Ideas and perspectives : why Holocene thermokarst sediments of the Yedoma region do not increase the northern peatland carbon pool

Introduction Conclusions References


Ideas and perspectives: why
Soils and sediments of the northern permafrost region have accumulated large stocks of organic carbon (OC) over millennia (Tarnocai et al., 2009).As the global climate warms there is a concern that thawing permafrost will expose soil organic matter (SOM) that was previously protected in permafrost to decomposition, causing a positive permafrost-carbon feedback to climate (Schuur et al., 2008(Schuur et al., , 2015)).Hugelius et al. (2014) provide the most recent integrated estimate of Northern circumpolar permafrost region soil and sediment OC stocks with total stocks estimated at 1307 Pg and a 95 % confidence interval of 1140-1476 Pg (updated from Strauss et al., 2013).Of this, roughly 800 Pg is perennially frozen with the remainder stored in active layer or talik deposits.A substantial part of the perennially frozen OC is stored in the Beringian Yedoma region with estimated permafrost deposit OC stocks of 213 Pg with an uncertainty range of 164-267 Pg.Schirrmeister et al. (2013) provide in depth discussion and review on various aspects of these deposits.Schuur et al. (2015), in a recent review of the permafrost carbon feedback, highlights that there is considerable spread in esti- Here we examine these important statements by evaluating the findings and data presented by Walter Anthony et al. (2014) against (1) a brief review of scientific definitions of peat, peatlands, organic soils and thermokarst sediments, (2) independent field data as well as independent geospatial databases showing the extent of organic soils and/or peatlands in the Siberian Yedoma region and (3) by analysing the spatial overlap between these new estimates and existing datasets of Yedoma region soil and sediment OC storage.

Methods
All geospatial analyses and quantification of areal extents of classes were calculated in equal area projections using Geographical Information Systems (software ArcGIS 10.3, ESRI, Redlands, California, USA).The extent of the Siberian Yedoma region was digitised from Grosse et al. (2013) and, where applicable, snapped to correspond to the Arctic Ocean coastlines of the NCSCDv2 (Hugelius et al., 2013b).Soil and nonsoil coverage within this region in the NCSCDv2 was extracted.To provide estimates of mapped coverage of peatlands and wetlands for the Siberian Yedoma region which can be considered as independent from the NCSCD the following international geospatial datasets were used: Nilsson et al. (2002); Bartalev et al. (2003); Lehner and Döll (2004) and Arino et al. (2012).The thematic classes that corresponded to peatlands were identified and their respective coverage in the Siberian Yedoma region quantified.See the Supplement for an overview of the detailed coverage of different classes in the respective geospatial datasets.
The independent field validation sites were classified and sampled using a transectbased semi-random approach during field campaigns in August (2010 and 2013), see Palmtag et al. (2015) and Siewert et al. (2015) for details.The starting point and direction of transects were chosen to cut across representative landscape types.After this, pedons (a pedon is a three-dimensional body of soil as sampled, described and Introduction

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Full classified in soil studies) were described and sampled at equidistant intervals.In three out of nine sites, the bottom of deep peat deposits was not reached when sampling.Calculations of overlap in soil carbon stocks between different estimates and datasets are based on data on soil and/or sediment carbon stocks from Tarnocai et al. (2009), Hugelius et al. (2013aHugelius et al. ( , b, 2014)), Walter Anthony et al. (2014) and Zimov et al. (2006).By using the reported depth ranges and soil carbon densities of the different studies, the overlap between estimates has been calculated following the same methods used in the original studies.The calculations assume that all Histels in the NCSCDv2 within the Siberian Yedoma region where located in alases and the remaining alas soil area was subdivided between the Turbel and Orthel suborders of the Gelisol soil order.The calculations are made assuming 15 % coverage of lakes/rivers in the Siberian Yedoma region.
Note that in this manuscript, the term alas is used in a wide sense to describe former thermokarst lake basins.Following initial permafrost degradation and thermokarst, these basin have typically been (partly) terrestrialized (e.g. through lake drainage or evaporation of lake water) and re-aggraded permafrost.

