Effects of an oceanic anoxic event on the stable carbon isotopic composition of early Toarcian carbon

Abstract

The Early Toarcian transgression is marked by the occurrence of organic carbon-rich shales in large parts of western Europe and in other parts of the world as well. Based on the positive carbon isotope excursion of pelagic limestones in the middle to upper part of the falciferum Zone in several of the Tethyan sections, the widespread occurrence of the Early Toarcian shales was explained by an Oceanic Anoxic Event (OAE) (Jenkyns, 1988; Jenkyns and Clayton, 1997). The rapid burial of large amounts of organic carbon, which is rich in 12C, would have led to a relative enrichment in 13C of the global carbon reservoir and hence to an increase in δ13C of the limestones. In Southwest Germany (and a number of other locations) both organic and inorganic carbon display a negative isotope excursion in the upper part of the tenuicostatum Zone and the lower part of the falciferum Zone synchronous to black shale deposition and predating the positive isotopic excursion (Küspert, 1982, 1983). It is presently unclear what the exact cause for this widespread negative isotopic excursion is (Jenkyns and Clayton, 1997). The objective of our study was to examine if the negative carbon isotope excursion of organic matter (OM) in the Southwest German Toarcian shales can be attributed to compositional changes of the OM, or if the excursion is related to variations in δ13C[CO2]aq in the photic zone. To this end, we analyzed the molecular and stable carbon isotope composition of OM in ten samples spanning the Early Toarcian (TOC 2.5-10.5 percent; HI 350-700). Biomarker distributions and kerogen pyrolysates differ only slightly among the samples, and δ13C values of primary production markers follow closely those of associated carbonate and kerogen (δ13CCARB/TOC). This strongly suggests that differences in δ13CTOC are related to differences in δ13C of [CO2]aq in the photic zone supporting Küspert's model of advection of isotopically light, organic matter-derived CO2 from anoxic bottom waters. Evidence for euxinic conditions extending into the photic zone was found by the presence of derivatives of carotenoids, such as isorenieratane, from anoxygenic phototrophic sulfur bacteria. Their concentrations are extremely low in black shales from the tenuicostatum Zone but are significant in the falciferum and bifrons zone with a maximum in the exaratum Subzone suggesting that photic zone euxinia (PZE) was a common feature of the Early Toarcian sea in Southwest Germany. Our data, therefore, support the Küspert model of advection of isotopically light, organic matter-derived CO2 from anoxic bottom waters to the upper part of the photic zone resulting in a negative isotope spike. The enrichment in 13C in the middle of the falciferum Zone, which followed the negative spike, may be attributed to the burial of isotopically light organic matter through worldwide black shale deposition.