•Plant wax components are used for paleoclimate, paleovegetation, and carbon cycle reconstruction.•The δ13C values are influenced by fractionation during photosynthesis and biosynthesis.•Depending on application, these competing influences may need to be constrained.•Characterizing uncertainty in plant biomarker-based reconstructions is advised.Terrestrial plant biomarkers and their carbon isotopes provide insights into carbon cycling, paleovegetation and paleoclimate, ranging in scale from local to global. Over the past decade, considerable efforts have been made to constrain the factors that influence plant biomarkers and their carbon isotope composition to improve their utility for paleo applications. Global and regional replication of time intervals of great interest, such as during carbon cycle perturbations, has increased the need to compare among sites, but doing so has also complicated interpretation of carbon cycle perturbations due to the differences among records. This has led to questions regarding the fidelity of isotope records, the sensitivity of the isotope record to climate, and the best practices for reconciling records. But, at the same time, it has led to new exciting information on ecosystem responses to climate change. By removing competing influences of climate, ecosystem and biology, modern biomarker and isotope calibrations provide a means of reconciling and improving paleorecords and placing quantitative constraints on their interpretation. Here, we review the factors that influence the concentration of plant biomarkers and their carbon isotope composition and provide best practices for reconciling biomarker carbon isotope records for interpreting climate, ecosystem, and carbon cycling in the geologic past.

•Dispersal and deposition of plant-derived terpenoids in modern river basin assessed.•Terpenoid composition and concentration inconsistent between leaves and substrates.•Sediment di-/triterpenoid ratios, after correcting for production, estimated plant community within 10–15%.•Transport of terpenoids by DOM and POM may influence sediment concentrations.•Recommendations for using terpenoids as paleovegetation proxies suggested.Tricyclic diterpenoids and non-steroid or non-hopanoid pentacyclic triterpenoids are almost exclusively taxon-specific terrestrial plant biomarkers produced by conifers and angiosperms, respectively. Due to this source specificity and their prevalence in the geologic record, these compounds are often used to reconstruct paleovegetation. However, the physical and chemical processes that influence the dispersal and fate of terpenoids in sedimentary archives are poorly constrained. Modern fluvial systems can be used as ancient river analogs to provide information on the utility of terrestrial plant terpenoids as paleovegetation proxies by defining their potential flux and identifying the processes that control their transport to, and deposition and degradation in, sediments. To determine if the contribution of terpenoids from vegetation is reflected in forested soil and river sediments and to constrain the dispersal of these compounds in fluvial systems, di- and triterpenoid concentrations in Miners River drainage basin (Upper Peninsula of Michigan, USA) were quantified. In the basin, evergreen conifers are less abundant than deciduous angiosperms, but yet contribute substantially more terpenoids to soils and river sediments when scaled for leaf litter production and present vegetation cover. The composition and relative concentration of di- and triterpenoids in source vegetation do not match those in soils and river sediments, suggesting that some process or processes result in the preferential removal of diterpenoids. While the soil and river sediment terpenoid concentration, corrected for differential terpenoid inputs, can closely predict the basin wide vegetation cover in Miners River drainage basin, the extent to which terpenoids can be used a paleovegetation proxy in other modern or geologic sediments remains unclear.

•Plant biomarkers (n-alkyl lipids and terpenoids) were artificially matured.•Triterpenoids were preferentially lost at lower temperature vs. diterpenoids.•Terpenoid starting composition is an important control on terpene production.•Carbon isotopes of n-alkanes remained unchanged < 200 °C.Plant biomarkers, such as terpenoids and leaf wax components (n-alkanes, n-alkanoic acids and n-alkanols), 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 triterpenoid yields decreased, with a greater loss of triterpenoids at lower temperature. Both diterpene and triterpene yield increased during maturation up to 310–320 °C. 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 may only be a qualitative vegetation proxy under many conditions. The n-alkane yield largely increased up to 320 °C, whereas the n-alkanol yield mainly decreased with increased maturity. The n-alkanoic acids initially increased, but then decreased. The stable carbon isotopic composition (δ13C) of the n-alkanes was generally, though not universally, constant up to 200 °C. Above this, the δ13C values of individual chain length hydrocarbons, for some species, changed by ca. 2‰. This suggests that n-alkane δ13C values should be unaltered in immature rocks, but can vary in the catagenic stage of maturation (oil window).

