| الوصف: |
In rivers draining the western Southern Alps, the organic carbon concentration (Corg) and stable isotopes of organic carbon (δ13Corg) are used to determine the source of riverine particulate organic carbon (POC). The rivers contain a mixture of POC derived from the terrestrial biosphere (vegetation and soils) and fossil POC in bedrock described by Corg and δ13Corg. The fraction of riverine POC derived recently from atmospheric CO2 (fraction non-fossil POC, Fnf) is quantified using this mixing relationship. Fnf, Corg suspended sediment transfer and water discharge (Qw) are used to calculate the erosion of POC from the terrestrial biosphere in these active mountain belts. The high yields (34±5 tC km-2 yr-1 in western Southern Alps, and 31±7 tC km-2 yr-1 in Taiwan) are presently sustained by the net primary productivity of the forest ecosystem in both mountain belts. With detailed knowledge of the source of riverine POC the processes responsible for its mobilisation and transfer are investigated. The primary tool is the detailed sample set collected from 15 rivers in Taiwan that combine the geochemistry of riverine suspended load POC with records of water discharge (Qw), SSC and precipitation. Remote sensing is used to provide additional insight. Three main routing processes are identified: 1. Landsliding – mobilising standing biomass and soils; 2. Overland flow – during precipitation organic materials are washed from hillslopes to the channel; and 3. Grinding – breakdown or coarse organic fragments during transport in turbid river waters. These processes cause the observed enrichment in non-fossil POC concentration in rivers (POCnf, mg L-1) during floods, which conflicts with a conventional view of dilution at high Qw and SSC. These processes lead to positive relationships between POCnf and Qw in Taiwanese rivers and this may be common throughout vegetated mountain catchments. The POC routing processes identified here lead to the high non-fossil POC yields in these mountain belts sustained by both tectonic uplift and climatically-driven erosion and sustenance of the terrestrial biosphere. Because of the observed power law relationship between POCnf and Qw, large floods caused by storms are responsible for the bulk of the riverine non-fossil POC transfer. It is recognised for the first time that storm-triggered floods optimise the delivery of non-fossil POC to depositional environments at times when it is likely to be sequestered in sediment. In Taiwan, the mobilisation of non-fossil POC from hillslopes significantly impacts the terrestrial biosphere. A move toward N-limitation on steep slopes (observed in δ15N of vegetation and soil) is driven by the physical removal of organic matter during rainfall and may influence the net primary productivity of the ecosystem. If common throughout mountains in tropical Oceania, this climatically driven phenomenon affects the ability of these forests to sequester atmospheric CO2 on decadal time-scales. |