Dynamic CO₂ evasion and colloidal control of trace metals in the Lower Lena River

Abstract. Large Arctic rivers integrate carbon and element fluxes across vast permafrost-dominated landscapes, yet the lower reaches of these systems remain poorly constrained in terms of greenhouse gas (GHG) emissions and solute organization. We investigated the Low Lena River over ~800 km during the beginning of summer baseflow, combining continuous in situ pCO₂ measurements, floating chamber flux determinations, and analyses of major and trace elements including colloidal size fractionation. CO₂ concentrations exhibited pronounced short-distance variability and weak northward decrease along the main stem. Diffusive CO₂ fluxes (0.1–1.3 g C m⁻² d⁻¹) were comparable to values reported for other large Siberian rivers, confirming the Lena as a persistent but moderate atmospheric CO₂ source during the open-water season. In contrast, CH₄ concentrations were low and spatially uniform, contributing <0.5 % to total carbon emissions. Notably, bulk DOC and DIC concentrations remained remarkably stable along the transect and were consistent with long-term monitoring records and previous expeditions, indicating strong buffering of dissolved carbon pools despite dynamic CO₂ evasion.

Major and trace elements segregated into two geochemical groups. Highly mobile major ions, Si, and selected oxyanion-forming trace elements were predominantly present in truly dissolved form (0–20 % colloidal fraction) and reflected groundwater connectivity and water–rock interaction. In contrast, lithogenic low-solubility elements – including trivalent and tetravalent hydrolysates – were strongly associated with Fe–Al–organic colloids (>70 %), indicating surface and suprapermafrost mobilization pathways. Multivariate statistics confirmed this dual organization of solute transport. These findings reveal a functional decoupling between structurally buffered dissolved carbon pools and dynamically regulated CO₂ exchange, a pattern likely characteristic of large Arctic rivers. Under ongoing warming, shifts in hydrological connectivity, discharge regime, and permafrost thaw may alter this balance, with implications for pan-Arctic carbon and element export to the Arctic Ocean.