September 1, 2013
I write as we are on the final stretch of our transit to Eureka. Finally, we got some really nice wind and sea conditions under blazing blue skies that allowed us to cut a few hours off the planned 46-hour transit.
We completed eight ROV dives and 12 wax-cores for glass during the leg, to complement the eight dives and 17 wax-cores accomplished during Leg 2. Five of the dives during Leg 2 and four during Leg 3 were roughly half-day dives, either planned as short dives or shortened due to poor weather. We collected 187 lava samples during the dives, plus small samples of volcanic glass from the tops of lava flows at 29 wax-core sites, 11 sediment scoop bags, and various animals that Linda Kuhnz has described in several logs. One-hundred and thirty-five push-, vibra-, and piston-cores of varying lengths up to 63 centimeters long were recovered, either to date underlying lava flows, or to sample the volcaniclastic deposits that drape the rim and outer flanks of the caldera at Axial Seamount.
All but one of the ROV dives were on Axial Seamount, with two dives on the north rift zone, three dives on the deep south rift zone, one dive on the upper south rift zone, and nine dives around the caldera. The one dive not at Axial Seamount was done on the CoAxial segment about 80 kilometers to the north.
The Axial lava flows erupted in 2011 were a major focus of our work with 22 of the 29 wax cores on these flows. Four dives also explored different parts of these lava flows at a pillow ridge deep on the south rift (29 samples), another pillow ridge on the upper south rift (three samples), and within the caldera (nine samples) and onto the east flank of the caldera (seven samples). These will be added to the 51 analyzed samples from the flows collected in 2011 (23 by us, four by Bill Chadwick from Oregon State University, and 24 by John Delaney and Deb Kelley of University of Washington, during three separate cruises). Samples from the deep south rift pillow ridge were dredged by the R/V Thompson during a transit from Seattle to Honolulu, also in 2011. The dredged samples, however, have different crystal content from the new samples and appear to be from an older nearby lava flow. This is now by far the most thoroughly sampled historical lava flow along the global mid-ocean ridge system. This sample coverage will be used to assess variations in lava composition and degassing along the eruptive fissure system and as the flows advanced away from the fissures. Next summer, we plan to complete the AUV post-eruption surveys of the deep south rift pillow ridge, the upper south rift pillow ridge, and the flows on the eastern rim of the caldera that were not mapped in 2011 following the eruption.
The volcaniclastic sediment deposits on the caldera rim are now sampled around the entire rim, although the full sequence was only recovered in a few places. The piston core system developed and constructed at sea by Francois Cazanave is a vast improvement over either vibra-coring or using long push-cores, but there are still some improvements we think we can make including using longer tubes, which appear to insert more easily using the ROV manipulator, and improvements in the seal of the piston in the non-uniform-sized tubing. Next year, we will most likely try to get the full section at locations where the volcaniclastic sediment is thickest. The age of the base of the deposit is important as it provides an estimate of the timing of major caldera formation at Axial Seamount. One thing we learned this year is how much this unconsolidated sediment compacts—during coring, during its ride for the remainder of the dive, and once on deck. We suspect to see further compaction during their transport back to Moss Landing in a truck. This is important to document if we are to be able to estimate what percent of the section we have recovered as opposed to the depth of core penetration, and presumed in situ deposit thickness.
The success of the cruise, as always, reflects the hard work and dedication of a lot of people on the ship and ashore. The ROV team of Knute Brekke, Mark Talkovic, Randy Prickett, Bryan Schaefer, and Ben Erwin performed their usual miracles collecting impossibly difficult glass rock samples, collecting delicate animals, and wrestling with various methods to collect the long cores. Captain Andrew McKee and his crew kept everything working well and fixed any problems quickly. The excellent station-keeping in heavy seas and strong winds is a tribute to their skills. Patrick Mitts kept us well fed and entertained with fish stories. Finally, the science party kept everything working smoothly in the control room and processing samples in the lab at the ends of dives. Our seasoned crew of Jenny, Linda, Brian, Ryan, Christoph, Mike, and Dorsey generally made it easy to be chief scientist. You have heard from our three first-time at-sea graduate students Iliya, Anita, and Will yesterday; they adjusted quickly to life at sea and assisted with nearly all the tasks that have to be done to make a cruise a success.
— Dave Clague