Molecular and isotopic biogeochemistry of the Miocene Clarkia Formation: hydrocarbons and alcohols

The Miocene lacustrine deposit at Clarkia, northern Idaho, U.S.A. is well known for its rich and exceptionally well preserved higher-plant leaf fossils. In the present study, the distributions, abundance and isotopic compositions of aliphatic hydrocarbons and alcohols isolated from two sediment samples and from two associated fossils (Magnolia latahensis and Platanus dissecta) are investigated. The general similarity of these characteristics in the sediments suggests a relatively uniform depositional environment and consistent terrestrial input. The sediment hydrocarbons are comprised mainly of compounds derived from higher plants and bacteria, whereas the alcohols reflect contributions from both higher plants and algae. A notable feature of the higher-plant derived terpenoid hydrocarbons is the prevalence of des-A-triterpenes, largely in the absence of their pentacyclic precursors, indicating the dominance of A-ring degradation for these pentacyclic triterpenoids. The des-A-triterpenes include several novel compounds, namely 19α(H)- and 19β(H)-28-nor-des-A-lup-17-ene and des-A-oleana-9,13(18)-diene. The sediment samples contain high abundances of C24---C32 long-chain alkanediols, consisting of a series of α,ω-diols, α,(α − 1)-diols, 1,15-diols and an unusual series of 1,3-diols. Both fossil leaves show clear differences in their compound distributions and concentrations from the associated sediments, which suggests that they retain a composition distinct from the sediment groundmass. The δ 13C values of C25---C33 n-alkanes and C24---C32 n-alcohols in sediments and fossils are consistent with an origin from terrestrial C3 higher plants. Algal lipids in the two sediments are represented by dinostanol (−24.6 and −25.9‰) inherited from dinoflagellates, and long-chain 1,15-alkanediols (−26.4 to −29.7‰) derived from microalgae. The long chain α,ω-, α,(ω − 1)-, and 1,3-alkanediols are more enriched in 13C (−21.8 to −26.4‰), suggesting an origin from sources other than higher plants, probably aquatic photoautotrophic organisms. The 13C depletion of some hopanoids (−35.9 to −61.0‰) attests to contributions from methanotrophic bacteria and suggests an active methane cycle at the time of deposition.