Last glacial maximum sediment record of the southern Weddell Sea shelf

Abstract: The history of grounded ice-sheet extent on the southern Weddell Sea shelf during the Last Glacial Maximum (LGM) and the timing of post-LGM ice-sheet retreat are poorly constrained. Several glaciological models reconstructed widespread grounding and major thickening of the Antarctic Ice Sheet in the Weddell Sea sector at the LGM. In contrast, recently published onshore data and modelling results concluded only very limited LGM-thickening of glaciers and ice streams feeding into the modern Filchner and Ronne ice shelves. These studies concluded that during the LGM ice shelves rather than grounded ice covered the Filchner and Ronne troughs, two deep palaeo-ice stream troughs eroded into the southern Weddell Sea shelf. Here we review previously published and unpublished marine geophysical and geological data from the southern Weddell Sea shelf. The stratigraphy and geometry of reflectors in acoustic sub-bottom profiles are similar to those from other West Antarctic palaeo-ice stream troughs, where grounded ice had advanced to the shelf break at the LGM. Numerous cores from the southern Weddell Sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments. These data sets give evidence that grounded ice had advanced across the shelf during the past, thereby grounding in even the deepest parts of the Filchner and Ronne troughs. Radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited, but indicate that the ice grounding occurred at the LGM and that ice retreat started before ~15.1 corrected 14C kyrs before present (BP) on the outer shelf and before ~7.7 corrected 14C kyrs BP on the inner shelf, which is broadly synchronous with ice retreat in other Antarctic sectors. The apparent mismatch between the ice-sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles (similar to those of "ice plains") that had advanced through Filchner Trough and Ronne Trough at the LGM. Considering the global sea-level lowstand of ~130 metres below present, a low surface slope of the expanded LGM-ice sheet in the southern Weddell Sea can reconcile grounding-line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland. This scenario implies that ice-sheet growth in the Weddell Sea sector during the LGM and ice-sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea-level fluctuations during these times.

Organic carbon accumulation in the South Atlantic Ocean

A compilation of 1118 surface sediment samples from the South Atlantic was used to map modern seafloor distribution of organic carbon content in this ocean basin. Using new data on Holocene sedimentation rates, we estimated the annual organic carbon accumulation in the pelagic realm (>3000 m water depth) to be approximately 1.8*10**12 g C/year. In the sediments underlying the divergence zone in the Eastern Equatorial Atlantic (EEA), only small amounts of organic carbon accumulate in spite of the high surface water productivity observed in that area. This implies that in the Eastern Equatorial Atlantic, organic carbon accumulation is strongly reduced by efficient degradation of organic matter prior to its burial. During the Last Glacial Maximum (LGM), accumulation of organic carbon was higher than during the mid-Holocene along the continental margins of Africa and South America (Brazil) as well as in the equatorial region. In the Eastern Equatorial Atlantic in particular, large relative differences between LGM and mid-Holocene accumulation rates are found. This is probably to a great extent due to better preservation of organic matter related to changes in bottom water circulation and not just a result of strongly enhanced export productivity during the glacial period. On average, a two- to three-fold increase in organic carbon accumulation during the LGM compared to mid-Holocene conditions can be deduced from our cores. However, for the deep-sea sediments this cannot be solely attributed to a glacial productivity increase, as changes in South Atlantic deep-water circulation seem to result in better organic carbon preservation during the LGM.

(Table 2) Ba/Ca ratios of Cibicides wuellerstorfi and Orbulina universa from Bahaman sediment cores

