Evolution of Sediment Fluxes and Ocean Salinity

Abstract

Knowledge of the rates of geological processes is an important aspect of basin modeling. Much of the surficial geology of the Earth is the result of erosion and deposition of sediment, Inspection of the inventory of sediments and sedimentary rocks existing today indicates that the global rates of these processes have changed markedly during the Phanerozoic. The mass-age distribution of Phanerozoic sediments and sedimentary rocks existing on Earth today has the general form of an exponential decay curve, reflecting the fact that new sediments are formed mostly from the erosion of older sediments. The dissolved salt in the ocean and in pore waters constitutes a special reservoir of the sedimentary system, representing part of the soluble matter derived from weathering. Additions to the total sedimentary system--sediments plus dissolved salts--come from the weathering of igneous and metamorphic rocks and from extraterrestrial sources but are relatively small in comparison with the overall rate of sediment cycling. Losses to the sedimentary system result from metamorphism and subduction but again are small in comparison with the overall rate of cycling of sedimentary materials. About a fourth of the total mass of sedimentary material has been subducted and replaced by new sedimentary material produced from the weathering of igneous and high-grade metamorphic rocks during the Phanerozoic. Sedimentary flux rates have changed by a factor of five through the Phanerozoic. It is most likely that these variations reflect changes in continental relief, but the high rates in the Early Paleozoic also were a function of the lack of plant cover to bind the soil and retard erosion. Because sedimentary strata accumulate in thin, widespread layers, erosion of sedimentary materials must proceed in such a way that young unconsolidated sediments arc more likely to be eroded than older rocks. Although some soluble rocks may be dissolved as they enter the active near-surface groundwater system, the amount of selective recycling of different lithologies is slight. There is little evidence for evolution of sedimentary materials on the continental blocks. Quartz sand has become less abundant in the later Phanerozoic, and since the mid Cretaceous the site of deposition of carbonates has shifted from the continental blocks to the deep sea. From the inventory of existing evaporites it is possible to use principles of sedimentary cycling to reconstruct the salinity of the ocean during the past. We conclude that average ocean salinities were almost 50parts per thousand in the Palcozoic and declined during the Mesozoic and Cenozoic to finally reach its modern value of 34.7parts per thousand.