Evaluation of the wider applications of the alkanol index BNA15 as temperature proxy in a broad distribution of peat deposits

Abstract

The BNA15 biomarker proxy, which reflects the relative abundance of the branched over the normal C15 alkan-1-ols, was recently proposed as a novel terrestrial paleothermometer and applied to ancient peat deposits to reconstruct past temperature variations across the Holocene. However, to date, a peat-specific calibration for this novel temperature proxy does not exist. Here we analyzed a set of surface and downcore peat samples from Chinese peatlands with a broad range in mean annual air temperature (MAAT) (–3.2 °C to 18.5 °C) and pH (4.7 to 8) to understand the relationship between the BNA15 proxy and temperature in peat deposits. Downcore samples from three peatlands outside China were also obtained to test its wider applicability. Our results show that the variations in BNA15 across multiple surface samples from a single peatland are relatively large, suggesting that we must be careful with the application of BNA15 to reconstruct small amplitude temperature variations. In addition, the BNA15 changes significantly with depth, with higher values in the anoxic catotelm in mid/low latitude peatlands and slightly higher values in the oxic acrotelm in high latitude/altitude peatlands. Combined with concentrations of alkan-1-ols in a subtropical peat core retrieved from central China, we propose that branched alkan-1-ols are predominantly produced at the acrotelm-catotelm boundary. BNA15 has a moderate correlation with the growth temperature (including the months that the daily temperature is over 0 °C) in surface peat samples. In contrast, a strong positive correlation exists between BNA15 and MAAT using catotelm samples from Chinese peatlands (MAAT = 2.03 × BNA15 – 2.33; R2 = 0.95, p < 0.01, RMSE = 2.3 °C). We find no clear correlation between BNA15 and pH in the surface peat samples. The application of the BNA15 index to two forest/woody wetlands from outside of China highlights that care should be taken when applying this index in these environments.