Calcium isotope evidence for dramatic increase of continental weathering during the Toarcian oceanic anoxic event (Early Jurassic)

International audienceThe early Toarcian was punctuated by pulses of massive carbon injection that are thought to have triggered, through increased greenhouse conditions, elevated continental discharge and nutrient input, marine anoxia, seawater acidification and species extinctions. Nevertheless, the mode and tempo of changes in continental weathering across this interval remains highly debated, leading to considerable uncertainty about the main causes of these perturbations. In this study we present calcium isotope measurements (delta Ca-44/40) of well-preserved brachiopods and bulk rock samples from the hemipelagic strata of Pliensbachian-Toarcian age of Peniche in Portugal in order to constrain changes in the calcium cycle and hence changes in continental weathering during the early Toarcian. The data reveal a similar trend as carbon isotope data from the same section and show negative excursions of about 0.5 parts per thousand at the Pliensbachian-Toarcian transition (PI-To) and at the base of the Toarcian Oceanic Anoxic Event (T-OAE) interval. The comparison of delta Ca-44/40 ratios recorded in brachiopods and bulk rock corrected for variable dolomite contribution indicates that these excursions reflect changes in the global isotopic composition of seawater rather than changes in the dominant mineralogy of calcifying organisms or in hydrological budget of the considered basin. Box modeling results suggest that the PI-To and T-OAE delta Ca-44/40 excursions can be explained by a transient 90% decrease of carbonate accumulation due to seawater acidification followed by a 500% increase in continental weathering rates. The sharp increases in continental weathering inferred from the delta Ca-44/40 ratios seem overall consistent with lower Toarcian sedimentological and biotic records that document rapid crises in carbonate production followed by episodes of increased calcium carbonate burial. Nevertheless, the maximum of carbonate burial recorded by most NW European basinal successions occurs several hundreds of kyrs after that predicted by box modeling results. This mismatch either implies that the European records of carbonate accumulation do not reflect global trends or that the fundamental processes related to the removal of excess alkalinity caused by increased continental weathering are more complex than previously appreciated. Based on the amount of Ca input simulated by box modeling, the injection of tens of thousands of gigatons of carbon with an isotopic composition (delta C-13) comprised between -6 parts per thousand and -14 parts per thousand appears as the most likely causes of the delta C-13 excursions characterizing these two events. These results indicate that environmental and biotic changes of the PI-To and T-OAE were mainly caused a cascade of environmental changes triggered by the massive carbon emissions from the Karoo-Ferrar volcanism. (C) 2014 Elsevier B.V. All rights reserved