Modal shift in North Atlantic seasonality during the last deglaciation (original) (raw)
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The Holocene, 2014
Holocene sea surface temperatures in the eastern Fram Strait are reconstructed based on Mg/Ca ratios measured on the planktic foraminifer Neogloboquadrina pachyderma (sin). The reconstructed sub sea surface temperatures (sSSTMg/Ca) fluctuate markedly during the earliest Holocene at ~11.7-10.5 ka BP. This probably is in response to the varying presence of sea ice and deglacial melt water. Between ~10.5-7.9 ka BP the sSSTMg/Ca values are relatively high (~4°C) and more stable reflecting high insolation and intensified poleward advection of Atlantic water. After 7.9 ka BP the sSSTMg/Ca decline to an average of ~3°C throughout the mid-Holocene. These changes can be attributed to a combined effect of reduced poleward oceanic heat advection and a decline in insolation as well as a gradually increased influence of eastward migrating Arctic Water. The sSSTMg/Ca increase and vary between 2.1-5.8°C from ~2.7 ka BP to the present. This warming is in contrast to declining late Holocene insolation and may instead be explained by factors including increased advection of oceanic heat to the Arctic region possibly insulated beneath a widening freshwater layer in the northern North Atlantic in conjunction with a shift in calcification season and/or depth habitat of N. pachyderma (sin).
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.
Subsurface water column dynamics in the subpolar North Atlantic were reconstructed in order to improve the understanding of the cause of abrupt IRD events during cold periods of the Early Pleistocene. We used Mg / Ca-based temperatures of deep-dwelling (Neogloboquadrina pachyderma sinistral) planktonic foraminifera and paired Mg / Caδ 18 O measurements to estimate the subsurface temperatures and δ 18 O of seawater at Site U1314. Carbon isotopes on benthic and planktonic foraminifera from the same site provide information about the ventilation and water column nutrient gradient. Mg / Ca-based temperatures and δ 18 O of seawater suggest increased temperatures and salinities during ice-rafting, likely due to enhanced northward subsurface transport of subtropical waters during periods of AMOC reduction. Planktonic carbon isotopes support this suggestion, showing coincident increased subsurface ventilation during deposition of ice-rafted detritus (IRD). Warm waters accumulated at subsurface would result in basal warming and break-up of ice-shelves, leading to massive iceberg discharges in the North Atlantic. Release of heat and salt stored at subsurface would help to restart the AMOC. This mechanism is in agreement with modelling and proxy studies that observe a subsurface warming in the North Atlantic in response to AMOC slowdown during the MIS3.
A Late Glacial–Early Holocene multiproxy record from the eastern Fram Strait, Polar North Atlantic
2014
The paleoceanographic development of the eastern Fram Strait during the transition from the cold Late Glacial and into the warm early Holocene was elucidated via a multiproxy study of a marine sediment record retrieved at the western Svalbard slope. The multiproxy study includes analyses of planktic foraminiferal fauna, bulk sediment grain size and CaCO 3 content in addition to Mg/Ca ratios and stable isotopes (δ 13 C and δ 18 O) measured on the planktic foraminifer Neogloboquadrina pachyderma. Furthermore paleo subsurface water temperatures were reconstructed via Mg/Ca ratios (sSST Mg/Ca) and transfer functions (sSST Transfer) enabling comparison between the two proxies within a single record. The age model was constrained by four accelerator mass spectrometry (AMS) 14 C dates. From 14,000 to 10,300 cal yr B.P. N. pachyderma dominated the planktic fauna and cold polar sea surface conditions existed. The period was characterized by extensive sea ice cover, iceberg transport and low sub sea surface temperatures (sSST Transfer ~2.1°C; sSST Mg/Ca ~3.5°C) resulting in restricted primary production. Atlantic Water inflow was reduced compared to the present-day and likely existed as a subsurface current. At ca. 10,300 cal yr B.P. Atlantic Water inflow increased and the Arctic Front retreated north-westward resulting in increased primary productivity, higher foraminiferal fluxes and a reduction in sea ice cover and iceberg transport. The fauna rapidly became dominated by the subpolar planktic foraminifer Turborotalita quinqueloba and summer sSST Transfer increased by ~3.5°C. Concurrently, the sSST Mg/Ca recorded by N. pachyderma rose only ~0.5°C. From ca. 10,300 to 8,600 cal yr B.P. the average sSST Mg/Ca and sSST Transfer were ~4.0°C and ~5.5°C, respectively. The relatively modest change in sSST Mg/Ca compared to sSST Transfer can probably be tied to a change of the main habitat depth and/or shift in the calcification season for N. pachyderma during this period.
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.
Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum
Nature, 2006
The Palaeocene/Eocene thermal maximum, ,55 million years ago, was a brief period of widespread, extreme climatic warming 1-3 , that was associated with massive atmospheric greenhouse gas input 4. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition 5. We show that sea surface temperatures near the North Pole increased from ,18 8C to over 23 8C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations 6 , but the absolute polar temperatures that we derive before, during and after the event are more than 10 8C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms-perhaps polar stratospheric clouds 7 or hurricane-induced ocean mixing 8-to amplify early Palaeogene polar temperatures. Stable carbon isotope (d 13 C) records of carbonate and organic carbon from numerous sites show a prominent negative carbon isotope excursion across the Palaeocene/Eocene thermal maximum (PETM) 2,9. The carbon isotope excursion is expressed as a .2.5‰ drop in d 13 C, which signifies an input of at least 1.5 £ 10 18 g of 13 C-depleted carbon, somewhat analogous in magnitude and composition to current and expected fossil fuel emissions. The PETM captures ,200 kyr (ref. 10) and is associated with profound environmental changes that are well-documented at low-to mid-latitudes (,608), including a 4-8 8C temperature rise of surface and deep ocean waters 1-3 and major terrestrial and marine biotic changes 11-13. Terrestrial mammal turnovers are consistent with mass migrations across Arctic regions resulting from high-latitude warming 14 , but no Arctic data have existed to evaluate this hypothesis. The Integrated Ocean Drilling Program Expedition (IODP) 302 (or the Arctic Coring Expedition) recently recovered a Palaeogene marine sedimentary record from Hole 4A (,878 52.00 0 N; 1368 10.64 0 E; 1,288 m water depth), on the Lomonosov ridge in
Nature Ecology & Evolution
Biodiversity is expected to change in response to future global warming. However, it is difficult to predict how species will track the ongoing climate change. Here we use the fossil record of planktonic foraminifera to assess how biodiversity responded to climate change with a magnitude comparable to future anthropogenic warming. We compiled time series of planktonic foraminifera assemblages, covering the time from the last ice age across the deglaciation to the current warm period. Planktonic foraminifera assemblages shifted immediately when temperature began to rise at the end of the last ice age and continued to change until approximately 5,000 years ago, even though global temperature remained relatively stable during the last 11,000 years. The biotic response was largest in the mid latitudes and dominated by range expansion, which resulted in the emergence of new assemblages without analogues in the glacial ocean. Our results indicate that the plankton response to global warmi...