SOLVING THE CONUNDRUM OF CARBON ISOTOPE SIGNATURE IN SPHAGNUM MOSS

Over the past 20 years, the carbon isotope signature of Sphagnum moss has been used as a proxy for surface wetness in peatlands. This geochemical proxy is a powerful tool for peat-based paleohydrological reconstructions, as the carbon isotope signature of moss follows that of the local hydrology at the seasonal scale, whereas plant assemblage reconstructions, for example, only provide a multi-decadal perspective on environmental change. However, conflicting interpretations of carbon isotope records have been recently published in the literature. These inconsistencies can be attributed to two main issues: (1) different biochemical markers (e.g., bulk plant, cellulose, leaf-wax) are being analyzed by different authors, and (2) the relative importance of different environmental factors (e.g., temperature, surface moisture, carbon source for photosynthesis) on carbon assimilation certainly varies among habitats and species growing in different regions of the world.

Here we address this conundrum by directly comparing the carbon isotope signature (δ13C) of 30 Sphagnum magellanicum samples that were collected in peatlands from southern Patagonia along a strong hydrological gradient, from very dry hummocks (80 cm above water table depth) to submerged hollows (5 cm below water surface). Cellulose and leaf-wax were extracted from the same samples and are currently being analyzed for δ13C. Results will be compared to (1) direct water table measurements that were made in the field, (2) Sphagnum moss moisture content as calculated in the laboratory, (3) testate amoebae assemblages, and (4) Sphagnum biochemical content (e.g., carbohydrates, amino acids, hydroxyphenols, cutin acids). This exercise is unique in that it allows addressing potential inconsistencies between leaf-wax vs. cellulose-based records of past hydrological conditions and linking them with environmental conditions (testate amoebae), moss resource allocation and associated isotopic fractionation, or both. The use of a single species allows for an easier assessment of the aforementioned effects. Overall, this modern calibration work should be of use for interpreting carbon isotope records from peatlands.