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RVIB Nathaniel B. Palmer
NBP-9507 Cruise Report
Week Four: November 17-23, 1995

During the fourth week of NBP9507, we worked in Bransfield Strait and made a brief stop at the Chilean Base, Frei, on King George Island. We acquired nearly 800 nautical miles of swath bathymetry, 50 nautical miles of single channel seismic data, lowered the ZAPS sled seven times, acquired four water sample rosettes, retrieved three sediment cores, took one dredge haul, and made one small boat excursion to collect uncontaminated seawater.


The Three Sisters Ridge
We started the week during a sled tow along the long middle ridge of the Three Sisters system which ended on the northern Sister. Another tow along the middle Sister was completed next. Then we took Kastenlot core #3 between the middle and north Sisters. After the core we collected swath bathymetry along the slope break on the southern margin of the central Bransfield basin en route to Maxwell Bay.

Base Frei, Maxwell Bay, King George Island
We had received confirmation the previous day that two packages we needed had been flown to King George Island by Chilean air support. We arrived at Maxwell Bay to find about 200 meters of fast ice along the shore. The Chileans had stopped walking on the ice about a week before but the only way for our party to get ashore was to beach the inflatable boat on the ice and then carefully walk across the ice. The intrepid landing party included Robert Kane, senior marine technician on Palmer, Jim Holik, the marine projects coordinator who is fluent in Spanish, Steve Stevenoski, our high school teacher from Wisconsin Rapids, Wisconsin, and Dick Von Herzen, senior science party member, from Woods Hole Oceanographic Institution. After some brief confusion as to exactly where the packages were and the obligatory social exchange and coffee drinking, the party returned via the Russian camp (Bellingshausen Base) which is next door to the Chilean base and closest to where the Zodiac was beached. Since the Chilean store, complete with tourist souvenirs, was closed because most of the base personnel were at the airport for the arrival of the new base commander, the shore party's shopping frenzy never materialized. Although each of the Russians ran a shop out of his house, the quality of the goods for sale was not great. Total time in Maxwell Bay was about two and one-half hours.

Two-fingered Jack
Immediately to the southwest of the middle Sister of the Three Sisters Ridge is a short ridge with a small volcano on its southern end. A sled tow along this ridge took about eight hours and covered about five kilometers. The tow was followed by a CTD lowering. Next, a systematic multibeam survey was begun of the area immediately to the west which had been covered by pack ice the previous week. Now the whole basin from the base of the northern slope to the southern edge was accessible. We found an amazing assortment of linear ridges and small lineated mounds and seamounts. We returned to the small volcano on the end of the Two-fingered Jack ridge where we did another CTD lowering, a sled tow and finally a dredge. The dredge was lowered on the northwest edge of the caldera rim, dragged across the crater center and up the southeast wall. The total elevation change was down about 40 meters, flat and then up about 60 meters. Unlike the first two dredges we did not recover any basalt. Rather we collected some compacted mud and some strange sea creatures. A few of the these grew attached to small rocks, which although volcanic appearing may have been drop stones. The amount of sediment cover leads us to think that this small crater may be older then the ridges and certainly older than the large volcano featured in the first weekly report. After the dredge was aboard, we headed to the northern margin of the basin, deployed the single channel seismic streamer and two airguns configured at a reduced capacity of 50 cu. inches each. We ran across a small seamount about 150 meters high on the northern margin of the basin and then across Two-fingered Jack to the southern edge of the deep part of the central Bransfield Basin. We then ran east-northeast, turned north-northwest and crossed the Three Sisters Ridge. The seismic data show faulting of the subsurface sediments, possibly as dikes were injected into the center of the basin.

Southwestern Ridge and Deception Island
After we retrieved the seismic gear we surveyed the steep northern margin of the basin and then turned south along the western margin of our previous westward survey. We did a quick survey along the axis of the next ridge to the southwest and then began a sled tow, a sled dip, a CTD lowering and finally a Kastenlot core. We then steamed towards Deception Island where we surveyed the entrance to the caldera. We then continued our multibeam survey of this region but ran into heavy fog and returned to the mouth of Deception caldera where a sled tow was undertaken. After tow we headed east to a flat spot between a circular mound about 300 meters high and a short ridge that is about 2 km long, a half km wide and 250 m high. There we lowered the sled for the last time, took the last CTD cast and a final Kastenlot core. Very early that morning during the final CTD cast, a large OSU contingent motored the inflatable boat away from the ship to collect uncontaminated seawater. We then got underway for a final multibeam survey of the northern part of the basin between the southwestern ridge, Deception Island and Livingston Island. Perhaps as an omen, we were treated to a glorious view of a cloud free Livingston Island. We could easily see a large fresh avalanche scar on the island that exposed a large stretch of rock.

