The isotopic composition of particulate organic carbon in mountain rivers of Taiwan

Abstract

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ13Corg) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ13Corg that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ13Corg from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ13Corg and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ13Corg, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ13Corg is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC. In order to quantify the fraction of suspended load POC derived from non-fossil sources (Fnf) as well as the isotopic composition of fossil POC (δ13Cfossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ13Corg that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, Fnf reproduces the fraction modern (Fmod) in our samples, determined from 14C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean Fnf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ13Cfossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ13Corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ13Corg observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ13Corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.