Abstract
The Cretaceous period (~145–65 m.y. ago) was characterized by intervals of enhanced organic carbon burial associated with increased primary production under greenhouse conditions. The global consequences of these perturbations, oceanic anoxic events (OAEs), lasted up to 1 m.y., but short-term nutrient and climatic controls on widespread anoxia are poorly understood. Here, we present a high-resolution reconstruction of oceanic redox and nutrient cycling as recorded in subtropical shelf sediments from Tarfaya, Morocco, spanning the initiation of OAE2. Iron-sulfur systematics and biomarker evidence demonstrate previously undescribed redox cyclicity on orbital time scales, from sulfidic to anoxic ferruginous (Fe-rich) water-column conditions. Bulk geochemical data and sulfur isotope modeling suggest that ferruginous conditions were not a consequence of nutrient or sulfate limitation, despite overall low sulfate concentrations in the proto–North Atlantic. Instead, fluctuations in the weathering influxes of sulfur and reactive iron, linked to a dynamic hydrological cycle, likely drove the redox cyclicity. Despite the potential for elevated phosphorus burial in association with Fe oxides under ferruginous conditions on the Tarfaya shelf, porewater sulfide generation drove extensive phosphorus recycling back to the water column, thus maintaining widespread open-ocean anoxia.
Highlights
Major perturbations to the global Earth system occurred during the mid-Cretaceous, resulting in repetitive d13C isotope excursions in organic carbon and carbonate linked to enhanced organic carbon burial (Jenkyns, 2010)
Water-Column Redox Reconstruction Elevated FeHR/FeT ratios are apparent throughout the analyzed core, suggesting persistent bottom-water anoxia prior to, and during, the onset and initial maximum of OAE2 (Fig. 1)
Throughout the majority of the interval, Fepy/FeHR ratios fall close to or above the 0.7 threshold, suggesting dominantly euxinic conditions. This is supported by the presence of the biomarker isorenieratane in all samples studied, which implies at least periodic incursions of sulfide into the lower photic zone, the variability in concentration suggests that the temporal extent or intensity of photic zone euxinia may have fluctuated (Fig. 1)
Summary
Major perturbations to the global Earth system occurred during the mid-Cretaceous, resulting in repetitive d13C isotope excursions in organic carbon and carbonate linked to enhanced organic carbon burial (Jenkyns, 2010). Coupled with more restricted basinal conditions and limited ocean circulation, enhanced primary production promoted extensive carbon burial, resulting in the widespread development of anoxic oceanic conditions (Trabucho Alexandre et al, 2010; Monteiro et al, 2012). If prevalent on a basinal or global scale, the development of ferruginous conditions with associated phosphorus burial would have had major implications for the persistence of elevated marine productivity and widespread anoxia. No other studies have evaluated the potential for rapid redox cycling between euxinic and ferruginous conditions during any of the Cretaceous OAEs, and the prevalence, controls, and implications of such conditions are unknown
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