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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 21 | Copyright
Biogeosciences, 15, 6637-6648, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Nov 2018

Research article | 08 Nov 2018

Spatiotemporal transformation of dissolved organic matter along an alpine stream flow path on the Qinghai–Tibet Plateau: importance of source and permafrost degradation

Yinghui Wang1,2, Robert G. M. Spencer3, David C. Podgorski4, Anne M. Kellerman3, Harunur Rashid1, Phoebe Zito4, Wenjie Xiao2, Dandan Wei1, Yuanhe Yang5, and Yunping Xu1 Yinghui Wang et al.
  • 1Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China
  • 2Key Laboratory for Earth Surface Processes of the Ministry of Education, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China
  • 3National High Magnetic Field Laboratory Geochemistry Group and Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA
  • 4Pontchartrain Institute for Environmental Sciences, Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA
  • 5State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China

Abstract. The Qinghai–Tibet Plateau (QTP) accounts for approximately 70% of global alpine permafrost and is an area sensitive to climate change. The thawing and mobilization of ice-rich and organic-carbon-rich permafrost impact hydrologic conditions and biogeochemical processes on the QTP. Despite numerous studies of Arctic permafrost, there are no reports to date for the molecular-level in-stream processing of permafrost-derived dissolved organic matter (DOM) on the QTP. In this study, we examine temporal and spatial changes of DOM along an alpine stream (3850–3207m above sea level) by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), accelerator mass spectrometry (AMS) and UV–visible spectroscopy. Compared to downstream sites, dissolved organic matter (DOM) at the headstream site exhibited older radiocarbon age, higher mean molecular weight, higher aromaticity and fewer highly unsaturated compounds. At the molecular level, 6409 and 1345 formulas were identified as unique to the active layer (AL) leachate and permafrost layer (PL) leachate, respectively. Comparing permafrost leachates to the downstream site, 59% of AL-specific formulas and 90% of PL-specific formulas were degraded, likely a result of rapid in-stream degradation of permafrost-derived DOM. From peak discharge in the summer to low flow in late autumn, the DOC concentration at the headstream site decreased from 13.9 to 10.2mgL−1, while the 14C age increased from 745 to 1560 years before present (BP), reflecting an increase in the relative contribution of deep permafrost carbon due to the effect of changing hydrological conditions over the course of the summer on the DOM source (AL vs. PL). Our study thus demonstrates that hydrological conditions impact the mobilization of permafrost carbon in an alpine fluvial network, the signature of which is quickly lost through in-stream mineralization and transformation.

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Short summary
With global warming, thawing of permafrost releases dissolved organic matter (DOM) into streams. By analyzing DOM along an alpine stream on the Qinghai–Tibet Plateau, we found DOM was mainly from the active layer, but with deepening of the active layer, the contribution of the deep permafrost layer increased, causing a change in the chemical composition of DOM. From the head- to downstream, DOM is undergoing rapid degradation, but some components are persistent and can be transported downstream.
With global warming, thawing of permafrost releases dissolved organic matter (DOM) into streams....