Water chemistry and sediment age of lake Terrasovoje, Amery Oasis, East Antarctica

The late Quaternary palaeoenvironmental history of the southern Windmill Islands, East Antarctica, has been reconstructed using diatom assemblages from two long, well-dated sediment cores taken in two marine bays. The diatom assemblage of the lowest sediment layers suggests a warm climate with mostly open water conditions during the late Pleistocene. During the following glacial, the Windmill Islands were covered by grounded ice preventing any in situ bioproductivity. Following deglaciation, a sapropel with a well-preserved diatom assemblage was deposited from ~10?500 cal yr BP. Between ~10?500 and ~4000 cal yr BP, total organic carbon (Corg) and total diatom valve concentrations as well as the diatom species composition suggest relatively cool summer temperatures. Hydrological conditions in coastal bays were characterised by combined winter sea-ice and open water conditions. This extensive period of glacial retreat was followed by the Holocene optimum (~4000 to ~1000 cal yr BP), which occurred later in the southern Windmill Islands than in most other Antarctic coastal regions. Diatom assemblages in this period suggest ice-free conditions and meltwater-stratified waters in the marine bays during summer, which is also reflected in high proportions of freshwater diatoms in the sediments. The diatom assemblage in the upper sediments of both cores indicates Neoglacial cooling from ~1000 cal yr BP, which again led to seasonally persistent sea-ice on the bays. The Holocene optimum and cooling trends in the Windmill Islands did not occur contemporaneously with other Antarctic coastal regions, showing that the here presented record reflects partly local environmental conditions rather than global climatic trends.

Age model, stable isotopes, major element proportions and endmember unmixing results from four western tropical Atlantic cores

We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial periods. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. Increased Amazonian rainfall reflects the intensification of the South American monsoon in response to enhanced land-ocean thermal gradient and moisture convergence. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganizations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.

Chemical composition of sediment core GeoB12309-5 from the northern Arabian Sea, its pore water profile and age depth relationship as well as CTD data obtained during cruise M74/3

The Arabian Sea off the Pakistan continental margin is characterized by one of the world's largest oxygen minimum zones (OMZ). The lithology and geochemistry of a 5.3 m long gravity core retrieved from the lower boundary of the modern OMZ (956 m water depth) were used to identify late Holocene changes in oceanographic conditions and the vertical extent of the OMZ. While the lower part of the core (535 - 465 cm, 5.04 - 4.45 cal kyr BP, Unit 3) is strongly bioturbated indicating oxic bottom water conditions, the upper part of the core (284 - 0 cm, 2.87 cal kyr BP to present, Unit 1) shows distinct and well-preserved lamination, suggesting anoxic bottom waters. The transitional interval from 465 to 284 cm (4.45 - 2.87 cal kyr BP, Unit 2) contains relicts of lamination which are in part intensely bioturbated. These fluctuations in bioturbation intensity suggest repetitive changes between anoxic and oxic/suboxic bottom-water conditions between 4.45 - 2.87 cal kyr BP. Barium excess (Baex) and total organic carbon (TOC) contents do not explain whether the increased TOC contents found in Unit 1 are the result of better preservation due to low BWO concentrations or if the decreased BWO concentration is a result of increased productivity. Changes in salinity and temperature of the outflowing water from the Red Sea during the Holocene influenced the water column stratification and probably affected the depth of the lower boundary of the OMZ in the northern Arabian Sea. Even if we cannot prove certain scenarios, we propose that the observed downward shift of the lower boundary of the OMZ was also impacted by a weakened Somali Current and a reduced transport of oxygen-rich Indian Central Water into the Arabian Sea, both as a response to decreased summer insolation and the continuous southward shift of the Intertropical Convergence Zone during the late Holocene.

