Rapid deglacial and early Holocene expansion of peatlands in Alaska

Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 04/2010; 107(16):7347-52. DOI: 10.1073/pnas.0911387107
Source: PubMed


Northern peatlands represent one of the largest biospheric carbon (C) reservoirs; however, the role of peatlands in the global carbon cycle remains intensely debated, owing in part to the paucity of detailed regional datasets and the complexity of the role of climate, ecosystem processes, and environmental factors in controlling peatland C dynamics. Here we used detailed C accumulation data from four peatlands and a compilation of peatland initiation ages across Alaska to examine Holocene peatland dynamics and climate sensitivity. We find that 75% of dated peatlands in Alaska initiated before 8,600 years ago and that early Holocene C accumulation rates were four times higher than the rest of the Holocene. Similar rapid peatland expansion occurred in West Siberia during the Holocene thermal maximum (HTM). Our results suggest that high summer temperature and strong seasonality during the HTM in Alaska might have played a major role in causing the highest rates of C accumulation and peatland expansion. The rapid peatland expansion and C accumulation in these vast regions contributed significantly to the peak of atmospheric methane concentrations in the early Holocene. Furthermore, we find that Alaskan peatlands began expanding much earlier than peatlands in other regions, indicating an important contribution of these peatlands to the pre-Holocene increase in atmospheric methane concentrations.

