Nov. 22: Thanksgiving Dives 4996 & 4997

This sunrise on Thanksgiving is usually vivid, and the memories of this dive day will also remain unusually vivid. The plan is to explore and sample a new site that the Sentry team has scouted out, a large hydrothermal feature north of Cathedral Hill, for now called Northern Cathedral hill, or NC. After this warm-up task, we will head over to the Osmosampler site near the eastern end of Cathedral hill, recover the osmosamplers, T-loggers, and some sediment cores.

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Today’s dive is piloted by Anthony Tarantino; the science observers are your blogger, and UNC Marine Science graduate student Chris Chambers. We fall somewhere between conspicuous bravado, and putting a brave face on things, not entirely inappropriate for today.

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The dive has barely started, but all of a sudden the variable ballast system has shorted and we need to return to the surface for repairs. The ballast pumps are essential for maintaining Alvin’s buoyancy and to adjust it to the payload that the sub is carrying; without it we would flounder around at the bottom, trying to keep afloat with the help pf the thrusters, which depletes the batteries and stirs up the notoriously silty bottom mud of Guaymas Basin. More on that topic soon… By 9:45 Alvin and dive team are in the water again, now with a new dive number; technically we all have two dives today.

Onward to the Northern Cathedral site – which turns out to be a large hydrothermal mound with perhaps the largest single Riftia garden in Guaymas Basin. The entire eastern side of the mound, ca. 10 meters high, is overgrown by Riftia and pillows of  Beggiatoa mats, and emits shimmering water everywhere. This must be the source of the massive hydrothermal plume that Sentry has recorded at this place two nights ago. Small chimneys, some extinct, some active, punctuate the Riftias; we pick this specimen fresh from the mound, and collect large chimneys fragments from the mounds base.Nov22SmallSpireSmall

After one hour, we head over to the osmosampler site, a huge hydrothermal area in the eastern Cathedral Hill region where strange small chimneys are surrounded by grey sediments and [maybe?] mats. At this unusual location, Mandy Joye and her team have deployed four osmosamplers and T-loggers during dive 4994; these are now recovered, together with adjacent sediment cores at osmosamplers 3 and 4. These sediment cores will soon turn out valuable and reveal geochemically odd, iron-rich, sulfide-poor sediments.Nov22OsmosetupSmall

The trouble begins as the osmosampler boxes are taken out and manoevred to the elevator, which is positioned nearby, wedged into a fairly small space between a row of chimneys and the osmosampler mat itself. The fine-scale movements of Alvin moving back and forth in a constrained space ultimately stir up the bottom sediment; here the sediment clouds are just starting to rise around the chimneys.

Nov22SiltedSpireSmallAfter a while, the elevator is enveloped in fog, and loaded and released only with great difficulty. Two osmosamplers ride to the surface on the elevator, two mores are held in place on the Alvin basket, on top of the coring boxes. When everything is done, Anthony and the observers breath a sigh of relief. Belatedly, after four, we start to the surface and arrive there just barely in daylight and ready for dinner.  Against the odds, the osmosamplers, T-loggers and cores all made it.  I had hoped for a more relaxing dive, but if this is what it takes to rescue the samples, be it… hopefully only today. The osmosamplers will be turned over and go back into a different location tomorrow.

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