Across different scientific disciplines (and countries) the definition of what is peat varies.
A commonly used definition states that peat is sedentarily accumulated material consisting of at least 30 % (dry weight) of dead organic material while peatlands are areas (with or without vegetation) with a naturally accumulated peat layer (Joostens and Clark, 2002).Many studies have employed a minimum depth criterion of the surface peat layer to the definition of peatland; most frequently 30 cm (Kivinen and Pakarinen, 1981;Lappalainen, 1996;Joostens and Clark, 2002).The Canadian definition of an Introduction

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Full organic wetland (or peatland) includes a depth of organic soil material (of 17 % OC or 30 % organic material) of at least 40 cm.Soil classification systems define organic soil material (or peat) based on organic carbon content, while the thickness of organic soil material in the upper soil column determines whether a soil is primarily considered to be a mineral soil or an organic soil.
The US soil taxonomy (Soil Survey Staff, 2010) and the World Reference Base for Soil Resources (IUSS Working Group WRB, 2007) defines waterlogged soil with more than 12 to 18 % OC (dry weight; range depending on clay content) as organic soil material while the Canadian System of Soil Classification (Soil Classification Working Group, 1998) defines soil with more than 17 % OC (or 30 % organic material; dry weight) as organic soil material.All these soil classification systems define a soil as an organic soil if there is 40 cm or more of accumulated organic soil material in the upper soil column (the Canadian system uses 60 cm for highly fibric moss-peat).
The literature describing sediments of thermokarst basins and lakes includes many different definitions of different facies or deposit types.These definitions are often not based on quantified physical or chemical properties of sediments, but rather reflect descriptive characteristics and the environments in which they formed.In addition to sedentary in situ peat, previous studies have described organic rich sedimentary thermokarst facies, including: (1) "detrital peat" described as layered organic deposits formed on beaches or in shallow waters (Murton, 1996) or as lee-shore deposits (Hopkins and Kid, 1988); (2) "organic rich silts" (or "lacustrine organic silt") where primarily mineral lake sediments are interspersed with sedentary or allochtonous detrital organic sediments layers (Murton, 1996;Kanevskiy et al., 2014) and (3) "Mud/muddy peat" which differs from detrital peat based on a higher mud content.These deposits may contain blocks of peat or other materials and typically form thick sediment layers in deep water thermokarst lake environments by suspension settling of fine and/or low-density material (Hopkins and Kidd, 1988;Murton et al., 1996).Introduction

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Classification of Kolyma lowland Holocene thermokarst sediments
The Beringian Yedoma region can be subdivided into areas of intact Yedoma (ca.30 % by area), areas that have been affected by thermokarst and subsequently re-aggraded permafrost (56 %) and areas of open water (14 %) which are commonly underlain by taliks (Strauss et al., 2013).The study by Walter Anthony et al. (2014) 2).Walter Anthony et al. (2014) claim that these sediments increase the previously recognized permafrost peat OC pool by 50 %.However, we argue that the use of imprecise terminology has caused misleading comparisons in relation to previous stock estimates.These deposits are very different from definitions of peat as being primarily organic material, usually of sedentary or shallow water origin (see 3.1 above).We consider it inappropriate and misleading to directly contrast these mineral lake sediments to organic peat stocks described following strict pedological definitions by e.g.Tarnocai et al. (2009).As these Holocene thermokarst deposits do not meet the criteria of peat (or organic soils) used in any regional or circumpolar peat carbon stock study they cannot be claimed to increase peat carbon stocks.They simply increase the stock of mineral alas sediments known to be of Holocene age.Introduction

