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The dissolution behavior of biogenic calcites in seawater and a possible role for magnesium and organic carbon.

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Title: The dissolution behavior of biogenic calcites in seawater and a possible role for magnesium and organic carbon.
Authors: Subhas, Adam V.1 asubhas@whoi.edu, Rollins, Nick E.1, Berelson, William M.1, Erez, Jonathan1, Ziveri, Patrizia1, Langer, Gerald1, Adkins, Jess F.1
Source: Marine Chemistry. Sep2018, Vol. 205, p100-112. 13p.
Subjects: Dissolution (Chemistry), Carbon content of seawater, Magnesium, Coccolithophores, Carbon content of water, Reaction mechanisms (Chemistry)
Abstract: Abstract We present the dissolution kinetics of mixed planktic foraminifera, the benthic foraminifera Amphistegina , the coccolithophore Emiliania huxleyi , and the soft coral Rhythismia fulvum in seawater. Dissolution rates were measured across a large range of saturation states (Ω = 0.99–0.2) by dissolving 13C-labeled calcites in natural seawater undersaturated with respect to calcite. 13C-label was incorporated into biogenic calcite by culturing marine calcifiers in 13C-labeled natural seawater. Net dissolution rates were calculated as the slope of seawater δ 13C versus time in a closed seawater-calcite system. All calcites show distinct, nonlinear, dependencies on seawater saturation state when normalized by mass or by specific surface area. For example, coccolith calcite dissolves at a similar rate to inorganic calcite near equilibrium when normalized by surface area, but dissolves much more slowly far from equilibrium. Mass loss from foraminiferal tests is correlated with a decrease in Mg/Ca of the solid, indicating that Mg-rich phases are preferentially leached out at even mild undersaturations. Dissolution also appears to strongly affect test B/Ca. Finally, we provide an interpretation of surface area-normalized biogenic calcite dissolution rates as a function of their Mg and organic carbon content. Near-equilibrium dissolution rates of all calcites measured here show a strong, nonlinear dependence on Mg content. Far-from-equilibrium dissolution rates decrease strongly as a function of organic carbon content. These results help to build a framework for understanding the underlying mechanisms of rate differences between biogenic calcites, and bear important implications for the dissolution of high-Mg calcites in view of ocean acidification. [ABSTRACT FROM AUTHOR]
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Abstract:Abstract We present the dissolution kinetics of mixed planktic foraminifera, the benthic foraminifera Amphistegina , the coccolithophore Emiliania huxleyi , and the soft coral Rhythismia fulvum in seawater. Dissolution rates were measured across a large range of saturation states (Ω = 0.99–0.2) by dissolving 13C-labeled calcites in natural seawater undersaturated with respect to calcite. 13C-label was incorporated into biogenic calcite by culturing marine calcifiers in 13C-labeled natural seawater. Net dissolution rates were calculated as the slope of seawater δ 13C versus time in a closed seawater-calcite system. All calcites show distinct, nonlinear, dependencies on seawater saturation state when normalized by mass or by specific surface area. For example, coccolith calcite dissolves at a similar rate to inorganic calcite near equilibrium when normalized by surface area, but dissolves much more slowly far from equilibrium. Mass loss from foraminiferal tests is correlated with a decrease in Mg/Ca of the solid, indicating that Mg-rich phases are preferentially leached out at even mild undersaturations. Dissolution also appears to strongly affect test B/Ca. Finally, we provide an interpretation of surface area-normalized biogenic calcite dissolution rates as a function of their Mg and organic carbon content. Near-equilibrium dissolution rates of all calcites measured here show a strong, nonlinear dependence on Mg content. Far-from-equilibrium dissolution rates decrease strongly as a function of organic carbon content. These results help to build a framework for understanding the underlying mechanisms of rate differences between biogenic calcites, and bear important implications for the dissolution of high-Mg calcites in view of ocean acidification. [ABSTRACT FROM AUTHOR]
ISSN:03044203
DOI:10.1016/j.marchem.2018.08.001