| Abstract: | A current popular model for the sporadic occurrence of ocean anoxic events (OAEs) in the Cretaceous ties hydrothermally-induced changes in ocean chemistry to increased surface productivity, followed by mid-to-deep water oxygen depletion and accumulation of organic-rich sediments. This proposed connection is far from accepted, and important unresolved aspects include the timing of events and yet-to-be-proved synchroneity of volcanism and OAEs, the sensitivity of phytoplankton to bio-limiting (and toxic) trace metals, the difference in biotic responses at various OAEs, and the source of the hydrothermal inputs (sea floor spreading centers or ocean plateaus?).
Important factors that could help distinguish ocean plateau (surfacing mantle plume) from sea floor spreading (new centers or increased rates) hydrothermal inputs to the oceans are: (1) the timing and duration of volcanic activity, (2) trace metal patterns created by degassing vs water-rock hydrothermal exchange, (3) depth of eruptions and thermal buoyancy of warm water plumes, and (4) scale of heating of deep ocean water. The abrupt nature of changes in redox conditions and sediment composition at OAEs is hard to reconcile with more gradual changes in sea floor spreading conditions. Trace metals enter a magmatic gas phase (governed by volatility) in different proportions to their partitioning in low temperature water-rock reactions. The construction of ocean plateaus to shallow water depths and the large volume of single eruptions favor warm water plumes that rise to the ocean surface; hydrothermal plumes from spreading centers do not rise above ~1km above the sea floor in today's ocean. The sensible heat and latent heat released by single large (~1000km3) lava flows could raise the temperature of a significant volume of the deep ocean, with possible effects being de-stabilization of gas hydrates and breakdown of water column density stratification.
Ocean drilling can make a major contribution to investigating the first two of these subjects. High resolution studies have and will continue to reveal the time scale and nature of biostratigraphic, lithologic and chemical changes at OAEs. An array of drilling sites designed to recover continuous sections for targeted OAEs, will document "near-field" and "far-field" effects, trace element abundance patterns, and Cretaceous ocean circulation. Advances in analytical methods (ICP-MS) open the way for detection of a wide range of potentially diagnostic elements from large numbers of samples. |