Mendenhall Glacier in Alaska. Photo by A. Stubbins.
The dissolved organic matter (DOM) in glacial runoff is relatively old and mostly made up of aliphatic molecules (i.e. molecules are built mainly from carbon chains rather than aromatic rings). However, the observed composition of DOM in glacial runoff is the opposite of what we would expect to observe based upon known sources of glacial DOM. We assume glacial DOM is primarily derived from organic matter in soils or deposited aerosols, like soot or other combustion products. When mixed with water, soils and aerosols release DOM that is compositionally dissimilar from DOM in glacial runoff … why is this?
In a recent study, led by Amy Holt (Florida State University) and co-authored by Sasha Wagner and others, determined that photodegradation of DOM in glacial environments may explain the difference in composition between glacial DOM sources (soils and aerosols) and DOM in glacial outflows. The researchers conducted a series of experiments, where soils and aerosols were leached in water, then exposed to sunlight in a solar simulator for several weeks. Ultrahigh resolution mass spectrometry was used to evaluate how the molecular “fingerprint” of glacial DOM sources changed during the photodegradation experiment. After extensive photo bleaching, the composition of DOM samples converged, becoming more similar to the molecular composition of DOM in glacial runoff. Taken together, the study findings suggest that photochemical degradation of aged, aromatic DOM sources could explain the observed composition of DOM in glacial outflows. This work is entitled “Assessing the role of photochemistry in driving the composition of dissolved organic matter in glacier runoff” and was published in the Journal of Geophysical Research: Biogeosciences.
