In a paper published in the journal Proceedings of the National Academy of Sciences, researchers examined lithium isotope compositions in the Ediacaran Doushantuo cap dolostone to test the plumeworld hypothesis after the Marinoan snowball Earth event.
Study: Lithium isotope evidence for a plumeworld ocean in the aftermath of the Marinoan snowball Earth. Image Credit: Starover Sibiriak/Shutterstock.com
Their findings supported diminished chemical weathering during glaciation and a meltwater plume that influenced salinity gradients, reflecting mixing processes between high lithium-7 isotope ratio (δ7Li) meltwater and low δ7Li hypersaline seawater. The data suggested weak continental weathering paired with significant reverse weathering on the seafloor.
Background
Past work investigated the Doushantuo cap dolostone overlying the Nantuo Formation in South China, highlighting sedimentary features like tepee structures and stomatitis-like cavities. Variations in dolostone thickness and karstic surfaces were linked to depositional environments, suggesting high-energy conditions nearshore and lower energy offshore. Comprehensive isotopic analyses of samples helped reveal geochemical variations across facies, underscoring the region’s dynamic post-glacial processes.
Doushantuo Cap Dolostone Analysis
The Doushantuo cap dolostone, ranging from 3 to 5 meters thick, lies atop the glaciogenic Nantuo Formation across the Yangtze platform. It features a karstic surface marked by barite fans, particularly pronounced in the inner shelf and shelf margin, indicating post-glacial exposure. The dolostone is composed of thin laminations, with tepee-like structures and cavities filled with botryoidal cement and pyrite, pointing to high-energy depositional environments nearshore.
Thickness variations reflect differences in carbonate supply and energy levels across facies. Isotopic analysis was conducted using precise methods, including leaching and continuous flow mass spectrometry, ensuring high precision in delta carbon-13 (δ13C) and delta oxygen-18 (δ18O) data acquisition.
The study discussed dolomitization and the impact of diagenetic alteration on δ7Li values in the Doushantuo cap dolostone. Experimental evidence suggested that microbial activity, low oxygen, and specific seawater chemistry facilitated dolomite precipitation in the Precambrian era. Despite diagenesis, petrographic and isotope analysis indicated minimal alteration, supported by consistent δ7Li values in samples and minimal clay contamination.
Elemental and isotope analyses further confirmed these findings. Differences between δ7Li values of dolomicrosparite and recrystallized dolomite were within error margins, contradicting typical diagenetic trends. The Monte Carlo method estimated minimal clay contamination (<0.5 wt.%), reinforcing the integrity of δ7Li data for paleoenvironmental interpretations.
The δ7Li values of cap dolostone samples provided critical insights into the chemical composition of seawater following the Marinoan glaciation. The relative stability of these values implied that lithium isotopes in the dolostone retained an original seawater signal, reflecting post-glacial ocean chemistry. This, combined with trace elemental data, suggested a significant contribution of weathering products from exposed continents into the ocean, highlighting an essential shift in the global carbon and nutrient cycles that followed one of Earth's most extensive glaciations.
The mixing model focuses on correcting the lithium isotope values of cap dolostone by accounting for contamination from clay minerals. The model assumes that the lithium isotopic data of cap dolostone are a mixture of contributions from clay minerals and dolomite. Key steps in the model include using clay mineral data from Marinoan glacial diamictite to estimate mass fractions and the lithium content of clay minerals, followed by an inversion to calculate the lithium content of carbonates.
The corrected lithium isotope values range from –1.9‰ to 23.9‰, with the clay mineral contamination not substantially altering the observed spatial pattern of δ⁷Li.
It discusses the mass balance model used to investigate the nearshore-offshore δ⁷Li gradient. The observed gradient of >10‰ is opposite the gradient between river water and seawater on modern Earth. The study suggests that the low δ⁷Li values observed in some samples can be attributed to synglacial distillation of seawater during the Marinoan snowball Earth event.
The model used to simulate the evolution of seawater δ⁷Li incorporates fluxes from river and hydrothermal inputs and outfluxes through authigenic clay mineral formation and low-temperature alteration, adjusting for isotopic fractionation. This model helps estimate how δ⁷Li values might have evolved during the Marinoan glaciation.
The authors discussed a sensitivity test using an analytical model to examine post-glacial seawater lithium isotope ratios. It showed that the syn-glacial isotope ratio stabilized over roughly 10 million years, enabling a steady-state assumption. The model is simplified to an equation that links isotopic and fractionation parameters.
By setting parameters like negligible riverine flux, the simplified equation accurately predicted variations in post-glacial lithium isotope ratios, validated by matching numerical results. The analysis showed that different isotopic fractionation and high-temperature lithium inputs led to ratios between 3‰ and 29‰, aligning with data and suggesting muted continental weathering or enhanced reverse weathering. This approach offered insights into post-glacial seawater chemistry and ancient Earth’s environmental dynamics.
Conclusion
To summarize, researchers examined lithium isotope compositions in the Doushantuo cap dolostone to test the plumeworld hypothesis. They revealed diminished chemical weathering and meltwater plumes influencing salinity gradients after the Marinoan glaciation.
The findings indicated stable δ7Li values, weak continental weathering, and significant seafloor reverse weathering. Corrected isotopic data and models highlighted post-glacial seawater chemistry, reflecting global carbon and nutrient cycle shifts and offering insights into ancient Earth's environmental dynamics.
Journal Reference
Gan, T., et al. (2024). Lithium isotope evidence for a plume would ocean in the aftermath of the Marinoan snowball Earth. Proceedings of the National Academy of Sciences, 121:46. DOI: 10.1073/pnas.2407419121, https://www.pnas.org/doi/10.1073/pnas.2407419121
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