GSA Connects 2021 in Portland, Oregon

Paper No. 20-5
Presentation Time: 9:05 AM


VOKHSHOORI, Natasha1, RICK, Torben2, BRAJE, Todd3 and MCCARTHY, Matthew D.1, (1)Ocean Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95060, (2)Anthropology, Smithsonian Institution National Museum of Natural History, 10th St. & Constitution Ave. NW, Washington, DC 20560, (3)Anthropology, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182

Stable isotope proxies measured in the proteinaceous fraction of archaeological bivalve shells contain important ecological information for reconstructing past trophic and climatic conditions of nearshore environments. A major issue, however, is controlling for diagenetic alteration in the isotope values. Whole tissue, or “bulk” isotope values of nitrogen (δ15N), and especially carbon (δ13C) has shown to decrease with increasing C:N ratios, resulting in unreliable values. Here, we measure the elemental, molecular and isotopic values in bulk and compound-specific isotopes of amino acids (CSI-AA) in carbon (δ13C-AA) and nitrogen (δ15N-AA) from the insoluble protein fraction of mussel shells from the California Channel Islands ranging in age (6,100 to 250 cal BP) and depositional environments (e.g. exposed coastal bluff, at depth, etc.). Archaeological mussel shells were compared to modern shells to test the preservation of δ13C-AA and δ15N-AA values. Our results show C:N ratios in modern shells ranged from 2.8 to 3.2 and 3.4 to 9.5 in archaeological shells. There was a strong negative correlation (R2=0.63) between C:N ratios and bulk δ13C values, but δ15N values did not show any relationship with C:N or weight %C. Grouped by C:N ratio, relative molar abundance revealed that Glycine and Alanine, the most abundant AAs, progressively decreased with increasing C:N. Normalized CSI-AA values showed that isotope proxies for “baseline” - isotopes that track the base of the food web (δ15N-Phenylalanine and average δ13C-Essential AAs) - were not statistically different from modern in all C:N groupings. In the structurally simplest AAs, Glycine and Serine, all C:N groups had significantly lower δ13C-AA values. For δ15N-AA, Aspartic Acid and Valine were significantly lower, and Serine was significantly higher; however, Glutamic Acid - AA used in calculating trophic level - was not altered. While bulk isotopes, particularly δ13C, might prove to be unreliable in archaeological shells with C:N ratios higher than ~4.0, isotope proxies from CSI-AA can definitively reconstruct past climatic and ecological conditions of the nearshore marine environment.