Evolution of South Atlantic density and chemical stratification across the last deglaciation.

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.We are grateful to I. Mather, J. Rolfe, F. Dewilde and G. Isguder for preparing and performing isotopic analyses, as well as C. Daunt, S. Souanef-Ureta and M. Greaves for technical assistance in performing trace element analysis. J.R. was funded jointly by the British Geological Survey/British Antarctic Survey (Natural Environment Research Council) and the University of Cambridge. J.G. was funded by the Gates Cambridge Trust. L.C.S. acknowledges support from the Royal Society and NERC grant NE/J010545/1. C.W. acknowledges support from the European Research Council grant ACCLIMATE/no 339108. This is LSCE contribution 5514. This work was funded (in part) by the European Research Council (ERC grant 2010-NEWLOG ADG-267931 HE). N.V.R. acknowledges support from EU RTN NICE (no. 36127). We thank the captain and crew of the RRS James Clark Ross for facilitating the collection of the marine sediment core GC528.This is the author accepted manuscript. The final version is available from PNAS via http://dx.doi.org/10.1073/pnas.151125211

Evolution of South Atlantic density and chemical stratification across the last deglaciation

The cause of the rise in atmospheric pCO2 over the last deglaciation has been a puzzle since its discovery in the early 1980s. It is widely believed to be related to changes in carbon storage in the deep ocean, but the exact mechanisms responsible for releasing CO2 from the deep-ocean reservoir, including the role of ocean density stratification, remains an open question. Here we reconstruct changes in the intermediate-deep density gradient in the South Atlantic across the last deglaciation and find evidence of an early deglacial chemical destratification and a late deglacial density destratification These results suggest that other mechanisms, besides deep-ocean density destratification, were responsible for the ocean–atmosphere transfer of carbon over the deglacial period

Atlantic circulation changes across a stadial-interstadial transition

[EN] We combine consistently dated benthic carbon isotopic records distributed over the entire Atlantic Ocean with numerical simulations performed by a glacial configuration of the Norwegian Earth System Model with active ocean biogeochemistry in order to interpret the observed Cibicides 13C changes at the stadial-interstadial transition corresponding to the end of Heinrich Stadial 4 (HS4) in terms of ocean circulation and remineralization changes. We show that the marked increase in Cibicides 13C observed at the end of HS4 between g1/42000 and 4200gm in the Atlantic can be explained by changes in nutrient concentrations as simulated by the model in response to the halting of freshwater input in the high-latitude glacial North Atlantic. Our model results show that this Cibicides 13C signal is associated with changes in the ratio of southern-sourced (SSW) versus northern-sourced (NSW) water masses at the core sites, whereby SSW is replaced by NSW as a consequence of the resumption of deep-water formation in the northern North Atlantic and Nordic Seas after the freshwater input is halted. Our results further suggest that the contribution of ocean circulation changes to this signal increases from g1/440g% at 2000gm to g1/480g% at 4000gm. Below g1/44200gm, the model shows little ocean circulation change but an increase in remineralization across the transition marking the end of HS4. The simulated lower remineralization during stadials compared to during interstadials is particularly pronounced in deep subantarctic sites, in agreement with the decrease in the export production of carbon to the deep Southern Ocean during stadials found in previous studies.This research has been supported by the Research Council of Norway (RNC – KLIMAFORSK contract no. 326603/E10 and Coordination and Support Activity contract no. 310328/E10). The research leading to these results derives from exchanges and collaborations between participants in the ACCLIMATE ERC project (FP7/2007-2013 grant agreement no. 339108) and ice2ice ERC project (FP7/2007-2013 grant agreement no. 610055). Guncheng Guo acknowledges support from the RCN-funded project ABRUPT (project no. 325333). Susana Lebreiro acknowledges funding from project CTM2017-84113-R. Jerry Tjiputra acknowledges RCN project INES (project no. 270061).Peer reviewe

Late Holocene paleoceanographic evidence of the influence of the Aleutian Low and North Pacific High on circulation in the Seymour-Belize Inlet Complex, British Columbia, Canada

