Late Glacial and Holocene data from sediment cores MD99-2236 and MD99-2256

The Laurentide Ice Sheet (LIS) was a large, dynamic ice sheet in the early Holocene. The glacial events through Hudson Strait leading to its eventual demise are recorded in the well-dated Labrador shelf core, MD99-2236 from the Cartwright Saddle. We develop a detailed history of the timing of ice-sheet discharge events from the Hudson Strait outlet of the LIS during the Holocene using high-resolution detrital carbonate, ice rafted detritus (IRD), d18O, and sediment color data. Eight detrital carbonate peaks (DCPs) associated with IRD peaks and light oxygen isotope events punctuate the MD99-2236 record between 11.5 and 8.0 ka. We use the stratigraphy of the DCPs developed from MD99-2236 to select the appropriate DeltaR to calibrate the ages of recorded glacial events in Hudson Bay and Hudson Strait such that they match the DCPs in MD99-2236. We associate the eight DCPs with H0, Gold Cove advance, Noble Inlet advance, initial retreat of the Hudson Strait ice stream (HSIS) from Hudson Strait, opening of the Tyrrell Sea, and drainage of glacial lakes Agassiz and Ojibway. The opening of Foxe Channel and retreat of glacial ice from Foxe Basin are represented by a shoulder in the carbonate data. DeltaR of 350 years applied to the radiocarbon ages constraining glacial events H0 through the opening of the Tyrell Sea provided the best match with the MD99-2236 DCPs; DeltaR values and ages from the literature are used for the younger events. A very close age match was achieved between the 8.2 ka cold event in the Greenland ice cores, DCP7 (8.15 ka BP), and the drainage of glacial lakes Agassiz and Ojibway. Our stratigraphic comparison between the DCPs in MD99-2236 and the calibrated ages of Hudson Strait/Bay deglacial events shows that the retreat of the HSIS, the opening of the Tyrell Sea, and the catastrophic drainage of glacial lakes Agassiz and Ojibway at 8.2 ka are separate events that have been combined in previous estimates of the timing of the 8.2 ka event from marine records. SW Iceland shelf core MD99-2256 documents freshwater entrainment into the subpolar gyre from the Hudson Strait outlet via the Labrador, North Atlantic, and Irminger currents. The timing of freshwater release from the LIS Hudson Strait outlet in MD99-2236 matches evidence for freshwater forcing and LIS icebergs carrying foreign minerals to the SW Iceland shelf between 11.5 and 8.2 ka. The congruency of these records supports the conclusion of the entrainment of freshwater from the retreat of the LIS through Hudson Strait into the subpolar gyre and provides specific time periods when pulses of LIS freshwater were present to influence climate.

Lithology, micropaleontology and isotope record of sediments from the Nordic Seas

On the basis of various lithological, mircopaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data reveal considerable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record, providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at ~26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic delta18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ~13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRD deposition and major input of meltwater ceased. During this time, two depletions in epibenthic delta13C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ~8 cal. kyr, warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to today's steep east-west surface temperature gradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.

Documentation and sedimentology of core PS1021-1 from the Antarctic continental margin off Kap Norvegia

Sedimentological analyses concerning ice rafted debris, grain size distribution, biogenous components, and clay mineral composition of four sediment cores from the Antarctic continental margin off Kapp Norvegia reveal a cyclical pattern of three different sediment facies. These are classified into warm and cold types representing warm and cold climatic periods and a short transition period from cold to warm events. The sedimentological parameters reflect the variations within the cryosphere and the hydrosphere, which are directly influenced by the climatic fluctuations. The unusually high content of carbonaceous planktonic and benthonic foraminifera in these polar sediments, as well as the interfingering of terrigeneous and biogeneous-rich sediments with increasing distance from the continent, might reflect the influence of the Weddell Sea Polynya and the oscillations of polynya, pack-ice and ice shelf extent during the late Pleistocene.

