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.

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.

Geochemistry of surface sediment samples from the South Atlantic Ocean

The aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity. For this purpose, 190 surface sediment samples from the South Atlantic Ocean were analysed for their barium and aluminium concentrations. Biogenic barium is estimated by subtracting the calculated terrigenous barium (obtained from the terrigenous Ba/Al ratio and the amount of Al in the sample) from the total Ba content in the sample. Based on the accumulation rates of biogenic barium, export production is estimated using three different algorithms proposed by [Paleoceanography 7 (1992) 163, doi:10.1029/92PA00181; Global Biogeochem. Cycles 9 (1995) 289, doi:10.1029/95GB00021; Geomar. Report 38 (1995) 105]. Primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters. Only the primary productions calculated on the basis of the algorithm of [Paleoceanography 7 (1992) 163, doi:10.1029/92PA00181] yield productivity values comparable to those existing in ocean surface waters. This study further reveals that it is not sufficient to use a constant, generally applicable organic carbon/biogenic barium ratio, as is postulated by [Global Biogeochem. Cycles 9 (1995) 289, doi:10.1029/95GB00021]. This ratio has to be assessed regionally. For the sediments of the Cape Basin in the eastern South Atlantic Ocean, a new algorithm is developed which gives plausible primary productivities for the overlying surface waters.

Sea-surface temperature reconstructions for the Quaternary western Atlantic

Paleotemperature estimates calculated by the SIMMAX Modern Analog Technique are presented for two gravity cores from the Rio Grande Rise, one from the Brazil Slope, and one from the Ceara Rise. The estimates are based on comparisons between modern and fossil planktonic foraminiferal assemblages and were carried out on samples from Quaternary sediments. Estimated warm-season temperatures from the Rio Grande Rise (at approx. 30° S) range from around 19°C to 24°C, with some coincidence of warm peaks with interglacial stages. The temperature estimates (also warm-season) from the more tropical Brazil Slope (at approx. 8° S) and Ceara Rise (at approx. 4° N) cores are more stable, remaining between 26°C and 28°C throughout most of their lengths. This fairly stable situation in the tropical western Atlantic is interrupted in oxygen isotope stage 6 by a significant drop of 2-3°C in both of these cores. Temperature estimates from the uppermost samples in all cores compare very well to the modern-day measured values. Affinities of some foraminiferal species for warmer or cooler surface temperatures are identified within the temperature range of the examined samples based on their abundance values. Especially notable among the warmer species are, Globorotalia menardii, Globigerinita glutinata, Globigerinoides ruber, and Globigerinoides sacculifer. Species indicative of cooler surface temperatures include Globorotalia inflata, Globigerina bulloides, Neogloboquadrina pachyderma, and Globigerina falconensis. A cluster analysis was carried out to assist in understanding the degree of variation which occurs in the foraminiferal assemblages, and how temperature differences influence the faunal compositions of the samples. It is demonstrated that fairly similar samples may have unexpectedly different estimated temperatures due to small differences in key species and, conversely, quite different assemblages can result in similar or identical temperature estimates which confirms that other parameters than just temperature affect faunal content.

Dialogo di Vincentio Galilei ... della musica antica, et della moderna.

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Fresh gleanings; or, A new sheaf from the old fields of continental Europe.

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Posteriora Magistri Petri de Alliaco : cum additionibus magistri Roberti Cenalis.

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