A NEW SEDIMENTARY MERCURY RECORD TO EVALUATE THE SILURIAN NIAGARA-SALINA TRANSITION MICHIGAN BASIN, USA

The transition from the Niagara to Salina (Silurian) in the Michigan Basin is characterized by an abrupt lithological change from open marine carbonates to evaporites, the cause unknown. Previous research suggests the Salina evaporite deposition may have been driven by local restriction. The published δ¹³C record in the Michigan Basin, however, exhibits several carbon isotope excursions (CIEs), that correlation to global CIEs suggesting a relationship between the global carbon cycle and evaporate deposition. Here, to further investigate the Niagara-Salina we apply two relatively new geochemical proxies: elemental mercury concentrations ([Hg]) and Hg isotopes.

[Hg] was measured for 88 rock samples from the State Kalkaska #2-15 core. [Hg] range from 0.11 to 0.62 mg/kg. Five [Hg] spikes in excess of the Phanerozoic carbonate [Hg] average were identified; four in the Niagara and one in the Salina. The two largest [Hg] spikes correlate with the onset and termination of the lower Mulde CIE, possibly suggesting some commonality. The [Hg] spike at the start of the lower Mulde is has Δ¹⁹⁹Hg values of -0.19‰, -0.22‰, and -0.19‰ and δ²⁰²Hg values of -0.91‰, -1.34‰, 1.44‰, suggesting a terrestrial source of Hg. In contrast, the [Hg] spike near the end of the lower Mulde is characterized by Δ¹⁹⁹Hg values of 0.07‰, -0.05‰, 0.04‰ and δ²⁰²Hg values of -2.70‰, -2.17‰, and -3.37‰, suggesting atmospheric Hg. Between the Niagara and the Salina, a negative shift in the mass dependent fractionation (MDF; δ²⁰²Hg) is observed, which suggests increased evaporation, consistent with the observed shift from carbonates to evaporites.

These data represent the first known [Hg] record in ancient evaporite deposits and suggest evaporitic basins may represent an important archive of the Hg cycle. The results also challenge the calculated [Hg] average for the evaporite interval (0.0078 mg/kg) is an order of magnitude lower than the assumed value for ancient evaporites (0.040 mg/kg). Last, the absence of time equivalent large igneous provinces during this interval also suggests that the assumed link between Hg spikes and LIPs may be tenuous.