Recently, supported by projects such as the National Natural Science Foundation and the National Key Research and Development Program, Liu Ben, a doctoral student, and researcher Wei Gangjian from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, conducted a series of adsorption experiments of iron manganese oxides, revealing the fractionation mechanism of stable strontium isotopes during the adsorption process. The related findings were published in "Geochemistry, Geophysics, Geosystems."
Silicate weathering significantly affects the geochemical composition of seawater by consuming atmospheric carbon dioxide and transporting soluble cations to rivers and oceans. Dissolved strontium in the ocean has a homogeneous isotopic composition, with its main sources and sinks being continental weathering inputs and marine carbonate precipitation, but it is also influenced by other factors such as submarine hydrothermal inputs and removal by iron manganese oxides. The radiogenic strontium isotopic composition (87Sr/86Sr) is the most classic indicator for tracing processes involving oceanic strontium sources and sinks. However, due to the similar 87Sr/86Sr ratios of marine carbonates and the surrounding seawater during their formation, relying solely on 87Sr/86Sr cannot accurately distinguish changes in the aforementioned processes. There is significant fractionation between stable strontium isotopes (δ88Sr) in marine carbonates and seawater. Therefore, combining radiogenic and stable strontium isotopic compositions in seawater allows for a more precise inference of changes in continental weathering intensity and carbonate burial rate, enhancing our understanding of global Sr cycling and its relationship with ocean chemistry, continental weathering, and climate change.
Commonly occurring sedimentary iron manganese oxides in the ocean serve as potential carriers for recording the history of seawater strontium isotopic variations. However, reconstructing stable strontium isotope records requires ensuring stable and well-defined fractionation between seawater and sediments. To investigate the stable strontium isotope fractionation mechanism between iron manganese oxide deposition and seawater, researchers conducted a series of adsorption experiments with iron manganese oxides. They synthesized the main minerals in iron manganese oxide deposits, mixed them with strontium-containing solutions, and observed stable strontium isotope fractionation behavior under different reaction times, pH levels, strontium concentrations, and ionic strengths.
The study found that manganese oxide has a much greater strontium adsorption capacity than iron oxide, and high pH, low strontium concentration, and ionic strength correspond to higher strontium adsorption rates. During the manganese oxide adsorption process, the light isotopes are preferentially adsorbed, with δ88Sr fractionations of -0.14‰ and -0.20‰ observed in low ionic strength solutions and synthetic seawater, respectively. The fractionation process conforms to equilibrium fractionation laws, and the magnitude of fractionation is not affected by time, pH, or strontium concentration. Adsorption experiments with synthetic seawater observed fractionation sizes consistent with the difference in isotopic compositions (-0.18±0.02‰) between natural iron manganese nodule samples and seawater, indicating similar fractionation mechanisms between marine iron manganese oxide deposition and seawater, whereby stable strontium isotopic fractionation remains constant.
This study demonstrates the potential of iron manganese oxide deposition to reconstruct the history of stable strontium isotopes in seawater, which is significant for deeper research into the evolution of oceanic strontium cycling, continental weathering history, and global carbon cycling.
Related Paper: https://doi.org/10.1029/2024GC011460
