Diverse styles of hydrothermalism on the ultraslow Arctic Mohns Ridge: the Fåvne, Ægir, Loki’s Castle, and Mohn’s Treasure vent fields

In recent decades, the deep-sea scientific community has learned that slow- to ultraslow-spreading ridges can host hydrothermal systems that are very diverse in their geologic setting, vent fluid geochemistry, and ecology compared to intermediate-/fast-spreading ridges. However, several regions of ultraslow/slow spreading seafloor are still undergoing early exploration, particularly the Arctic Mid-Ocean Ridges (AMOR). In this presentation I will detail chemical compositions of high and low temperature hydrothermal fluids collected from several newly explored active vent fields on the ultraslow spreading Mohns Ridge, in particular Fåvne, Ægir and a reevaluation of time-series fluid chemistries from the well-known Loki’s Castle vent field. I will also describe very newly discovered focused, low temperature venting (ca.38°C) sampled from a vent field long presumed to be extinct - Mohn’s Treasure. These systems are situated in different bare-rock settings within the wide (~20 km) rift zone. While Loki’s Castle and Ægir are located on central neovolcanic ridges, Fåvne and Mohn’s Treasure are located on bounding rift valley walls. Indeed, all 4 display highly diverse fluid compositions with drastic differences in relative geochemical influences of ultramafic-mafic rock versus subsurface sediment alteration, in addition to microbial processes. These differences emphasize the critical role of local geologic architecture in delimiting hydrothermal circulation pathways, which in turn influences contributions of local sediment and subseafloor host rock to resulting fluid chemistries and ecosystems. The diversity of fluid styles emanating along such a short (<300 km) length of Mohns Ridge illustrates that the AMOR hosts an unusual diversity of geological-geochemical processes affecting hydrothermal activity, giving rise to highly individual seafloor vent ecosystems.