Sedimentology of cores from the Bellingshausen Sea, Antarctic

On the continental rise west of the Antarctic Peninsula there are nine large mounds interpreted as sediment drifts, separated by turbidity current channels. Drift 7 is 150 km long, 70 km wide and up to 700 m high and is asymmetric, with steep sides on the south-east (towards the continent) and south-west, and gentle slopes to north-west and north-east. Cores on the gentle sides of the drift show a cyclicity between brown, bioturbated, diatom-bearing mud with foraminifera and radiolarians, and grey, laminated, barren mud. Biostratigraphic evidence is consistent with a Late Quaternary age. Detailed lithostratigraphy and magnetic susceptibility data allow precise correlation over distances of tens of kilometres. On the basis of chemostratigraphy, the brown sediment is interpreted as interglacial (isotope stages 1 and 5) and the grey as glacial (stages 2-4 and 6). Sedimentation rates are 3.0-5.5 cm/ka. Cores on the steep sides of the drift recovered a condensed section with thinner cycles and hiatuses. Fine grain size, very poor sorting and the absence of a mode in the silt size range indicate deposition from suspension with only weak current activity, There is little evidence for cyclic changes in bottom current strength. Supply of sediment to the benthic nepheloid layer was by entrainment ofmud from turbidity currents, and by settling ofpelagic material (biogenic grains, IRD, sediment suspended in meltwater plumes). Cyclic changes in sediment supply include more biogenic supply in interglacials with less sea ice cover, more terrigenous supply from turbidites in glacials with ice sheets grounded to the shelf edge, and changes in IRD content.

Oceanographic investiations from the Sea of Ochotsk

The Sea of Okhotsk is a marginal sea of the Pacific Ocean, which is characterized by strong variations in the productivity and sediment supply due to sea ice transport and river input. Furthermore the variations in the hydrological cycle determine the formation of the SOIW (Sea of Okhotsk Intermediate Water) which plays an important role in the ventilation processes in the intermediate water of the N-Pacific. Isotope data measured on planktonic and benthic foraminifera, sedimentological and geochemical studies of sediment cores and surface samples from the Sea of Okhotsk are used to reconstruct the paleoceanography during the past 350.000 years. The dating and correlation of the sediments are based on oxygen isotope stratigraphy, absolute ages, magnetic susceptibility as well as a detailled tephrachronology of the entire basin. The sedimentation rates are characterized by temporal and spatial variations. The maximum sedimentation rate takes place at the continental slope off Sakhalin due to the input of the Amur River, the sea ice drift and the high productivity. The sedimentation rate in the eastern part of the Sea of Okhotsk is generelly high because of the influence of the nutrient-rich Kamchatka Current. In the central and northern parts of the Sea of Okhotsk, areas with low productivity and reduced terrestrial supply, the sedimentation rate is the lowest. The analyses of the surface sediment samples make it possible to characterize the (sub)- recent sediment supply and transportation processes. The bulk sediment measurements, isotope data and the accumulation rate of ice-rafted debris (IRD) show a dominant sea ice cover and a region with a high productivity as well as a high Amur River input in the western part of the sea. The eastern part of the Sea of Okhotsk, however, is marked by the predominance of warm and nutrient-rich water masses coming from the Kamchatka Current which restricts the sea ice cover. This is reflected in low content of ice-rafted debris and high productivity proxies as well as in isotope data. The deposits of the Sea of Okhotsk are characterized by terrestrial, biogenic and volcanogenic sediment input which varies temporally and spatially. Here, the sedimentation pattern is dominated by the terrestrial input. Bulk sediment measurements and sample analyses of the > 63 micron particle input make it possible to distinguish glacial and interglacial fluctuations. The sedimentation processes during glacial times are determined by a high content of ice-rafted debris, whereas the primary production is higher during interglacial periods. During the last glacial/interglacial cycle the IRD-distribution pattern indicates a strong sea ice transport in the western part and in large areas of the open sea in the eastern part of the Sea of Okhotsk with a relatively constant ice-drift system. The IRD flux in sediments of the oxygen isotope Stage 6 reflects a new sedimentation pattern in the eastern part of the sea. This high IRD accumulation rate indicates ice advances beyond the shelf margin and an iceberg transport from NE-E direction into the Sea of Okhotsk. The several large, brief, negative anomalies in d13C values of Neogloboquadrina pachyderma (s) show releases of methane from basin sediments which correspond to periods of relative sea level falls. The high sedimentation rates on the Sakhalin slope allow insights into the climatic history in Holocene and indicate shorter-scale variations oscillation in Stage 3, which correlate with the global climatic changes. These variations are described as Dansgaard-Oeschger cycles in Greenland ice cores and as Heinrich-Events in several marine sediment cores from the N-Atlantic.

