Mendenhall Glacier, Southeast Alaska, study site for our work on the glacier carbon cycle.
Glaciers and ice sheets represent the second largest reservoir of water in the global hydrologic system and glacier ecosystems cover 10% of the earth. Yet the carbon dynamics underpinning these ecosystems remain poorly understood. Improving knowledge of glacier biogeochemistry is particularly important as they are among the environments most sensitive to climate warming and industrial forcing. Most notably, glacier wastage is accelerating due to rising temperatures, changing precipitation patterns and the deposition of black carbon, which darkens glacier surfaces enhancing their absorption of light and heat. Glacier ecosystems were recently identified as a significant source of ancient, yet highly bioavailable dissolved organic carbon to downstream aquatic ecosystems. This finding runs counter to logical perceptions of age-reactivity relationships, in which the least reactive material withstands degradation the longest and is therefore the oldest. The remnants of ancient peatlands and forests since overrun by glaciers have been invoked as a source of this ancient, labile organic carbon. Results upon which this study is based, challenge the peatland/forest source hypothesis, indicating instead that glacier organic carbon is predominantly from aerosol deposition and enters glaciers in a pre-aged form and were recently published in Nature Geoscience. This study will determine the contribution to the glacial organic carbon pool made by fossil fuel derived aerosols, verify whether this organic carbon is indeed ancient and labile, and quantify the extent to which it is being exported to downstream ecosystems. The proposed view of the glacier carbon cycle is presented below.
Today, around 60% of organic aerosols are derived from anthropogenic activities, indicating that organic deposition has also increased dramatically since the industrial revolution. Therefore, if the organics found on, within and being exported from Gulf of Alaska glaciers are from aerosols, the glacier ecosystem structure we observe today is founded upon heterotrophy fueled by the waste products of industrial activity occurring thousands of miles away. If this is the case, then the organic carbon which is exported and feeds heterotrophy in ecosystems downstream of glaciers would also be of anthropogenic origin, suggesting these receiving ecosystems are also transformed relative to their pre-industrial status. As deposition of combustion products is a global phenomenon, all ecosystems may be receiving this ancient, labile carbon subsidy. In vibrant ecosystems, the labile carbon windfall is presumably rapidly processed and its signal is lost. In frigid glacier environments, these inputs stand out, making glaciers sentinel ecosystems for the detection and study of perturbation to global ecosystems through increasing anthropogenic deposition. Thus, although the study focuses upon glaciers along the Gulf of Alaska, findings will be highly relevant to any ecosystem receiving depositional inputs.