CAUSAL MECHANISMS FOR OAE'S III: THE ROLE OF PHOSPHOROUS?

Increased rates of marine primary production due to enhanced nutrient flux is one of the favored mechanisms proposed to account for the initiation of Cretaceous Oceanic Anoxic Events. Among the possible candidates phosphorus (P) is believed to be important because of arguments that it is the key biolimiting nutrient on geologic timescales. What remains unclear in this hypothesis is the mechanism by which P delivery to surface waters might have been enhanced. Did the terrigenous P flux increase due to a change in chemical weathering rates? Or did the proportion of P recycled from decomposing marine organic matter change due to a shift in early diagenetic conditions. In this study a sequential extraction procedure (SEDEX) is used to separate 5 phases of P known to occur in modern marine sediments from strata spanning the Cenomanian – Turonian (C-T) OAE II. The study area is the Western Interior Basin (WIB) of North America, where a terrigenous P signal, if present, should be readily detectable. Sampled localities represent a number of sites representing different depositional environments. The sample interval begins in strata representing open marine, but intermittently strongly stratified conditions on sub-orbital timescales that preceded OAE II and extends through the deeper marine OAE II period during which mixed and stratified conditions oscillated on orbital timescales. Development of a new timescale integrating radiometric and orbital components allows concentration data to be converted to accumulation rates. The WIB P data demonstrates a significant increase in the accumulation rates of both detrital and authigenic phases immediately prior to OAE II. The increase in detrital P suggests a change in the P budget related to an increase in the weathering flux of P. A comparison of detrital P accumulation rates in a distal – proximal transect supports the conclusion that the detrital phase is not overestimated authigenic phase as has been suggested. Once OAE II initiates, however, accumulation rates of both phases drop to between 20 – 50% of pre-OAE II values. This suggests a major change in the P burial rate and provides a possible source of the excess P that maintained increased primary production during OAE II. This talk will conclude by considering the relationship between reactive Fe availability and P burial rate.