UTIG’s Sean Gulick sent a final report covering the last two weeks of research work with the Totten Expedition aboard the RVIB Nathaniel B. Palmer. We have divided the report for length considerations, with Part I below covering Marine Geology and Geophysics, and Seismic Operations. Part II will be posted on this blog tomorrow (March 12).
Dr. Gulick’s report:
During our last approximate two weeks of work, we have continued our interdisciplinary focus on the Moscow University Ice Shelf polynya, trying to decipher the short- and long-term history of this very complex part of the Antarctic margin. Our survey region has expanded, with a dedicated effort (1) to define oceanic circulation patterns, pathways, and processes, (2) to link modern oceanography to the sediment record, (3) to utilize strategic coring to understand depositional processes and histories of features observed via surface swath and sub-surface mapping, (4) to understand controls on the flow of sub-glacial meltwater, and (5) to map out the geologic evolution of the Sabrina Coast, as imaged via multi-channel seismic data acquisition.
The ice fronting the Totten Glacier withdrew sufficiently to warrant a third attempt to gain access to the glacier front, if only for a brief period. However, we have learned that ice conditions change rapidly with changes in intensity and direction of the wind. Unfortunately, en route, increasingly difficult ice conditions prevented our westward progress and we returned to the polynya.
Marine Geology and Geophysics – During the final two weeks, we completed multibeam mapping of the juncture of crystalline bedrock with the sedimentary strata of the continental shelf. This mapping was done in order to resolve the nature of bedrock channels which appear to serve as subglacial meltwater conduits during times of expanded ice sheet cover. The channel networks are complex, reflect structural character of the crystalline rocks (i.e., joints, faults, and contrasts in rock type), and range in depth from hundreds to over a thousand meters. It is important to establish the geometry of these features in order to understand how subglacial melt water sculpted and was/is conveyed towards the margin, as offshore features give evidence for periodic outbursts of flowing water of a considerable magnitude. We feel we have sufficiently characterized this seafloor terrain with an extended multibeam survey, despite continual problems with the Kongsberg EM120 multibeam system. In the sedimentary terrain, we continued to work with the UTIG group to help define the geologic evolution of the margin, which is quite remarkable from other Antarctic margins. The diversity of seismically imaged deposystems and association with a multitude of new types of seafloor morphologies has had us continually discussing various interpretations while the data stream evolves.
It is quite exciting science – challenging, but exciting. One of the clearest things we have observed is that the margin has indeed preserved a continuum including: non-glacial, partly glaciated, to fully glaciated depo- and erosional systems. We suspect that this is due to the rather pronounced subsidence history that this margin has experienced in combination with its juxtaposition at the seaward end of the Aurora subglacial basin. To this end we have continued our strategic selection of single barrel jumbo piston cores to acquire samples of strata that outcrop on the seafloor. Two such samples were acquired below and above a prominent fluvial deltaic complex that lies in the part of the section deposited just prior to any evidence of glacial processes. We also completed the multibeam mapping and added a pair of gravity cores on the top of two moraine complexes that resemble feathers on a bird wing (feather moraines); these cores were acquired in order to characterize the physical properties and till rheology, aspects that will help us to understand the depositional processes leading to the development of these rather unique features.
Our earlier discovery of “mega-dune” bed forms at 600m water depth was followed up with a bottom camera survey and a Smith-McIntyre grab. We recovered well-sorted medium to coarse-grained sand, which appears to blanket the seafloor with a surficial scattering of ice rafted boulders (all heavily encrusted with epifaunal organisms). Subsequent short core efforts also revealed that the sand is interbedded with mud in the near subsurface. Hence, the “sand plains basin” appears to have a more complex history of sand deposition and dune formation than we had originally supposed. This type of seafloor appears to be even more extensive in other basins, those that were partly imaged and which lay farther seaward on the continental shelf. We continue to work on correlations amongst our growing number of jumbo piston cores and kasten cores using physical properties and lithofacies descriptions. There appears to be a systematic succession of lithofacies related to alternating biogenic vs. fine grained siliciclastic deposition along with a rather “sand-rich” facies transition from glacigenic (till) deposits. We are confident, based on the number of foraminifers observed in Holocene sediments from the region, that we will be able to date the deglacial transition within the inner shelf based upon the cores we have collected.
