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.
Driving mechanisms of organic carbon burial in the Early Cretaceous South Atlantic Cape Basin (DSDP Site 361)
