An analysis of 1516 radiocarbon dates demonstrates that the development of the current circumarctic peatlands began approximately 16.5 thousand years ago (ka) and expanded explosively between 12 and 8 ka in concert with high summer insolation and increasing temperatures. Their rapid development contributed to the sustained peak in CH4 and modest decline of CO2 during the early Holocene and likely contributed to CH4 and CO2 fluctuations during earlier interglacial and interstadial transitions. Given the decreased tempo of peatland initiation in the late Holocene and the transition of many from fens (which generated high levels of CH4) to ombrotrophic bogs, a neoglacial expansion of northern peatlands cannot explain the increase in atmospheric CH4 that occurred after 6 ka.

Interpolar methane gradient (IPG) data from ice cores suggest the "switching on" of a major Northern Hemisphere methane source in the early Holocene. Extensive data from Russia's West Siberian Lowland show (i) explosive, widespread peatland establishment between 11.5 and 9 thousand years ago, predating comparable development in North America and synchronous with increased atmospheric methane concentrations and IPGs, (ii) larger carbon stocks than previously thought (70.2 Petagrams, up to approximately 26% of all terrestrial carbon accumulated since the Last Glacial Maximum), and (iii) little evidence for catastrophic oxidation, suggesting the region represents a long-term carbon dioxide sink and global methane source since the early Holocene.

[1] The West Siberian Lowland (WSL) contains the world's most extensive peatlands and a substantial fraction of the global terrestrial carbon pool. Despite its recognition as a carbon reservoir of great significance, the extent, thickness, and carbon content of WSL peatlands have not been analyzed in detail. This paper compiles a wide array of data into a geographic information system (GIS) to create a high-resolution, spatially explicit digital inventory of all WSL peatlands and their associated physical properties. Detailed measurements for nearly 10,000 individual peatlands (patches) are based on compilation of previously unpublished Russian field and ancillary map data, satellite imagery, previously published depth measurements, and our own field depth and core measurements taken throughout the region during field campaigns in 1999, 2000, and 2001. At the patch level, carbon storage is estimated as the product of peatland area, depth, and carbon content. Estimates of peatland area are validated from RESURS-01 satellite images, and the quality of the Russian peatland depth and carbon content data is independently confirmed by laboratory analysis of core samples. Through GIS-based spatial analysis of the peat areal extent, depth, and carbon content data, we conservatively estimate the total area of WSL peatlands at 592,440 km 2 , the total peat mass at 147.82 Pg, and the total carbon pool at 70.21 Pg C. Our analysis concludes that WSL peatlands are more extensive and represent a substantially larger carbon pool than previously thought: Previous studies report 9,440-273,440 km 2 less peatland area and 15.11-30.19 Pg less carbon than found in this analysis. The complete digital database is freely available for scientific use at

The Russian treeline is a dynamic ecotone typified by steep gradients in summer temperature and regionally variable gradients in albedo and heat flux. The location of the treeline is largely controlled by summer temperatures and growing season length. Temperatures have responded strongly to twentieth-century global warming and will display a magnified response to future warming. Dendroecological studies indicate enhanced conifer recruitment during the twentieth century. However, conifers have not yet recolonized many areas where trees were present during the Medieval Warm period (ca AD 800-1300) or the Holocene Thermal Maximum (HTM; ca 10 000-3000 years ago). Reconstruction of tree distributions during the HTM suggests that the future position of the treeline due to global warming may approximate its former Holocene maximum position. An increased dominance of evergreen tree species in the northern Siberian forests may be an important difference between past and future conditions. Based on the slow rates of treeline expansion observed during the twentieth century, the presence of steep climatic gradients associated with the current Arctic coastline and the prevalence of organic soils, it is possible that rates of treeline expansion will be regionally variable and transient forest communities with species abundances different from today's may develop.

a b s t r a c tWe present a database of late-Quaternary plant macrofossil records for northern Eurasia (from 23 to 180 E and 46 to 76 N) comprising 281 localities, over 2300 samples and over 13,000 individual records. Samples are individually radiocarbon dated or are assigned ages via age models fitted to sequences of calibrated radiocarbon dates within a section. Tree species characteristic of modern northern forests (e.g. Picea, Larix, tree-Betula) are recorded at least intermittently from prior to the last glacial maximum (LGM), through the LGM and Lateglacial, to the Holocene, and some records locate trees close to the limits of the Scandinavian ice sheet, supporting the hypothesis that some taxa persisted in northern refugia during the last glacial cycle. Northern trees show differing spatio-temporal patterns across Siberia: deciduous trees were widespread in the Lateglacial, with individuals occurring across much of their contemporary ranges, while evergreen conifers expanded northwards to their range limits in the Holocene.

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