Millennial scale evolution of the Southern Ocean chemical divide (original) (raw)
Related papers
Eocene/Oligocene ocean de-acidification linked to Antarctic glaciation by sea-level fall
Nature, 2008
One of the most dramatic perturbations to the Earth system during the past 100 million years was the rapid onset of Antarctic glaciation near the Eocene/Oligocene epoch boundary 1-3 (,34 million years ago). This climate transition was accompanied 3 by a deepening of the calcite compensation depth-the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution. Changes in the global carbon cycle 4 , rather than changes in continental configuration 5 , have recently been proposed as the most likely root cause of Antarctic glaciation, but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear. Here we use a global biogeochemical box model to test competing hypotheses put forward to explain the Eocene/Oligocene transition. We find that, of the candidate hypotheses, only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor. In our simulations, glacioeustatic sea-level fall associated with the growth of Antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves, leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth. At the same time, fresh limestones are exposed to erosion, thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater d 13 C. Our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the Cenozoic era.
Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores.
Paleoceanography, 2002
The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice-rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO2 variation depends on the age model as 14C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean.
Resolving a late Oligocene conundrum: Deep-sea warming and Antarctic glaciation
Palaeogeography Palaeoclimatology Palaeoecology, 2006
Changes in ice volume and resulting changes in sea level were determined for the late Oligocene (26–23 Ma, Astronomical Timescale, ATS) by applying δ18O-to-sea-level calibrations to deep-sea δ18O records from ODP Sites 689, 690, 929, 1090, and 1218. Our results show that maximum global ice volume occurred during two late Oligocene δ18O events, Oi2c (24.4 Ma) and Mi1 (23.0 Ma) (inferred glacioeustatic lowering), with volumes up to ~25% greater than the present-day East Antarctic Ice Sheet (EAIS). Ice volume during glacial minima was on the order of about 50% of the present-day EAIS. This is supported by late Oligocene stratigraphic records from Antarctica that contain evidence of cold climates and repeated episodes of glaciation at sea level and grounding lines of glacial ice on the Antarctic continental shelf in the Ross Sea and Prydz Bay. In contrast, composite deep-sea δ18O records show a significant decrease (≥ 1‰) between 26.7 and 23.5 Ma, which have long been interpreted as bottom-water warming combined with deglaciation of Antarctica. However, a close examination of individual δ18O records indicates a clear divergence after 26.8 Ma between records from Southern Ocean locations (i.e., Ocean Drilling Program Sites 689, 690, 744) and those of other ocean basins. High δ18O values (2.9‰–3.3‰) from these Southern Ocean δ18O records are consistent with an ice sheet on the East Antarctic continent equivalent to present-day values and cold bottom-water temperatures (≤ 2.0 °C). These differences suggest a reduction in deep-water produced near the Antarctic continent (i.e., proto-Antarctic Bottom Water, proto-AABW), which were quickly entrained and mixed with warmer (and presumably more saline) bottom-water originating from lower latitudes. Expansion of a warmer deep-water mass and the weakening of the proto-AABW may explain the large intra-basinal isotopic gradients that developed among late Oligocene benthic δ18O records. These conclusions are also supported by ocean modeling suggesting a reduction of deep-water formed in the Southern Ocean, strengthening of deep-water from the northern hemisphere, and decreasing temperatures in high southern latitudes occurred as the Drake Passage opened to deep-water. Low δ18O values reported from deep-sea locations other than the Southern Ocean are shown to bias estimates of Antarctic ice volume, calling for a re-evaluation of the notion that Antarctic ice volume was significantly reduced during the late Oligocene.
