A continental-weathering control on orbitally driven redox-nutrient cycling during Cretaceous Oceanic Anoxic Event 2

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. INTRODUCTION 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). Although the precise driving mechanisms varied for each of these perturbations, extreme greenhouse conditions were a common feature, leading to enhanced hydrological cycling and oceanic nutrient (phosphorus) inputs, particularly in equatorial regions (Wagner et al., 2013). Coupled with more restricted basinal conditions and limited ocean circulation, enhanced primary production promoted extensive carbon burial, ultimately resulting in the widespread development of anoxic oceanic conditions (Trabucho Alexandre et al., 2010; Monteiro et al., 2012). Redox-sensitive element and biomarker records imply widespread euxinic (sulfidic) conditions during these oceanic anoxic events (OAEs), which intermittently extended from bottom waters into the lower photic zone (Sinninghe Damsté and Köster, 1998; Hetzel et al., 2009). There is evidence to suggest, however, that euxinic conditions fluctuated with ferruginous conditions on orbital time scales during OAE3 (Coniacian-Santonian) in the deep-sea proto–North Atlantic (März et al., 2008). The OAE3 black shales highlight a classic effect of redox fluctuations on phosphorus cycling (März et al., 2008), whereby efficient recycling of phosphorus to the water column during euxinic periods (positive productivity feedback) contrasts with extensive burial of water-column phosphorus during ferruginous intervals (negative productivity feedback). 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. To date, however, no other studies have evaluated the potential for rapid redox cycling between euxinic and ferruginous conditions during any of the Cretaceous OAEs, and hence the prevalence, controls, and implications of such conditions are unknown. To address this, we present a redox and nutrient reconstruction of an expanded shallowmarine black shale section of CenomanianTuronian age (OAE2) from the northwest African shelf at Tarfaya, Morocco (see the GSA Data Repository1 for details of the geologic setting). A positive organic carbon isotope excursion of ~3‰ (Fig. 1) marks the onset of OAE2 (Tsikos et al., 2004), while distinct sedimentary 1 GSA Data Repository item 2015320, methods and data, including geologic setting, model parameters and mineralogical analyses, is available online at www.geosociety.org/pubs/ft2015.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. cycles broadly relate to fluctuations in total organic carbon (TOC) content (Kuhnt et al., 2005). Obliquity forcing has recently been suggested to be the dominant driver of this cyclicity, with eccentricity forcing possibly being a secondary component in parts of the record (Meyers et al., 2012). We focus on the onset of OAE2, as documented in records of centimeter-scale (millennial-scale) resolution from core S57 obtained from near the shelf basin center (Kuhnt et al., 2005). We utilize Fe-based redox proxies and molecular biomarkers to provide a detailed evaluation of oceanic redox conditions, and subsequently investigate controls on these conditions. Finally, we evaluate the behavior of phosphorus cycling during redox fluctuations on the Tarfaya shelf to provide new mechanistic insight into the maintenance of widespread anoxia during OAE2.