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. The Qinghai–Tibet Plateau (QTP) accounts for approximately70 % of global alpine permafrost and is an area sensitive to climatechange. The thawing and mobilization of ice-rich and organic-carbon-richpermafrost impact hydrologic conditions and biogeochemical processes on theQTP. Despite numerous studies of Arctic permafrost, there are no reports todate for the molecular-level in-stream processing of permafrost-deriveddissolved organic matter (DOM) on the QTP. In this study, we examine temporaland spatial changes of DOM along an alpine stream (3850–3207 m above sealevel) by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS), accelerator mass spectrometry (AMS) and UV–visible spectroscopy.Compared to downstream sites, dissolved organic matter (DOM) at theheadstream site exhibited older radiocarbon age, higher mean molecularweight, higher aromaticity and fewer highly unsaturated compounds. At themolecular level, 6409 and 1345 formulas were identified as unique to theactive layer (AL) leachate and permafrost layer (PL) leachate, respectively.Comparing permafrost leachates to the downstream site, 59 % ofAL-specific formulas and 90 % of PL-specific formulas were degraded,likely a result of rapid in-stream degradation of permafrost-derived DOM. Frompeak discharge in the summer to low flow in late autumn, the DOCconcentration at the headstream site decreased from 13.9 to10.2 mg L −1 , while the 14C ageincreased from 745 to 1560 years before present (BP), reflecting an increasein the relative contribution of deep permafrost carbon due to the effect ofchanging hydrological conditions over the course of the summer on the DOM source(AL vs. PL). Our study thus demonstrates that hydrological conditions impactthe mobilization of permafrost carbon in an alpine fluvial network, thesignature of which is quickly lost through in-stream mineralization andtransformation.

biogeosciences

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