New high resolution geochemistry of Lower Jurassic marine sections in western North America: A global positive carbon isotope excursion in the Sinemurian?

- New osmium isotope and carbon isotope data for two Early Jurassic marine sections.
- Gradual positive CIE in upper Leslei Zone at Last Creek.
- Last Creek positive CIE likely reflects a global carbon isotope signature.
- Locally driven abrupt negative CIE also observed at Last Creek.
- Os isotopes show that both LC and FCD were deposited in contrasting environments.

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.