GLACIAL ISOSTATIC ADJUSTMENT PROCESSES AND ORBITAL FORCING MAY EXPLAIN THE ORIGIN OF THERMALLY ANOMALOUS MOLLUSCAN FOSSILS OF EARLY AND MIDDLE PLEISTOCENE MARINE TERRACE FAUNAS, SAN NICOLAS ISLAND, CALIFORNIA

"Thermally anomalous" faunas in the Quaternary marine terrace fossil record of California contain mixes of both southern (warm water) and northern (cool water) species. Such faunas are a challenge to simple paleozoogeographic interpretations and the origin of such assemblages has been debated for decades. In the late Quaternary marine terrace record, it is now known that in areas of low uplift rate, a relatively high paleo-sea level at ~100 ka (marine isotope stage [MIS] 5c) resulted in capture of the outer parts of ~120 ka (MIS 5e) marine terraces, due to glacial isostatic adjustment (GIA) processes. This capture mixed cool-water mollusks (~100 ka) with warm-water mollusks (~120 ka). It is hypothesized that similar sequences of events could have taken place earlier in the Quaternary, particularly prior to the mid-Pleistocene transition, when glacial-interglacial cycles were of much shorter duration. On San Nicolas Island, California, the 3rd, 4th, 6th, 8th, 9th, 10th, 11th, and 14th terraces have Sr isotope ages of ~0.42, 0.57, 0.78, 0.86, 1.08, 1.15, 1.15, and 1.21 Ma, respectively, permitting possible correlations with MIS 11, 15, 19, 21, 29, 31 [or 33], 31 [or 33], and 35. Deposits of the 8th terrace contain at least 5 northern species, but also 7 southern species of mollusks. Deposits of the 10th terrace contain at least 9 northern species of mollusks, but also at least 19 southern species. It is likely that capture of older marine terraces by younger high-sea stands occurred during the middle and early Quaternary, as well as during the late Quaternary. Capture of terraces was probably due to GIA processes and/or orbital forcing (shorter interglacial/glacial cycles), superimposed on a relatively low, long-term uplift rate. As a consequence, fossils from cooler high-sea stands were mixed with fossils from warmer high-sea stands, explaining thermally anomalous faunas.