CENOMANIAN VOLCANISM AND ITS INFLUENCE ON CARBON CYCLING DURING OCEANIC ANOXIC EVENT 2 IN THE TROPICAL REALM

Secular episodes of enhanced anoxia in the Mesozoic ocean, collectively termed oceanic anoxic events-OAEs, led to the deposition of organic-rich sediments in a wide range of marine environments. A cause-effect relationship between volcanism, ocean anoxia, enhanced organic carbon burial, and carbon cycling disruption has been proposed for the OAEs. Among these events, the Oceanic Anoxic Event 2 is characterized by a well-developed positive d¹³C anomaly both in carbonate and organic substrates. Recently, the evolution of the d¹³C_TOC excursion has been presented in a highly-resolved chronostratigraphic framework along the Western Interior Seaway. Stratigraphic correlations between this area and other localities worldwide enable the identification of a typical d¹³C_TOC anomaly consisting of a first positive d¹³C_TOC maximum interrupted by a sudden return to more negative d¹³C_TOC values, a return to heavier and sustained d¹³C_TOC values, and finally a gradual return to pre-excursion values. Less well-studied are the precursor events, periods of fluctuating positive and negative peaks in d¹³C_TOC, prior to the main positive anomaly. Two Cenomanian-Turonian organic-rich mudstone successions from the Upper Magdalena Valley, Colombia display d¹³C_TOC variability that is typical of OAE 2, in addition to the precursor events. Thin ash beds are commonly found (ca. 60 beds) and provide clues to the coupling between carbon cycling and regional volcanism. The coincidence of the d¹³C_TOC variability prior to OAE 2 and the increasing occurrence of ash layers suggests a link between volcanic activity and the carbon cycle preceding the onset of the isotopic anomaly. Interestingly, after its onset, the remaining isotopic anomaly and its short-term fluctuations seem to not be fully driven by volcanism. We suggest that internal feedback dynamics in the climate-ocean system are most likely the responsible mechanism leading to the return of the carbon cycle to pre-event levels.