Second Jason dive
By Bill Chadwick
After being kept out of the water for 3 days due to high winds, we were all happy to get ROV Jason wet again on Thursday August 20. This was only our second Jason dive at Axial Seamount on this expedition, but we were able to accomplish a lot in one dive. We started at a site called Marker 33 Vent (see map) where Jason filled the incubators on the fluid sampler to conduct the microbial growth experiments described in the previous blog post. Marker 33 Vent (FIG. 1) is notable because it has been a persistent area of diffuse hydrothermal venting on Axial’s South Rift Zone with diverse microbe populations despite being buried by lava twice. Before the 1998 eruption, the area was observed to be venting with extensive fields of lush tubeworm “communities,” but in 1998 it was buried by lava. However, soon afterward the vent site re-established itself with warm water flowing from a crack in the 1998 lava flow that was quickly re-colonized by vent animals. Then in 2011 the site was buried again by 4 meters (13 ft) of lava, but the vent site survived and once again it has been quickly re-colonized by vent animals. Because of this remarkable history, Marker 33 Vent is one of our best long-term sites where we can study how the volcano perturbs the hydrothermal systems and their biological communities.
The next stop on the dive was a vent field called the International District (FIG. 2), which has an assortment of tall sulfide chimneys, many of which are actively venting, high-temperature “black smokers.” Vent fluid samples were collected at several of these, including a 15-m tall smoker chimney named “El Guapo,” which vents mineral-laden fluid as hot as it can get – right at the boiling point for seawater (348° C) at this depth (1510 m). After sampling high-temperature vent fluids at two other gas-rich anhydrite chimneys (Diva and Castle vents) to extend chemical time-series to look for changes due to the 2015 eruption. Afterwards, Jason transited about 4 km to the north.
The second half of the dive consisted of a traverse from south to north across the 2015 lava flow that erupted in the NE part of the caldera (FIG. 3). Interestingly, this lava flow was very different in appearance from the larger and thicker 2015 flows we explored on the first Jason dive. The NE caldera flow is very thin lobate lava (in many places just 1 meter thick and less than 10 m (32 ft) at the thickest), and includes channels and collapse areas in some areas where the lava clearly flowed rapidly downslope (FIG. 4). Consequently, we saw no warm water escaping from the lava flow, apparently because it had already cooled since the eruption in April. The lava is black and glassy almost everywhere, with little or none of the “eruption mat” that was so prominent on the much thicker (and still warm) lava flows on Axial’s North Rift Zone. Also, we did not cross any obvious eruptive fissures, so presumably they were off to the side of our dive track. The detailed bathymetry that AUV Sentry collected over this lava flow may help to locate where the lava was erupted. We collected samples of the 2015 lava for chemical analysis and to compare with those from the North Rift Zone and to lavas from previous eruptions (FIG. 5).
At the end of the dive we visited the CASM hydrothermal vent site (FIG. 6), which is located where the North Rift Zone intersects the caldera. The high-temperature vents at CASM are located down inside an older eruptive fissure that is 20 m (64 ft) wide and 10 m (32 ft) deep. We had not visited CASM since 2007, so it will be interesting to see if the chemistry of the vents has changed since then, especially since the 2015 eruption was located along the North Rift Zone (although no lava came out at CASM). The chimneys at CASM were spectacularly beautiful and were completely covered with lush tubeworms and other vent animals that looked like very healthy communities. If anything, the vent animals at CASM seem to have been rejuvenated from the injection of magma beneath the rift zone.