Abstract
The spatial distribution and depth of permafrost are changing in response to warming and landscape disturbance across northern Arctic and boreal regions. This alters the infiltration, flow, surface and subsurface distribution, and hydrologic connectivity of inland waters. Such changes in the water cycle consequently alter the source, transport, and biogeochemical cycling of aquatic carbon (C), its role in the production and emission of greenhouse gases, and C delivery to inland waters and the Arctic Ocean. Responses to permafrost thaw across heterogeneous boreal landscapes will be neither spatially uniform nor synchronous, thus giving rise to expressions of low to medium confidence in predicting hydrologic and aquatic C response despite very high confidence in projections of widespread near-surface permafrost disappearance as described in the 2019 Intergovernmental Panel on Climate Change Special Report on the Ocean and Cryosphere in a Changing Climate: Polar Regions. Here, we describe the state of the science regarding mechanisms and factors that influence aquatic C and hydrologic responses to permafrost thaw. Through synthesis of recent topical field and modeling studies and evaluation of influential landscape characteristics, we present a framework for assessing vulnerabilities of northern permafrost landscapes to specific modes of thaw affecting local to regional hydrology and aquatic C biogeochemistry and transport. Lastly, we discuss scaling challenges relevant to model prediction of these impacts in heterogeneous permafrost landscapes.
Highlights
Permafrost, ground remaining below 0◦C for more than 2 years, contains roughly a third of the world’s soil organic carbon (Schuur et al, 2015; Meredith et al, 2019) and covers nearly one-fourth of the terrestrial Northern Hemisphere (Brown et al, 2002), including much of the boreal forest and tundra biomes (Figure 1)
Water and carbon become available for increased biogeochemical processing and transport via deeper subsurface pathways
Shallow permafrost thaw processes have proportionally large hydrologic and C-cycle impacts in systems with continuous permafrost coverage, whereas systems having discontinuous to sporadic permafrost coverage tend to be progressively more impacted by deeper permafrost thaw processes, including supra-permafrost talik development and complete permafrost loss
Summary
Permafrost, ground remaining below 0◦C for more than 2 years, contains roughly a third of the world’s soil organic carbon (Schuur et al, 2015; Meredith et al, 2019) and covers nearly one-fourth of the terrestrial Northern Hemisphere (Brown et al, 2002), including much of the boreal forest and tundra biomes (Figure 1). This overview explores these complexities through synthesis of recent work in northern latitudes, at the continuous (>90% coverage) to discontinuous (50–90% coverage) permafrost transition, where shifts from predominately shallow, warm-season flow processes toward deeper year-round flow processes are expected to impact hydrologic C lateral transport Based on this synthesis, we offer a framework for assessing vulnerabilities of water and aquatic C cycles in unstudied Arctic-boreal areas by addressing landscape, subsurface hydrologic, and C source factors that influence coupled hydrologic and biogeochemical responses to five distinct modes of permafrost thaw. Thickening of the active layer increases potential shallow groundwater storage and promotes infiltration, thereby reducing runoff via overland flow and deepening subsurface flowpaths (Table 2, H1, H2) These responses act to lengthen delivery times of water and solutes to stream and river networks (Ala-aho et al, 2018). H5: sub-permafrost flow o o CPL and OT enhance deep groundwater circulation Rowland et al, 2011; Walvoord and connectivity between shallow and deep, et al, 2012; McKenzie and Voss, sub-permafrost water sources
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