Wood-Based Carbon Storage in the Mackenzie River Delta: The World's Largest Mapped Riverine Wood Deposit

The Mackenzie River Delta (MRD) has been recognized as an important host of river-derived wood deposits, and Mackenzie River wood has been found across the Arctic Ocean. Nevertheless, we lack estimates of the amount of carbon stored as wood and its age in the delta, representing a gap in carbon cycle estimates. Here, we use very high-resolution satellite imagery and deep learning to map wood deposits in the MRD, combining this with field data to measure the stock and age of wood-based carbon. We find >400,000 individual large wood deposits, collectively storing 3.1 x 10(12) g-C, equating to 2 x 10(6) g-C ha(-1) across the delta. Sampled wood pieces date from 690 AD to 2015 AD but are mostly young with similar to 40% of the wood samples formed after 1955 AD. These estimates represent a minimum bound on an important surficial, potentially reactive, carbon pool compared to other deeper carbon stocks in permafrost zones. Plain Language Summary The Arctic is warming rapidly, and this can increase landscape erosion. Consequently, carbon can be transferred into rivers and transported toward the Arctic Ocean. To date, work has tracked finer material dissolved in river water and the size of sand grains but has missed large pieces of wood that fall or slide into rivers. Wood is an important carbon-rich material that may break down differently from the finer carbon pools, however, the amount of wood stored in Arctic river deltas has not been measured before. We also don't know the age of wood in those deposits. Here we study the Mackenzie River Delta, where river-sourced wood is common and deposits extensive. We use very high-resolution satellite images which show individual pieces of wood and use a machine learning technique to map wood across the delta. We also visited the deposits to measure their size and collected samples for radiocarbon dating. We find the wood is very young compared to other carbon pools carried by the river, and that the stocks of carbon are regionally important. Our work calls for further work to understand this overlooked carbon pool in river deltas and coastal regions of the Arctic.