Pan‐Arctic Riverine Dissolved Organic Matter: Synchronous Molecular Stability, Shifting Sources and Subsidies

• Published in: Global biogeochemical cycles 2021-04, Vol.35 (4), p.n/a
• Main Authors: Behnke, M. I., McClelland, J. W., Tank, S. E., Kellerman, A. M., Holmes, R. M., Haghipour, N., Eglinton, T. I., Raymond, P. A., Suslova, A., Zhulidov, A. V., Gurtovaya, T., Zimov, N., Zimov, S., Mutter, E. A., Amos, E., Spencer, R. G. M.
• Format: Article
• Language: English
• Subjects: Arctic; Arctic climate changes; Base flow; Bioavailability; Biodegradation; Carbon cycle; Carbon isotopes; Carbon sequestration; Carbon sinks; Chemical composition; Climate change; Cyclotron resonance; Dissolved organic carbon; Dissolved organic matter; Fingerprints; Frozen ground; FT‐ICR MS; Gases; Greenhouse effect; Greenhouse gases; Groundwater; Groundwater discharge; Leaching; Mass spectrometry; Mass spectroscopy; Microbial degradation; Microorganisms; Ocean warming; Oceans; Organic carbon; Permafrost; radiocarbon; Rivers; Seasonal variation; Seasonal variations; Seasonality; Spring; Spring (season); Stability; Subsidies; Substrates; Sulfur; Sulphur; Surface water; Thawing; Water analysis
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2020GB006871

Climate change is dramatically altering Arctic ecosystems, leading to shifts in the sources, composition, and eventual fate of riverine dissolved organic matter (DOM) in the Arctic Ocean. Here we examine a 6‐year DOM compositional record from the six major Arctic rivers using Fourier‐transform ion cyclotron resonance mass spectrometry paired with dissolved organic carbon isotope data (Δ14C, δ13C) to investigate how seasonality and permafrost influence DOM, and how DOM export may change with warming. Across the pan‐Arctic, DOM molecular composition demonstrates synchrony and stability. Spring freshet brings recently leached terrestrial DOM with a latent high‐energy and potentially bioavailable subsidy, reconciling the historical paradox between freshet DOM's terrestrial bulk signatures and high biolability. Winter features undiluted baseflow DOM sourced from old, microbially degraded groundwater DOM. A stable core Arctic riverine fingerprint (CARF) is present in all samples and may contribute to the potential carbon sink of persistent, aged DOM in the global ocean. Future warming may lead to shifting sources of DOM and export through: (1) flattening Arctic hydrographs and earlier melt modifying the timing and role of the spring high‐energy subsidy; (2) increasing groundwater discharge resulting in a greater fraction of DOM export to the ocean occurring as stable and aged molecules; and (3) increasing contribution of nitrogen/sulfur‐containing DOM from microbial degradation caused by increased connectivity between groundwater and surface waters due to permafrost thaw. Our findings suggest the ubiquitous CARF (which may contribute to oceanic carbon sequestration) underlies predictable variations in riverine DOM composition caused by seasonality and permafrost extent. Plain Language Summary Climate change is causing the Arctic to warm, leading to the thaw of permafrost (permanently frozen ground) and shifting the distribution of plants, which in turn impact the quantity and quality of dissolved organic matter (DOM) in rivers. The fate of the carbon found in river DOM depends on its source as this determines its composition – a smorgasbord of organic substrates with some destined to be rapidly turned into greenhouse gases through respiration, while others may be exported to and sequestered in the ocean for millennia. Understanding the sources and composition of DOM presently in Arctic rivers will help predict future changes in the global carbon cycle. In this s