This geochemical investigation utilizes Ba/Ca in the benthic foraminifer Cibicides wuellerstorfi from cores taken from the Bahama Banks and the Caribbean Sea to reconstruct changes in basal thermocline ventilation (800-1000 m) and middepth thermohaline circulation (1000-2000 m) in the western North Atlantic during the last glacial period, focusing on the deglacial transition. Previous studies show that an increase in ventilation of the North Atlantic subtropical gyre during the Last Glacial Maximum (LGM) caused a 30-60% decrease in labile nutrients within the thermocline layer. Using foraminiferal Ba/Ca as a proxy of refractory nutrients, increased ventilation during the LGM produced a depletion of less than 20% compared to Holocene values. Following glaciation, the production of Glacial North Atlantic Intermediate Water (GNAIW) shut down owing to the presence of meltwater in the surface ocean, which resulted in a decrease in ventilation, as seen by an enrichment of barium in the basal thermocline. GNAIW was subsequently replaced by barium-rich southern component water in the middepth western North Atlantic. Foraminiferal Ba/Ca data suggest a 38% contribution from southern component water to a depth as shallow as 1475 m and a 14% contribution at 1123 m during deglaciation.

Sedimentary record of the western Amundsen Sea Embayment after the last glacial maximum

Reliable dating of glaciomarine sediments deposited on the Antarctic shelf since the Last Glacial Maximum (LGM) is very challenging because of the general absence of calcareous (micro-) fossils and the recycling of fossil organic matter. As a consequence, radiocarbon (14C) ages of the acid-insoluble organic fraction (AIO) of the sediments bear uncertainties that are very difficult to quantify. In this paper we present the results of three different chronostratigraphic methods to date a sedimentary unit consisting of diatomaceous ooze and diatomaceous mud that was deposited following the last deglaciation at five core sites on the inner shelf in the western Amundsen Sea (West Antarctica). In three cores conventional 14C dating of the AIO in bulk sediment samples yielded age reversals down-core, but at all sites the AIO 14C ages obtained from diatomaceous ooze within the diatom-rich unit yielded similar uncorrected 14C ages ranging from 13,517±56 to 11,543±47 years before present (yr BP). Correction of these ages by subtracting the core-top ages, which are assumed to reflect present-day deposition (as indicated by 21044 Pb dating of the sediment surface at one core site), yielded ages between ca. 10,500 and 8,400 calibrated years before present (cal yr BP). Correction of the AIO ages of the diatomaceous ooze by only subtracting the marine reservoir effect (MRE) of 1,300 years indicated deposition of the diatom-rich sediments between 14,100 and 11,900 cal yr BP. Most of these ages are consistent with age constraints between 13.0 and 8.0 ka BP for the diatom-rich unit, which we obtained by correlating the relative palaeomagnetic intensity (RPI) records of three of the sediment cores with global and regional reference curves for palaeomagnetic intensity. As a third dating technique we applied conventional 53 radiocarbon dating of the AIO included in acid-cleaned diatom hard parts that were extracted from the diatomaceous ooze. This method yielded uncorrected 14C ages of only 5,111±38 and 5,106±38 yr BP, respectively. We reject these young ages, because they are likely to be overprinted by the adsorption of modern atmospheric carbon dioxide onto the surfaces of the extracted diatom hard parts prior to sample graphitisation and combustion for 14C dating. The deposition of the diatom-rich unit in the western Amundsen Sea suggests deglaciation of the inner shelf before ca. 13 ka BP. The deposition of diatomaceous oozes on other parts of the Antarctic shelf around the same time, however, seems to be coincidental rather than directly related.

