Influence of temperature and C₄ abundance on n-alkane chain length distributions across the central USA

The distributions of long chain n-alkanes are often utilized as plant-derived paleoenvironmental proxies. However, there is debate regarding the degree to which photosynthetic pathway, temperature and/or hydrology drive the variation in chain length distribution observed among different environments. In order to assess the influence of temperature and photosynthetic pathway on plant n-alkanes, this study examined n-alkane chain length distributions in plants and soils along a transect across mid-continental USA spanning >20°C mean annual temperature with a limited range of mean annual precipitation. We found that (i) longer chain length in plants and soils correlated with higher growing season temperature, with relative humidity also a possible driver, (ii) soils exhibited a stronger correlation with temperature than did individual plants, which may reflect the averaging of plant input to soils and (iii) average chain length values in soils did not correlate with the predominance of C₄ plants. The data suggest that the spatial and temporal variation in chain length distributions observed in studies of sediment archives may be driven in large part by growing season temperature and/or aridity rather than photosynthetic pathway (C₃ or C₄). The findings call for further research into the mechanisms and effects of leaf wax composition on water permeability under different environmental conditions, the relationship between leaf wax n-alkanes and relative humidity, and the incorporation of n-alkanes into soils and sediments.