A complex record of last interglacial sea-level history and paleozoogeography, Santa Rosa Island, Channel Islands National Park, California, USA

Abstract

Studies of marine terraces and their fossils can yield important information about sea level history, tectonic uplift rates, and paleozoogeography, but some aspects of terrace history, particularly with regard to their fossil record, are not clearly understood. Marine terraces are well preserved on Santa Rosa Island, California, and the island is situated near a major marine faunal boundary. Two prominent low-elevation terraces record the ∼80 ka (marine isotope stage [MIS] 5a) and ∼120 ka (MIS 5e) high-sea stands, based on U-series dating of fossil corals and aminostratigraphic correlation to dated localities elsewhere in California and Baja California. Low uplift rates are implied by an interpretation of these ages, along with their elevations. The fossil assemblage from the ∼120 ka (2nd) terrace contains a number of northern, cool-water species, along with several southern, warm-water species, a classic example of what has been called a thermally anomalous fauna. Low uplift rates in the late Pleistocene, combined with glacial isostatic adjustment (GIA) processes, could have resulted in reoccupation of the ∼120 ka (MIS 5e), 2nd terrace during the ∼100 ka (MIS 5c) high-sea stand, explaining the mix of warm-water (∼120 ka?) and cool-water (∼100 ka?) fossils in the terrace deposits. In addition, however, sea surface temperature (SST) variability during MIS 5e may have been a contributing factor, given that Santa Rosa Island is bathed at times by the cold California Current with its upwelling and at other times is subject to El Niño warm waters, evident in the Holocene SST record. Study of an older, high-elevation marine terrace on the western part of Santa Rosa Island shows more obvious evidence of fossil mixing. Strontium isotope ages span a large range, from ∼2.3 Ma to ∼0.91 Ma. These analyses indicate an age range of ∼500 ka at one locality and ∼ 600 ka at another locality, interpreted to be due to terrace reoccupation and fossil reworking. Consideration of elevations and ages here also yield low, long-term uplift rates, which in part explains the potential for terrace reoccupation in the early Pleistocene. In addition, however, early Pleistocene glacial-interglacial cycles were of much shorter duration, linked to the ∼41 ka obliquity cycle of orbital forcing, a factor that would also enhance terrace reoccupation in regions of low uplift rate. It is likely that other Pacific Coast marine terrace localities of early Pleistocene age, in areas with low uplift rates, also have evidence of fossil mixing from these processes, an hypothesis that can be tested in future studies.