Sedimentary Mo isotope variability reflects climate-driven water column oxygenation changes in ferruginous Lake Towuti, Indonesia over the last ∼30 ka BP

Abstract

Previous studies have shown the utility of Molybdenum (Mo) concentrations and isotopic compositions as sensitive recorders for changes in redox conditions in marine environments. Compared to marine settings, the much lower dissolved Mo concentrations in freshwater settings could result in enhanced sensitivity of Mo isotopic composition to the temporal variability in redox conditions. Additionally, unlike the dissolved Mo isotopic compositions in open marine environments, freshwater settings are highly influenced by site-specific factors such as catchment characteristics and in-lake processes. Here, we present sedimentary Mo concentration ([Mo]) and isotope (δ⁹⁸Mo) data from presently stratified Lake Towuti (Indonesia) to provide insight into Mo behaviour in ferruginous (Fe-rich) and non-sulfidic freshwater settings. We find large variations in [Mo] (0.12–1.11 μg⋅g⁻¹) and δ⁹⁸Mo (−0.14 ‰ to −1.13 ‰) in lake surface sediments, which are distinct from previously reported variations found in weathering profiles of the ultramafic catchment. The [Mo] variation can partly be attributed to hydrodynamic sorting, with Mo enrichment at pelagic deep water sites distant from the major lake inflows. Sedimentary δ⁹⁸Mo shows a distinct pattern, with generally higher values in sediments deposited in deep anoxic water compared to lower δ⁹⁸Mo values in sediments deposited in shallow water sites above the oxycline. We attribute this δ⁹⁸Mo pattern in Lake Towuti predominantly to redox-controlled processes. These involve the reductive dissolution of suspended laterite-sourced Fe oxides, with Mo liberation under anoxic conditions shortly below the sediment-water interface and/or water column oxycline. Above the oxycline, the formation of amorphous Fe oxides promotes the preferential adsorption of isotopically light Mo. This partial scavenging of isotopically light Mo leads to the formation of isotopically heavier dissolved Mo reservoirs. The scavenging of isotopically heavy dissolved Mo by organic matter (OM) coupled with higher OM burial rates in anoxic sediments promotes higher sedimentary δ⁹⁸Mo ratios in sediment deposited below the oxycline, where OM is better preserved and amorphous Fe oxides are preferentially dissolved. When applying this framework of modern lake processes to a sediment piston core extending back ∼30 ka, we find δ⁹⁸Mo variations that are similar to those observed in modern surface sediments, both above and below the oxycline. Low δ⁹⁸Mo values in sediments deposited between ∼30 and ∼14 cal ka BP suggest deep oxygenation of the water column to the sediment-water interface. In contrast, Mo isotope signatures between ∼14 cal ka BP and the present are similar to those of present-day deep water anoxic sites, suggesting water column stratification throughout the Holocene. The water column oxygenation pattern inferred from δ⁹⁸Mo aligns well with other independent sedimentary indicators that indicate a cooler, drier last glacial maximum with deep water column mixing, and a warmer, wetter Holocene with a stratified water column. Thus, our results imply that δ⁹⁸Mo can be applied as an effective tracer of climate-driven changes in water column oxygenation in ferruginous and non-sulfidic settings.