Sedimentary record of the western Amundsen Sea Embayment

The Amundsen Sea Embayment (ASE) drains approximately 35% of the West Antarctic Ice Sheet (WAIS) and is one of the most rapidly changing parts of the cryosphere. In order to predict future ice-sheet behaviour, modellers require long-term records of ice-sheet melting to constrain and build confidence in their simulations. Here, we present detailed marine geological and radiocarbon data along three palaeo-ice stream tributary troughs in the western ASE to establish vital information on the timing of deglaciation of the WAIS since the Last Glacial Maximum (LGM). We have undertaken multi-proxy analyses of the cores (core description, shear strength, x-radiographs, magnetic susceptibility, wet bulk density, total organic carbon/nitrogen, carbonate content and clay mineral analyses) in order to: (1) characterise the sedimentological facies and depositional environments; and (2) identify the horizon(s) in each core that would yield the most reliable age for deglaciation. In accordance with previous studies we identify three key facies, which offer the most reliable stratigraphies for dating deglaciation by recording the transition from a grounded ice sheet to open marine environments. These facies are: i) subglacial, ii) proximal grounding-line, and iii) seasonal open-marine. In addition, we incorporate ages from other facies (e.g., glaciomarine diamictons deposited at some distance from the grounding line, such as glaciogenic debris flows and iceberg rafted diamictons and turbates) into our deglacial model. In total, we have dated 78 samples (mainly the acid insoluble organic (AIO) fraction, but also calcareous foraminifers), which include 63 downcore and 15 surface samples. Through careful sample selection prior to dating, we have established a robust deglacial chronology for this sector of the WAIS. Our data show that deglaciation of the western ASE was probably underway as early as 22,351 calibrated years before present (cal 44 yr BP), reaching the mid-shelf by 13,837 cal yr BP and the inner shelf to within c.10-12 km of the present ice shelf front between 12,618 and 10,072 cal yr BP. The deglacial steps in the western ASE broadly coincide with the rapid rises in sea-level associated with global meltwater pulses 1a and 1b, although given the potential dating uncertainty, additional, more precise ages are required before these findings can be fully substantiated. Finally, we show that the rate of ice-sheet retreat increased across the deep (up to1,600 m) basins of the inner shelf, highlighting the importance of reverse slope and pinning points in accelerated phases of deglaciation.

Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum

Based on the quantitative study of diatoms and radiolarians, summer sea-surface temperature (SSST) and sea ice distribution were estimated from 122 sediment core localities in the Atlantic, Indian and Pacific sectors of the Southern Ocean to reconstruct the last glacial environment at the EPILOG (19.5-16.0 ka or 23 000-19 000 cal yr. B.P.) time-slice. The statistical methods applied include the Imbrie and Kipp Method, the Modern Analog Technique and the General Additive Model. Summer SSTs reveal greater surface-water cooling than reconstructed by CLIMAP (Geol. Soc. Am. Map Chart. Ser. MC-36 (1981) 1), reaching a maximum (4-5 °C) in the present Subantarctic Zone of the Atlantic and Indian sector. The reconstruction of maximum winter sea ice (WSI) extent is in accordance with CLIMAP, showing an expansion of the WSI field by around 100% compared to the present. Although only limited information is available, the data clearly show that CLIMAP strongly overestimated the glacial summer sea ice extent. As a result of the northward expansion of Antarctic cold waters by 5-10° in latitude and a relatively small displacement of the Subtropical Front, thermal gradients were steepened during the last glacial in the northern zone of the Southern Ocean. Such reconstruction may, however, be inapposite for the Pacific sector. The few data available indicate reduced cooling in the southern Pacific and give suggestion for a non-uniform cooling of the glacial Southern Ocean.

Age, 232Th, 230Th, 238U, CaCO3, organic carbon, lithogenic fraction and opal concentrations of 4 sediment cores from the southwest Pacific, with calculated 230Th-normalized mass flux

No clear scenario has yet been able to explain the full carbon drawdown that occurred during the Last Glacial Maximum (LGM); however, increased export production (EP) in the Subantarctic Zone (SAZ) of the Southern Ocean due to iron (Fe) fertilisation has been proposed to have provided a key mechanism affecting the air-sea partitioning of carbon. We chronicle changes in marine EP based on four sediment cores in Subtropical Waters (STW) and SAZ around New Zealand since the LGM. For the first time in this region, we present 230-Thorium normalised fluxes of biogenic opal, carbonate (CaCO3), excess Barium (xsBa), and organic Carbon (Corg). In STW and SAZ, these flux variations show that EP did not change markedly since the LGM. The only exception was a site in the SAZ close to the STF, where we suggest the STF shifted over the core site, driving increased EP. To understand why EP was mostly low and constant we investigated dust deposition changes by measuring lithogenic fluxes at the four sites. These data are coherent with an increased dust deposition in the southwest Pacific during the LGM. Additionally, we infer an increased lithogenic material discharge from erosion and glacier melts during the deglaciation, limited to the Campbell Plateau. Therefore, we propose that even though increased glacial dust deposition may have relieved Fe limitation within the SAZ, the availability of silicic acid (Si(OH)4) limited any resultant increase in carbon export during the LGM. Consequently, we infer low Si(OH)4 concentrations in the SAZ that have not significantly changed since the LGM. This result suggests that both Si(OH)4 and Fe co-limit EP in the SAZ around New Zealand, which would be consistent with modern process studies.