Paleoceanography, 2015
Changes in ocean circulation structure, together with biological cycling, have been proposed for trapping carbon in the deep ocean during glacial periods of the Late Pleistocene, but uncertainty remains in the nature and timing of deep ocean circulation changes through glacial cycles. In this study, we use neodymium (Nd) and carbon isotopes from a deep Indian Ocean sediment core to reconstruct water mass mixing and carbon cycling in Circumpolar Deep Water over the past 250 thousand years, a period encompassing two full glacial cycles and including a range of orbital forcing. Building on recent studies, we use reductive sediment leaching supported by measurements on isolated phases (foraminifera and fish teeth) in order to obtain a robust seawater Nd isotope reconstruction. Neodymium isotopes record a changing North Atlantic Deep Water (NADW) component in the deep Indian Ocean that bears a striking resemblance to Northern Hemisphere climate records. In particular, we identify both an approximately in-phase link to Northern Hemisphere summer insolation in the precession band and a longer-term reduction of NADW contributions over the course of glacial cycles. The orbital timescale changes may record the influence of insolation forcing, for example via NADW temperature and/or Antarctic sea ice extent, on deep stratification and mixing in the Southern Ocean, leading to isolation of the global deep oceans from an NADW source during times of low Northern Hemisphere summer insolation. That evidence could support an active role for changing deep ocean circulation in carbon storage during glacial inceptions. However, mid-depth water mass mixing and deep ocean carbon storage were largely decoupled within glacial periods, and a return to an interglacial-like circulation state during marine isotope stage (MIS) 6.5 was accompanied by only minor changes in atmospheric CO 2. Although a gradual reduction of NADW export through glacial periods may have produced slow climate feedbacks linked to the growth of Northern Hemisphere ice sheets, carbon cycling in the glacial ocean was instead more strongly linked to Southern Ocean processes. Evidence on past Atlantic Ocean circulation derived from carbon isotope reconstructions has been used to suggest that ocean circulation is primarily responding to, rather than driving, Pleistocene climate change on orbital timescales. For example, Imbrie et al. [1992] placed deep Atlantic ventilation within a "late response" group of variables, with increased ventilation occurring~8 kyr behind precessional maxima in Northern Hemisphere insolation. More recently, Lisiecki et al. [2008] similarly proposed a lag of 6-11 kyr WILSON ET AL.
Nature, 1998
The nitrogen-isotope record preserved in Southern Ocean sediments, along with several geochemical tracers for the settling fluxes of biogenic matter, reveals patterns of past nutrient supply to phytoplankton and surface-water stratification in this oceanic region. Areal averaging of these spatial patterns indicates that reduction of the CO 2 'leak' from ocean to atmosphere by increased surface-water stratification south of the Polar Front made a greater contribution to the lowering of atmospheric CO 2 concentration during the Last Glacial Maximum than did the increased export of organic carbon from surface to deep waters occurring further north.
Nature Communications, 2013
The Southern Ocean plays a prominent role in the Earth's climate and carbon cycle. Changes in the Southern Ocean circulation may have regulated the release of CO 2 to the atmosphere from a deep-ocean reservoir during the last deglaciation. However, the path and exact timing of this deglacial CO 2 release are still under debate. Here we present measurements of deglacial surface reservoir 14 C age changes in the eastern Pacific sector of the Southern Ocean, obtained by 14 C dating of tephra deposited over the marine and terrestrial regions. These results, along with records of foraminifera benthic-planktic 14 C age and d 13 C difference, provide evidence for three periods of enhanced upwelling in the Southern Ocean during the last deglaciation, supporting the hypothesis that Southern Ocean upwelling contributed to the deglacial rise in atmospheric CO 2. These independently dated marine records suggest synchronous changes in the Southern Ocean circulation and Antarctic climate during the last deglaciation.
