Conifers are a major source of sedimentary leaf wax n-alkanes when dominant in the landscape: Case studies from the Paleogene

Abstract

Plant wax n-alkanes are valuable paleoclimate proxies because their carbon (δ13C) and hydrogen (δ2H) isotopes track biological and environmental processes. Angiosperms produce higher concentrations of n-alkanes than conifers, with some exceptions. Vegetation source is significant because in similar climates, both taxa produce n-alkanes with unique δ13C and δ2H values due to different physiological strategies. To test whether conifers contribute significantly to sediment n-alkanes and result in distinctive isotopic signatures, we collected sediment samples from a suite of Paleogene paleobotanical sites in North America with high and low conifer abundances. To disentangle the source of sediment n-alkanes, we measured the δ13C values of nonsteroidal triterpenoids (angiosperm biomarkers) and tricyclic diterpenoids (conifer biomarkers) to determine angiosperm and conifer end member δ13C values. We then compared these end member values to n-alkane δ13C values for each site to estimate their major taxon sources. At sites dominated by conifer macrofossils, δ13C values of n-alkanes indicate a conifer source. At mixed conifer and angiosperm sites, conifer contributions increased with increasing n-alkane chain length. At sites where conifers were not as abundant as angiosperms, the δ13C values of n-alkanes indicate a predominant angiosperm source with some sites showing a conifer contribution to n-C33 and n-C35 alkanes. This suggests that conifers in the Paleogene contributed to longer chain n-alkanes (n-C33 and n-C35) even when not the dominant taxa, but this likely differs for other geographic locations and taxa. This new approach allows unique floral information to be extracted when chain length is carefully considered in the absence of other paleobotanical data and necessitates having some paleovegetation constraints when interpreting carbon and hydrogen isotopes of plant wax-derived n-alkanes.