POSSIBLE IMPACT OF HYDROTHERMAL BRINE DISCHARGE EVENTS ON SULFUR ISOTOPES IN MARINE BASINS AND THE GLOBAL OCEANS

The hydrothermal brines that formed sediment-hosted massive sulfide and barite (sedex-type) deposits may have induced changes in the composition of overlying basin water, and ultimately in the global oceans (Emsbo 2004, submitted). To test whether sedex events could have impacted marine sulfate we mathematically simulated the concentration and δ³⁴S value of water column sulfate as a function of export to massive sulfide, barite and biogenic pyrite, and import from rivers. Massive sulfide was assumed to form from H₂S from bacterially-mediated anaerobic oxidation of discharged methane or hydrocarbons (model of Johnson et al. 2004).

The simulation was applied to the basin-scale problem by combining observations of 2 Paleozoic localities, the Kuna basin in NW Alaska for which basin size and ore tonnage are well known, and the Selwyn Basin in Canada for which analyses of sedimentary pyrite in sedex-correlative strata suggest basin water δ³⁴S excursions of up to 35 ‰. The results of the simulation (using the published marine sulfate concentration of ~10 % modern, modern global fluxes for biogenic pyrite formation and riverine input scaled to the Kuna water mass, deposit growth rates from modern analogs) suggest that formation of the ore deposits did not impact basin water sulfate directly. δ³⁴S is sensitive only to sulfate concentration (halving gives a 2 ‰ rise) and biogenic pyrite production (tripling gives a 2 ‰ rise) which suggests that sedex events could have changed δ³⁴S only if they affected these parameters.

A similar simulation was applied to the global oceans to evaluate possible links between the Kuna sedex event in Mississippian time and a synchronous global δ³⁴S rise of 1 ‰, and between a series of 7 known ore districts of late Devonian age and the steep 5 ‰ rise in that time period. A key feature of these periods is that δ³⁴S correlates positively with δ¹³C consistent with sulfate reduction by an agent other than organic C. The simulation suggests that the observed δ³⁴S increase could be attributed to brines if their discharge stimulated organic C preservation and biogenic pyrite formation simultaneously and globally. This could have been a consequence of the large inventories of reduced chemical species (including methane/hydrocarbons), metals, and other nutrients that the brines are known to have carried.