Abstract
Abstract. Extensive black shale deposits formed in the Early Cretaceous South Atlantic, supporting the notion that this emerging ocean basin was a globally important site of organic carbon burial. The magnitude of organic carbon burial in marine basins is known to be controlled by various tectonic, oceanographic, hydrological, and climatic processes acting on different temporal and spatial scales, the nature and relative importance of which are poorly understood for the young South Atlantic. Here we present new bulk and molecular geochemical data from an Aptian–Albian sediment record recovered from the deep Cape Basin at Deep Sea Drilling Project (DSDP) Site 361, which we combine with general circulation model results to identify driving mechanisms of organic carbon burial. A multimillion-year decrease (i.e., Early Aptian–Albian) in organic carbon burial, reflected in a lithological succession of black shale, gray shale, and red beds, was caused by increasing bottom water oxygenation due to abating hydrographic restriction via South Atlantic–Southern Ocean gateways. These results emphasize basin evolution and ocean gateway development as a decisive primary control on enhanced organic carbon preservation in the Cape Basin at geological timescales (> 1 Myr). The Early Aptian black shale sequence comprises alternations of shales with high (> 6 %) and relatively low (∼ 3.5 %) organic carbon content of marine sources, the former being deposited during the global Oceanic Anoxic Event (OAE) 1a, as well as during repetitive intervals before and after OAE 1a. In all cases, these short-term intervals of enhanced organic carbon burial coincided with strong influxes of sediments derived from the proximal African continent, indicating closely coupled climate–land–ocean interactions. Supported by our model results, we show that fluctuations in weathering-derived nutrient input from the southern African continent, linked to changes in orbitally driven humidity and aridity, were the underlying drivers of repetitive episodes of enhanced organic carbon burial in the deep Cape Basin. These results suggest that deep marine environments of emerging ocean basins responded sensitively and directly to short-term fluctuations in riverine nutrient fluxes. We explain this relationship using the lack of wide and mature continental shelf seas that could have acted as a barrier or filter for nutrient transfer from the continent into the deep ocean.
Highlights
The Early Cretaceous epoch (∼ 145–100.5 Ma) has long been a focus of interest for geologists and the petroleum industry due to the widespread occurrence of marine black shales, Published by Copernicus Publications on behalf of the European Geosciences Union.W
We broadly subdivide the black shale unit into “low-TOC black shales” (i.e., 0.7 %–6 % TOC) and “high-TOC black shales” (i.e., > 6 % TOC) to differentiate two organic carbon (OC) burial end-member states and to facilitate the discussion of related changes in paleo-environmental conditions. The former subunit comprises the majority of black shale samples (i.e., 70 %), which cluster around a mean of 3.5 % TOC (Fig. 3a) and appear to constitute background sedimentation during the Early Aptian (Fig. 2c)
We reconstruct the evolution of paleo-redox conditions, OC composition, and sediment provenance in the Aptian–Albian Cape Basin based on Fe, S, and TOC systematics; distribution of redox-sensitive trace metals; lipid biomarker data; and the inorganic geochemical composition of sediments at Site 361
Summary
Dummann et al.: Driving mechanisms of OC burial in the Cretaceous South Atlantic which were preferentially deposited in emerging ocean basins that developed in the wake of the breakup of Pangaea (Stein et al, 1986; Zimmerman et al, 1987) Within these basins, organic carbon (OC) burial was controlled by a complex interplay of productivity, preservation, and dilution (Pedersen and Calvert, 1990; Tyson, 2005; Arthur and Sageman, 1994; Bralower and Thierstein, 1984), which was affected by multiple processes acting on different timescales. During brief episodes ≤ 1 Myr, marine black shales were deposited regionally to globally and accompanied by intense perturbations in ocean chemistry and ecology (Jenkyns, 2010; Weissert et al, 1998; Erba, 1994). OAE 1a, one of the most severe OAEs (Jenkyns, 2010, and references therein), occurred during the Early Aptian (Coccioni et al, 1992) and was marked by a distinct negative stable carbon isotope excursion at its onset (Menegatti et al, 1998), evidence for rapid warming (O’Brien et al, 2017, and references therein) and elevated atmospheric CO2 concentrations (Naafs et al, 2016)
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