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The extent of organic soils in the Siberian Yedoma Region
The discussion by Walter Anthony et al. (2014) surrounding Holocene-aged alas deposits in the Yedoma region are based on the assumption that these alases are peat deposits.They conclude that the NCSCD is underestimating the spatial coverage of Histels (permafrost peatland soils) and that the pedon dataset of the NCSCD is biased towards non-alas soils (see Walter Anthony et al., 2014;Sect. 3.5 in the Supplement).Here we evaluate these statements against independent analyses of geospatial datasets and field inventory data.Both these sources show a limited extent of organic soils in the Siberian Yedoma region (Fig. 1).The Histel coverage in the Siberian Yedoma region in the NCSCD is 9 %.This is comparable, but somewhat higher, than peatland coverage estimated from independent geospatial databases of 3 to 6 % (Fig. 1).It is notable that the degree of overlap between independent datasets is limited, indicative of difficulties with classifications and differences in class definitions when mapping peatland extent (Fig. 1).Our independent compilation of field sites located in alases or thermoerosional gullies from across the Siberian Yedoma region reveals that only 9 out of 49 sites are peatlands (Fig. 1).The surface peat depth of these nine peatland sites was ≥ 1.3 ± 1.1 m (mean± std), with a range of 0.4 m to > 3.7 m.
A spatial overlay analyses of regional land-cover and wetland characterization maps (Nilsson et al., 2002;Stolbovoi, 2002) suggest that ∼ 3 % of the region is covered by deep peat bogs while 19 % is characterized as swamps with very shallow peat (0.1-0.5 m).
Walter Anthony et al. (2014) base the argument that there is a bias towards non-alas soils in the NCSCD pedon dataset v2 on the fact that only nine out of 60 pedons in the Siberian Yedoma region are classified as Histels.However, the assumption that all pedons in alas deposits should be classified as Histels is erroneous and likely based on a misunderstanding caused by conflicting terminologies between fields.Descriptions of geomorphological settings of the pedons presented by Hugelius et al. (2013a) actually describe 13 additional mineral soil pedons sampled in alases or thermokarst deposits Introduction

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Full in the Siberian Yedoma region.The geomorphological setting of the remaining sites of the NCSCDv2 cannot be fully resolved since many of them lack metadata describing their geomorphological location (Hugelius et al., 2013a).All of these combined lines of evidence support an interpretation that peatlands are locally present in alases of the Siberian Yedoma region, but rarely cover large surfaces.The alases are dominated by mineral soils formed into e.g.reworked yedoma or lacustrine sediments.This interpretation is also supported by previous scientific studies from this region (e.g.Czudek et al., 1970;Veremeeva and Gubin, 2009;Wetterich et al., 2009;Schirrmeister et al., 2011;Morgenstern et al., 2013).We find no support to the claim that the maps or pedon dataset of the NCSCD are systematically biased.

Overlap between soil C estimates in Yedoma region alases
Spatial overlap between different studies of soil carbon stocks may mislead data users and cause significant errors in estimates.Previous integrated estimates of carbon stocks in the Beringian Yedoma region (Tarnocai et al., 2009;Hugelius et al., 2014) are based on soil maps linked to field-based soil data for the upper three m and generalized estimates of Yedoma region deposits for deeper deposits (Zimov et al., 2006;Strauss et al., 2013).The Holocene thermokarst deposits described by Walter Anthony et al. ( 2014) overlap these previous estimates in space, but they differ in their characterisation of the sediment (Fig. 2).An important difference compared to previous studies is that Walter Anthony et al. (2014) include ∼ 24 Pg carbon in Holocene deposits assumed to occur in taliks (perennially thawed ground) under present day lakes and rivers.We recognize that these estimates are new, but they are also outside the a ∼ 20 Pg net increase.However, given that the geographical distribution of the sites in the new estimate by Walter Anthony et al. (2014) is limited to the Kolyma river lowlands (see Fig. 1) we do not consider this estimate more robust than previous estimates.For alas deposits below three meters, the estimate by Walter Anthony et al. ( 2014) includes ∼ 60 Pg of Holocene OC and ∼ 155 Pg of Pleistocene OC which overlaps with recent estimates of ∼ 110 Pg of OC in refrozen thermokarst sediments (Strauss et al., 2013;updated in Hugelius et al., 2014).The differences between estimates are primarily caused by methodological differences in how stocks are calculated rather than large differences in field data or terminologies.Walter Anthony et al. (2014) provide an account for how these estimates overlap.In brief, earlier studies did not include a pool of OC stored taberites, an in situ thawed, diagenetically altered Yedoma deposit, and applied bootstrapping approaches to calculate OC stocks while Walter Anthony et al. ( 2014) use arithmetic means.There is currently an Action Group of the International Permafrost Association dedicated to more in-depth analyses and synthesis on the extent and properties of Yedoma region sediments (led by Dr. J. Strauss, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research) and this issue is not further discussed here.