Foraminiferal and thecamoebian faunas from the Seymour-Belize Inlet Complex (SBIC), a fjord network situated on the mainland coast of British Columbia, were studied to assess climatic cycles and trends impacting the area through the ∼ AD 850–AD 2002 interval. Ocean circulation patterns prevalent in the SBIC are strongly linked to precipitation, which is closely linked to the relative strength and position (center of action; COA) of the seasonally developed Aleutian Low (AL) and North Pacific High (NPH) atmospheric circulation gyres. Through interpretation of cluster analysis and ordination methods, a period of weak estuarine circulation was recognized to have impacted the SBIC area between ∼ AD 850 and AD 1500. During this time waters in the SBIC were dysoxic to anoxic and the sediment–water interface was comprised of a depauperate foraminiferal fauna consisting of low diversity agglutinated forms. These reduced oxygen conditions came about as a result of diminished precipitation in the SBIC catchment as the COA of the AL progressively migrated westward over time, resulting in greatly reduced estuarine circulation and only infrequent and feeble incursions of well oxygenated open ocean water into the SBIC basin. By ∼AD 1575, following a gradual transition period of ∼75 years when circulation patterns in the inlet were unstable, very strong estuarine circulation developed in the SBIC, concomitant with the onset of the Little Ice Age (LIA) in western Canada. In the SBIC this interval was characterized by higher levels of precipitation, which greatly enhanced estuarine circulation resulting in frequent incursions of cold, well oxygenated ocean currents into the bottom waters of the SBIC and the development of a diverse calcareous foraminiferal fauna. This circulation pattern began to break down in the late 19th century AD and by ∼AD 1940 conditions similar to those that existed in the inlet prior to ∼AD 1500 had redeveloped, a process that continues at present

An Illustrated Guide to Fjord Foraminifera from the Seymour-Beliz Inlet Complex, Northern British Columbia, Canada

Detailed taxonomic descriptions and illustrations of 94 foraminiferal species found in the Seymour-Belize Inlet Complex (SBIC), a fjord network situated on the north coast of British Columbia, are presented as an aid to future researchers. This treat-ment includes a few planktic foraminiferal taxa that carried into the SBIC from the open ocean. In addition, ten freshwater thecamoebian species that were washed into the inlet from the nearby adjacent shore are also described

Interglacials of the last 800,000 years

Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end-members of glacial cycles. Based on a sea level definition, we identify eleven interglacials in the last 800,000 years, a result that is robust to alternative definitions. Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (last interglacial) and 11c (∼400 ka ago) were globally strong (warm), while MIS 13a (∼500 ka ago) was cool at many locations. A step change in strength of interglacials at 450 ka is apparent only in atmospheric CO2 and in Antarctic and deep ocean temperature. The onset of an interglacial (glacial termination) seems to require a reducing precession parameter (increasing Northern Hemisphere summer insolation), but this condition alone is insufficient. Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and temperature to external astronomical forcing. The precise timing of events may be modulated by millennial-scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere. A variety of temporal trends is observed, such that maxima in the main records are observed either early or late in different interglacials. The end of an interglacial (glacial inception) is a slower process involving a global sequence of changes. Interglacials have been typically 10-30 ka long. The combination of minimal reduction in northern summer insolation over the next few orbital cycles, owing to low eccentricity, and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future. ©2015. The Authors.This paper arose as a result of a succession of workshops of the Past Interglacials Group (PIGS), sponsored by the Past Global Changes Project (PAGES). The authors acknowledge the contributions of all participants at those workshops, of whom the listed authors are only a subset. Numerous funding agencies have contributed to the work of this paper including NSF (USA), NERC and The Royal Society (UK), F.R.S-FNRS (Belgium), and SNF (Switzerland).Peer reviewe

Age versus component declination and inclination of natural remanent magnetization (NRM) of u-channel samples recording the Laschamp excursion in Core MD07-3076 recovered during Marion Dufresne cruise VT90/SOUC

Component declination and inclination, determined from stepwise alternating field demagnetization data recording the Laschamp excursion from u-channel samples from Core MD07-3076 (see Channell et al., 2017). Age models for records of the Laschamp and Iceland Basin excursions were derived from radiocarbon dates, and from matching sea-surface temperature records to Antarctic (EPICA) air-temperature records from ice cores. The onset of the Laschamp excursion occurred during Antarctic Isotopic Maximum (AIM) 10, consistent with its occurrence during Greenland Interstadial 10. The end of the Laschamp excursion occurred prior to AIM 9 in Greenland Stadial 10. The age model is supported by synchroneity of directional and relative paleointensity manifestations of the Laschamp excursion in the marine core with peaks in EPICA10Be and nitrate flux. The Iceland Basin excursion is synchronous with the final phase of the transition from marine isotope stage (MIS) 7a to MIS 6e as recorded in the EPICA δD record. The onset of the Laschamp and Iceland Basin excursions, defined here by component inclinations >-40°, occurred at 41.4 ka and 190.0 ka, and durations are ~1 kyr and ~ 3.5 kyr, respectively, although these estimates depend on the criteria used to define the directional excursions. By comparison with Laschamp and Iceland Basin excursion records from the North Atlantic Ocean, the two excursions are synchronous at centennial timescales between the two hemispheres, based on synchronization of the GICC05 and AICC2012 age models for Greenland and Antarctic ice cores