The Role of Photochemistry in Driving the Composition of Dissolved Organic Matter Found in Glacier Environments

Dissolved organic matter (DOM) in glacier environments is aliphatic-rich, yet studies have proposed DOM originates from allochthonous, aromatic and often aged material. Allochthonous organic matter (OM) is exposed to ultraviolet radiation both in atmospheric transport and post-deposition on the glacier surface. Thus, we evaluate photochemistry as a mechanism to account for the compositional disconnect between allochthonous OM sources and glacier DOM composition. Six solid endmember OM sources were leached in ultrapure water and photo-irradiated for 28-days in a solar simulator, until > 90 % of initial chromophoric DOM was removed. Ultrahigh-resolution mass spectrometry was used to compare the molecular composition of leachates pre- and post-irradiation to DOM sampled from supraglacial and bulk runoff from the Greenland Ice Sheet and Juneau Icefield, respectively. Photo-irradiation drove molecular level convergence between the initially aromatic-rich leachates and aromatic-poor glacial samples, selectively removing aromatic compounds (-80 ± 19 %) and producing aliphatics (+75 ± 35 % relative abundance). The molecular level composition of glacial DOM was statistically indistinguishable to post-irradiation leachates. Furthermore, Bray-Curtis analysis showed substantial similarity in the molecular formulae present between glacier samples and post-irradiation leachates. Post-irradiation leachates contained 84 ± 7.4 % of the molecular formulae, including 72 ± 17 % of the aliphatic formulae, detected in glacier samples. Therefore, photodegradation provides a mechanistic pathway which may account for the disconnect between proposed aromatic, aged sources of OM and the aliphatic-rich fingerprint of glacial DOM. More work is required to quantify rates and magnitudes of photodegradation possible in transit to and on glacier surfaces. Nonetheless, given the high ultraviolet conditions of both these environments, photochemical alteration of DOM is likely significant. Thus, photochemical transformation of allochthonous OM may mediate the composition of DOM found in glacier environments and thus this mechanism should be considered when assessing the DOM chemistry therein.