Carbonate sediments, which consist largely of the skeletal remains of microplankton (calcareous nannoplankton, planktonic foraminifers, pteropods and heteropods) cover nearly 47% or 126 million sq km of the ocean floor. Their distribution is particularly dominant in the warm-water belt between approximately 45°N and 45°S and over submarine topographic elevations such as the mid-oceanic ridge systems and isolated seamounts. This distributional pattern is affected both by conditions controlling production of microplankton in the overlying surface waters and by their postmortem dissolution or destruction in the water column, either at the sediment-water surface or within the sediment column. In areas where the dissolution rate of carbonates exceeds supply, a "compe sation condition" is reached and, in general, carbonates are not accumulating in depths greater than 4,000 m. Because of low productivity of calcareous microplankton in surface waters coupled with greater water depth, the lowest sedimentation rates of biogenous carbonates occur under the North and South Pacific central water masses. A general distribution pattern of carbonate sedimentation in the past can be pieced together on the basis of nearly 1,000 pre-Pleistocene piston cores and from the results of the U.S. Deep Sea Drilling Project. A considerable part of the abyssal floor once received calcareous sediment. Carbonate sediments were later replaced by clays as the basin deepened and the dissolution rate exceeded supply. The resulting facies boundary is broadly time-transgressive wit increasing age away from the axes of the mid-oceanic ridge systems.

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