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Tethyan–Mediterranean organic carbon-rich sediments from Mesozoic black shales to sapropels



The Jurassic to Holocene record of black shale deposition in the Tethys-Mediterranean region is unrivalled by that of any other ocean basin, either in land sections or drill cores. The term ‘black shale’ is used here broadly for sediments with elevated organic carbon concentrations (> 1%), including the Pliocene to Recent sapropels. Most of the black shales are devoid of benthonic organisms, are laminated, and were deposited in distinct rhythms during periods when the deep waters of the ocean basins were anoxic or dysoxic. The Tethyan black shale records have become essential in studies of the transfer of organic carbon into the sediment record and for astronomical tuning and geological time scales. These records have been central in understanding climate control on ocean dynamics and biogeochemical cycles. The Mesozoic black shales were deposited within well-defined time envelopes of around 0·5 to 1 Myr. These black shales, which were confined to certain chronostratigraphic intervals in the Jurassic and Cretaceous, were recognized as expressions of global Oceanic Anoxic Events in the mid-1970s and subsequently named after prominent researchers (Bonarelli, Selli, Goguel). The black shale episodes were dated by biostratigraphic methods and by high-resolution chemostratigraphy and cyclostratigraphy. Mesozoic black shales are now interpreted as the oceanographic expression of major perturbations of the global carbon cycle and climate. Research into younger (Pliocene to Pleistocene) sedimentary cycles (including black shales, termed sapropels) exposed in land sections, or found in pelagic and hemipelagic marine sediment cores of Late Quaternary age, started in the 1950s. Main threads pursued from this end of the record were the climatic control of oceanic processes that permitted the development of highly detailed and precise time scales tuned to the astronomical clock of insolation changes, palaeoclimate evolution of the circum-Mediterranean area and biogeochemical dynamics in anoxic basins. After 50 years of intensive research, the Tethyan and Mediterranean black shales remain subjects of fascination in Earth Science. Tracing the origins and building upon recent progress, the current hypotheses on their formation are reviewed here. It is a panorama of complex interplays between global and regional tectonics, climate dynamics during both Ice House and Greenhouse states of global and regional climate, oceanographic responses to these climate changes, and biogeochemical adaptations that were all needed to shape an extraordinary archive of global change in the absence of human activity.