[M-IS14] Biogeochemistry

[MIS14-11] Dissolved and colloidal organic carbon and trace metals in thermokarst lakes of the Western Siberia Lowland.

*Dahedrey Payandi-Rolland^1,2, Liudmila S. Shirokova^2,3, Pascale Bénézeth², Rinat M. Manasypov⁴, Oleg S. Pokrovsky^2,4 (1.Department of Geography, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan, 2.Géosciences Environnement Toulouse, GET – CNRS – CNES – IRD – OMP – Université de Toulouse, France, 14 Avenue Edouard Belin, 31400 Toulouse, France, 3.N. Laverov Federal Center for Integrated Arctic Research of the Ural Branch or RAS, Arkhangelsk, Nab Severnoi Dviny 23, Russia, 4.BIO-GEO-CLIM Laboratory, Tomsk State University, 35 Lenina Pr., Tomsk, Russia)

Keywords:peatland, surface waters, organo-mineral colloids

In subarctic regions, the peat soil, constituted of partially decomposed vegetation, is highly rich in carbon (C). Part of this C is trapped in the permafrost, with an estimation of 1460 to 1600 Pg of C in the northern permafrost peatlands. The thawing of permafrost creates numerous thermokarst lakes that are increasing in area due to the abrasion of lake edges. In those regions, thermokarst lakes physicochemical characteristics are mainly driven by the size of the lake itself, from small acidic highly rich in organic matter lakes to large, close to neutral pH and rich in organic matter lakes. In this study we classified thermokarst lakes depending on their areas: with an area of less than 1,000 m² thermokarst lakes are considered “small”, between 1,000 and 10,000 m² they are called “medium” and above 10,000 m² they are labelled as “large”. The peat edge abrasion during increasing lake size induces a large input of C in lake waters, and dissolved organic carbon (DOC) is the main form of terrestrial organic C exported from soils to surface waterbodies. Another main aspect of those northern regions is that river waters carry sizable amounts of trace metals such as Fe, Al, and Pb, mainly during high water season. Considering those two main characteristics of surface waters in permafrost peatlands and the fact that organic matter and trace elements tend to form organo-mineral colloids in humic waters, a size fractionation study of colloids in thermokarst lake waters from the Western Siberia Lowland has been lead. The goal was to evaluate the link between DOC and trace metals.
Waters from 9 thermokarst lakes of different sizes were sampled in July 2018, filtrated at 0.45 µm, and ultrafiltrated (via centrifugal ultrafiltration) at 100, 50, 3, and 1 kDa. The pH, conductivity, DIC, and DOC concentrations, UV-visible absorbance, and trace metals concentrations were determined for all those waters. The main results show that DOC in those thermokarst lakes can be classified under 3 groups: the low molecular weight (LMW) fraction under 3 kDa, the small colloidal fraction (between 3 and 100 kDa), and the high molecular weight (HMW) colloidal fraction (between 100 kDa and 0.45 µm). Those results also highlight that DOC mainly remains under 100 kDa, with 44% in the small colloidal fraction and 35% in the LMW fraction. On the other hand, the HMW colloidal fraction contains only 19% of the DOC in all thermokarst lakes. The size of thermokarst lakes does not impact the amount of DOC in the HMW colloidal fraction but controls the distribution of DOC in the small colloidal and the LMW fraction (with larger lakes exhibiting lower DOC concentrations of both small colloidal and LMW fractions). At the same time, trace metals such as Fe, Al, Pb, Si, As, and Ni are significantly correlated with the DOC concentration in the 0.45 µm filtrated waters. It can also be noticed that Al, Fe, and Pb can be found respectively at 53, 60, and 63% of their total concentrations in the small colloidal fraction.
The correlation between DOC and trace metals (such as Al, Fe, and Pb), and their concentrations in the small colloidal fraction (notably in large thermokarst lake, where Pb concentration reaches 80% of its total dissolved concentration in the small colloidal fraction) are particular features of the Western Siberia Lowland thermokarst lakes. Those features can be considered as important factors for the formation of organo-mineral colloids which are more stable and capable of reaching the Arctic Ocean during spring floods, when lakes discharge their waters into streams and rivers.