The isotopic composition and fluxes of particulate organic carbon exported from the eastern margin of the Tibetan Plateau

Erosion of organic carbon from the terrestrial biosphere and sedimentary rocks plays an important role in the global carbon cycle across a range of timescales. Over geological timescales (>10(4) years), erosion and burial of particulate organic carbon (POC) from the terrestrial biosphere (POCbiosphere) is an important CO2 sink, while oxidation of organic carbon derived from sedimentary rocks (petrogenic, POCpetro) releases CO2 to the atmosphere. Over decadal to millennial timescales, the balance between POCbiosphere production and degradation affects atmospheric CO2 concentrations. To better constrain the controls on erosional carbon transfers, here we quantify POCbiosphere and POCpetro fluxes in a mountain range with relatively low runoff, the Longmen Shan, which drains the eastern margin of the Tibetan Plateau. We measure total organic carbon content ([OCtotal]) and the carbon isotopic compositions (C-13/C-12 expressed as delta C-13; C-14/C-12 expressed as fraction modern or F-mod) of organic matter in suspended sediments collected from six gauging stations on the Min Jiang, a tributary of the Yangtze River, from 2005 to 2012. We find that POCpetro has a large range of delta C-13, from 26.2 parts per thousand to 13.2 parts per thousand. This POCpetro mixes with POCbiosphere to set the delta C-13 of POC in river sediments. Binary mixing models reveal the possibility of aged POCbiosphere at two gauging stations which drain the high elevations of the eastern Tibetan Plateau, with modelled biospheric F-mod values of 0.82 +/- 0.09 and 0.84 +/- 0.08. This is consistent with prior suggestions of aged biospheric carbon being eroded from the plateau. The annual POCpetro yields range from 0.04 +/- 0.02 tC km(-2) yr(-1) to 1.69 +/- 0.56 tC km(-2) yr(-1) across the five study catchments, with basin average yield that appears to be linked to catchment average slope as a likely proxy for erosion rate. Here, the variability in the petrogenic organic carbon content of rocks masks the signal of the weathering and oxidation of this rock-derived organic carbon. The annual POCbiosphere yields range from 0.21 +/- 0.04 tC km(-2) yr(-1) to 3.33 +/- 0.57 tC km(-2) yr(-1). These values are towards the lower end of those measured in mountain ranges around the world, which we suggest not only reflects the relatively low erosion rates of the Longman Shan, but also the low annual runoff (<1 m yr(-1)). Across this region, the river POCbiosphere discharge is related to the intensity of runoff events. Our data suggest that a wetter (and/or stormier) climate could increase the erosional export of POCbiosphere in this tectonically-active mountain range. Depending on the fate of POCbiosphere downstream in larger river systems, this could act as carbon-cycle climate feedback over geological timescales. (C) 2019 Elsevier Ltd. All rights reserved.