Hydrothermal Activity at the Ultraslow‐Spreading Mohns Ridge: New Insights From Near‐Seafloor Magnetics

Abstract

Hydrothermal circulation is a process fundamental to all types of mid-ocean ridges that largely impacts the chemical and physical balance of the World Ocean. However, diversity of geological settings hosting hydrothermal fields complicates the exploration and requires thorough investigation of each individual case study before effective criteria can be established. Analysis of high-resolution bathymetric and magnetic data, coupled with video and rock samples material, furthers our knowledge about mid-ocean-ridge-hosted venting sites and aid in the interpretation of the interplay between magmatic and tectonic processes along the axial volcanic ridges. The rock-magnetic data provide constraints on the interpretation of the observed contrasts in crustal magnetization. We map the areal extent of the previously discovered active basalt-hosted Loki's Castle and inactive sediment-hosted Mohn's Treasure massive sulfide deposits and infer their subsurface extent. Remarkably, extinct hydrothermal sites have enhanced magnetizations and display clear magnetic signatures allowing their confident identification and delineation. Identified magnetic signatures exert two new fossil hydrothermal deposits, MT-2 and MT-3. The Loki's Castle site coincides with negative magnetic anomaly observed in the 2-D magnetic profile data crossing the deposit. First geophysical investigations in this area reveal the complexity of the geological setting and the variation of the physical properties in the subsurface.

Benthic Communities on the Mohn’s Treasure Mound: Implications for Management of Seabed Mining in the Arctic Mid-Ocean Ridge

“…Although previously described as a massive sulfide deposit (Pedersen et al, 2010b), no resource exploration has been conducted at Mohn's Treasure to date. In addition, our ROV observations and magnetic data collected during the MarMine cruise (Lim et al, 2019) indicate that this is a highly sedimented system, which would not be, in principle, interesting to mineral exploration in the current technological and economical context. However, Mohn's Treasure is situated only 30 km to the south of Loki's Castle active hydrothermal vent field and other active vents with potential sulfide deposits, which have been identified along the Mohn's Ridge since 2015 (Olsen et al, 2016).…”

“…Although Mohn's Treasure has been described as a massive sulfide deposit, the only evidence of mineralization found during the entire MarMine survey over the area (13 ROV dives) was a few meters of oxidized basalt outcrops comprising minor pyrites and features possibly formed by diffuse discharges (Ludvigsen et al, 2016). Subsequent analyses of magnetic data collected during the cruise suggest the presence of deposits buried under 15 m of sediments (Lim et al, 2019). The general seascape was that of a sedimented rift valley, with the seafloor composed mainly of soft sediment with basalt outcrops.…”

“…Although mining exploitation on Mohn's Treasure seems unlikely currently, as any potential resources would be covered by a thick (ca. 15 m) sediment layer (Lim et al, 2019), new sites may be found in proximity. The potential exploitation of mineral resources in the region may result in harmful effects on the crinoid and sponge communities that we observed and on the ecosystem services they provide, such as nutrient regeneration and habitat provision.…”

“…Mohn's Treasure is located at the edge of an inner rift wall, 30 km southwest of Loki's Castle, and was described as a massive inactive sulfide deposit based on hydrothermal deposits collected in a rock dredge in 2002 (Pedersen et al, 2010b). Lim et al (2019) provided magnetic data of the fossil sedimented-hosted hydrothermal deposit, delineating a deposit of approximately 200 × 150 m buried under 15 m of sediments, as well as two new deposits. The ecology of AMOR vents has been studied for the shallow active vent fields and Loki's Castle.…”

Oceanic Core Complex or not? When partial spreading asymmetry triggers seafloor diversity

“…The ship deployed the Autonomous Underwater Vehicle (AUV) Hugin, which carried out a dive over the hydrothermal site to collect highresolution bathymetric and magnetic data. The near-sea oor magnetic data were acquired by a uxgate magnetometer from Ocean Floor Geophysics mounted to the AUV (15) (Methods and Supplementary Information). The AUV routes were 150 m apart and 100 m above the sea oor (Fig.…”