The late Hauterivian Faraoni "Oceanic Anoxic Event" at Río Argos (southern Spain): an assessment on the level of oxygen depletion.
Résumé
An integrated study of multiple geochemical proxies (TOC, δ13Ccarb, trace metallic elements, iron speciation) on the late Hauterivian Faraoni Oceanic Anoxic Event has been performed along the hemi-pelagic section of Río Argos (south-eastern Spain) in order to better constrain the environmental perturbations linked to this event in the Subbetic domain. In the studied section, the TOC is relatively low (b0.5 wt.%) except for one sample at the base of the Faraoni horizon, where it reaches 1.5 wt.%. The δ13Ccarb signal is characterized by a minor long-term increase, primarily attributed to an enhanced organic matter burial. However, both organic and isotopic signals are probably affected by dilution effect of carbonates and a possible diagenetic overprint to primary organic and isotopic signal, due to a possible alteration of organic matter during early diagenesis, cannot be ruled out. For the inorganic signal, the trace metallic elements show contrasted record during the late Hauterivian. While detrital proxies (Th, Ti, Zr) record rather constant signal, the productivity (Cu, Ni, P) and redox (U, V, Mn, Fe) proxies show noticeable variations. The organic-rich interval at the base of the Faraoni horizon records higher values of Cu, Ni and P as well as of U and V, linked to bottom-water oxygen depletion and enhanced primary productivity. The rest of the Faraoni horizon records punctual peaks of productivity and redox proxies but rather constant TOC concentration. As for organic and isotopic signals, parts of inorganic signal, linked to organic matter (as those of productivity proxies), have been probably reworked and partly influenced by carbonates. However, inorganic geochemical proxies are consistent suggesting rather oxygenated water-masses associated with moderate productivity during the late Hauterivian. Based on redox ratios (U/Th vs. V/(V+Ni), FeHR/FeT, Fepy/FeHR), oxygen concentrations were determined and it clearly appears that only the organic-rich interval at the base of the Faraoni horizon might record ferruginous anoxic conditions. Compared to other hemi-pelagic Mediterranean regions (the Ultrahelvetic Basin from Switzerland and the Vocontian Trough from France), the Río Argos section shows good similarities concerning TOC and δ13Ccarb signals, suggesting moderate organic matter burial during the Faraoni Event in proximal environments. The main difference concerns the redox variations. Contrary to Italy and France sections where anoxic bottom-water conditions persist up to the end of the Faraoni Event, the Río Argos section shows no truly anoxia onset except at the base of the Faraoni horizon, which we rather interpret as local intra-basinal chemical variations.