Peatlands Are a Key Component of the Global Carbon Cycle

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH4 Alberto V. Reyes1,2,3 and Colin A. Cooke1,2,4 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E3 Edited by James P. Kennett, University of California, Santa Barbara, CA 93106, and approved January 14, 2011 (received for review September 13, 2010) 14 Peatlands are a key component of the global carbon cycle. Chron- Compilations of peatland basal C dates make the implicit ologies of peatland initiation are typically based on compiled basal assumption that the compiled dates faithfully reflect the timing peat radiocarbon (14C) dates and frequency histograms of binned of peatland initiation, but peat can be difficult to date reliably 14 calibrated age ranges. However, such compilations are problematic (e.g., 14) and many C dates used in such compilations are because poor quality 14C dates are commonly included and because decades old and have large analytical uncertainty terms. More frequency histograms of binned age ranges introduce chronologi- critically, radiocarbon dates must be calibrated to calendar years 14 cal artefacts that bias the record of peatland initiation. Using a because production of C, and its relative distribution in the published compilation of 274 basal 14C dates from Alaska as a case oceans, atmosphere, and terrestrial biosphere, is not constant study, we show that nearly half the 14C dates are inappropriate through time (Fig. 2A and ref. 15). The calibration process yields for reconstructing peatland initiation, and that the temporal potentially large, non-Gaussian, distributions of likely calendar 14 structure of peatland initiation is sensitive to sampling biases and ages that present interpretive challenges for C-dated compila- 14 tions of peatland initiation. treatment of calibrated C dates. We present revised chronologies 14 of peatland initiation for Alaska and the circumpolar Arctic based Here, we use the recent compilation of basal peat C dates 14 from Alaska (9) as a case study to examine how different treat- on summed probability distributions of calibrated C dates. These 14 revised chronologies reveal that northern peatland initiation ment of C dates affects interpretation of temporal trends for peatland initiation, before demonstrating the broad applicability lagged abrupt increases in atmospheric CH4 concentration at the 14 start of the Bølling–Allerød interstadial (Termination 1A) and the of our results for global peatland C datasets and associated C end of the Younger Dryas chronozone (Termination 1B), suggesting cycle implications. Our revised chronologies reveal that northern that northern peatlands were not the primary drivers of the rapid peatland initiation lagged abrupt deglacial and early Holocene increases in AMC, and thus could not have been the principal increases in atmospheric CH4. Our results demonstrate that subtle methodological changes in the synthesis of basal 14C ages lead to driver for the AMC increases. substantially different interpretations of temporal trends in peat- Results and Discussion land initiation, with direct implications for the role of peatlands in Of the 274 14C dates presented by Jones and Yu (9) (hereafter the global carbon cycle. 10 termed JY all) we rejected 115 dates (Fig. 1, Dataset S1, and SI Text) based on poor suitability of the dated material, lack radiocarbon dating ∣ peatland carbon ∣ ice core methane ∣ paleoclimate of supporting stratigraphic context, poorly constrained stratigra- phy in the original source, or because the most parsimonious eatlands play an important role in millennial-scale climate stratigraphic interpretation does not involve peatland initiation. Pchange and the carbon (C) cycle because they are important For example, we deemed inappropriate the inclusion of 14C dates C sinks (1, 2) as well as a substantial methane (CH4) source (3, 4). on peat interbedded within fluvial sediments or thin peat strin- Atmospheric CH4 concentrations (AMC) rose quickly during gers overlying till (e.g., I-10469 and CAMS-41410 in Dataset S1); deglaciation and following the Younger Dryas chronozone, but such dates record, at best, the timing of geomorphic processes then declined steadily until ∼5 ka (ka ¼ 103 cal yr BP; cal yr BP ¼ or glacial recession, and are unrelated to peatland initiation. 