Gorda Ridge

MBARI Expedition -http://www.mbari.org/expeditions/GordaRidge/August6.htm

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Dave Clague writes: Dive 187 explored a section of the northern Gorda Ridge hypothesized to have had an eruption in 1996. After a thorough search, we did not locate any new flow and conclude that the 1996 eruption occurred only in the more northerly of the two sites inferred to have been active.

During the dive we also examined and sampled the largest flat-topped volcano along the Gorda Ridge and found that the steep sides are entire rock talus and that the flat top has many collapsed areas characteristic of lava ponds, as we have proposed in a recent paper on similar features around Hawaii. We also collected samples from about a dozen eruptive vents, most of which appeared relatively old due to their thick sediment cover. However, the recovered samples are almost all remarkably fresh, indicating their youth and suggesting that age inferences based on photographic data alone are misleading in this region of rapid sedimentation. We also discovered several areas in the neovolcanic zone that are sliced up by abundant faults and fissures, indicating that parts of the neovolcanic zone have not been volcanically active for longtime periods.

All day Monday, the winds built up and Tuesday began with choppy seas and strong winds, so we are postponing today''s dive for a while at least, so our night coring operations will be continued this morning

Dave Clague writes: Despite high seas and winds, we completed seven successful deployments of the rock crusher overnight and into the morning. Dive 188 began about 1 pm and dove to the "Seacliff" hydrothermal site, where we had done a short dive on August 6. We mapped the extent of the entire vent field by running transects along contours and measuring distances to a beacon we had left 2 days ago.

The field is not as large as previously thought and is elongate upslope-downslope rather than along contours, as previously mapped. The field has a central region of anhydrite chimneys venting 304 degree centigrade low-salinity fluids. The temperature probe worked perfectly, although we melted some of the plastic potting that connects the probe to the cable. We collected two gas-tight water samples easily but had difficulty triggering the Ti-major water sample bottles and the two samples collected are only partly high-temperature fluid. Surrounding the area of high-temperature vents and chimneys is an extensive region of tubeworms and other vent biota. We collected a large clump of pogonophoran tubeworms, some more of the unusual blue bacterial mats (attached to both tubeworm tubes and hydrothermal crusts), some red anemones, several types of worms, several types of gastropods, more limpets, and more pycnogonids.

At the very end of the dive we used a push-core, modified by the addition of a core-catcher, to collect some grainy hydrothermal sediment. The 23-cm core consists mostly of amorphous silica, with anhydrite near the top. We will be working on ways to make it easier to deploy the major water samplers before we arrive in Escanaba Trough, where we will be collecting additional hydrothermal vent fluids as part of Rob Zierenberg''s NURP-funded program. Tomorrow''s dive will be to sample and map some small cones that are the youngest expression of the President Jackson Seamount Chain.


Dave Clague writes: Dive 189 explored the youngest volcanoes in the President Jackson Seamount chain. This chain consists of a linear array of large volcanoes with multiple, offset, large calderas. At the southeastern end of the chain is a region of scattered small cones that represent the early stage of one of the larger volcanoes. We examined and collected samples from 9 of these cones.

The ocean crust beneath these small cones is about 1.9 million years old and the cones are thought to be about 1.2 million years old. We hope these samples, erupted directly from the mantle with no shallow storage in the ocean crust, will help us understand magma generation under the seamount chain and under the nearby ridge crest.

Despite the geologically young age estimates for these cones, they have thick sediment cover and the exposed pillow lavas have manganese-oxide crusts up to several centimeters thick. These crusts made sample collection a challenge, since the manganese cements the lava fragments together. We still managed to collect 24 lava samples, many with fresh glass rinds beneath the manganese. We also collected some benthic animals including a different large, predatory tunicate than those found on the seamounts offshore Monterey; a small Venus-flytrap anemone, also different from the ones seen on the seamounts offshore Monterey; a worm with brass-colored bristles (nicknamed the "golden cockroach"); a gorgonian; and a few small sponges.

The weather continues to subside and the seas are beginning to flatten out. Tomorrow we are diving near the base of another of the President Jackson Seamounts to collect a suite of generated-generated basalts exposed on a 200-m tall ridge-parallel fault. We will then proceed to examine and sample some additional small cones near the base of the easternmost large seamount. The wind continues to subside and the seas are slowly flattening out-a trend we hope continues

Log Entry: We returned to the "Seacliff" hydrothermal vents today to collect fluidsfrom more vents and to measure temperatures at the vents, with secondary objectives to collect additional vent fauna and volcanic samples.