Quaternary Science Reviews, 2002
We review the various methods which have been applied to estimate the change of seawater d 18 O (dw) between the Last Glacial Maximum (LGM) and the Holocene. The most accurate constraints on these estimates are provided by the measurement of pore waters d 18 O and by high resolution records of benthic foraminifer d 18 O in the high latitude oceans of both hemispheres. They show that the d 18 O of seawater in the deep ocean during the LGM was 1.0570.20% heavier than today, with significant regional variations. Constraints resulting from ice sheet models are less accurate, because both the volume and isotopic composition of each ice sheet are still poorly known. The amplitude of the benthic d 18 O change between the LGM and the Holocene, together with the d 18 O and d 13 C values of the benthic foraminifera genus Cibicides during the LGM, show that the Southern Ocean deep waters were extremely cold, close to the freezing point. During this time, deep waters of the South Atlantic and the Pacific oceans were at least 1.31C warmer than those of the Southern Ocean. Overall, the glacial deep ocean, below 2500 m, was characterized by extremely cold temperatures, everywhere lower than 01C. d 18 O values of benthic foraminifer from the North Atlantic are highly variable. This variability suggests that deep Atlantic waters were not homogeneous, probably because they resulted from the sinking of different surface water masses at various locations during winter. The deep waters in the North Atlantic were at most 21C warmer than in Southern Ocean. Alternatively, they could have been nearer the freezing point with a d 18 O value lighter than the mean ocean water. Brine formation during winter would preserve such light d 18 O values of the northern North Atlantic surface water.
Middle Eocene to Late Oligocene Antarctic glaciation/deglaciation and Southern Ocean productivity
Paleoceanography, 2014
During the Eocene-Oligocene transition, Earth cooled significantly from a greenhouse to an icehouse climate. Nannofossil assemblages from Southern Ocean sites enable evaluation of paleoceanographic changes and, hence, of the oceanic response to Antarctic ice sheet evolution during the Eocene and Oligocene. A combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes. A cooling trend, started in the Middle Eocene, was interrupted by warming during the Middle Eocene Climatic optimum and by short cooling episodes. The cooling episode at 39.6 Ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from~39.1 to~36.2 Ma. We suggest that oligotrophic conditions were associated with increased water mass stratification, low nutrient contents, and high efficiency of the oceanic biological pump that, in turn, promoted sequestration of carbon from surface waters, which favored cooling. After 36.2 Ma, we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance. This turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient Antarctic ice sheet and/or to iron delivery to the sea surface. Eutrophic conditions were maintained throughout the Oligocene, which was characterized by a nannofossil assemblage shift toward cool conditions at the Eocene-Oligocene transition. Finally, a warm nannofossil assemblage in the Late Oligocene indicates a warming phase.
Two Modes of Change in Southern Ocean Productivity Over the Past Million Years
Science, 2013
Export of organic carbon from surface waters of the Antarctic zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric CO2 concentrations, signaling reduced exchange of CO2 between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes and thus suggesting iron fertilization of Subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic zone is compiled with parallel Subantarctic data over the last million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial/interglacial atmospheric CO2 record, including during the interval of "lukewarm" interglacials between 450 and 800 thousand years ago. Antarctic ice core measurements reveal that regional air temperatures and atmospheric pCO 2 were tightly correlated over glacial-interglacial cycles of the past 800 kyrs (1). Many studies have inferred a dominant role for the Southern Ocean in modulating glacial-interglacial variability of atmospheric pCO 2 ((2) and references therein). The central role of the Southern Ocean is thought to reflect its leverage on the global efficiency of the biological pump, in which the production, sinking, and deep
Paleoceanography , 2017
Enhanced vertical gradients in benthic foraminiferal δ 13 C and δ 18 O in the Atlantic and Pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of North Atlantic sourced interior waters; nonetheless, our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete. Here we compare high-resolution depth transects of Cibicidoides spp. δ 13 C and δ 18 O from the Southwest Pacific and the Southwest Atlantic to examine relative changes in northern and southern sourced deep waters during the Last Glacial Maximum (LGM) and deglaciation. During the LGM, our transects show that water mass properties and boundaries in the South Atlantic and Pacific were different from one another. The Atlantic between~1.0 and 2.5 km was more than 1‰ enriched in δ 13 C relative to the Pacific and remained more enriched through the deglaciation. During the LGM, Atlantic δ 18 O was~0.5‰ more enriched than the Pacific, particularly below 2.5 km. This compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation. We attribute these changes to a "deep gateway" effect whereby northern sourced waters shallower than the Drake Passage sill were unable to flow southward into the Southern Ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east-west circumpolar pressure gradient above the sill depth. We surmise that through the LGM and early deglaciation, shoaled northern sourced waters were unable to escape the Atlantic and contribute to deep water formation in the Southern Ocean.