Continental drift and Phanerozoic carbonate accumulation in shallow-shelf and deep-marine settings

Knowledge of past rates of transfer of rock-forming materials among the principal geologic reservoirs is central to understanding causes and magnitudes of change in earth surface processes over Phanerozoic time. To determine typical rates of global sediment cycling, we compiled information on area, volume, and lithology of shallow-water sediments by epoch for both terrigenous clastics and marine carbonates. Data on amounts of surviving continental terrigenous rock (as opposed to deep oceanic, and including terrestrial, marginal marine, and marine deposits) exhibit positive age/area trends wherein greatest areas and volumes of conglomerate, sandstone, and shale are represented by younger sequences. Global volumes of terrigenous-clastic sediment yield a mean cycling rate of 0.00124/m.yr., similar to that determined for Eurasian (0.00127), North American (0.00058 [A. B. Ronov], 0.00352 [T. D. Cook and A. W. Bally]), African (0.0017), and South American (0.0021) clastic sequences. Surficial erosion results in the mean destruction of similar to0.124% of terrigenous rock volume per million years of reservoir age. In contrast, surviving epicratonic and shelf-margin carbonate sequences yield negative cycling rates of about -0.164%/m.yr. Surviving areas and volumes increase with sequence age; that is, the amount of limestone and dolostone preserved in shallow-water settings increases back in time to maximal areal extents in the Middle and Upper Cambrian. Mass/age data on terrigenous-clastic successions indicate generally constant rates of crustal erosion over Phanerozoic time. Decrease in size of the shallow-marine carbonate reservoir forward in time therefore suggests generally invariant rates of global limestone accumulation and a shift in sites of accumulation from shallow-cratonic to deep-oceanic settings over much of the past 540 m.yr. Causes of this eon-scale depositional translocation of carbonate sediment from shallow-to deep-marine settings cannot be satisfactorily linked to either changes in global sea level or the evolution of carbonate-producing plankton because neither exhibits a pattern of unidirectional change in position or abundance since the Early Phanerozoic. However, tabulation of long-term variation in areas of continental shelves from paleogeographic maps reveals a generally uniform decrease in low-latitude (<30) platform area with decreasing age over most of the Phanerozoic. Moreover, rate of change of low-latitude shelf area (similar to 52.6 x 10(3) km(2)/m.yr.) is almost exactly the rate of change in the area of shallow-water carbonate sequences (similar to 63.0 x km(2)/m.yr.) over the same interval. This agreement suggests that poleward movement of the major continents over the past 540 m.yr. has had a substantial impact on patterns of global carbonate accumulation. The long-term effect of decreasing low-latitude shelf area has been augmented, particularly over the last 100 m.yr., by eustatic sea level fall and the diversification of open-ocean calcifiers. The evolution of calcareous plankton in the Late Paleozoic or Early Mesozoic may have been facilitated, or at least permitted, by increasing saturation state of the oceans associated with progressive decline in shallow-water carbonates.