Heading Home
At 1200 local on the 21st of November, we concluded science in the Bransfield Strait region and set sail for Lyttelton, New Zealand. We then ran down the margin of the Antarctic Peninsula to about 68 degrees W where we slipped off the continental slope into deep water. Soon after we crossed the first of what will be many fracture zones. The first was about 30 km west of where it had been previously identified. On the 23rd we celebrated Thanksgiving at sea. Six turkeys are history and the stewards put on a great feast.

ZAPS operations summary
The survey for hydrothermal activity in the Bransfield Strait ended this week with several successful operations. In total this project carried out 32 sled lowerings and drifts and 11 rosette casts in the eastern and central Bransfield basins covering all the major volcanic features between Clarence and Deception Islands. Strong indications of hydrothermal activity, including coincident temperature and turbidity anomalies, were detected on three different ridge segments in the central basin. A dissolved manganese anomaly was detected in a plume from one of these sites. All three sites lie along the perceived axis of rifting. This last week was spent isolating these sites with a series of tow-yo and bottom drift operations. This work will aid future geological and geophysical investigations of these areas. Bottom temperature maps were created at two of these sites which narrow the search area to a square kilometer of sea floor. We completed our survey with a hydrographic section of the waters near the breach in the caldera at Deception Island. Sixty measurements of radon gas were made onboard and over 300 water samples were collected for laboratory analyses of dissolved manganese, total dissolvable manganese, and rare earth elements. An additional 55 samples were taken for the measurement of helium concentration and isotopic composition back on shore.

222Rn in sea water
(Mark Rudnicki of Oregon State University is studying radon in seawater on this cruise. He has contributed the following section discussing his work.)

222Rn is a naturally occurring radioactive isotope with an oceanographically short half life of 3.8 days. It is formed in-situ in sea water and in sediments from the decay of its parent isotope 226Ra, thus a water column profile will typically show a bottom enrichment, the extent of which reflects the balance between diffusion from the source and advection by bottom currents. It has traditionally been used to investigate circulation and mixing problems, but has gained new applications following the measurement of large concentrations of 226Ra and 222Rn in the Galapagos Rift hydrothermal vents- the first vents discovered in 1977. Since then, vent and plume 222Rn have been infrequently measured at hydrothermal sites both on the Northern Mid-Atlantic Ridge and in the northeast Pacific on the Juan de Fuca Ridge. With a knowledge of the primary ratio of 222Rn to heat of fluids carried to the non-buoyant plume, and a measure of the 'standing crop', that is, the steady state amount of 222Rn overlying a vent-field, an estimate can be made of the high temperature hydrothermal fluid and heat fluxes.

222Rn is measured by alpha scintillation counting after preconcentration from 10-20 litres of sea water. The sample is drawn directly from the rosette bottles into an evacuated glass bottle. Ultrapure helium is circulated in a closed system through the sample- any radon present is adsorbed onto an activated charcoal column held at about -70 degrees C. After stripping, the charcoal column is heated to about 450 degrees C This expels the radon, which is then drawn under vacuum into a scintillation cell for alpha counting.

The relatively large sample volume requirement for the method results from the low concentrations of 222Rn in sea water. The typical background activity of about 30dpm/100kg (disintegrations per minute, per 100 kg of sea water) corresponds to a 222Rn concentration of about 10^^21 mol/kg. The greater the volume of sea water extracted, the shorter the counting time necessary to obtain good counting statistics (counting errors are generally <3%, overall accuracy is about 10%). After the sea water is stripped for 222Rn, is it saved for later analysis of the 226Ra content: a mismatch between the two concentrations may indicate a source for either element.

In total, 10 CTD-rosette casts were dedicated to the collection of samples for 222Rn analysis. 6 samples were collected from each rosette cast. In addition, 1 litre unfiltered, 1 litre filtered and filter samples were also collected for subsequent analysis to provide a geochemical setting for the 222Rn measurements. 60 samples of 12-15 litres were saved for shore analysis of 226Ra. The sampling was designed to target particle-rich layers in the water column that might have a hydrothermal origin, and also to sample the water masses present in the Bransfield Strait.

Lawver, Klinkhammer, and the Shipboard Scientific Party