<p>Variation in molybdenum (Mo) concentration and isotope composition is an established tracer for redox changes in marine environments. Here we apply Mo as a proxy of past water column oxygenation in ancient ferruginous and hyposulfidic Lake Towuti. Lake Towuti is >1.2 Myrs old, up to 200m deep, weakly stratified and anoxic below ~100 m water depth, and surrounded by an ultramafic bedrock-dominated catchment in South Sulawesi, Indonesia. Despite the current permanent stratification, deeper water mixing and oxygenation occurred periodically in the past due to Towuti’s sensitivity to climate change. To better reconstruct the redox changes through time, we present Mo concentration and isotope data from laterite profiles overlying ultramafic bedrock, lake surface sediments, and a ~30 kyr sediment piston core from Lake Towuti, Indonesia.</p><p>Although Mo concentrations in laterite profiles are overall higher compared to the underlying bedrock, the absence of a significant Mo isotopic variability with values close to the unweathered ultramafic protolith, varying from –0.16‰ to –0.04‰ δ<sup>98</sup>Mo<sub>NIST3134</sub>, suggests low aqueous mobility of Mo during weathering due to the formation of laterite Fe-oxides. In contrast to the laterite samples, Mo isotopic variability in lake surface sediments show a larger variability varying from –1.15‰ to –0.13‰ δ<sup>98</sup>Mo<sub>NIST3134</sub> with a significantly lighter Mo isotopic composition in sediments deposited under oxic bottom water conditions. This light Mo isotopic composition is likely caused by early diagenetic redox cycling of Mo and Fe at the sediment-water interface. In the deeper, anoxic parts of the basin, Mo isotopic compositions show values close to the laterite input with elevated sedimentary Mo concentrations which are likely the result of an authigenic Mo enrichment from the water column. Mo isotope variability in sediments taken from a piston core in the deep part of the lake is in the range of Mo isotope compositions from modern surface sediments, varying from –0.14‰ to –0.66‰ δ<sup>98</sup>Mo<sub>NIST3134</sub>. Interestingly, this variability is well correlated with local and global indicators of climate change from previous studies. Sediments deposited between ~30 kyr and ~10 kyr exhibit Mo isotope signatures similar to present-day oxygenated shallow water sites, thereby suggesting enhanced lake mixing and bottom water oxygenation under drier and colder climate conditions of the last glacial period. Sediments deposited since ~10 kyr under wetter and warmer climate conditions exhibit Mo isotope signatures similar to present-day anoxic deeper water settings. These suggest that Mo isotope compositions of lake sediments are potential quantitative indicators of past climate-driven water column oxygenation.</p>

<p>Molybdenum (Mo) isotopes are known as sensitive recorders for changes in redox conditions because the oxidized form of Mo (Mo VI) is more soluble, whereas its reduced form is more particle reactive. Previous studies suggest that Mo isotopic fractionation during the weathering process is controlled by atmospheric input, Mo host, and bedrock composition. However, Mo isotopic variation and processes influencing fractionation in weathering profiles overlying ultramafic bedrock, the early Earth analog, have yet to be explored. This study explores for the first time (1) Mo behavior and (2) isotopic fractionation in two representative and intensely-weathered lateritic profiles overlying ultramafic bedrock of the East Sulawesi Ophiolite, Indonesia. Mo concentrations measured on samples obtained from laterite successions studied here range between 60 - 537 ppb and are overall higher compared to bedrock values ranging between 9 - 45 ppb. The Mo isotope compositions of laterite samples vary between -0.043‰ to -0.161‰ δ<sup>98</sup>Mo<sub>NIST3134</sub>. The overall close to mantle Mo isotopic composition of the laterite samples, their small Mo isotope variability, and the covariation between Mo and Ti concentrations suggest low mobility of Mo during chemical weathering and laterite formation. This low Mo mobility is likely a consequence of a) the low Mo concentration of the ultramafic protolith and b) adsorption of Mo to secondary Fe-Oxides during laterite formation under oxic weathering conditions.</p>