Paleoclimate significance of n-alkane molecular distributions and δ2H values in surface peats across the monsoon region of China

Abstract

Leaf wax molecular distributions and isotopic values are generally considered paleohydrologic proxies. Here we evaluate the sensitivity of alkane chain length distributions and compound-specific hydrogen isotopic compositions (δ2Halk) in surface peats to environmental parameters across the monsoon region of China. The alkane average chain length (ACL), carbon predominance index (CPI) and δ2Halk values show relatively large variations in multiple samples from a single site, highlighting the complexity of the processes affecting these ratios at small spatial scales. Nonetheless, the site-averaged CPI values correlate significantly with the mean annual temperature (r=−0.57, p(α)<0.001) and precipitation (r=−0.53, p(α)<0.001), suggesting that climate is important to control CPI values, together with other influences. The large variation of δ2Halk values in a single peatland suggests that δ2Halk values may be affected by factors in addition to biosynthetic isotopic discrimination and the D/H ratio of the water available to the plants. It is interesting to note that the site-averaged δ2Halk values correlate poorly with the modeled yearly δ2Hp values. In addition, the site-averaged apparent hydrogen isotope fractionation between C31n-alkanes and precipitation correlates with mean annual temperature. Together, our results show a relatively complex spatial pattern of δ2Halk values in a single peatland or along a climate transection. Thus, we caution that δ2Halk values would be affected by a series of factors, including the growth season length, the lipid synthesis time, the plant life forms, and/or the hydrogen isotope discrimination during lipid biosynthesis. More study is clearly required to elucidate the major controllers.