BENTHIC FORAMINIFERAL ASSEMBLAGE FORMATION, ORGANIC-CARBON FLUX AND OXYGEN CONCENTRATIONS ON THE OUTER CONTINENTAL-SHELF AND SLOPE

A model is presented that defines the interaction of benthic foraminif eral shell production and taphonomy over a range of organic carbon flu x and bottom water oxygen values. This model applies to outer continen tal shelf and slope environments where the sediments are hemipelagic m ud, there is net sediment accumulation and limited current transport, and sediment column properties are primarily controlled by sea-bed org anic carbon flux and bottom water oxygen content. Data on sediment bio geochemistry from the Gulf of Mexico and the central California margin are used to define an organic carbon flux-bottom water oxygen field. Within this field, observations of foraminiferal standing stocks and a bsolute test abundances indicate that there are four different product ion-taphonomic regimes. These are: (1) the high organic matter flu (mo re than 6 to 7 gC/m(2)/ yr)-low bottom water oxygen (less than 1 ml/L) setting where there is heterogenous shell generation in the sediment column, limited oxic taphonomy and inefficient sediment mixing resulti ng in highly variable assemblages on small spatial scales; (2) the mod erate organic carbon flux (between 2 and 6 gC/m(2)/yr)-moderate bottom water oxygen (between 1.5 and 3 ml/L) setting where there is heteroge nous shell production in the sediment column, oxic taphonomy in the to p 5 cm of sediments and efficient sediment mixing; (3) the moderate or ganic carbon flux-high bottom water oxygen (greater than 3.5 ml/L) set ting where there are vertically stratified microhabitats, a distinct t aphonomic zone in the upper 3 to 4 cm of the sediments, significant in faunal shell generation below the taphonomic zone and sediment mixing that cannot blend completely across that zone; and (4) the low organic carbon flux-high bottom water oxygen setting where shell production i s close to the sediment-water interface, taphonomy occurs in the upper 1 to 2 cm of the sediments and effective mixing reaches to sediment d epths of 5 to 8 cm. The microhabitats available to the foraminifera an d the changes in the taphonomic-sediment mixing regime within these se ttings can help explain the significant fossil assemblage composition differences across the organic carbon flux-bottom water oxygen field t hat are reported in the literature.