Sediment Cycling During Earth History

Abstract

The mass-age relations of sedimentary rocks are roughly consistent with a model in which the rate of sedimentation has been approximately constant through time, averaged over very long time intervals. The total mass of sedimentary rocks is assumed to be constant through time, and the probability of erosion of sedimentary rocks of a given period is assumed to be proportional to the mass/year of that age interval remaining. The model is not entirely cannibalistic; subduction of sediments, return of their volatiles to the atmosphere, and production of new sediments by reaction of these volatiles with crystalline rocks is an important process.

This model, with multiple turnovers of the sedimentary mass, without striking changes in its gross composition, has led to the concept of an average steady state among the reservoirs of chemical sediments. Although there seem to be few important secular elemental trends in the total sedimentary mass, there are important reciprocal changes among the mineral reservoirs, notably reciprocal changes between sulfide and sulfate, balanced by changes between carbonate and organic carbon.

The earth surface environment, so dependent on oxygen and carbon dioxide levels in the ocean and atmosphere, is dependent, in the long term, on the transfers of carbon and sulfur from one reservoir to another. The ocean-atmosphere contents of oxygen and carbon dioxide are small relative to the amounts of carbon and sulfur that have been transferred back and forth between the mineral reservoirs. The ocean and atmosphere, over geologic time, are media of transfer and not reservoirs. The above relations have led to mathematical models of sediment cycling which, initially, assumed constancy of ocean-atmosphere. The models, using the isotopic composition of the carbon of carbonates and organic carbon on the one hand, and that of sulfide and sulfate on the other as “ground truth” have yielded results in terms of transfer rates between sulfide-sulfate-carbonate-organic carbon reservoirs that probably are accurate within an order of magnitude, and indicate general trends through Phanerozoic time. The models have recently been expanded to include transfers among all the major chemical sediment reservoirs, and functional relations have been developed on transfer rates as functions of reservoir sizes, temperatures, land area, sea floor spreading rates, oceanic circulation through ridges, and metamorphic and volcanic processes in general.