Evaluating the 'conservative' behavior of stable isotopic ratios (δ~(13)C, δ~(15)N, and δ~(18)O) in humic acids and their reliability as paleoenvironmental proxies along a peat sequence

摘要

Although several studies have used bogs in order to reconstruct paleoclimatic conditions and the historical trends of pollutants, scientific literature is still rather controversial about the role of ombrotrophic bogs as reliable record of past environmental changes. Consequently, understanding whether all vegetational and climatic "information" are effectively preserved in peat deposits during humification becomes an essential aspect to be tested before using bogs as natural archives. The present work focuses on stable isotopic ratios, i.e., ~(13)C/~(12)C (δ~(13)C), ~(15)N/~(14)N (δ~(15)N) and ~(18)O/~(16)O (δ~(18)O), and is based on the assumption that, if bogs are consistent archives of environmental changes, these types of "information" should be recorded also into humic acids (HA), i.e. the fraction of peat more recalcitrant and refractory to degradation.Thus, an 81-cm long peat core, covering the last 2000years, was collected from the Etang de la Gruère bog (Jura Mountains, Switzerland), cut into 3cm slices, and HA were isolated from each age dated layer. Stable isotopic ratios (δ~(13)C, δ~(15)N and δ~(18)O) were then determined in bulk peat and corresponding HA samples. An increase in the humification degree and a decrease of the C/N ratio were observed along the profile. The δ~(13)C of both peat and HA showed a significantly similar trend with depth (p=0.0001), and the same significant correlation was observed for the δ~(15)N (p<0.0001). Also the ratio between δ~(13)C and δ~(15)N closely resembled the C/N ratio observed in the peat and in HA (p<0.0001), thus underlining that the trend of these isotopic ratios is preserved along the studied bog profile. Consequently, our data seem to support both the role of HA as recalcitrant, stable molecules with a long-term residence time, and the potential of ombrotrophic bogs to be used as "archives" of vegetational changes occurring (at least) in the last 2000years. Although a certain relationship (p<0.05) between peat and corresponding HA was found also for δ~(18)O, our data did not allow the solving of the issue of its reliability as paleoenvironmental proxy.