Age determination and stable isotope ratios of three sediment cores from the Great Belt

The marine transgression Into the Baltic Sea through the Great Belt took place around 9,370 calibrated C-14-years B.P. The sedimentary sequence from the early brackish phase and the change to marine conditions has been investigated in detail through C-14-datings, and oxygen and carbon isotope measurements, and is interpreted by comparison with modern analogs. The oldest brackish sediments are the strongly laminated clays and silts rich in organic carbon followed by non-laminated heavily bioturbated silts. The bedding and textural characteristics and stable isotope analyses on Ammonia beccarii (dextral) and A. beccarii (sinistral) show that the deposltlonal conditions respond to a change at about 9,100 cal. a B.P. from an unstratified brackish water environment in the initial stage of the Littorina Transgression to a thermohaline layered milieu in the upper unit. The oxygen isotope results indicate that the bottom waters of this latter period had salinities and temperatures comparable to the present day Kiel Bay waters. The isotopic composition of the total organic carbon and the d13C-values of A. beccarii reveal a gradual change from an initially lacustrine/terrestrial provenance toward a brackish/marine dominated depositional environment. A stagnation of the sea level at around 9,100 to 9,400 B.P. is indicated.

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.