Alpine ecosystem response to climate warming: Long-term monitoring data of stream chemistries revisited

Alpine ecosystems are considered to be more sensitive to climate change than are downstream habitats because they exhibit less resilience to climate warming. Long-term monitoring of stream chemistries serves as a powerful tool to discern environmentally driven changes, in the context of cryospheric processes. According to the research hypothesis, the chemistry trends in the Green Lakes Valley, Colorado Front Range resulted from bedrock weathering in response to permafrost thawing coupled with exogenous sources. The median pH values during the 36 years of monitoring (1984-2020) have increased significantly in all five sampling sites from the headwater (5.7 at ~3800 m a.s.l) to the outlet (6.9 at ~3345 m a.s.l). The ANC increased significantly from a median of 10.6 μeq L^-1 at the headwater to a median of 129 μeq L^-1 at the outlet. These trends have occurred in spite of high sulfate concentrations in meltwater generated by oxidative weathering of pyrite (OWP). Analysis of the major cations and anions in the downstream sites revealed fairly consistent multi-segment trends. The first segment from 1984 to the mid 1990s exhibited a slight decrease in the electrolytes concentrations. Remarkably rapid upswings in electrolyte concentrations were observed in the mid 1990s to the mid 2000s. However, these trends significantly decreased from the mid 2000s onward. If climate warming is the mechanism that controls the rate of mineral weathering, then the electrolytes release rates from meltwater and permafrost thawing should increase over time, rather than flatten off or even decrease, as observed here. These spatiotemporal patterns of calcium and sulfate can be explained by a combination of mineral weathering and exogenous additions. Permafrost thawing is an important mechanism that adversely affects the OWP; consequently, it releases sulfuric acid and increases mineral weathering. However, the influence of eolian dust on alpine stream chemistry should not be overlooked.