PHOSPHORUS CYCLING DURING TRANSGRESSIVE PHASE OF THE CENOMANIAN-TURONIAN GREENHORN CYCLOTHEM - WESTERN INTERIOR BASIN

Although the ultimate cause of Mid to Late Cretaceous Oceanic Anoxic Events (OAE's) remains unknown, there is a general consensus that increased rates of marine primary production were involved. Since enhanced production requires increased delivery of nutrients to surface waters, a better understanding of nutrient cycling during OAE's may provide a means to identify the ultimate driving mechanism. This study uses sequential extraction techniques to speciate five separate phases of phosphorus (P) and calculates both concentration and mass accumulation rate for each phase. The objective is to determine the role of P as a key limiting nutrient for primary production in the Western Interior Basin (WIB) during the Cenomanian – Turonian (C-T) OAE II. The OAE II interval occurs in this and other basins just prior to the peak highstand of a long term rise in sea level; maximum transgression occurs immediately after OAE II.

The long-term Cenomanian sea level rise is represented in the WIB by a succession of facies reflecting 1) high fresh water input, a predominantly stratified water column, and dominance of terrestrial organic matter (early transgression); 2) open marine conditions, an intermittently strongly stratified water column, and dominant marine algal organic matter (mid-transgression); and 3) open marine conditions with periods of orbitally influenced oscillation between well mixed and stratified water columns (late transgression and OAE II). Analysis of samples representing these different depositional conditions allows a test of the effect that changes in terrestrial input, marine chemistry, and circulation had on P cycling and rates of organic matter production and burial in the WIB. Preliminary evidence indicates a P cycle dominated by terrestrial influence during early transgression, but with continued sea level rise detrital P decreases, most likely trapped in coastal estuaries. This suggests that the P needed for enhanced production during OAE II was generated by recycling within bottom and pore waters or external sources. Data on P concentration and mass accumulation rates under different redox conditions suggests that P recycling was not exceptional in the WIB during OAE II and therefore that external P sources may have been necessary to account for the enhanced productivity associated with this event.