Most of the silica dissolved in sea water comes from silica-rich interstitial waters of marine sediments and from rivers carrying the products of subaerial weathering. Silica-secreting microplankton, diatoms and radiolarians, extract enough opal each year to strip the oceans of dissolved silica in about 250 years. Because most of the microscopic tests dissolve rapidly in the water column and at the sea floor, however, only about 4% survive long enough to he buried, and only about 2% avoid post-depositional dissolution and enter the geologic record. Opal-rich sediments are deposited beneath biologically productive surface regions where nutrient-rich deep waters upwell to the photic zone, rather than in areas where the silica enters the sea. The opal-rich deposits ultimately undergo diagenetic transformation to chert. The occurrence of volcanic material with siliceous deposits in the geologic recurd does not reflect a direct cause-offect relation, but rather the association of volcanism with the same tectonic processes that modify oceanographic and depositional environments so as to induce high productivity of silica-secreting organisms and preservation of the tests in protected basins. At present, 85-90% of the opaline silica incorporated in marine sediments is deposited in near-shore environments where most of it is masked by terrigenous debris. Deposition of silica by inorganic precipitation appears virtually impossible since the late Mesozoic and unlikely since the Precambrian.
Figures & Tables
This volume represents some of the papers presented at the SEPM Research Symposium GeologicHistory of the Oceans at the Annual Meeting, March 1971, in Houston, Texas. Knowledge of oceanic sediments has been acquired in two ways: 1) directly by sampling and observation, and 2) indirectly through seismic investigations. Until the past decade, direct sampling and observation techniques could only provide information on the surficial materials of the ocean floor. The development of the piston corer has permitted oceanographic vessels to sample the upper 20 meters, and more recently the upper 30 meters, of the ocean floor, but such cores rarely penetrate the Pleistocene and enter older sediments. Until recently, most knowledge of the deeper sedimentary materials in the ocean basins was obtained through seismic reflection studies. The purpose of this volume is to present a number of observations, ideas, interpretations, and speculations which will be of value in considering the meaning of the increasing volume of data from older deep sea deposits.