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.

Abstract

The normal fatty acids (C 14C 32), ω-hydroxy acids (C 12C 32), (ω-1)-hydroxy acids (C 28 to C 32), 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., C 22C 32 n-acids) and differences (e.g., C 16 and C 18 n-acids, C 22 ω- and C 28 and C 30 (ω-1) hydroxy acids) between the individual fossils and sediment matrix. These δ 13C values are distinctive among compound classes. The molecular distributions and the ranges of δ 13C values (−27.2 to −32.2‰) for long chain n-fatty acids (C 22C 32), terpenoid acids and ω-hydroxy acids (except for C 22 ω-hydroxy acids) are in accord with derivation from mainly C 3 terrestrial higher plants. The (ω-1) hydroxy acids (C 28 and C 30 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 CO 2 limited. The C 22 ω-hydroxy acid shows high abundance in sediments but is not detected in association with leaf fossils and has distinctive δ 13C values (ca. −33‰), depleted in 13C by about 4–5‰ compared with the other ω-hydroxy acid homologues (C 24C 28), and is probably derived from micro-organisms living below the oxycline. Bacterial hopanoids are dominated by isomers with 22 R-17β(H),21β(H) configuration, consistent with the low thermal maturity of the sediment, and also display rather negative δ 13C 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.