Uranium and Strontium geochronology data for marine terraces on Santa Rosa Island, Channel Islands National Park, California, USA

Studies of marine terraces and their fossils can yield important information about sea level history, tectonic uplift rates, and paleozoogeography. The marine terrace record on Santa Rosa Island, California is complex. Two prominent low-elevation terraces appear to record the ~80 ka (MIS 5a) and ~120 ka (MIS 5e) high-sea stands, based on U-series dating of fossil corals, but interpretations are tentative because of clear indications of open-system behavior with respect to U-series nuclides. Nevertheless, low uplift rates are implied by a preferred interpretation of the ages. It is inferred that low late Pleistocene uplift rates, combined with glacial isostatic adjustment (GIA) processes likely resulted in reoccupation of the ~120 ka 2nd terrace during the ~100 ka (MIS 5c) high-sea stand. Study of a high-elevation marine terrace on the western part of Santa Rosa Island also shows evidence of fossil mixing. Strontium isotope ages of fossil mollusks indicate an age range of ~500 ka at one locality and ~600 ka at another locality. Consideration of elevations and ages here also yield low, long-term uplift rates, which explains, at least in part, 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.