Abstract
Glacier ecosystems are a significant source of bioavailable, yet ancient dissolved organic carbon (DOC). Characterizing DOC in Mendenhall Glacier outflow (southeast Alaska) we document a seasonal persistence to the radiocarbon-depleted signature of DOC, highlighting ancient DOC as a ubiquitous feature of glacier outflow. We observed no systematic depletion in Δ14C-DOC with increasing discharge during the melt season that would suggest mobilization of an aged subglacial carbon store. However, DOC concentration, δ13C-DOC, Δ14C-DOC and fluorescence signatures appear to have been influenced by runoff from vegetated hillslopes above the glacier during onset and senescence of melt. In the peak glacier melt period, the Δ14C-DOC of stream samples at the outflow (−181.7 to −355.3‰) was comparable to the Δ14C-DOC for snow samples from the accumulation zone (−207.2 to −390.9‰), suggesting that ancient DOC from the glacier surface is exported in glacier runoff. The pre-aged DOC in glacier snow and runoff is consistent with contributions from fossil fuel combustion sources similar to those documented previously in ice cores and thus provides evidence for anthropogenic perturbation of the carbon cycle. Overall, our results emphasize the need to further characterize DOC inputs to glacier ecosystems, particularly in light of predicted changes in glacier mass and runoff in the coming century.
Highlights
Glacier ecosystems cover approximately 10% of the Earth’s surface and contribute large volumes of runoff to rivers and the coastal ocean
Watersheds in coastal and high-latitude regions can lead to a substantial watershed flux of dissolved organic matter (DOM) despite the typically low concentrations of dissolved organic carbon (DOC) reported for glacier ecosystems (Hood and Scott 2008, Hood et al 2009, Bhatia et al 2013)
Glacier environments can be important drivers of regional biogeochemical change and these ecosystems will increase in importance over the coming decades in areas where glacial water fluxes and DOM are increasing with climate warming
Summary
Glacier ecosystems cover approximately 10% of the Earth’s surface and contribute large volumes of runoff to rivers and the coastal ocean. Specific conductivity for the Mendenhall outflow ranged from 8.01 to 40.80 (μS cm−1) and was significantly higher (t-test, p < 0.01) during the transition periods (n = 11; mean = 24.80; σ = 7.41 μS cm−1) compared to the melt period (n = 14; mean = 12.64; σ = 4.13 μS cm−1) (figure 2(a); table 1).
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