However, relating the detailed chronology of the complex of “feather moraines” to a firm absolute time scale will be more challenging due to the multitude of surfaces – far too many for us to core in the time available. Several larger and more typical morainal bank systems have also been mapped yet they similarly provide rather unique signatures including a spectacular bank collapse and slide block terrains wherein large portions of the grounding zone crest has detached and slid seaward by several kilometers producing a rubble field and glide block terrain in the downslope (seaward) direction. The causal mechanism(s) for this instability are being discussed.
We successfully completed two interdisciplinary water columns to sediment sampling stations in the northeast region of the polynya. At these stations, we used a suite of sampling devices to characterize the physical oceanography, water column properties, and surface to deep sedimentation. Such studies will enable ecologic investigations, paleoceanographic proxy development, and will enable us to improve understanding of how and why sediment is deposited at each site. We were originally drawn to the locations for jumbo piston coring because the Knudsen 3.5 KHz data revealed 8 to 14 meters of sediment at each site. The geophysics team worked closely with the marine geology team to identify precise coring targets. At Station 27 (~540m), we collected two 3m kasten cores (2.9 and 2.7m) comprised of laminated diatomaceous ooze and mud; the second core recovered an intact sediment water interface.
These deployments were followed by the deployment and recovery of a 13m Jumbo Piston Core (JPC). Five days later, we returned to the same site and deployed a rosette CTD, two McLean pumps, and a Megacore. The rosette CTD was intended to characterize the physical oceanography of the region and revealed warm modified Circumpolar Deep Water at depth. ADCP data revealed a local recirculation feature associated with the local bottom topography. The McLean pumps were then deployed for three hours at 480m (core of mCDW) and at 25m (surface) to filter large volumes of mCDW and surface water at 0.2ïm for DNA/RNA and geochemical analyses of viruses, archaea, and phytoplankton in the water column as well as those associated with each water mass. The deep pump achieved minimum flow and shut down after pumping ~50L. The surface pump pumped 380 L before reaching minimum flow, and shutting down. Upon recovery, filters were removed and stored at -80°C.
Upon completion of pumping, we deployed a megacore and recovered 11 tubes with 35cm of sediment in each tube. Megacores were subsampled for DNA, foraminifers, diatoms, sedimentology, radiocarbon, 210Pb, and organic geochemistry. Several days later, we returned to the site for a third time and recovered a 5.0m jumbo kasten core with an intact sediment water interface. This core enabled us to extend the DNA/RNA sampling downcore to 5m; this type of sampling must be conducted immediately upon recovery and samples are immediately frozen and kept at -80°C. At the second water column to sediment station, we collected an 8.76m jumbo piston core and a 2.5m long kasten core that also revealed a sequence of laminated diatom oozes and muds and an intact sediment water interface. We then conducted a rosette CTD and discovered the warmest water at depth in the polynya (~450m). We also deployed a megacore and recovered 12 tubes of sediment (37cm each) with excellent sediment water interfaces; one tube included a small fish. Following the megacore deployment, the McLean pumps were deployed for 3 hours at depths of 450m and 25m to collect particulate matter associated with the mCDW and surface waters, respectively.
Seismic Operations – We acquired an additional 266nm of seismic reflection data. All of these lines used the new operational protocols in terms of minimizing the times the guns were on deck in the cold temperatures, use of no-tox in advance of deployment, and firing gun 3 a few times on the turns. Additionally the adjustments on the mount for the blast phone on gun 2 proved stable. These changes were so successful that we had no failures of guns 1 or 2 for the all of these profiles. All of these profiles continued to be acquired with a 5s shot rate, sampling interval of 0.5ms, and record length of 2.5s.
