LEAF WAX N-ALKANE SIGNALS IN SEDIMENTARY RECORDS: WHAT ARE THE KEY DRIVERS?

Leaf wax n-alkanes (C25-C35) provide insights into past ecosystems. However, to interpret them we need to understand their temporal and spatial scale of integration in soil. n-Alkanes are thought to be highly refractory in sediments, yet subject to rapid rates of turnover in soils. Inputs to soils can come from both distant wind-blown and local leaf-fall sources. To explore the taphonomy of leaf waxes, we conducted field and experimental studies to examine whether n-alkane signals in soils reflect the current local plant community, and whether post-depositional degradation in soils affect signatures.

Modern plants and soils were obtained from 20 long-term monitoring plots distributed across Australia. n-Alkanes were extracted from the three most dominant species and the surface soil at each plot. Average chain length (ACL) and carbon preference index (CPI) were measured for each plant and used to model ACL and CPI inputs to soils, weighted by concentration and percent coverage. Modelled ACL and CPI were generally higher than actual values indicating that local and recent inputs do not dominate soil n-alkane signals. We attribute this to temporal and/or spatial averaging of inputs, with the local plant contributor signals being overwhelmed by an accumulation in the soils of past and/or distant contributors.

We explored whether post-depositional modification of n-alkanes in soils accounts for the observed model-data mismatch in an experimental study utilising incubated soil samples mixed with different organic amendments such as composts. Samples at 0 and 18 months incubation were analysed for n-alkane ACL, CPI and concentration. The concentration of n-alkanes decreased markedly with losses of up to 90% but ACL remained unchanged. CPI decreased significantly, suggesting that CPI may be a useful indicator of n-alkane degradation. Our results imply that inputs to soils have a greater influence on n-alkane ACL than post-depositional modification.