DO PLANT N-ALKANE DISTRIBUTIONS RECORD CLIMATE MORE THAN PLANT TYPE?

Long chain (C₂₁-C₃₇) n-alkanes are among the most long-lived and widely utilized terrestrial plant biomarkers. Though long studied, most paleoenvironmental applications of n-alkane chain length distributions have been based on a scattered set of modern data that have not addressed intra- and inter-plant variability and causes of variation in n-alkane distributions. To address this, we present the results of a study of modern trees, as well as a meta-analysis of published data on modern plant n-alkane distributions to examine the generalities of n-alkane parameters (e.g. average chain length) across plant growth types. From these studies, we found no significant effects of either canopy position or time of leaf collection for mature leaves of trees and also that angiosperms as a group produce more n-alkanes than gymnosperms. Both findings have implications for paleoenvironmental interpretations of plant communities. We also tested whether certain n-alkane chain lengths are representative of particular plant groups: namely, C₂₃ and C₂₅ in Sphagnum mosses, C₂₇ and C₂₉ in woody plants, and C₃₁ in graminoids (grasses). Chain length distributions are found to be highly variable in vascular plant groups, but Sphagnum mosses show a distinctive predominance of C₂₃ and C₂₅. This supports the use of C₂₃ as a robust proxy for Sphagnum, but not the use of C₂₇, C₂₉, and C₃₁ to separate graminoids and woody plants. Instead, chain length distributions may be more directly driven by local environmental conditions. The same dataset was re-analyzed for geographical location and tested against modeled environmental variables. Chain length distributions correlate significantly with temperature and latitude, supporting the hypothesis that longer chain lengths with greater hydrophobicity are favored in warmer environments. It is plausible that previously observed correlations between changes in sedimentary n-alkane chain length abundances and changes in local floral composition (e.g. from wooded to grass-dominated) are less a reflection of the shift in the local plant groups and more a direct manifestation of climatic influences. n-Alkane distributions could potentially serve as a valuable proxy of past continental climate conditions, and further, more directed testing on climate drivers of modern distributions is ongoing.