Signatures of Molecular Unification and Progressive Oxidation Unfold in Dissolved Organic Matter of the Ob-Irtysh River System along Its Path to the Arctic Ocean

Abstract

The Ob-Irtysh River system is the seventh-longest one in the world. Unlike the other Great Siberian rivers, it is only slightly impacted by the continuous permafrost in its low flow. Instead, it drains the Great Vasyugan mire, which is the world largest swamp, and receives huge load of the Irtysh waters which drain the populated lowlands of the East Siberian Plain. The central challenge of this paper is to understand the processes responsible for molecular transformations of natural organic matter (NOM) in the Ob-Irtysh river system along the South-North transect. For solving this task, the NOM was isolated from the water samples collected along the 3,000 km transect using solid-phase extraction. The NOM samples were further analyzed using high resolution mass spectrometry and optical spectroscopy. The obtained results have shown a distinct trend both in molecular composition and diversity of the NOM along the South-North transect: the largest diversity was observed in the Southern “swamp-wetland” stations. The samples were dominated with humic and lignin-like components, and enriched with aminosugars. After the Irtysh confluence, the molecular nature of NOM has changed drastically: it became much more oxidized and enriched with heterocyclic N-containing compounds. These molecular features are very different from the aliphatics-rich permafrost NOM. They witnesses much more conservative nature of the NOM discharged into the Arctic by the Ob-Irtysh river system. In general, drastic reduction in molecular diversity was observed in the northern stations located in the lower Ob flow.

Pan‐Arctic Riverine Dissolved Organic Matter: Synchronous Molecular Stability, Shifting Sources and Subsidies

“…The compositional space occupied by the CARF reflects a wide variety of compound classes, including those with probable terrestrial sources such as polyphenolics and condensed aromatics (Behnke et al., 2020; Kellerman et al., 2018; Seidel et al., 2015; Sleighter & Hatcher, 2008) and those with possible autochthonous sources such as aliphatics and sugar‐like formulae (Antony et al., 2017; Feng et al., 2016; Johnston et al., 2018). The CARF is similar to the common formulae identified in all samples along a transect of the Ob'‐Irtysh river system and contained a similar number of formulae in a comparable central region of van Krevelen space (1,328 vs. 1,629 formulae, respectively), suggesting that the CARF could appear throughout the upstream extents of large Arctic rivers as well as near their termini (Perminova et al., 2019).…”

Landscape, Soil, Lithology, Climate and Permafrost Control on Dissolved Carbon, Major and Trace Elements in the Ob River, Western Siberia

“…It is important to note that, unlike it was reported for water pCO2 and molecular composition of riverine DOM (i.e., 17,18,19], the Irtysh River does not play a regulatory role of the lower Ob River water chemistry in terms of concentrations of other major and trace elements including organomineral colloids. Instead, the hydrochemical conditions of the Ob River are shaped by integration of multiple tributaries those chemical composition gradually changes along the permafrost and landscape gradient, as it is known for other river basins of western Siberia such as Taz and Pur [36].…”

“…In this work, we used a ship ('OM-341' vessel, see ref. [19]) route sampling and collected 21 main stem water samples going from the north to the south between 8 July 2016 and 23 July 2016 (Fig. 1).…”

“…These included DOC time series observation by molecular-level techniques [15] and via remote sensing [16] and quantification of particulate organic matter export [7]. In contrast, spatial coverage of the river main stem and tributaries is rather low, with just a few studies of the dissolved carbon and related CO2 and CH4 emissions [17,18] and one study of molecular composition of DOC [19]. Large amount of data are available from systematic State Rosgidromet monitoring of OC and major ions on four gauging stations of the Ob River (Salekhard, Belogor'e, Aleksandrovskoe and Kolpashevo) during 1970-2010 and measurements by Tomsk Politechnical University as summarized in ref.…”

“…In order to reveal the mechanisms of element transport in the Ob River and its tributaries, we assessed the colloidal and truly dissolved (low molecular weight) concentration of carbon, all major and trace elements in selected samples. We hypothesized drastic change in organic colloids after the confluence of the Ob and Irtysh rivers, following recent study of DOM pattern in the Ob River main stem [19]. We anticipate that acquiring this new knowledge on riverine major and trace elements over large climate, permafrost and vegetation gradient of the Ob River basin will allow empirical but straightforward prediction of future changes in river water chemistry, following a well-established substituting space for time approach.…”