Research

U isotope paleoredox proxy

I. Primary calcium carbonates

Abiotic aragonite and calcite

Chen et al. (2016) — We conducted abiotic calcium carbonate coprecipitation experiment to examine the U isotope fractionation during uranyl incorporation into aragonite and calcite. Our results demonstrated a small but detectable U isotope fractionation during uranyl incorporation into aragonite at pH ~8.5 with heavy U isotopes preferentially enriched in the solid. In contrast, we did not observe U isotope fractionation in calcite. The magnitude of the isotope fractionation scaled with the fraction of the neutrally-charged U species—Ca₂UO₂(CO₃)₃(aq). We proposed that equilibrium isotope fractionation among aqueous uranyl species and preferential incorporation of the isotopically heavy U species into calcium carbonate caused the preferential partition of heavy U isotopes in the solid phase.

Biogenic calcium carbonate: corals, calcareous algae, mollusks

Chen et al. (2018) — This paper developed a high-precision U isotope analysis method and examined the U isotope fractionation in various primary biogenic calcium carbonates. We found variable U isotope fractionation in primary biogenic carbonates, suggesting biological effects. The magnitude of U isotope fractionation depends on the extent of the isolation of the calcification center from ambient seawater.

U isotope fractionation induced by aqueous U speciation

Chen et al. (2017) — This paper applied the speciation-dependent isotope fractionation observed in abiotic calcium carbonate coprecipitation experiments in Chen et al. (2016) to explore the U isotope fractionation in abiotic calcium carbonates induced by changes in aqueous U speciation due to changes by secular variations in seawater chemistry over the Phanerozoic. Our results revealed a variable isotope fractionation of ~ 0.10 – 0.23‰ during abiotic carbonate precipitation process, leading to significant underestimate of extent of oceanic anoxia.

II. Calibration

Shallow-water carbonate sediments

Chen et al. (2018) — This work explored U isotope fractionation during different types of carbonate diagenesis (meteoric, phreatic, and marine burial diagenesis) and possible fractionation mechanisms. Our results demonstrated that carbonate diagenesis cause an isotopic offset of +0.27 ± 0.14‰ relative to primary calcium carbonate precipitates. Meteoric diagenesis caused the largest U isotope fractionation.

Authigenic accumulation of isotopically heavy U(IV) below the sediment-water interface is the predominantly mechanism that drive U isotope fractionation during carbonate diagenesis. This study addresses the importance of considering the carbonate depositional environments when using carbonate δ²³⁸U proxy to reconstruct paleoredox conditions.

Anoxic depositional environments

Chen et al. (2021) — We used Fayetteville Green Lake, New York, as an analogue of a redox-stratified ocean to explore the effects of anoxic depositional environments on carbonate δ²³⁸U proxy. In the redox-stratified FGL, calcite was formed in the oxic surface water and deposited under sulfidic bottom waters. Our results demonstrated that δ²³⁸U in carbonate sediments was significantly heavier than primary calcite, reflecting the typical U isotope fractionation (~0.6‰) in organic-rich sediments deposition under anoxic conditions.

In addition, our δ²³⁸U data in sediment traps in FGL demonstrated U(VI) reduction in the reducing water column for the first time, challenging the previous finding that no U(VI) reduction occurred in anoxic water columns. Our finding is crucial for our understanding of U isotope geochemistry in oceans and requires further water column U speciation studies.

Primary calcium carbonate mineralogy

Chen et al. (2022) — This paper explored the effect of primary carbonate mineralogy on U isotope fractionation during carbonate diagenesis. Since U in primary calcite is typically two orders of magnitude lower than that in aragonite, we expected that δ²³⁸U in primary calcite is more susceptible to diagenesis compared to primary aragonite.

Our data confirmed our expectation and demonstrated that variations of δ²³⁸U in carbonate sediments with a primary calcitic mineralogy would more likely reflect the local redox state of the depositional and early diagenetic environments. The δ²³⁸U in carbonate sediments that have a primary calcitic mineralogy increased with increasing U/Ca due to the authigenic accumulation of isotopically heavy U(IV) during diagenesis. Thus, it is crucial to consider primary carbonate mineralogy when applying this proxy.

Mo isotope paleoredox proxy

Mo isotope fractionation during MoO₄^2- incorporation into abiotic and microbial calcite

Chen et al. (2018) — It is the first experimental study of Mo isotope fractionation during molybdate incorporation into calcite. The partition coefficient of molybdate in our abiotic calcite was 0.000015, consistent with low Mo concentration in natural primary calcite precipitates. Our experiment revealed a constant Mo isotope fractionation of ~0.15‰ between calcite and the aqueous precipitating fluid with preferential partition of light-mass Mo isotopes in the solid phase.

Molybdenum isotopic compositions in microbial calcite and modern marine carbonates tend to be close to or lighter than abiotic calcite and modern seawater. Molybdenum concentrations in these carbonates were typically higher than abiotic calcite, suggesting other biogeochemical processes involved in these carbonates. Compiling all carbonate Mo isotope data, we found that Fe and Mn oxides, organic matter, detrital materials, and local redox conditions significantly affected carbonate δ⁹⁸Mo. Thus, it is crucial to consider these effects when applying carbonate δ⁹⁸Mo proxy.

Tl and V isotope paleoredox proxy

Fe and Mn shuttle on Tl and Mo isotope paleoredox proxy

Chen et al. (2022) — This study examined the effect of Fe and Mn shuttle on Tl and V isotope proxy in organic-rich marine sediments. Our results revealed that Fe and Mn shuttle has no effect on sedimentary authigenic Tl and V isotope signatures in Black Sea due to shallow chemocline (~100 m) in the 2000-m deep redox-stratified water column and slow deep-water renewal rate.

Trace element incorporation into carbonates

Aqueous speciation on U incorporation into foraminiferal calcite

Chen (2020) — This study explored the possible mechanism that lead to the decrease of U/Ca in calcium carbonates with increasing pH and/or [CO₃^2-] by examining which aqueous U species is most likely to be incorporated into foraminiferal calcite precipitates. My results revealed that the dominant aqueous U species, Ca₂UO₂(CO₃)₃(aq), CaUO₂(CO₃)₃^2-, and MgUO₂(CO₃)₃^2-, were invariant under variable pH and [CO₃^2-], and that the minor aqueous U species, UO₂(CO₃)₂^2-, decreased with increasing pH and [CO₃^2-]. Consequently, UO₂(CO₃)₂^2- is the most likely aqueous U species incorporated into foraminiferal calcite.