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Ron Brown Vents 98 Leg 1
July 30 - August 15, 1998

Reports from sea:

August 13, 1998

Our work at Axial is winding down, and we've seen enough plumes to last till next year. Today we recovered the short-term current meter/MAPR mooring we deployed on Saturday at the Sonne vent field. A quick look at the data indicates very large changes in the plume intensity, which comes as no surprise. We plan to take a quick look at the long-term MTR data tomorrow. The MTRs, from moorings near North Sonne and Axial Gardens, came back heavily coated with a brown film, something we haven't seen before. Here's some summary comments about what we've learned so far...

1. Hydrothermal activity seems centered at the south end of the caldera, generally in the vicinity of ASHES and Axial Gardens/Sonne. We'll need to compare current meter data (from the short term mooring, and hopefully, the still-to-be-recovered long term mooring) with the plume observations to determine where the most intense plumes were coming from when observed. It's still difficult to say whether ASHES or the SE corner of the caldera is the larger source.

2. There is an abundance of sources throughout the caldera. We have seen many, many plumes, some only a few meters thick, with a variety of optical/heat ratios. We towed the CTD at 20 mab along the eastern wall of the caldera and down the south rift zone to the first SeaBeam anomaly, finding many instances of apparently buoyant plumes. Greatest concentration was at the southern end of the caldera.

3. There have been many changes since February... a. Max T anomalies have declined from ~0.2C to ~0.05C. This decrease might be due to hydrothermal cooling or to hydrographic masking by sources of phase-separated fluid, though it's hard to believe there are low-S fluids everywhere. Plume rise heights in the caldera are only a little less than in Feb.
b. Optical anomalies have decreased also, though not nearly as much. I said in my last report that optical anomalies here were the largest we've seen, but in fact they were higher in Feb (different instruments fogged my memory). However, the very high optical/heat ratio is indicative of microbe-rich plumes as seen at other eruption sites. The very largest anomalies were only seen in the SE corner of the caldera, some in thin plumes >150 mab.
c. Plume strength now falls off with distance much more rapidly than in Feb, when plumes 20 km "downstream" were as intense as in the caldera. We have been unable to locate a similarly strong far-field plume this trip. Plumes within 10 km are highly variable in time. We occupied 12 stations successively at 30deg intervals around the compass, with a radius of 20 km from Sonne. No strong plume signals were found and no real pattern. We did, however, find distinct plume remnants along the north and south rift zones far from the caldera. these appeared to be remnants of the caldera plume (same density horizon). My guess is that the winter flow field was steadier and more energetic than now.

4. We found no clear evidence of any present discharge at either of the SeaBeam anomalies.

5. Hydrothermal activity here may be decreasing more slowly than at CoAxial. At Floc, max dT values decreased from ~0.1C to 0.03C in 6 months. Rise height has not changed much since Feb. Sounds like a good place for a seafloor observatory.

That's it for now. We are flooded with data, approaching 30 casts and 20 tows, and looking forward to passing the baton to ROPOS.

Cheers, Ed and the water column gang
Ron Brown Vents 1998 Leg 1

August 7, 1998
Work completed at Cleft on 8/5 and Brown currently at Axial. 3 vertical CTD casts completed around Sonne vent field and 1 tow from eastern wall of caldera down to sourth rift zone & the 2 bathymetry anomalies. Greatest plume so far are over and around Sonne vent field. Optical anomalies are greater than ever previously measured, but temperature anomalies small. Visual inspection of filters suggests most plumes not composed of mineral precipitates, likely bacterial. H2S scent remarkably strong from bottles. Plume height over Sonne at 200m maximum with many individual layers. Particle anomalies at Sonne more intense than Wecoma response cruise immediately after event.
After 6 months since event, hydrothermal discharge still robust. Probability of long lasting activity good, indicating Axial is a good choice for NeMO Observatory and time-series observations will be unusually productive. Judging by intensity of optical signals, deep hot biosphere is alive and well at Axial.

