MID-CRETACEOUS OCEANIC ANOXIC EVENTS AND PLANKTON EVOLUTION

Oceanic Anoxic Events were short-lived episodes of organic carbon burial during the mid-Cretaceous. Turnover (extinction plus speciation) in the planktic foraminifera closely tracks that of the radiolaria, and both groups display the greatest rates of turnover at or near the OAEs. The calcareous nannoplankton were most strongly affected by the early Aptian OAE1a and the Cenomanian/Turonian boundary OAE2. These two events were the most widespread of the OAEs. Both were associated with increased marine productivity, and both may have been triggered by iron fertilization associated with submarine volcanism. Latest Aptian-early Albian OAE1b is also linked to increased submarine volcanism and marine productivity, but over a protracted interval of time in contrast to the single black shale horizons of OAE1a and OAE2. Carbonate dissolution during all three of these OAEs was caused by the combined influences of volcanically-derived CO2 in the water column and organic carbon accumulation on the seafloor. OAE1a seriously affected the heavily calcified nannofossils (the nannoconids), while both nannoconids and the largest and most heavily calcified planktic foraminifera were either eradicated or severely depleted by OAE1b. OAE1b was a watershed event in the evolution of planktic foraminifera, and it signaled a major change in ocean chemistry favoring calcium carbonate. OAE1b anchors the long-term trend towards rising sea level and warmer climate, both of which peaked in the early Turonian (~92-93 Ma). OAE1b also marks the beginning of the end of widespread black shale deposition and the initiation of chalk deposition. This lithologic transition records a fundamental shift in oceanic water column structure, nutrient partitioning, and plankton ecosystem dynamics which favored stratified, oligotrophic to mesotrophic, diverse, carbonate-based pelagic and benthic ecosystems that came to characterize the widespread chalk seas of the Late Cretaceous. These findings suggest that there were important linkages between submarine volcanism, plankton evolution, and the cycling of carbon through the marine biosphere.