(Table 1) Age determination on monospecies planktonic foraminifera of sediment core AN25-934

Reconstruction of regional climate and the Okhotsk Sea (OS) environment for the Last Glacial Maximum (LGM), deglaciation and Holocene were performed on the basis of high-resolution records of ice rafted debris (IRD), CaCO3, opal, total organic carbon (TOC), biogenic Ba (Ba_bio) and redox sensitive element (Mn, Mo) content, and diatom and pollen results of four cores that form a north-southern transect. Age models of the studied cores were earlier established by AMS 14C data, oxygen - isotope chronostratigraphy and tephrochronology. According to received results, since 25 ka the regional climate and OS environmental conditions have changed synchronously with LGM condition, cold Heinrich event 1, Bølling -Allerød (BA) warming, Younger Dryas (YD) cooling and Pre-Boreal (PB) warming recorded in the Greenland ice core, North Atlantic sediment, and China cave stalagmites. Calculation of IRD MAR in sediment of north-south transect cores indicate an increase of sea ice formation several times in the glacial OS as compared to the Late Holocene. Accompanying ice formation, increased brine rejection and the larger potential density of surface water at the north shelf due to a drop of glacial East Asia summer monsoon precipitation and Amur River run off, led to strong enhancement of the role of the OS in glacial North Pacific Intermediate Water (NPIW) formation. The remarkable increase in OS productivity during BA and PB warming was probably related with significant reorganisation of the North Pacific deep water ventilation and nutrient input into the NPIW and OS Intermediate Water (OSIW). Seven Holocene OS millennial cold events based on the elevated values of the detrended IRD stack record over the IRD broad trend in the sediments of the studied cores have occurred synchronously with cold events recorded in the North Atlantic, Greenland ice cores and China cave stalagmites after 9 ka. Diatom production in the OS were mostly controlled by sea ice cover changes and surface water stratification induced by sea-ice melting; therefore significant opal accumulation in sediments of this basin begin from 4-6 ka ago simultaneously with a remarkable decrease of sea ice cover.

Sedimentology of the Okhotsk Sea

On the basis of two sedimentary records from the central Sea of Okhotsk, we reconstruct the closely coupled glacial/interglacial changes in terrigenous flux, marine productivity, and sea ice coverage over the past 1.1 Myr. The correspondance of our sedimentary records to the China loess grain size record (China loess particle timescale, CHILOPARTS) suggests that environmental changes in both the Sea of Okhotsk area and in SE Asia were closely related via the Siberian atmospheric high-pressure cell. During full glacial times our records point to a strong Siberian High causing northerly wind directions, the extension of the sea ice cover, and a reduced Amur River discharge. Deglacial maxima of terrigenous flux were succeeded by or synchronous to high-productivity events. Marine productivity was strengthened during glacial terminations because of an effective nutrient utilization at times of enhanced water column stratification and high nutrient supply from fluvial runoff and sea ice thawing. During interglacials, SE monsoonal winds prevailed, analogous to today's summer situation of a pronounced Mongolian Heat Low and a strong Hawaiian High. Strong freshwater discharge induced by high precipitation rates in the Amur drainage area and a seasonally reduced and mobile sea ice cover favored marine productivity (although being considerably lower than during the terminations) and a lowered flux of ice-rafted detritus.

