This is another day (1 August) under the influence of a large low pressure systems that has been over us for the past couple of days. Pressure gradients are low, however, so winds are relatively light. We just arrived at station #18, (-67° .29.475S; -69° 31.483W) and it is first light at 0846. Wind is out of the northwest (315) at 7 to 10 kts and the air temperature is -9.8°C. The CTD has been deployed in an open lead in the pack ice and half a dozen seals are swimming in a lead not far away. A large iceberg sits a 1/4 mile to our stern and there are several more in the vicinity. The cloud deck is just high enough so that the flanks of the rugged mountains of southwestern portion of Adelaide Island are visible to 15 miles or so to our east.
This report will cover two days of work, 30 and 31 July.
During 30 July, the work along transect 2 was completed and transect 3 started in weather that was mostly cloudy and windy - around 30 kts out of the northeast (045) - with a light snow off and on throughout the day. Most of the day was taken with work at the offshore deep water station #12. In addition to a CTD, the MOCNESS-10 was towed for the first time by Jose Torres. It was a long deep haul to 1000 m. As is now becoming expected in towing in the pack ice, the towing wire had a number of ice hang ups while shooting and hauling the net, and on a couple of occasions to free the wire, the ship had to stop and back up until the net was hanging almost straight up and down. But the tow was successfully completed. Not successful was the towyoing of BIOMAPER-II. The same problems with the towing wire being caught by large ice cakes in the wake that have affected the net operations occurred with BIOMAPER-II during the transit between stations #12 and 13, resulting in the towed body sustaining serious damage and needing substantive repairs, as described below.
The 31st of July found us working on the middle to inner portion of the continental shelf on transect 3. While the air temperature was around zero, which made for much more comfortable working conditions on the deck, it was very cloudy and light snow fell most of the day, associated with a low pressure system that had moved in over us. Winds were moderate all day, in the range of 12 to 18 kts. Two stations, #15 and 17, had a number of data collecting events scheduled besides the customary CTD. At station #15, a 1-m2 MOCNESS tow was successfully completed. After the MOCNESS tow, Jose Torres and company went diving under the ice to collect krill for experimental work on the Palmer. While they were there, a crab eater seal came in under the ice to see what they were doing and so also did an emperor penguin, which popped up next to the zodiac and then hopped up on the ice to see what was going on. Following the dive, the ice collectors, led by Frank Stewart, went to work on an ice cake next to the ship. They were put onto the ice using a personnel basket attached to the crane to get them out away from the ship. During the work at station 15, we learned that the L.M. Gould was stuck in the ice some 60 miles away just to the south of Adelaide Island and for a while it looked like we would have to steam down to assist them break free. But a couple of hours later, they reported that they were freed and did not need our assistance.
At station #17, in addition to a CTD, a 1-m2 MOCNESS tow and an ROV deployment were successfully completed. This station had quite a lot of open water in leads that were interspersed between pack ice that was a mixture of some that was newly formed and some that was older. The underside of the newly formed ice was smooth while the older had a more rugged bottom. The ROV was driven by Scott Gallager back and forth under the two bottom types looking for differences in krill abundance.
John Klinck reports that on 30 July, the CTD group did one CTD cast, but it was a deep one. This station completed the second line of stations (460 line). We have also begun to look at nutrient concentrations and calibration for the CTD.
Station #12 (3090 m) was the first full-depth station off the shelf. Conditions here are not remarkably different from those over the shelf, in contrast to many other continental shelf systems. The mixed layer extends to about 90 m with almost no vertical structure. Below the pycnocline is the warm layer with temperature maximum at about 300 m, the source Upper CDW for this shelf. The O2 minimum is above this layer at about 250 m. The weak salinity maximum, associated with Lower CDW occurs at about 1000 m but this is only evident on a T-S plot and is hard to see on a plot of salinity vs. depth. A deep temperature minimum occurs at about 2800 m; a deep O2 max occurs at about 2900 m. This combination of temperature minimum and O2 maximum indicates abyssal water leaking from the Weddell around the tip of the Peninsula and southward along the coast. This is only of pedantic interest as these bottom waters do not affect the western Peninsula shelf region.
Analysis of all data from this section shows an intrusion of warm water (UCDW) to about the mid-shelf. The low salinity mixed layer, which is at the coast on the 500 line, is displaced a bit away from the coast on this line. This shift is likely due to strong vertical mixing at the coast, increasing the surface salinity at the coast. The pycnocline is basically level across the shelf, but the isopycnals spread at the coast indicating enhanced mixing there compared to that in mid-shelf or offshore.
