Mercury Isotope Geochemistry in Ediacaran Cap Carbonates and Cretaceous Oceanic Red Beds

       Mercury (Hg), a highly volatile metal, is capable of tracing volcanism through geological history as LIP events transiently emit large amounts of Hg. There are two indicators that make Hg a unique tool for geochemistry, the Hg to total organic carbon ratio (Hg/TOC) and mass-independent fractionation (Hg-MIF, defined as Δ¹⁹⁹Hg). Owing to the affinity of Hg to organic matter, anomalous high Hg/TOC ratios in sediments can better reveal large volcanic eruptions. The anomaly of Hg-MIF is mainly observed in Hg photoreactions, providing a fingerprints of specific reaction pathways of Hg. Volcanic Hg usually has Δ¹⁹⁹Hg ~ 0, but photochemical processes in the surface environment can alter this signal, resulting in positive Δ¹⁹⁹Hg in marine systems (e.g., seawater and marine sediments) and negative Δ¹⁹⁹Hg in terrestrial systems (e.g., soil and vegetation).

         Here, we examined the Hg records in Ediacaran cap carbonates in South China and Upper Cretaceous oceanic red beds (ORBs) in southern Tibet and the North Atlantic, to obtain their sedimentary material sources and the cause of the termination of Marinoan glaciation and Cretaceous oceanic anoxic events.

       (1) The cap carbonates show higher Hg concentrations (4.9 to 405 ppb), most of which are comparable to that observed in carbonates deposited during non-LIPs periods. The lack of Hg/TOC anomalies in these cap carbonates suggests that background volcanic activity, rather than a short-term large igneous province event, drove the Marinoan deglaciation. The cap carbonates show positive Δ¹⁹⁹Hg values (0.18 to 0.34 ‰) in slope settings and slightly negative to slightly positive Δ¹⁹⁹Hg values (0.16 to 0.11 ‰) in shelf settings, suggesting a binary mixing of seawater- and terrestrial-derived Hg in early Ediacaran Ocean. We infer that the accumulation of greenhouse gases, due to ongoing volcanic emissions of CO₂ and enhanced release of gas hydrates, triggered global warming. This warming led to melting of sea ice cover, enhanced terrestrial inputs, and large-scale dissolution of atmospheric CO₂ into seawater, driving widespread deposition of Ediacaran cap carbonates.

       (2) In southern Tibet and the North Atlantic, black/gray shales (typical deposition of oceanic anoxic events) show much higher Hg concentrations and Hg/TOC values than ORBs, indicating enhanced Hg flux to global oceans during time of black/gray shale deposition. Black/gray shales show lower Fe³⁺/Fe²⁺ and positive Δ¹⁹⁹Hg, suggesting a significant input of Hg into the anoxic/dysoxic ocean via atmospheric deposition. The isotope values are consistent with a volcanic source for this excess Hg. ORBs show high Fe³⁺/Fe²⁺ and negative shifts of Δ¹⁹⁹Hg, suggesting that the dominant source of Hg into the oxic oceans was via terrestrial runoff. These results suggest that volcanism was an important driver of the climate/ocean dynamics during the Late Cretaceous.

       To sum up, in addition to indicating short-strong volcanic activities, Hg can also trace the source of sedimentary materials under weak magmatism. Moreover, Hg offers a more accurate depiction of the interactions and exchanges among the Earth’s atmosphere-ocean-land system.