Constraining seasonal active layer dynamics and chemical weathering reactions occurring in North Slope Alaskan watersheds with major ion and isotope (δ34SSO4, δ13CDIC, 87Sr/86Sr, δ44/40Ca, and δ44/42Ca) measurements

Abstract

Rising air temperatures in the Arctic may destabilize a large pool of organic carbon stored in permafrost, thereby causing a positive feedback to global climate warming. Permafrost thaw could also deepen hydrologic flow paths and expose previously frozen rock and mineral fragments to chemical weathering. Future shifts in the inorganic solute geochemistry of Arctic rivers may signal changes in soil processes that also affect organic carbon storage. Tracing permafrost thaw with dissolved riverine loads requires understanding the spatial and seasonal variation of chemical weathering reactions and other biogeochemical phenomena that affect elemental mass-transport. To help identify connections between mineral weathering and active layer processes, we studied the major ion and isotope (δ34SSO4, δ13CDIC, 87Sr/86Sr, δ44/40Ca, and δ44/42Ca) geochemistry of five streams draining the North Slope of Alaska. Continuous permafrost underlies all streams, but the Atigun River, Roche Moutonnée Creek, and Trevor Creek primarily drain bare bedrock outcrops in the Brooks Range, while the Upper Kuparuk River and Imnavait Creek primarily drain tundra. In total, we collected 546 water samples spanning the spring freshet through fall freeze-up in 2009 and 2010. We also analyzed snow, rock, sediment, soil, and plant samples.Major ion ratios and δ13CDIC values point to the overall dominance of carbonate weathering by carbonic and sulfuric acids, with additional influences from atmospheric deposition, plant decay, sulfate salt dissolution, and silicate weathering by carbonic acid. δ13CDIC values may also reflect partial equilibration with soil and atmospheric CO2. All streams display large seasonal variations in major ion ratios and δ13CDIC values that are consistent with progressive deepening of the seasonally thawed zone over the summer. In the mountain watersheds, carbonate weathering dominates during the spring and summer, while sulfate salt (primarily CaSO4 and MgSO4) dissolution dominates during the fall. Riverine δ34SSO4 values reveal that the sulfate salts are secondary precipitates. We propose a conceptual model where cryoconcentration in soils during the late fall and winter causes secondary mineral formation at depth and re-exposure during subsequent thaw seasons produces the observed geochemical signals in rivers. The tundra streams lack definitive evidence for sulfate salt dissolution, presumably because thick peat soils limit the exposure and weathering of underlying glacial sediment where the salts are expected to form and dissolve. Appearance of a sulfate salt dissolution signal in tundra streams may correlate with future permafrost degradation. Carbonate weathering dominates riverine 87Sr/86Sr ratios, but the compositional heterogeneity of bedrock limits interpretation of the data. All rivers have higher δ44/40Ca values compared to bedrock, likely due to plant uptake of lighter Ca isotopes. In the tundra watersheds, freshet δ44/40Ca values were 0.10–0.20‰ lower than summer and fall values. These trends likely reflect contributions from plant decay, as comparison between δ44/40Ca and δ44/42Ca values suggests that all isotopic variation is mass-dependent with minimal radiogenic 40Ca inputs from the weathering of old silicate minerals with high K/Ca ratios.