Searching for Serpentinization-Driven Hydrothermal Venting on Oceanic Core Complexes of the Mid-Atlantic Ridge

By David Butterfield

Joan Alfaro-Lucas

Our cruise to the Mid-Atlantic Ridge has gone extremely well so far. We are gathering many kinds of data non-stop. Below is a brief summary of what we have found so far. You can follow our progress on the SOI web site www.schmidtocean.org, see the video on YouTube, and see updates on Instagram and Facebook. We are at our third study site now, and have some clear targets for ROV dives. The next one should start tomorrow, March 27, in the late afternoon Seattle time.

Notable Finds:

Metal sulfide deposits from extinct hydrothermal deposits on mid-ocean ridges are a target for mineral exploration. The absence of endemic vent fauna on such ancient deposits provides a rationale (untested) that mining them would not pose a risk to chemosynthetic communities that are dependent on hydrothermal flow as the basis for the food web that supports diverse communities. Regional Environmental Management Plans for regulating ocean mining require accurate scientific data on the presence of animal communities and an understanding of how sites are colonized.

Puy des Folles is a volcano on the Mid-Atlantic Ridge with reported, inactive metal sulfide deposits. Previous expeditions have explored the volcano with human-occupied vehicles, towed cameras, and dredges, finding no active venting, but noting that hydrothermal plumes were present. The plumes indicated to us that there must be active venting. Puy des Folles was a perfect site to apply our multi-stage exploration methods for finding hydrothermal vents and the unique fauna they support.

In 48 hours at Puy des Folles:

• Mapped the volcano with the Falkor (too) EM124 Multibeam sonar system.
• Surveyed the water over the volcano with two crossing CTD tow-yos.
• Deployed two AUVs and mapped 21 km2 (80% of the volcano’s summit) at 1-meter scale.
• Launched ROV SuBastian within 3 hours of recovering the AUVs.
• Collected in-situ methane concentration data with a new capillary absorption spectrometer.
• Identified probable chimneys by AUV bathymetry and active vent areas by water column properties.
• Found active diffuse venting and one large, very active hydrothermal vent with abundant and diverse vent fauna.
• We have at least 2 other targets that look like active hydrothermal chimneys.
• Collected diffuse vent fluids for chemistry and microbiology.

When mantle rocks that are normally covered by volcanic ocean crust are exposed to seawater at oceanic core complexes (OCCs), they react to form serpentine and other minerals and produce hydrogen gas. The reactions remove dissolved CO₂ from seawater. If conditions are right, carbonate chimney towers can be formed by venting of high-pH fluids. There are very few examples of this type of venting. Our project is studying an oceanic core complex in detail to understand the geology and to search for hydrothermal venting related to serpentinization. This type of venting is difficult to find because it does not produce the particle plumes that are associated with high-temperature venting (‘black smokers’). The AUV sonar can resolve hydrothermal chimney features, and we are using AUVs with sensors that respond to hydrogen to search for venting related to serpentinization.

In nine days at the EMARK area:

• Mapped the inside corner high (ICH) out to the Kane fracture zone and the MAR at 15-20m scale with the Falkor (too) EM124 multibeam sonar.
• Mapped ~106 km2 of the ICH to 1-meter scale with MBARI AUVs.
• Identified a detachment fault within the study area.
• Conducted 4 CTD tows and 13 vertical CTD casts to search for plumes.
• Used ROV SuBastian for geologic mapping, rock sampling, biological observations, fauna sampling, and chemistry sampling.
• Discovered a large high-temperature vent field between 3900 and 4000 meters on the east side of the ICH close to the MAR axis. The field has 5 distinct smoker vents.
• Collected rock samples including metal suflide chimneys, lavas, gabbros, and serpentinites.
• Collected hot fluid samples. Gas chemistry of sampled fluids indicates reaction with mantle rocks.
• Collected hydrothermal plume samples for chemistry and microbiology/virus studies.
• Sampled shrimp, anemones, and gastropods for identification and comparison with fauna from other vent fields.
• Extracted and sequenced environmental DNA (on board ship) from near vents to compare with detailed observations of the chemosynthetic communities.