Exploring linkages between dissolved organic matter and dissolved silicon in Arctic rivers influenced by a gradient of permafrost degradation

Abstract:

The silicon (Si) and carbon (C) cycle are intricately linked at the global scale through several well identified means, such as silicate weathering, uptake of Si by aquatic primary producers in freshwater and marine systems, and uptake of Si by terrestrial vegetation. Here we examine another potential linkage in between Si and C – riverine dissolved organic matter (DOM) and Si exports in fluvial systems across the boreal-Arctic transition. Using a space-for-time study design, we quantify linkages between DOM and DSi biogeochemistry in six rivers spanning a 700 km latitudinal gradient in north-central Alaska, examining how shifts in watershed active layer depth and vegetation cover impacts these relationships. We collected samples at each site every three days during the spring freshet of 2016, the period of most dynamic discharge in high latitude rivers, allowing us to determine how shifts in discharge influence these relationships. To assess DOM source and composition, we used absorbance and fluorescence spectroscopy to measure chromophoric DOM (CDOM) properties, lignin biomarker analyses to evaluate vascular plant contribution to the DOM pool, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to assess compositional changes. We assess these relationships to discharge-weighted mean concentrations of dissolved and biogenic Si concentrations using principal component analysis and linear regression across the full range of the freshet hydrograph. Together, our analysis sheds light on previously overlooked relationships between Si and C in fluvial material exports in a region of the globe experiencing rapid shifts chemical weathering rates, active layer depth, and vegetation cover.