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

摘要

Plant wax n-alkanes are valuable paleoclimate proxies because their carbon (delta C-13) and hydrogen (delta H-2) 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 delta C-13 and delta H-2 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 nalkanes, we measured the delta C-13 values of nonsteroidal triterpenoids (angiosperm biomarkers) and tricyclic diterpenoids (conifer biomarkers) to determine angiosperm and conifer end member delta C-13 values. We then compared these end member values to n-alkane delta C-13 values for each site to estimate their major taxon sources. At sites dominated by conifer macrofossils, delta C-13 values of n-alkanes indicate a conifer source. At mixed conifer and angiosperm sites, conifer contributions increased with increasing nalkane chain length. At sites where conifers were not as abundant as angiosperms, the delta C-13 values of n-alkanes indicate a predominant angiosperm source with some sites showing a conifer contribution to n-C-33 and n-C-35 alkanes. This suggests that conifers in the Paleogene contributed to longer chain nalkanes (n-C-33 and n-C-35) 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. (C) 2020 Elsevier Ltd. All rights reserved.