Abstract
Climate change is causing extensive warming across arctic regions resulting in permafrost degradation, alterations to regional hydrology, and shifting amounts and composition of dissolved organic matter (DOM) transported by streams and rivers. Here, we characterize the DOM composition and optical properties of the six largest arctic rivers draining into the Arctic Ocean to examine the ability of optical measurements to provide meaningful insights into terrigenous carbon export patterns and biogeochemical cycling. The chemical composition of aquatic DOM varied with season, spring months were typified by highest lignin phenol and dissolved organic carbon (DOC) concentrations with greater hydrophobic acid content, and lower proportions of hydrophilic compounds, relative to summer and winter months. Chromophoric DOM (CDOM) spectral slope (S275-295) tracked seasonal shifts in DOM composition across river basins. Fluorescence and parallel factor analysis identified seven components across the six Arctic rivers. The ratios of ‘terrestrial humic-like’ versus ‘marine humic-like’ fluorescent components co-varied with lignin monomer ratios over summer and winter months, suggesting fluorescence may provide information on the age and degradation state of riverine DOM. CDOM absorbance (a350) proved a sensitive proxy for lignin phenol concentrations across all six river basins and over the hydrograph, enabling for the first time the development of a single pan-arctic relationship between a350 and terrigenous DOC (R2 = 0.93). Combining this lignin proxy with high-resolution monitoring of a350, pan-arctic estimates of annual lignin flux were calculated to range from 156 to 185 Gg, resulting in shorter and more constrained estimates of terrigenous DOM residence times in the Arctic Ocean (spanning 7 months to 2½ years). Furthermore, multiple linear regression models incorporating both absorbance and fluorescence variables proved capable of explaining much of the variability in lignin composition across rivers and seasons. Our findings suggest that synoptic, high-resolution optical measurements can provide improved understanding of northern high-latitude organic matter cycling and flux, and prove an important technique for capturing future climate-driven changes.
Highlights
Northern high-latitude regions contain substantial quantities of organic carbon in perennially and seasonally frozen soils, comprising more than half the entire global carbon soil stock (Tarnocai et al, 2009)
We show that simple absorbance proxies (a350, S275–295), which can be measured with in-situ techniques, are capable of tracing dissolved lignin concentrations ( 8) and seasonal changes in geochemical dissolved organic matter (DOM) composition (e.g., 8 and percent hydrophobic organic acid (HPOA)) occurring across the six major Arctic rivers
We demonstrate that lignin phenol biomarkers appear capable of providing information on the biogeochemical cycling of the hydrophobic dissolved organic carbon (DOC) fraction, knowledge on a major proportion of the aquatic DOM pool
Summary
Northern high-latitude regions contain substantial quantities of organic carbon in perennially and seasonally frozen soils, comprising more than half the entire global carbon soil stock (Tarnocai et al, 2009). Large Arctic rivers play an increasingly recognized role in regional carbon cycling by transporting a proportion of this terrigenous material from land to the ocean, whilst acting as sites for active carbon metabolism and transformation (Striegl et al, 2005; Holmes et al, 2012; Mann et al, 2015; Spencer et al, 2015). Arctic riverine export is substantial enough (∼10% of the global freshwater discharge) that it imparts estuarine-like water quality characteristics throughout the Arctic Ocean, influencing coastal salinity structure on a localized basis (Aagaard and Carmack, 1989; Serreze et al, 2006; McClelland et al, 2011). Future changes in the fluxes and composition of terrigenous DOM released to and exported from Arctic rivers are likely. Tracing future alterations in the composition as well as concentration of riverine DOM is crucial for understanding the effects of climate change
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