Biogeochemical attributes of ice glaciers and rock glaciers in low latitude alpine ecosystems

Background/Question/Methods

Alpine ice glaciers in the American West are projected to disappear within the next 100 years, though rock glaciers may persist for longer. Both ice glaciers and rock glaciers have been shown to contribute metals, nutrients, and contaminants to downstream waters. A better understanding of what the differences of the inputs of these frozen masses to mountain headwaters in the United States is needed. Rock glaciers, which are heterogeneous masses of ice and rock are an order of magnitude more abundant than ice glaciers in the mountain ranges of the western United States. In our 2012-2014 comparative study of outflow chemistry and biology from 70 glaciers and rock glaciers across the Cascades, Sierra Nevada, and Rocky Mountains, we hypothesized that rock glaciers would have higher meltwater temperatures due to lower elevations, greater metal concentrations from increased chemical and physical weathering, lower nutrients due to greater biological activity and uptake, and more complex organic chemistry due to multiple carbon sources. 

Results/Conclusions

There were significant differences in the temperature and chemistry of meltwaters between ice and rock glaciers. Although there was variation among and within regions, in general, rock glaciers had higher temperatures, pH, electrical conductivities, metal concentrations, nitrate and total nitrogen concentrations than ice glaciers. Dissolved organic carbon (DOC) concentrations were low but the structure (observed through fluorescence spectroscopy), varied between paired rock-ice glacier sites. Results were suggestive of a microbial source in ice glaciers and a combination of microbial and vascular plant source in rock glaciers. NH₄⁺ levels were near or at detection limit for both types of glaciers, however both NO₃^- and NH₄⁺ values appeared to be greater in regions with higher atmospheric N deposition. We also investigated differences in microbial community structure among rock and ice glaciers with 16S gene surveys. Our analysis suggests meltwater from glaciers and rock glaciers are actively contributing metals and nutrients to downstream waters, with the potential to affect microbial communities at the glacial terminus and potentially the nature of reactive elements that are delivered to headwater aquatic ecosystems.