ABSTRACT: Early Diagenesis of Benthically Derived Carbonates and Their Importance in the Carbon Cycle

Fred T. Mackenzie, Christopher L. Sabine

Benthically derived shoal-water carbonate mineralogies are an important source of sediment for oceanic slopes. In the Pacific, mid-depth banks (average depth between 50 and 100 m) are sites of significant benthic carbonate production. Because of their open morphology, these banks act as carbonate particle sources for the open ocean.

The potential early diagenesis of these particles in the open-ocean realm is discussed in light of their solubility, dissolution rates, and importance in the global cycle of carbonate in the ocean. These carbonate particles are more soluble and kinetically reactive than their pelagic counterparts. From sediment trap and seawater carbonate chemistry investigations on Penguin Bank, Necker, and Maro reefs in the northwestern Hawaiian Islands, and a tentative model of the global cycle of inorganic carbon in the ocean, we conclude the following. (1) Benthically derived shallow-water carbonate particles can account for about 10% of the global sedimentation flux of open-ocean carbonate sediments. Much of this sedimentation occurs on oceanic slopes. (2) This flux of benthic carbonates, if dis olved in the open ocean, can represent an additional sink for fossil fuel CO₂, amounting to a few percent of the atmospheric increases in CO₂ of 1.4 ppmv/y^-1. (3) This component of inorganic carbon in the ocean may account for 25% of the observed alkalinity excess of the intermediate-depth alkalinity maximum of the western North Pacific. (4) The relative importance and degree of coupling of shoal-water and open-ocean carbonate accumulation are important factors in terms of the Phanerozoic history of atmospheric CO₂ levels.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990