Abstract
Abstract. Mobilization of dissolved organic carbon (DOC) and related trace elements (TEs) from the frozen peat to surface waters in the permafrost zone is expected to enhance under ongoing permafrost thaw and active layer thickness (ALT) deepening in high-latitude regions. The interstitial soil solutions are efficient tracers of ongoing bio-geochemical processes in the critical zone and can help to decipher the intensity of carbon and metals migration from the soil to the rivers and further to the ocean. To this end, we collected, across a 640 km latitudinal transect of the sporadic to continuous permafrost zone of western Siberia peatlands, soil porewaters from 30 cm depth using suction cups and we analyzed DOC, dissolved inorganic carbon (DIC), and 40 major elements and TEs in 0.45 µm filtered fraction of 80 soil porewaters. Despite an expected decrease in the intensity of DOC and TE mobilization from the soil and vegetation litter to the interstitial fluids with the increase in the permafrost coverage and a decrease in the annual temperature and ALT, the DOC and many major and trace elements did not exhibit any distinct decrease in concentration along the latitudinal transect from 62.2 to 67.4° N. The DOC demonstrated a maximum of concentration at 66° N, on the border of the discontinuous/continuous permafrost zone, whereas the DOC concentration in peat soil solutions from the continuous permafrost zone was equal to or higher than that in the sporadic/discontinuous permafrost zone. Moreover, a number of major (Ca, Mg) and trace (Al, Ti, Sr, Ga, rare earth elements (REEs), Zr, Hf, Th) elements exhibited an increasing, not decreasing, northward concentration trend. We hypothesize that the effects of temperature and thickness of the ALT are of secondary importance relative to the leaching capacity of peat, which is in turn controlled by the water saturation of the peat core. The water residence time in peat pores also plays a role in enriching the fluids in some elements: the DOC, V, Cu, Pb, REEs, and Th were a factor of 1.5 to 2.0 higher in mounds relative to hollows. As such, it is possible that the time of reaction between the peat and downward infiltrating waters essentially controls the degree of peat porewater enrichments in DOC and other solutes. A 2° northward shift in the position of the permafrost boundaries may bring about a factor of 1.3 ± 0.2 decrease in Ca, Mg, Sr, Al, Fe, Ti, Mn, Ni, Co, V, Zr, Hf, Th, and REE porewater concentration in continuous and discontinuous permafrost zones, and a possible decrease in DOC, specific ultraviolet absorbency (SUVA), Ca, Mg, Fe, and Sr will not exceed 20 % of their current values. The projected increase in ALT and vegetation density, northward migration of the permafrost boundary, or the change of hydrological regime is unlikely to modify chemical composition of peat porewater fluids larger than their natural variations within different micro-landscapes, i.e., within a factor of 2. The decrease in DOC and metal delivery to small rivers and lakes by peat soil leachate may also decrease the overall export of dissolved components from the continuous permafrost zone to the Arctic Ocean. This challenges the current paradigm on the increase in DOC export from the land to the ocean under climate warming in high latitudes.
Highlights
Boreal and subarctic regions of the Northern Hemisphere are among the most vulnerable areas to ongoing climate warming (Natali et al, 2011, 2015; Schuur et al, 2015; Vonk et al, 2015b; Pries et al, 2016)
A snapshot of peat soil water chemistry allowed for quantifying the distribution of dissolved organic carbon (DOC) and major and trace elements in peat porewaters at the end of the active period across a sizeable gradient of permafrost
We did not confirm a trend of diminishing DOC and metal concentration in peat porewaters northward, despite a decrease in mean annual temperature, vegetation density, and the active layer thickness
Summary
Boreal and subarctic regions of the Northern Hemisphere are among the most vulnerable areas to ongoing climate warming (Natali et al, 2011, 2015; Schuur et al, 2015; Vonk et al, 2015b; Pries et al, 2016). Because of sizeable carbon storage in frozen soils of Siberia (Botch et al, 1995; Krementski et al, 2003; Frey and Smith, 2007; Beilman et al, 2009; Tarnocai et al, 2009; Gentsch et al, 2015), the warming in this region is especially important for global projections of the carbon balance on the planet (Smith et al, 2004; Frey and Smith, 2005; Feng et al, 2013) In this regard, permafrostbearing part of Western Siberia Lowland (WSL) is highly sensitive to soil warming, due to (i) the dominance of discontinuous, sporadic, and intermittent permafrost coverage compared to continuous and discontinuous permafrost of central and eastern Siberia and Canada High Arctic; (ii) the surface layer temperature of the WSL permafrost is often between 0 and −2 ◦C, which is warmer than in other regions of the world (Romanovsky et al, 2010); (iii) essentially flat area of the WSL and high impact of flooding and thermokarst development; and, most importantly, (iv) high stock of ancient and recent organic carbon in the form of partially frozen peat deposits of 1 to 4 m thickness. The peat land zones have received significant attention (Haapalehto et al, 2011; Olefeldt and Roulet, 2012; Charman et al, 2013; Quinton and Baltzer, 2013; Muller et al, 2015; Morison et al, 2017), notably via natural manipulation experiments in order to assess the responses of peat carbon to simulated warming and oxidizing (Dielemann et al, 2016; Liu et al, 2016), water table manipulation (Blodau and Moore, 2003; Strack et al, 2008; Goldberg et al, 2010), and drought (Clark et al, 2012)
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