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Lithospheric organic carbon ("petrogenic"; OC petro ) is oxidized during exhumation and subsequent erosion of mountain ranges. This process is a considerable source of carbon dioxide (CO 2 ) to the atmosphere over geologic time scales, but the mechanisms that govern oxidation rates in mountain landscapes are poorly constrained. We demonstrate that, on average, 67 ± 11% of the OC petro initially present in bedrock exhumed from the tropical, rapidly eroding Central Range of Taiwan is oxidized in soils, leading to CO 2 emissions of 6.1 to 18.6 metric tons of carbon per square kilometer per year. The molecular and isotopic evolution of bulk OC and lipid biomarkers during soil formation reveals that OC petro remineralization is microbially mediated. Rapid oxidation in mountain soils drives CO 2 emission fluxes that increase with erosion rate, thereby counteracting CO 2 drawdown by silicate weathering and biospheric OC burial., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere. Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (CO2) release. However, some of this soil organic carbon may be eroded and transferred to rivers. If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological CO2 sink. Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean, and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC. Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers. From the measured biospheric POC content and variability in annual sediment yield, we calculate a biospheric POC flux of 2.2(+1.3)(-0.9) teragrams of carbon per year from the Mackenzie River, which is three times the CO2 drawdown by silicate weathering in this basin. Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological CO2 sink.

The oxidation of organic carbon contained within sedimentary rocks ("petrogenic" carbon, or hereafter OCpetro) emits nearly as much C2 as is released by volcanism, thereby playing a key role in the long-term global C budget. High erosion rates in mountains have been shown to increase OCpetro oxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OCpetro oxidation in mountains versus floodplains remains difficult to assess as disparate methods have been used in the different environments. Here, we investigate the sources and fluxes of rhenium (Re) in the Rio Madre de Dios to quantify OCpetro oxidation from the Andes to the Amazon floodplain using a common approach. Dissolved rhenium concentrations (n = 131) range from 0.01 to 63 pmol L-1 and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OCpetro oxidation and that this proportion increases downstream. We estimate that in the Andes, OCpetro oxidation releases 11.2+4.5/-2.8 tC km-2 y-1 of C2, which corresponds to ~41% of the total OCpetro denudation (sum of oxidized and solid OCpetro). A Re mass balance across the Rio Madre de Dios shows that 46% of OCpetro oxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OCpetro oxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OCpetro oxidation and C2 release. [ABSTRACT FROM AUTHOR]

In mountain ranges, earthquakes can trigger widespread landsliding and mobilize large amounts of organic carbon by eroding soil and vegetation from hillslopes. Following a major earthquake, the landslide-mobilized organic carbon can be exported from river catchments by physical sediment transport processes or stored within the landscape where it may be degraded by heterotrophic respiration. The competition between these physical and biogeochemical processes governs a net transfer of carbon between the atmosphere and sedimentary organic matter, yet their relative importance following a large landslide-triggering earthquake remains poorly constrained. Here, we propose a model framework to quantify the post-seismic redistribution of soil-derived organic carbon. The approach combines predictions based on empirical observations of co-seismic sediment mobilization with a description of the physical and biogeochemical processes involved after an earthquake. Earthquake-triggered landslide populations are generated by randomly sampling a landslide area distribution, a proportion of which is initially connected to the fluvial network. Initially disconnected landslide deposits are transported downslope and connected to rivers at a constant velocity in the post-seismic period. Disconnected landslide deposits lose organic carbon by heterotrophic oxidation, while connected deposits lose organic carbon synchronously by both oxidation and river export. The modeling approach is numerically efficient and allows us to explore a large range of parameter values that exert a control on the fate of organic carbon in the upland erosional system. We explore the role of the climatic context (in terms of mean annual runoff and runoff variability) and rates of organic matter degradation using single pool and multi-pool models. Our results highlight the fact that the redistribution of organic carbon is strongly controlled by the annual runoff and the extent of landslide connection, but less so by the choice of organic matter degradation model. In the context of mountain ranges typical of the southwestern Pacific region, we find that model configurations allow more than 90 % of the landslide-mobilized carbon to be exported from mountain catchments. A simulation of earthquake cycles suggests efficient transfer of organic carbon out of a mountain range during the first decade of the post-seismic period. Pulsed erosion of organic matter by earthquake-triggered landslides is therefore an effective process to promote carbon sequestration in sedimentary deposits over thousands of years. [ABSTRACT FROM AUTHOR]

[i] Erosion of particulate organic carbon (POC) occurs at very high rates in mountain river cstchnsents, yet the proportion derived recentty from atmospheric CO2 in the terrestrial biosphere (POCnon-fossil) remains poorly constrained. Here we examine the transport of POCnon-fossil in mountain rivers of Taiwan and its climatic and geomorphic controls. In II catchments we have combined previous geochemical quantification of POC source (accounting for fossil POC from bedrock), with measurements of water discharge (Qw) and suspended sediment concentration over 2 yesrs. In these catchments, POC non-fossil concentration (mg L-1) was positively correlated with Qw, with enhanced loads at high flow attributed to rainfall driven supply of POCnon-fossil, from forested hillslopes, This climatic control on POCnon-fossil transport was moderated by catchment geomorphology: the gradient of a linear relation of POCnon-fossil concentration and Qw increased as the proportion of steep hillsloprs (>35°) in the catchment increased. The data suggest enhanced supply of POCnon-fossil, by erosion processes which act most efficiently on the steepest sections of forest. Across Taiwan, POCnon-fossil, yield was correlated with suspended sediment yield, with a mean of2l ± 10 tC km-2 yr-1. Al this rate, export of POCnon-fossil impacts an upper bound on the time available for biospheric growth, of 800 yr. Over longer time periods, POCnon-fossil transferred with large amounts of elastic sediment can contribute to sequestration of atmospheric CO2 if buried in marine sediments. Our results show that this carbon transfer should be enhanced in a wetter and stornsicr climate, and the rales moderated on geological timescales by the regional tectonic setting. [ABSTRACT FROM AUTHOR]

We investigate the routing and transfer of particulate organic carbon (POC) from the western Southern Alps, New Zealand, using organic carbon (Corg) and nitrogen (Norg) concentrations and stable carbon isotopes (δ13Corg). In this active mountain belt, sediment discharge is dominated by landslide-derived material. Landsliding acts to homogenize the geochemically diverse hillslope POC, mixing POC from the standing biomass and soil with the fossil POC from bedrock. As a result, the POC in river sediment at the mountain front is a binary mixture of fossil and nonfossil carbon sourced from many landslide deposits. We calculate that nonfossil biogenic POC makes up 63 ± 7% of the total POC in the suspended load of rivers draining the western Southern Alps. The erosional flux of biogenic POC from these catchments represents a transfer of 39 tC km-2 a-1 of atmospheric CO2 averaged over the west flank of the mountain belt. If more than 10% of this POC is preserved in sediments on geological timescales, then this process is the most significant way in which the Southern Alps and similar, tectonically active mountain belts with restricted alluvial aprons consume atmospheric CO2. [ABSTRACT FROM AUTHOR]