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

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.

Biogeochemical sulfur cycling during Cretaceous oceanic anoxic events: A comparison of OAE1a and OAE2

“…There are various lines of evidence presented here and elsewhere [i.e., Lowenstein et al, ; Wortmann and Chernyavsky , ; Adams et al, ; Owens et al, ; Poulton et al, ] that indicate that Cretaceous marine sulfate concentrations were lower than modern concentrations (~28 mM). The model that best reproduced the data indicates that sulfate concentrations increased from ~6 to ~11 mM prior to the evaporite burial event (~116–114 Ma) and ranged from <1 to ~7 mM in the aftermath.…”

“…The latter estimated range is below the Lower‐Upper Cretaceous (~112–94 Ma) fluid‐inclusion‐derived estimate of 6 to 14 mM [ Lowenstein et al, ] and the ~7 mM pre‐CIE estimate of Owens et al []. However, there is some overlap between our estimated range and fluid inclusion data and our estimate is in agreement with the pre‐CIE estimate of ~2 to 4 mM of Adams et al [] and the ~3 to 7 mM estimate of Poulton et al []. These previously published estimates indicate that the Cretaceous marine sulfate reservoir was at least <50% of the modern marine sulfate reservoir.…”

Cyanobacteria Proliferation in the Cenomanian‐Turonian Boundary Interval of the Apennine Carbonate Platform: Immediate Response to the Environmental Perturbations Associated With OAE‐2?

“…In terms of mechanism, volcanic activity associated with LIP emplacement would have led to rapid increase in atmospheric CO 2 and temperature (Forster et al, 2007;Barclay et al, 2010;van Bentum et al, 2012, Figure 8) and release of Fe and other biolimiting micronutrients to oceanic waters (Jenkyns, 2010;Jenkyns et al, 2017;Poulton et al, 2015) and to an increase in weathering (Blätter et al, 2011;Hetzel et al, 2009;Pogge von Strandmann et al, 2013). Increased runoff associated with increased rates of weathering would have enhanced the supply of nutrient P to the oceans (Mort et al, 2007;Ruvalcaba Baroni et al, 2014;Figure 8).…”

“…Large emissions of CO 2 were released during this time due to intense volcanic activity associated with the emplacement of Large Igneous Provinces (LIPs; Caribbean and High Arctic LIPs). This would have enhanced terrestrial weathering rates, leading to an increase in the quantity of nutrients delivered to the oceans, which fueled primary productivity (Du Vivier et al, 2015;Flögel et al, 2011;Jenkyns, 2003Jenkyns, , 2010Jenkyns et al, 2017;Leckie et al, 2002;Owens et al, 2013;Pogge von Strandmann et al, 2013;Poulton et al, 2015;Sweere et al, 2018;Thomas & Tilghman, 2014;Van Helmond et al, 2014). Following, the CO 2 emitted to the atmosphere would have increased temperature and precipitation extremes (Heimhofer et al, 2018;Van Helmond et al, 2014;Wagner et al, 2013).…”

“…Following, the CO 2 emitted to the atmosphere would have increased temperature and precipitation extremes (Heimhofer et al, 2018;Van Helmond et al, 2014;Wagner et al, 2013). This would have enhanced terrestrial weathering rates, leading to an increase in the quantity of nutrients delivered to the oceans, which fueled primary productivity (Du Vivier et al, 2015;Flögel et al, 2011;Jenkyns, 2003Jenkyns, , 2010Jenkyns et al, 2017;Leckie et al, 2002;Owens et al, 2013;Pogge von Strandmann et al, 2013;Poulton et al, 2015;Sweere et al, 2018;Thomas & Tilghman, 2014;Van Helmond et al, 2014). Primary productivity might also have been stimulated by the release of large quantities of hydrothermal iron and other micronutrients due to submarine volcanogenic activity (Adams et al, 2010), by enhanced nitrogen fixation (Junium & Arthur, 2007;Ruvalcaba Baroni et al, 2015), or by phosphorus regeneration from organic-rich sediments (Monteiro et al, 2012;Mort et al, 2007;Ruvalcaba Baroni et al, 2014).…”