Rapid, effective screening of tar seep fossils for radiocarbon and stable isotope analysis

Tar seeps trap and preserve diverse fossil assemblages that reflect unique environmental histories. While the macro preservation of the fossils is usually good, preservation of organic bone collagen is often variable. Radiocarbon dating and stable isotope analysis of tar seep taxa can reveal distinct paleoecological insights but are complicated by the contamination from tar infiltration. Additionally, the removal of tar is complex and time-consuming. Therefore identifying fossils with preserved collagen prior to further investigation minimizes unnecessary damage to fossil collections and improves success rates for analyses that require organic preservation. We used tar pit fossils where the collagen preservation state was independently known to test non-destructive (visual inspection) and minimally-destructive (infrared spectroscopy; FTIR) methods to determine the most reliable methods to identify bones with well preserved collagen. We found that while collagen is less often preserved in heavily weathered bones, visual cues alone are not a reliable indicator. Instead, the Water-Amide-on-Phosphate FTIR index is highly sensitive and specific at identifying fossils suitable for radiocarbon dating and stable isotope analysis. While our protocol and analysis code were developed using fossils from two California tar seep localities (McKittrick and Rancho La Brea), they are likely to be broadly applicable to other tar seep localities that have yielded fewer fossils, thus requiring even greater care when selecting specimens for further analysis.