Controls on carbonate mineral accumulation in Bahamian basins and adjacent Atlantic Ocean sediments

Abstract

Carbonate mineralogical studies of surface Bahamian periplatform ooze, containing pelagic calcite as well as bank-derived aragonite and magnesian calcite, were coupled with studies of the carbonate chemistry of the waters overlying these sediments. The presence of metastable aragonite and magnesian calcite in the periplatform ooze presents a unique opportunity to assess the relation between the extent of carbonate saturation in the water column and the disappearance of metastable carbonate minerals in an area of the North Atlantic Ocean where intermediate and deep waters are saturated with respect to calcite. Aragonite is present in the fine fraction of the surface sediment to a water depth of 4,800 m, but decreases sharply below 4,000 m. This is also the depth limit of the occurrence of pteropod tests and fragments in the coarse fraction of the sediment. The aragonite compensation depth should be (by extrapolation) between 5,000 and 5,200 m. The saturation depth with respect to aragonite was estimated to be located between 3,800 and 4,500 m. This corresponds fairly well with the depth of a sharp decrease of the fine aragonite as well as the depth of pteropod disappearance. Magnesian calcite in the fine fraction of the sediment decreases irregularly with depth and disappears between 3,700 and 4,000 m. The calculated saturation depth with respect to magnesian calcite (13 mole % MgCO 3 ) is between 900 and 1,500 m. At this depth a significant drop in the concentration of magnesian calcite occurs. This decrease of magnesian calcite from 25 to 15 percent could be caused by both dissolution and the increasing distance of deeper waters from the banks. However, the higher values of specific alkalinity of the waters ranging between 750 and 2,000 m, when compared with the alkalinity values of the Sargasso Sea at the same latitude, are interpreted to be indicative that some dissolution of magnesian calcite occurs in the Bahamian basins at these water depths. Our results show depressed levels, by 500 m or more, of the aragonite saturation depth in the water column as well as of the depth of aragonite disappearance in the surface sediment, when compared with open ocean studies in the Northwest Atlantic. In addition, our results indicate that fine magnesian calcite (12 to 13 mole % MgCO 3 ) disappears 1,200 to 1,500 m above the extrapolated disappearance depth of aragonite. This observation is consistent with results for experimental studies, showing that aragonite is more stable in seawater than a magnesian calcite of 12 to 13 mole % MgCO 3 .