Late-Holocene ecosystem dynamics and climate sensitivity of a permafrost peatland in Northeast China

Northern peatlands play an important role in the global carbon cycle. Here we use multi-core and multi-proxy records from a peatland near Darbin Lake in the Arxan region of Northeast China-near the southernmost limit of circum-Arctic permafrost and the northern limit of East Asian summer monsoon influence-to document peatland development and carbon accumulation histories and their responses to past climate changes during the last 2500 years. Our macrofossil results show that the peatland was characterized by a sedge-dominated fen from 500 BCE to 1450 CE, changed to a Sphagnum-dominated poor fen or bog with abundant shrubs between 1450 and 1960 CE, and finally became predominated by Sphagnum mosses after 1960 CE. The time-weighted mean apparent carbon accumulation rates from three cores ranged from 19.5 to 53.0 g C m- 2 yr- 1 with a mean value of 32.4 g C m- 2 yr- 1, but increased rapidly to 139.2 g C m- 2 yr- 1 during the last several decades. During the early stage of the peatland development, three coring sites that are only 50 m apart were all in the fen phase but they had highly variable bulk densities. The fen-bog transition at these locations occurred during the Little Ice Age but at different times owing to the various influences of local hydrology, permafrost dynamics, or fire disturbance. These observations suggest that fens are highly heterogeneous, not only in peat properties but also in ecosystem dynamics. The dramatic increases in apparent carbon accumulation rates during the late stage of the bog phase after 1960 CE cannot be entirely explained by the limited decomposition of recently accumulated young peat. Instead, our analysis suggests that this was likely due to the increasing Sphagnum dominance and low decom-position of Sphagnum-derived litter, supporting the notion of the important role of vegetation changes in con-trolling carbon accumulation rates of peatlands. Around the 1990s CE, an increase in allogenic carbon accumulation rate-after removing the effect of age-related long-term autogenic decay of peat-coincides with a period of increased regional summer precipitation, suggesting the high sensitivity of continental ombrotrophic bog ecosystems to hydroclimate changes at decadal timescales.