HG ISOTOPES AS TRACERS OF DEPOSITIONAL ENVIRONMENTS IN MARINE SEDIMENTS FROM THE PRECAMBRIAN TO THE PHANEROZOIC

Mercury (Hg) is a volatile, chalcophilic, photochemically-active, redox-sensitive metal with complex biogeochemistry that displays a wide range of mass-dependent (MDF) and mass-independent (MIF) stable isotopic fractionation. Throughout geologic time, there have been many events and environmental shifts capable of modifying the global Hg cycle, including the eruption of large igneous provinces (LIPs), variations in atmospheric and oceanic redox conditions, and changes in the biosphere (e.g. the evolution of terrestrial flora). Here, we use Hg isotopes (in particular, the Hg-MIF signatures) to investigate how and if such changes are reflected in the isotopic composition marine sediments through time. In the Phanerozoic, we focus on marine sediments deposited during periods of massive volcanism (e.g., before, during, and after the emplacement of the Central Atlantic Magmatic Province, coincident with the end-Triassic mass extinction). The Precambrian sediments span in age from 600 Ma to ~3.3 Ga and represent a wide variety of depositional environments and Earth surface conditions. Our results indicate Phanerozoic successions do record isotopic shifts in response to the eruption of LIPs, with a reduction in Hg-MIF consistent with an overwhelming igneous source. Phanerozoic sediments (deposited prior to, or following LIPs) and Precambrian sediments tend preserve small amounts of odd-isotope Hg-MIF and have ∆¹⁹⁹Hg/∆²⁰¹Hg ratios consistent with the production of MIF via the magnetic isotope effect. The direction of MIF appears correlated with depositional environment, with deep water successions generally having odd isotope excesses (e.g. positive ∆¹⁹⁹Hg values) and sediments deposited in shallower, near shore environments generally having odd-isotope deficits (e.g. negative ∆¹⁹⁹Hg values). These signatures are consistent with sediments dominated by Hg inputs from atmospheric Hg²⁺ deposition versus terrestrial runoff, respectively. The similar Hg-MIF signatures of Precambrian and Phanerozoic sediments also suggest that the pathways and sources of Hg to marine sediments (i.e., atmospheric deposition and terrestrial runoff) may have remained similar over much of Earth history.