The mid-Cretaceous North Atlantic nutrient trap: Black shales and OAEs

Organic-rich sediments are the salient marine sedimentation product in the mid-Cretaceous of the ocean basins formed in the Mesozoic. Oceanic anoxic events (OAEs) are discrete and particularly organic-rich intervals within these mid-Cretaceous organic-rich sequences and are defined by pronounced carbon isotope excursions. Marine productivity during OAEs appears to have been enhanced by the increased availability of biolimiting nutrients in seawater due to hydrothermal alteration of submarine basalts in the Pacific and proto-Indian oceans. The exact mechanisms behind the deposition of organic-rich sediments in the mid-Cretaceous are still a matter of discussion, but a hypothesis which is often put forward is that their deposition was a consequence of the coupling of a particular paleogeography with changes in ocean circulation and nutrient supply. In this study, we used a global coupled climate model to investigate oceanic processes that affect the interbasinal exchange of nutrients as well as their spatial distribution and bioavailability. We conclude that the mid-Cretaceous North Atlantic was a nutrient trap as a consequence of an estuarine circulation with respect to the Pacific. Organic-rich sediments in the North Atlantic were deposited below regions of intense upwelling. We suggest that enhanced productivity during OAEs was a consequence of upwelling of Pacific-derived nutrient-rich seawater associated with submarine igneous events

Seismic interpretation of dinantian carbonate platforms in the Netherlands; implications for the palaeogeographical and structural development of the Northwest European Carboniferous Basin

The Northwest European Carboniferous Basin is characterized by a series of carbonate platforms and intervening shale-dominated troughs during the Dinantian Sub-period. These structures have been mainly found along the margins of the basin. Here we present the results of an investigation of high-quality 3D seismic surveys, which has allowed the construction of a palaeogeographical and structural framework of the Dinantian Sub-period in the centre of the Northwest European Carboniferous Basin. Analysis of these data has revealed a series of structures characterized by flat tops, well-developed slopes and onlap by overlying strata. These geometrical features suggest that the structures represent Dinantian carbonate platforms, an interpretation that is supported by recently published well data. These findings suggest the following sequential depositional and structural model for the area: (1) during the Tournaisian Stage, carbonate sedimentation took place in basinal areas; (2) continued subsidence, probably induced by rifting, resulted in the gradual onlap of carbonate deposition onto topographically higher areas; (3) during the Late Viséan Stage, rimmed shelf deposition took place, by analogy with observations from the UK and Belgium. The seismically mapped structures represent these rimmed-shelf carbonate platforms. © 2010 Geological Society of London