A first comparison of dissolved oxygen with the CTD sensor is promising. The CTD O2 sensor gives concentrations that are a little higher than those determined from titration. At high concentrations (around 8 ml-l) the difference is 1-2% while at low concentrations (4 ml-l) the difference can be as large as 10%. As we gather more samples, we will analyze these differences. However, given the large differences in oxygen in various layers, the CTD clearly indicates water masses. Age or mixing calculations with these data will require a full calibration analysis.
A first look at nutrient measurements is taking place and will be discussed in a later daily report.
On 31 July, the CTD group did 4 casts (stations #13-16) along the 420 line. These stations extend from the shelf break to the inner shelf lacking one station (17) to complete the section. In addition, we have started to look at and plot the nutrient samples taken on the first dozen stations. Finally, comments are included regarding oxygen and salinity calibration and ADCP records.
Station #13 (844 m) is at the shelf break on the 420 line. The mixed layer extends to 100 m without structure. The permanent pycnocline spans 150 m down to the oxygen minimum zone (from 300 to 460 m). There are two temperature maxima (340 m and 420 m) within the extended O2 min layer. A weak salinity maximum occurs at 700 m indicating LCDW. One temperature reversal occurs in the middle of the pycnocline spanning about 20 m of thickness. This seems to be significant lateral water mass intrustion, but the details are not clear at this point.
Station #14 (525 m) has two mixed layers each with a thickness of 50 m. Although the temperature is constant to 100 m, the salinity increases in a step by .05 to .07 at 50 m. The temperature maximum occurs at about 270 m. There is considerable layering structure in the pycnocline and an abrupt temperature drop below the temperature maximum.
Station #15 (510 m) has a mixed layer down to 90 m with no structure. Continuous layering occurs in the pycnocline with layer thicknesses exceeding 5 m. A weak temperature maximum exists at 280 m. A deeper temperature minimum (250 m) and temperature maximum (260 m) indicate lateral intrusion. Oxygen increases abruptly at 260 m by almost 0.2 ml-l separating two different water types. The source and movement direction of these waters is at present unclear.
Station #16 (512 m) has a mixed layer to 80 m, but there is an increase in salinity from 40 to 50 m producing a two layer mixed layer. A warm (0°C) and salty layer of water occurs just below the mixed layer (90 - 110 m) which looks like a horizontal intrusion of water from offshore (due to the higher temp and sal). The temperature maximum occurs at the base of the pycnocline at about 170 m. Some variation in temperature occurs throughout the remainder of the cast. As luck would have it, the CTD logging stopped (computer locked up) after the sensor was below 200 m, so the CTD was brought to the surface and a second cast made. Both records have the same feature, so this is not a sensor glitch but is real. I have not seen such a change in the temperature in the pycnocline in all previous data from this region.
Station #17 (298 m) is near the coast with a mixed layer to about 100 m, but there is a clear linear salinity, hence density, gradient in the mixed layer. A temperature maximum layer occurs below 200 m to the bottom. There is some indication of temperature reversals below 150 m.
Without looking at the last station, it is clear that this is a strange section. There seems to be active layered interleaving throughout this region with some layer thicknesses exceeding 10 m. The temperature maximum layer is either split or shifts in depth from one station to the next. This line of stations is along the north side of the depression ("canyon") that we have focused on which may have some bearing on the different character of these stations compared to the other ones on the shelf. This issue clearly needs consideration as more data are gathered.
Nutrients (nitrate, nitrite, silicate, phosphate and ammonium) are available for the first two sections, thanks to the efforts of Rob and Yulia and the process of making section plots is underway.
More oxygen titrations have been compared to the CTD sensor with the result that the response over the range of 4 to 8 ml-l is parallel and linear, but the CTD gives concentrations that are higher by about 0.15 ml-l.
We have started to measure the salinity of the bottle samples for calibration of the CTD. After having run about 45 samples yesterday, we find that both salinometers are showing instability in that the first two measurements are close and then the third and following measurements have increasing values. Because of this neither salinometer can be standardized. Both salinometers were worked on at the port call. Jonnette is trying to bring these sensors back to proper behavior. Thus, we have limited information on salinity calibration, at this time.
We have been struggling with acoustics in the various forms that we use them throughout this cruise. In addition to cluttering the sea beam data with junk (Don't you love to ping edit?), ice is keeping the ADCP from obtaining reliable statistics. The ping recovery rate is 5 to 10%, so the velocities are unreliable while underway. While on station, observation are taken so maybe some information will come from this sensor. These data are stored and backed up but may require extensive after cruise work to be useful.
Ari Friedlaender reports that on 30 July, observations were limited to 1500-1600 as the ship was stopped at station 12 for the better part of the daylight hours. Ice conditions were similar to the previous day, although more new snow had fallen, and there appeared to be more small floes about. Visibility was limited to 0.5 miles at best, as the weather had changed to 30 knot northeasterlies with fog and blowing snow. The air temperature rose continually throughout the day from -12 to -3.0° . Likewise, barometric pressure fell precipitously all day. No sightings were made during observation effort.