We had done some minor modifications to the hydraulic ram on the Raptor manipulator and on the water bottles to make triggering the bottles more reliable.

On Dive 191, we relocated the vents we had found previously and then quickly collected 2 major water samples from the vent where we had obtained good gas-tight samples 3 days ago. The temperature was measured and seemed too low, indicating that the temperature probe was not functioning properly. At this point, the Raptor manipulator failed and we terminated the dive in order to repair it and replace the temperature probe. After rapid repairs during lunch, we redeployed Tiburon on Dive 192

We again found the vents, re-measured the temperatures and got 300-305 degrees centigrade at three different vents. During this operation the pan-and-tilt on the starboard camera failed and we had to collect the water samples without being able to fully utilize the camera. Despite this handicap, the pilots collected two major and two gas-tight water samples from a different vent. We then collected a fragment of the chimney structure and the underlying hydrothermal mound in a push core, collected additional samples of the blue mat that grows on low-temperature hydrothermal crust, and cored the material beneath the crust. During the coring operation, the Raptor manipulator failed as it had on the Dive 191. We terminated the dive for repairs at this point.

Despite a day when things did not go well, we completed the primary and secondary dive objectives and several lower priorities as well. With the work done in the hydrothermal field we have now collected good fluid samples from three vents in the hydrothermal field, determined that the fluids have low salinities and temperatures of about 300 degrees centigrade. The hydrothermal precipitates at the high temperature vents are dominantly anhydrite, with minor clays and sulfides, whereas those at the lower temperature vents are dominantly barite. The mounds around the vents consist dominantly of clays and silica, after the anhydrite dissolves. We also mapped the extent the hydrothermal field and collected a wide range of vent fauna. We have found some evidence that an off-axis eruption occurred in the area, since some of the thin crusts near the margins of the hydrothermal field appear to be hyaloclastite and some thin sheet-like flows drape part of the upper slope. Rock coring continues in the axis of the rift valley at night. When we head south tonight, we will have completed 17 rock cores, of which 16 collected glass. Tomorrow we will dive in the transform at the north end of the Escanaba Trough

Log Entry: Dive 193 was located near the intersection of the Escanaba Trough and the first transform fault that offsets the ridge axis to the east. The dive started in the western valley floor and found young ridge-parallel fissures cutting the sediment cover. The underlying basalt flow was exposed in some of these fissures.

The base of the scarp bounding the valley on the west side consisted of a series of small stair steps with outcrop on the steep faulted steep sections and sediment draping the flat sections. Many of the flat sections are actually back-tilted so the traverse was up steeply followed by a gentle down slope followed again by a steep slope upwards. The outcrops in the lower section are mainly altered dikes and massive lava that has been strongly sheared and fractured. Most of the rocks from this section are low-grade metamorphic rocks with chlorite alteration and some quartz veins.

As we moved upslope the steeper sections were pillow basalts or bedded sediments. The upper section of the dive, at about 2600 m depth, consisted of alternating sediments and talus of pillow fragments with little outcrop. We also encountered a zone of hydrothermally altered lava with bright yellow-orange iron oxidation. Samples from this zone have pyrite present.

The results from this dive suggest that future work here may be able to examine the pillow basalt/dike transition in the ocean crust. At other locations along the transform fault, we may also be able to find exposed ocean-crust layer-3 gabbro. Future dives will be directed at this objective.

The dive also recovered a number of non-vent fauna including two gorgonians, neither seen on previous dives, a second specimen of the "golden cockroach" worm, a small gastropod, and a large seastar with webbing between the arms. We also saw more specimens of the Taney Seamount tunicate, the large "windsock" tunicate we collected earlier, and another tunicate, similar to the Taney one, except more flattened into a disc-shape.

Sunday''s dive will be directed at mapping and sampling one of the youngest flows in the Escanaba Trough. The Simrad data suggests that the eruptive fissure is neither ridge-parallel nor located near the neovolcanic zone.

Dave Clague writes: Dive 194 was our first dive in the Escanaba Trough area. The dive was planned to map the vents and extent of a lava flow erupted near the western edge of the northernmost Escanaba Trough that was identified by high backscatter in both Simrad and AMS-60 side-scan data.

