WHY DO LEAF WAX N-ALKANE DISTRIBUTIONS CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM? (Invited Presentation)

Leaf wax n-alkanes (C25-C35) can be preserved for millions of years and retain isotopic and compositional information from past vegetation. While the isotopic ratios of leaf waxes are readily interpreted in terms of photosynthetic pathway and hydroclimate, the meaning of shifts in their distribution has been debated. The distribution of n-alkanes, often summarized as Average Chain Length (ACL), was initially interpreted as a record of changes in the abundance of woody versus graminoid vegetation. This paradigm was challenged, however, by a meta-analysis showing no distinction in ACL between woody and graminoid taxa at a global scale. An alternative hypothesis for observed correlations between climate change and shifts in ACL is that chain-length distributions are driven directly by climate. The prevailing model for how cuticular waxes control permeability supports the notion that longer, more narrowly distributed chain lengths should reduce non-stomatal water loss, conferring an advantage in dry and/or hot conditions.

Here, we present a range of modern transect and time-series studies to characterize within-species and among-species variations in ACL with climate, and to examine whether these variations are fixed within populations or respond plastically. We then consider the implications of these observations for our understanding of the increase in ACL during the Paleocene-Eocene Thermal Maximum (PETM) in the Bighorn Basin, WY, that is associated with both climate change and plant community change. We conclude that framing the problem in terms of either climate or plant type as the sole driver of ACL shifts represents a false dichotomy. We argue that ACL reflects the combined outcome of adaptation to climate and variation among taxa (but not plant type). Thus, ACL is neither a proxy for vegetation type (e.g. tree vs grass) nor for climate. Instead, n-alkane ACL represents an integrated measure of plant migrations in response to climate change.