EXPLORING THE COMPLEX MARINE TERRACE PALEOZOOGEOGRAPHIC RECORD OF EARLY PLEISTOCENE INTERGLACIAL PERIODS ON THE CALIFORNIA COAST: AN EXAMPLE FROM SAN NICOLAS ISLAND

Studies of last interglacial (~120,000 yr ago) marine terrace fossil faunas on the California coast have shown that many contain mixes of extralimital northern and southern species of mollusks (called “thermally anomalous” faunas). However, paleozoogeographic studies of early Pleistocene fossil faunas have been limited. Here, results from geochronological and paleontological studies of the 10^th emergent terrace (~236 m above sea level) on San Nicolas Island (SNI) provide an older example of a thermally anomalous fauna. Nineteen fossil specimens of Epilucina californica from SNI terrace 10 gave Sr isotope ages ranging from 1.11 Ma to 1.24 Ma, with a mean age of 1.18 ± 0.04 Ma. These ages indicate probable correlation with marine isotope stage (MIS) 35, although correlations with MIS 37 and 33 cannot be ruled out. The age is consistent with the presence of two extinct mollusks, Pusio fortis and Lottia edmitchelli. Based on the alkenone record from Ocean Drilling Project (ODP) core 1012 from East Cortes Basin (Federov et al, 2013, Nature, doi:1038/nature12003), ~140 km from SNI, sea surface temperatures (SST) during MIS 35 were warmer than present and even warmer than interglacials of MIS 5e and 11. Broadly consistent with this, faunal data from three localities on the 10^th terrace on SNI do include 16 southward-ranging species, but only one possible extralimital southern species (Crepidula cf. C. excavata). Unexpectedly, the deposit also hosts 7 entirely extralimital northern species (Acmaea scutum, Haliotis kamtschatkana, Harfordia harfordi, Lacuna carinata, Lirabuccinum dirum, Neostylidium eschrichtii, Turbonilla valdezi) and 4 additional northward-ranging species. Reworking likely does not explain this, as most of the cool-water forms are not found in higher (older) terraces (Vedder & Norris, 1963, USGS PP 369). SNI at present experiences substantial cold-water upwelling and hosts a modern molluscan fauna that has more northern affinities compared to mainland localities at the same latitude. The cool-water forms present in early Pleistocene marine deposits on SNI imply that upwelling not only was active during a major interglacial period, but actually more intense than is the case at present.