COUPLED ISOTOPIC EVIDENCE FOR ELEVATED pCO2 AND NITROGEN LIMITATION ACROSS THE SANTONIAN-CAMPANIAN TRANSITION

The Santonian-Campanian (S-C) boundary (ca. 83.5 Ma) notably coincides with the termination of the Cretaceous Normal Superchron that lasted nearly 40 million years, as well as a positive carbon isotope anomaly and enhanced preservation of organic matter associated with an oceanic anoxic event (OAE 3). The driving factors for the OAEs must include higher temperatures, 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 ~3x the size of Lake Superior) that preserves two thick oil shale horizons and a positive carbon isotope anomaly near the S-C boundary. This freshwater basin is one of many that formed as a consequence of Mesozoic and Cenozoic 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 analyses suggest maximal fractionation (ε_TOC) at the S-C boundary that may result from enhanced burial of organic carbon and inputs from anoxygenic photoautotrophs and chemoautotrophs, or from the buildup of CO₂ in the lake environment. On the other hand, the nitrogen isotope data suggest that the lake became nitrogen limited 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 describe from the Songliao Basin are recognized in the Cretaceous OAEs. Insofar as climatically-sensitive lakes are typically supersaturated with CO₂ relative to the atmosphere, we speculate whether warming may have released enough greenhouse gas (CO₂, CH₄, and NO_x) into the atmosphere to enhance global temperatures and widespread anoxia in the oceans.