δ34S of dissolved sulfate uncovers groundwater dynamics in Grand Canyon karst systems

The groundwater systems in the Grand Canyon, the main source of water for the surrounding communities and park visitors, are thought to be highly sensitive to climatic change. As a result, they have been subject for multiple hydrogeochemical studies, including the investigation of deeply sourced hydrothermal fluids and microbial activity impacting the groundwater chemistry. However, the complexity of this system has precluded a clear determination of water sources and flow paths. This study uses sulfate sulfur isotopes (δ³⁴S_SO4) as a new approach to separating water sources and mixing processes in karst systems.

To investigate groundwater dynamics on a smaller regional scale we analyzed the δ³⁴S ratios and sulfate concentrations in spring water samples from below the Shivwits Plateau, the northwestern boundary of the canyon. Combined sulfate concentrations and δ³⁴S_SO4 ratios distinguish a lower and an upper aquifer system. Most water samples from the lower aquifer show δ³⁴S_SO4 values around 11-12 ‰ V-CDT and with that they match the seawater values from the Upper Mississippian to lower Permian strata that the springs emerge from. For one spring close to the central part of the Canyon, the influence of magmatic sulfur (δ³⁴S = -5 to +5 ‰) was observed.

The results from the upper aquifer provide a more complex mixing pattern with three endmembers emerging: 1) sulfate reflecting dissolved gypsum from Permian evaporite layers (around 11-12 ‰ V-CDT), 2) a lighter sulfate phase (less than -20‰ V-CDT) indicating oxidized pyrite or hydrogen sulfide of microbial origin, and 3) sulfate with increased δ³⁴S values of +18‰ V-CDT or more resulting from ongoing microbial sulfate reduction in close vicinity to high gypsum dissolution. These results will be combined with the δ¹⁸O_SO4 and δ¹³C_DIC to further isolate the influence of these environments. Overall, δ³⁴S_SO4 discerns different water sources, microbial activity, and inorganic fluid-rock interactions on a more detailed scale than other geochemical proxies (δ¹³C, ⁸⁷Sr/⁸⁶Sr) previously applied for groundwater systems.