Planktic Foraminiferal Porosity Analysis as a Tool for Paleoceanographic Reconstruction, Mid-Cretaceous Western Interior Sea

Abstract

Based on study of modern foraminifera, changes in test po-rosity of ancient planktic foraminiferal specimens may be interpreted to reflect primarily changes in water tempera-ture. To test this hypothesis, planktic foraminiferal porosity data (percent pore space per unit area) of Hedbergella del-rioensis from three localities in the Cretaceous Western In-terior basin were analyzed in the context of basin history (e.g., relative sea-level). These data record spatial and tem-poral changes in water masses at the average living depth of H. delrioensis. A significant increase in porosity between the upper Cenomanian M. mosbyense biozone and lower S. gracile biozone suggests an increase in water mass temper-ature, reflecting northward advection of a central core of warmer (less dense) water into the central seaway. The spread of this water mass toward the north, and laterally east and west, correlates with an independent record of ris-ing sea-level. In addition to this secular porosity trend, higher frequency fluctuations may reflect transient oceano-graphic events related to changes in sea-level or circulation. In this study, porosity measurement was improved utilizing a scanning electron microscope and image analysis soft-ware. The work confirms prior results that as few as five fo-raminifera are required to produce a statistical assessment of average sample porosity. Use of pore concentration (num-ber of pores per unit area), previously suggested as a tem-perature proxy, was not supported. The major interpretative contribution is a temporally precise record of spatial chang-es in Western Interior water masses.