New insights in the pattern and timing of the Early Jurassic calcareous nannofossil crisis
Abstract The Toarcian calcareous nannofossils crisis associated with the early Toarcian Oceanic Anoxic Event (T-OAE) in the Early Jurassic period is thought to represent one of the most important biocalcification crises during the Mesozoic, occurring simultaneously with profound disturbance of the carbon cycle. However, the causes are still under debate, particularly with regard to the pattern, timing of the biocalcification crisis, relative roles of intrinsic and extrinsic processes as drivers of the crisis, and also causal mechanisms of the T-OAE. In this study, a new quantification of Toarcian calcareous nannofossil abundance and size is presented for the Sancerre borehole (Paris Basin, France). Beyond the recognition of a severe biocalcification crisis defined by the major drop in abundance, and the reduction in size of the most important pelagic carbonate producer Schizosphaerella punctulata , for the first time, this study proposes an insight into the pace and timing of the nannoplankton crisis. At Sancerre, the carbonate production of the lower Toarcian sediments previously attributed to obliquity forcing of climate allows estimating a duration of ~ 210 kyr for the biocalcification crisis and of ~ 120 kyr for the shift towards lower carbon isotope values. The onset of the biocalcification crisis marked by a fertility event lasted ~ 60 kyr, and the calcium carbonate values remained low for ~ 150 kyr; the subsequent recovery of carbonate and nannoplankton lasted ~ 60 kyr and > 550 kyr, respectively. Additionally, a link between the biocalcification crisis, the seawater palaeotemperature, and the carbon isotope steps can be demonstrated. This covariance provides compelling evidence of fundamental change in the response of the climatic warming and the carbon cycle systems triggering the biocalcification crisis. These observations indicate that the biocalcification crisis can be regarded as a direct or indirect consequence of a global warming. Moreover, a deficiency of the biological pump is proposed here, as a complementary causal mechanism for explaining the negative carbon-isotope excursion.