Mercury isotopes in sedimentary rocks as a paleoenvironmental proxy

The co-evolution of Earth's environment and life is one of the most fundamental questions in Earth science. Precise paleoenvironmental reconstruction is the key to understanding the evolution of life, which help predict the future prospect of Earth's habitability. In recent years, mercury (Hg) and its stable isotopes in ancient sedimentary rocks have emerged as irreplaceable tools to reconstruct Earth's paleoenvironment, opening a new frontier in Earth science. This study summaries the current state of Hg isotope geochemistry in paleoenvironmental research, and reviews the status and prospects of this promising research field. Mercury and its stable isotopes have been primarily applied as proxies for large-scale volcanic activities (e.g., large igneous provinces, LIPs), terrestrial soil erosion, and oceanic redox conditions, facilitating the evaluation of the roles of these processes in critical environmental and biotic crises, such as mass extinctions. For example, LIPs are able to emit large quantities of Hg that are readily dispersed globally through the atmosphere, leading to widespread Hg enrichment in marine and terrestrial sediments. Mercury isotopes can distinguish between volcanic and non-volcanic Hg sources, owing to the unique mass-independent fractionation (MIF, represented by Delta Hg-199) that this element undergoes. During LIP events, the Delta Hg-199 value can display either positive or negative shifts relative to the pre-volcanic values, or shifts towards the MIF value of direct volcanic emission (Delta Hg-199 approximate to 0). A positive shift is typically observed in sections where Hg emitted by volcanism has undergone long-distance transport before deposition, because the photochemical transformations during atmospheric dispersion typically result in positive Delta Hg-199 in oxidized Hg species that deposit more easily to ocean. A negative shift is typically explained as the result of enhanced inputs of terrestrial Hg associated with soils and plants, which are characterized by negative Delta Hg-199 in modern environments. For example, most marine and terrestrial sections of the end-Permian mass extinction (EPME, similar to 252 Ma) display significant negative Delta Hg-199 shifts (up to -0.20%) during the extinction interval. It is hypothesized that the eruptions of Siberian Traps LIP and large-scale volcanism in South China led to abrupt climate and environmental changes that enhanced terrestrial weathering and soil erosion. The Delta Hg-199 value can also shift towards the signature of direct volcanic emission if local volcanism is the dominant source of Hg. Mercury from local volcanism has not undergone long-range atmospheric transport and thus is more likely to retain the original MIF signals of volcanic sources. However, the variations of Hg MIF during volcanic events do not always follow the above framework. Significant differences in the direction and degree of the Delta Hg-199 shifts can be found either between different volcanic events or between different sedimentary environments. Thus, the mechanism controlling the variations of Hg isotopes during different volcanic events requires further study. In addition to being a proxy for volcanism, Hg isotopes were also recently recognized as a promising proxy for photic zone euxinia (PZE), the enrichment of toxic H2S in the marine photic zone. PZE is particularly detrimental to shallow marine inhabitants because the photic zone is the critical zone that supports marine primary productivity and hosts the majority of marine life. Therefore, the occurrence of PZE has been considered as a potent kill mechanism during almost all the Phanerozoic mass extinction events. A previous pilot study found that ancient marine sedimentary rocks deposited under PZE exhibit more negative odd-MIF and more positive MDF than those formed under non-PZE conditions. This observation was explained by two possible mechanisms: photoreduction of Hg(II) complexed by reduced sulfur in a sulfide-rich photic zone, and enhanced sequestration of atmospheric Hg(0) by sulfidic surface water. However, application of Hg isotopes in this aspect is still relatively limited. Overall, as an emerging paleoenvironmental proxy, Hg isotopes have shown great potential in reconstructing the importance of ancient volcanism and ocean redox evolution. However, this field is still at a beginning stage. It faces various challenges, notably the lack of a mechanistic, coherent, and quantitative understanding on how Hg isotopes indicate volcanism and oceanic redox variations. For example, various factors could affect the interpretation of Hg MIF values in sedimentary rock, including the type of volcanism (e.g., local vs. global, subaerial vs. submarine), the transformation processes of Hg during atmospheric and marine cycling, the lithology and Hg speciation, transformation and migration of Hg during diagenesis. However, these influences have received little investigation. Future research should focus on revealing the underlining principle of Hg isotopes as a paleoenvironmental proxy by clarifying its fractionation mechanisms and calibrating its use in various depositional settings, thus advancing this new research field.

Automated summary

The co-evolution of Earth's environment and life is a fundamental question in Earth science. Precise paleoenvironmental reconstruction using the proxy of mercury isotopes in ancient sedimentary rocks has emerged as a powerful tool for understanding the evolution of life and predicting the future habitability of Earth.