The disappearance of relict permafrost in boreal north America: Effects on peatland carbon storage and fluxes (original) (raw)
Ecosystems, 2012
Recent warming at high-latitudes has accelerated permafrost thaw in northern peatlands, and thaw can have profound effects on local hydrology and ecosystem carbon balance. To assess the impact of permafrost thaw on soil organic carbon (OC) dynamics, we measured soil hydrologic and thermal dynamics and soil OC stocks across a collapse-scar bog chronosequence in interior Alaska. We observed dramatic changes in the distribution of soil water associated with thawing of ice-rich frozen peat. The impoundment of warm water in collapse-scar bogs initiated talik formation and the lateral expansion of bogs over time. On average, Permafrost Plateaus stored 137 ± 37 kg C m -2 , whereas OC storage in Young Bogs and Old Bogs averaged 84 ± 13 kg C m -2 . Based on our reconstructions, the accumulation of OC in near-surface bog peat continued for nearly 1,000 years following permafrost thaw, at which point accumulation rates slowed. Rapid decomposi-tion of thawed forest peat reduced deep OC stocks by nearly half during the first 100 years following thaw. Using a simple mass-balance model, we show that accumulation rates at the bog surface were not sufficient to balance deep OC losses, resulting in a net loss of OC from the entire peat column. An uncertainty analysis also revealed that the magnitude and timing of soil OC loss from thawed forest peat depends substantially on variation in OC input rates to bog peat and variation in decay constants for shallow and deep OC stocks. These findings suggest that permafrost thaw and the subsequent release of OC from thawed peat will likely reduce the strength of northern permafrost-affected peatlands as a carbon dioxide sink, and consequently, will likely accelerate rates of atmospheric warming.
Earth System Dynamics, 2011
Northern peatlands contain a large terrestrial carbon pool that plays an important role in the Earth's carbon cycle. A considerable fraction of this carbon pool is currently in permafrost and is biogeochemically relatively inert; this will change with increasing soil temperatures as a result of climate warming in the 21st century. We use a geospatially explicit representation of peat areas and peat depth from a recently-compiled database and a geothermal model to estimate northern North America soil temperature responses to predicted changes in air temperature. We find that, despite a widespread decline in the areas classified as permafrost, soil temperatures in peatlands respond more slowly to increases in air temperature owing to the insulating properties of peat. We estimate that an additional 670 km 3 of peat soils in North America, containing ∼33 Pg C, could be seasonally thawed by the end of the century, representing ∼20 % of the total peat volume in Alaska and Canada. Warming conditions result in a lengthening of the soil thaw period by ∼40 days, averaged over the model domain. These changes have potentially important implications for the carbon balance of peat soils.
Carbon accumulation in permafrost peatlands in the Northwest Territories and Nunavut, Canada
The Holocene, 2000
Average long-term apparent rates of carbon (C) accumulation (LARCA) were estimated for four peat cores from Arctic and Subarctic Canada. Detailed analyses of dry bulk-density and C content were used to determine variations in C accumulation rates throughout the cores. LARCA range from 12.5 to 16.5 g C m −2 yr −1 over the past 6700-10 000 years. Rates are lower for the surface layers of Arctic high-centred peat polygons, at 5.3 to 7.1 g C m −2 yr −1 for the last 3500-4500 years. By comparison, the rate for the near-surface peat from a Sphagnum fuscum hummock in the high Subarctic was considerably higher, at 24.1 g C m −2 yr −1 . The highest carbon accumulation rates were from core segments older than 4500 BP, which represent fen stages according to palaeoecological analysis. The average LARCA in our study are considerably lower than recent estimates of average carbon accumulation in Boreal peatlands. This difference is attributable partly to lower carbon percentages in our cores compared to the mean or estimated values of 50 to 51.7% used in those studies. Another factor is the presence of ground ice, which exaggerates the apparent peat depth and leads to erroneously high values if cumulative carbon estimates are based on depth. Using cumulative dry bulk-density, as we have done, eliminates the influence of ground ice and thus makes more accurate estimates possible.
Boreal peatland C fluxes under varying permafrost regimes
Soil Biology and Biochemistry, 2002
Discontinuous permafrost in peatlands has recently been melting across western Canada, creating wet Sphagnum-Carex lawns (internal lawns) interspersed within drier ombrotrophic bog. Permafrost degradation alters peat hydrology, thermal regimes and plant species assemblages, all of which could affect gaseous C emissions in peatlands. We quanti®ed CO 2 and CH 4¯u xes across the peat-atmosphere boundary using dark static chambers in adjacent internal lawns, continental bogs and frost mounds in an area of localized permafrost in northcentral Saskatchewan. Carbon dioxide and CH 4¯u xes ranged from 0.2 to 14.6 mmol CO 2 m 22 d 21 and from 224 to 344 mmol CH 4 , respectively, and differed signi®cantly among peatland types and sampling dates. Our estimates of CH 4¯u x are low compared to previous estimates from boreal wetlands, with net consumption of CH 4 typically in frost mounds. Permafrost melt in our study area is associated with 1.6-and 30-fold increases in CO 2 and CH 4 emissions, respectively. More widespread thaw across the discontinuous permafrost region will be an important consideration to boreal C budgets with future climatic changes. q
The long-term fate of permafrost peatlands under rapid climate warming
Scientific reports, 2015
Permafrost peatlands contain globally important amounts of soil organic carbon, owing to cold conditions which suppress anaerobic decomposition. However, climate warming and permafrost thaw threaten the stability of this carbon store. The ultimate fate of permafrost peatlands and their carbon stores is unclear because of complex feedbacks between peat accumulation, hydrology and vegetation. Field monitoring campaigns only span the last few decades and therefore provide an incomplete picture of permafrost peatland response to recent rapid warming. Here we use a high-resolution palaeoecological approach to understand the longer-term response of peatlands in contrasting states of permafrost degradation to recent rapid warming. At all sites we identify a drying trend until the late-twentieth century; however, two sites subsequently experienced a rapid shift to wetter conditions as permafrost thawed in response to climatic warming, culminating in collapse of the peat domes. Commonalities...