We began the dive near the edge of the flow, but landed in mud. After crossing what we thought would be the young lava flow, but where all we saw was a mud-covered bottom, we concluded that the flow was shallowly buried beneath sediments and that we were unlikely to be able to do any work on it with Tiburon.

We modified the dive plan and drove about 1 kilometer to the western valley wall to do a transect up the scarp. The scarp consists of sandstones and siltstones about halfway up from the bottom, with lava fragments as talus and finally as a flow sequence at the top of the nearly 700-m scarp. Many of the siltstone outcrops were near vertical and fractured so that they resembled dikes or pseudo-columnar jointing in thick flows or sills. The stratigraphy exposed in the scarp is similar to that within parts of the present-day Escanaba Trough, with thick turbidites from the continental margin overlain by surficial lava flows. Considering that these sedimentary rocks are younger than about one million years old, they are remarkably lithified. We collected about 10 sedimentary and 5 volcanic rocks, as well as rock chips in 5 pushcores. We also collected some non-vent fauna including a large sponge, a flytrap anemone, and the disc-shaped tunicate.

Monday''s dive will be located at the Northern Escanaba hydrothermal site where the Ocean Drilling Program drilled a series of holes. Objectives are to collect water and temperatures of the fluids, vent fauna, and to map the hydrothermal deposits at these vents

Dave Clague writes: Our dive today went to the hydrothermal vent field at the northern Escanaba Trough (NESCA). This site was drilled by the Ocean Drilling Program (ODP) and one of our objectives was to see if the drill-holes had become hydrothermal vents. We started well, landing almost within sight of a marker left during a previous ALVIN dive and only a short distance from the first of the drill-holes we wanted to visit. This drill hole is located near some high-temperature vents, which we relocated and sampled for fluid chemistry after measuring their temperatures. These vents were still about the same 215 degree centigrade temperature they were in 1988 when they were last sampled. The samples collected for gas analyses are the first ever collected here.

The vents support large clumps of Riftia tubeworms and associated vent fauna and form chimney structures several meters tall. After completing the fluid collection (4 bottles), we searched for the first drill hole and found it colonized by vent fauna and emitting a small amount of shimmering water. We then drove down slope to a site where 108 degree C water had been sampled in 1988, but the mound was inactive and no longer supported vent fauna. We located another drill hole, discovering a previously unknown field of inactive barite chimneys on the way. This hole was not venting any fluids. We returned to the first site, where we had left a homer beacon, and then traversed across the summit of a large ridge of massive sulfide to two more drill holes, neither of which was venting fluids. We then visited the top of the hill where the hole was drilled to basaltic basement, but no fluid was venting here either. We continued to the southeast towards a region where active vents with chemosynthetic clams have been seen in the past.

On our way to this site we encountered a drill-string that was dropped during the ODP operations here. The ship was located more than half a kilometer from the site where we encountered the drill pipe, indicating that the pipe is more-or-less laid out across the seafloor rather than in a pretzel-shaped pile. The pipe was not in a straight line but had buckled and pipe was crossing pipe and we decided to avoid the area for fear of entangling the tether. At this point, we triggered the last major water sampler to obtain a background seawater sample. We then went to the northern flank of the hill and searched for new vent sites and clams. Numerous large inactive sulfide mounds were seen, but no active vent sites and no live clams. We returned to the area where we had started and collected samples of sulfide chimneys and mounds, vent fauna, and pushcores of bacterial

Compared to the submarine landscape I have seen on the Gorda Ridge (the ridge segment south of the Juan de Fuca Ridge) or on the seamounts offshore California, the landscapes on these dives have been the most dramatic I have seen on the ocean floor. Today�s dive was the third on the unusual big lava pond complex on Axial Seamount. Like the previous dives, we traversed pillow talus covered slopes with varying amounts of sediment that surrounded depressions or craters. The craters were bounded by steep cliffs of truncated pillow and lobate lavas with drainage shelves. Most of the lava surfaces we traversed were exceedingly rough, and chaotic looking, resembling �a�a flows on Hawaii, but of extremely fluid lavas. Some mounds and spires appeared to be composed of spatter, and the samples we collected confirmed this, although spatter had been presumed not to be able to form at these depths.
Biology was fairly sparse with sudden islands of abundance. We saw flocks of Flabelligerid polychaete worms congregating for reasons only known to them. Rare lava spires had colonies of sponges, corals, and crinoids

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