Differential mobilization of terrestrial carbon pools in Eurasian Arctic river basins

Abstract

Mobilization of Arctic permafrost carbon is expected to increase with warming-induced thawing. However, this effect is challenging to assess due to the diverse processes controlling the release of various organic carbon (OC) pools from heterogeneous Arctic landscapes. Here, by radiocarbon dating various terrestrial OC components in fluvially and coastally integrated estuarine sediments, we present a unique framework for deconvoluting the contrasting mobilization mechanisms of surface vs. deep (permafrost) carbon pools across the climosequence of the Eurasian Arctic. Vascular plant-derived lignin phenol (14)C contents reveal significant inputs of young carbon from surface sources whose delivery is dominantly controlled by river runoff. In contrast, plant wax lipids predominantly trace ancient (permafrost) OC that is preferentially mobilized from discontinuous permafrost regions, where hydrological conduits penetrate deeper into soils and thermokarst erosion occurs more frequently. Because river runoff has significantly increased across the Eurasian Arctic in recent decades, we estimate from an isotopic mixing model that, in tandem with an increased transfer of young surface carbon, the proportion of mobilized terrestrial OC accounted for by ancient carbon has increased by 3-6% between 1985 and 2004. These findings suggest that although partly masked by surface carbon export, climate change-induced mobilization of old permafrost carbon is well underway in the Arctic.

Keywords: compound-specific 14C; fluvial mobilization; hydrogeographic control.

Fig. 1.

Eurasian Arctic transect and cartoon of hydrological mobilization of terrestrial carbon into rivers. (A) Map of the rivers (black lines) with permafrost distribution (modified from refs. and 6) and sampling locations (red circles). (B) Illustration of the western Eurasian Arctic characterized by extensive moss-dominated wetlands underlain by discontinuous permafrost and ubiquitous deep groundwater conduits. (C) Illustration of eastern Eurasian Arctic characterized by a wide distribution of Yedoma ice complex, a thin seasonally thawing active layer, and thick continuous permafrost below. Blue arrows indicate hydrological transport of carbon from different physiogeographic regimes.

Fig. 2.

Hydrogeographic characteristics of the Eurasian Arctic rivers (A) and contrasting radiocarbon contents (expressed as Δ¹⁴C and conventional ¹⁴C age) of terrestrial markers compared with bulk OC in the estuarine surface sediments (B). Runoff rate = discharge/basin area. Detailed hydrogeographic data are listed in Table S1 [compiled from refs. , , , and “watersheds of the world” (http://www.wri.org/publications)]. The ∆¹⁴C values of terrestrial markers represent concentration-weighted averages with the SEs of analytical measurement propagated. Lignin phenols refer to vanillyl and syringyl phenols (detailed data are provided in Fig. S1). Hydroxy phenols refer to p-hydroxybenzaldehyde, p-hydroxyacetophenone, and p-hydroxybenzoic acid. Plant wax lipids constitute n-alkanes (C_27,29,31) and n-alkanoic acids (C_24,26,28) (6).

Fig. 3.

Hydrological and physiogeographic controls on the age of terrestrial markers in the integrating Eurasian Arctic estuaries: correlation of ∆¹⁴C_{lignin phenols} with runoff rate (A), ∆¹⁴C_{plant wax lipids} with continuous permafrost coverage (B), ∆¹⁴C_{hydroxy phenols} with wetland coverage (C), and ∆¹⁴C_{hydroxy phenols} with runoff rate (D). The blue dotted line in D represents linear correlation for the data of four eastern rivers (P < 0.05, R² = 0.81). *Runoff rate = discharge/basin area. Contents of terrestrial markers are defined in Fig. 2. Further statistical analyses can be found in Fig. S2.