Stable isotope ratios on foraminifera and age determinations from sediments of BP00 and BP01 cruises to the Kara Sea

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Mg/Ca ratios of Neogloboquadrina pachyderma (s) and Globigerina bulloides in Irminger Sea sediment trap time series

iven the importance of high-latitude areas in the ocean-climate system, there is need for a paleothermometer that is reliable at low temperatures. Here we assess the applicability of the Mg/Ca-temperature proxy in colder waters (5-10?°C) by comparing for the first time the seasonal Mg/Ca and d18O cycles of N. pachyderma (s) and G. bulloides using a sediment trap time-series from the northern North Atlantic. While both species show indistinguishable seasonal d18O patterns that clearly track the near surface temperature cycle, their Mg/Ca are very different. G. bulloides Mg/Ca is high (2.0-3.1 mmol/mol), but varies in concert with the seasonal temperature cycle. The Mg/Ca of N. pachyderma (s), on the other hand, is low (1.1-1.5 mmol/mol) and shows only a very weak seasonal cycle. The d18O patterns indicate that both species calcify in the same depth zone. Consequently, depth habitat differences cannot explain the contrasting Mg/Ca patterns. The elevated Mg/Ca in pristine G. bulloides might be due to the presence of high Mg phases that are not preserved in fossil shells. The contrasting absence of a seasonal trend in the Mg/Ca of N. pachyderma (s) confirms other studies where calcification temperatures were less well constrained. The reason for this absence is not fully known, but may include species-specific vital effects. The very different seasonal patterns of both species' Mg/Ca underscore the importance of parameters other than temperature in controlling planktonic foraminiferal Mg/Ca. Our results therefore lend further caution in the interpretation of Mg/Ca-temperature reconstructions from high northern latitudes.

Seaweed fails to prevent ocean acidification impact on foraminifera along a shallow-water CO2 gradient

Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow-water CO2 seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH 8.19) to one dominated by agglutinated foraminifera at elevated levels of CO2 (pH 7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.

Sediments in Arctic sea ice

Despite the Arctic sea ice cover's recognized sensitivity to environmental change, the role of sediment inclusions in lowering ice albedo and affecting ice ablation is poorly understood. Sea ice sediment inclusions were studied in the central Arctic Ocean during the Arctic 91 expedition and in the Laptev Sea (East Siberian Arctic Region Expedition 1992). Results from these investigations are here combined with previous studies performed in major areas of ice ablation and the southern central Arctic Ocean. This study documents the regional distribution and composition of particle-laden ice, investigates and evaluates processes by which sediment is incorporated into the ice cover, and identifies transport paths and probable depositional centers for the released sediment. In April 1992, sea ice in the Laptev Sea was relatively clean. The sediment occasionally observed was distributed diffusely over the entire ice column, forming turbid ice. Observations indicate that frazil and anchor ice formation occurring in a large coastal polynya provide a main mechanism for sediment entrainment. In the central Arctic Ocean sediments are concentrated in layers within or at the surface of ice floes due to melting and refreezing processes. The surface sediment accumulation in central Arctic multi-year sea ice exceeds by far the amounts observed in first-year ice from the Laptev Sea in April 1992. Sea ice sediments are generally fine grained, although coarse sediments and stones up to 5 cm in diameter are observed. Component analysis indicates that quartz and clay minerals are the main terrigenous sediment particles. The biogenous components, namely shells of pelecypods and benthic foraminiferal tests, point to a shallow, benthic, marine source area. Apparently, sediment inclusions were resuspended from shelf areas before and incorporated into the sea ice by suspension freezing. Clay mineralogy of ice-rafted sediments provides information on potential source areas. A smectite maximum in sea ice sediment samples repeatedly occurred between 81°N and 83°N along the Arctic 91 transect, indicating a rather stable and narrow smectite rich ice drift stream of the Transpolar Drift. The smectite concentrations are comparable to those found in both Laptev Sea shelf sediments and anchor ice sediments, pointing to this sea as a potential source area for sea ice sediments. In the central Arctic Ocean sea ice clay mineralogy is significantly different from deep-sea clay mineral distribution patterns. The contribution of sea ice sediments to the deep sea is apparently diluted by sedimentary material provided by other transport mechanisms.

