Development of new isotope proxies | Zheng Isotope Geochemistry Laboratory

A systematic investigation of stable cerium (Ce) isotope systematics and its potential application to reconstruction of paleoenvironmental conditions

The rare earth elements (REEs, including from La to Lu) are characterized by their systematic geochemical behaviors resulting from similar electronic configurations and decreasing ionic radii along atomic numbers. Considering the sensitivity to redox change, cerium (Ce) can act as a multivalent element to transform between soluble Ce³⁺ and insoluble Ce⁴⁺. Therefore, Ce anomalies (designated as Ce/Ce*) compared with its neighboring elements (La and Pr) have been widely applied to trace the fluctuations of oxygen level both in modern and deep time environments.

However, the preservation of REEs signals and Ce anomalies in rock archives is prone to be influenced by diagenetic alterations and remobilizations, which cannot be easily revealed by solely Ce concentrations. Recent data show the large fractionation of stable Ce isotopes (¹⁴²Ce/¹⁴⁰Ce) during Ce adsorption onto Mn oxides, indicating the potential application to trace the Earth’s Mn cycle. In addition, ¹³⁸La-¹³⁸Ce geochronology allows for dating of depositional age of sedimentary strata to examine if the Ce remained in the closed system or not since its deposition.

Development of stable potassium isotopes as a novel tracer for the global K cycle

The variation of stable potassium isotope ratios (41K/39K) among marine and terrestrial materials has exhibited a promising sign to study geochemical processes intertwined with the marine-terrestrial interface such as silicate weathering, various hydrothermal reactions, and water-rock interactions. The application of stable potassium isotope was enabled by prominent occurrence of potassium (K) in silicate minerals, its abundant in the global ocean, and a unique stable potassium isotope ratio of seawater that is isotopically heavier than most bulk silicate rocks.

However, the K isotope mass balance among its major reservoirs and fluxes has not been fully constrained, hindering such interpretation of K isotopes data. Therefore, our main mission has been to develop a comprehensive understanding of stable potassium isotopes in the terrestrial-marine systems.