Efficient transport of terrestrial particulate carbon in a tectonically-active marginal sea off southwestern Taiwan

Abstract

This study investigates the fluvial and marine burial fluxes of particulate carbon in the source-to-sink system of the mountainous Gaoping (GP) river system to elucidate the efficient transport of terrestrial particulate carbon via a submarine canyon in an active margin. The river fluxes of total suspended matter (TSM), particulate organic carbon (POC) and particulate inorganic carbon (PIC) were much higher in the wet season than in the dry season, mainly because of extremely high sediment yield associated with typhoons in the summer. The TSM, POC and PIC were carried mainly in <63μm particles and showed difference in size distribution between the dry and wet seasons for TSM and POC but not for PIC. The river transported around 226GgCyr−1 POC (64% particulate carbon (PC)) and 125GgCyr−1 PIC (36% PC) into the Gaoping coastal sea during the study period. The POC flux accounts for only 0.074−0.164% of the global river flux, but the POC yield (69.2tonCkm−2yr−1) is about one order of magnitude higher than the global average, and may be the second highest among global small mountainous rivers. If integrated over an area of 3045km2 around the Gaoping Canyon with a maximum water depth of 1500m, the mean burial fluxes of total organic carbon (TOC) and inorganic carbon (TIC) were 13.0gCm−2yr−1 and 9.5gCm−2yr−1, respectively. The buried PC comprised ~58% TOC (39.6GgCTOCyr−1) and ~40% TIC (28.9GgCTICyr−1). From the isotopic composition (δ13C) of TOC in the surface sediments, around 62% of the TOC was estimated to be sourced from the river. The diagenetic and benthic flux of dissolved carbon (DIC+DOC) may account for 18.1% of the deposited TOC flux or 12.9% of the deposited total carbon (TOC+TIC) flux. The burial efficiency of terrestrial TOC was therefore estimated to be only 10.9% of the riverine POC input. Despite the lack of direct evidence, the ratio of buried terrestrial TIC to riverine PIC input may be roughly equal to that of terrestrial TOC, as river POC/PIC and sedimentary TOC/TIC ratios did not differ significantly from each other. The low accumulation of terrestrial sediment and particulate carbon suggests that most POC and PIC that were derived from the Gaoping River may have been recycled and/or moved out of the study area. The active margin associated with the canyon appears to act as an efficient conduit for the transfer of terrestrial POC and PIC into the deep ocean.