Effect of thermal maturation on plant-derived terpenoids and leaf wax n-alkyl components

Abstract

Plant biomarkers, such as terpenoids and leaf wax components (n-alkanes, n-alkanoic acids and nalkanols), are frequently found in sediments and can be used, often in association with stable carbon (and hydrogen) isotope measurements, as paleovegetation and paleoclimate proxies. However, few controlled studies have monitored plant biomarker alteration to determine if certain plant biomarkers are preferentially lost relative to more recalcitrant forms.To investigate the role of selective alteration and degradation of plant biomarkers, hydrous pyrolysis was used to artificially mature leaves from four plant species, including the deciduous angiosperms Acer rubrum and Platanus occidentalis, the deciduous conifer Taxodium distichum and the evergreen conifer Pinus sylvestris. Leaves were artificially matured at temperatures ranging from 150 to 330 °C for 72 h to simulate maturation. With increasing temperature, functionalized di-and triterpenoidyields decreased, with a greater loss of triterpenoids at lower temperature. Both diterpene and triterpene yield increased during maturation up to 310 to 320 ºC. A greater amount of diterpenes and triterpenes was generated for P. sylvestris and A. rubrum, respectively, and might be related to differences in terpenoid starting composition. Terpenols were preferentially converted to terpenes over terpenoic acids. Taken together, hydrous pyrolysis of plant biomarkers indicates that paleovegetation reconstruction from terpenoids can be informative, but only as a qualitative vegetation proxy. The n-alkane yield largely increased up to 320 ºC,whereas the n-alkanol yiels mainly decreased with increased maturity. The n-alkanoic acids initially increased, but then decreased. The stable carbon isotopic composition (δ 13 C) of the n-alkanes was generally, though not universally, constant up to 200 ºC. Above this, the δ 13 C values of individual chain length hydrocarbons, for some species, changed by ca. 2‰. This suggests that n-alkane δ 13 C values should be unaltered in immature rocks, but can vary in the catagenic stage of maturation (oil window).

Extracting the most from terrestrial plant-derived n-alkyl lipids and their carbon isotopes from the sedimentary record: A review

“…Upon heating and increasing maturity, n-alkane δ 13 C values appear to not be altered up to the pre-oil generation window. This was determined with hydrous pyrolysis experiments using fresh leaves from Acer rubrum (red maple), Platanus occidentalis (American sycamore), Pinus sylvestris (Scots pine) and Taxodium distichum (bald cypress) at various temperatures (Diefendorf et al, 2015c). In that study, changes in long chain n-alkane δ 13 C values were not observed until hydrous pyrolysis temperatures increased to 250 ºC and above.…”

“…For immature sediments, n-alkanoic acids and n-alkanols are polymerized to insoluble macromolecules through diagenetic alteration. Evidence for this polymerization and formation of kerogen comes from extensive use of confined and hydrous pyrolysis experiments using fresh leaf or arthropod tissue (Stankiewicz et al, 2000;Gupta et al, 2007a;Gupta, 2014;Diefendorf et al, 2015c) and from observations made on fossils (e.g., Mösle et al, 1997Mösle et al, , 1998. For immature sediments and rocks, nalkanes appear to be rather immobile.…”

Insights into the timing, intensity and natural setting of Neanderthal occupation from the geoarchaeological study of combustion structures: A micromorphological and biomarker investigation of El Salt, unit Xb, Alcoy, Spain

“…Above 330–360 °C an increasing depletion in 12 C takes place, which can be attributed to the cleavage of 12C-12C bonds [155]. On the other hand, Diefendorf et al [156] report a shift of compound-specific n -alkane δ 13 C values up to 2 ‰ in either direction depending on species and chain length, at a temperature higher than 200–250 °C. Thus, a shift in the compound-specific n -alkane δ 13 C values would corroborate that BL and WL represent in fact burning if all other parameters are ruled out.…”

Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

“…At significantly higher thermal exposure within the realm of gas generation, previous studies have shown that thermal cleavage affecting longer n-alkane compounds could increase the δ 13 C values of residual long-chain n-alkanes by up to 4‰ (Clayton & Bjorøy, 1994;Odden et al, 2002;Tang et al, 2005), due to the preferential breakage of 12 C-12 C bonds that leave remaining structures enriched in 13 C (Andresen et al, 1995;Lewan, 1986;Sackett, 1978). The temperatures based on our T max represent early oil generation (Riber, 2009), well below those that would alter alkane 13 C compositions by loss due to thermally driven cracking by more than 1‰-2‰ (Diefendorf et al, 2015). Because the sampled section is within a similar level of thermal maturity, approximately early oil-window, and thus thermal effects, if present, were similar across all samples.…”