Typhoon-Mediated Organic Carbon Export in the Western Pacific: The Role of Steep Mountainous Rivers

Abstract:

Steep, mountainous rivers draning recently uplifted lithology on high standing islands within the Inter Tropical Convergence Zone export a disproportionately high amount of sediment given their small area; up to 20% of global export in 3% of the total exorheic land area. Typhoons strongly influence climate in this region, leading to increased sediment discharge, hyperpycnal flow, and efficient deposition of terrestrial particulate organic carbon in marine sediments. If this material escapes benthic remineralization – as is expected due to the lack of a continental shelf – typhoon mediated export of plant-derived POC is a net atmospheric CO₂ sink.

Here we report results from high-resolution (~hours) sampling on the LiWu River, Taiwan, during three sequential typhoons in 2008, and re-examine previous soil samples using more in-depth methods. Using a combination of bulk carbon isotopes (δ¹³C, ∆¹⁴C), vascular plant biomarker concentrations and δ¹³C, and ramped oxidation radiocarbon analysis, we describe the source and age of exported POC over a range of discharge conditions. We show that previous results indicating an increase in biospheric POC export during typhoons is an oversimplification; pre-aged soil OC complicates export. We offer an updated view of steep, mountainous rivers as a negative feedback loop to increased atmospheric CO₂ and estimate for the first time the contribution and residence time of pre-aged soil OC in such a system.