Foraminiferal Analysis of Holocene Sea Level Rise within the Trinity River Incised Paleo-Valley, Offshore Galveston Bay, Texas

Patricia Standring , Christopher Michael Lowery, Jacob Burstein, John Swartz, JOHN A GOFF , Sean Gulick

Sea-level is expected to continue to rise in the next century, and as society prepares to deal with this hazard it is critically important to understand how coastal systems will respond, especially in regions with rapid rates of coastal erosion and relative sea-level rise like the Gulf of Mexico Texas coast. Tide gauge records in Galveston Bay, Texas, indicate that local sea level rise rates are more than twice the global average, raising important questions about the long-term stability of the barrier islands protecting the bay and how the estuary and coastline will respond to sea-level rise. However, tide gauge records only go back to the beginning of the last century, and longer records are needed to provide insight into dynamic coastal response to sea-level fluctuations. Here, we combine geophysical (chirp sub-bottom profiler) surveys and sediment cores (providing sedimentological and micropaleontological data constrained by radiocarbon dating) to characterize paleoenvironmental change in the Holocene estuary system offshore modern Galveston Bay over the last ~10 kyr; with the first 4 kyr of this time span undergoing a period of rapid sea level rise more than twice the modern rate. Our foraminiferal analysis provides ecological context on the stability of these paleoenvironments and the timing of coastal change over the last ~10 kyr. We provide a model of Holocene shoreline change differing from existing interpretations of rapid landward shifts with asymmetric coastal geometry to one composed of more gradual transitions matching modern coastal geometry and argue for an overall stable paleoestuarine environment throughout the middle Holocene (~6.9 ka – 8.8 ka). Subsequent shoreline shifts occurred after global sea level rise slowed below modern rates, indicating hydroclimate impacts on sediment flux likely had a greater influence on the earlier stability of the estuarine system and later shoreline retreat than rates of sea-level rise.