Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes

The Early Toarcian Oceanic Anoxic Event (T-OAE), about 183 myr ago, was a global event of environmental and carbon cycle perturbations, which deeply affected both marine biota and carbonate production. Nevertheless, the long-term environmental conditions prevailing prior to the main phase of marine extinction and carbonate production crisis remain poorly understood. Here we present a ∼ 8 myr-long record of Early Pliensbachian–Middle Toarcian environmental changes from the Lusitanian Basin, Portugal, in order to address the long-term paleoclimatic evolution that ultimately led to carbonate production and biotic crises during the T-OAE. Paleotemperature estimates derived from the oxygen isotope compositions of well-preserved brachiopod shells from two different sections reveal a pronounced ∼ 5 °C cooling in the Late Pliensbachian (margaritatus–spinatum ammonite Zones boundary). This cooling event is followed by a marked ∼ 7–10 °C seawater warming in the Early Toarcian that, after a second cooling event in the mid-polymorphum Zone, culminates during the T-OAE. Calcium carbonate (CaCO3) contents, the amount of nannofossil calcite and the mean size of the major pelagic carbonate producer Schizosphaerella, all largely covary with paleotemperatures, indicating a coupling between climatic conditions and both pelagic and neritic CaCO3 production. Furthermore, the cooling and warming episodes coincided with major marine regressions and transgressions, respectively, suggesting that the growth and decay of ice caps may have exerted a strong control on sea-level fluctuations throughout the studied time interval. This revised chronology of environmental changes shows important similarities with Neogene and Paleozoic episodes of deglacial black shale formation, and thus prompts the reevaluation of ice sheet dynamics as a possible agent of Mesozoic events of extinction and organic-rich sedimentation.