Holocene peatland carbon dynamics in the circum-Arctic region: An introduction

Abstract

Peatlands represent the largest and most concentrated carbon pool in the terrestrial biosphere, and their dynamics during the Holocene have had significant impacts on the global carbon cycle. In this Introduction paper, we provide an overview of the contributions presented in this Special Issue on Holocene peatland carbon dynamics. We also provide a brief history and current status of peat-core-based research on peatland carbon dynamics. Finally, we identify and discuss some challenges and opportunities that would guide peatland carbon research in the near future. These challenges and opportunities include the need to fill data gaps and increase geographic representations of peat carbon accumulation records, a better understanding of peatland lateral expansion process and improved estimate of peatland area change over time, developing regional carbon accumulation histories and carbon pool estimates, and projecting and quantifying overall peatland net carbon balance in a changing world.

Keywords

biogeochemistry carbon climate Holocene paleoclimate peatlands wetlands

The importance of peatlands in the global carbon cycle

There has been increasing focus on carbon cycle dynamics and feedbacks to the climate system in peatland research over the past 30 years. Schlesinger (1977) provided the first comprehensive review on carbon accumulation and carbon release in detritus (dead organic matter) of the world’s terrestrial ecosystems, including wetlands. Sjörs (1981) was one of the first to discuss the role of northern peatlands as a global, long-term carbon sink. His study was followed by those of Billings et al. (1982), who evaluated the fate of soil carbon in the Alaskan tundra under warming climates. The emphasis on peatland sensitivity to climate change grew rapidly throughout the 1990s and 2000s, in sync with the prominence of scientific studies concerned with global warming (Chambers and Brain, 2002). Gorham (1991) popularized the role of northern peatlands in the global carbon cycle by presenting one of the first carbon pool estimates and discussing uncertainties related to current carbon stocks and fluxes. Since Gorham’s (1991) seminal work, much effort has been put toward understanding and quantifying the role of peatlands in the global carbon cycle in the past, present, and future.

A short exercise using a large bibliographic database illustrates the rapid increase in the number of publications on carbon and climate in peatland research. The Science Citation Index Expanded™ from 1970 to 2013 was mined via Web of Science™ Core Collection using the keywords peat*, carbon, and climat*. This collection contains data from over 8500 of the world’s leading scientific journals. Citation indices for ‘peat*’ and ‘carbon’ indicate that while less than 10% of all peat-related publications published prior to 1990 were concerned with carbon cycling, 33% of them had ‘carbon’ listed as a main topic in 2012–2013 (Figure 1). Likewise, the number of publications discussing peatlands in association with climate (‘peat*’, ‘climat*’) increased from 8% in 1990–1991 to 27% in 2012–2013. The number of studies relating peatland to carbon and climate also increased, from 1% in 1991–1992 to 14% in 2012–2013. Finally, 41% of papers concerned with peat and carbon were related to climate in 2012–2013, compared with only 10% in 1990–1991.

Figure 1.

Number of publications on peatland carbon dynamics and carbon–climate interactions, as derived from The Science Citation Index Expanded™. The publication numbers shown are for 2-year totals. See the text for more detail of citation change over time.

Over the last couple of decades, peat-core analysis has been extensively used to provide a long-term perspective on Holocene peatland carbon dynamics and to refine carbon stock estimates at local, regional, and global scales (e.g. Clymo et al., 1998; Loisel et al., 2014; Turunen et al., 2002; Yu et al., 2010). Individual peat-core records, as presented in some of the contributions in this Special Issue, are essential for large-scale synthesis. Recent compilations of such peatland carbon datasets have improved our understanding of Holocene carbon dynamics in the circum-Arctic region, the region with the most abundant peatlands in the world (Loisel et al., 2014; Yu et al., 2009). For example, combining basal peat ages (e.g. MacDonald et al., 2006) with peat-core carbon accumulation records offers a means to estimate peatland carbon pool over time (Yu et al., 2010), thereby providing an alternative to the approach based on peat depth and volume estimates (Gorham, 1991; Turunen et al., 2002). Furthermore, peatland carbon sequestration (net carbon balance) history during the Holocene can be assessed using these large databases (Yu, 2011). The net carbon balance estimates provide the opportunity to directly evaluate the impacts of peatland carbon fluxes and carbon burial rates on atmospheric carbon dioxide (CO₂) concentration (Yu, 2011), in addition to providing the opportunity to assess and compare the role of peatlands with that of other carbon reservoirs in the Earth system over time (e.g. Kleinen et al., 2010; Spahni et al., 2013). These new insights provide essential constraints on balancing the Holocene global carbon budget (Ciais et al., 2013; Ruddiman et al., 2011). Among all the carbon reservoirs (land and ocean) and processes discussed in the IPCC AR5 Report published in 2013, peat accumulation is one of the most important carbon fluxes in controlling the atmospheric CO₂ concentration over the last 7000 years (Ciais et al., 2013; Figure 2).

