Pleistocene foraminifera assemblages as a proxy for temperature in the Weddell Sea, ODP Site 693A (original) (raw)
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Quaternary Science Reviews, 2011
Stable carbon and oxygen isotopes from benthic and planktic foraminifers, planktic foraminifer assemblages and ice rafted debris from the North Atlantic Site U1314 (Integrated Ocean Drilling Program Expedition 306) were examined to investigate orbital and millennial-scale climate variability in the North Atlantic and its impact on global circulation focusing on the development of glacial periods during the mid-Pleistocene (ca 800e400 ka). Glacial initiations were characterized by a rapid cooling (6e10 C in less than 7 kyr) in the mean annual sea surface temperature (SST), increasing benthic d 18 O values and high benthic d 13 C values. The continuous increase in benthic d 18 O suggests a continuous ice sheet growth whereas the positive benthic d 13 C values indicate that the flow of the Iceland Scotland Overflow water (ISOW) was vigorous. Strong deep water formation in the Norwegian Greenland Sea promoted a high transfer of freshwater from the ocean to the continents. However, low SSTs at Site U1314 suggest a subpolar gyre cooling and freshening that may have reduced deep water formation in the Labrador Sea during glacial initiations. Once the 3.5& threshold in the benthic d 18 O record was exceeded, ice rafting started and ice sheet growth was punctuated by millennial-scale waning events which returned to the ocean part of the freshwater accumulated on the continents. Ice-rafting events were associated with a rapid reduction in the ISOW (benthic d 13 C values dropped 0.5e1&) and followed by millennial-scale warmings. The first two millennial-scale warm intervals of each glacial period reached interglacial temperatures and were particularly abrupt (6e10 C in w3 kyr). Subsequent millennial-scale warm events were cooler probably because the AMOC was rather reduced as suggested by the low benthic d 13 C values. These two abrupt warming events that occurred at early glacial periods were also observed in the Antarctic temperature and CO 2 records, suggesting a close correlation between both Hemispheres. The comparison of the sea surface proxies with the benthic d 18 O record (as the Southern sign) indicates the presence of a millennial-scale seesaw pattern similar to that seen during the Last Glacial period.
The last deglaciation in the Southern Ocean
Paleoceanography, 1989
The isotopic and micropaleontological deglacial records of three deep-sea cores from 44øS to 55øS have been dated by accelerator mass spectrometry. The available records did not allow accurate dating of the initiation of the deglaciation. By 13,000 years B.P., sea surface temperatures reached values similar to the present values. A cool oscillation abruptly interrupted this warm phase between 12,000 and 11,000 years B.P. Initiation of this cooling therefore preceded the northern hemisphere Younger Dryas by approximately 1000 years. Complete warming was reached by 10,000 years B.P., more or less synchronous with the northeast Atlantic 1D6partement de G6ologie et Oc6anographie, Universit6 de Bordeaux 1, Talence, France. 2Centre des Faibles Radioactivit6s, Laboratoire mixte thousand years) in the southern ocean than in the northern Atlantic, both for the last and the penultimate glacialinterglacial transitions [Hays, 1978; Morley and Robinson, 1980; CLIMAP Project Members, 1984]. Yet quantitative informations on the timing of the southern hemisphere deglacial records are still insufficient to explore the reasons for this lead and understand the interhemispheric teleconnections of climate. In this paper we document the timing of the changes in surface water temperature and foraminiferal isotopic ratio during the last deglaciation between 44øS and 54øS by 14C AMS dating of planktonic foraminifera in three cores located in the Indian sector of the southern ocean, between Crozet and Heard islands (MD 73-025 at 43ø49'S, 51ø19'E, 3284 m; MD 84-527 at 43ø49.3'S, 51ø19.1'E, 3269 m, and MD 84-551 at 55ø00.5'S, 73ø16.9'E, 2230 m). TI-IE SOUTI-IERN OCEAN PALEOCLIMATIC RECORDS Two types of climatic signals have been considered in this study: the oxygen isotope measurements (•5180 versus PeeDee Belemnites Standard (PDB)) and the sea surface temperatures (SST) estimated from foraminiferal-based transfer function (T. •.): The fractionation between calcium carbonate and water during foraminiferal growth depends strongly on the seawater temperature. The b180 of foraminifera shells is therefore a function not only of the global variations in the isotopic composition of the oceans due to changes in the volume of ice stored over the continents, but also of the seawater temperature. In the case of the last deglaciation the total ice volume effect is thought to account for _+ 1.1%o [Labeyrie et al., 1987]. The local seawater b180 depends also upon salinity variations and meltwater discharges in the proximity of the Antarctic ice sheet [Labeyrie et al., 1986]. The temperature contribution to the foraminiferal •180 signal may not, therefore, be calculated directly.
