Development and validation of elemental ratio proxies in foraminifera and corals, using laboratory cultures, inorganic precipitation experiments, numerical small scale modelling and field investigations
Zusammenfassung der Projektergebnisse
The aim of "Paleoprox" was to bring together the expertise of Dutch and German research groups from several disciplines in order to develop and verify a mechanistic understanding of major and trace element proxies in key organisms used in paleo-climatology and -oceanography (foraminifera and corals). Towards this goal, we operated at six levels: 1) Provide a baseline for divalent cation incorporation using inorganic precipitation experiments. 2) Conduct laboratory experiments under controlled conditions with foraminifera to quantify "vital effects". 3) Conduct field experiments (stratified plankton tows, sediment traps) to determine "when and where" proxies are incorporated. 4) Employ numerical modeling to verify our "conceptual understanding" and to provide new questions. 5) Determine the impact of dissolution on elemental ratio proxies. 6) Develop new methods for analysis. "Paleoprox" has successfully been completed. More than 30 peer reviewed papers and seven PhD thesis have been published or are in the process of being finished. The consortium has generated many new national and intemational programmes and projects in which the Dutch and German partners continue to collaborate. In the following we will describe our major findings with respect to the six topics ouflined above: 1) Inorganic precipitation experiments We could demonstrate that the Sr/Ca ratio in calcite reflects the combined effects of the supersaturation and stoichiometry of the aqueous phase on the crystal growth kinetics. Both are effectively modulated by cellular processes in calcifying organisms, and may therefore contribute to the "vital effect" on Sr/Ca ratios. 2) Quantify "vital effects". Using laboratory experiments with the benthonic foraminifer, Ammonia tepida, we have demonstrated important differences with respect to the incorporation of Mg and Sr in their tests. While the Mg incorporation follows proposed thermodynamic considerations, albeit with a lower slope, is the Sr incorporation dependent on additional factors. The Sr incorporation increases with increasing salinity and can be explained by an increase in the rate of calcification which is induced by the associated increase of the carbonate saturation state with increasing salinity. The increase of the Sr incorporation with increasing temperature, on the other hand, may be explained by the increase of the calcite saturation state, independent of changes in the rate of calcification. Apparently, kinetic processes regulate the incorporation of Sr into foraminiferal calcite. 3) Determine "when and where" proxies are incorporated. We field-demonstrate for the first time that shell deposition fluxes of Hastigerina pelagica are synchronous with lunar periodicity. By contrast, no endogenous or exogenous lunar periodicity was observed in the deposition flux or size distribution of any of the 27 other shell species. The replacement of the foraminiferal population by a new generation, was determined to be a matter of days to weeks. Depth stratified plankton tows, sediment traps and box core surface observations from the same site indicate that of G. glutinata and G. trilobus are excellent indicators for sea surface temperature. G. ruber is rather an indicator of temperature of deep chlorophyll maximum zone, while size fractions of left-coiled G. truncatulinoides, --400 pm and ~250 pm, respectively, provide a good estimate of the summer sea temperature at 340 and 180 meter. 4) Numerical modeling to verify our "conceptual understanding" and to provide new questions. A novel modeling approach, CD-MUSIC, was developed to describe the chemical structure of, and small-scale processes at carbonate mineral-aqueous solution interfaces. The model has been used to successfully simulate the development of surface charges and surface potentials on divalent carbonate minerals as a function of the aqueous solution composition. We have also developed a mechanistic model for the incorporation of divalent cations into foraminifera tests, the "applied vital effects model". This model is suited for modelling Mg incorporation into foraminiferal calcite but requires the use of very small time steps, which will be implemented in the future. 5) Determine the impact of dissolution on elemental ratio proxies. Using a novel method to simulate natural seafloor dissolution, we demonstrated that the breakdown of microstructures on shell surfaces is systematic and may be useful to estimate the preservation state of shells deposited in sediments. Certain elements are preferentially leached out and analyses of Mg/Ca in partially dissolved shells reproduced previously observed shifts that lead to an underestimate of paleotemperatures. 6) Develop new methods for analysis. We have developed analytical methods not used before in the field of paleoceanography. We can now, for the first time, determine the boron isotope ratio in single foraminiferal shells using resonant post-ionization. Further projects are currently undertaken to expand this very promising collaboration with experimental physicists.
