Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance

Abstract

Productive forests of the Andes are subject to high erosion rates that supply to the Amazon River sediment and carbon from both recently photosynthesized biomass and geological sources. Despite this recognition, the source and discharge of particulate organic carbon (POC) in Andean Rivers remain poorly constrained. We collected suspended sediments from the Kosñipata River, Peru, over 1 year at two river gauging stations. Carbon isotopes (14C, 13C, and 12C) and nitrogen to organic carbon ratios of the suspended sediments suggest a mixture of POC from sedimentary rocks (POCpetro) and from the terrestrial biosphere (POCbiosphere). The majority of the POCbiosphere has a composition similar to surface soil horizons, and we estimate that it is mostly younger than 850 14C years. The suspended sediment yield in 2010 was 3500 ± 210 t km−2 yr−1, >10 times the yield from the Amazon Basin. The POCbiosphere yield was 12.6 ± 0.4 t C km−2 yr−1 and the POCpetro yield was 16.1 ± 1.4 t C km−2 yr−1, mostly discharged in the wet season (December to March) during flood events. The river POCbiosphere discharge is large enough to play a role in determining whether Andean forests are a source or sink of carbon dioxide. The estimated erosional discharge of POCpetro from the Andes is much larger (~1 Mt C yr−1) than the POCpetro discharge by the Madeira River downstream in the Amazon Basin, suggesting that oxidation of POCpetro counters CO2 drawdown by silicate weathering. The flux and fate of Andean POCbiosphere and POCpetro need to be better constrained to fully understand the carbon budget of the Amazon River basin.

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

“…These patterns are consistent with observations from other studies of mountain rivers (Hilton et al, 2010;Marwick et al, 2015). The relationship can be explained by a mixing of POC petro from rocks, which here is low in [OC total ], 13 C enriched and 14 C depleted, with POC biosphere from the vegetation and surface soils, which is high in [OC total ], generally 13 C depleted and 14 C enriched (Leithold et al, 2006;Hilton et al, 2015;Clark et al, 2017). This mixing is also suggested by the linear relationships of F mod with δ 13 C org and with 1/[OC total ] of the suspended sediment samples (Fig.…”

“…Four suspended load POC samples from station U2 have δ 13 C org values >-20‰, which are higher than the most enriched values reported from other mountain rivers where C3 plants dominate (Marwick et al, 2015). The river bed materials are dominated by POC petro and could represent an averaged composition of POC petro due to the mixing of rock clasts during sediment transport (Hilton et al, 2010;Wang et al, 2016;Clark et al, 2017). The measured δ 13 C org of the river bed material collected within the same catchment is -15.6 ± 2.7‰, consistent with the 13 C-enriched POC petro .…”

“…Radiocarbon activity has proven to be one of the most robust ways to constrain POC sources because of the difference between POC petro (which will have no measurable 14 C above analytical background for sample ages of >60 ka) and POC biosphere which contains 14 C derived from atmospheric CO 2 during recent photosynthesis (Blair et al, 2003;Komada et al, 2004;Galy et al, 2007;Hilton et al, 2008b). Other bulk geochemical analyses, such as elemental (e.g., C/N) and stable C isotopic ratios, can also be used to constrain the source of POC biosphere , and can provide complementary evidence for aged POC biosphere Clark et al, 2017).…”

“…The high δ 13 C org values of bedrocks in the Longmen Shan give perhaps the most extreme example to date of how POC petro can impact the stable carbon isotope composition of POC carried by mountain rivers (Komada et al, 2004;Hilton et al, 2010;Marwick et al, 2015;Clark et al, 2017). If this POC petro persists downstream and contributes to sedimentary deposits upon exiting the mountain belt, it may influence the bulk  13 C org of detrital material (Hilton et al, 2010;Kao et al, 2014).…”

Identifying and quantifying geogenic organic carbon in soils – the case of graphite

“…The fate of this geogenic graphite under weathering and soil formation has rarely been studied, possibly due to the lack of methods for determining and quantifying geogenic graphite beyond the background of soil OC (OC). There are some 20 indications that a substantial part of the geogenic graphitic C is actually lost in the pathway from rock weathering to (marine) sedimentation (Galy et al, 2008;Clark et al, 2017). In a recent study, Hemmingway et al (2018) estimated 2/3 of the graphitic C to get oxidized during soil formation, strongly facilitated by soil microbial activity.…”