•Plant-derived terpenoids in Paleogene sediments investigated.•Paleocene and Eocene fluvial sediments in the Bighorn Basin studied.•Di-/triterpenoid ratios tested against fossils as paleovegetation proxies.•Terpenoid proxies overestimated conifer contribution vs. fossil estimates.•Diterpenoid/alkane biomarker proxy for paleovegetation estimation proposed.Plant-derived terpenoids, long recognized as biomarkers, can help reveal the major taxonomic groups of land plants present in ancient environments, even if rocks and sediments do not preserve plant macro- or microfossils. Previous studies have used simple di- to triterpenoid ratios to reconstruct floral changes in the geologic past, but few have compared terpenoid ratios with estimates of floral composition from fossils. Further, reconstructions have not taken into account differences in biomarker production (i.e. concentration relative to leaf biomass) between different types of plants. Here, we have examined terpenoids from early Cenozoic fluvial rocks from the Bighorn Basin (Wyoming, USA), where fossil flora has been studied in detail. We analyzed the distributions of diterpenoids, triterpenoids and n-alkanes from leaf wax in a total of 43 samples from 15 stratigraphic horizons of late Paleocene (63 Ma) to early Eocene (53 Ma) in age. In nearly all samples, triterpenoids, derived from angiosperms, were significantly lower in abundance than conifer-specific diterpenoids, a finding that contrasted with plant fossil evidence for the same rocks. This suggests that di- to triterpenoid ratios severely underestimate the abundance of angiosperms in paleovegetation. Angiosperms dominated n-alkane production among modern plants, and we propose a new paleovegetation proxy based on the ratio of diterpenoids (conifers) to n-alkanes (angiosperms), corrected for lipid production estimated from extant vegetation. Using diterpenoids and alkanes, we infer the composition of paleovegetation to be similar to that inferred from plant fossils. Although the approach works well for the Bighorn Basin, we stress the new paleovegetation proxy will need to be evaluated for other time periods, communities, paleogeography and depositional environments with pollen or megafossil data available.

Leaf waxes (i.e., n-alkyl lipids or n-alkanes) are land-plant biomarkers widely used to reconstruct changes in climate and the carbon isotopic composition of the atmosphere. There is little information available, however, on how the production of leaf waxes by different kinds of plants might influence the abundance and isotopic composition of n-alkanes in sedimentary archives. This lack of information increases uncertainty in interpreting n-alkyl lipid abundance and δ13C signals in ancient settings. We provide here n-alkyl abundance distributions and carbon isotope fractionation data for deciduous and evergreen angiosperm and gymnosperm leaves from 46 tree species, representing 24 families. n-Alkane abundances are significantly higher in angiosperms than gymnosperms; many of the gymnosperm species investigated did not produce any n-alkanes. On average, deciduous angiosperms produce 200 times more n-alkanes than deciduous gymnosperms. Although differences between angiosperms and gymnosperms dominate the variance in n-alkane abundance, leaf life-span is also important, with higher n-alkane abundances in longer-lived leaves. n-Alkanol abundances covary with n-alkanes, but n-alkanoic acids have similar abundances across all plant groups. Isotopic fractionation between leaf tissue and individual alkanes (εlipid) varies by as much as 10‰ among different chain lengths. Overall, εlipid values are slightly lower (−4.5‰) for angiosperm than for gymnosperm (−2.5‰) n-alkanes. Angiosperms commonly express slightly higher Δleaf (photosynthetic discrimination) relative to gymnosperms under similar growth conditions. As a result, angiosperm n-alkanes are expected to be generally 3–5‰ more depleted in 13C relative to gymnosperm alkanes for the same locality. Differences in n-alkane production indicate the biomarker record will largely (but not exclusively) reflect angiosperms if both groups were present, and also that evergreen plants will likely be overrepresented compared with deciduous ones. We apply our modern lipid abundance patterns and εlipid results to constrain the magnitude of the carbon isotope excursion (CIE) at the onset of the Paleocene–Eocene Thermal Maximum (55.8 Ma). When Bighorn Basin (WY) sediment n-alkanes are interpreted in context of floral changes and modern n-alkane production estimates for angiosperms and gymnosperms, the CIE is greater in magnitude (−5.6‰) by ∼1‰ compared to previous estimates that do not take into account n-alkane production.