Abstract
Hummocks develop by cryoturbation in fine-grained frost-susceptible soils and their stage of maturity may affect the translocation of organics in Cryosols. This study examines the distribution and morphology of hummocks in the Chuck Creek Trail Valley (northern British Columbia) and determines the quantity, distribution, and composition of organic matter in their soils. Hummocks occupy about 5%–20% of the valley and their morphology is largely affected by their silt content. Cryoturbated intrusions, radiocarbon dated to 2814 and 1648 cal year B.P., suggest that hummock development was initiated during the cooler late Holocene. Hummocks have an average soil organic carbon density of 16.3 kg m−2 in the uppermost 1 m, with 62% stored in the top 25 cm. Organics are mainly present as particulate organic matter in the O-horizon (25%–80%), characterized by degradable alkyl C and O/N-alkyl groups, but occur as mineral-associated organic matter (96%–98%) composed of recalcitrant aromatic and aliphatic C groups in the underlying B and C horizons. Minor differences in organic content and composition occur between hummock tops and troughs, and between hummocks showing different stages of maturity. In the absence of an observed frost table, contemporary hummock activity is attributed to seasonal freezing and thawing.
Highlights
Near-surface soils in permafrost regions store about 1600 Pg of soil organic carbon (Tarnocai et al 2009; Hugelius et al 2014)
Continued warming of permafrost soils may make these large stocks of organic carbon available for microbial decomposition, contributing to a positive feedback in the climate–carbon system (Ping et al 2015; Schuur et al 2015)
The magnitude of microbial decomposition of soil organic carbon depends on the type of organic carbon and on cryoturbation, a process that translocates near-surface organics deeper in the active layer where microbial activity becomes limited because of lower soil temperature and higher soil moisture (Bockheim 2007; Ping et al 2015)
Summary
Near-surface soils in permafrost regions store about 1600 Pg of soil organic carbon (Tarnocai et al 2009; Hugelius et al 2014). Earth hummocks, which are sub-meter- to meter-scale circular to oval mounds that develop in fine-grained frost-susceptible sediments (Mackay 1980), are widely distributed landforms indicative of cryoturbation. Their presence has been reported from subarctic to high Arctic regions (e.g., Lundqvist 1962; Zoltai 1975; Schunke 1977; Tarnocai and Zoltai 1978; Zoltai et al 1978; Scotter and Zoltai 1982; Ellis 1983; Kojima 1994; Bockheim and Tarnocai 1998; Kokelj et al 2007) and their development has been attributed to repeated freeze–thaw processes and differential frost heave (e.g., Mackay 1980; Ballantyne 1986; Schunke and Zoltai 1988; Lewis et al 1993; Van Vliet-Lanoë 2004). Following the degradation and collapse of hummocks, the bowl-shaped permafrost table degrades and thaw subsidence occurs, redistributing the organic matter; this process could make labile organic carbon previously trapped in permafrost available for biodegradation
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