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Erosion drives the export of particulate organic carbon from the terrestrial biosphere (POC biosphere ) and its delivery to rivers. The carbon transfer is globally significant and can result in drawdown of atmospheric carbon dioxide (CO 2 ) if the eroded POC biosphere escapes degradation during river transfer and sedimentary deposition. Despite this recognition, we lack a global perspective on how the tectonic and climatic factors which govern physical erosion regulate POC biosphere discharge, obscuring linkages between mountain building, climate, and CO 2 drawdown. To fill this deficit, geochemical (δ 13 C, 14 C and C/N), hydrometric (water discharge, suspended sediment concentration) and geomorphic (slope) measurements are combined from 33 globally-distributed forested mountain catchments. Radiocarbon activity is used to account for rock-derived organic carbon and reveals that POC biosphere eroded from mountain forests is mostly < 1300 14 C years old. Annual POC biosphere yields are positively correlated with suspended sediment yields, confirming results from Taiwan and a recent global analysis, and are high in catchments with the steepest slopes. Based on these relationships and the global distribution of slope angles (3-arc-second), it is suggested that topography steeper than 10° (16% of the continental area) may contribute ~ 40% of global POC biosphere erosional flux. Climate is shown to regulate POC biosphere discharge by mountain rivers, by controlling hydrologically-driven erosion processes. In catchments where discharge measurements are available (8 of the 33) a significant relationship exists between daily runoff (mm day − 1 ) and POC biosphere concentration (mg L − 1 ) ( r = 0.53, P < 0.0001). The relationship can be described by a single power law and suggests a high connectivity between forested hillslopes and mountain river channels. As a result, annual POC biosphere yields are significantly correlated with mean annual runoff ( r = 0.64, P < 0.0001). A shear-stress POC biosphere erosion model is proposed which can explain the patterns in the data. The model allows the climate sensitivity of this carbon flux to be assessed for the first time. For a 1% increase in annual runoff, POC biosphere discharge is predicted to increase by ~ 4%. In steeper catchments, POC biosphere discharge increases more rapidly with an increase in annual runoff. For comparison, a 1% increase in annual runoff is predicted to increase carbon transfers by silicate weathering solute fluxes in mountains by 0.4–0.7%. Depending on the fate of the eroded POC biosphere , river export of POC biosphere from mountains may act as an important negative feedback on rising atmospheric CO 2 and increased global temperature. Erosion of carbon from the terrestrial biosphere links mountain building and climate to the geological evolution of atmospheric CO 2 , while the carbon fluxes are sensitive to predicted changes in runoff over the coming century. [ABSTRACT FROM AUTHOR]

Rapid erosion in mountain forests results in high rates of biospheric particulate organic carbon (POC) export by rivers, which can contribute to atmospheric carbon dioxide drawdown. However, coarse POC (CPOC) carried by particles >~1 mm is rarely quantified. In a forested pre-Alpine catchment, we measured CPOC transport rates and found that they increase more rapidly with water discharge than fine POC (<1 mm) and dissolved organic carbon (DOC). As a result, decadal estimates of CPOC yield of 12.3 ± 1.9 t C km^yr"1 are higher than for fine POC and DOC, even when excluding 4 extreme flood events. When including these floods, CPOC dominates organic carbon discharge (~80%). Most CPOC (69%) was water logged and denser than water, suggesting that CPOC has the potential to contribute to long-term sedimentary burial. Global fluxes remain poorly constrained, but if the transport behavior of CPOC shown here is common to other mountain streams and rivers, then neglecting CPOC discharge could lead to a large underestimation of the global transfer of biospheric POC from land to ocean. [ABSTRACT FROM AUTHOR]

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 () in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean that ranges from − to − (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 from − to − between the major geological formations. Using coupled 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 , resulting in more variable and lower values than elsewhere. In all other catchments, we have found that variability in 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 () as well as the isotopic composition of fossil POC () 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 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, reproduces the fraction modern () in our samples, determined from measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean 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 in suspended POC varied between − and − from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of 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 observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in 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. [Copyright &y& Elsevier]