Figure 2.

Peat accumulation and other mechanisms contributing to CO₂ concentration change during the last 7000 years as presented in the IPCC AR5 report (Ciais et al., 2013). Filled black circles represent individual model-based estimates for individual mechanisms (ocean, land, geological or human), while solid color bars represent expert judgment. Confidence levels for each mechanism are indicated in the right column (H for high, M for medium, and L for low confidence). See Ciais et al. (2013) for references and detail.

Overviews of contributions to the Special Issue

This Special Issue was initiated during the most recent Holocene peatland carbon synthesis effort for the circum-Arctic (Loisel et al., 2014). We present a total of 13 new contributions focusing on regional syntheses of major peatland regions, presenting case studies at individual sites, and using novel proxy and modeling approaches to investigate peatland carbon dynamics. Several contributions also address important peatland processes, including peatland initiation, paludification, lateral expansion, and net carbon balance. Locations of study sites and regions for most contributions are shown in Figure 3. Below, we provide a brief overview of each contribution to the Special Issue.

Figure 3.

Map showing the extent of circum-Arctic peatlands and locations of carbon accumulation records. The previous synthesis of carbon accumulation records was based on 33 sites (yellow triangles; Yu et al., 2009), and the recently updated synthesis consists of 127 sites (black circles; Loisel et al., 2014). Peatland extent is from Yu et al. (2010). New peatland sites and regions discussed in the contributions of this Special Issue (shown as open boxes from western Europe to eastern Canada) are from 1 – Van der Linden et al. (2014); 2 – Mathijssen et al. (2014); 3 – Pluchon et al. (2014); 4 – Zhao et al. (2014); 5 – Nichols et al. (2014); 6 – Yu et al. (2014); 7 – Packalen and Finkelstein (2014); 8 – Holmquist and MacDonald (2014); 9 – Shiller et al. (2014); 10 – Magnan and Garneau (2014); and 11 – Garneau et al. (2014).

Circum-Arctic and regional peatland syntheses

Loisel et al. (2014) describe an expanded database of peat properties and carbon and nitrogen accumulation rates across the circum-Arctic region. The database consists of 268 peat cores from 215 sites, including data from some major peatland regions that were identified as data gaps in a previous synthesis (Yu et al., 2009), such as the Hudson Bay Lowlands, West Siberia Lowlands, and Kamchatka (Figure 3). This synthesis presents the most comprehensive compilation of peat properties, such as organic matter content, bulk density, and carbon content, that will be useful for large-scale scaling up and modeling of peatland carbon dynamics. The expanded database of peat carbon accumulation records confirms the earlier observation (Yu et al., 2009) that the highest carbon accumulation rates occur during the early Holocene, when climate was warmer in the northern extra-tropical region.

Garneau et al. (2014) present a regional synthesis of carbon accumulation records from 21 peat cores, including 16 cores with detailed accumulation history analysis, from six ecoclimatic regions across Québec in eastern Canada. They found that climate is a major control of carbon accumulation, resulting in the highest accumulation rates in the mid-Holocene and a lower rate during the Neoglacial climate cooling in the late Holocene. The combination of spatial and temporal analysis using a suite of records to address the question of climate control on peat carbon accumulation is a major strength of this contribution.

Magnan and Garneau (2014) compare carbon accumulation records using eight bog cores between two maritime regions along the St Lawrence North Shore in eastern Canada, in more detail than discussed in Garneau et al. (2014). They found that temporal variations in carbon accumulation rates at individual sites are controlled by local factors and peatland internal dynamics, but the difference between two regions is likely caused by different bioclimatic conditions.

Packalen and Finkelstein (2014) provide a synthesis of Holocene carbon dynamics for the Hudson Bay Lowlands, Canada. They combine carbon accumulation records (to derive carbon accumulate rates) and basal peat ages (to reconstruct peatland initiation history as a proxy for peatland area change) to provide an estimate of peatland carbon stocks in the Hudson Bay Lowlands. They also use an empirical modeling approach to generate a regional-scale reconstruction of net carbon balance to evaluate peatland impact on the global carbon cycle during the late Holocene, especially in regard to atmospheric methane (CH₄) change.