Lines 14 through 18 were acquired to image the more eastern area closer to the Dalton Ice Tongue. Lines 14 and 15 crossed from the older sedimentary section across the deeper embayment that leads to the western glacial trough and onto outcropping basement. Line 16 was shot from shallower water southwest towards the deeper area. A brief loss of communication with the geode resulted in a loss of a few shots and duplicate shot point numbering requiring ~2/3 of the profile to be named 16a. Line 17 was a longer line across sedimentary older sedimentary sequence and the region of shallow eroded features that may be dune-like features before turning back west across a moraine lying closer to the ice tongue. A surprising result was a clearly imaged delta complex on this profile within the dipping older strata. This last line was Line 18 which was acquired first west and then southeast and thus during processing was broken in 18 and 18a. All of these lines showed a significantly more eroded seafloor with little till and virtually absent Holocene strata in this eastern area.
Lines 19-25 were acquired to first cross our existing profiles within the older strata and then turn north to cross one of the large moraines including an area of intriguing topography originally thought to be crenulations and then continue northwards into an area of newly opened ice well north on the shelf. Line 19 was westerly followed by Line 20, which was southeasterly, to link to Line 21 that was the long northern line. The unusual topography turned out to include evidence of slump scars consisting of till where slump blocks, also consisting of till, were observed in the bathymetry to the west of the line. Alone Line 21 we continued to observe the deeper dipping strata getting progressively younger as we proceeded northwards. Within the upper younger strata there were 2-3 erosional surfaces that were clearly both glacial in origin and dipping northwards along with the underlying older strata; these glacial surfaces were distinctly different in scale, facies, and dip from the overlying regional unconformity and moraines and till that make up the seafloor related features. Within the northern part of the shelf more difficult ice cover resulted in Lines 22-25 being shot eastward then looped back southwestward and then crossed back west.
Both Lines 22 and 25 cross an ~2 km wide glacial trough the tended northwards towards the shelf edge. During Julian Day 57, four hours of daylight were available for seismic operations during which Line 26 was acquired to cross previous lines in a westerly direction but farther north than previous crossing lines had been acquired. This line was acquired to cross over one of the delta features in the subsurface and end by crossing a potential buried glacial moraine observed on Line 21.
The next morning seismic operations resumed with the acquisition of Lines 27 to the northeast crossing Line 21 a little farther south within the region of slump blocks and then turning northwest along Line 28 crossing Line 21 within a region where the till was thinner. While acquiring this line, a new ice image was transmitted to the ship that showed the region to the west of all our previous profiles was ice-free. Thus Line 29 was acquired southward keeping about 1 nm from the ice edge and this line reached to the entrance to the deeper western glacial trough. Line 29 showed three clear back-stepping moraines, the southernmost one of these was the edge of a “feather moraine.” When ice was too thick to proceed farther south, Line 30 was acquired back east across the thickest till adjacent to the deeper contact with basement within which numerous glacial surfaces were observed.
The final seismic survey was undertaken during our transit out of the polynya and included Lines 31-35. At the start of Line 31 we had two marine mammal shutdowns, both for less than 7 minutes. The first was due to a seal on an ice floe and the second due to a seal in the water, but neither were in the water within the 100m radius when the guns were operating. Line 31 crossed 18a and Line 21 heading northwest to provide an additional tie line and then due to ice was forced to turn northwards on Line 32. We then turned east on Line 33 when a ship-wide power outage occurred. The repairs and safety checks on the power outage resulted in missing the critical data to cross Line 21 at the northernmost moraine on that line. Therefore we chose to circle back west during marine mammal observations and re-drive to the east on Line 33a. Once we crossed Line 21 and unable to proceed farther east due to ice from the Dalton Ice Tongue, we turned northwards on Line 34. This line showed parallel reflectors within the moraine and the underlying dipping reflectors continuing to young towards the shelf edge. We chose for our last line, Line 35, to drive northwest and were able to continue until the edge of the moraine associated with the Dalton system.