Preservation state of metastable magnesian calcite in periplatform sediments from the Caribbean Sea over the last million years

Abstract

Carbonate‐rich periplatform sediments represent an active carbon reservoir containing metastable aragonite and magnesian calcite (Mg‐calcite, > 4 mol % MgCO3). Since Mg‐calcite is highly soluble, the preservation state of this mineral provides information on past carbonate systems at water depths shallower than the lysocline. The mineralogy and geochemistry of the carbonate‐rich fine fraction (<63 μm) was determined for the last 940 ka in a sediment core collected from the Walton Basin (Northern Caribbean Sea, 846 m water depth). Mg/Ca ratios for Mg‐calcite showed clear glacial‐interglacial cycles with high values (12 mol % MgCO3) during interglacials. Glacial Mg/Ca ratios were approximately 8 mol % MgCO3 for the period from 940 ka to 400 ka, and approximately 10 mol % MgCO3 for the last 400 ka. The Mg/Ca shift is concomitant with a preferential loss of Mg‐calcite relative to aragonite. The preservation state of Mg‐calcite revealed that the bottom water mass of the studied site was slightly more corrosive for the earlier period, possibly relating to a composition change in intermediate water and/or to the ventilation rate in the Atlantic Ocean.