Holmquist and MacDonald (2014) use peat-core records from eight peatlands along a south–north transect in the James Bay Lowlands in northern Ontario, Canada, to investigate peatland initiation, fen-to-bog transition, and carbon accumulation dynamics. They found that peatland initiation lagged land availability for up to several millennia and that old inland peatlands show earlier fen-to-bog transitions, suggesting a dominant control of local and autogenic processes on this ecological succession. Furthermore, their carbon accumulation rates vary among sites and over time, suggesting dominant ecological rather than climatic controls.

Yu et al. (2014) provide a regional synthesis of peat properties and carbon accumulation rates from nine cores at six fen sites in Alberta, Canada. They found that carbon accumulation rates are much higher than global northern peatland averages, suggesting that continental fens are more effective carbon sinks than oceanic peatlands.

Studies on peatland processes

Shiller et al. (2014) present multiple paleoecological data from a temperate peatland in southern Ontario, Canada, to evaluate the relative importance of climate and autogenic processes in controlling Holocene carbon accumulation. They found that autogenic processes driven by the accumulation of organic matter have played a primary role in causing vegetation change from herbaceous (sedge-dominated) to Sphagnum-dominated peatlands, influencing carbon accumulation during the Holocene, while climate has played only a secondary, but important, role in changing species abundance.

Van der Linden et al. (2014) present peat-core analysis results from four bogs along a latitudinal gradient from northern Germany to northern Sweden to investigate climate influence on peat carbon accumulation during the last millennium. They found that southern sites have higher accumulation rates than northern sites, suggesting the importance of temperature in affecting carbon accumulation.

Pluchon et al. (2014) study peatland paludification and its impacts on ecosystem carbon storage in the boreal forest region of northeastern European Russia. They found that substrate slope is the main control over lateral expansion rates in the late Holocene. Their finding that carbon storage decreases initially after paludification of boreal forest demonstrates that peatlands are more effective carbon sinks than forests over millennial timescales, but not at shorter timescales.

Zhao et al. (2014) use new and previously published basal peat dates to investigate peatland initiation and lateral expansion histories in the Zoige Basin on the eastern Tibetan Plateau. They also found that landscape slope is a dominant control of lateral expansion rates, as documented in Korhola (1994), Loisel et al. (2013), and Pluchon et al. (2014). They attribute the higher initiation and accumulation rates in the early Holocene to greater influence of the Asian summer monsoon.

Novel approaches in peatland studies

Nichols et al. (2014) use a suite of novel organic geochemical datasets from peat to investigate vegetation change, carbon accumulation, and peatland hydrology at a Sphagnum-dominated peatland in southeastern Alaska during the Holocene. They found that species composition, which is partly dependent on regional climate, is the dominant control of peat carbon accumulation, with Sphagnum-dominated peatland in the early and late Holocene accumulating carbon at higher rates than sedge-dominated peatlands during the warm and dry mid-Holocene.

Mathijssen et al. (2014) provide a case study at a subarctic fen in northern Finland to investigate peat vertical growth and lateral expansion history during the Holocene. They found that carbon accumulation and lateral expansion slowed down during the warm and dry mid-Holocene. Also, on the basis of contemporary carbon flux measurements at the site, they reconstruct Holocene CO₂ and CH₄ fluxes and model their radiative forcing.

Frolking et al. (2014) use a well-established process-based model of peatland dynamics to examine the relationship between net carbon balance and apparent carbon accumulation rate as derived from peat cores. They simulated two Holocene precipitation time series representing eastern Canada, in addition to carrying out simulations of two hypothetical late-Holocene drying scenarios. They found that elevated decomposition from persistent dry periods would influence apparent carbon accumulation rates of peat deposited long before the occurrence of these dry periods.

Holocene carbon dynamics and carbon stocks of northern peatlands

The circum-Arctic peatland synthesis as presented in Loisel et al. (2014) shows a generally similar temporal pattern as in a previous synthesis with limited sites (Yu et al., 2009), indicating that the highest apparent rates of carbon accumulation occurred during the Holocene thermal maximum (HTM) in the early Holocene (Figure 4a). However, two main differences were found between these synthesis products. First, the mean Holocene rate of peat carbon accumulation presented by Loisel et al. (22.9 g C/m²/yr) is higher than the estimate from Yu et al. (18.6 g C/m²/yr). Second, the difference in carbon accumulation rates between the early and late Holocene was smaller in the updated synthesis (Loisel et al.) than in the previous one (Yu et al.). These differences are likely related to the very different numbers of sites and differences in geographic representation of sites included in each one of these datasets, but also to different site selection criteria.

Figure 4.