Conclusions and recommendations
We conclude that Holocene OC stocks in Siberian Yedoma region alases are primarily stored in mineral soils and lacustrine sediments rather than peat.There is no evidence or reasoning to suggest that these deposits increase the northern peatland pool or that the NCSCD is systematically biased against upland soils.It is relevant and important to contrast the Holocene accumulation of carbon in alas sediments to that estimated for peatlands.But it is important to note that the Holocene thermokarst sediments described by Walter Anthony et al. (2014) do not add to circumpolar permafrost peat carbon stocks.

BGD Introduction
Full We emphasize that our concerns regarding use of terminology and spatial overlap of estimates in the discussed study in no way affects the validity of their other important findings regarding the Holocene carbon dynamics of these ecosystems.We attribute the misunderstandings to overlap between terminologies in the respective fields of science that study soils and sediments in periglacial landscapes.We suggest that a careful and exhaustive review of these terminologies would help future studies to harmonize classifications and definitions.

Information about the supplement
The Supplement contains information on the analyses of land cover from different geospatial databases within the Siberian Yedoma region.All the pedons used as independent ground truth sites will be archived in the open access database of the International Soil Carbon Network (http://iscn.fluxdata.org/Pages/default.aspx).Pending publication of data in other venues, all data is available from the authors upon request.

The
mates of Yedoma region permafrost OC stocks.In a study describing the Holocene C dynamics of Siberian thermokarst lakes Walter Anthony et al. (2014) estimate a pool of 456 ± 45 Pg C in the Beringian Yedoma region.This estimate includes a previously unrecognized pool of 159 ± 29 Pg OC accumulated in Holocene aged sediments deposited in drained thermokarst-lake basins (hereafter called alases) of the Yedoma region.They conclude that these alas sediments increase the previously recognized circumpolar permafrost peat OC pool by 50 %.It is further stated that previous integrated studies of the permafrost OC pool (Tarnocai et al., 2009; Hugelius et al., 2013a) have failed to account for these deposits because of biases in the Northern Circumpolar Soil Carbon Database (NCSCD).Walter Anthony et al. (2014) argue that the field site data of the NCSCD is biased towards non-alas sites and that the soil maps on which the database is based are too generalized to show the distribution of deposits in alases of the Yedoma region.Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | scope of the studies by Tarnocai et al. (2009) and Hugelius et al. (2014) as they are per definition not soils, nor are they typically permafrost deposits.This leaves ∼ 135 Pg of Holocene carbon to be reconciled with previous estimates for soil/sediment that occupy the same physical space.For the upper three meters, Walter Anthony et al. estimate ∼ 76 Pg of Holocene carbon.This overlaps soils from previous estimates with carbon stocks of 53-58 Pg (Tarnocai et al., 2009; Hugelius et al., 2013a, b; 2014) resulting in Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 .Figure 2 .
Figure1.Overview of field sites and estimated coverage of peatlands in the Siberian Yedoma region.Graduated colours within the region show coverage of peatlands in four global/regional map products that are independent from the NCSCD (see Methods).The coverage is shown cumulatively so that the colours reflect how many of the four products that map peatlands in any given location.Points show locations of the Holocene alas profiles used by Walter Anthony et al. as well as independent soil profiles for validation (classified as mineral soils or peatlands).All of the independent validation points are known to be located in alases or thermoerosional gullies.
Anthony et al. (2014)tial subdivision of this region but uses different data and computational methods to estimate the volume and OC stocks of the various sediments and deposit types in the region.WalterAnthony et al. (2014)present valuable new data from Yedoma and thermokarst deposits in the Kolyma Lowlands.Extrapolated from 28 sampled exposures WalterAnthony et al. (2014)estimate that 159 Pg of OC has accumulated in deep Holocene thermokarst deposits across the Yedoma region.The bulk of this Holocene OC has accumulated in sediment facies they descriptively call "Stratified muddy peat".The authors state that this facies corresponds to strata that previous au- thors have called "Mud/muddy peat" (see 3.1 above).These facies are described as deep-water lake sediments, predominantly of minerogenic origin and with an OC content of only 3 to 4 % by weight (Walter Anthony et al., 2014; Fig. 2 and extended data Table