Dynamic CO ₂ evasion and colloidal control of trace metals in the Lower Lena River

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 distributions. We investigated the Lower Lena River over ∼ 1500 km during the beginning of summer baseflow, combining continuous in situ p CO ₂ measurements, floating chamber flux determinations, and analyses of major and trace elements including colloidal size fractionation. p CO ₂ exhibited pronounced short-distance variability and decreased weakly northward along the main stem. Diffusive CO ₂ 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 CO ₂ source during the open-water season. In contrast, CH ₄ 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 stability of bulk dissolved carbon concentrations despite dynamic CO ₂ evasion. Dissolved (< 0.45 µm) major and trace elements formed two major geochemical groups. Highly mobile major ions, Si, and selected oxyanion-forming trace elements were predominantly present in low molecular weight (< 1 kDa) 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 indicate a functional decoupling between stable bulk dissolved-carbon concentrations and dynamically regulated CO ₂ 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.