Influence of temperature and CO2 on the strontium and magnesium composition of coccolithophore calcite

[摘要] Marine calcareous sediments provide a fundamental basis for palaeoceanographicstudies aiming to reconstruct past oceanic conditions and understand keybiogeochemical element cycles. Calcifying unicellular phytoplankton(coccolithophores) are a major contributor to both carbon and calcium cyclingby photosynthesis and the production of calcite (coccoliths) in the euphoticzone, and the subsequent long-term deposition and burial into marinesediments. Here we present data from controlled laboratory experiments onfour coccolithophore species and elucidate the relation between the divalentcation (Sr, Mg and Ca) partitioning in coccoliths and cellular physiology(growth, calcification and photosynthesis). Coccolithophores were culturedunder different seawater temperature and carbonate chemistry conditions. Thepartition coefficient of strontium (D_Sr) was positivelycorrelated with both carbon dioxide (pCO₂) and temperature butdisplayed no coherent relation to particulate organic and inorganic carbonproduction rates. Furthermore, D_Sr correlated positively withcellular growth rates when driven by temperature but no correlation waspresent when changes in growth rates were pCO₂-induced. Our resultsdemonstrate the complex interaction between environmental forcing andphysiological control on the strontium partitioning in coccolithophorecalcite and challenge interpretations of the coccolith Sr / Ca ratio fromhigh-pCO₂ environments (e.g. Palaeocene–Eocene thermal maximum). Thepartition coefficient of magnesium (D_Mg) displayedspecies-specific differences and elevated values under nutrient limitation.No conclusive correlation between coccolith D_Mg and temperaturewas observed but pCO₂ induced a rising trend in coccolithD_Mg. Interestingly, the best correlation was found betweencoccolith D_Mg and chlorophyll a production, suggestingthat chlorophyll a and calcite associated Mg originate from the sameintracellular pool. These and previous findings indicate that Mg istransported into the cell and to the site of calcification via differentpathways than Ca and Sr. Consequently, the coccolith Mg / Ca ratio should bedecoupled from the seawater Mg / Ca ratio. This study gives an extendedinsight into the driving factors influencing the coccolith Mg / Ca ratio andshould be considered for future palaeoproxy calibrations.

[学科分类] 地球化学与岩石