NSFGEO-NERC: WOOD-BASED CARBON DISCHARGE TO THE ARCTIC OCEAN

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Abstract: Erosion mobilizes and delivers organic carbon from soils and terrestrial vegetation to soils. The resultant river discharge of organic carbon to oceans is globally significant. Current estimates of this discharge, however, do not account for material larger than about 0.5 millimeters and thus do not account for pieces of large wood. Because few studies have investigated the drainage-basin-scale patterns of wood delivery, decay, and transport, we have limited knowledge of how large wood influences organic carbon discharge to the ocean. This knowledge gap leads to our primary research goals, which are to (i) quantify annual coarse particulate organic carbon discharge in the form of large wood versus other sources of dissolved and particulate organic carbon from the Mackenzie River drainage of Canada to the Arctic Ocean and (ii) estimate the storage volume, residence time, and decay rates of large wood in the drainage basin. We are conducting this work in the Mackenzie River drainage because it is a relatively pristine large river drainage basin that is known for high rates of wood export to the ocean.Our primary research aim is to quantify the discharge of large wood (LW) and assess its contribution to the organic carbon (OC) budget of the Mackenzie and the Arctic Ocean, while determining the dynamics of LW transport and its residence time in the river network. We have two specific objectives: (1) Quantify annual biospheric particulate organic carbon flux by LW using ground-based time-lapse photography and high-resolution satellite imagery. (2) Determine LW transport dynamics and the residence time, of LW using ground-based measurements of LW accumulations and satellite imagery. We will characterize residence time and decay rates with a 14C-based chronology of LW. The research plan includes five primary components over two years: At key locations within the Mackenzie River drainage, we will (1) measure LW fluxes, (2) estimate LW recruitment and storage, (3) measure LW volume and age distribution, (4) sample fine suspended sediment to improve particulate OC flux estimates, and (5) collect tree cores that can be used to constrain LW age, provenance, and physical and chemical degradation of LW during transport. The transformative aspects of the proposed research are: (1) quantification of proportions of OC exported as wood versus finer material from a large river, providing insight into the relative importance of different sources of OC under contemporary conditions, and (2) the first assessment of the relative ages of material across the full range of particulate sizes carried by a large river from 500 microns to large wood, providing a clearer basis for predicting how changes in the fate of coarse POC may impact the carbon cycle under a warming climate. This project will also substantially advance understanding of wood dynamics in large river basins under a warming climate that will alter wood recruitment, transport, and deposition processes, altering timing, volume, and placement of wood exports to the Mackenzie Delta and the Arctic.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.