LATE QUATERNARY SEA-LEVEL HISTORY AND THE ANTIQUITY OF THE PYGMY MAMMOTH (MAMMUTHUS EXILIS), CHANNEL ISLANDS NATIONAL PARK, CALIFORNIA, USA

Fossil proboscideans have been found on a number of islands worldwide. Three of the islands in Channel Islands National Park, California have records of fossil mammoths, both Columbian mammoths [Mammuthus columbi (Falconer, 1857)] and pygmy mammoths [Mammuthus exilis (Stock and Furlong, 1928)]. Most fossils of both species on the islands date to the last glacial period (~22-12 ka, in cal ¹⁴C yr), but early observations of mammoth remains within the lowest (youngest) subaerially exposed marine terrace deposits indicated the possibility of a pre-last-glacial age for the earliest arrivals. In the course of marine terrace work on Santa Rosa Island, a tusk of Mammuthus exilis was found in eolian/colluvial deposits overlying thin marine deposits. Radiocarbon analyses on land snails (Helminthoglypta ayresiana) gave five ages, ranging from 46-41 ka (¹⁴C yr), all of which are regarded as minimum ages. Nevertheless, the find documents a pre-last-glacial occurrence of pygmy mammoths on the Channel Islands. A more definitive age was obtained when another well-preserved Mammuthus exilis tusk was found in deposits of the lowest marine terrace, along with marine mollusks and marine mammal remains. Uranium-series dating of six fossil corals from deposits of this terrace yielded ages ranging from 83.8 ± 0.6 ka to 78.6 ± 0.5 ka. All but one of the corals have back-calculated initial ²³⁴U/²³⁸U values that fall within the range of modern seawater, indicating the ages are robust. These ages correlate the deposit with marine isotope stage (MIS) 5.1, a time of relatively high sea level during the latter part of the last interglacial complex. Thus, mammoths have been on the Channel Islands prior to the last glacial period and probably migrated to the islands by swimming during the penultimate glacial period (~150 ka), or MIS 6. This was a time when sea level was low and the island-mainland distance was minimal, as was also the case during the last glacial period (MIS 2). The timing of initial mammoth migration had to occur not only during a glacial period, but also late enough in the Quaternary that uplift of both the islands and the mainland decreased the swimming distance to a range that could be accomplished by mammoths.