Abstract
Recognising variations in the carbon isotope compositions of marine organic-rich sedimentary rocks can provide insight into changes in ocean chemistry throughout geological time. Further, identification of global excursions in the carbon isotope record has proved to be valuable as a chronostratigraphic correlation tool.This investigation presents new high-resolution organic carbon isotope data (δCorg13) for marine sediments from 2 regions in North America (Last Creek, British Columbia, Canada and Five Card Draw, Nevada, USA). The carbon isotope profiles demonstrate that there were significant differences between the carbon reservoirs at Five Card Draw and Last Creek, notably in the upper part of the Leslei Zone. The δCorg13 values show a gradual positive CIE (∼2‰) at Last Creek in the upper part of the Leslei Zone. This corresponds to a coeval positive CIE of similar duration in Dorset, UK (upper Turneri Zone; Jenkyns and Weedon, 2013), suggesting that this may be a global marine carbon isotope signature, and likely reflects a widespread increase in primary productivity during the Early Sinemurian. In addition, a brief negative CIE is observed in the uppermost Lower Sinemurian at Last Creek. This negative excursion is not recorded in the Dorset section, suggesting localised upwelling of 12C-rich bottom-waters at Last Creek. Further, the signals identified at Last Creek are not present in coeval sections at Five Card Draw, thus highlighting a significant difference between these localities. Osmium (Os) isotope data (initial 187Os/188Os values) provide a quantitative determination of the contrasting depositional environments of Five Card Draw and Last Creek (at least partially restricted with high levels of continental inundation and open-ocean, respectively). This demonstrates that basinal restriction may act as a major factor that controls isotopic stratigraphic signatures, thus preventing the identification of global or widespread regional excursions.
Highlights
Some of the main intervals of hydrocarbon source rocks are supposed to be linked to the postulated Oceanic Anoxic Events (OAEs), which have been defined (Jenkyns and Clayton, 1986, 1997) on the basis of the presence of black shale facies (>5wt.%Total Organic Carbon (TOC); Bates and Jackson, 1987) worldwide
The lithological succession, sequence stratigraphy, curve of the gamma-ray measured figures, profiles of the δ18Obel and δ13Cbel values obtained from belemnite calcite and the curve of Total Organic Carbon (TOC) variations have been plotted against the 562 measured beds of the Rodiles section (Fig. 3)
The Upper Sinemurian, Pliensbachian and Lower Toarcian deposits of the Rodiles section are constituted by couplets of bioclastic lime mudstone to wackestone, occasionally containing bioclastic packstone facies commonly concentrated in gutters, and grey to black marls, deposited in an open marine external platform environment
Summary
Some of the main intervals of hydrocarbon source rocks are supposed to be linked to the postulated Oceanic Anoxic Events (OAEs), which have been defined (Jenkyns and Clayton, 1986, 1997) on the basis of the presence of black shale facies (>5wt.%TOC; Bates and Jackson, 1987) worldwide. The major forcing event behind these OAEs (Jenkyns et al, 2002), has been attributed to abrupt rises in temperature, induced by rapid influx of CO2 into the atmosphere from volcanogenic and/or methanogenic sources (Jenkyns, 2010). Following these ideas, palaeoclimatological studies, oriented to search for source rocks linked to warming periods could be used as an additional tool to the commonly used in hydrocarbons exploration. The Late Pliensbachian represents an outstanding cooling interval containing a positive δ13Cbel excursion interrupted by a small negative δ13Cbel peak, and the Early Toarcian represented an exceptional superwarming period pointed as the main cause of a prominent mass extinction (Gómez and Goy, 2011)
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