Magnesium/Calcium ratios in benthic foraminifera from surface sediments

We used modern epibenthic foraminifer tests of Cibicidoides mundulus and Planulina wuellerstorfi from South Atlantic core top sediments in order to establish Mg/Ca-temperature relationships for the temperature range from 0 to 15°C. We obtained the following calibrations: Mg/Ca (mmol/mol) = 0.830*exp(0.145*BWT (°C)) for P. wuellerstorfi, and Mg/Ca (mmol/mol) = 0.627*exp(0.143*BWT (°C)) for C. mundulus. However, a number of tests, especially those bathed in North Atlantic Deep Water, revealed higher Mg/Ca ratios than predicted from the calibration. Our data suggest that d[CO3 2-] of bottom water exerts a significant control on dMg/Ca (temperature-corrected) of C. mundulus (dMg/Ca = 0.017*d[CO3 2-] -0.14), while dMg/Ca of P. wuellerstorfi is more likely to be governed by TCO2 (dMg/Ca = -0.007*TCO2 + 15). Since both d[CO3 2-] and TCO2 are closely linked to [CO3 2-], it is inferred that carbonate ion acts as secondary control, after temperature, on benthic shell Mg/Ca below -4°C. A drop in [CO3 2-] by 25 ?mol/kg at 4 km water depth, as suggested for the Last Glacial Maximum, would decrease Mg/Ca by up to 0.4 mmol/mol, which leads to an underestimation of bottom water temperature by -3.5°C. Therefore our results indicate that the Mg/Ca thermometer should be used cautiously for benthic foraminifers where changes in the carbonate chemistry are present in the paleoceanographic record.

Granulometry, calcium carbonate content and radiocarbon ages of soil profiles obtained in Quinghai Province, northeast Tibet

Colluvial deposits consisting of silts and loams were detected in several climatologically different areas of NE Tibet (3200-3700 m a.s.l.). Layering, distinct organic content and low content of coarse matter as well as location in the relief revealed an origin from low-energy slope erosion (hillwash). Underlying and intercalated paleosols were classified as Chernozems, Phaeozems, Regosols and Fluvisols. Fifteen radiocarbon datings predominant on charcoal from both colluvial layers and paleosols yielded ages between 8988 ± 66 and 3512 ± 56 uncal BP. Natural or anthropogenic factors could have been the triggers of the erosional processes derived. It remains unclear which reason was mainly responsible, due to controversial paleoclimatic and geomorphic records as well as insufficient archaeological knowledge from this region. Determinations of charcoal and fossil wood revealed the Holocene occurrence of tree species (spruce, juniper) for areas which nowadays have no trees or only few forest islands. Thus large areas of NE Tibet which are at present steppes and alpine pastures were forested in the past.

Shell preservation of Limacina inflata in surface sediments of the Central and South Atlantic

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inflata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass configurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inflata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inflata per sample were found). This is most probably due to extensive "metabolic" aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inflata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between different water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived fine aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble surficial calcium carbonate data and delta13C and carbonate ion concentration profiles in the water column. Moreover, preservation of L. inflata within AAIW and UCDW improves significantly to the north, whereas carbonate corrosiveness diminishes due to increased mixing of AAIW and UNADW. The close relationship between LDX values and aragonite contents in the sediments shows much promise for the quantification of the aragonite loss under the influence of different water masses. LDX failure and uncertainties may be attributed to (1) aragonite dissolution due to bottom water corrosiveness, (2) aragonite dissolution due to additional CO2 release into the bottom water by the degradation of organic matter based on an enhanced supply of organic matter into the sediment, (3) variations in the distribution of L. inflata and hence a lack of supply into the sediment, (4) dilution of the sediments and hence a lack of tests of L. inflata, or (5) redeposition of sediment particles.

Compilation of 220 sediment core d13C data from the glacial Atlantic Ocean

We compare a compilation of 220 sediment core d13C data from the glacial Atlantic Ocean with three-dimensional ocean circulation simulations including a marine carbon cycle model. The carbon cycle model employs circulation fields which were derived from previous climate simulations. All sediment data have been thoroughly quality controlled, focusing on epibenthic foraminiferal species (such as Cibicidoides wuellerstorfi or Planulina ariminensis) to improve the comparability of model and sediment core carbon isotopes. The model captures the general d13C pattern indicated by present-day water column data and Late Holocene sediment cores but underestimates intermediate and deep water values in the South Atlantic. The best agreement with glacial reconstructions is obtained for a model scenario with an altered freshwater balance in the Southern Ocean that mimics enhanced northward sea ice export and melting away from the zone of sea ice production. This results in a shoaled and weakened North Atlantic Deep Water flow and intensified Antarctic Bottom Water export, hence confirming previous reconstructions from paleoproxy records. Moreover, the modeled abyssal ocean is very cold and very saline, which is in line with other proxy data evidence.

Data on the biogeochemical cycle of methane in the ocean

Geological, mineralogical and microbiological aspects of the methane cycle in water and sediments of different areas in the oceans are under consideration in the monograph. Original and published estimations of formation- and oxidation rates of methane with use of radioisotope and isotopic methods are given. The role of aerobic and anaerobic microbial oxidation of methane in production of organic matter and in formation of authigenic carbonates is considered. Particular attention is paid to processes of methane transformation in areas of its intensive input to the water column from deep-sea hydrothermal sources, mud volcanoes, and cold methane seeps.

Temperatures, salinity, wind speed, grain sizes and foraminifera abundance in the Great Belt Channel, western Baltic Sea

The Great Belt, the largest inlet to the Baltic Sea, has a deep and well defined channel system. A distinct thermohaline layer at roughly 18 to 20 m of water depth separates the saltier and generally cooler deeper North Sea water from the brackish and warmer surface water. It is practically a current dominated area, with the strongest bottom currents due to prolonged west winds. The size and shape of the surface sediments and their grain size distributions show a close relationship with the prevailing hydrographical conditions. Southerly current marks predominate while northerly directions are confined to 10 to 14 m of water depth. The degree of bioturbation is highest in the uppermost sedimentary cover where practically all original stratification has been destroyed. Various bioturbate structures have been identified with the fauna. Coiling ratios of Ammonia beccarii (Linnaeus) have been successfully applied for correlation in the postglacial sediments of the early Littorina Transgression. The succession shows that in the Boreal brackish water conditions were probably followed by peat and limnic sediments as the sea regressed. With the Littorina Transgression, the sea again entered the area and high sedimentation rates resulted in the major deposits of the Great Belt. At least for the last 4000 years, sedimentation rates had been very low. Present day currents sweep out the sediments, mainly to the southern marginal areas.