Modern Sphagnum δ 13 C signatures follow a surface moisture gradient in two boreal peat bogs, James Bay lowlands, Québec

Carbon isotopic composition of Sphagnum macrofossils can potentially be used as a palaeohydrological tool for peat‐based climatic studies since a relationship between Sphagnum δ 13 C values and peatland surface moisture has been presented in previous studies. In order to verify this hypothesis, modern Sphagnum δ 13 C values were measured along a moisture (microtopographic) gradient in two boreal peat bogs. Isotopic measurements were performed on bulk material of S. fuscum , S. magellanicum , S. capillifolium and S. pulchrum . Isotopic variations found within and between Sphagnum species along the microtopographic gradient were compared using analysis of variance. A significant positive correlation ( P  < 0.0001) was found between Sphagnum δ 13 C values and their position along the surface moisture gradient. Results show that 13 C‐depleted values are related to low water table depths (WTD), while 13 C‐enriched values correspond to a water table that is close to the peat surface. Although the mechanisms underlying carbon fractionation processes in mosses are not well understood, we demonstrate that water resistance to CO 2 diffusion is an important fractionation process that is observed in bulk Sphagnum δ 13 C measurements, since drier and wetter samples exhibit consistent and very different isotopic signatures. Copyright © 2008 John Wiley & Sons, Ltd.