Age determination of sediment cores from the Shelf Edge off Mauritania, Northwest Africa

The impact of late glacial changes on the sedimentary record was investigated in two long vibracores, collected from the shelf edge off Mauritania, northwest Africa. Lithology and radiocarbon dates indicate that the sedimentary sequences were mainly controlled by sea-level changes on the shelf. The upper Pleistocene sequence is characterized by deposition in coastal environments, while the Holocene sequence represents deposition in shelf environments. During low sea level, much sediment was supplied to the present outer shelf, and the data imply an average accumulation rate of up to 43.0 cm/1000 yrs during the late Pleistocene, which is substantially higher than the Holocene rate. Shelf sediments were continuously reworked and redistributed on a regional scale during falling and rising sea level. The presence of reworked material results in radiocarbon ages which are too old. The mollusc. Venus striatula, which presently is found north of, but not along, the Mauritanian coast, occurs in the upper Pleistocene sequence, suggesting cooler water conditions in the shelf during late glacial times. This species probably migrated to the south during late glacial times, following the southward extension of the cold Canary Current. Radiocarbon dates of the shells broadly coincide with a lowstand of sea level over this part of the continental shelf.

Foraminiferal trace element ratios from South Atlantic sediments

The rate of uranium accumulation in oceanic sediments from seawater is controlled by bottom water oxygen concentrations and organic carbon fluxes-two parameters that are linked to deep ocean storage of CO2. To investigate glacial-interglacial changes in what is known as authigenic U, we have developed a rapid method for its determination as a simple addition to a procedure for foraminiferal trace element analysis. Foraminiferal calcite acts as a low U substrate (U/Ca < 15 nmol/mol) upon which authigenic U accumulates in reducing sediments. We measured a downcore record of foraminiferal U/Ca from ODP Site 1090 in the South Atlantic and found that U/Ca ratios increase by 70-320 nmol/mol during glacial intervals. There is a significant correlation between U/Ca records of benthic and planktonic foraminiferal species and between U/Ca and bulk sediment authigenic U. These results indicate that elevated U/Ca ratios are attributable to the accumulation of authigenic U coatings in sediments. Foraminiferal Mn/Ca ratios were lower during the glacial intervals, suggesting that the observed U accumulation on the shells is not directly linked to U incorporation into secondary manganese phases. Thus, foraminiferal U/Ca ratios may provide useful information on past changes in sediment redox conditions.

Barium in sediments off northwest Africa

High-resolution down-core analyses of the solid phase content of total barium (Batot) and total organic carbon (TOC) back to 25 kyr B.P. were performed on a gravity core from the upper continental slope off Cape Yubi (Morocco). The observed discrepancy between the two potential paleoproductivity proxies, Batot and TOC, initiated supplementary examinations of the pore water, the geochemistry of the clay fraction, X-ray diffraction analyses, and the application of a sequential Ba extraction method of selected samples. Additionally, we analyzed down-core samples of the planktonic foraminifera Turborotalita quinqueloba and Globorotalia inflata for their Ba/Ca ratios. These analyses, which were performed for the first time on these species, were used to reconstruct past oceanic Ba concentrations. We suggest that in the study area, which is characterized by high accumulation rates, the preserved TOC content is a valuable proxy for past primary productivity, whereas the solid phase Batot contents appear to be affected by other mechanisms and factors. Peaks of total barium content in the clay fraction and of Ba/Ca ratios in the planktonic foraminifera shells found during the Younger Dryas and the Heinrich 1 event are likely to result from increased meltwater influx into the northern North Atlantic. We suggest that Ba-enriched meltwater was transmitted by the eastern boundary current system from higher latitudes to the region of the Canary Islands. Total barium contents of the clay fraction (Batot,clay) and Ba/Ca in planktonic foraminifera shells seem to be reliable proxies for this discharge of meltwater.