Abstract: Living Shelled Microzooplankton (Radiolarians, Foraminifers, and Pteropods) as Indicators of Oceanographic Processes in Water of Outer Continental Shelf of South Texas

Ann Leavesley, Mary Bauer, Ken McMillen, Richard Casey

Radiolarians, planktonic and benthonic foraminifers, and pteropods collected in plankton tows from waters overlying the outer continental shelf of South Texas have been correlated with biologic, physical, chemical, and geologic data collected during the same period, 1975 through 1977. Planktonic foraminifers and radiolarians appear to indicate open-marine conditions and occur in abundance over the shelf during periods of intrusion of shallow or deep (upwelling) water from the open gulf. Pteropods and benthonic foraminifers characterize the water column of shelfal waters. Globigerina falconensis and Globigerina quinqueloba were the dominant planktonic foraminifers in the well-mixed water (to a depth of 70 m) of the winter. In late winter or early spring of 1977, presence o the radiolarian Spongotrochus glacialis indicated mid-shelf upwelling of deep open-gulf water. Continental runoff dominated the shallow water during the spring and carried Bolivina lowmani beyond the shelf break into water overlying the pycnocline. This continental runoff seems to create an "open-ocean estuarine type of upwelling" by bringing mesopelagic forms onto the shelf under the pycnocline. Globigerina bulloides and Globigerinoides ruber dominate shelf waters above the thermocline in summer, and offshore radiolarians and Globigerina falconensis occur beneath the thermocline. During the summer of 1976, an anticyclonic gyre (detached from the loop current) grounded itself on the shelf. This gyre carried radiolarians indicative of subtropical underwater onto the shelf, and its rotati n seems to have been the dominant circulation pattern during that summer. The gyre transported a population of Bolivina lowmani northward. The shelled microzooplankton appear to be excellent indices for current and water-mass movements on the shelf and for intrusions of open-ocean water masses. These studies, therefore, should aid in deciphering similar parameters on comparable ancient shelves.

AAPG Search and Discovery Article #90965©1978 GCAGS and GC Section SEPM, New Orleans, Louisiana