Dissolution Kinetics of Biogenic Carbonates in Seawater
Adam Subhas, Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, United States, Jonathan Erez, Hebrew University of Jerusalem, Earth Sciences, Jerusalem, Israel, Nick Rollins, University of Southern California, Earth Sciences, Los Angeles, United States, Jess F Adkins, California Institute of Technology, Pasadena, CA, United States, William Berelson, University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States, Patrizia Ziveri, Ph.D.., Universitat Autònoma de Barcelona, Institute of Environmental Science and Technology, Bellaterra, Spain and Gerald Langer, University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom
Abstract:
We present here laboratory-based dissolution kinetic studies of inorganic, foraminiferal, and coccolith calcite, as well as high-Mg calcite from the soft coral
Rhythismia fulvum. Dissolution rate measurements were conducted using a novel
13C-based tracer measurement, which traces mass loss via isotopic enrichment in a closed system of calcite and seawater. Inorganic calcite dissolution rates are highly nonlinear with respect to saturation state, and imply multiple dissolution mechanisms. Our work validates early characterizations of highly nonlinear dissolution rates, and also agrees with far-from-equilibrium rate determinations.
Dissolution rates of biogenic materials are also highly nonlinear, although the relationship between undersaturation and dissolution rate often does not resemble that of inorganic calcite. Coccoliths dissolve slowly compared to inorganic calcite after surface area normalization, even when treated with bleach to remove their organic matter. High-Mg calcite from soft corals dissolves quickly, and the relationship between dissolution rate and undersaturation seems to be complicated by the presence of two (Mg-carbonate and Ca-carbonate) phases. These biogenic dissolution rates provide a new constraint on the sensitivities of these minerals to saturation state, both in the deep ocean water column and in the surface ocean as saturation state is decreasing due to ocean acidification.
