Isotope and molecular evidence for the diverse origins of carboxylic acids in leaf fossils and sediments from the Miocene Lake Clarkia deposit, Idaho, U.S.A.

The normal fatty acids (C14---C32), ω-hydroxy acids (C12---C32), (ω-1)-hydroxy acids (C28 to C32), terpenoid acids and hopanoid acids in higher plant leaf fossils and enclosing sediments from the Miocene lacustrine Clarkia deposit (17–20 Ma) show a wide range (−18.5 to −55.2‰ vs PDB) of carbon isotopic compositions. Both molecular distributions and isotopic compositions display highly informative similarities (e.g., C22---C32 n-acids) and differences (e.g., C16 and C18 n-acids, C22 ω- and C28 and C30 (ω-1) hydroxy acids) between the individual fossils and sediment matrix. These δ13C values are distinctive among compound classes. The molecular distributions and the ranges of δ13C values (−27.2 to −32.2‰) for long chain n-fatty acids (C22---C32), terpenoid acids and ω-hydroxy acids (except for C22 ω-hydroxy acids) are in accord with derivation from mainly C3 terrestrial higher plants. The (ω-1) hydroxy acids (C28 and C30 homologues), however, show a significant 13C enrichment (−18.5 to −22.8‰) in the sediments, consistent with an origin in aquatic photosynthetic organisms whose habitats are severely CO2 limited. The C22 ω-hydroxy acid shows high abundance in sediments but is not detected in association with leaf fossils and has distinctive δ13C values (ca. −33‰), depleted in 13C by about 4–5‰ compared with the other ω-hydroxy acid homologues (C24---C28), and is probably derived from micro-organisms living below the oxycline. Bacterial hopanoids are dominated by isomers with 22R-17β(H),21β(H) configuration, consistent with the low thermal maturity of the sediment, and also display rather negative δ13C values (−38 to −55‰), suggesting their derivation from methanotrophic bacteria. From the isotopic data we infer the operation of a methane cycle, which is also supported by the palaeontological evidence for anoxia in the water column. A novel compound, des-A-olean-28-oic acid was also tentatively identified.