TESTING THE PRESERVATION OF ISOTOPE SIGNATURES IN AMINO ACIDS FROM DIAGENETICALLY ALTERED ARCHAEOLOGICAL SHELLS

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.