From leaf to soil: <i>n</i>-alkane signal preservation, despite degradation along an environmental gradient in the tropical Andes

Abstract

<p><strong>Abstract.</strong> Plant wax <i>n</i>-alkane biomarkers obtained from ancient soils and sediments have been used to reconstruct past environmental changes. However, the interpretation of these ancient <i>n</i>-alkane patterns relies primarily on our understanding of modern plant wax <i>n</i>-alkane patterns measured from leaves. Very little is known about how <i>n</i>-alkane patterns might be altered during the process of transfer from leaves into soil. Therefore our interpretations of the ancient <i>n</i>-alkane biomarker signal could be confounded by an unobserved bias caused by degradation processes. Here we present the <i>n</i>-alkane patterns extracted from leaves, necromass and soil samples to clarify whether the <i>n</i>-alkane pattern, the <i>n</i>-alkane signal, and the local environmental information reflected in the <i>n</i>-alkane signal degrade, as the plant source material degrades in the tropical Andes. We find that the <i>n</i>-alkane patterns do degrade, but that the <i>n</i>-alkane patterns and signal remain similar across sample types. We find that the <i>n</i>-alkane patterns primarily reflect changes in longer vs. shorter <i>n</i>-alkanes, captured by the average chain length (ACL) and the C<sub>31</sub> / (C<sub>29</sub> + C<sub>31</sub>) ratio (ratio), regardless of sample type. Additionally, soil sample <i>n</i>-alkanes secondarily reflect changes in carbon preference index (CPI) whereas leaf and necromass <i>n</i>-alkanes do not. We find that in all sample types the primary observed <i>n</i>-alkane signals correlate significantly with the environment, temperature in particular, but that soil <i>n</i>-alkane correlations are muted compared to leaf <i>n</i>-alkanes. The secondary <i>n</i>-alkane signal (CPI) in soils also correlates significantly with the environmental signal, temperature in particular. Our results are an important step towards better understanding the taphonomy of the <i>n</i>-alkane signal in the tropics, and suggest that environmental information is preserved in the <i>n</i>-alkane signal, despite the observed degradation.</p>