Elemental sulfur formation in the Kemp Caldera hydrothermal system, Scotia Sea

The Kemp Caldera is a submarine arc caldera volcano and belongs to the southernmost part of the South Sandwich island arc, located in the Scotia Sea. In 2009, the caldera was discovered by the R/V James Clark Ross research cruise JR224 during a geophysical survey. At this time, first hydrothermal activities were observed within the caldera. Around a resurgent cone in the center of the caldera, extinct chim­neys and whi­te smo­ker vent fiel­ds are found. A special feature of the Kemp Caldera hydrothermal system is the occurrence of elemental sulfur (S⁰) at uncommonly high pH values. Sulfur samples of the white smoker vent fields “Great Wall” and “Toxic Castle” at the eastern flank of the resurgent cone were recovered with a remotely operated vehicle during the R/V Polarstern PS119 expedition in 2019. These two sites are no more than 80 m apart, but the occurrence of S⁰ is different: at Great Wall, the sulfur is crystalline, while at Toxic Castle the sulfur is liquid and forms amorphous, pearl-like structures. Both sites are characterized by fluids with pH_{25 °C} values > 5 and show a temperature range from 63 to > 200 °C. Most interesting, however, are the δ³⁴S values of elemental sulfur, ranging from +5.2 to +5.8 ‰.

Disproportionation of magmatic SO₂ commonly explains the formation of S⁰ in arc/back-arc systems. Elemental sulfur precipitates from highly acidic hydrothermal fluids with pH-values ≤ 1 and show negative δ³⁴S values due to isotope fractionation. However, this formation mechanism cannot explain the moderate pH of the fluids and the lack of significantly negative δ³⁴S values for sulfur that would indicate SO₂ disproportionation. We suggest that the formation of sulfur in the Kemp Caldera is a result of SO₂ and H₂S synproportionation. From a thermodynamic point of view, this formation mechanism is possible, but it has not yet been demonstrated that it actually takes place in hydrothermal systems. Our study focuses on the formation of elemental sulfur in the Kemp Caldera hydrothermal system and shows that the diversity of hydrothermal arc/back-arc systems may be greater than previously assumed.