Stable carbon and oxygen isotopes in sub-fossil Sphagnum: Assessment of their applicability for palaeoclimatology

Abstract

To investigate the potential of stable isotopes of Sphagnum peat deposits for palaeoclimate research and to inform sampling strategies, we present results from a study of selected Sphagnum plant constituents. We report a combined stable carbon and oxygen isotope record of cellulose separately extracted from Sphagnum branches and stem sections manually sampled from a ∼ 4000 year old peat, deposited in a small dry maar crater located in the Westeifel Volcanic Field, Germany. We have determined the species composition of each individual sample of Sphagnum branches to address the sensitivity of the stable isotope records to potential changes in Sphagnum assemblages. The youngest approximately 3000 years old peat section consists of scarcely decomposed Sphagnum plant material. From the bog's surface to a depth of ∼ 60 cm the predominant species is Sphagnum magellanicum. Between ∼ 60 and ∼ 550 cm the peat predominantly consists of Sphagnum capillifolium var. rubellum. At greater depths the decomposition status increases, species identification is, however, solely achievable if the record under examination consists of moderately decomposed peat. The stable carbon and oxygen isotope values of cellulose from Sphagnum stem sections are significantly lighter than those of the branches. Both isotopic offsets between the different plant compounds exhibit a strong degree of correlation, are statistically highly significant and observable down-core. The stable carbon isotope offset averages to 1.5‰, however, presumably decreases with increasing age of the plant material. In contrast, the averaged oxygen isotope offset of 0.9‰ is consistent in time. Our results imply that if no differentiation into Sphagnum branches and stem sections prior to stable isotope analyses is possible, erroneous interpretations of the isotope records are likely, since down-core changes in the ratio of branches to stem sections in the peat profile are most likely. This also implies that the removal of all non-Sphagnum plants or plant fragments is insufficient to retrieve stable isotope signals from peat deposits exclusively reflecting palaeo-environmental conditions. Even isotopic records from bulk Sphagnum cellulose comprise of two different signals: firstly, an environmental signal based on the plant response to external controls. This signal is, however, masked by a second plant physiological signal originating from the isotopic offset between branches and stem sections.

Introduction

The stable carbon and oxygen isotopic composition of plant organic matter has frequently been used for palaeoclimatic and palaeoenvironmental reconstructions. Wood is the preferred material for such studies which found empirical relationships between plant isotopic ratios and different climatic parameters, including temperature and relative humidity (e.g. DeNiro and Epstein, 1981, Edwards et al., 1985, Sternberg et al., 1986). Bulk plant tissue is composed of a number of chemical components that differ in their proportions between species. In dendroclimatological research, plant cellulose is usually used for stable isotope studies since this single chemical fraction allows linking the isotopic signal to a specific growth period in the annual cycle of wood formation (Helle and Schleser, 2004). Moreover, isolation of a single chemical component reduces problems associated with changes in the relative proportion of chemical compounds over time (Rinne et al., 2005).

Despite the great potential of peat bogs as climate archives, to date only few stable isotope studies focus on cellulose derived from continuously accumulated peat deposits. These archives provide records of environmental changes and vegetation dynamics over time, are widely distributed, and cover a large part of the earth's land surface often within human habitat (Charman, 2002). They can have relatively high accumulation rates and botanical fossils such as pollen and plant macrofossils are often well preserved due to the oxygen poor conditions in peat. Peat deposits are, therefore, an ideal archive for combining isotope-geochemical studies with palynological and palaeoecological approaches (Brenninkmeijer et al., 1982). Sphagnum is the most abundant peat forming genus in ombrotrophic bogs and fens in northern and western Europe and abundant in peat records throughout large time spans of the Holocene (Clymo, 1970). Recent studies have systematically investigated the relationship between climate parameters and cellulose isotope ratios in modern Sphagnum plants (e.g. Proctor et al., 1992, Aucour et al., 1996, Ménot and Burns, 2001, Ménot-Combes et al., 2002, Zanazzi and Mora, 2005, Skrzypek et al., 2007). Also a number of studies have attempted to use the isotopic compositions of sub-fossil peat material to infer environmental and/or climatic changes over time (Brenninkmeijer et al., 1982, Aucour et al., 1996, Turney et al., 1997, Hong et al., 2000, Hong et al., 2001, Jędrysek and Skrzypek, 2005). These studies, however, focus on bulk peat material and provided somewhat ambiguous results. Peat deposits are usually a mixture of partly decayed plant materials from different species. Use of bulk peat material, therefore, can be problematic due to contributions from multiple plants and fungi and/or selective degradation of isotopically distinct compounds (Pancost et al., 2003). A further problem must be seen in the fact that peat bogs are composed of a variety of different microenvironments that exhibit considerable variations in terms of bog wetness and moisture level. Bog vegetation, thus, typically consists of different Sphagnum species and sedges. Since different peatland species have been shown to be related to environmental- or climatic parameters in a different degree, analysing bulk peat material ignores the possible complicating factor of changes in Sphagnum species or species composition in a peat deposit (Ménot-Combes et al., 2002). Despite the great potential for environmental reconstructions, the application of sub-fossil Sphagnum cellulose of bulk organic matter from peat deposits as a valid palaeoclimate proxy, therefore, is still uncertain. An advanced contribution to test the potential of stable carbon isotopes of Sphagnum as palaeoclimate proxy was accomplished by Loader et al. (2007). The authors investigated inter- and intra-plant stable carbon isotopic variability of different physical components of individual modern Sphagnum plants. The results revealed a significant isotopic offset between branches and stems.

To explore the potential of stable isotopes of selected Sphagnum peat constituent for palaeoclimate research, we present results from a study of cellulose stable isotopes from selected Sphagnum constituents. We report the first combined δ¹³C and δ¹⁸O record of Sphagnum cellulose separately extracted from Sphagnum branches and stem sections manually separated from an approximately 4000 year old peat deposit. The main goals of this study are as follows. First, we sought to better understand the isotopic variability of different physical Sphagnum plant components in sub-fossil peat material. The manual separation of branches and stem sections prior to stable isotope analyses should allow determining possible stable isotope offsets between these different plant components. Second, we attempt to investigate the species composition of each individual sample down core to address the sensitivity of the stable isotope records to potential changes in Sphagnum assemblages. Since different Sphagnum species are assumed to be related to changes in bog ecology in a different degree, this approach also address the sensitivity of the records to potential changes in bog ecosystem variations. The aim of this study is, moreover, to inform sampling strategies for application of stable isotopes from Sphagnum plant material to the palaeorecord. An improved understanding of the isotope variability in sub-fossil Sphagnum plant components will significantly contribute to the evaluation of the potential of stable isotope signals from peat deposits and address to the reliability of such archives to record climatic and/or environmental information on the basis of stable isotope analysis.