MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 23 update from Rob Zierenberg:
Tiburon dive 884, GR-14 vent field.
Leg three is off to a great start following a highly productive dive at the Sea Cliff or GR-14 hydrothermal vent field. We dove here with Tiburon in 2000 and 2002 and collected samples of hydrothermal fluids from 305° C vents. The primary objective of today’s dive was to collect tubeworms from the far corners of the vent field to check for genetic diversity. We also collected several Galatheid crabs (squat lobsters) to compare the genetics to similar crabs from other vent fields.
Revisiting this site allowed us to assess the extent of change in the hydrothermal field. This field seems to be changing very little with similar numbers of hydrothermal vents issuing fluids at temperatures slightly above 300° C. The vents have chosen the non-smoking section. Even though the fluids coming out of the vents are very hot, they are clear rather then “smoky.” Analysis of the vent fluids in this area previously collected by our colleague Karen Von Damm at the University of New Hampshire has shown that the fluids contain low levels of dissolved metals. The reason is that the pH of the fluids is higher than in most vent fields, so they are not acidic enough to dissolve a lot of metal from the basalt volcanic rocks that underlie the vent field. Therefore, the fluids do not precipitate sulfide minerals when they emerge into cold sea water (this is what makes the black “smoke” that was seen above the chimneys on the Juan de Fuca Ridge during the previous leg of this cruise). The chimneys here just vent hot shimmering water.
After we completed our sampling of hydrothermal vent fauna and collected some altered volcanic rocks for analysis, we dropped down to the bottom of the vent field to see if the octopus brooding ground we observed in 2002 was still being used. In a small area with outcropping basalt rock we observed more than 40 octopuses brooding eggs. The mother octopuses fiercely guarded their eggs, but we convinced a few to move enough that we could see that there were dozens of eggs being guarded by each octopus.
There was clearly something special about the area where the octopuses were nesting. Compared to other areas of basalt outcrop in the area, this area was literally crawling with brittle stars and had many more attached animals such as anemones. In some areas there was warm water leaking out of the seafloor, and at least two octopuses were observed to be nesting with their eggs directly in the hydrothermal flow. Many others may be bathed in hydrothermal fluid that was flowing too slowly to be observed. Unfortunately, our temperature probe was damaged while measuring some of the high temperature vents, so we were not able to measure the temperatures where the octopuses were nesting.
We did observe a few very small Ridgeia tubeworms in this area, but most of the conspicuous vent fauna were absent, suggesting that perhaps the hydrogen sulfide levels in the vent fluids are too low to support normal chemosynthetic fauna.
The vent field where we dove today is a bit unusual, but not unique, in that it occurs in somewhat older rocks away from the spreading axis. These rocks have been moved by faulting to up above the main rift valley floor. Tomorrow we will descend into the rift valley and look at an area that erupted in 1996 forming a ridge of pillow basalts. How will the seafloor have changed since then? Will any animals have colonized the new flow? Tune in tomorrow and we’ll let you know what we find.