Particulate Organic Matter Dynamics in a Permafrost Headwater Stream and the Kolyma River Mainstem.

Lisa Bröder,Anya Davydova,Nikita Zimov,Negar Haghipour,Timothy I Eglinton,Sergey Davydov,Jorien E Vonk

doi:10.1029/2019jg005511

Abstract

Ongoing rapid arctic warming leads to extensive permafrost thaw, which in turn increases the hydrologic connectivity of the landscape by opening up subsurface flow paths. Suspended particulate organic matter (POM) has proven useful to trace permafrost thaw signals in arctic rivers, which may experience higher organic matter loads in the future due to expansion and increasing intensity of thaw processes such as thermokarst and river bank erosion. Here we focus on the Kolyma River watershed in Northeast Siberia, the world's largest watershed entirely underlain by continuous permafrost. To evaluate and characterize the present‐day fluvial release of POM from permafrost thaw, we collected water samples every 4–7 days during the 4‐month open water season in 2013 and 2015 from the lower Kolyma River mainstem and from a small nearby headwater stream (Y3) draining an area completely underlain by Yedoma permafrost (Pleistocene ice‐ and organic‐rich deposits). Concentrations of particulate organic carbon generally followed the hydrograph with the highest concentrations during the spring flood in late May/early June. For the Kolyma River, concentrations of dissolved organic carbon showed a similar behavior, in contrast to the headwater stream, where dissolved organic carbon values were generally higher and particulate organic carbon concentrations lower than for Kolyma. Carbon isotope analysis (δ13C, Δ14C) suggested Kolyma‐POM to stem from both contemporary and older permafrost sources, while Y3‐POM was more strongly influenced by in‐stream production and recent vegetation. Lipid biomarker concentrations (high‐molecular‐weight n‐alkanoic acids and n‐alkanes) did not display clear seasonal patterns, yet implied Y3‐POM to be more degraded than Kolyma‐POM.

Highlights

Unprecedented warming at high latitudes, presently twice as fast as the global average, has the potential to affect global biogeochemical cycles (Richter‐Menge et al, 2018)
We focus on the particulate organic matter (POM) fraction transported by the Kolyma River mainstem close to its delta near Cherskiy, and from a nearby small headwater catchment (Y3) draining Yedoma permafrost soils
This high‐resolution time‐series, comprising a total of 36 POM samples for the lower Kolyma mainstem and 35 POM samples for headwater stream Y3 collected during the open‐water season in 2013 and 2015, offers detailed insights into the fluvial carbon dynamics of a major, permafrost‐dominated arctic watershed

Summary

Introduction

Unprecedented warming at high latitudes, presently twice as fast as the global average, has the potential to affect global biogeochemical cycles (Richter‐Menge et al, 2018). The thaw of frozen soils (permafrost) may trigger a positive feedback loop, since permafrost soils store about half of the global soil organic matter (OM) (Hugelius et al, 2014; Schuur et al, 2015), which upon thaw is exposed to decomposition, thereby generating greenhouse gases that fuel further global warming Permafrost thaw occurs both gradually from the top down and through more abrupt thaw processes (thermokarst and thermoerosion), leading to collapsing BRÖDER ET AL. The extent of thermokarst disturbances is predicted to grow across arctic landscapes, resulting in increased hydrologic conductivity and potentially enhanced release of OM to aquatic environments (e.g., Olefeldt et al, 2016) Hydrological changes, such as higher contributions of groundwater and base flow, predicted as a consequence of permafrost thaw have already been observed (Walvoord et al, 2012; Walvoord & Striegl, 2007), yet their effect on the OM loads of arctic rivers remains unclear. Increasing fluxes of dissolved organic carbon (DOC) and inorganic constituents were measured for the Mackenzie River from 1972–2012 (Tank et al, 2016), whereas a decrease in DOC and bicarbonate discharge with progressing thaw was reported for the Yukon River, possibly caused by enhanced OM decomposition in the soils or sorption to mineral soils (Striegl et al, 2005)

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