Reconciliation of geochronology and paleozoogeography for Quaternary marine terraces, San Luis Obispo Bay area, California, USA

In the San Luis Obispo Bay area of central California, interpretations of marine terrace ages have been hampered by inconsistent results from geochronological indicators (U-series ages of corals and correlations using amino acid racemization of mollusks) and seemingly contradictory paleozoogeographic aspects of fossil faunal assemblages. New U-series ages of corals, amino acid data from mollusks, and detailed analyses of fossil mollusk assemblages are presented that reconcile many of the apparently discordant results from previous studies. The two lowest-elevation terraces are Q1 (lower) and Q2 (upper); both are thought to date from high-sea stands of marine isotope stage (MIS) 5, the last interglacial complex. A combination of U-series dating and amino acid racemization results indicates that the Q1 terrace probably dates to the ∼80 ka high-sea stand of MIS 5a. U-series analyses of corals from the Q2 terrace show open-system histories, but consideration of two alternative open-system histories indicates that this terrace likely hosts corals dating to the high-sea stands of MIS 5c (∼105–95 ka) and MIS 5e (∼130–115 ka). Amino acid ratios in the fossil bivalve Leukoma staminea (Conrad, 1837) support the age differences between the two terraces and the open-system models suggested by the U-series data. Alkenone studies along much of the coast of California have shown that sea surface temperatures (SSTs) of the eastern Pacific Ocean were warmer than present during MIS 5e. During MIS 5c, on the other hand, SSTs were either similar to the present or cooler, but in either case, much cooler than during MIS 5e. The paleozoogeographic aspects of the molluscan faunas from the Q2 terrace support an interpretation that these deposits contain a mixture of fossils from both the MIS 5c and 5e high-sea stands. Fossils from the Q2 terrace include southern or southward-ranging species (likely dating to MIS 5e) and northern or northward-ranging species (likely dating to MIS 5c). A mixture of MIS 5e and MIS 5c fossils on the Q2 terrace is consistent with similar data reported for other localities in California. This phenomenon is explained by glacial isostatic adjustment (GIA) processes, which brought about a higher-than-present sea level during MIS 5c. GIA processes, combined with a low uplift rate, allowed mixing of MIS 5e fossils during the MIS 5c high-sea stand through capture of an MIS 5e terrace that had experienced minimal uplift in ∼20,000 years. Terrace reoccupation and fossil mixing such as that in the San Luis Obispo Bay area can be expected along any part of the Pacific Coast of North America that was affected by GIA processes and where uplift rates are low.