Tectonic Subsidence of California Estuaries Increases Forecasts of Relative Sea-Level Rise

Abstract

Even along the generally uplifting coast of the Pacific US, local geologic structures can cause subsidence.In this study we quantify Holocene-averaged subsidence rates in four estuaries (Carpinteria Slough, Goleta Slough, Campus Lagoon, and Morro Bay) along the southern and central California coast by comparing radiocarbon-dated estuarine material to a regional sea-level curve. Holocene-averaged rates of vertical motion range from subsidence of 1.4+/-2.4 mm/yr, 1.2+/-0.4 mm/yr, and 0.4+/-0.3 mm/yr in Morro Bay, Carpinteria Slough, and Goleta Slough, respectively, to possible uplift in Campus Lagoon (-0.1+/-0.9 mm/yr). The calculated rates of subsidence are of the same magnitude as rates of relative sea-level rise experienced over the late Holocene and effectively double the ongoing rates of relative sea-level rise experienced over the last five decades on other parts of the coast. The difference in rates of vertical motion among these four estuaries is attributed to their geological settings. Estuaries developed in subsiding geological structures such as synclines and fault-bounded basins are subsiding at much higher rates than those developed within flooded river valleys incised into marine terraces. Restoration projects accounting for future sea-level rise must consider the geologic setting of the estuaries and, if applicable, include subsidence in future sea-level rise scenarios, even along the tectonically uplifting US Pacific Coast.

Shelf evolution along a transpressive transform margin, Santa Barbara Channel, California

“…5A and 12) that might be a broad lowstand paleochannel system. Simms et al (2016a) suggested that a combination of synclinal folding and downdropping on the Rincon Creek fault is responsible for active subsidence (~1.2 mm/yr) in the Carpinteria slough. Johnson et al | Shelf evolution, Santa Barbara Channel GEOSPHERE | Volume 13 | Number 6…”