Arctic, Antarctic, and Alpine Research, 2000
Carbon and peat accumulation rates over the past 1200 yr were measured in relation to permafrost aggradation, maturity, ground fires, and degradation in a peatland with discontinuous permafrost near Fort Simpson, N.W.T., Canada. The White River volcanic ash layer, deposited 1200 yr ago, was used as a chronostratigraphic marker to compare peat and carbon accumulation among peat cores collected along transects over a consistent period of time, The aggradation of permafrost results in a change from unfrozen bog to forested peat plateau, and approximate decreases of 50 and 65% in carbon and vertical peat accumulation rates, respectively. Carbon and peat accumulation continue to decrease significantly with both increasing permafrost maturity and the number of ground fires. The transition from peat plateau to collapse bog through internal permafrost degradation results in up to a 72 and 200% increase in carbon and vertical peat accumulation rates, respectively. Permafrost degradation at the margins of a peat plateau can result in the formation of collapse fens, in which vertical peat accumulation increases significantly yet the carbon accumulation rates remain similar to the peat plateau. A warming climate may result in a shift towards higher carbon accumulation rates in peatlands associated with bog vegetation following peat plateau collapse, yet warmer peat temperatures, greater soil aeration, greater rates of peat decomposition, and an increase in burning may provide limits to the increase.
Changes in peat chemistry associated with permafrost thaw increase greenhouse gas production
Proceedings of the National Academy of Sciences, 2014
Carbon release due to permafrost thaw represents a potentially major positive climate change feedback. The magnitude of carbon loss and the proportion lost as methane (CH 4 ) vs. carbon dioxide (CO 2 ) depend on factors including temperature, mobilization of previously frozen carbon, hydrology, and changes in organic matter chemistry associated with environmental responses to thaw. While the first three of these effects are relatively well understood, the effect of organic matter chemistry remains largely unstudied. To address this gap, we examined the biogeochemistry of peat and dissolved organic matter (DOM) along a ∼40-y permafrost thaw progression from recently-to fully thawed sites in Stordalen Mire (68.35°N,.05°E), a thawing peat plateau in northern Sweden. Thaw-induced subsidence and the resulting inundation along this progression led to succession in vegetation types accompanied by an evolution in organic matter chemistry. Peat C/N ratios decreased whereas humification rates increased, and DOM shifted toward lower molecular weight compounds with lower aromaticity, lower organic oxygen content, and more abundant microbially produced compounds. Corresponding changes in decomposition along this gradient included increasing CH 4 and CO 2 production potentials, higher relative CH 4 /CO 2 ratios, and a shift in CH 4 production pathway from CO 2 reduction to acetate cleavage. These results imply that subsidence and thermokarst-associated increases in organic matter lability cause shifts in biogeochemical processes toward faster decomposition with an increasing proportion of carbon released as CH 4 . This impact of permafrost thaw on organic matter chemistry could intensify the predicted climate feedbacks of increasing temperatures, permafrost carbon mobilization, and hydrologic changes.
Accelerated thawing of subarctic peatland permafrost over the last 50 years
Geophysical Research Letters, 2004
1] In this study we provide a quantification of the main patterns of change of a subarctic peatland caused by permafrost decay monitored between 1957 and 2003. Up-thrusting of the peatland surface due to permafrost aggradation during the Little Ice Age resulted in the formation of an extensive peat plateau that gradually fragmented into residual palsas from the 19th century to the present. Only about 18% of the original surface occupied by permafrost was thawed in 1957, whereas only 13% was still surviving in 2003. Rapid permafrost melting over the last 50 years caused the concurrent formation of thermokarst ponds and fen-bog vegetation with rapid peat accumulation through natural successional processes of terrestrialization. The main climatic driver for accelerated permafrost thawing was snow precipitation which increased from 1957 to present while annual and seasonal temperatures remained relatively stable until about the mid-1990s when annual temperature rose well above the mean. Contrary to current expectations, the melting of permafrost caused by recent climate change does not transform the peatland to a carbon-source ecosystem as rapid terrestrialization exacerbates carbon-sink conditions and tends to balance the local carbon budget.