The Valanginian delta C-13 excursion may not be an expression of a global oceanic anoxic event

摘要

Marine and terrestrial sediments of the Valanginian age display a distinct positive delta C-13 excursion, which has recently been interpreted as the expression of an oceanic anoxic episode (OAE) of global importance. Here we evaluate the extent of anaerobic conditions in marine bottom waters and explore the mechanisms involved in changing carbon storage on a global scale during this time interval. We asses redox-sensitive trace-element distributions (RSTE; uranium, vanadium, cobalt, arsenic and molybdenum) and the quality and quantity of preserved organic matter (OM) in representative sections along a shelf-basin transect in the western Tethys, in the Polish Basin and on Shatsky Rise. OM-rich layers corresponding in time to the delta C-13 shift are generally rare in the Tethyan sections and if present, the layers are not thicker than several centimetres and total organic carbon (TOC) contents do not surpass 1.68 wt..%. Palynological observations and geochemical properties of the preserved OM suggest a mixed marine and terrestrial origin and deposition in an oxic environment. In the Polish Basin, OM-rich layers show evidence for an important continental component. RSTE exhibit no major enrichments during the delta C-13 excursion in all studied Tethyan sections. RSTE enrichments are, however, observed in the pre-delta C-13 excursion OM-rich "Barrande" levels of the Vocontian Trough. In addition, all Tethyan sections record higher Mn contents during the delta C-13 shift, indicating rather well-oxygenated bottom waters in the western Tethys and the presence of anoxic basins elsewhere, such as the restricted basins of the North Atlantic and Weddell Sea. We propose that the Valanginian delta C-13 shift is the consequence of a combination of increased OM storage in marginal seas and on continents (as a sink of C-12-enriched organic carbon), coupled with the demise of shallow-water carbonate platforms (diminishing the storage capacity of C-13-enriched carbonate carbon). As such the Valanginian provides a more faithful natural analogue to present-day environmental change than most other Mesozoic OAEs, which are characterized by the development of ocean-wide dysaerobic to anaerobic conditions.