Dissolution rates of calcium carbonate in the deep ocean; an in-situ experiment in the North Atlantic Ocean |
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| Authors: | Susumu Honjo Jonathan Erez |
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| Institution: | Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 U.S.A. |
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| Abstract: | An in-situ water circulator (ISWAC) was developed to allow accurate measurement of dissolution rates of various carbonate particles with minimal stagnation and mechanical weight loss. Three ISWAC packages were deployed at 3600, 4800 and 5518 m for 79 days in the Sargasso Sea (Northwest Atlantic). Weight losses for different particles during 79 days at 5518 m were as follows: pteropods and synthetic aragonite, 72.8%; reagent calcite, 57.5%; foraminifera, 23–36%; coccoliths, 11.3–24%, and diatoms, 12%. These weight losses are 2.5–7.5 times higher than those reported in earlier in-situ experiments. Normalization of weight losses with respect to BET specific surface area for different calcite particles yielded specific dissolution rates that differed by more than 2 orders of magnitude. Bleached biogenic particles dissolve significantly faster than non-bleached although their surface area is identical. We suggest that the BET surface area does not represent the reactive surface area available for dissolution, especially in biogenic calcite particles. Coatings, probably of organic matter, may reduce the reactive surface area and thus retard dissolution rates.The existence of a chemical lysocline in the Northwest Atlantic was confirmed. However, it seems that different kinds of particles have different lysoclines. The origin of the lysocline cannot be attributed to water flow or to the thermodynamic transition from supersaturation to undersaturation (Ω = 1). It seems to be a kinetic phenomenon.A simple model comparing the complete dissolution time and the residence time of a particle on the sediment-water interface suggests that coccoliths can be preserved in the sediments of the deep Northwest Atlantic below the CCD, in good agreement with SEM observations. |
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