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Available from: Miriam C. Jones
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    • "Most of the peatland-initiation dates come from south-central Alaska and north of the Brooks Range, where the age-frequency patterns are similar. In addition to high rates of peatland initiation, high rates of vertical accumulation of well-preserved peat occurred between 11.5 and 9.0 ka in four peatlands on the Kenai Peninsula, indicating that conditions were optimal for their growth (Jones and Yu, 2010).Gajewski et al. (2001)found that the relative abundance ofSphagnum spores, as represented in the North American Pollen Database, peaked around 9 ka in eastern Beringia. However, some peatlands did not contain Sphagnum in early initiation stages, and the presence of Sphagnum spores does not correlate well with changes in Sphagnum abundance (LaCourse and Davies, 2015). "
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    ABSTRACT: Reconstructing climates of the past relies on a variety of evidence from a large number of sites to capture the varied features of climate and the spatial heterogeneity of climate change. This review summarizes available information from diverse Holocene paleoenvironmental records across eastern Beringia (Alaska, westernmost Canada and adjacent seas), and it quantifies the primary trends of temperature- and moisture-sensitive records based in part on midges, pollen, and biogeochemical indicators (compiled in the recently published Arctic Holocene database, and updated here to v2.1). The composite time series from these proxy records are compared with new summaries of mountain-glacier and lake-level fluctuations, terrestrial water-isotope records, sea-ice and sea-surface-temperature analyses, and peatland and thaw-lake initiation frequencies to clarify multi-centennial- to millennial-scale trends in Holocene climate change. To focus the synthesis, the paleo data are used to frame specific questions that can be addressed with simulations by Earth system models to investigate the causes and dynamics of past and future climate change. This systematic review shows that, during the early Holocene (11.7–8.2 ka; 1 ka = 1000 cal yr BP), rather than a prominent thermal maximum as suggested previously, temperatures were highly variable, at times both higher and lower than present (approximate mid-20th-century average), with no clear spatial pattern. Composited pollen, midge and other proxy records average out the variability and show the overall lowest summer and mean-annual temperatures across the study region during the earliest Holocene, followed by warming over the early Holocene. The sparse data available on early Holocene glaciation show that glaciers in southern Alaska were as extensive then as they were during the late Holocene. Early Holocene lake levels were low in interior Alaska, but moisture indicators show pronounced differences across the region. The highest frequency of both peatland and thaw-lake initiation ages also occurred during the early Holocene. During the middle Holocene (8.2–4.2 ka), glaciers retreated as the regional average temperature increased to a maximum between 7 and 5 ka, as reflected in most proxy types. Following the middle Holocene thermal maximum, temperatures decreased starting between 4 and 3 ka, signaling the onset of Neoglacial cooling. Glaciers in the Brooks and Alaska Ranges advanced to their maximum Holocene extent as lakes generally rose to modern levels. Temperature differences for averaged 500-year time steps typically ranged by 1–2 °C for individual records in the Arctic Holocene database, with a transition to a cooler late Holocene that was neither abrupt nor spatially coherent. The longest and highest-resolution terrestrial water isotope records previously interpreted to represent changes in the Aleutian low-pressure system around this time are here shown to be largely contradictory. Furthermore, there are too few records with sufficient resolution to identify sub-centennial-scale climate anomalies, such as the 8.2 ka event. The review concludes by suggesting some priorities for future paleoclimate research in the region.
    Full-text · Article · Jan 2016 · Quaternary Science Reviews
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    • "i. O'Reilly et al., 2014 Alaska BLB07-01 j. , FLB13-1B j. , KG07-2 IV45-46 k. , NL 10-2 l. , NNC 07-01 k. , OP13-2 j. , WH13-1 j. , YD-2 m. j. Yu et al., Unpublished; k. Jones and Yu, 2010; l. Hunt et al., 2013; m. "
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    ABSTRACT: Databases of basal radiocarbon (14C) ages from peatlands have been used extensively at regional and global scales to examine peatland initiation, land-cover change and carbon cycle dynamics. Many dates collected and analyzed before the 1990s are from bulk peat samples of organic-rich sediments, and such basal radiocarbon ages might have been either too young or too old due to inclusion of non-contemporary carbon via translocation from higher horizons. However, there is rarely a systematic assessment of this problem, especially for large datasets. Here we used AMS 14C dating of both bulk peat and individual macrofossils from the same basal horizon at each of 40 peatland cores across North America and Siberia to evaluate the differences between the two sample types. Our results show that there is no significant or systematic difference between ages derived from bulk material and plant macrofossils. We find that the greatest age overlap of 2σ calibrated age distributions occurred between bulk peat and aboveground macrofossils such as moss fragments, seeds, and herbaceous leaves, suggesting that the bulk material is contemporaneous with the aboveground biomass and active carbon uptake. Dates including wood fragments showed wider divergence compared to moss fragments, seeds, and leaves. We find no evidence for statistically significant and consistent bias introduced by 14C dating of bulk basal peat.
    Full-text · Article · Oct 2015 · Quatemary Geochronology
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    • "We interpret this to reflect relatively high terrestrial productivity, which coincides with the latter part of the HTM in Alaska (Kaufman et al., 2004) and with the peak in sea surface temperatures in the northwest Pacific Ocean and Bering Sea (Max et al., 2012) over the last 15,000 years. Regionally, high productivity also coincides with the peak in thermokarst lake development across the circum Arctic (Walter et al., 2007) and peatland initiation across Alaska (Jones and Yu, 2010). Holocene to late Pleistocene age solifluction deposits mapped immediately north of Burial Lake (Hamilton, 2010) represent a local source of autochthonous organic matter delivered to the lake from permafrost degradation at this time. "
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    ABSTRACT: Sediment cores from Burial Lake located in the western Brooks Range in Arctic Alaska record paleoenvironmental changes that span the last 37,000 calendar years before present (cal yr BP). We identified four distinct lithologic subunits based on physical properties (dry bulk density, magnetic susceptibility), sediment composition, and geochemical proxies (organic matter, biogenic silica, C/N, organic matter δ13C and δ15N, and elemental data from scanning X-ray Fluorescence). The multi-proxy approach and relatively high temporal resolution (at multi-decadal to centennial time scales) of our proxy analysis, compared with previous studies of intermediate water depth cores from Burial Lake, provides new insights into the paleoenvironmental history of the region spanning the period prior to the Last Glacial Maximum. Relatively high lake-levels and gradually decreasing in-lake and terrestrial productivity occur during the mid-Wisconsin interstadial from 37,200 to 29,600 cal yr BP. The subsequent period is defined by falling and lower lake-levels with decreasing effective-moisture, windier conditions, and sustained low aquatic productivity throughout the LGM between 29,600 and 19,600 cal yr BP. The last deglaciation that commenced by 19,600 cal yr BP is characterized by gradual changes in several sediment physical and geochemical proxies, including increasing C/N ratios and terrestrial productivity, decreasing magnetic susceptibility and clastic sediment flux, along with rising and relatively higher lake-levels. A decrease in aeolian activity after 16,500 cal yr BP is inferred from the appearance of fine (very fine sandy silt) sediment, compared to coarse sediments through the LGM and last deglaciation. The highest levels of terrestrial inputs along with increasing and variable aquatic productivity occurs during the Lateglacial to early Holocene interval between 16,500 and 8,800 cal yr BP. The absence of multi-proxy evidence for a strong climatic reversal during the Younger Dryas from Burial Lake sediments contrasts with some paleorecords showing cooler temperatures and/or dry conditions in northern Alaska at this time. Peak levels of sediment organic content and terrestrial productivity at Burial Lake between 10,500 and 9,900 cal yr BP coincide with the early Holocene summer insolation maxima, which likely represents summertime warming and an enhanced flux of watershed derived organic matter from permafrost degradation. The remainder of the Holocene (since 8,800 cal yr BP) at Burial Lake is characterized by relatively high and stable lake levels, landscape stabilization, and relatively high and variable levels of aquatic productivity.
    Full-text · Article · Aug 2015 · Quaternary Science Reviews
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