KEYNOTE: OCEANIC ANOXIC EVENTS: CARBON AND NITROGEN ISOTOPE SIGNATURES AND THEIR IMPLICATIONS

Mesozoic “Oceanic Anoxic Events” coincide with rising sea level and warm, generally stably stratified seas. The related mechanisms that induce widespread deposition of organic-carbon rich sediments (“black-shales”) involve some combination of elevated productivity and enhanced organic carbon preservation under oxygen-depleted water masses. The trigger(s) for these events is (are) unknown, but flood basalt volcanism and (or) decomposition of clathrates have been implicated.

We also do not know what sustains organic-carbon burial for events lasting as long as 1 million years. OAE-associated black shales are characterized by high orgC/Ptot and orgC/Ntot ratios, in many cases much higher than Redfield stoichiometry. This suggests preferential release of phosphate and nitrogen from particles in the water column or during early diagenesis as a result of anaerobic degradation of organic matter and reduction of metal oxide particles. Recent research suggests that a substantial proportion of what has typically been considered “organic P” is sorbed to metal oxide coatings on cell walls and, therefore, is easily liberated in dysaerobic to anaerobic conditions. Such rapid recycling of P, in particular, could be a strong positive feedback to productivity in surface waters of anoxic basins. However, availability of N is limited because of N consumption during denitrification or annamox reactions in the shallow chemocline of anoxic basins. Thus, without further addition of N, productivity is limited. However, N isotope and limited biomarker data from some black shales seems to indicate that N fixation becomes a dominant mode of organic matter production because of the enhanced P availability and N limitation. The carbon isotopic signature of organic and carbonate carbon deposited during OAEs provides some constraints on the causes and consequences of these events. Such data cannot simply be interpreted as reflecting increasing burial of organic carbon during OAEs. Numerical models and a globally extensive Cretaceous geochemical data set will be used to illustrate these principles.