Planktic and benthic 14C reservoir ages, calibrated by a suite of 14C plateaus in the glacial-to-deglacial Suigetsu atmospheric 14C record

We here present a compilation of planktic and benthic 14C reservoir ages for the Last Glacial Maximum (LGM) and early deglacial from 11 key sites of global ocean circulation in the Atlantic and Indo-Pacific Ocean. The ages were obtained by 14C plateau tuning, a robust technique to derive both an absolute chronology for marine sediment records and a high-resolution record of changing reservoir/ventilation ages (Delta14C values) for surface and deep waters by comparing the suite of planktic 14C plateaus of a sediment record with that of the atmospheric 14C record (Sarnthein et al., 2007, doi:10.1029/173GM13). Results published thus far used as atmospheric 14C reference U/Th-dated corals, the Cariaco planktic record, and speleothems (Fairbanks et al., 2005, doi:10.1016/j.quascirev.2005.04.007; Hughen et al., 2006, doi:10.1016/j.quascirev.2006.03.014; Beck et al., 2001, doi:10.1023/A:1008175728826). We have now used the varve-counted atmospheric 14C record of Lake Suigetsu terrestrial macrofossils (Ramsey et al., 2012, doi:10.1126/science.1226660) to recalibrate the boundary ages and reservoir ages of the seven published records directly to an atmospheric 14C record. In addition, the results for four new cores and further planktic results for four published records are given. Main conclusions from the new compilation are: (1) The Suigetsu atmospheric 14C record on its varve counted time scale reflects all 14C plateaus, their internal structures and relative length previously identified, but implies a rise in the average 14C plateau age by 200-700 14C yr during LGM and early deglacial times. (2) Based on different 14C ages of coeval atmospheric and planktic 14C plateaus, marine surface water Delta14C may have temporarily dropped to an equivalent of ~0 yr in low-latitude lagoon waters, but reached >2500 14C yr both in stratified subpolar waters and in upwelled waters such as in the South China Sea. These values differ significantly from a widely assumed constant global planktic Delta14C value of 400 yr. (3) Suites of deglacial planktic Delta14C values are closely reproducible in 14C records measured at neighboring core sites. (4) Apparent deep-water 14C ventilation ages (equivalents of benthic Delta14C), deduced from the sum of planktic Delta14C and coeval benthic-planktic 14C differences, vary from 500 up to >5000 yr in LGM and deglacial ocean basins.

Age determination and model of sediment core RAPiD-15-4P

Greenland ice core records indicate that the last deglaciation (~7-21 ka) was punctuated by numerous abrupt climate reversals involving temperature changes of up to 5°C-10°C within decades. However, the cause behind many of these events is uncertain. A likely candidate may have been the input of deglacial meltwater, from the Laurentide ice sheet (LIS), to the high-latitude North Atlantic, which disrupted ocean circulation and triggered cooling. Yet the direct evidence of meltwater input for many of these events has so far remained undetected. In this study, we use the geochemistry (paired Mg/Ca-d18O) of planktonic foraminifera from a sediment core south of Iceland to reconstruct the input of freshwater to the northern North Atlantic during abrupt deglacial climate change. Our record can be placed on the same timescale as ice cores and therefore provides a direct comparison between the timing of freshwater input and climate variability. Meltwater events coincide with the onset of numerous cold intervals, including the Older Dryas (14.0 ka), two events during the Allerød (at ~13.1 and 13.6 ka), the Younger Dryas (12.9 ka), and the 8.2 ka event, supporting a causal link between these abrupt climate changes and meltwater input. During the Bølling-Allerød warm interval, we find that periods of warming are associated with an increased meltwater flux to the northern North Atlantic, which in turn induces abrupt cooling, a cessation in meltwater input, and eventual climate recovery. This implies that feedback between climate and meltwater input produced a highly variable climate. A comparison to published data sets suggests that this feedback likely included fluctuations in the southern margin of the LIS causing rerouting of LIS meltwater between southern and eastern drainage outlets, as proposed by Clark et al. (2001, doi:10.1126/science.1062517).