Abstract:
During the last Quaternary sea level fall (120-20 ka), Baffin Bay was formed by down-cutting of the Los Olmos, San Fernando, and Petronila Creeks of south Texas. When sea level rose, this incised valley was then filled with mixed siliciclastic/carbonate sediments that record coastal environmental change over the past 10 ky. Previous sedimentological and seismic analysis shows that Baffin Bay contains atypical depositional environments as a result of its semi-arid climate setting and isolation from the Gulf of Mexico. Three prominent stratigraphic surfaces can be recognized within the bay deposits, and are chronostratigraphically constrained using radiocarbon dates. The purpose of the present study is to use foraminifera to create a separate account of change and to determine if foraminiferal data corroborate sedimentological evidence for sea level and climate fluctuations. Foraminifera were sampled at 20 cm intervals from a 14.4 m dated core and from surface and subsurface sediments of five cores along a dip transect. Multiple discriminate analysis was used to compare sections of the core by species proportions, and clearly delineates three different foraminiferal communities: deltaic, open-bay, and hypersaline. Breaks between these communities coincide with two of the surfaces observed in the core, one at about 8.0 ky and the other around 5.5 ky. Rapid sea level rise at the 8.0 ky flooding surface corresponds with a shift from a deltaic to an open-bay foraminiferal assemblage, while faunal change across the 5.5 ky surface is due to the formation of a large barrier island (Padre Island) and the onset of more arid climate conditions. Prior to the isolation of Baffin Bay at 5.5 ky, foraminiferal assemblages do not correspond to climate change records, perhaps because open circulation with the Gulf of Mexico tempered regional climate effects on bay salinity. After 5.5 ky, changes in foraminiferal assemblages correspond to independently derived records of the El Niño-Southern Oscillation and North American monsoon. Foraminiferal analysis supports sedimentological interpretations in that assemblages and sediments track climate change.
