Marine black shale deposition and Hadley Cell dynamics: A conceptual framework for the Cretaceous Atlantic Ocean

Understanding the controls that determine the spatial distribution and internal heterogeneities of black shales in the Mesozoic ocean has been a focal point of research over many decades. The consensus is that atmosphere–land–ocean interactions influenced variations in marine biogeochemistry and sediment supply, thus exerting fundamental controls on the richness and quality of sedimentary organic matter (OM) and ultimately on petroleum source rock distribution and its generation potential. Internal, small-scale heterogeneities in black shales that have been reported from all ocean settings were often linked to orbitally-driven fluctuations in continental runoff and marine upwelling. The two processes are generically related under the ascending (tropical) and descending (subtropical) limbs of the palaeo-Hadley Cells, with fluctuations at variable time (seasonal, orbital, geological) and spatial (shelf, margin, deep basin) scales. These dynamic variations translate into characteristic patterns of OM quantity and quality, best preserved near the continents where the forcing effects are strongest. The expression of these orbital-scale interactions are not well constrained at the basin scale, however, they may hold a key to better understand the distribution of heterogeneities in black shales. This study presents a conceptual framework that links OM quality and quantity in Cretaceous Atlantic sediments with the dominant processes that operated under the Hadley Cells. Using a comprehensive compilation of bulk organic geochemical data – total organic carbon concentration (TOC), hydrogen index (HI), oxygen index (OI), and kerogen type – we explore how basic geochemical patterns can be used to identify the underlying generic processes. We utilise published and new data from deep ocean sites of the DSDP/ODP program, as well as one palaeo-shelf setting (Tarfaya), spanning a latitudinal transect from the outer subtropics to the palaeo-equator during the Albian, the Cenomanian–Turonian, and the Coniacian–Santonian. This study emphasises the potential of integrating orbital scale datasets and wide spatial coverage as a predictive tool for black shale formation across ocean basins.