Sedimentology of the northwestern Weddel Sea continental shelf and slope

Geologie cores on two profiles oriented normaly to the continental shelf and slope, have been investigated to reconstruct the Quaternary sedimentary history of the southeast continental border of South Orkney (NW Weddell Sea). The sediments were described macroscopically and their fabric investigated by use of X-radiographs. Laboratory work comprised detailed grain-size analysis, determination of the watercontent, carbonate, organic carbon and sand fraction.composition. Stable oxygen and carbon isotopes have been measured On planktonic foraminifera. Palaeomagnetism, analysis of 230Th-content and detailed comparison of the lithlogic Parameters with the oxygen isotope stages (Martinson curve) were used for stratigraphic classification of the sediments. The sediment cores from the continental slope comprise a maximum age of 300,000 years B. P.. Bottom currents, ice rafting and biogenic input are the main sources of sediment. Based on lithologic parameters a distinction between glacial and interglacial facies is possible. Silty clays without microfossils and few bioturbation characterise the sediments of the glacial facies. Only small amounts of icerafted debris can be recognized. This type of sediment was accumulated during times of lower sea-level and drastically reduced rate of bottom water production. Based on grain-size distribution, bottom current velocities of 0.01 cmls were calculated. Thick sea-ice coverage reduced biogenic production in the surface water, and as consequence benthic communities were depleted. Because of the reduced benthic life, sediments are only slithly bioturbated. At the beginning of the interglacial Stage, the sea-level rised rapidly, and calving rate of icebergs, combined with input of ice-rafted material, increased considerably. Sediments of this transition facies are silty cliiys with a high proportion of coarse ice-rafted debris, but without microfossils. With the onset of bottom water production in connection with shelf ice water, sediments of interglacial facies were formed. They consist of silty clays to clayey silts with considerable content of sand and gravel. Sediments are strongly bioturbated. Based On the sediment caracteristics, current velocities of the bottom water were calculated to be of 0.96 cmls for interglacials. At the southern slope of a NW/SE-striking ridge, bottom water current is channelized, resulting in a drastic increase of current velocities. Current velocities up to 7.5 cm/s lead to formation of residual sediments. While the continental slope has predominantly fine sediments, the South Orkney shelf are mainly sandy silts and silty sands with a high proportion of gravel. These sediments were formed dominantly by ice-rafting during Brunhes- and Matuyama-Epoch. Currents removed the fine fraction of the sediments. Based on microfossil contents it was not possible to differentiate sediments from glacial to interglacial. In the upper Parts of the cores graded sequences truncated by erosion were observed. These sequences were formed during Brunhes-Epoch by strong currents with velocities decreasing periodically from about 7.5 cm/s to about 1 cm/s. Sediments with a high proportion of siliceous microfossils but barren of foraminifera compose the lower part of the shelf cores. These sediments have formed during the warmer Matuyama-Epoch.

Sedimentology on two cores from Scoresby Sund, Greenland

Reconstruction of the postglacial palaeoenvironmental evolution was the main objective of marine geological investigations in the Scorcsby Sund fjord system. For this purpose, samples of marine sediments, taken on RV Polarstern cruises ARK-V/3b and ARK-VII/3b in 1988 and 1990, have been analysed. All investigated fjord sediments are paratills. However, remarkable changes in sediment fabric and composition occur with depth in cores. They are attributable to different modes of sediment deposition. Therefore, a subdivision of the postglacial palaeoenvironmental history into periods of considerably different sedimentary conditions is feasible. The change of sedimentary fades with time is interpreted by deposition under changing climatic conditions during the postglacial. Displacements of cyclonic and anticyclonic centers in the atmosphere change amount of precipitation at the east coast of Greenland. Precipitation strongly influences extension of local ice caps of coastal areas and duration of coverage of the fjords by sea ice. These factors again control the sedimentary regime in the fjord system.