Ana Sirovic reports that on 30 July, she deployed only one sonobuoy (14th of the trip), as we were approaching station #12. (One of SIO moorings is very close to this station - at 65° 58.4 S and 71° 4.1 W - so she wanted to have a recording from its vicinity.) The sonobouy was monitored for five and a half hours. Again, no whales or other biological noise was heard in the lower frequency range (up to 6 kHz) from this buoy.
On 31 July, two difar sonobuoys were deployed. The first one was deployed 2.8 mi before station 15. Its signal was lost after 42 minutes of transmission. The next buoy was tossed in just as we were leaving station 15 and it stopped transmitting its signal quite suddenly after only 8 minutes. That ended the buoy deployments for the day. No biological noise was heard on any of the buoys. The rest of the day was spent stripping sonobuoys for future deployment (taking them out of their packaging and disposing of all the superfluous plastic) and going over some recordings from previous days (especially 29 July, when two minkes were sighted). Again no biological noise were found in the recordings.
Chris Ribic and Erik Chapman reported that on 30 July (JD-210), they
surveyed for just 43 minutes in deep water off the shelf as we steamed
back to station 12 from the termination point of the 10-m2 MOCNESS.
Ice conditions were between 8 and 10 tenths and the primary ice cover was
first year cake ice. During today's short survey, they saw a relatively
high number of Snow and Antarctic Petrels within the 300 m transect. These
species are typical of heavy ice within the pack. Off the transect, a single
Southern Giant Petrel was seen.
|Southern Giant Petrel||2|
They surveyed for an hour last night from the bridge and made no sightings.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, and S. Gallager):
The ice pack is beautiful and wonderful to look at, but it comes with a set of hazards for towing that we did not fully appreciate until now. On the evening of 30 July around 1800, 40 minutes after being deployed at the end of station #12, we got a lesson in this big time. BIOMAPER-II was down around 200 m with 480 m of wire out, when a large ice cake caught the wire and took it aft at the surface. With the ship's speed at about 5 kts, within a minute or two, the fish was pulled up from depth and into the bottom of an ice cake. The last pictures taken by the VPR before it died were of the underside of the ice that it ran up into. Shortly after the VPR died, power to the fish was lost. There was not enough reaction time to get the ship stopped even though those "flying" the towed body knew the fish was coming up too fast and called to have the ship slow down. It happened in a minute or two. Once the ship did get stopped, we went through what is becoming a ritual of backing down the ship while pulling in the wire until we get to a place where the wire is where it is supposed to be near the transom and the ice can be flushed out from around the wire using the ship's propellers. When we got the fish on board. It was a mess. The tail assembly was gone and the feet too, The VPR support frame was mangled and the cameras damaged, at least one may not be fixable on this cruise, but Scott Gallager is working on it. The towing bail was twisted badly. There were a number of electrical cables running around the telemetry bottle that were cut. All of the acoustics, however, were OK as were most of the environmental sensors except for the transmissometer that got destroyed - top lopped off.
We immediately set out to do repairs, and the repair work continued into 31 July. By early on the 31 July, the electrical system was back up and running, and a number of the mechanical parts were repaired. The Raytheon techs working with members of the ships engine room have done a remarkable job of straightening out the towing bail and getting the camera mounting frame straight enough to be put back into service. A back-up tail assembly has been installed, and a new horizontal stabilizer and a pair of elevators were fabricated in the ship's shop. A pair of "feet" were also constructed and installed on the towed body. We anticipate having the towed body ready for deployment again near station #19.
One-m2 MOCNESS tows were taken at stations #15 and 17. In both cases, we had trouble with the wire being caught by a ice chunks and the dragging the wire back at the surface. But we have instituted a policy of not paying out more wire than water depth, so even if we have to back the ship down to free the wire, the net system will not hit the bottom. But this limits our tow depth to much less than wanted, so to get around that limitation, we have been slowing the ship down and letting the net sink to the desired depth after all the wire has been payed out. Then, the ship's speed is increased to get the net coming up and the nets are opened and closed as the net system kites up. When the net plateaus, hauling of wire is begun. For both tows, this strategy worked well, in spite of snagging the wire on ice on a number of occasions during the tows.
The MOCNESS tow at station 15 went to 400 meters and the one at station #17 went to 250 m. At station #17, ctenophores were observed at all depths. The 150-250 m depth range was characterized by chaetognaths, Euchaeta, and Thyanoessa. The 100-150 m range contained copepods, and furcilia. The 75-100 m range contained very little. Furcilia, Thyanoessa and pteropods were seen in the 50-75 m range and the upper 5 m contained E. superba and small copepods.