The record of global change in mid-Cretaceous (Barremian-Albian) sections from the Sierra Madre, northeastern Mexico

Our current understanding of mid-Cretaceous global change is largely based on investigations of pelagic sections from southern Europe and deep sea dri lling sites. Much less information exists from other continents and from he mipelagic sections deposited on continental margins. This investigation see ks to broaden our understanding of mid-Cretaceous global change by focusing on the record from hemipelagic sections deposited along the continental ma rgin of northeastern Mexico. The major goals are to compare the record, tim ing, and extent of the Oceanic Anoxic Events (OAEs) in Mexico and other are as, and to determine the relationship between these events and the global b urial of organic material using carbon isotopes. We have investigated four sections from the Sierra Madre Oriental, integrat ing biostratigraphy, magnetostratigraphy and carbon isotope stratigraphy. C arbon isotopes, measured on the organic carbon (C-org) fraction, show ident ical stratigraphic changes to curves from Barremian to lower Albian Europea n and Pacific deep-sea sections. Our results add new detail to the C-isotop e stratigraphy of the middle and late Albian interval. Three abrupt peaks i n C-org content correlate with OAE1a (early Aptian), OAE1b (early Albian) a nd an event in the late Aptian Globigerinelloides algerianus Zone. Ail thre e events are marked by short-term, 0.5-3 per mil decreases in C-isotope val ues followed by increases of similar magnitude. The decreases may reflect c hanges in the type of C-org, the nature of carbon cycling, or an increase i n hydrothermal activity. The increases in C-isotope values reflect widespre ad burial of C-org. The similar shape of the C-isotope curves in Mexico and other areas, and the response of C-isotopes to the OAEs, indicate that the late Aptian episode was extensive, and that OAE1a and OAE1b were global. The three anoxic events appear to correlate with rising relative sea level. OAE1a also corresponds to major changes in nannofossil assemblages; the we ll-known "nannoconid crisis" can be easily recognized in the Mexican sectio ns. This event is characterized by an increase in abundance of nannofossils and foraminifera in sediments, possibly reflecting a decrease in dilution as a result of the rise in relative sea level.