The role of deep ocean circulation in setting glacial climates
1] The glacial cycles of the Pleistocene involve changes in the circulation of the deep ocean in important ways. This review seeks to establish what were the robust patterns of deep-sea water mass changes and how they might have influenced important parts of the last glacial cycle. After a brief review of how tracers in the modern ocean can be used to understand the distribution of water masses, I examine the data for biogeochemical, circulation rate, and conservative tracers during glacial climates. Some of the robust results from the literature of the last 30 years include: a shoaled version of northern source deep water in the Atlantic, expanded southern source water in the abyss and deep ocean, salt (rather than heat) stratification of the last glacial maximum (LGM) deep-sea, and several lines of evidence for slower overturning circulation in the southern deep cell. We combine these observations into a new idea for how the ocean-atmosphere system moves from interglacial to glacial periods across a single cycle. By virtue of its influence on the melting of land-based ice around Antarctica, cooling North Atlantic Deep Water (NADW) leads to a cold and salty version of Antarctic Bottom Water (AABW). This previously underappreciated feedback can lead to a more stratified deep ocean that operates as a more effective carbon trap than the modern, helping to lower atmospheric CO 2 and providing a mechanism for the deep ocean to synchronize the hemispheres in a positive feedback that drives the system to further cooling.
Paleoceanography, 1992
A set of numerical equations is developed to estimate past sea surface temperatures (SST) from fossil Antarctic diatoms. These equations take into account both the biogeographic distribution and experimentally derived silica dissolution. The data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments. Factor analysis of 166 samples (124 Holocene core top and 42 artificial samples) resolved four factors. Three of these factors depend on the water mass distribution (one Subantarctic and two Antarctic assemblages); factor 4 corresponds to a "dissolution assemblage". Inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and 1D6partement de G6ologie et Oc•anographie, CNRS URA. 197, Universit6 de Bordeaux 1, Paper number 92PA00709. 0883-8305/92/92PA-00709510.00 gives accurate estimates of SST over a wide range of biosiliceous dissolution. A transfer function (DTF 166/34/4) is derived from the distribution of these factors versus summer SST. Its standard error is + IøC in the-1 to +10 øC summer temperature range. This transfer function is used to estimate SST changes in two southern ocean cores (43øS and 55øS) which cover the last climatic cycle. The time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios. The reconstructed SST records present strong analogies with the air temperature record over Antarctica at the Vostok site, derived from changes in the isotopic ratio of the ice. This similarity may be used to compare the oceanic isotope stratigraphy and the Vostok time scale derived from ice flow model. The oceanic time scale, if taken at face value, would indicate that large changes in ice accumulation rates occurred between warm and cold periods. 90006,Paleoclimatological and chronological implications of the Vostok core dust record, Nature, 343, 56-58, 1990. Pichon, J.J., M. Labracherie, L.D. Labeyrie, and J. Duprat, Transfer functions between diatom assemblages and surface hydrology in the 318 southern ocean, Paleogeogr. Paleoclimatol. Paleoecol., 61, 79-95, 1987. A seasonally recurrent patch of Antarctic planktonic diatoms, Search, 16(1-2), 48, 1985. Robinson, S.G., The late Pleistocene paleoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements, Phys. Earth Planet Inter., 42, 22-47, 1986. Shackleton, N.J., Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uvigerina: Isotopic changes in the ocean during the last glacial, in les mdthodes quantitatives d'•tude des variations du climat au cours du pleistocene, edited by J. Labeyrie, pp. 203-210, Colloque CNRS
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.
Forcing of the deep ocean circulation in simulations of the Last Glacial Maximum
Paleoceanography, 2002
1] From the interpretation of different proxy data it is widely believed that the North Atlantic thermohaline circulation during the maximum of the last ice age $21,000 years ago was considerably weaker than today. Recent equilibrium simulations with a coupled ocean-atmosphere-sea ice model successfully simulated a reduction in North Atlantic Deep Water (NADW) formation consistent with reconstructions. Here we examine the influence of different air-sea fluxes on simulated changes in the deep ocean circulation between the Last Glacial Maximum and present day. We find that changes in the oceanic surface freshwater fluxes are the dominant forcing mechanism for the reduced Atlantic overturning. Diminished export of freshwater out of the Atlantic drainage basin through the atmosphere decreases surface salinities in the North Atlantic, leading to less NADW formation in the colder climate. Changes in heat fluxes, which lead to increased sea surface densities in the North Atlantic and therefore to an enhanced overturning, are of secondary importance. Wind stress variations seem to play a negligible role. The degree to which the Atlantic freshwater export and hence the NADW formation are reduced depends on the formulation of the atmospheric hydrological cycle and on the strength of the overturning in the present-day simulation. Simulated changes in sea surface properties for a large variety of overturning strengths are compared with different reconstruction data sets. The results depend strongly on the data set used. Sea surface temperature reconstructions from Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) and earlier salinity reconstructions based on planktonic foraminifera are most consistent with a significant reduction of the circulation, while recent reconstructions using dinocyst assemblages allow no unequivocal conclusion.