Tracing weathering intensity across a permafrost gradient in the Yukon River

Abstract:

The Yukon River is an ideal basin to study the interactions of permafrost with weathering processes at the watershed scale. The entire Yukon basin lies within the permafrost zone, albeit with a range of extent and depth. Large increase in solute concentrations have been observed at the Yukon River mouth over the last decades but the link between these elemental changes, permafrost degradation and weathering processes is ambiguous. Here, we analyze 130 samples collected during the summer from most major tributaries of the Yukon River for major and trace elements, strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) and lithium isotope composition (δ⁷Li). Elemental and isotopic geochemistry displays strong spatial patterns that primarily correlate with climatic and geological conditions over the watershed. We find that δ⁷ Li values correlate with both mean annual temperature and permafrost cover of the sampled catchments. However this relationship is not linear but shows a threshold at -5°C. For river samples collected in catchments with a mean annual temperature below -5°C, δ⁷Li values are relatively constant around 19 ‰ and largely independent of geology. However, above this temperature threshold, the δ⁷Li values become more variable and more dependent on geology and topography. We argue that permafrost covered catchments are at steady-state in term of weathering with relatively high denudation rate due to repeated freeze-thaw cycles. This weathering regime promotes high dissolution rate of primary minerals but low rate of secondary mineral formation. Conversely, in areas with less permafrost cover, erosion rates decrease and favor the formation of soils through enhanced secondary mineral formation rates leading to higher but more variable δ⁷Li values. Deeper weathering front also favor the enhanced connectivity of surface and subsurface waters. Lastly δ⁷Li values are high (>25‰) in all the tributaries draining volcanic units suggesting that secondary mineral formation is not inhibited by permafrost in volcanic setting. Our study demonstrates the importance of permafrost in controlling erosion and weathering processes in northern rivers and highlights the vulnerability of these processes to temperature thresholds.