Deep-sea pelagic sediments and palaeo-oceanography: a review of recent progress

Summary Understanding of present and past patterns of sedimentation, circulation, fertility, biogeography and chemistry in the oceans has advanced greatly in the last decade. In achieving this a diverse range of geological and geophysical techniques has been drawn on. Egress zones of active hydrothermal systems venting sulphide-laden hot water at spreading centres, long predicted on theoretical and other grounds, were discovered using deep-towed geophysical packages and have been fruitfully studied using submersibles. The chemistry of the venting solutions, and the apparent scale of the hydrothermal processes, have engendered a rethink of the oceanic fluxes of many elements. Continuing deep-sea drilling increasingly refines reconstructions of continental-drift events and the consequent changes in circulation patterns. The Deep-Sea Drilling Project planning has become more problem-specific, and on ‘palaeo-environment’ legs, sediment successions have been cored more continuously than was earlier the case. As a result, palaeo-oceanographers have been provided with a flood of data germane to the reconstruction of past behaviour of the CCD, to appreciation of past states of oxygenation of the oceans and of carbon preservation patterns (the ‘black-shale problem’), to documentation and interpretation of hiatus patterns, to calculation of global sediment budgets through time, and to calibration of the overall cooling of the Cenozoic hydrosphere. Isotopic determinations and organic geochemistry have become key tools in these studies. Global sea-level stands, clearly important as a regulator of conditions in the pelagic realm (though to an arguable extent) have been documented using seismic stratigraphy. Attempts at synthesis of the complex spectrum of palaeo-oceanographic changes evident through time in the pelagic relam stress the rhythmicity with which the oceans have oscillated between broadly uniformitarian and non-uniformitarian conditions. Syntheses build towards identification of driving forces and responses.