Geochemical characterisation of living planktonic foraminifera in the Caribbean
Final Report Abstract
The trace metal composition and stable oxygen isotope ratios of the calcite tests of planktonic foraminifers are widely used for reconstructions of environmental conditions in the geological past. For applications in palaeothermometry, Mg/Ca ratios were calibrated with annual average sea surface temperatures and the Mg/Ca ratios of planktonic foraminifers from underlying surface sediments, which contain a mixture of tests from hundreds of years to several millennia. At high sea surface temperatures, as they prevail in the Caribbean Sea today, the calibration data sets show a large scatter. This reduces the accuracy of palaeotemperature estimates to several degrees centigrade, much higher than the seasonal sea surface temperature variability in the study region. Our initial hypothesis giving the impetus to this study was that this calibration can be significantly improved once: (1) the habitat and calcification depths of living planktonic foraminifera in the Caribbean is precisely known, (2) temperatures and salinities were accurately measured at their living depths, (3) water samples were taken from their habitats and analysed for their stable isotopic and trace metal composition. Furthermore, the data would offer to comprehensively describe abundance and regional distribution patterns of near-surface, thermocline, and deep-dwelling planktonic foraminifera in the Caribbean. Such a ground-truthing survey has not been attempted to date. The regional database would also reveal the species’ response to low salinities in the vicinity of the Orinoco mouth, the third-largest river shedding into the Atlantic Ocean. We collected in total 164 plankton samples, 41 sediment samples, and 131 water samples from the southeastern and central Caribbean, Yucatan and Florida Straits, and from the Gulf of Mexico on R/V Meteor cruise M78/1, M94, M95 and during a research visit at Magueyes Island Marine Station, Puerto Rico, for our project. Globigerinoides sacculifer, Globorotalia glutinata and Globigerinoides ruber were the dominant species of the Caribbean fauna in the upper water column in spring, and they also dominated the subfossil assemblages in surface sediments. Local acme occurrences and distinct depth distributions were recognised in many species. Such variegated distribution patterns in time and space presumably mirrored the scattered distribution of food sources and light intensities, which are important for symbiont-bearing foraminifera. Low surface water salinity and a presumably high turbidity expelled these symbiont-bearing planktonic foraminifera from the Orinoco River plume. We found many evidences for vertical migrations of species during their life cycle. The usual reproduction around full moon in surface waters can be postponed once the water is too murky from fine-grained sediments stirred up during storm events. Stormy weather also stimulated a massive floating of meroplanktonic as well as genuine benthic foraminiferal species, a response pattern we could observe during the transit of Hurricane “Sandy” at Puerto Rico for the first time. Geochemical data analyses and isotope measurements of water samples are still in progress. Preliminary results on G. sacculifer suggest a systematic stable oxygen isotope offset of -0.4 ‰ with reference to equilibrium calcite values of the ambient seawater. The tests were mainly formed in near-surface waters even though the specimens may have been caught at deeper levels. To our surprise, Mg/Ca ratios of living G. sacculifer show a large scatter at a given temperature, and the variability is to the same scale as reported from near-surface sediments in the literature. Laser ablation measurements revealed that the scatter is also mirrored in successive chambers of a single specimen, probably due to influences of yet unknown physiological or environmental factors. A refinement of the Mg/Ca temperature calibration can therefore not be achieved by analysing living specimens, the geochemical proxy data are at their limits. If more precise palaeo sea-surface temperatures are required, estimates from modern analogue techniques or neuronal network applications of planktonic foraminiferal census data should be used. If the physiological or environmental factors influencing the trace metal composition of the tests are to be investigated, oceanic mesocosm experiments are the most prospective strategy, even though many new technical developments are needed before successful experiments with living planktonic foraminifera floating in mesocosms could be made.
Publications
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2013. Comparison of Ba/Ca and δ18OWATER as freshwater proxies: a multi-species core-top study on planktonic foraminifera from the vicinity of the Orinoco River mouth. Earth and Planetary Science Letters, 383: 45-57
Bahr, A., Schönfeld, J., Hoffmann, J., Voigt, S., Aurahs, R., Kucera, M., Flögel, S., Jentzen, A., Gerdes, A.
