Controls on the dissolution rates of natural and synthetic calcites under seafloor-like conditions
Abstract
The preservation of calcite and low Mg-calcites in deep-sea sediments depends on the balance between accumulation and dissolution rates at the sediment-water interface (SWI). If, at the SWI, calcite accumulation is faster than its dissolution, calcitic particles can be buried. Conversely, below the calcite compensation depth, at 5 km-depth in the present ocean, calcite dissolution is faster than accumulation, and all the calcite reaching the seafloor is dissolved at the SWI. Thus, an accurate description of the rate of calcite dissolution at the SWI is required to properly interpret both the past and the present CaCO3 sediment record.
The scarce in-situ measurements of the dissolution rate of a calcite bed at the SWI diverge widely from the kinetics observed in laboratory studies performed on suspended particles, under hydrodynamic and geometric conditions that are not representative of those encountered at the seafloor. Using a newly designed flow-through rotating-disk reactor and simulated deep-sea sediment disks, we provide a quantitative description of the calcite dissolution kinetics under more representative hydrodynamic conditions and unravel the mechanisms ruling this dissolution. Among all the investigated oceanographic variables that may potentially influence the dissolution rate, i.e., sediment porosity, permeability, grain size and specific surface area, biogenic or synthetic particles, seawater temperature, current speed and carbonate ion concentration [CO32-], we determined that only seawater [CO32-] and the current speed have a statistically significant influence on calcite dissolution rate. The current-speed dependency is explained by the presence of a diffusive boundary layer (DBL) above the SWI. Based on these observations, we argue that geologically long- or medium-term changes in bottom-current velocities, such as those that may be induced by changes in continental positions or evolving climate, should be considered along with variations of calcite deposition fluxes or seawater acidification when interpreting calcite preservation data through time.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP54A..05S
- Keywords:
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- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 3675 Sedimentary petrology;
- MINERALOGY AND PETROLOGYDE: 4863 Sedimentation;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL