Glaciers and rock glaciers (coarse rock fragments bound together and lubricated by ice) are melting worldwide from climate change, mobilizing ice-locked organic matter, minerals, and nutrients. The release of these meltwater constituents has implications for downstream chemical cycling and biological activity. Past studies have found that headwater alpine lakes and streams fed by glaciers have higher nutrient concentrations than headwaters fed only by snow. In a new USGS study, researchers were able to extend this finding by showing that different types of glaciers differ in their biogeochemical contributions. The results suggest that different types of glaciers affect their local ecosystems and have the potential to alter fundamental ecological aspects in important headwater ecosystems.

Not all glaciers are created equal. This study compared the different meltwater compositions of ice glaciers and rock glaciers in the western U.S. and asked whether there were differences in physical, chemical and microbial characteristics in meltwater. An extensive field sampling effort over three summers of 25 alpine glaciers and 24 rock glaciers found in the Cascades, Sierra Nevada, and Rocky Mountains provided the data for comparison.

There were clear differences between the meltwater compositions of glaciers and rock glaciers, and there were also geographic differences. The type of glacier (ice or rock) influences the downstream concentration of weathering products such as silica, calcium, and strontium. Glacier type also affected the complexity of organic matter exported via meltwaters, with glaciers producing organic matter that appears to originate as a byproduct of microbes and rock glaciers providing organic matter from plant decomposition as well as microbes. The geology of each mountain range controls the chemistry of the weathering products released; geology and climate influences the rate and intensity of weathering. The location of the glaciers relative to human settlement affects the compounds found in ice that come from atmospheric deposition. For example, while rock glaciers produced more nitrate than glaciers at all sample sites, the nitrate concentrations in the Rocky Mountain samples were very high because of proximity to agricultural and metropolitan sources of nitrogen. Unexpectedly, both the type of glacier and geographic location influence microbial diversity. The greatest microbial diversity was in rock glaciers, but there were many microbes that were found both in rock and ice glaciers. The Rocky Mountains had higher microbial diversity than the Cascades, but there were common microbes across all the mountains.  

Glaciers and rock glaciers sit at the interface of atmospheric and terrestrial environments, where inputs are captured, stored, altered, and then released to alpine headwaters. With 10,000 rock glaciers identified in the U.S., they are five times more common than ice glaciers. In coming centuries, rising temperatures will melt rock glaciers and ice glaciers alike. Some, ice glaciers may become rock glaciers as they shrink. Because rocks insulate rock glaciers, they melt more slowly, and will likely exist after ice glaciers disappear. Knowing the meltwater composition of both glaciers and rock glaciers will help predict how they will alter downstream ecosystems in the future.

The paper, "The Differing Biogeochemical and Microbial Signatures of Glaciers and Rock Glaciers", was published in JGR-Biogeosciences. 

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