Calcareous Organisms and Sediment Mineralogy on a Mid-Depth Bank in the Hawaiian Archipelago

Abstract

The dominant calcareous organisms on Penguin Bank, a middepth bank (40-100 m) off the southwestern tip of the island of Molokai, Hawaii, are red and green algae, benthic foraminifera, and bryozoans. The sediments on Penguin Bank are a mixed mineralogic assemblage of benthically derived magnesian calcite and aragonite. A low pelagic input of foraminifera and coccolithophorids to the sediments was indicated by the small percentage of low magnesian calcite found only in the smallest size fractions and the lack of recognizable particles of these organisms in these size fractions. The benthic community on Penguin Bank, composed ofcoralline algae, benthic foraminifera, and bryozoans, produces magnesian calcite with a range in magnesium content of about 6-16 mole % MgC03. Calcareous green algae (predominantly Halimeda) are the dominant producers of aragonite. Sediments on Penguin Bank are dominated by magnesian calcite particles in all size fractions (<45-3962 mm). The ratio of the percentage of high magnesian calcite (>5 mole %) to aragonite increases in the smaller size fractions and with increasing water depth from 40 to 93 m. The magnesium content of the sediments decreases within the same depth range. Mid-depth banks may be potential sources of highly chemically reactive carbonate particles to the open ocean. The magnitude of this input has not been quantitatively assessed but may be important in global biogeochemical cycles of calcium and carbon in the ocean reservoir.