New insights into the early Bajocian (Middle Jurassic) carbon cycle perturbation

Mesozoic strata record numerous negative and positive carbon isotope excursions (CIEs). The Middle Jurassic records a negative ~0.5 ‰ CIE at the Aalenian-Bajocian boundary followed by a positive ~1.5 ‰ CIE covering the entire early Bajocian. Although these CIEs are recorded in northern and southwestern Tethys and may reflect perturbations of the global carbon cycle, they remain poorly investigated. In this study, we present new geochemical and sedimentological data from the Chaudon-Norante section in France and Murtinheira section in Portugal to better constrain the origin of the lower Bajocian CIEs. Associated with the previously published carbon isotopic composition of bulk carbonate (δ13Cbulk carbonate) we provide new δ13Corg records, as well as total phosphorus content, phosphorus accumulation rates (AR), and CaCO3 and organic matter data. In contrast with previous interpretations, our results show no evidence of a carbonate production crisis during the early Bajocian. A slight increase in siliciclastic and phosphorus AR would argue for oceanic fertilization but without a parallel increase in organic matter AR. The obtained stratigraphic δ13Cbulk carbonate vs. of the carbon isotopic composition of bulk organic matter (δ13Corg) patterns are distinctive and mimic previous box modeling results simulating an increase in productivity forced by higher phosphorus rates from riverine and weathered-carbon input burial. The subsequent organic carbon burial produced counter-greenhouse conditions, which in turn produced a cooling by CO2 uptake. Our results indicate that the lower Bajocian event shows several similarities with the late Valanginian positive CIE (also known as the Weissert event). An accurate comparison of both events may help for a better understanding of the origin and consequence of such carbon cycle perturbations.