0 Ma.
Develop d18O stratigraphic scheme: base of 8 zones correlates with 18O maximum: (Oi1-Oi2 and Mi1-Mi6: Oligocene/Miocene oxygen isotope zones) from benthic foraminifera. Note: Wright and Miller (1990) identify 3 zones; Mi1a, Mi1b, Mi7. Interpret benthic-planktonic covariance to reflect ice volume increases at the base of Zones Mi2 (16.1), Mi3 (13.6) and possibly Mi5 (11.3). Large continental ice sheets probably existed in Antarctica from beginning of Oligocene. They go on to evaluate temp/salinity relationship using modern temp-18O-salinity relationships. Conclude that an ice-house world existed. They recognize 7 periods of glacial development in Early to Mid Miocene (zones Mi1-Mi5, Mi1a and Mi1b) correlating to extensive glaciomarine sedimentation. Factoids: Burial depth at western equatorial Pacific site 289 exceeds 500 m and oxygen signal may be altered. First order 18O/temp relationship is ≈0.23/1˚C. A 3 psu increase in deepwater salinity results in a 9-12 psu decrease in surface waters (due to reservoir change).
A new model has been constructed for calculating the level of atmospheric CO2 over Phanerozoic time which is much simpler mathematically than the BLAG model, but more complex geologically and biologically. Mathematical simplification comes about by following the cycle of carbon only, lumping all carbonate minerals together, combining the ocean and atmosphere into one reservoir, and calculating atmospheric CO2 level as a series of successive ocean-atmosphere steady states. Results suggest that there has been a notable pattern of varying atmospheric CO2 level over the past 570 my with high levels during the Mesozoic and early Paleozoic and low levels during the Permo-Carboniferous and late Cenozoic. -from Author
Previous interpretation of the Tertiary delta 18O record of planktic and benthic foraminifers has emphasized comparison to the modern ocean, assumed an ice-free world prior to middle Miocene time, and thereby calculated surprisingly cool temperatures for the tropical sea surface. We propose an alternative interpretation, which compares Tertiary data to average late Pleistocene, assumes constant tropical sea-surface temperature, and thereby estimates global ice volume. This approach suggests that Earth has had a significant ice budget (and therefore glacio-eustatic sea-level fluctuations) at least since Eocene and perhaps even throughout much of Cretaceous time. -Authors
A suite of middle Miocene Deep Sea Drilling Project sites in the
southwest Pacific reveals large-scale changes in deepwater circulation
associated with East Antarctic Ice Sheet (EAIS) variations from
˜16.5 to 13.8 Ma. Oxygen and carbon isotopic records based on
Cibicidoides benthic foraminifera from a depth transect (sites 590B,
588A, 591B, and 206 from 1200- to 3150-m paleodepth at ˜35°S
paleolatitude) and from a meridional transect (sites 588A, 590B, 593,
and 594 from 30° to 48°S paleolatitude at intermediate water
depth) allow detailed examination of southwest Pacific deepwater
circulation from ˜17.5 to 12 Ma. Significantly, intervals of low
δ18O from 16.5 to 16.3 Ma and perhaps at 15.7 Ma were
marked by similar δ18O values at upper bathyal
(˜1200-1500 m; sites 588A and 590B) and midbathyal (˜2100 m;
site 591B) water depths. Small vertical δ18O gradients
during δ18O minima may indicate warm saline deep water
(WSDW) at midbathyal depths in the southwest Pacific during intervals of
inferred global warmth and low global ice volume. Increased vertical
δ18O gradients after ˜15.6 Ma and especially
after 13.8 Ma indicate increased production of Southern Component Water
(SCW) in association with EAIS growth. These data are consistent with
the hypothesis (Woodruff and Savin, 1989) that major EAIS growth was
fostered by diminished meridional heat transport to the high southern
latitudes related to the termination of Tethyan Indian Saline Water
(TISW) and an increase in SCW production during the early middle Miocene
after ˜15.6 Ma. Further, a maximum vertical carbon isotopic
gradient of ˜0.8 ‰ at 13.6 Ma suggests that Southern
Component Intermediate Water (SCIW) production and Pacific Deep Water
(PDW) strength were each at a maximum at this time and were critical to
major EAIS growth. The establishment of near-modern
δ13C and δ18O gradients following
major EAIS growth from ˜14.0 to 13.8 Ma marks a major step in the
development of the Neogene ocean/cryosphere system.