Holocene carbon accumulation and initiation histories of northern peatlands: (a) apparent carbon accumulation rates in 1000-year bins from Yu et al. (2009; n = 33 sites) and Loisel et al. (2014; n = 127 sites), with error bars as standard errors of means in both cases; (b) cumulative basal peat ages representing peatland initiation histories used to approximate peatland area change over time (dashed line from MacDonald et al., 2006; n = 1516; solid line from combined database in MacDonald et al., 2006; Gorham et al., 2007; and Korhola et al., 2010; as shown in Yu et al., 2013; n = 2577).

The circum-Arctic synthesis presented by Loisel et al. (2014) reflects the overall Holocene pattern of peat carbon accumulation in northern peatlands. However, at the regional scale, this pattern may differ. For example, peatland sites in eastern Canada (Garneau et al., 2014) and in southern Alaska (Jones and Yu, 2010) show pronounced maximum accumulation during the HTM, although at different timings because of the influence of waning Laurentide ice sheet on insolation-driven climate change (Kaufman et al., 2004; Renssen et al., 2009). However, this HTM pattern is less obvious or entirely absent from other regions, such as the James Bay Lowlands (Holmquist and MacDonald, 2014) and western continental Canada (Yu et al., 2014). Furthermore, different peatland regions as represented in this Special Issue show different Holocene-scale mean carbon accumulation rates, with the highest rate in Alberta, Canada, and the lowest in the James Bay Lowlands, Canada (Table 1).

Table 1.

Peatland carbon accumulation rates at regional scales in northern peatlands as reported in this Special Issue.

Region	Mean apparent C accumulation rates (gC/m²/yr)	Time period in the Holocene	Reference
Northern peatlands	22.9	The entire Holocene	Loisel et al. (2014)
Québec, eastern Canada	26.1	Last 7000 years	Garneau et al. (2014)
Hudson Bay Lowlands, Canada^a	24	Last 7500 years	Packalen and Finkelstein (2014)
James Bay Lowlands, Ontario, Canada^b	23.8	Last 7000 years	Holmquist and MacDonald (2014)
Alberta, western Canada	32.5	Last 9000 years	Yu et al. (2014)
Zoige Basin, Tibetan Plateau, China^c	31.1	The Holocene	Zhao et al. (2014)

Calculated from 17 peat cores with multiple dates.

Calculated from the mean LARCA and basal age of each of eight cores as shown in their Tables 2 and 3 (and in their Figure 4b and c).

Depth-based estimate assuming uniform bulk density and C%.

The circum-Arctic peatland synthesis provides an opportunity to update the peatland carbon pool estimates. Using the carbon accumulation history from the expanded database (Figure 4a; Loisel et al., 2014) and the most comprehensive basal age database (Figure 4b) of MacDonald et al. (2006), Gorham et al. (2007), and Korhola et al. (2010) combined, we generate three new estimates of carbon pools using the time history approach of Yu et al. (2010) with different total area of present peatlands and peatland area histories from basal age databases. These carbon pool estimates range from 436 to 604 GtC (Table 2), which are still within the rounded value of 500 ± 100 GtC as the most likely figure, but with greater uncertainties (Yu, 2012).

Table 2.

Selected estimates of carbon pools in northern (circum-Arctic) peatlands using two different approaches.^a.

A. Time (carbon accumulation) history approach
Mean and range of apparent C rate (gC/m²/yr)^b	Present peatland area (×10⁶ km²)^c	Dataset for peatland area change over time		C pool (GtC)	Reference
18.6 (13.4–25.6)	4	MacDonald et al. (2006)		546	Yu et al. (2010)
18.6 (13.4–25.6)	3.7	M+G+K dataset^d		436	This paper
22.9 (19.7–28.7)	4	M+G+K dataset^d		604	This paper
22.9 (19.7–28.7)	3.7	M+G+K dataset^d		559	This paper
B. Peat volume approach
Mean peat depth (m)	Present area (×10⁶ km²)	Bulk density (g/cm³)	C proportion	C pool (GtC)	Reference
2.3	3.42	0.112	0.517	455	Gorham (1991)
1.1	3.46	0.081–0.091	0.5	270–370	Turunen et al. (2002)
2.3^e	3.42^e	0.118	0.47	436	Loisel et al. (2014)

See Yu (2012) for discussion of different approaches to estimating peatland carbon pools.

From two synthesis studies on Holocene C accumulation rates in northern peatlands (Yu et al. 2009: 18.6 gC/m²/yr, and Loisel et al. (2014): 22.9 gC/m²/yr), and ranges are from 1-kyr binned values.

Present peatland area from Maltby and Immirzi (1993; 4 × 10⁶ km²) and Joosten and Clarke (2002; 3.7 × 10⁶ km²).

