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