[Eoas-seminar] Saha - MS Defense

[eoas-seminar at lists.fsu.edu]

Good morning,

Please join us for Sayantan Saha's defense on October 28, 2026 at 2 pm in EOAS 4067.

Title: Geochemical and thermodynamic investigation into the fate of graphitic carbon and sulfides during orogenesis: a field-based approach in the Proterozoic Ottawan Orogeny, Ontario, Canada.


Name: Sayantan Saha

Date: October 28, 2026 from 2-4 pm Eastern Time (US and Canada)

Location: EOAS 4067

Major Professor: Dr. Emily Stewart


Abstract: Metamorphic degassing has turned out to be an often-overlooked flux when considering the global carbon and sulfur cycles. Sediments containing carbon-based and sulfidic minerals undergo the very slow and diffuse process of metamorphic decarbonation and desulfidation. This makes it very difficult to quantify the amount of carbon and sulfur being released into the atmosphere via surface measurements, and yet both of these fluxes have profound effects on planetary habitability of the Earth over time. The release of these volatiles into the atmosphere also have opposite effects on the global temperature but operate at different timescales. While CO2 with a residence time of ~4 years in the atmosphere (Harde, 2017) has on overall warming effect, any SO2 released has a net cooling effect and stays in the troposphere for anywhere between 20 hours and 32 days (Barrie & Hoff, 1984; Henmi & Reiter, 1978) depending on the latitude. Based on the rate of devolatilization, the net effect on habitability will shift.

This study focuses on the Flinton Group, which is part of the Mazinaw Domain in Ontario, Canada and was deposited between 1180 – 1150 Ma (Kinsman & Parrish, 1990; Sager-Kinsman & Parrish, 1993). Special emphasis is given to the Myer Cave Formation for this study, the lithology of which is defined by sulfidic and graphitic schists and pelites, calcitic and dolomitic marbles and marble clast breccias (Easton, 2006). The mineralogies of the schists are characterized by quartz, calcite, dolomite, feldspars, muscovite, biotite, hornblende, sillimanite, graphite, pyrite and pyrrhotite. Traces of retrograde chlorite has also been confirmed. Unlike the rest of the region that has undergone two phases of metamorphism, the Flinton Group has been subjected to a single medium to high grade event at 976 ± 4 Ma (McCarron et al., 2014).
Decarbonation of graphitic carbons and carbonates is recorded as a negative shift in δ13C values of both. We find that the δ13Ccarb and δ13Cred vary within the range of 2.64 to – 10.04 ‰ and – 28.4 to –19.2 ‰ respectively on the VPDB scale. Oxygen isotopes are known to show a negative shift under metamorphic conditions and the δ18O values for this study lie between 28.5 and 11.7 ‰ on the VSMOW scale. Rayleigh fractionation calculations show that at least part of the samples follow a Rayleigh trend on their devolatilization paths, while the rest of the samples can be attributed to fluid infiltration and/or local isotopic equilibration. Thermodynamic models show much earlier equilibration in T-a(CO2) space. Sulfur isotope values for both pyrite and pyrrhotite are very similar and show a range between 10 to – 20 ‰ on the VCDT scale, which provides evidence for minimal fractionation between pyrite and pyrrhotite. Based on this study, the release of volatiles seems to be negligible and the impact on planetary habitability little.

Best,
Adea


Adea Arrison

Sr. Academic Program Specialist

Department of Earth, Ocean & Atmospheric Science

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