DON JOHNSON’S ROLE IN THE EARLY DAYS OF AMINOSTRATIGRAPHIC STUDIES ON THE CALIFORNIA COAST

Amino acid racemization (AAR) has been applied to numerous studies of Quaternary deposits around the world. In the mid-1970’s, uplifted marine terraces from central and southern California provided a natural laboratory for the testing and evaluation of the utility of AAR for mapping coastal landforms and assessment of late Quaternary deformation rates. Existing collections from classic terrace flights on San Nicolas Island and the Palos Verdes Hills were used in initial studies published in 1977. Motivated by these initial observations, in 1978 Don Johnson collected mollusks from sites on Anacapa, San Miguel, Santa Cruz, and Santa Barbara Islands as part of his developing studies of the geomorphology of the Channel Islands. These samples were the focus of one part of a larger PhD research project (Keenan, 1982) focused on evaluation of AAR using both field samples and laboratory experiments. 21 samples from the four islands were analyzed using currently available gas chromatographic methods, yielding D/L values for between four and seven amino acids. Sample quality and sample preparation requirements prevented additional analyses. Results from San Miguel Island verified that D/L values increase with increasing terrace elevation, a fundamental criterion of aminostratigraphy, also seen in at least some results for multiple terraces on San Nicolas Island. For other islands, the limited D/L data suggested substantial variation in elevation for single-age terraces and, in one instance, reworking of an older shell onto a younger terrace. Observed D/L values suggested a preponderance of MIS 5 terraces, with some older (perhaps to MIS 9) deposits in some locations. The early results, limited in quantity and quality, nevertheless justified further efforts, pursued primarily by Dan Muhs (USGS). These more recent and more comprehensive Channel Island AAR studies (Muhs and others, 1983-present), some involving Johnson’s original collection sites and all using more advanced procedures, have yielded much greater insight into terrace chronologies of the Islands, as well as added perspective on the viability of AAR using the taxa commonly found on the island terraces. With a few exceptions, early results agree with the more reliable results obtained in the past decade.