HOLOCENE SEA LEVEL RISE AND PALEO-ENVIRONMENTAL CHANGE WITHIN TRINITY RIVER PALEO-VALLEY OFFSHORE GALVESTON BAY, GULF OF MEXICO

As the Texas Gulf Coast becomes more vulnerable to sea level rise, it is increasingly important to constrain the response of different parts of the coastal environment to this forcing. Differing effects are expected due to variable rates of sediment supply and subsidence. Paleo records can augment historical data to determine long-term sea level histories of the Texas shelf. Just as the modern Trinity River empties into Galveston Bay, offshore beneath the modern seafloor lies the Trinity River paleo-valley and overlying paleo-estuarine fill which record sea level rise following the Last Glacial Maximum. In addition to a record of the marine transgression, the paleo-river valley contains fluvial sand deposits (e.g., point bars) which represent a crucial resource on the sand-poor Texas shelf and may be used for coastal rejuvenation projects. Here, we present paleo-environmental data from biostratigraphic analysis of sediment cores from the Trinity River paleo-valley to provide a more complete reconstruction of the former river valley. Carbon-dated macrofauna found in the sediment cores provide exact timing on relative sea level rise and place these cored depths in context. Geophysical (chirp) survey data image sedimentary strata which record the evolution of the Texas shelf during the Holocene transgression. Sedimentary and benthic foraminifera analysis of piston cores reveal a rise in sea level and a change in the paleo-environment from fluvial to estuarine over a several kyrs. Our data indicate the development and long-term stability of an estuary offshore of the modern Galveston Bay. Samples shift between the predominance of the genera Ammonia (indicating a middle bay environment) and Elphidium (indicating an outer bay environment), within low diversity assemblages, which indicates an estuarine environment filled the paleo-valley that shifted from middle to outer bay. These data indicate fluctuations in relative sea level and/or salinity, but overall also imply long-term stability of an estuarine environment during the Holocene. Outer bay facies are overlain by the modern seafloor, less than 20 km offshore, indicating either rapid sea level rise or a sudden shift in sediment supply terminated the paleo-estuary.

Keywords: Foraminifera, Galveston Bay, Holocene, Sea Level Change