Quantifying rates of coastal subsidence since the last interglacial and the role of sediment loading

The rate of sea-level rise is expected to increase over the next century. In many areas, increasing rates of sea-level rise are exacerbated by subsidence. In order to develop proper mitigation strategies for coastal change, better estimates for the rates of subsidence are needed. In this study we outline a strategy for calculating long-term subsidence rates for coastlines based on the differential elevations of modern shorelines and their last interglacial (LIG) equivalent geomorphic features. We apply this strategy to the LIG shoreline of the USA Texas coast. We first obtained optically stimulated luminescence ages of features long conjectured to be LIG, but, until now have remained undated. We use a digital elevation model to calculate the difference in elevations between the modern and MIS5e shorelines. This difference is corrected for glacial-hydro-isostatic adjustments to the Texas coast over the last 120 ky. Our analysis shows spatial variability in the rate of subsidence that increases seaward and at locations closer to the Brazos/Colorado delta. The lowest rates of subsidence were 0.03 mm/yr at the most inland site. The highest rates were 0.09 mm/yr near the modern Brazos/Colorado Delta. The spatial pattern of subsidence suggests that most of the long-term vertical motion along the Texas coast is due to sediment loading. The rates of subsidence along the portions of the Texas coast are equal to, and in some places greater than, glacial-isostatic adjustments (GIA), thus highlighting the importance of other vertical motions such as sediment loading when using sea-level data to constrain GIA models even in the absence of active tectonics. In addition, these rates are two orders of magnitude less than modern rates of relative sea-level rise recorded at tide gauges along the Texas coast, highlighting the importance of Holocene compaction and fluid withdrawal in accelerating rates of subsidence along the Texas coast.