The Kemp Caldera hydrothermal system, Scotia Sea – Morphological, mineralogical and geochemical characteristics

Abstract

Submarine calderas are a unique form of hydrothermal systems, which are not yet well understood in terms of how they form or how they develop over time. One of the least explored submarine calderas is the Kemp Caldera, which is located in the Scotia Sea in the rear-arc region of the southernmost part of the South Sandwich island arc. Since its discovery in 2009, the caldera has been of great interest primarily to bioscience researchers, but within the last few years, the Kemp Caldera has also been increasingly studied from a geoscientific perspective. One of the objectives of the R/V Polarstern PS119 expedition in 2019 was the investigation of the Kemp Caldera and its hydrothermal system in more detail. New bathymetric data together with visual seafloor observations and rock samples now show that the caldera was formed by two collapse events, resulting in a prominent morphology. The shape of the resurgent cone, which occurs in the central part of the caldera, and the results of rock analyses indicate a dacitic post-caldera eruption that was responsible for the formation and development of several vent fields. Two of these hydrothermal fields, Great Wall and Toxic Castle, located on the eastern slope of the central resurgent cone, are of particular interest. Here, contrary to other hydrothermal systems in this area, elemental sulfur occurs not only in fine-crystalline, but also in liquid form. Sampling and later investigation of the sulfur and other hydrothermal precipitates showed that the elemental sulfur is isotopically heavy and thus cannot be attributed to the generally accepted formation by SO2 disproportionation. Instead, the observed isotopic composition of sulfur must be the result from synproportionation of SO2 and H2S. Although this reaction has not been documented from other hydrothermal systems, the use of a thermodynamic computation software and a Rayleigh fractionation model demonstrated that synproportionation is thermodynamically possible both at Great Wall and Toxic Castle, and capable of producing the observed sulfur isotopic composition. Moreover, experiments carried out under hydrothermal conditions produced artificially generated sulfur, whose measured isotope values matched those predicted by the Rayleigh fractionation model for synproportionation. This thesis therefore proves, for the first time, that synproportionation can form sulfur in hydrothermal environments and might thus represent a previously unknown source of isotopically heavy sulfur.