Long‐term changes in organic matter and mercury transport to lakes in the sporadic discontinuous permafrost zone related to peat subsidence

Abstract

AbstractPermafrost‐supported peatlands near the southern limit of permafrost are experiencing dramatic landscape changes as a result of recent climate warming, which have the potential to impact aquatic ecosystems through changes in terrestrial run‐off. Our objectives were to determine how terrestrial organic matter inputs to aquatic ecosystems in the southern Northwest Territories (Canada) changed as a result of peat subsidence, and whether terrestrial organic matter can be linked to sedimentary mercury. To accomplish this, we analyzed lipid biomarkers, lignin‐derived phenols, and other geochemical proxies in sediment cores from two lakes (KAK‐1 and TAH‐7) affected by recent peat subsidence. Both lakes experienced substantial shifts in organic matter proxies through time, but the trajectory of change differed, reflecting local heterogeneity in hydrological setting and other environmental factors. In KAK‐1, recent peat subsidence corresponded to a decrease in lignin‐derived phenol yield, increased inferred lignin oxidation, and δ15N depletion. In TAH‐7, peat subsidence was likely initiated by a local forest fire, and resulted in an increase in the n‐alkanol ratio C30/(C30 + C28) (consistent with a warmer, wetter climate), lignin‐derived phenol yield (mainly syringyls), and δ13C depletion. In TAH‐7, total mercury inputs were positively correlated to terrestrial carbon inputs, suggesting allochthonous carbon is an important vector for mercury transport to the lake. In both lakes, an increase in the C23/(C23 + C29) n‐alkane ratio was observed and suggests increased organic matter input from Sphagnum mosses. Our results demonstrate how terrestrial landscape changes occurring as a result of peat subsidence can influence carbon accumulation in aquatic ecosystems in discontinuous permafrost zones.