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Word from the NBP!

12/21/2010, 12:00 AM by Lollie Garay
One of many incredible bergs in Iceberg Alley(2007)<br/><br/>Credit: Lollie Garay, PolarTREC 2007, Courtesy of ARCUS
One of many incredible bergs in Iceberg Alley(2007)
Photo Credit: Lollie Garay, PolarTREC 2007, Courtesy of ARCUS

December 20, 2010
 72°43’S, 114°12’W
 In transit to meet up with IB Oden
 Onboard RIB Nathaniel B. Palmer

Apparently the ASPIRE Team and the NBP been very busy! In Tish’s words: “Please forgive the lateness and length of this report- it has been a thrillingly hectic week onboard the NBP!”

The 20th marks the halfway point of their journey and they expect to rendezvous with the Oden later today.

Their primary objective this past week was to recover the moorings deployed in the region by Stan Jacobs during NBP 09-01.Their focus was on the western and central polynya moorings first since they had the best chance for recovery under conditions at that time.  However as they approached BSR#12 and #13 on the slope break, conditions indicated that ice cover was likely too heavy for successful recovery, and the final approach too time consuming to warrant further efforts. So they focused on getting to BSR#14.On the way, a series of XBTs were fired hourly as they approached the shelf break. 

They arrived at BSR#14 on the morning of Dec 13 and were able to successfully recover the mooring despite heavy ice cover. Tish writes: “I was completely awed by the talents of Povl Abrahamsen and Raul Guerrero as they located the site of the mooring to within about 100 m and then released it perfectly into a small hole aft of the ship.  I had never seen anything like it.  The equipment was then skillfully brought onboard by the Captain, crew, and RPS MTs. “

 A conventional CTD cast was also made at the site; and the ASPIRE team took advantage of the opportunity to assess the trough as a conduit between the polynya and the deep sea.  They sampled core physical and biogeochemical parameters from the conventional rosette and then deployed the Trace Metal CTD system and in situ pumps for additional trace metal samples.

 Late on Dec 13, the NBP broke ice for an additional 40 nm, entered the Amundsen Sea polynya, and headed toward BSR#8 near the Getz Ice Shelf.  With the help of good imagery, it took the NBP a total of about 2 days to break into the polynya from the outer marginal ice zone.  Once in open water, the ASPIRE team began sampling hourly from the ship's underway system for a suite of core biological parameters and fired  XBTs hourly.

 Upon arrival at the Getz Ice Shelf (Dec 14), Tish says: “we had beautiful, calm weather and open water”. BSR#8 was located and communicated with.  A release command was issued, but the mooring did not rise to the surface.  They  spent the day dragging for the mooring, but it was not recovered.  That evening, ASPIRE used the opportunity to sample core parameters with 2 trace metal and 1 conventional CTD casts.


Next they set their sites on BSR#7 - 70nautical miles away.  Again they sampled hourly from the underway during the transit. BSR#7 was  in the trough leading to the Dotson and Getz ice shelves, near the ice edge at the northern part of the polynya. A large area of open water at the site allowed for a successful recovery of the mooring after only a few minutes on Dec 15. More CTD casts followed.


 As they transited toward BSR#9 at the Dotson Ice Shelf, ASPIRE achieved two of its primary objectives for the week:  to deploy a glider and a moored sediment trap to the polynya .  The Slocum-Webb Glider was successfully launched on Dec 15 and sent  on a two-week mission to sample continuous vertical profiles along a series of transects within the polynya.  These 0-100 m continuous profiles of temperature, salinity, and optics (including fluorescence) will  complement the shipboard station plan.   The glider has already completed nearly 60 nm of profiles, revealing very high biomass in the upper 30-50 m of the NE polynya region.

 ASPIRE scientist Anton Post, with the collaborative assistance of many (including the Jacobs mooring team, the Crab team engineers, and the RPS MTs), deployed  ASPIRE's moored sediment trap to 800 m without a hitch.  A CTD profile was collected near the trap and then they steamed southeast to the Dotson Ice Shelf.

On Dec 16, Tish reports: “BSR#9 was located under open water, very calm seas, and bright sunny skies. The acoustic release reported that it was tilted, however, and no mooring equipment was visible in the water column using sonar. Three attempts at dragging for the mooring were made, and the Crab Team's towed camera was deployed to attempt to find any remnants of the mooring on the seabed, to ascertain the orientation of the mooring line, and shed some more light on why the mooring was horizontal. Unfortunately the mooring line itself was not sighted during three passes with the sled (though the plough marks from the first dragging attempt were visible). It is possible that shards from the flotation were visible in some of the images.”

“During a pause in the mooring recovery operation, ASPIRE successfully executed an along-shelf survey of the Dotson Ice Shelf that included bottom mapping of the seafloor and ice shelf draft, vertical ocean temperature profiles every 1.25 nm along its 25 nm ice shelf front.  This high resolution ‘curtain’ of thermal measurements was anchored by vertical ocean profiles of physical, biological and biogeochemical properties, including trace metal measurements at strategically located stations.  This effort would not have been possible without the expert boat handling by Captain and Crew, and for Kathleen Gavahan (RPS), who was at the helm of the multibeam mapping effort, and who helped brainstorm this novel application during a previous cruise NBP09-01 to Pine Island Bay.  These measurements define a critical boundary for the ASPIRE study as it will help quantify the amount of trace metals associated with basal ice shelf melt and resuspended sediments being exported through what we are now calling ‘the iron curtain’.  Further, Kathleen's preliminary calculations indicate that the Dotson Ice shelf draft is between 175 and 225 m.  Compared with data collected on NBP09-01, it also appears that the ice shelf itself has extended northward about 100 m since 2009.”

 Upon completion of the work near the Dotson Ice Shelf, the ship steamed north to rendezvous with the Oden (on Dec 20th).  The Crab Team will transfer over to complete its science plan onboard Oden.  On Dec 18th a  "long station" which included a full suite of biological process studies, net tows, and higher resolution sampling was completed successfully.   “The boat was absolutely buzzing with happy scientists” who found the water column productivity higher than expected.

They also deployed the CrabCam in the open waters of the polynya to look for  indications of benthic-pelagic coupling to form the basis for future studies in the Amundsen Sea polynya.


On Dec 19 an eastern transect into ‘Iceberg alley’  gave the team an opportunity  to measure another important water mass and trace metal endmember.   Tish adds: “The eastern edge of the Amundsen Sea polynya is a very shallow region of the continental shelf that extends north from the Thwaites Iceberg Tongue.  This shallow area is a perfect trap for icebergs, which remain stranded until enough basal melt occurs to set them adrift once again.  Our transects will help map the spatial distribution of iceberg-derived meltwater and trace metal contributions to the Amundsen Sea polynya from the east and will help determine the degree of water mass modification by these point sources.”

As you can see, it truly has been a full week of successful science for the team. Maneuvers and physical operations aside, there are long periods of lab work that precede and follow the daily operations. Long stations usually are 24 hours long so water filtering, sample preparations, data input, etc. take up most of the waking hours of the day. Its hard work but all indications are that the team is working efficiently and collaboratively:)

In the next post I hope to describe the ship-to-ship transfer between the NBP and the Oden and have new images to share!

Lollie

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Question of the Day

  • What are Polynyas and why are they important to study?

    Polynyas, are recurring areas of seasonally open water surrounded by ice.

    Energy and material transfer between the atmosphere, polar surface ocean, and the deep sea in polynas provide polar ecosystems with just the right ingredients needed for high productivity and intense biogeochemical recycling.

    Polynyas may be the key to understanding the future of Polar Regions since their extent is expected to increase with anthropogenic warming.