Carbon and oxygen isotope records from the southern Eurasian Seaway following the Triassic-Jurassic boundary: parallel long-term enhanced carbon burial and seawater warming

Abstract

Fossil shells of benthos and nektobenthos have been shown to be faithful recorders of seawater carbon- and oxygen-isotope geochemistry, and thus also useful to track the relationship between carbon cycle and palaeotemperature. In this study we present an extensive dataset from Lower Jurassic (Hettangian and lower Sinemurian) mollusc and brachiopod hard parts collected from biostratigraphically well-calibrated UK coastal outcrops (Bristol Channel and Hebrides basins). These basins lay palaeogeographically in the southern part of the Laurasian Seaway that connected the Tethys and Boreal oceans. All samples have been subject to screening for diagenesis on the basis of elemental composition, light microscopy, and SEM observations. In the case of some localities within the Hebrides Basin, alteration by hydrothermal systems around Paleogene intrusions has led to re-setting of carbonate oxygen isotopes, but the original carbon isotope values from the shells are largely preserved. Above the prominent and apparently short-lived, ~3 per mil δ13Ccarb amplitude positive carbon-isotope excursion (CIE) that occurs immediately above the Triassic-Jurasic (T-J) boundary (in the tilmanni ammonite biozone), a pronounced negative CIE (the so-called Main Negative CIE) spans the entire Hettangian Stage. At the Hettangian-Sinemurian boundary, and through the lower Sinemurian, the carbon-isotope values of the skeletal carbonate again trend towards progressively more positive values, but representing a time of several million years. The heaviest δ13Ccarb values of about ~ +4.3 per mil are evident towards the top of the lower Sinemurian, and are comparable with values observed from the tilmanni Zone, and from the lower Toarcian, higher in the Jurassic. This long-term positive hump, which confirms trends derived from bulk organic matter carbon-isotope records, is supporting evidence of prolonged enhanced organic carbon burial that is inferred to have occurred in the extensive system of lacustrine and marine rifts that traversed a fragmenting Pangaea after emplacement of the Central Atlantic Magmatic Province. In parallel, oxygen-isotope values of the skeletal carbonate show a continuous downward trend from the lower part of the Hettangian (~ −1 per mil δ18Ocarb in the planorbis Zone) to the top of the lower Sinemurian (~ −4 per mil δ18Ocarb in the higher turneri Zone). Oxygen-isotope values may be interpreted as due to gradually increasing palaeotemperature, and/or addition of a meteoric or cryospheric water component; in the case of the Laurasian Seaway, palaeoceanographic and palaeoecological considerations point towards a dominant palaeotemperature signal. Consequently, any atmospheric carbon-dioxide drawdown effect on global palaeotemperatures, as suggested by progressively increasing δ13Ccarb values, and assuming a constant silicate weathering sink, was more than counterbalanced in the seaway by regional processes that led to significantly warmer bottom water temperatures.