Changements climatiques et écologiques dans le nord de l’Alaska au cours de la glaciation du Wisconsinien : le Yedoma de la rivière Itkillik

Le climat continental et froid de la Béringie lors de la glaciation du Wisconsinien a conduit à la formation d’une forme relique de pergélisol syngénétique nommé yedoma. Ces dépôts ont permis la préservation d’indicateurs environnementaux très diversifiés qui peuvent être employés pour reconstituer la dynamique climatique et écologique de la Béringie avant le dernier maximum glaciaire. À ce jour, peu d’études ont été réalisées au nord de la chaîne de montagnes Brooks (Alaska) et l’hétérogénéité écologique régionale de la Béringie Est lors de la glaciation du Wisonsinien reste mal définie. Ce mémoire porte sur une reconstitution paléoenvironnementale de plus de 39 ka du nord de l’Alaska réalisée à partir de sédiments provenant du Yedoma de la rivière Itkillik. Les objectifs sont (1) de reconstituer l’histoire de la végétation avec l’analyse pollinique; (2) de reconstituer les températures de juillet, le contraste de température saisonnier et l’ensoleillement de juillet avec la technique des analogues modernes et (3) de mettre les données biogéochimiques et glaciologiques du site en lien avec le climat reconstitué. L’étude montre que vers 35 ka BP (Interstade du Wisconsinien Moyen), des conditions climatiques semblables à l’actuel ont favorisé l’accumulation de tourbe riche en carbone organique. À partir de 29,7 ka BP, les températures de juillet reconstituées diminuent, alors que la continentalité du climat semble augmenter. Le contenu en glace des sédiments est plus alors plus faible et la pluie pollinique devient dominée par Poaceae, Artemisia et autres herbacés non graminoïdes. Ces indicateurs suggèrent des conditions environnementales plus xériques qu’aujourd’hui. Les anomalies isotopiques de 18O, 2H et l’excès de deutérium confirment un épisode d’avancée glaciaire (Wisconsinien Tardif). Après 17,9 ka BP (Tardiglaciaire), les températures de juillet et le contraste saisonnier augmentent. Les valeurs de contenu en carbone organique des sédiments sont plus élevées et la plus grande disponibilité en eau favorise l’établissement d’un couvert herbacé moderne dominé par les Cyperaceae.

(Table 1) Age determination of sediment core KC073

We present a high-resolution paleoceanographic record of deglaciation based on diatom assemblages from a core located just south of the Polar Front in the southwest Atlantic. Core KC073 is from a sediment drift at the mouth of the Falkland Trough and contains sediments from the Last Glacial Maximum (LGM) to present, dated using radiocarbon dates on bulk organic matter and radiolarian stratigraphy. The site lies along the path of the Antarctic Circumpolar Current (ACC) and immediately downstream of where North Atlantic Deep Water (NADW) is entrained into the ACC. Significant variations in ocean conditions are reflected in high-amplitude changes in diatom concentrations and assemblage composition. The diatom assemblage at the LGM indicates that winter sea ice extent was at least 5° farther north than present until at least 19.0 ka (calendar years) and summer sea ice may have occasionally extended over the site, but for the most part it lay to the south. During deglaciation, Chaetoceros resting spores (CRS) dominate the diatom assemblage with valve concentrations in excess of 500 * 10**6 valves per gram. Submillennial-scale variations in the numbers of CRS and Thalassiosira antarctica occur throughout the late deglacial and dominate the changes in diatom concentration. We propose that the influx of CRS is controlled by the flow of NADW over the Falkland Plateau. As such our data provide unique evidence that NADW impacted on this sector of the Southern Ocean during deglaciation. During the Holocene the sedimentation rate dramatically reduced. We suggest that the ACC flow increased over the site and inhibited settling and winnowed the surface sediments.

