COMPARISON OF PHOSPHROUS, ORGANIC CARBON, CARBONATE, AND IRON BURIAL IN CENOMANIAN-TURONIAN STRATA OF THE WESTERN INTERIOR BASIN

This study focuses on the marine phosphorus (P) cycle and its control on organic matter burial in Cretaceous strata of the Western Interior seaway. In recent years, discussion surrounding the genesis of organic rich strata has focused on three major driving processes: productivity, preservation, and bulk sediment accumulation. These processes may be linked when changes in bulk sediment accumulation and bottom water redox conditions influence the rate of nutrient regeneration in bottom waters and sediments. Phosphate released from organic matter during diagenesis is either taken up by bacteria for use as an energy source or trapped at the redox boundary by iron oxides formed from ferrous iron ions (Fe²⁺) diffusing upwards from reducing sediment of the sulfate reduction zone (SRZ). When the SRZ is at the sediment water interface (SWI) or within the lower water column, phosphate may be released, thus increasing bottom water P concentrations. Upon mixing of the water column, this P becomes available for primary production.

It has been suggested by paleoceanographers that Cretaceous oceans were oligotrophic and that ancient epeiric seas would have acted as extensions of the surface ocean during sea level highstands. Under such conditions, would additions of regenerated P to the background fluvial input have significant effects on production? How would such changes compare to the influence of oxygen minimum zone waters advected into the basin? In this study trends in the concentration and accumulation rate of P in strata of the Western Interior are compared with similar data for carbonate, organic carbon, and iron. Accumulation rate calculations are based on recent development of an orbital time scale for the study interval. An onshore-offshore data transect allows the influence of fluvial P sources to be evaluated, and a stratigraphic data set across the Cenomanian-Turonian boundary reveals the record of changes in P burial associated with major shifts in water masses and benthic redox conditions (i.e., Oceanic Anoxic Event II). The results allow a preliminary model summarizing changes in the Western Interior P-cycle under different oceanographic and biogeochemical scenarios to be proposed.