14 10 14 calendar years before A.D. 1950) before rising again during the The vetted compilation of C dates (JY vet) contains 159 C late Holocene (5). However, controversy surrounds the temporal dates. Of these, 70 were sampled from sites within last glacial pattern of peatland initiation and expansion, the role of peatlands maximum (LGM) ice limits and only 76 were sampled from areas in deglacial and early Holocene atmospheric CH4 fluctuations, with at least 5% areal coverage by peat (Fig. 1 and Table S1). and what, if any, role peatlands played in the middle Holocene The histogram method (HIST; see Methods), employed in 14 reversal of AMC (6–9). most previous compilations of basal peat C dates (6, 7, 9, 10, Compilations of radiocarbon (14C) dates on basal peat deposits 12), uses the summed number of calibrated age ranges within provide the foundation for efforts aimed at assessing the long- a given year, or range of years, as a proxy time series for peatland term role of northern peatlands in the climate system and the initiation; cumulative initiation curves are then used to infer C cycle (6–12). In a landmark study, MacDonald et al. (7) com- changes in total peatland area over time (e.g., 9, 16). Using piled 1,516 14C dates on basal peat from across middle- and high- latitude Asia, Europe, and North America. They found that Author contributions: A.V.R. and C.A.C. designed research, performed research, analyzed boreal and northern peatlands expanded rapidly between 12 data, and wrote the paper. and 8 ka, and proposed a direct link between northern peatland The authors declare no conflict of interest. expansion and the early Holocene rise in AMC. This article is a PNAS Direct Submission. Using similar compilations of 14C dates, others have noted 1A.V.R. and C.A.C. contributed equally to this work. the relation between deglaciation and peatland initiation (13), 2To whom correspondence may be addressed. E-mail: [email protected] or colin.cooke@ and the implications of lateral peatland expansion for late Holo- yale.edu. cene terrestrial C cycling (8). Most recently, Jones and Yu (9) 3Present address: Department of Geoscience, University of Wisconsin, Madison, WI 53706. compiled new and previously published basal 14C dates from 4Present address: Department of Geology and Geophysics, Yale University, New Haven, Alaskan peatlands (Fig. 1), and suggested that they were an CT 06520-8109. important CH4 source during the initial abrupt rise in AMC at This article contains supporting information online at www.pnas.org/lookup/suppl/ Termination 1B. doi:10.1073/pnas.1013270108/-/DCSupplemental. 4748–4753 ∣ PNAS ∣ March 22, 2011 ∣ vol. 108 ∣ no. 12 www.pnas.org/cgi/doi/10.1073/pnas.1013270108 Downloaded by guest on September 23, 2021 Fig. 1. Map of Alaska show- ing locations of sampling sites for basal peat 14C dates in Alaska (9). Last glacial maximum ice limits (17) show that large areas of interior Alaska remained un- covered by ice. Organic-rich soils of the Histosol order and Histel suborder (18) are mapped as an approxima- tion of peatland extent in Alaska. Shaded relief bathy- metry and elevation maps from ETOPO2 and the Alas- ka Geospatial Data Clearing- house, respectively. 10 10 14 HIST,both JY all and JY vet exhibit similar temporal structure, initiation events, of different ages, based on one C date. Sec- with a pronounced mode of peatland initiation between ∼10 ond, by weighing equally the 50-yr bins at the extremes of the and 12.5 ka (Fig. 2B). The magnitude of this mode is muted calibrated age range, the HIST method assumes that the tails of 10 in JY vet, though cumulative peatland development for both the probability density function for a given calibrated age range 10 10 JY all and JY vet is similar. Both datasets show peatland are just as likely to represent the true calendar age of the basal initiation commencing ∼18–16 ka and increasing steadily until peat sample as the mode or modes of the probability density ∼11 ka, with short-lived drops in peatland initiation ∼11.2 ka. function (e.g., Fig. S2). Finally, because large calibrated age The rate of peatland initiation slows precipitously in both records ranges generate more bins (i.e., more peatland initiation events) after ∼11–10 ka. By 9 ka, the cumulative curves associated than small calibrated age ranges, the HIST method biases the GEOLOGY with the HIST method for both datasets show almost 70% of 14C record of peatland initiation toward the late Pleistocene present peatland area was established, and only ∼15% of Alaska and early Holocene. This is because older 14C dates tend to peatland area formed after 5 ka. have larger associated laboratory uncertainties (Table S1 and The HIST method generates n ¼ r · b−1 peatland initiation Fig. S3), in turn yielding larger calibrated age ranges and more events for each calibrated age range of an individual radiocarbon numerous bin entries, particularly for calibrated dates affected date, where r and b are the length in years of the calibrated age by 14C plateaus. range and the histogram bins, respectively; thus, the number of As an alternative to the HIST method of summarizing the peatland initiation events generated by the HIST method is basal peat 14C dataset for Alaska, we calculated the sum of the SCIENCES much higher than the number of basal peat 14C dates used in the probability density functions associated with each calibrated age ENVIRONMENTAL 10 analysis (e.g., Fig. S1). For example, JY all yields 5099 peatland range (hereafter termed the PROB method).

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