THE MARINE TERRACES OF SANTA CRUZ ISLAND, CALIFORNIA: IMPLICATIONS FOR GLACIAL ISOSTATIC ADJUSTMENT MODELS OF LAST-INTERGLACIAL SEA-LEVEL HISTORY

Glacial isostatic adjustment (GIA) models hypothesize that along coastal California, last interglacial sea level could have been as high as +12 m to +14 m at ~120 ka, much higher than a simple eustatic rise of +6 m. Areas with low uplift rates can test whether such models are valid. Marine terraces on Santa Cruz Island have previously been reported to occur at low (<10 m) elevations, but ages of many localities are not known. Using LiDAR imagery as a base, marine terraces on Santa Cruz Island were newly mapped, elevations were measured, fossils collected for U-series dating (corals), amino acid geochronology (mollusks), and paleozoogeography (all taxa). U-series ages of corals from the western part of the island indicate that low-elevation terraces north of the Santa Cruz Island fault are 120 ka. These localities contain mostly neutral, but also some distinctly warm-water taxa, similar to other 120 ka localities in California and Mexico. Where corals are lacking for some low-elevation terrace localities, amino acid ratios and faunal aspects support correlation of these occurrences to the ~120 ka high stand of sea. Elevations of most terrace localities north of the fault, both in western and eastern Santa Cruz Island, range from 5.75 m to 8 m above sea level, well below the modeled paleo-sea level range. Subsidence is ruled out as a mechanism for explaining the lower-than-modeled elevations, because higher-elevation terraces are present along much of the Santa Cruz Island coast north of the fault, indicating long-term tectonic uplift as the dominant tectonic style. Other localities on the Pacific Coast, dated to 120 ka, also have marine terrace elevations below the modeled level, including Cayucos, Point San Luis, Anacapa Island, and Santa Barbara Island (in California) and Isla de Guadalupe, Punta Santa Rosalíllíta, Bahia de Magdalena, Cabo Pulmo, and Puerto Penasco (in Mexico). It is concluded that GIA models may have overestimated last interglacial sea level by a substantial amount and need to be revised if used for forecasts for future sea-level rise.