Plate tectonic trigger of changes in pCO2 and climate in the Oxfordian (Late Jurassic): Carbon isotope and modeling evidence

The transition from the Middle to the Late Jurassic was characterized by significant changes in oceanography and climate and by changes in global carbon cycle as shown in the C-isotope record. A prominent mid-Oxfordian positive excursion in bulk carbonate carbon isotope values (δ13Ccarb) with an amplitude of more than 1‰ has been documented from many sections in the Northern Tethys realm. In this study we present new bulk organic matter C-isotope data (δ13Corg) from northwestern Tethys that do not record the mid-Oxfordian positive excursion in carbonate carbon. On the contrary, δ13Corg decreases during the interval of the most rapid increase in δ13Ccarb. We demonstrate that this decrease is not due to a changing marine–terrestrial organic carbon partitioning but that the contrasting isotope trends record peculiar environmental and climate changes which occurred near the beginning of the Late Jurassic. Using a simple carbon cycle model we show that an increase in atmospheric pCO2 starting at modern levels could be the cause of contrasting trends in δ13Ccarb and δ13Corg. We suggest that a reorganisation of ocean currents related to the opening and/or widening of the Tethys–Atlantic–Pacific seaway, and a massive spread of shallow-sea carbonate production led to higher pCO2. Model simulations indicate that this increase in pCO2 may have triggered changes in the biological carbon pump and in organic carbon burial that can explain the Middle Oxfordian C-isotope record.