Combined dataset from basal peat ages (n = 2577) in MacDonald et al. (2006), Gorham et al. (2007) and Korhola et al. (2010), as presented in Yu et al. (2013).

Peatland depth and area data from Gorham (1991).

Challenges and opportunities

From the most recent synthesis and community-wide effort, the following challenges and associated opportunities can be identified:

Peatland data gaps and geographic representations. The expanded database (Loisel et al., 2014) now includes peatland carbon accumulation records from two of the largest peatland regions in the world, the Hudson Bay Lowlands, and West Siberia Lowlands. Additional records from these and other peatland regions are needed to increase their representation. However, remaining data gap regions include East Siberia and European Russia (Figure 3). Peatlands tend to be shallow and are often influenced by permafrost in East Siberia, but their carbon accumulation histories will be useful to understand carbon dynamics under different climate conditions in the interior of the largest Eurasian continent. This understanding will provide insights into projecting response of soil carbon pools to future climate change in one of the fastest warming regions on Earth.

Lateral expansion dynamics and the challenge of scaling up. We now have a good understanding of peatland initiation and carbon accumulation histories on the basis of large-scale databases that have expanded rapidly in recent years. However, we still lack a good understanding of peatland lateral expansion process, partly because of the high cost and time-intensive nature of site-specific studies. Some studies, including two papers in this Special Issue, show dominant controls of local topography, especially landscape slopes, on lateral expansion rates (e.g. Korhola, 1994; Loisel et al., 2013; Pluchon et al., 2014; Zhao et al., 2014). As a result, it is challenging to generalize these observations to formulate a conceptual model for scaling up to regional and circum-Arctic scales. Lateral expansion history will likely better represent the change of peatland area over time than peatland initiation history from basal peat ages that have been used for global (Yu et al., 2010) and regional carbon stock estimates (Packalen and Finkelstein, 2014). Further analysis of large radiocarbon-derived peatland age databases as presented in Korhola et al. (2010) and Ruppel et al. (2013) would help generate a better empirical history of peatland area change than simply using the basal initiation ages (Figure 4b). Also, a process-based modeling approach using digital elevation data will help address the uncertainties in estimating peatland area change over time and peatland carbon pool estimates at regional and global scales.

Regional peatland accumulation histories and carbon pool estimates. With the continued increase in availability of peat carbon accumulation records across circum-Arctic peatlands, we will be able to generate more representative carbon accumulation histories at the regional scale, which will be better suited for discussing the role of regional bioclimatic conditions in controlling carbon sequestration rates. Also, a reliable estimate of the global peatland carbon pool can probably be better achieved through regional carbon pool assessments based on a time history approach. This method allows for easy updates of the peatland carbon pool estimate as new peat carbon records and better approximations of peatland area change over time become available. Packalen and Finkelstein (2014) provide a case study for the Hudson Bay Lowlands.

Projecting and quantifying overall peatland carbon balance in the future. The emerging pattern of rapid peatland carbon accumulation under warm climates in the Holocene (Charman et al., 2013; Yu et al., 2010) has important implication for assessing the fate of large peatland carbon pools in future warming climate. However, we need to recognize that peatlands near the southern limits of peatland domains (e.g. temperate peatlands) likely show different and complex responses to warmer climates because of increasing evapotranspiration, reduced precipitation, and resultant moisture deficits. An overall carbon balance of northern peatlands in the future would need to consider both likely elevated carbon sequestration in peatlands further north and carbon losses from peatlands near their southern distribution ranges.

Footnotes

Acknowledgements

We thank all the reviewers for evaluating the manuscripts included in this Special Issue. They are Adam Ali, Matt Amesbury, Ilka Bauer, Lisa Belyea, Anne Bjune, Jane Bunting, Frank Chambers, Sarah Finkelstein, Paul Glaser, Paul Hughes, Miriam Jones, Edgar Karofeld, Atte Korhola, Martin Lavoie, John Matthews, Dmitri Mauquoy, Erin McClymont, Philip Meyers, Maara Packalen, Britta Sannel, Renato Spahni, Maria Strack, Minna Väliranta, Simon van Bellen, Guoping Wang, and Catherine Yansa.

Funding

Some ideas presented here benefited from discussions at a peatland synthesis and training workshop held at Lehigh University in October 2013, supported by NSF (ARC-1107981), INQUA (Project 1303 ‘Holocene Global Peatland Carbon Dynamics’) and PAGES (Working Group ‘C-PEAT: Carbon in Peat on EArth through Time’). ZY and JL were supported by NSF grant ARC-1107981, and DJC by NERC grant NE/I012915/1.

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