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Diagenesis of tricyclic diterpenoids used as biomarkers in an anoxic lake Nielsen, Bente


Diterpenes with a tricyclic skeleton and two other hydrocarbons, perylene and 6-methyl hexadecane were extracted from anoxic sediments of Powell Lake, B.C. The aim of the project was to establish the precursor to product relationships between biosynthesized resin acids of the abietane skeleton from trees and the defunctionalized and aromatized compounds found in these sediments. A Douglas Fir (Pseudotsuga menziesii) from U.B.C. Research Forest, Maple Ridge provided the resin acids for the comparison to the sedimentary diterpenes. Perylene and 6-methyl hexadecane were selected as they are known to be of different origin than the resin acids and were assumed to show a different isotopic signature from the C₃-plant acids when analyzed by Stable Carbon Isotope Mass Spectrometry (SCIMS). A range of values had been estimated previously for the various groups of compounds and it was expected that the resin acids would show δ¹³C values of -21 to -33‰ (plus an additional -8‰ for lipids). The dehydroabietic, abietic, and pimaric acids extracted from the Douglas Fir wood had a δ¹³C[sub PDB] of -29.5±0.3‰ as expected. However, the degraded diterpenes extracted from the lake sediments showed a range of values from dehydroabietane, δ¹³C[sub PDB] -22.7±0.4‰ to retene, δ¹³C[sub PDB] -24.5 ±0.3‰. It was estimated that the loss of CH₃-groups at C-4 and C-10 and COOH at C-4 should generate a degradation product that would be up to 2‰ lighter than the precursor compound, since these groups were heavier in the original acetyl-CoA used in the biosynthesis of these compounds. It was, furthermore, thought that a small isotope effect might be associated with the degradation of these acids. However, the degraded compounds from the sediments were heavier than the original acids and it is concluded that dehydroabietane may be a residual product produced by the conifer trees, with an original heavier isotopic signature than the resin acids. The lighter fraction of the diterpenoids is suggested to have been totally degraded to short chain acids and it is the partially degraded heavier fraction of the original acids that are observed in the sediments, showing an isotope signature less negative than the resin acids. The δ¹³C values for 6-methyl hexadecane (known to be a cyanobacterial product) given in the literature are quite varied, depending upon source of CO₂. In the sediments this compound has a δ¹³C[sub PDB] value of -25.1±0.1‰ which compares well with values for whole cell material from freshwater, eutrophic lake plankton (δ¹³C = -26 to -30‰) and with the range of whole cyanobacterial mat material (δ¹³C[sub PDB] = -16 to -32‰). Perylene is an ubiquitous compound in both terrestrial and marine environments but its origin is unknown. It is believed that in Powell Lake sediments the origin is most likely fungal—a reduced fungal pigment, cercosporin or perylene-quinone. The δ¹³C[sub PDB] value of -26.0±0.7‰ reflects the terrestrial source of the carbon used and shows the expected lighter isotopic signature. It is unlikely that perylene is of anthropogenic origin as its signature should be lighter, less than -28‰. Based on their δ¹³C values it was established: (1) that the diterpenes in the sediments and their acid precursors with an abietane skeleton belong to the same family of compounds; (2) that they are microbially degraded showing slight isotope effects; (3) that no definite route of degradation could be seen, but the more oxidized and/or reduced diterpenes generally have lighter isotopic signatures; (4) that aromatization and defunctionalization take place rapidly and early during diagenesis limiting their use as biomarkers in the assessment of thermal maturation; and (5) that single organic compounds defined by their isotopic signatures could be used as tracers in environmental geochemistry studies. For perylene and 6-methyl hexadecane it was shown that their isotopic ratios reflect the terrestrial origin of their CO₂ (HCO₃⁻), lighter than would be expected for either fungal or cyanobacterial whole cell material, but consistent with a terrestrial, freshwater environment.

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