PRODUCTION AND ACCUMULATION OF CALCIUM-CARBONATE IN THE OCEAN - BUDGET OF A NONSTEADY STATE

Present-day production of CaCO3 in the world ocean is calculated to be about 5 billion tons (bt) per year, of which about 3 bt accumulate in sediments; the other 40 % is dissolved. Nearly half of the carbonate sediment accumulates on reefs, banks, and tropical shelves, and consis ts largely of metastable aragonite and magnesian calcite. Deep-sea car bonates, predominantly calcitic coccoliths and planktonic foraminifera , have orders of magnitude lower productivity and accumulation rates t han shallow-water carbonates, but they cover orders of magnitude large r basin area. Twice as much calcium is removed from the oceans by pres ent-day carbonate accumulation as is estimated to be brought in by riv ers and hydrothermal activity (1.6 bt), suggesting that outputs have b een overestimated or inputs underestimated, that one or more other inp uts have not been identified, and/or that the oceans are not presently in steady state. One ''missing'' calcium source might be groundwater, although its present-day input is probably much smaller than that of rivers. If, as seems likely, CaCO3 accumulation presently exceeds terr estial and hydrothermal input, this imbalance presumably is offset by decreased accumulation and increased input during lowered sea level: s hallow-water accumulation decreases by an order of magnitude with a 10 0 m drop in sea level, while groundwater influx increases because of h eightened piezometric head and the diagenesis of metastable aragonite and magnesian calcite from subaerially exposed shallow-water carbonate s.