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Geochemistry of benthic foraminifera grown under different carbonate system conditions
Insights into past marine carbon cycling and water mass properties can be obtained through controlled laboratory experiments with accurate seawater carbonate system (C-system) manipulations. Here, we explored the use of strontium/calcium ratio (Sr/Ca) of the calcite shells of benthic foraminifera as a potential seawater C-system proxy through a controlled growth experiment with two deep-sea species (Bulimina marginata and Cassidulina laevigata) and one intertidal species (Ammonia T6). To this aim, we decoupled carbonate chemistry in controlled growth experiments, i.e., changing pH at constant dissolved inorganic carbon (DIC) and changing DIC at constant pH. Four climatic chambers were used with different controlled concentrations of atmospheric pCO2 (180 ppm, 410 ppm, 1000 ppm, 1500 ppm). Our results demonstrated that low pH conditions (7.56-7.73) did not influence the survival and growth rates of the three species. However, low DIC conditions (879 µmol kg-1) negatively affected B. marginata and C. laevigata through low growth rate, whereas no effect was observed for Ammonia T6. Our results also showed a significant positive correlation between Sr/Ca and the C-system (DIC - bicarbonate ion concentration) for Ammonia T6 and B. marginata. For these two species, the regression models were coherent with published data (existing so far only for Ammonia T6) and showed overall similar slopes but different intercepts, implying species-specific effect. Furthermore, the Sr/Ca - C-system relationship was not impacted by ontogenetic trends between chamber stages, which is a considerable advantage for paleo-applications. However, no correlation with any of the C-system parameters was observed for Sr/Ca in C. laevigata. This might imply either a strong species-specific effect and/or a low tolerance to laboratory conditions leading to physiological stress, thereby impacting the Sr incorporation into the calcite lattice.
Carbonate system, Culture experiment, pCO2, pH, DIC, Total Alkalinity, Bicarbonate ions, Carbonate ions, Sr/Ca, Benthic foraminifera, Ocean acidification, Biomineralisation
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