Assessing Silicate Weathering in Permafrost-Dominated Catchments Using Lithium Isotopes: The Lena River, Siberia

Abstract:

Rising global temperatures have the potential to influence the Earth’s climate feedback cycles due to permafrost thawing, altering the freshwater input and trace metal and carbon fluxes into the ocean and atmosphere. Riverine lithium isotope ratios (d⁷Li) are a tracer of silicate weathering processes, which are key in the removal of atmospheric CO₂ over geological timescales.

Despite this, little is known about the effects of permafrost thawing on d⁷Li variations. Strong seasonal changes in the thawed active layer thickness dictate surficial water flow paths, which may influence intra-annual riverine d⁷Li signatures. We present a study of the dissolved d⁷Li from the large permafrost-dominated watersheds of the Lena River (Siberia), which drain into the Arctic Ocean. This work comprises a temporal study during the May 2015 spring flood, from ice breakup through peak flooding, thus monitoring changes in water-rock and water-soil interaction, both processes that control weathering and hence Li isotopes.

Before riverine ice started to break up, high [Li] are observed as the river signature is governed by winter base flow conditions. As the river ice breaks up, surface runoff flows over the impermeable permafrost, interacting with leaf litter, diluting the [Li]. We compare d⁷Li over the spring flood period with a greater spatial study conducted over two summer field seasons (2012/2013) of the main Lena River channel and its tributaries, which drain a variety of lithologies/topographies. During the summer, the thawed active layer promotes deeper water flow paths, greater water-rock interaction and enhanced secondary minerals formation which preferentially take up ⁶Li. Summer riverine d⁷Li typically fall between +14.5 ‰ to +28.5 ‰, with rivers draining the Central Siberian Plateau typically exhibiting high [Li], but similar δ⁷Li to rivers draining the Verkhoyansk Mountain Range.

Overall, this study demonstrates how Li isotopes respond to weathering in a permafrost-dominated region, and provides rates on how quickly water-rock interaction can affect silicate weathering.