Glacier Outflow Dissolved Organic Matter as a Window Into Seasonally Changing Carbon Sources: Leverett Glacier, Greenland

Abstract

The Greenland Ice Sheet is losing mass at a remarkable rate as a result of climatic warming.This mass loss coincides with the export of dissolved organic matter (DOM) in glacial meltwaters. However, little is known about how the source and composition of exported DOM changes over the melt season, which is key for understanding its fate in downstream ecosystems. Over the 2015 ablation season, we sampled the outflow of Leverett Glacier, a large land-terminating glacier of the Greenland Ice Sheet. Dissolved organic carbon (DOC) concentrations and DOM fluorescence were analyzed to assess the evolution of DOM sources over the course of the melt season. DOC concentrations and red-shifted fluorescence were highly associated (R 2 > 0.95) and suggest terrestrial inputs from overridden soils dominated DOM early season inputs before progressive dilution with increasing discharge. During the outburst period, supraglacial drainage events disrupted the subglacial drainage system and introduced dominant protein-like fluorescence signatures not observed in basal flow. These results suggest that subglacial hydrology and changing water sources influence exported DOC concentration and DOM composition, and these sources were differentiated using fluorescence characteristics. Red-shifted fluorescence components were robust proxies for DOC concentration. Finally, the majority of DOM flux, which occurs during the outburst and postoutburst periods, was characterized by protein-like fluorescence from supraglacial and potentially subglacial microbial sources. As protein-like fluorescence is linked to the bioavailability of DOM, the observed changes likely reflect seasonal variations in the impact of glacial inputs on secondary production in downstream ecosystems due to shifting hydrologic regimes.

Molecular Signatures of Glacial Dissolved Organic Matter From Svalbard and Greenland

“…Furthermore, the western margin of the GrIS in the Kangerlussuaq region is postulated to be underlain by paleosols from growth during the last glacial minimum (Levy et al., 2017). The GrIS has active subglacial hydrology with extensive regions of saturated sediments (Jordan et al., 2018; MacGregor et al., 2016), which allows for microbial activity and may contribute more aromatic DOM signatures at the onset of melt (Kellerman et al., 2020b; O'Donnell et al., 2016). Svalbard glaciers, on the other hand, are a mixture of cold‐ and polythermal‐based glaciers (Sevestre et al., 2015).…”

“…The high protein‐like fluorescence in both depositional and microbially produced DOM (Mladenov et al., 2011; Musilova et al., 2017) and high spatial and temporal variability in DOC concentration in the supraglacial environment (Holland et al., 2019; Musilova et al., 2017) make deconvoluting large scale controls on DOM composition problematic with the data at hand. Consideration also needs to be given to the timing of sampling during the melt seasons for supraglacial systems, as well as in glacial outflow that has temporally and spatially variant upstream inputs (Kellerman et al., 2020b). The wide range of supraglacial sample types (snow, ice, cryoconite holes, and supraglacial rivers and ponds) and microbial production over the course of the ablation season, result in spatially and temporally heterogeneous DOC concentrations (Holland et al., 2019; Musilova et al., 2017).…”

“…Glacial DOM sources include ancient organic matter previously stored cryogenically or held in subglacial stores, atmospheric deposition, and recently produced DOM in supraglacial and subglacial ecosystems (Hood et al., 2009; Spencer, Guo, et al., 2014; Spencer, Vermilyea, et al., 2014; Wadham et al., 2019). DOM characteristics have been shown to be indicative of hydrological evolution at a polythermal glacier, from outflow dominated by water interacting with the basal environment early in the melt season to supraglacial melt flowing through channelized drainage at peak melt (Kellerman et al., 2020b). Furthermore, spatial and temporal variability of dissolved organic carbon (DOC) concentrations have been observed over the ablation season on the Greenland Ice Sheet (GrIS) (Holland et al., 2019; Musilova et al., 2017), potentially due to increasing algal production with surface melt (Nicholes et al., 2019).…”

Seasonal Changes in Dissolved Organic Matter Composition in a Patagonian Fjord Affected by Glacier Melt Inputs

“…Samples for DOC measurement were filtered to 0.45 µm using either Whatman R Polycap GW PES filter capsules (fjord surface samples), Geotech Versapor filter capsules (fjord subsurface samples) or Whatman R Puradisc AQUA syringe filters (river samples) and stored frozen in acid-clean HDPE bottles (Nalgene R ). DOC concentrations were determined for all fresh and marine water samples on a Shimadzu TOC-L CHN analyzer equipped with the saline sample kit (e.g., Kellerman et al, 2020). The limits of detection (LoD = LoB + [1.645 × SD of low concentration sample]; where LoB = limit of blank) and quantification (LoQ = LoB + [5 × SD of low concentration sample]) were calculated as 2.7 µM and 3.7 µM, respectively (Armbruster and Pry, 2008).…”

Measuring pH in low ionic strength glacial meltwaters using ion selective field effect transistor (ISFET) technology

“…Meltwater from the Greenland ice sheet and other land ice in the Arctic is contributing approximately 600 km 3 yr −1 to the oceans (Bamber et al 2018). The chemical composition of these waters has the potential to significantly influence the oceans, by changing carbon and nutrient budgets (Hood et al 2009, 2015; Hawkings et al 2015; Hopwood et al 2020; Kellerman et al 2020), influencing marine food webs (Arrigo et al 2017; Hopwood et al 2018) and potentially controlling CO 2 dynamics (Fransson et al 2015; Beaird et al 2018; St. Pierre et al 2019). The nature of glacial meltwaters means that they are challenging to monitor, because of low temperatures, low ionic strength, rapidly changing discharge, and high turbidity and sedimentation.…”