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This study presents lithium and magnesium isotope ratios of soils and their drainage waters from a well-characterised weathering experiment with two soil cores, one with olivine added to the surface layer, and the other a control core. The experimental design mimics olivine addition to soils for CO2 sequestration and/or crop fertilisation, as well as natural surface addition of reactive minerals such as during volcanic deposition. More generally, this study presents an opportunity to better understand how isotopic fractionation records weathering processes. At the start of the experiment, waters draining both cores have similar Mg isotope composition to the soil exchangeable pool. The composition in the two cores evolve in different directions as olivine dissolution progresses. Mass balance calculations show that the water δ26Mg value is controlled by congruent dissolution of carbonate and silicates (the latter in the olivine core only), plus an isotopically fractionated exchangeable pool. For Li, waters exiting the base of the cores initially have the same isotope composition, but then diverge as olivine dissolution progresses. For both Mg and Li, the transport down-core is significantly retarded and fractionated by exchange with the exchangeable pool. This observation has implications for the monitoring of enhanced weathering using trace elements or isotopes, because dissolution rates and fluxes will be underestimated during the time when the exchangeable pool evolves towards a new equilibrium.

In nature, primordial 236 U has long since decayed to concentrations below detection. However, measurement of 236 U produced in-situ by neutron capture on 235 U in high-grade uranium deposits is made possible by recent advances in accelerator mass spectrometry (AMS). The detection of appreciable quantities of 236 U in groundwaters may reflect local uranium mineralisation, and thus prove useful in uranium exploration and potential age and ore grade estimations. Nine mineralised sediments from the South Australian Beverley North sandstone-hosted uranium deposits have 236 U/ 238 U ratios ranging from (1.57 ± 0.43) × 10 − 12 to (9.09 ± 0.55) × 10 − 12 , and U concentrations that vary by almost three orders of magnitude, ranging from 78.9 to 24,200 μg/g. Overall, the samples with the highest [U] have higher 236 U/ 238 U ratios, consistent with the generation of higher neutron fluxes with one notable exception with anomalously high [U] and a relatively low 236 U/ 238 U ratio. The observed variability in the 236 U/ 238 U ratio both within the deposits themselves, and between deposits may reflect heterogeneous mineralogy, elemental composition and water contents, which can affect the neutron flux generated within the samples. A single groundwater sampled within mineralisation from the Pepegoona West deposit yielded a 236 U/ 238 U ratio of (6.57 ± 2.97) × 10 − 12 . This is the first published data detecting natural, non-anthropogenic 236 U in groundwater in contact with a uranium deposit. The 236 U/ 238 U isotopic composition of the single groundwater sample is indistinguishable from that of the mineralised sediments from the same deposit. This is interpreted to reflect isotopic equilibration between the mineralisation and groundwater, rather than the in-situ production of 236 U by neutron capture on dissolved 235 U in the waters due to the low [U] typical of these highly reducing groundwaters. 236 U appears to have limited mobility in the Pepegoona West groundwater system, as evidenced by the lack of signature in groundwaters sampled from nearby wells in low-grade and un-mineralised portions of the deposit. This suggests that the detection of 236 U in the highly reducing groundwaters prevalent in this area may not be applicable as a proxy for uranium mineralisation. However, use of this technique as a potential exploration tool may have greater success in other areas with different hydrogeological conditions, specifically where the groundwaters are oxidising and uranium has a greater solubility as U(VI) complexes.

Silicate weathering is the primary control of atmospheric CO2 concentrations on multiple timescales. However, tracing this process has proven difficult. Lithium isotopes are a promising tracer of silicate weathering. This study has reacted basalt sand with natural river water for ~9 months in closed experiments, in order to examine the behaviour of Li isotopes during weathering. Aqueous Li concentrations decrease by a factor of ~10 with time, and δ7Li increases by ~19‰, implying that Li is being taken up into secondary phases that prefer 6Li. Mass balance using various selective leaches of the exchangeable and secondary mineral fractions suggest that ~12–16% of Li is adsorbed, and the remainder is removed into neoformed secondary minerals. The exchangeable fractionation factors have a Δ7Liexch-soln = −11.6 to −11.9‰, while the secondary minerals impose Δ7Lisecmin-soln = −22.5 to −23.9‰. Overall the experiment can be modelled with a Rayleigh fractionation factor of α = 0.991, similar to that found for natural basaltic rivers. The mobility of Li relative to the carbon-cycle-critical cations of Ca and Mg changes with time, but rapidly evolves within one month to remarkably similar mobilities amongst these three elements. This evolution shows a linear relationship with δ7Li (largely due to a co-variation between aqueous [Li] and δ7Li), suggesting that Li isotopes have the potential to be used as a tracer of Ca and Mg mobility during basaltic weathering, and ultimately CO2 drawdown.