Date of Award

12-2021

Degree Name

Master of Science

Department

Geological and Environmental Sciences

First Advisor

Dr. Stephen E. Kaczmarek

Second Advisor

Dr. Theodore Them II

Third Advisor

Dr. Andrew Caruthers

Keywords

Niagara, salina, mercury, carbon isotopic excursions, Michigan

Access Setting

Masters Thesis-Open Access

Abstract

The Niagara-Salina boundary in the Michigan Basin is marked by an abrupt transition from carbonates to evaporites. Though the cause is uncertain, previous work suggests the onset of Salina evaporites was driven by basin restriction, but the presence of several global carbon isotope excursions (CIE) suggest a global driver. This study builds on this discussion using two relatively new geochemical proxies - elemental mercury concentrations [Hg] and Hg isotopes.

Mercury concentrations [Hg] were measured in 88 samples from the State Kalkaska #2-15 core from 6604.03 ft to 6797.42 ft. [Hg] ranges from 0.11 to 0.62 mg/kg and [Hg]/TOC from 0.038 to 0.50 mg/kg/wt. The average [Hg] in the Niagaran carbonates is 0.0324 mg/kg and the average [Hg] in the A-1E evaporite is 0.0078 mg/kg, lower than a previously reported value of 0.040 mg/kg for evaporites. Five [Hg] spikes greater than the Phanerozoic carbonate [Hg] average are identified. The largest two correlate with onset and end of the lower Mulde CIE, indicating a shared environmental cause. The [Hg] spike near the onset of the lower Mulde has δ202Hg values of -0.91‰ to 1.44‰, and Δ199Hg values -0.19‰ to -0.22‰, suggesting a terrestrial Hg source. The [Hg] spike near the end has Δ199Hg values of 0.07‰, to -0.05‰ and δ202Hg values of -2.17‰, to -3.37‰, suggesting an atmospheric source. Hg mass dependent fractionation values characterized by a negative shift from Niagara to Salina, implying increased evaporation. This study provides a novel [Hg] record in ancient evaporites. These data suggest that evaporites may be an important archive of atmospheric [Hg] and highlights uncertainties with the [Hg] record as a proxy for paleo volcanism.

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