Unsuccessful recovery of mooring 97V103 (with current meter) at Sonne vent field (site of largest plume anomalies so far). Possibility of mooring damaged by high-temperature fluids during event or mundane mechanical problems. Would like Brown Vents leg 2 with ROPOS to attempt recovery. Tomorrow planned recovery of 2 VSM's and deployment of Lavelle's short-term mooring monitoring diffuse flow at Sonne.

Final Cleft notes: Plumes common everywhere at Cleft, N. Cleft temperature anomalies higher than for past several years (max. approach 0.05C). Vertical casts at Pipe Organ and Monolith vents found buoyant plumes and a scent of H2S. Plume rise along N. Cleft also higher than 1997, typically 200m or more. S. Cleft anomalies appear relatively unchanged to previous years, with stronger particle plumes but weaker temperature anomalies than N. Cleft. All moorings and instruments recovered on Cleft.

August 3, 1998
Ron Brown currently at Cleft segment of Juan de Fuca, expected to work 1-2 more days at Cleft before moving to Axial.

Cleft: Spectacular weather led to recovery of 6 moorings (Cascadia and 5 at S. Cleft), 4 vertical casts and 2 tows at S. Cleft, and 2 tows at N. Cleft. Planned recovery of 3 N. Cleft moorings today. Plume at S. Cleft optically intense and widespread. Substantial plumes from 44 30'-44 48'N, transport is to west.

R/V Atlantis and ALVIN dives at Axial
July 14-18 1998

Preliminary Science Report on Axial ALVIN Dives
July, 1998

posted 8/10/98

As part of a ALVIN dive series funded by the NSF LExEn (Life in Extreme Environments) Program -- Jim Cowen (University of Hawaii),and Paul Johnson (U. Washington)--Jim Cowen--Chief Scientist--, two additional dives were funded by the NSF RIDGE Program and the NOAA EOI Program to conduct surveys of the Axial Volcano seismic event area and to recover samples of vent fluids. Three of the dives visited ASHES for all or a portion of the dive, one spent about a third of the time on the east side and the final one was devoted entirely to the the east side of the caldera (northernmost south rift zone).

ASHES-- There was surprisingly little physical change at ASHES. In fact, the temperatures appeared to be back to near their 1980s values. We obtained a reasonable sampling of vent fluids (with gas samples at Virgin and Inferno), a good video survey (the navigation was excellent), and installed two low temperature probes (at crack vents and the base of Inferno) and two high temperature HOBO probes (at Inferno and Virgin mound). The LexEn effort at this site resulted in a very successful emplacement of a year-long column to collect organic material from the subsurface

East Caldera-- The lava fields that were encountered varied from very lightly sedimented to those with distinct pockets of pelagic sediment and a coating of sediment on their surface. Very young flows characterized by glassy surfaces and only a dusting of gray sediment were observed in some places. However, stalked organisms were sometimes observed on these flows, suggesting an age of at least several years. The best candidate for 98 flows were those (particularly a site observed at the end of the last dive) with bright yellow pockets of Fe-rich sediment. These deposits have been observed forming on still-cooling lavas at the CoAxial eruption (1993) and the Gorda Ridge eruption (1996). Although no lavas of unequivocal 1998 vintage were found, we did see some striking signs of new activity. There was abundant floc in the water column and collecting in drifts on the seafloor as we saw at CoAxial in 1993 (but not at Gorda). There were a lot of dying vents (characterized by large tube worm bushes with many dead residents) near what what appeared to be new vents which were characterized by thick coatings of white bacterial mats. We got water samples at three of these vents, laid markers at same, and left two temperature monitors. Areas of intense floc venting was passed over in drain pits, but we were not able to find the source in the confusing terrain and amidst our own sediment wake.

Water Column Plumes- Several CTD tows along northernmost south rift zone and near the walls of the caldera found intense plumes within 200 meters of the seafloor. The tow along the East side of the caldera and upper north rift zone passed through extremely intense particle plumes (up to 8% transmission loss), consistent in location with the concentration of seafloor venting observed on ALVIN.