Sources and Residence Times of Exported Organic Carbon From High-standing Tropical Island

Abstract:

Steep, mountainous rivers draining high standing islands within the ITCZ 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 burial of terrestrial particulate organic carbon (POC) in marine sediments. Here, we investigate the exported POC from high-resolution (~hours) sampling on the LiWu River, Taiwan, during three sequential typhoons in 2008 as well as from soil profiles taken throughout the catchment. We utilize the recently developed Ramped Pyrox technique to investigate the chemical structure and isotopic distribution (¹⁴C, δ¹³C) of different OC sources by separating OC components based on thermo-stability.

We show that even the most labile OC_bio can exhibit protracted storage in soils (up to ca. 2000 ¹⁴C years), and that soil OC displays a large increase in apparent age with increasing thermal stability, likely due to contribution by microbially re-worked petrogenic carbon. Additionally, exported POC_bio is too depleted and POC_petro too enriched in δ¹³C to be sourced from upper-catchment soil biomass. Rather, exported POC during large storm events can be best explained by a downstream source. Our results therefore offer an updated view on the mechanisms of organic carbon export from steep mountainous rivers by showing that: (i.) high erosion rates do not preclude significant pre-ageing of soil OC and (ii.) exported POC during large storm events is dominated by a local, downstream signal.