The Role of Glacier Erosion in Riverine Particulate Organic Carbon Export

Biospheric particulate organic carbon (POCbio) burial and rock petrogenic particulate organic carbon (POCpetro) oxidation are opposing long-term controls on the global carbon cycle, sequestering and releasing carbon, respectively. Here, we examine how watershed glacierization impacts the POC source by assessing the concentration and isotopic composition (delta C-13 and Delta C-14) of POC exported from four watersheds with 0%-49% glacier coverage across a melt season in Southeast Alaska. We used two mixing models (age-weight percent and dual carbon isotope) to calculate concentrations of POCbio and POCpetro within the bulk POC pool. The fraction POCpetro contribution was highest in the heavily glacierized watershed (age-weight percent: 0.39 +/- 0.05; dual isotope: 0.42 (0.37-0.47)), demonstrating a glacial source of POCpetro to fjords. POCpetro was mobilized via glacier melt and subglacial flow, while POCbio was largely flushed from the non-glacierized landscape by rain. Flow normalized POCbio concentrations exceeded POCpetro concentrations for all streams, but surprisingly were highest in the heavily glacierized watershed (mean: 0.70 mgL(-1); range 0.16-1.41 mgL(-1)), suggesting that glacier rivers can contribute substantial POCbio to coastal waters. Further, the most heavily glacierized watershed had the highest sediment concentration (207 mgL(-1); 7-708 mgL(-1)), and thus may facilitate long-term POCbio protection via sediment burial in glacier-dominated fjords. Our results suggest that continuing glacial retreat will decrease POC concentrations and increase POCbio:POCpetro exported from currently glacierized watersheds. Glacier retreat may thus decrease carbon storage in marine sediments and provide a positive feedback mechanism to climate change that is sensitive to future changes in POCpetro oxidation.

Automated summary

This research examines the impact of glacierization on the source and contribution of particulate organic carbon (POC) in Southeast Alaska. The study finds that glacier melting releases more rock petrogenic POC, while non-glacier areas primarily flush POC through rainfall. However, glacier rivers still play a significant role in POC export, especially in heavily glacierized regions. Additionally, glacier retreat can lead to a decrease in carbon storage and create a positive feedback mechanism to climate change.