Geochemical variations across the Jurassic/Cretaceous boundary in central Mexico. Insights for correlation with Tethyan areas

A multiproxy study of a 29 m-thick carbonate succession in the Apulco area, Puebla State in central Mexico including pyrite framboids, delta C-13(carb), delta O-18(carb), Total Inorganic Carbon (TIC), Total Organic Carbon (TOC) and Organic Matter (OM), reveals minor geochemical changes across the Jurassic/Cretaceous boundary. Anoxic bottom conditions during the Tithonian were registered by high values of TOC and OM, and low values of delta C-13(carb) and TIC. Near the Jurassic/Cretaceous boundary, signatures of all measured proxies changed drastically due to processes related with the organic matter preservation in the basin. These changes can be associated with the late Tithonian sea level drop and paleoclimatic variations passing from dry to semi-arid conditions. Otherwise, on late early to late Berriasian facies, values of all proxies tend to the stabilization indicating more oxygenated bottom conditions. The delta C-13(carb) goes to positive values while delta O-18(carb) and TIC remain constant, the organic matter and total organic carbon decrease to the lowest values. The geochemical behavior of the Apulco section is similar to those reported from other Tethyan sections as Brodno and DSDP 534A and can be preliminary correlated by its chemostratigraphic signal. Additionally, a central age of 141.65 Ma (Sr-87/Sr-86) in the very base of the calpionellid Elliptica Subzone strengthens the calpionellid biostratigraphic framework for Mexican sections.