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  1. The sources of atmospheric methane (CH4) during the Holocene remain widely debated, including the role of high latitude wetland and peatland expansion and fen-to-bog transitions. We reconstructed CH4 emissions from northern peatlands from 13,000 before present (BP) to present using an empirical model based on observations of peat initiation (>3600 14C dates), peatland type (>250 peat cores), and contemporary CH4 emissions in order to explore the effects of changes in wetland type and peatland expansion on CH4 emissions over the end of the late glacial and the Holocene. We find that fen area increased steadily before 8000 BP as fens formed in major wetland complexes. After 8000 BP, new fen formation continued but widespread peatland succession (to bogs) and permafrost aggradation occurred. Reconstructed CH4 emissions from peatlands increased rapidly between 10,600 BP and 6900 BP due to fen formation and expansion. Emissions stabilized after 5000 BP at 42 ± 25 Tg CH4 y-1 as high-emitting fens transitioned to lower-emitting bogs and permafrost peatlands. Widespread permafrost formation in northern peatlands after 1000 BP led to drier and colder soils which decreased CH4 emissions by 20% to 34 ± 21 Tg y-1 by the present day.
  2. null (Ed.)
  3. Abstract

    Thermokarst lakes accelerate deep permafrost thaw and the mobilization of previously frozen soil organic carbon. This leads to microbial decomposition and large releases of carbon dioxide (CO2) and methane (CH4) that enhance climate warming. However, the time scale of permafrost-carbon emissions following thaw is not well known but is important for understanding how abrupt permafrost thaw impacts climate feedback. We combined field measurements and radiocarbon dating of CH4ebullition with (a) an assessment of lake area changes delineated from high-resolution (1–2.5 m) optical imagery and (b) geophysical measurements of thaw bulbs (taliks) to determine the spatiotemporal dynamics of hotspot-seep CH4ebullition in interior Alaska thermokarst lakes. Hotspot seeps are characterized as point-sources of high ebullition that release14C-depleted CH4from deep (up to tens of meters) within lake thaw bulbs year-round. Thermokarst lakes, initiated by a variety of factors, doubled in number and increased 37.5% in area from 1949 to 2009 as climate warmed. Approximately 80% of contemporary CH4hotspot seeps were associated with this recent thermokarst activity, occurring where 60 years of abrupt thaw took place as a result of new and expanded lake areas. Hotspot occurrence diminished with distance from thermokarst lake margins. We attribute older14C ages of CH4released from hotspot seepsmore »in older, expanding thermokarst lakes (14CCH420 079 ± 1227 years BP, mean ± standard error (s.e.m.) years) to deeper taliks (thaw bulbs) compared to younger14CCH4in new lakes (14CCH48526 ± 741 years BP) with shallower taliks. We find that smaller, non-hotspot ebullition seeps have younger14C ages (expanding lakes 7473 ± 1762 years; new lakes 4742 ± 803 years) and that their emissions span a larger historic range. These observations provide a first-order constraint on the magnitude and decadal-scale duration of CH4-hotspot seep emissions following formation of thermokarst lakes as climate warms.

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  4. Thoman, R.L. ; Richter-Menge, J. ; Druckenmiller, M.L. (Ed.)
    Since the early 2000s, observations from 14 coastal permafrost sites have been updated, providing a synopsis of how changes in the Arctic System are intensifying the dynamics of permafrost coasts in the 21st Century. Observations from all but 1 of the 14 permafrost coastal sites around the Arctic indicate that decadal-scale erosion rates are increasing. The US and Canadian Beaufort Sea coasts have experienced the largest increases in erosion rates since the early-2000s. The mean annual erosion rate in these regions has increased by 80 to 160 % at the five sites with available data, with sites in the Canadian Beaufort Sea experiencing the largest relative increase. The sole available site in the Greenland Sea, on southern Svalbard, indicates an increase in mean annual erosion rates by 66 % since 2000, due primarily to a reduction in nearshore sediment supply from glacial recession. At the five sites along the Barents, Kara, and Laptev Seas in Siberia, mean annual erosion rates increased between 33 and 97 % since the early to mid-2000s. The only site to experience a decrease in mean annual erosion (- 40%) was located in the Chukchi Sea in Alaska. Interestingly, the other site in the Chukchi Seamore »experienced one of the highest increases in mean annual erosion (+160%) over the same period. In general, a considerable increase in the variability of erosion and deposition intensity was also observed along most of the sites.« less