Patterns and drivers of riverine particulate organic carbon transport in an Andean valley
Keywords: rivers
hydrology
Particulate organic carbon
landslides
Amazon
erosion
sediment
Peru
Andes
POC

Physical erosion can mobilise particulate organic carbon (POC) from vegetation and soil, representing an export of primary productivity from ecosystems, and a lateral transfer of carbon recently-derived from the atmosphere. These carbon transfers are thought to be enhanced in mountain forests where erosion rates are high. However, the rates and controls on POC transfer remain poorly constrained, as does the impact of POC export on carbon cycling at regional and global scales. This thesis takes an interdisciplinary approach to address this issue, using remote sensing, river geochemistry, river hydrology, and geomorphic mapping in the Kosñipata Valley, in the Central Andes of Peru. Its main aims are to: 1) estimate stream discharge throughout the year and to evaluate the water balance and sources; 2) quantify the source of riverine POC, accounting for POC derived from sedimentary rocks (POCfossil) to examine the POC eroded from soils and vegetation (POCnon-fossil); 3) quantify river POC yields; 4) assess the hillslope processes that erode POC; and 5) assess how POC export impacts the carbon balance of mountain forest, and how fluvial transfer impacts the wider carbon cycle.
Stream flow was monitored from January 2010 to February 2011 at two newly installed river gauging stations in the Kosñipata Valley at 2250 m (Wayqecha, 48.5 km2) and 1360 m (San Pedro, 164.4 km2). Then annual water balance for the San Pedro catchment was quantified. Rainfall inputs of 3028 mm and cloud water inputs of 308 ± 97 mm were balanced by outputs via stream runoff (2721 mm) and actual evapotranspiration (907 mm), leaving a residual of -294 ± 97 mm (< ~10 % of water inputs).
The source of POC in river suspended sediment samples was quantified using radiocarbon (Δ14C, ‰), stable carbon isotopes, and the nitrogen to carbon ratio. This revealed that river POCnon-fossil was sourced from very young organic carbon in the valley (Δ14C ~50 ‰) and that POCfossil comprised 43 % of total POC. Combining the hydrometric measurements with river samples, annual particulate load fluxes were quantified. The vast majority (73 % to 77 %) of the annual suspended sediment transfer and POC (both POCfossil and POCnon-fossil) occurred in the wet season over a period of 4 months. The suspended sediment yield for the valley (960 – 1200 t km-2 yr-1) was consistent with those for the Andean portion of the Madre de Dios River into which the Kosñipata River drains. The river POCnon-fossil yield was 5.2 – 6.9 tC km-2 yr-1.
Landslides are likely to have played an important role in the mobilisation of POCnon-fossil. A detailed landslide mapping using 25 years of remote sensing data revealed that on average 0.09 % of the valley per year is impacted by this mass-wasting process. These landslides mobilise ~28 tC km-2 yr-1 of soil and vegetation valley-wide. The discrepancy between the landslide erosional flux and fluvial POCnon-fossil export suggests an important fraction of the POCnon-fossil harvested by landslides is either exported as coarse debris (not quantified in the fluvial POCnon-fossil flux), remains buried onsite, or is degraded and respired onsite. Landslides also played an important ecosystem function, turning over some sections of the mountain forest within ~625 years, with a 1200 year valley-wide mean.
On the basin scale, the Madre de Dios River drains ~ 6 % of the Amazonian Andes. This study enables estimation of the delivery of POC to the lowland Amazon Basin. Using the observation that POCnon-fossil and POCfossil fluxes were closely linked with suspended sediment transfer, total yields of ~0.22 MtC yr-1 and ~0.17 MtC yr-1, respectively, were estimated from this section of the Andes. The export of POCnon-fossil from mountain forests by rivers represents 0.4 – 1.0 % yr-1 of the net primary productivity of Andean forest and so even if only a small portion of this is buried in sedimentary deposits, it may promote the Andes as a carbon sink. These results demonstrate the long-term influence of erosional processes in the cycling of carbon in the Amazon Basin.