What's Happening Today - May 20

We're transiting south in fair seas making between 10-12 knots. We hope to reach Pt. Ayora, on Santa Cruz Island in the Galápagos on Thursday afternoon. Everyone on board is busy analyzing data from the recent Alvin and ABE dives and testing continued on the towed camera and rock corer system.

Ken Macdonald and Rachel Haymon have provided the synopsis below of their dives on this cruise.

DIVES 3785 (May 17) and 3787 (May 19)

Ken Macdonald and Rachel Haymon - UC- Santa Barbara

Many dives on the East Pacific Rise (EPR) at 9°-10° N have taken place on
top of the rise crest, where the great plates of lithosphere that floor the
Pacific Ocean are spreading apart. Little exploration has been done on the
adjacent flanks of the rise, where faulting parallel to the EPR breaks the
plates into long slivers that are alternately upthrown to form elongate
hills (known as "abyssal hills") and downthrown to form linear valleys
between the hills. In 1994, Prof. Ken Macdonald (University of California,
Santa Barbara) and his colleague Dr. Jeff Fox (now director of the U. S.
Ocean Drilling Program at Texas A&M) used Alvin to show that abyssal hills
are created by faulting in a zone approximately 1.5-5 km away from the top
of the EPR, and are modified near the rise crest by volcanic flows pouring
downhill from the summit of the EPR. Once formed, the abyssal hills travel
along with the plates, which are spreading away from the EPR here at a rate
of about 5.5 cm/year. Slippage on the faults bounding the abyssal hills
continues out to distances of up to 50 km from the rise crest. This
slippage increases the heights of the abyssal hills to as much as 200 m
above the adjacent valleys. Sediment accumulates with time on top of the
hills and in the intervening valleys. Hundreds of km from the EPR crest,
after millions of years of plate travel, the old hills and valleys become
completely buried beneath a smooth blanket of sediment.

During their 1994 dives, Macdonald and Fox found some interesting features
on a tall abyssal hill located about 26 km west of the EPR summit at
latitude 9° 27'N. The sedimented top of the hill was decorated with many
small (less than 10 feet across x 4 feet high), whitish mounds of unknown
origin. A surprising abundance of mobile animals were seen, and the bottom
waters contained many particles. The rocky fault scarp on the east side of
hill was covered with a mossy material, possibly microbial in origin.
Altogether, these observations suggested possible hydrothermal venting from
the abyssal hill, a phenomenon which is common on the volcanically-active
crest of the EPR, but previously has not been found within the vast area of
the EPR flanks. Based on these findings, the National Science Foundation
funded two dives in 2002 to investigate possible hydrothermal activity on
the abyssal hill visited previously by Macdonald and Fox.

On May 17 and May 19, 2002, Prof. Macdonald and Prof. Rachel Haymon (who
conducts research on EPR hydrothermal systems at University of California,
Santa Barbara) used Alvin to explore the curious, possibly hydrothermal,
features found by Macdonald and Fox in 1994. On Dives 3785 and 3787,
Macdonald and Haymon found many mounds of light colored sediment sitting on
top of the darker sediments that blanket the top of the abyssal hill (see
Photo 1). The mounds usually lie at the margins of crater-like depressions
in the sediments, approximately 0.5-1 m across, and appear to contain
ejected chunks of sediment. The Alvin pilots (Phil Forte and Blee Williams)
collected "push cores" for the scientists of the light colored sediments
comprising the mounds (Photo 2), and also of the sediments within the
craters, and the darker sediments outside mounds. Temperature gradients
were measured in the sediments, and these measurements showed that there is
a significant flux of heat from the top of the abyssal hill (approximately
15 heat flow units), even though no visible venting of fluids from the
seafloor were observed during the dives. During Dive 3785, the divers
encountered structures on the fault scarp that appeared to be old (inactive)
hydrothermal chimneys (Photo 3). These structures were very hardened and
difficult to sample, and it was frustrating not to be able to break off a
sample. If they are hydrothermal chimneys, as they appear to be, it means
that focused hydrothermal fluid flow has occurred along the fault. Other
samples collected during the dives include "slurp" samples of the mossy
material on the fault scarp, and rock samples with moss-covered surfaces.

After the dives, Haymon and UCSB graduate students Sara Benjamin and Jeff
Blasius were busy until midnight taking samples from the push cores, while
Eric Olson (University of Washington) analyzed some of the samples for
methane and hydrogen. More analyses of the sediments and pore fluids in the
cores will be done this summer at UCSB to look for clues about the origin of
the mounds and craters. Preliminary observations of the layering in the
cores suggest that the light-colored sediment was ejected from the craters
and deposited on top of the surrounding sediments. Chemical and
mineralogical analyses of the cores will provide more information about how
and why these "blow outs" occurred.

Photo 1: Seafloor photo of the mysterious whitish sediment mounds
(foreground) perched at the edge of a small depression (shadowed area behind
mound). There are scores of these mounds and depressions on top of an
abyssal hill located about 25 km west of the EPR crest at 9° 27'N. These
structures may be produced by "blow outs" of some kind that ejected material
out of the depressions and deposited them on top of the surrounding
sediments.

Photo 2: Shipboard photo of one of the push cores recovered with Alvin from
the mystery mound in Photo 1. The white mound material sits on top of the surrounding sediment which is dark brown. Samples were extracted from these cores for
chemical and mineralogical analyses ashore back at UCSB.

Photo 3: Seafloor photo of a possible inactive hydrothermal chimney found
on the fault scarp bounding the east side of the abyssal hill explored
during Dive 3785. This structure was composed of hardened material that
proved impossible to sample with the robot arm of Alvin, thus its
composition remains unknown. However, its presence here indicates that
focused hydrothermal fluid flow has occurred along this scarp.

ABE just before it's recovery last night.Hans (left) and Maurice are all smiles at the succesful results of the short cruise.

Patrick Hennessy, one of Atlantis's Able Seamen, hauling in the tuna he caught today as we transit to the Galápagos Islands. Dana Yoerger, meantime, was battling the Killer White Rabbit.

Paul Johnson going to the seafloor in Alvin

Alvin slurping bacterial mats near one of the hydrothermal vents. This large sea anemone was living on the edge of a large collapse pit in the lobate lava at about 2510 meters depth.

Maurice Tivey (left) and Blee Williams, one of the Alvin pilots talk about the successful dives.