地球が化石炭素をどのように循環させているかに光を当てた研究(Study sheds light on how Earth cycles fossil carbon)

2023-11-14 ライス大学

＜関連情報＞

• https://news.rice.edu/news/2023/study-sheds-light-how-earth-cycles-fossil-carbon
• https://www.pnas.org/doi/full/10.1073/pnas.2306343120

アンデスの堆積物がアマゾンの前陸-氾濫原を通過する際に持続する、岩石有機炭素の高い酸化速度 High rates of rock organic carbon oxidation sustained as Andean sediment transits the Amazon foreland-floodplain

Mathieu Dellinger, Robert G. Hilton, J. Jotautas Baronas, Mark A. Torres, Emily I. Burt, Kasey E. Clark, Valier Galy, Adan Julian Ccahuana Quispe, and A. Joshua West
Proceedings of the National Academy of Sciences  Published:September 19, 2023
DOI:https://doi.org/10.1073/pnas.2306343120

Significance

Erosion and weathering play key roles in Earth’s carbon cycle, controlling climate over millions of years by transferring CO₂ to and from the atmosphere. Weathering of sedimentary rocks in mountains can release CO₂ when ancient organic carbon in rocks is chemically broken down. However, mountain erosion also moves rock organic carbon that has not been broken down and is transported by rivers to lowland floodplains to an unknown fate. Here, we use rhenium in rivers to quantify rock organic carbon weathering in the Andes and adjacent floodplain. Erosion in the Andes leads to high rates of CO₂ release. However, CO₂ release doubles when including the floodplains. The presence or absence of floodplains next to mountains governs CO₂ release by weathering.

Abstract

The oxidation of organic carbon contained within sedimentary rocks (“petrogenic” carbon, or hereafter OC_petro) emits nearly as much CO₂ 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 OC_petro oxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OC_petro 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 OC_petro 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 OC_petro oxidation and that this proportion increases downstream. We estimate that in the Andes, OC_petro oxidation releases 11.2^+4.5/_−2.8 tC km⁻² y⁻¹ of CO₂, which corresponds to ~41% of the total OC_petro denudation (sum of oxidized and solid OC_petro). A Re mass balance across the Rio Madre de Dios shows that 46% of OC_petro oxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OC_petro oxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OC_petro oxidation and CO₂ release.