Stratigraphy and Whole-Rock Amino Acid Geochronology of Key Holocene and Last Interglacial Carbonate Deposits in the Hawaiian Islands

We evaluated the utility of whole-rock amino acid racemization as a method for the stratigraphic correlation and dating of carbonate sediments in the Hawaiian Islands. D-alloisoleucine/L-isoleucine (A/I) ratios were determined for carbonate sand and sandstone samples from 25 localities in the archipelago. The superposition of A/I ratios within stratigraphic sections and the regional concordance of ratios within geological formations support the integrity of the method. To correlate the A/I ratios with an absolute chronology, comparisons were made with previously published uranium series dates on corals and with 14C dates on carbonate sand and organic material, including several new dates reported herein. The A/I mean from four marine isotope stage (MIS) 5e V-series calibration sites was 0.505 ± 0.027 (n = 11), and 12 "test sites" of previously uncertain or speculative geochronological age yielded an A/I mean of 0.445 ± 0.058 (n = 17). Similarly, extensive Holocene dunes on Moloka'i and Kaua'i were correlated by a mean A/I ratio of 0.266 ± 0.022 (n = 8) and equated with a 14C bulk sediment mean age of 8600 yr B.P. Our results indicate that the eolian dunes currently exposed in various localities in the Islands originated primarily during two major periods of dune formation, the last interglacial (MIS 5e) and the early Holocene (MIS 1). MIS 5e and MIS 1 A/I ratios from the Hawaiian Islands show close agreement with previous whole-rock studies in Bermuda and the Bahamas. We discuss these results in terms of their relevance to models of lithospheric flexure and to imposing constraints on the time frame for the extinction of fossil birds. GEOCHRONOLOGY AND SEA-LEVEL history have been particularly important in the scientific history of the Hawaiian Islands. Thermal ionization mass spectrometric (TIMS) and alpha V-series dates augmented by 14C dates from the Holocene (marine isotope stage 1 [MIS 1]) provide a temporal framework for important events in geology and paleobiology during the late Quaternary. More than 70 V-series dates have been obtained from 1 Manuscript accepted I February 2000. 2 School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia (paul.hearty@ jcu.edu.au). 3 Departments of Geology and Environmental Sciences, Northern Arizona University, Flagstaff, Arizona 86011-4099 ([email protected]). 4National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560. last interglacial (MIS 5e, ca. 125 ka [thousand years] old) deposits from O'ahu (Ku et al. 1974, Easton and Ku 1981, Muhs and Szabo 1994, Szabo et al. 1994). 14C dates from terrestrial sites on O'ahu, Moloka'i, Hawai'i, Maui, and Kaua'i (Olson and James 1982a,b, James et al. 1987, Paxinos 1998; this study, Table 1) have placed constraints on the recent extinctions of dozens of species of fossil birds. Nonetheless, the age of many important sites remains uncertain. V-series dating is most effective on coral samples, ideally those collected in growth position and composed of pure aragonite. However, like other relatively stable carbonate-producing areas (Bermuda and the Bahamas), the majority of deposits in the Hawaiian Islands are not corals or coral reefs, but carbonate grainstone deposited in