Ferruginous oceans during OAE1a and collapse of the marine sulfate pool

Carbon pump dynamics and limited organic carbon burial during OAE1a

“…OAE1a sediments are highly enriched in Fe relative to Al with ~80% of the samples having Fe/Al compositions higher than the PAAS, confirming widespread anoxia throughout the western Tethys (Figure 5). This observation is further supported by redox sensitive trace element distributions from multiple sites deposited at comparable depths in the western Tethyan basin (Gorgo a Cerbara, Glaise, and Cassis la Bédoule, Roter Sattel), which are largely incompatible with sediment deposition under sulfidic conditions and thus imply persistent and widespread ferruginous bottom waters in the western Tethyan basin during OAE1a (Bauer et al, 2021, 2022). Additional studies on the Fe‐speciation of sediment deposited during OAE1a are needed to confirm the extent of ferruginous waters across multiple sites.…”

“…Iron speciation ratios of Fe HR /Fe Tot > 0.38 and Fe Pyr /Fe HR < 0.70 record sediment deposition beneath anoxic and ferruginous waters (Raiswell et al, 2018). Coeval with the onset of OAE1a in the Cismon core, Fe‐speciation displays a transition to Fe HR /Fe Tot values < 0.38 to Fe HR /Fe Tot values > 0.38 and Fe Pyr /Fe HR < 0.6–0.7 (Figure 5a), diagnostic of sediment deposition beneath ferruginous waters at the Cismon location (Bauer et al, 2022; Poulton, 2021). Ferruginous conditions persist throughout OAE1a before returning to pre‐excursion ratios following OAE1a termination (Figure 5a) (Bauer et al, 2022).…”

“…Coeval with the onset of OAE1a in the Cismon core, Fe‐speciation displays a transition to Fe HR /Fe Tot values < 0.38 to Fe HR /Fe Tot values > 0.38 and Fe Pyr /Fe HR < 0.6–0.7 (Figure 5a), diagnostic of sediment deposition beneath ferruginous waters at the Cismon location (Bauer et al, 2022; Poulton, 2021). Ferruginous conditions persist throughout OAE1a before returning to pre‐excursion ratios following OAE1a termination (Figure 5a) (Bauer et al, 2022). The use of Fe/Al ratios can also be employed to infer anoxic (and ferruginous) conditions.…”

“…To relate OC burial fluxes ( OC Acc ) to potential changes in seawater redox chemistry during OAE1a, we also compiled geochemical data from multiple sites in the western Tethys (Figure 1). We focused on sites that were included in our OC export flux calculations and that are associated with previously published Fe, Al, P, and redox sensitive trace element (RSTE) data sets (Cismon, Glaise, Gorgo a Cerbara) (Bauer et al, 2021, 2022; Stein et al, 2011; Westermann et al, 2013). The Fe, P, and Al geochemical data is tabulated (Table S1) with a detailed RSTE compilation appearing in Bauer et al (2022) using data from a number of different sources (Charbonnier et al, 2018; Föllmi, 2012; Westermann et al, 2013).…”

“…Early Cretaceous OAE1a (~120 Ma) is associated with severe global ocean deoxygenation and the coeval expansion of anoxia in multiple ocean basins (Pacific, Tethys, proto‐Atlantic) (Bauer et al, 2021; Jenkyns, 2010). During OAE1a, the western Tethys Ocean was characterized by widespread ocean anoxia and the deposition of OC‐rich sediment, often classified and referred to as “black shales”, for more than a million years (e.g., Arthur et al, 1990; Bauer et al, 2021, 2022; Bodin et al, 2013; Föllmi, 2012; Westermann et al, 2013). While the OC‐rich nature of the western Tethyan strata implies a connection to expanded ocean anoxia, the links between marine productivity, changes in the BCP, and the drivers of ocean deoxygenation, as well as their corresponding climate and ecological impacts, remain ambiguous.…”