Coupled isotopic evidence for elevated pCO2 and nitrogen limitation across the Santonian-Campanian transition

Abstract

The Santonian–Campanian (S–C) transition notably coincides with the termination of the Cretaceous Normal Superchron, a positive carbon isotope anomaly, and the enhanced preservation of organic matter associated with an oceanic anoxic event at ca. 84 Ma. The driving factors for the OAEs should include global warming, widely believed to be from volcanic sources, the delivery of nutrients to surface oceans from the continents, anoxic sediments and atmosphere, and variations in the configuration of ocean basins. To further investigate this phenomenon and evaluate whether there was a teleconnection between the marine and continental realm, we have studied a lacustrine succession in northeastern China (the Songliao Basin, which is ~3× the size of Lake Superior) that preserves two thick horizons of oil shales and a positive carbon isotope anomaly recorded in ostracode carbonate near the S–C boundary. This freshwater basin is one of the many that formed as a consequence of Mesozoic extensional tectonics in eastern Asia. The stratigraphic trends illustrate broad excursions in the isotopic compositions of carbonate, total organic carbon (including black carbon), and total nitrogen. The carbon isotope data suggest that maximal fractionation (εTOC) at the S–C boundary is caused by enhanced burial of organic carbon and inputs from anoxygenic photoautotrophs and chemoautotrophs, or from the buildup of CO2 in a lake environment. On the other hand, the nitrogen isotope data suggest that the lake became limited in nitrogen immediately before the deposition of the younger oil shale, resulting in the addition of new nitrogen to the system, likely as the result of diazotrophy or ammonium assimilation. To some extent, all of the features we described from the Songliao Basin have been recognized in Jurassic and Cretaceous OAEs. Insofar as climatically-sensitive lakes are typically supersaturated with CO2 relative to the atmosphere, we speculate that nutrient-driven primary productivity in massive lake systems may have released enough greenhouse gas (CO2, CH4, and N2O) into the atmosphere and therefore played a role in enhancing global temperatures and driving widespread anoxia in the Mesozoic oceans.