Development of a mechanistic understanding of elements incorporation into biogenic calcite (foraminifera)

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Dissard, D. (2009): Development of a mechanistic understanding of elements incorporation into biogenic calcite (foraminifera) , PhD thesis, PhD thesis.
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The elemental composition of biogenic carbonates has become, in the last decade, an important tool to reconstruct past oceanic conditions. My thesis aimed to optimise these reconstructions by developing a mechanistic understanding of the biotic processes that control divalent cation concentration into calcitic foraminiferal tests. On timescales much shorter than their residence time, the elements strontium and magnesium occur in seawater with nearly constant ratios to calcium. Variation in Sr/Ca and Mg/Ca in foraminiferal tests can then be explained as a function of environmental parameters that control their incorporation into the tests. For foraminifera, temperature appears to be the dominant parameter controlling the incorporation of Mg, however, a better understanding of the possible impact of other parameters such as pH or [CO32-], and salinity is needed to increase the accuracy of element ratio proxies. Culture studies carried out under controlled physico-chemical conditions can rule out microhabitat effects and provide the best opportunity to deconvolve potential environmental effects. However, laboratory experiments with benthic foraminifera are long-lasting as their life cycle is typically longer than that of other unicellular marine calcifier, and therefore fastidious. Analytical improvement allows to determine elemental concentration on a single chamber. The latter is crucial in the investigation of benthic foraminifera since the growth of new chambers under laboratory conditions do not face the difficulties associated with experiments involving reproduction. The addition of chambers can be recognized by the incorporation of the fluorescent dye calcein. In order to use this experimental technique, the potential impact of calcein on the elemental composition of calcium carbonate (both biotic and abiotic) was investigated. In that aim, specimens of the shallow water benthic foraminifera Ammonia tepida were cultured in the presence and absence of calcein (15°C, salinity 33), and Mg and Sr in newly formed chambers were analysed. Additionally, the impact of calcein on Mg and Sr incorporation in inorganically precipitated calcium carbonate crystals was quantified. Results show that presence of calcein does not significantly impact the incorporation of Mg and Sr into biologically and inorganically precipitated calcium carbonate (publication I). In the light of results from publication I, several culture experiments were performed in order to quantify in isolation the effect of temperature and salinity on Mg and Sr incorporation into benthic foraminifera calcite. Individuals of Ammonia tepida were cultured under three different salinities (20, 33 and 40) and two different temperatures (10 and 15°C). Weights and elemental composition were determined. Results indicate that both Mg and Sr incorporation are enhanced with increasing temperatures and increasing salinity (salinity increase of 2 results in enhanced Mg incorporation equivalent to 1°C temperature increase). However, the temperature dependency for Sr disappears when the distribution factor DSr is plotted as a function of calcite saturation state (Ω). This suggests that a kinetic process related to Ω is responsible for the observed dependency of Sr incorporation on sea water temperature. Alternatively, Mg incorporation appeared independent of calcification rate. (publication II). To assess the sensitivity of benthic foraminifera to changing carbon dioxide levels and subsequent alteration in seawater carbonate chemistry, a second series of experiment were performed. Individuals of Ammonia tepida were maintained under two concentrations of atmospheric CO2 (120 and 2000 ppm) and two temperatures (10°C and 15°C). Shell weights and elemental compositions were determined. Results indicate that shell weights decrease with decreasing [CO32-], and increase with decreasing temperature. Changes in [CO32-] or total dissolved inorganic carbon do not affect the Mg partition coefficient. On the contrary, Sr incorporation is enhanced under increasing [CO32-] (publication III). Finally, Mg/Ca, Sr/Ca ratio and oxygen isotope composition of Globigerinoides sacculifer specimens collected in surface waters (0-10m) along an eastern transect of the tropical Atlantic Ocean were measured, in order to test and further calibrate geochemical proxies based on planktonic foraminifera species. This allowed for establishment of (1) a new calibration for the paleo-temperature reconstruction equation considering Mg/Ca but also Sr/Ca ratios and, (2) a new calibration for the oxygen isotopes paleo-temperature equation for surface waters collected G. sacculifer specimens. Subsequently, several paleo-reconstruction scenarios were tested in which, one, two, or three reconstruction equations were considered. Results indicate that foraminiferal Mg/Ca ratios allow for an accurate reconstruction of surface water temperature. In contrast, δ18Ow can only be reconstructed within a precision of about ±0.5. No reliable salinity reconstruction could be performed in this study (publication VI).

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