Investigations on sediment coring site PS1823 in the Riiser-Larsen Sea, Southern Ocean

Physical and sedimentological investigations were carried out on a 14 m long gravity core and a 0.5 m long box core from 4440 m water depth off Queen Maud Land, East-Antarctica. Strongly bioturbated hemipelagic muds of predominantly terrigenous origin and a very small biogenic part build up the 'Normal-Facies'. Several sandy to silty layers are inserted in the 'Normal-Facies'. These layers are seperated by lithology, structure and the investigated parameters of this study and are interpreted as turbidites. The source area for the turbidity currents is supposed to be at the uppermost continental margin, close to the shelf break and there is evidenee for this gravity transport within the erosive Ritscher-Canyon, which extends close to the core position. The distribution of biogenic components indicates an age of 1.3 million years or more, with an average sedimentation rate of about 1 cm/1000 years. Early diagenetic proeesses caused water loss by compaction, errosion and dissolution of biogenic components and precipitation and recrystallization of manganese micronodules. Cyclic fluctuations of the sediment-parameters within the 'Normal-Facies' enable the distinction of a 'Glazial'- and an 'Interglazial'-Facies. The 'Glazial'-Facies reflects glacial sedimentary conditions and shows a dark olive gray colour, high susceptibility, low silt/clay-ratios, only a few biogenic components and the regular occurence of interrelated turbidite layers. In contrast, the 'Interglazial'-Facies is dominated by a light olive or olive-brown colour, low susceptibility, high silt/clay-ratios and an increased number of biogenic components. This facies corresponds to interglacial conditions. Three main processes are supposed to have been responsible for the observed facies changes: (1) the bottom water mass circulation, (2) the gravity transport by turbidity currents and (3) the biogenic surface production. These processes are related to the quaternary climatic changes. The extension of the ice shelves directed the gravity transport to the deep sea and the formation of Antarctic Bottom Water, which in turn influenced the silt/clay-ratios in the sediment record. Fluctuations in sea ice coverage controlled the biogenic surface production.

Sedimentology on cores off Filchner Shelf Ice, Weddell Sea, Antarctica

The sediments of 14 box cores and 7 gravity cores, mainly taken directly in front of the Filchner(-Ronne) ice shelf northwest of Berkner Island (Weddell Sea), allowed to distinguish six sediment types. On the one hand,the retreat of the at first grounded and then floated ice from the last glacial maximum is documented. On the other hand,the sediments give an insight into extensive Holocene sediment deposition and remobilization northwest of Berkner Island. The ortho till was deposited directly by the grounded ice sheet and is lacking any marine influence. After floating of the ice shelf, partly very weIl stratified, partly unstratified, non-bioturbated paratill is deposited beneath the ice shelf. Lack of IRD-content in the paratill immediately above the orthotill indicates freezing at the bottom of the ice, at least for a short period after the ice became afloat. The orthotill and paratill contain small amounts of fragmented Tertiary diatoms, which allow the conclusion, that glacial-marine sediments in the accumulation area of the Ronne ice shelf will be eroded and later deposited by ice in the investigation area. Starting of bioturbation and therefore change in sedimentation from paratill to bioturbated paratill,is caused by the retreat of the ice shelf to its actual position. Isostatic uplift of the sea-bed after the Ice Age causes minor water depths with higher current velocities. The fine-fraction is eroding and mean particle-size will increase. Maybe, also isostatic uplift is responsible for repeated great advances of the floated ice shelf as shown in an erosional horizon in some cores containing bioturbated paratill. Postglacial sediment-thicknesses exceed 3 m. Assuming floating of the ice 15.000 YBP, accumulation rates reach nearly 20cm/lOOO years. Following the theories about sediment input in front of wide ice shelves, this was not expected. In the shallower water depths of Berkner Bank, the oscillations of the ice shelf are recorded in the sediments. Sorting and redistribution by high current velocities from beneath the ice up to the calving line, lead to the deposition of the weIl to very weIl sorted sandy till. In front of the calving line the finer fraction will settle down. Remobilization is possible by bioturbation and increasing current-velocity. According to the intensity of mixing of the sandy till with the fine fraction, modified till or muddy till results.