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

The Early Toarcian transgression is marked by the occurrence of organic car bon-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 li mestones in the middle to upper part of the falciferum Zone in several of t he 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 C-12, would have led to a relative enrichment in C-13 of the gl obal carbon reservoir and hence to an increase in delta(13)C of the limesto nes. In Southwest Germany (and a number of other locations) both organic an d inorganic carbon display a negative isotope excursion in the upper part o f the tenuicostatum Zone and the lower part of the falciferum Zone synchron ous to black shale deposition and predating the positive isotopic excursion (Kuspert, 1982, 1983). It is presently unclear what the exact cause for th is widespread negative isotopic excursion is (Jenkyns and Clayton, 1997). The objective of our study was to examine if the negative carbon isotope ex cursion of organic matter (OM) in the Southwest German Toarcian shales can be attributed to compositional changes of the OM, or if the excursion is re lated to variations in delta(13)C([CO2]aq) in the photic zone. To this end, we analyzed the molecular and stable carbon isotope composition of OM in t en 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 delta(13)C values of primary production markers follow clo sely those of associated carbonate and kerogen (delta(13)C(CARB/TOC)). This strongly suggests that differences in delta(13)C(TOC) are related to diffe rences in delta(13)C of [CO2](aq) in the photic zone supporting Kuspert's m odel of advection of isotopically light organic matter-derived CO2 from ano xic bottom waters. Evidence for euxinic conditions extending into the photi c zone was found by the presence of derivatives of carotenoids, such as iso renieratane, from anoxygenic phototrophic sulfur bacteria. Their concentrat ions are extremely low in black shales from the tenuicostatum Zone but are significant in the falciferum and bifrons zone with a maximum in the exarat um Subzone suggesting that photic zone euxinia (PZE) was a common feature o f the Early Toarcian sea in Southwest Germany. Our data, therefore, support the Kuspert model of advection of isotopically light, organic matter-deriv ed CO2 from anoxic bottom waters to the upper part of the photic zone resul ting in a negative isotope spike. The enrichment in C-13 in the middle of t he falciferum Zone, which followed the negative spike, may be attributed to the burial of isotopically light organic matter through worldwide black sh ale deposition.