Lithium Isotopes as Proxy of Continental Silicate Weathering

Abstract:

Chemical weathering has an important influence on continental crust evolution, as weathering of basalt preferentially removes soluble elements, such as Mg and Ca, and can shift the crust towards more andesitic compositions, thus helping to solve the crustal composition paradox. The isotopic compositions of soluble elements (e.g., Li and Mg) provide a monitor of chemical weathering of the continents through time.

Here we evaluate the factors influencing the abundance, [Li], and isotopic composition of riverine Li delivered to the oceans through analyses and modeling of [Li] and d⁷Li in streams and groundwaters draining a single continental lithology, the Columbia River Basalts (CRBs). The streams were sampled in different climate zones that lie on the dry and wet sides of the Cascades Mountains, and during two different seasons (summer and late winter) in order to evaluate climatic and seasonal influences on Li isotopes in rivers. Dissolved Li (δ⁷Li = +9.3 to +30.4) is systematically heavier than that of fresh or weathered CRBs, suspended loads (-5.9 to -0.3), and shallow groundwaters (+6.7 to +9.4). Continued isotopic fractionation between stream water and suspended and/or bed loads has a major influence on riverine δ⁷Li as indicated by the heavier Li in streams, compared to the shallow groundwaters that feed them. Seasonal δ⁷Li variation is observed only for streams west of the Cascades, where the difference in precipitation rate between the seasons is greatest. Reactive transport model simulations reveal that riverine δ⁷Li is strongly controlled by subsurface residence times and Li isotope fractionation occurring within rivers. The varying residence times for groundwaters feeding the western streams in summer (long residence times, higher δ⁷Li, greater weathering) and winter (short residence times, lower δ⁷Li, less weathering) explains the observed seasonal variations. A global, negative correlation between δ⁷Li and Li/Na for streams and rivers draining basaltic catchments reflects the overall transport time, hence the amount of silicate weathering. Based on our results, the increase of δ⁷Li in seawater during the Cenozoic is unlikely related to changing climate, but may reflect mountain building giving rise to increased silicate weathering.