ELEVATED ANTHROPOGENICALLY DRIVEN SUBSIDENCE RATES WITHIN GALVESTON BAY PROVIDES AN OPPORTUNITY TO DETERMINE IF SEDIMENTATION WITHIN A COASTAL PLAIN ESTUARY KEEPS PACE WITH RAPID SUBSIDENCE

Galveston Bay (GB) is the second largest estuary in the Gulf of Mexico, with the watershed containing one of the largest concentrations of petroleum and chemical industries globally, particularly within the lower 15 km of the San Jacinto River/Houston Ship Channel (SJR/HSC). Throughout the last century, extensive groundwater extraction to support these industries and an expanding population has resulted in significantly enhanced land subsidence (0.6-3.0 cm yr^-1). More distal areas in East and West GB have subsidence rates on the order of 0.2 cm yr^-1 , comparable to eustatic rates. The elevated rates fall within the range of elevated sea level rise rates for the next century estimated by Intergovernmental Panel on Climate Change (IPCC), of 0.26-0.55 cm yr^-1. In order to examine the impact of these anthropogenic alterations to the system, 22 vibracores were collected throughout the bay and ²¹⁰Pb and ¹³⁷Cs radioisotope geochronologies and grain size distributions were determined. Historical data documenting land subsidence rates for the last century revealed 3 distinctive regions along the shore of GB with elevated anthropogenically driven subsidence, which are: 1) the San Jacinto River Bayhead Delta; 2) Houston Ship Channel and 3) Texas City. Sediment accumulation rates show a gradient with the highest rates in SJR/HSC, and decreasing both seaward (along salinity gradient) and towards low subsidence regions. Average sediment accumulation rate ranges from less than 0.1 cm yr^-1 in the distal portions of the bay to 1.9 ± 0.5 cm yr^-1, with highest rate proximal to Houston Ship Channel. These results indicate sedimentation rates are dramatically higher (1.4-1.9 cm yr^-1) in areas with elevated Relative Sea Level Rise (RSLR) and are on the same order as subsidence rates, but, in general, sedimentation rates are lower (as much as 50%) than RSLR.