Modern plant-derived terpenoids in an upper Michigan river basin and implications for interpreting the geologic record

Di- and triterpenoids are taxonomically specific plant biomarkers, which are produced by conifers and angiosperms, respectively. Because of this source specificity, terpenoids are often used for paleovegetation reconstruction. However, few studies have evaluated weather terpenoid ratios in modern river systems reflect the surrounding plant community. It is likely that various processes that bias terpenoid ratios as they are transported from plants to sediments. To learn more about these important geologic biomarkers, we used a modern fluvial system as an ancient river analog to provide information on the utility of terpenoids as quantitative paleovegetation proxies. Di- and triterpenoid concentrations were quantified in plants, sediments, and particulate and dissolved organic matter in a small river in the Upper Peninsula of Michigan to (1) determine if the contribution of terpenoids from source vegetation is reflected in forested soil and river sediments, and (2) constrain the dispersal of these compounds in fluvial systems. In Miners River drainage basin, evergreen needleleaf conifers are six times less abundant than deciduous broadleaf angiosperms, yet contribute five times more terpenoids to the sediments, when scaled for leaf litter production and present vegetation cover. Thus, using sediment terpenoid ratios alone (ie. no corrections for production differences between major taxonomic groups) to reconstruct vegetation will drastically over represent evergreen conifer populations. Sediment di-/triterpenoid ratios are considerably lower than the expected terpenoid flux from vegetation, suggesting these compounds are preferentially lost between source and sink. In Miners River, terpenoids are transported in the particulate and dissolved organic matter (POM and DOM) fractions of river water. Fluvial transport of terpenoids does not appear to influence river sediment terpenoid concentrations in fresh water systems, like Miners River, however, transport by POM and DOM may affect terpenoid concentrations in estuarine and marine sediments. Despite the challenges of using terpenoids as paleovegetation proxies, when corrected for plant production, basin-wide terpenoid concentrations are useful in predicting the present plant community composition (based on plant census data) within 10-15%. This study suggests that the sedimentary record of terpenoids can broadly reconstruct paleovegetation, when corrected for differential terpenoid production and transport.