Abstract:
The Aptian/Albian oceanic anoxic event 1b contains the record of several perturbations in the global carbon cycle and multiple black shale levels, particularly in the Western Tethys. The local lithological expression of an oceanic anoxic event depends on paleogeographical and depositional setting as well as on regional paleoclimate. Marine sediments at a particular location may therefore be more or less organic-rich (or not at all) and they may consist of different lithologies. In most studies, however, much of the lithological variability associated with oceanic anoxic events is left unaccounted for and, thus, the exact processes leading to the enrichment of organic matter in these marine sediments and their subsequent preservation in the geological record are unknown. This study focuses on the local sedimentary processes behind the deposition of organic-rich sediments at Deep Sea Drilling Project Site 545 and Ocean Drilling Program Sites 1049 and 1276 in the North Atlantic during oceanic anoxic event 1b. Although specifically dealing with the sediments deposited during this particular event at these localities, it is expected that the same processes were responsible for determining the exact sedimentary products at localities in similar settings, as well as during other similar events in the Mesozoic. Here, it is shown that the deposition of organic-rich sediments during oceanic anoxic event 1b was a consequence of the enhanced productivity favoured by upwelling and by riverine nutrient input, or aeolian fertilization of the euphotic zone depending on geographical location. Slope instability processes resulted in the transfer of part of these organic-rich sediments from the shelf to deep sea depocenters as mud-laden organic-rich turbidites, especially in the northern North Atlantic. The so-called "black shales" are much more varied than their name implies. The end product of sedimentation during an oceanic anoxic event at a particular location is commonly the result of several equifinal processes acting on a local scale rather than the direct result of basinal or even global mechanisms. Abstract Copyright (2010), International Association of Sedimentologists.