Plant type effect overweighs seasonal variation in n-alkanoic acid biomarker on regional Loess Plateau of China

Abstract

The effect of plant type on n-alkane biomarker has been widely studied, but the influence of plant type on n-alkanoic acids (n-FAs) has received less attention. Understanding the effect of the distribution of n-FAs and their hydrogen isotope composition (δ²H_FA) is critical for interpreting sedimentary δ²H_FA values as proxies of hydroclimate and/or vegetation changes. In this study, we systematically investigated the distribution of n-FAs and δ²H_FA values across seasons on the Chinese Loess Plateau (CLP). Our results showed that there were no significant seasonal differences in n-FAs distribution and δ²H_FA values, but there were significant differences in δ²H_FA values across plant types. The δ²H_FA values ranged from −185‰ to −125‰ in dicots (−161‰±28‰), and from −215‰ to −170‰ in monocots (−196‰±22‰) on the CLP. Isotope fractionation between δ²H_FA values and precipitation δ²H (ε_FA-P) was also ²H-enriched in dicots relative to monocots. At the regional scale of the CLP, δ²H_FA values were not correlated with δ²H values of precipitation. However, δ²H_FA values from the CLP were similar to expectations from a global comparison, both in terms of their mean values and high variability among plants growing in sites with similar δ²H values of precipitation. As some of this variability in ε_FA-P is due to the effect of plant type, a careful assessment of the likely sources of waxes in terrestrial sediments is critical when using δ²H_FA values for paleohydroclimate reconstruction.