Iodine-129 of Appalachian Basin Brines: Implications for Microbial Fractionation and Fluid Migration

Iodine isotopes have been used as tracers of organic matter and fluid flow in fractured crystalline rock and sedimentary basins. However, little is known about iodine biogeochemistry and the possibility of fractionation in subsurface environments. Improved knowledge of this novel isotope system will advance scientific interpretation and application of results aimed at understanding the source(s) of organic matter, large-scale fluid migration, and microbially mediated redox reactions in deep groundwater basins.

Ten brine samples were collected from active oil and gas wells producing from Mississippian, Devonian, and Silurian age formations at the northern Appalachian basin margin (W. NY, N.W. PA, and E. KY) to understand fluid migration. Samples were analyzed for ¹²⁹I/I, δ¹³C_DIC, and elemental composition. Measured ¹²⁹I/I values (28-1890 X 10^-15) are 5 to 9 orders of magnitude higher than calculated cosmogenic values (10^-19 to 10^-21), given the depositional age of the organic matter source (~350 to ~450 Ma). Thus, cosmogenic ¹²⁹I is negligible. The maximum fissiogenic ¹²⁹I/I value (50-100 X10^-15) in Devonian organic-rich shales was calculated from published ²³⁸U (spontaneous fission to ¹²⁹I) data, and represents local background levels. One sub-set of samples has relatively invariant ¹²⁹I/I with depth, close to background fissiogenic ¹²⁹I/I levels. A second sub-set of samples show increasing ¹²⁹I/I from background levels with depth. The increasing ¹²⁹I/I pattern is opposite to that expected if anthropogenic ¹²⁹I were significant. The high measured ¹²⁹I/I may be accounted for by large ²³⁸U deposits in S.E. PA and NJ (regional source), followed by basin-scale brine migration to its present location (likely during orogeny). Alternatively, the second subset of samples with increasing ¹²⁹I/I are associated with relatively negative δ¹³C_DIC values (<-15‰), and decreasing sulfate concentrations with depth, indicating that microbially mediated sulfate reduction may be an important process fractionating ¹²⁹I in the subsurface.