Abstract
Abstract. Peat humification or decomposition is a frequently used proxy to extract past time changes in hydrology and climate from peat bogs. During the past century several methods to determine changes in peat decomposition have been introduced. Most of these methods are operationally defined only and the chemical changes underlying the decomposition process are often poorly understood and lack validation. Owing to the chemically undefined nature of many humification analyses the comparison of results obtained by different methods is difficult. In this study we compared changes in peat decomposition proxies in cores of two peat bogs (Königsmoor, KK; Kleines Rotes Bruch, KRB) from the Harz Mountains (Germany) using C / N ratios, Fourier transform infrared spectra absorption (FTIR) intensities, Rock Eva® oxygen and hydrogen indices, δ13C and δ15N isotopic signatures and UV-absorption (UV-ABS) of NaOH peat extracts. In order to explain parallels and discrepancies between these methods, one of the cores was additionally analysed by pyrolysis gas chromatography mass spectrometry (pyrolysis-GC-MS). Pyrolysis-GC-MS data provide detailed information on a molecular level, which allows differentiation of both changes attributed to decomposition processes and changes in vegetation. Principal component analysis was used to identify and separate the effects of changes in vegetation pattern and decomposition processes because both may occur simultaneously upon changes in bog hydrology. Records of decomposition proxies show similar historical development at both sites, indicating external forcing such as climate as controlling the process. All decomposition proxies except UV-ABS and δ15N isotopes show similar patterns in their records and reflect to different extents signals of decomposition. The molecular composition of the KK core reveals that these changes are mainly attributed to decomposition processes and to a lesser extent to changes in vegetation. Changes in the molecular composition indicate that peat decomposition in the KK bog is mainly characterized by preferential decomposition of phenols and polysaccharides and relative enrichment of aliphatics during drier periods. Enrichment of lignin and other aromatics during decomposition was also observed but showed less variation than polysaccharides or aliphatics, and presumably reflects changes in vegetation associated with changes in hydrology of the bogs. Significant correlations with polysaccharide and aliphatic pyrolysis products were found for C / N ratios, FTIR-band intensities and for hydrogen index values, supporting that these decomposition indices provide reasonable information. Correlations of polysaccharide and aliphatic pyrolysis products with oxygen index values and δ13C was weaker, assumingly indicating carboxylation of the peat during drier periods and enrichment of isotopically lighter peat components during decomposition, respectively. FTIR, C / N ratio, pyrolysis-GC-MS analyses and Rock Eval hydrogen indices appear to reflect mass loss and related changes in the molecular peat composition during mineralization best. Pyrolysis-GC-MS allows disentangling the decomposition processes and vegetation changes. UV-ABS measurements of alkaline peat extracts show only weak correlation with other decomposition proxies and pyrolysis results as they mainly reflect the formation of humic acids through humification and to a lesser extent mass loss during mineralization.
Highlights
Many studies have used ombrotrophic bogs as archives to reconstruct climate or environmental conditions of the past (Barber, 2006; Barber and Langdon, 2007)
The main aim of this study is to evaluate the extent to which common peat decomposition proxies compare with peat structural components and to discuss their value as indicators of historical changes in peat decomposition
Results of C and N analyses of plants show highest C / N ratios for Sphagnum as compared to Calluna vulgaris (Ericaceae) and Eriophorum vaginatum (Cyperaceae), which are considered as the most important peat-forming plants in the Harz bogs (Table 1). δ13C values range between −27.09 ‰ and −29.21 ‰ and are similar in the three investigated species, but significantly lighter in C. vulgaris
Summary
Many studies have used ombrotrophic bogs as archives to reconstruct climate or environmental conditions of the past (Barber, 2006; Barber and Langdon, 2007). Changes of the physical–chemical conditions through time or changes in vegetation offer different proxies for past environmental conditions (Chambers et al, 2012; Charman et al, 2009, and references therein) Some of these proxies, such as pollen, dust, volcanic ash or trace elements and pollutants from anthropogenic sources, are related solely to atmospheric deposition and may be considered as “external” proxies, reflecting changes on a local or regional scale. The degree of peat decomposition in the historic profile reflects the hydrological conditions during the time of peat formation, or more exactly at the time of peat burial in the catotelm It has to be noted, though, that repeated dryfalling and re-wetting cycles might affect deeper and older peat layers and modify and superimpose the historic record of peat decomposition. This latter process, named secondary decomposition, can result in a decoupling of the age versus depth relationship between peat formation and peat decomposition (Biester et al, 2007; Steinmann et al, 2006)
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