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