Dissolution index of Globigerina bulloides in recent and Last Glacial Maximum sediments

The modern Atlantic Ocean, dominated by the interactions of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), plays a key role in redistributing heat from the Southern to the Northern Hemisphere. In order to reconstruct the evolution of the relative importance of these two water masses, the NADW/AABW transition, reflected by the calcite lysocline, was investigated by the Globigerina bulloides dissolution index (BDX?). The depth level of the Late Glacial Maximum (LGM) calcite lysocline was elevated by several hundred metres, indicating a more corrosive water mass present at modern NADW level. Overall, the small range of BDX? data and the gradual decrease in preservation below the calcite lysocline point to a less stratified Atlantic Ocean during the LGM. Similar preservation patterns in the West and East Atlantic demonstrate that the modern west-east asymmetry did not exist due to an expansion of southern deep waters compensating for the decrease in NADW formation.

Geochemistry for sediment core MD02-2496

Variations in chalcophile and redox-sensitive trace elements are examined at high-resolution intervals from a ~50 kyr long sediment core (MD02-2496) from the Vancouver Island margin. Enrichments of Ag, Cd, Re, U, and Mo above lithogenous levels, signifying sedimentary suboxia and anoxia, occurred during the early Holocene and Bølling/Allerød, and during warm interstadial events of Marine Isotope Stage (MIS) 3. Down-core trace element profiles co-vary with productivity proxy records (opal, CaCO3, and marine organic carbon), and with sedimentary nitrogen isotope ratios, which reflect variably enriched nitrate upwelled from intermediate waters that were transported northward from the Eastern Tropical North Pacific. The similarity of the MD02-2496 record with records from the southern portion of the California Current System (CCS), and to the Greenland ice core oxygen isotope record during warm climate intervals, suggests that sedimentary redox conditions along the California Current responded to local productivity, to North Atlantic climate change and to tropical Pacific surface water processes via long-distance teleconnections. Concentrations of trace elements and productivity proxies were relatively depleted during the Younger Dryas, cool stadial events of MIS 3, and in two episodes of glaciomarine sedimentation from ~14.7 to 30.5 kyr BP (last glacial maximum, LGM), and from 44 to 50.4 kyr BP. Cordilleran Ice Sheet advancement onto the Vancouver Island continental shelf during the LGM led to intervals of increased terrigenous sedimentation and greatly reduced productivity not seen in the southern portion of the CCS, and along with ventilation of North Pacific Intermediate Waters, resulted in brief sedimentary oxic conditions.

Smear slide analysis of sediment core GeoB5836-2

Laminated sediments spanning the last 20,000 years (though not continuously) in the Shaban Deep, a brine-filled basin in the northern Red Sea, were analyzed microscopically and with backscattered electron imagery in order to determine laminae composition with emphasis on the diatomaceous component. Based on this detailed study, we present schematic models to propose paleoflux scenarios for laminae formation at different time-slices. The investigated core (GeoB 5836-2; 26°12.61'N, 35°21.56'E; water depth 1475 m) shows light and dark alternating laminae that are easily distinguishable in the mid-Holocene and at the end of the deglaciation (13-15 ka) period. Light layers are mainly composed of coccoliths, terrigenous material and diatom fragments, while dark layers consist almost exclusively of diatom frustules (monospecific or mixed assemblages). The regularity in the occurrence of coccolith/diatom couplets points to an annual deposition cycle where contrasting seasons and associated plankton blooms are represented (diatoms-fall/winter deposition, coccoliths-summer signal). We propose that, for the past ~15,000 years, the laminations represent two-season annual varves. Strong dissolution of carbonate, with the concomitant loss of the coccolith-rich layer in sediments older than 15 ka, prevents us from presenting a schematic model of annual deposition. However, the diatomaceous component reveals a marked switch in species composition between Last Glacial Maximum (LGM) sediments (dominated by Chaetoceros resting spores) and sediments somewhat younger (18-19 ka; dominated by Rhizosolenia). We propose that different diatom assemblages reflect changing conditions in stratification in the northern Red Sea: Strong stratification conditions, such as during two meltwater pulses at 14.5 and 11.4 ka, are reflected in the sediment by Rhizosolenia layers, while Chaetoceros-dominated assemblages represent deep convection conditions.

