MOLECULAR AND ISOTOPIC BIOGEOCHEMISTRY OF THE MIOCENE CLARKIA FORMATION - HYDROCARBONS AND ALCOHOLS

The Miocene lacustrine deposit at Clarkia, northern Idaho, U.S.A. is w ell known for its rich and exceptionally well preserved higher-plant l eaf fossils. In the present study, the distributions, abundance and is otopic compositions of aliphatic hydrocarbons and alcohols isolated fr om two sediment samples and from two associated fossils (Magnolia lata hensis and Platanus dissecta) are investigated. The general similarity of these characteristics in the sediments suggests a relatively unifo rm depositional environment and consistent terrestrial input. The sedi ment hydrocarbons are comprised mainly of compounds derived from highe r plants and bacteria, whereas the alcohols reflect contributions from both higher plants and algae. A notable feature of the higher-plant d erived 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 19 alpha (H)- and 19 beta(H)-28-nor-des-A-lup-17-ene and des-A-oleana-9,13(18)- diene. The sediment samples contain high abundances of C-24-C-32 long- chain alkanediols, consisting of a series of alpha,omega-diols, alpha, (omega - 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 d elta C-13 values of C-25-C-33 n-alkanes and C-24-C-32 n-alcohols in se diments and fossils are consistent with an origin from terrestrial C-3 higher plants. Algal lipids in the two sediments are represented by d inostanol (-24.6 and -25.9 parts per thousand) inherited from dinoflag ellates, and long-chain 1,15-alkanediols (-26.4 to -29.7 parts per tho usand) derived from microalgae. The long chain alpha,omega-, alpha,(om ega - 1)-, and 1,3-alkanediols are more enriched in C-13 (-21.8 to -26 .4 parts per thousand), suggesting an origin from sources other than h igher plants, probably aquatic photoautotrophic organisms. The C-13 de pletion of some hopanoids (-35.9 to -61.0 parts per thousand) attests to contributions from methanotrophic bacteria and suggests an active m ethane cycle at the time of deposition.