Expedition Report: iMAGE-CREATE Trainee Carlos Braga Reports from the R/V Sonne

R/V Sonne Research Cruise SO-311 – Expedition to the Brothers Volcano in the Southern Kermadec Arc

Expedition report by iMAGE-CREATE PhD trainee Carlos Braga

Hydrothermal systems associated with massive sulfide deposits are highly variable depending on their geologic setting (e.g., mid-ocean ridges, arcs, back-arcs, etc.). In arc settings, the hydrothermal fluids that cause the deposition of metals below or at the seafloor are associated with volcanic edifices and have a large component derived from magmatic sources, which has important controls on chemistry, biota and mineralization. Thus, unveiling the magmatic hydrothermal signature of these modern systems may provide important clues to understanding their analogs, such as epithermal gold deposits and porphyry copper deposits mined on land.

Brothers Volcano on the Kermadec Arc, 400 km north of New Zealand, is an important representative of arc volcanoes, formed by the subduction of the Pacific plate under the Australian plate and has been a target of research since the 1990s. It is the most hydrothermally active volcano on the Kermadec Arc and hosts six sites of venting (five active, one inactive), with some of its chimneys having ~30 wt.% copper and ~90 g/ton of gold. For that reason, it was chosen as the target for drilling during the SO311 expedition aboard the German RV Sonne from March 26^th, 2025, to May 1^st, 2025. Drilling Brothers would provide the 3^rd dimension (depth), which is crucial to understanding subseafloor architecture, permeability, fluid flow, and the extent of Earth’s deep biosphere. PhD student and iMAGE-CREATE trainee Carlos Braga was sent from the University of Ottawa to assist carrying out scientific activities at Brothers and the surrounding areas.

Besides Carlos, the team included scientists from the University of Bremen, the University of Erlangen-Nuremberg, the Max Planck Institute, the Berlin Institute of Technology, the University of Genoa, the University of Canterbury and GNS Science (Te Pū Ao). Dr. Wolfgang Bach was the chief scientist, and Dr. Manuel Keith deputy chief scientist. The group was composed of geologists, geophysicists, biologists, technicians, and engineers, including many MSc and PhD students who were for their first time conducting research at sea.

The main activity of the cruise was drilling selected sites at Brothers with MeBo200, a robotic drill developed by MARUM (Center for Marine and Environmental Sciences) at the University of Bremen. MeBo has been operating since 2014 and has the capability to retrieve core down to 200 m below the seafloor. Additional goals included sampling with:

• TV-grab: a heavy hydraulic claw tethered to the ship equipped with a TV camera for live transmission, capable of sampling loose rocks, sediment and organisms from the seafloor:
• CTD: Conductivity-temperature-depth sensors tethered to the ship, which can sample water at different depths and detect heat and particle anomalies, which may help pinpoint the location of a hydrothermal vent field.
• Dredge: a heavy chain-bag with steel dents tethered to the ship capable of scraping samples from the bottom of the ocean and bringing them back to the ship’s deck.

Multibeam echosounder (MBES), parasound, and gravimetry surveys were also carried out daily during the cruise. MBES is an active sonar system mounted on a ship’s hull used to map the topography of the ocean floor at resolutions of tens of meters per pixel. Parasound is a sonar system capable of penetrating the first few tens of meters below the seafloor, which can identify lithological variations in deep-sea sediment layers. Lastly, gravitometers were used to measure local variations on Earth’s magnetic field, which can help in characterizing Brothers and other areas, and may improve more general gravity models of the area.

Drilling proved to be the main challenge during the cruise. Brothers is a volcano composed mainly of hard dacite and loose tephra layers. Consequently, although core was retrieved from multiple sites, recovery tended to be low, with the hard rocks jamming the drill process and much of the loose material being washed away during recovery. However, since development of seafloor drill rigs is an ongoing process, this experience provided the MeBo crew with valuable information to help MARUM in upgrading their current and future instruments to the challenges that drilling objects such as Brothers imply.

In summary, even though drilled cores did not have the desired recovery, the cruise provided the team with tens of samples of hydrothermal vents and volcanic rocks, including active and extinct massive sulfide chimneys from Brothers, volcanic rocks full of native sulfur from different areas along the Kermadec, lavas with different degrees of alteration, and active chimneys from a newly discovered vent field at the Ngatoroirangi volcano. These samples, together with the recovered cores, may provide valuable insights into the evolution of volcanic and hydrothermal processes at Brothers and other sampled areas on the Kermadec. Some of the rocks still contain abundant barite crystals (BaSO₄), which may be used to date hydrothermal vents using Ra-Ba methods. Furthermore, many of the samples taken by the biologists may further reveal how bacteria and other microorganisms are involved in hydrothermal processes occurring at the deep-sea.