Radiocarbon dating and alkenone-based proxies of sediment core GeoB7702-3

In this study we utilize two organic geochemical proxies, the Uk'37 index and TEX86, to examine past sea surface temperatures (SST) from a site located near the Nile River Delta in the eastern Mediterranean (EM) Sea. The Uk'37 and TEX86 records generally are in agreement and indicate SST ranges of 14°C-26°C and 14°C-28°C, respectively, during the last 27 cal ka. During the Holocene, TEX86-based SST estimates are usually higher than Uk'37-based SST estimates, which is likely due to seasonal differences between the timing of the haptophyte and crenarchaeota blooms in the EM and is related to the onset of the modern flow regime of the Nile River. Both records show that SST varied on centennial to millennial timescales in response to global climate events, i.e., cooling during the Last Glacial Maximum (LGM), Heinrich event 1 (H1), and the Younger Dryas (YD) and warming during the Bølling-Allerød and in the early Holocene during deposition of sapropel S1. The H1 cooling was particularly severe and is marked by a drop in SST of ~4.5°C in comparison to pre-H1 SST, with temperatures >1°C cooler than during the LGM. In contrast to high-latitude and western Mediterranean records, which indicate both an abrupt onset and termination of the YD event, the transition from the YD to the Holocene was much more gradual in the EM.

Cadmium analytical results from sediment and foraminifera shells

There has been a major contradiction between benthic foraminiferal Cd/Ca and d13C data concerning the labile nutrient chemistry of the Southern Ocean during the Last Glacial Maximum (LGM). Cd data indicates that LGM South Atlantic nutrient concentrations were as low as they are today, indicative of a persistent influx of nutrient-depleted North Atlantic Deep Water (NADW). d13C data indicates that LGM South Atlantic nutrient concentrations were much higher than at present (even higher than anywhere else in the ocean at that time), and these data have been interpreted as signifying the complete shutdown ofthe export of NADW into the global ocean. This paper examines both true geochemical differences and various confounding foraminiferal artifacts for both tracers. While many different processes and artifacts affect both tracers in the margin, we conclude the discrepancy is mainly due to the "Mackensen Effect" of low foraminiferal d13C as a result of high carbon flux to the sediments, and that LGM Atlantic Sector Southern Ocean nutrient concentrations remained similar to the levels encountered today.

(Table 1) Radiocarbon ages and different calibrated ages for sediment core HLY0503-18TC obtained during the HOTRAX expedition

A short sediment core from a local depression forming an intra basin on the Lomonosov Ridge, was retrieved during the Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX). It contains a record of the Marine Isotope Stages (MIS) 1-3 showing exceptionally high abundances of calcareous microfossils during parts of MIS 3. Based on radiocarbon dating, linear sedimentation rates of 7-9 cm/ka persist during the last deglaciation. The Last Glacial Maximum (LGM) is partly characterized by a hiatus. Planktic foraminiferal abundance variations of Neogloboquadrina pachyderma sinistral and calcareous nannofossils reflect changes in Arctic Ocean summer sea ice coverage and probably inflow of subpolar North Atlantic water. Calibration of the radiocarbon ages, using modeled reservoir corrections from previous studies and the microfossil abundance record of the studied core, results in marine reservoir ages of 1400 years or more, at least during the last deglaciation. Paired benthic-planktic radiocarbon dated foraminiferal samples indicate a slow decrease in age difference between surface and bottom waters from the Lateglacial to the Holocene, suggesting circulation and ventilation changes.