No evidence for anoxia during the Valanginian carbon isotope event-An organic-geochemical study from the Vocontian Basin, SE France

摘要

The Valanginian time interval (Early Cretaceous) is characterized by a positive carton isotope excursion (CIE) which represents the first of several prominent Cretaceous δ~(13)C anomalies. A combined chemostratigraphic and organic-geochemical approach has been chosen to investigate the composition and distribution of sedimentary organic matter (OM) deposited before the Valanginian CIE, during its onset and plateau-phase. This was done to test whether this CIE is accompanied by changes in marine primary production and/or OM preservation. Biostratigraphically well-calibrated deposits from two hemipelagic sections located i 1 the Vocontian Basin of SE France are used as sedimentary archives. A newly established high-resolution δ~(13)Crecord covering the composite succession shows a characteristic Valanginian pattern and enables a detailed correiation with existing carbon isotope curves from the northern Tethyan margin. The analyzed solvent extractabie fraction of the sedimentary OM is mainly composed of a marine origin with an admixture of land plant material, Variations in specific bio-markers for cyanobacteria (2α-methyl-hopanes), dinoflagellates (dinosterane or- 4-desmethyl-23,24-dimethyl steranes) and terrigenous plant-derived OM (odd-numbered long-chain n-alkanes) as well as the sterane/ hopane ratio, the C_(35) hopane index and the isoprenoids pristane and phytane were investigated. In contrast to the well-studied mid-Cretaceous Oceanic Anoxic Events (OAEs), neither significant OM enrichment nor prominent fluctuations in the selected biomarker abundances can be observed during he build-up phase of the Valanginian CIE. This points to relatively stable marine paleoenvironmental conditions with well-oxygenated bottom waters. Prior to the CIE, four cm-thick, finely laminated, dark layers (known as Barrande layers) with total organic carbon content reaching up to 4% show an exception from the generally stable biomarker pattern. Sedimentological and biomarker evidence support deposition under less oxygenated conditions for the Barrande layers. However, their occurrence clearly predates the onset of the positive δ~(13)C_(caed) shift (by about 180 kyrs). Contrary to the subsequent mid-Cretaceous CIEs, the occurrence of widesprread anoxia associated with the Valanginian CIE cannot be confirmed for the Vocontian Basin.