For the first half of the report, including Marine Geology/Geophysics and Seismic Operations summaries, click here.
Dr. Sean Gulick of the University of Texas Institute for Geophysics (UTIG) concluded his summary of the final two weeks of the Totten Expedition:
This is a final update from the Totten Expedition (or, as we have gotten fond of saying, the “not-Totten Expedition” since we never actually reached the Totten). It encompasses the last 2 weeks of work in the Moscow University Ice Shelf area and we are now halfway through the 8-day transit back to Hobart.
Before I summarize the science side I should announce the results of the exciting (RVIB Nathaniel B.) Palmer cornhole tournament. We had four Longhorn teams make it to the quarterfinals and then the finals consisted of Bruce Frederick’s team and Steffen Saustrup’s team. It was a day of significant and fairly unpredictable deck rolls and pitches, which played to the strength of Steffen and his teammate, 3rd mate Pete. They took home the trophies.
Links to Aerogeophysical data – Over the past week the NBP doubled the total line kilometers of scalar magnetics data acquired by the project in the Sabrina Coast area. These new data were collected during seismic operations using the NBP’s towed system. Although the data require final processing, the preliminary results indicate notable changes in magnetic intensity where the continued underway gravimetry dataset reveal column-averaged density variations, as well.
Acquiring magnetics data in ice-choked areas like this is difficult because the sensor uses a 300m-long cable to maintain adequate distance from the electromagnetic noise of the vessel. Despite this, the 610km of scalar magnetics data acquired in the survey area at least partially samples most of the region. In total, 5,230km of dual-instrument gravimetry data were collected over the Sabrina Coast continental shelf. The compiled Free Air Disturbance and Bouguer Anomaly data reveal a notable density variation in the bedrock along the Dalton Ice Shelf, low-density material near the Dalton Rise, and interesting high-density regions in between. Notably, the UTIG and CMG Ops-provided GT2M gravimeter has produced exceptional data with less than 1mgal crossover error and 765m spatial resolution after just preliminary processing. The southern extent of the NBP’s coverage connects to existing ICECAP airborne geophysical data that used the same gravimeter and magnetics sensors over what the NBP’s multibeam system reveals to be exposed bedrock along the calving front of the Dalton Ice Shelf.
Moorings – Temporary mooring M1 was recovered and yielded full records for all deployed instruments. A clear tidal signal is evident, and so too is an event that we have tentatively identified as a strike by a passing iceberg. Mooring M2 stubbornly remained covered by ice. Three visits were made to the site when the available imagery indicated at least a chance that we might be able to recover the mooring.
Unfortunately, recovery was impossible, so M2 has been reluctantly left in place to be recovered next year by the R.V.I.B. Aurora Australis. Two year-long moorings were deployed, the CSIRO mooring T3 (ADCP with SBE Microcat recorders) and a sediment trap mooring. The initial idea for the last Australian mooring was for an outer shelf deployment, with the hope of targeting both a bathymetric trough feature, and outflow water.
Given the uncertainty of finding either of these within the accessible area north of the ice, the plan was changed and the mooring was instead deployed inside the polynya on an assumed inflow path between the two previously deployed Australian moorings, and at the confluence of isobaths running from the northwest and from the north. Additional weight was given to this chosen location after interesting findings from underway CTD work the previous day. The deployment was anchor first, smooth and trouble-free, and the separate gyroscope package was successfully recovered after 2 hours on the bottom. This final deployment completed the Australian mooring program for the cruise, and in each case good alignment data were obtained from the recovered gyroscope package, thus giving the landing orientation of the bottom-mounted ADCP at each location.
Mooring recoveries a year from now are keenly anticipated. Three conical single collector traps on the sediment trap mooring are complemented by a CTD with transmissometer, an SBE microcat to record temperature, pressure and salinity, and an SBE39 temperature-pressure recorder. Changing sea state and ice conditions dictated that we deploy the moorings anchor first. This required an on-the-fly redesign of the sediment trap mooring, and a new, smaller anchor provided by the ASC marine technicians and ECO deck crew. Many thanks to them for providing an essential element on such short notice! As usual, the deployments went smoothly thanks to skillful support from the ASC and ECO staff.
Physical Oceanography – Since the last weekly report, we collected about one-fourth (28%) of the total number of CTD profiles (131) from the new underway system installed on the Palmer for this cruise. A series of four short synoptic transects revealed much detail of the currents structure and water mass stratification along the path of the Antarctic Coastal Current.
Rapid cross-current CTD sections were able to effectively capture the strong nature of this southward-flowing boundary current, all the way from its entrance to the continental shelf off Sabrina Coast near the shelf break, to its westward turn near the eastern end of the Moscow Ice Shelf. Our final five rosette CTD/LADCP casts allowed us to provide water samples for calibration of the CTD as well as samples for diatoms, virus sampling, oxygen isotopes and alkalinity with dissolved inorganic carbon (for E Shadwick, ACECRC [Antarctic Climate and Ecosystems Cooperative Research Centre]) We also took the opportunity to attach the three underway CTD probes to the rosette on several occasions for intercalibration.
Water samples for calibration of the CTD salinity and oxygen were collected and analyzed on board. The work load for collecting and analyzing the samples was handled by two groups: Mark Rosenberg, Jamin Greenbaum and Natalie Zielinski managed the salinity samples, and Bruce Huber and David Gwyther managed sampling and titrating on 147 samples, to measure oxygen concentration in water collected by the CTD. For most members of both teams, the sampling and analyses were new experiences, and we thank Amy Westman, the ASC MLT, for her patience and support in helping us with the analytical equipment.
The ship ADCP system performed well throughout the cruise. We have found that the highest quality data were obtained during the seismic lines. In examining the ADCP/seismic tracks, we have found evidence of small recirculation features, most notably near station 27 as noted in the section above describing the water column to sediment activities. While the broad features of the circulation in the basin can be guessed at from the distribution of water properties, the addition of repeated ADCP sections helps us identify what may be important small-scale circulation features often associated with changes in the bottom topography.
Other notes – During the last couple of weeks and especially now on the transit we have seen some truly amazing auroras. We also had several days of remarkable winds within the polynya but got to experience the impressive ability of ice to completely dampen any waves from those winds. As we were completing our work, fall had truly arrived and we were breaking through pancake ice, shuga ice and new ice, sometimes several inches think. It actually turned out to be perfect for seismic work, as once broken through it wouldn’t close in behind us and its presence prevented any swell noise, so we got great data. The cook continues to be great in the food he prepares and the fact he was able to keep apples, pears and oranges to the end of the trip; apparently this was done by moving their position within the large fridge and rotating each piece of fruit each day to prevent any problems. Also the coffee supplies have held out. Texas Independence Day was celebrated out here, including running the Texas flag, which Jamin supplied, up the Palmer’s flagpole. We look forward to being back at port on March 16 for the end of cruise party (which Don B. will also be in town for and Mike Coffin will be coming to represent University of Tasmania) and enjoying a St. Patty’s day beer the next day.
We express appreciation for the opportunity we’ve had to explore the Sabrina Coast, and with thanks to all involved in NBP1402 scientific party, ASC support staff led by MPC Eric Hutt and ECO crew members led by Captain Sebastian Paoni. The success of this project is a reflection of the hard work, long hours and positive spirit of our team.