Pyroclastic Eruption Boosts Organic Carbon Fluxes Into Patagonian Fjords

Fjords and old‐growth forests store large amounts of organic carbon. Yet the role of episodic disturbances, particularly volcanic eruptions, in mobilizing organic carbon in fjord landscapes covered by temperate rainforests remains poorly quantified. To this end, we estimated how much wood and soils were flushed to nearby fjords following the 2008 eruption of Chaitén volcano in south‐central Chile, where pyroclastic sediments covered >12 km 2 of pristine temperate rainforest. Field‐based surveys of forest biomass, soil organic content, and dead wood transport reveal that the reworking of pyroclastic sediments delivered ~66,500 + 14,600/−14,500 tC of large wood to two rivers entering the nearby Patagonian fjords in less than a decade. A similar volume of wood remains in dead tree stands and buried beneath pyroclastic deposits (~79,900 + 21,100/−16,900 tC) or stored in active river channels (5,900–10,600 tC). We estimate that bank erosion mobilized ~132,300 +21,700 / −30,600  tC of floodplain forest soil. Eroded and reworked forest soils have been accreting on coastal river deltas at >5 mm yr −1 since the eruption. While much of the large wood is transported out of the fjord by long‐shore drift, the finer fraction from eroded forest soils is likely to be buried in the fjords. We conclude that the organic carbon fluxes boosted by rivers adjusting to high pyroclastic sediment loads may remain elevated for up to a decade and that Patagonian temperate rainforests disturbed by excessive loads of pyroclastic debris can be episodic short‐lived carbon sources.