Dispatches from Antarctica – First Month’s Progress

David Honig is a graduate student in marine science at Duke University in the lab of Dr. Cindy Van Dover. He is participating in LARISSA, a 2 month multinational expedition to study the causes and consequences of the ice shelf collapse. He will be posting regular updates on the expedition exclusively for Deep Sea News readers!

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2 February 2010

One month into LARISSA, one month to go: a summary of progress and an outline of remaining cruise objectives

It has been one month since the “LARsen Ice Shelf System, Antarctica” (LARISSA) science team left port in Punta Arenas, Chile bound for the rapidly-disappearing Larsen Ice Shelf System. Our main target was the embayment formerly occupied by the Larsen B Ice Shelf—a 75-story-tall floating slab of ice the size of Rhode Island that disintegrated over the course of just four weeks in 2002—and the even larger Larsen C Ice Shelf immediately to the south that may be nearing a catastrophic, Larsen-B-style collapse. LARISSA plans to use the Larsen B and C Ice Shelves as a model system for understanding why Antarctic ice shelves collapse, how often they collapse, and what effect such rapid disintegration has on underlying marine communities.

Unfortunately, it is now February and we have yet to reach the Larsen B embayment. The Weddell Sea was choked with rotten fast ice in January and the Palmer could not push far enough south to reach the Larsen B. “Fast ice” forms on the sea surface (as opposed to ice shelves which form on land as glaciers) and can be anywhere from a few inches to tens of feet thick. “Rotten” fast ice contains pockets of slush. The RVIB in RVIB Nathaniel B. Palmer stands for “research vessel ice breaker,” but the ship is designed to break through hard fast ice that cracks into rigid fragments the ship can easily push aside into piles. Trying to break rotten fast ice is like pushing aside toothpaste—progress is slow and pressure the ice exerts against the hull can reach dangerously high levels.

Instead of fighting a perilous and futile battle against miles of rotten fast ice, we moved into fjords west of the Antarctic Peninsula. From there, we reasoned we could use the two helicopters on board to install monitoring equipment on the glaciers that flow into the Larsen B and C embayments. Meanwhile, our ship-bound scientists could extract sediment, invertebrates, and other data from the fjords relevant to LARISSA objectives. After nearly three weeks west of the Antarctic Peninsula, we have now returned to the Weddell Sea for a final assault on the Larsen B. It is a good time to review our progress thus far. What have we accomplished? What remains to be done?

Work thus far has contributed to several key LARISSA objectives:

Constrain past configuration of the Antarctic Peninsula Ice Sheet. The Larsen Ice Shelf System is the remnant of a vast ice sheet known as the Antarctic Peninsula Ice Sheet (APIS) that stretched into South America 20,000 years ago during the peak of the last ice age, a time known as the Last Glacial Maximum. Since the last glacial maximum, the APIS has retreated to its current position. How quickly did the retreat take? Was it punctuated by collapse events similar to the disintegration of the Larsen B?

Our marine and quaternary geosciences team has extracted sediment cores from several locations west of the Antarctic Peninsula that address these questions by constraining the lateral extent of the APIS through time. By examining diatom abundances in radiocarbon-dated sediment layers, we can infer when the area was covered by ice shelves (no diatoms present) and when it was not (diatoms present).

To measure the vertical extent of ice shelves through time, geologist Dr. Greg Balco (UC Berkeley) has recovered “glacially-rafted erratic”—rocks whose mineralogy do not match the underlying bedrock because they were deposited by receding glaciers—from an ice-free plateau north of the Larsen A. These rocks will undergo cosmogenic dating, a technique that infers how long a rock has been exposed aboveground by measuring the concentration of exotic, cosmic ray-generated isotopes locked within its crystal matrix. Exposure-times of erratics can be used to infer how long ago glaciers receded.

Two GPS stations were installed west of the Larsen B to measure the upward surge of the Antarctic Peninsula as it rebounds from the massive burden of the APIS. The speed of this rebound can be used to infer APIS thickness, complementing cosmogenic dating work.

Multibeam bathymetry—a technique that measures seafloor depth via sound waves—has been conducted constantly from an instrument mounted on the bottom of the ship. Seafloor morphology reveals where ice shelves interacted with the seafloor. Smooth, scoured seafloor was once sanded by ice sheet while rougher seafloor was not. Lines between rough and smooth seafloor are known as “grounding lines” and indicate where the past ice shelf transitioned from being in contact with the seafloor to floating on the ocean surface.

Link past APIS configuration to paleoclimate. Correlating past climatic conditions with Larsen-B-style collapse events during the APIS retreat will lend insight into why ice shelves are disintegrating today.

After over a month of drilling through the heart of a massive glacier perched on the spine of the Antarctic Peninsula, a contingent of LARISSA glaciologists extracted the last segment of a 445-meter-long ice core that contains climate data stretching back into the Holocene. The core will receive a battery of tests to parse-out past climate conditions. To name a few: bubbles locked in the ice reveal the concentration of gases such as CO2 in the ancient atmosphere, oxygen stable isotope ratios approximate past atmospheric temperature, and concentrations of certain salts in the ice can be used to infer net patterns of atmospheric circulation (e.g. high chloride concentration suggests winds blew primarily from cold ocean to the west rather than ice and land to south and the east).

Characterize glacial response to ice shelf collapse. Ice shelves are glaciers that have spilled out into the ocean. The Larsen B was fed by several tributary glaciers that now terminate at open water. How do these glaciers respond to loss of the Larsen B?

In addition to indicating lateral ice shelf extent through time, sediment cores record climate data. Molecular remains of pelagic microbes called “crenarchaea,” for example, approximate sea surface temperature. Crenarchaea predictably alter the composition of membrane lipids to suit water temperature and, since the dominant flux of certain lipids to the seafloor is of crenarchaeotal origin, the distribution of these compounds in sediment layers reflects the mean sea surface temperature when the layer was deposited. Examining crenarchaeotal lipid compositions and other past climate proxies in sediments extracted from west of the Antarctic Peninsula will provide insight into ancient ocean conditions and will complement the atmospheric record obtained from the ice core.

• To address this question, our glaciology team has thus far installed one of six planned AMIGOS (“Automated Meteorological-Ice-Geophysics Observation Stations”) on glaciers feeding into the Larsen B embayment. The AMIGOS will simultaneously track glacier movement, appearance (e.g. presence/absence of melt ponds, large cracks, etc.), and weather to understand how the effects of ice-shelf collapse propagate out onto the surrounding land.

In addition to ticking-off a few geology and glaciology-related cruise objectives, our time west of the Peninsula allowed us to complete . . .

Opportunistic oceanography in Western Antarctic Peninsula fjords. Ice-shelf disintegration is propagating south along the eastern side of the Antarctic Peninsula. Are glaciers that empty into fjords on the western side of the Antarctic Peninsula experiencing a similar phenomenon? How are marine ecosystems changing in response? Our marine ecosystems team took advantage of time in the fjords to capture a snapshot of currents, primary production, and benthic diversity using the following tools:

An acoustic Doppler current profiler (ADCP) mounted on the hull of the Palmer and another lowered into the water column measured shallow- and deepwater currents, respectively, by bouncing sound waves off particles in the water column. The orientation of the ADCP and the delay and Doppler shift in the reflected signal indicates the speed and direction of particle and, in turn, the current.

A conductivity-temperature-depth (CTD) sensor package profiled water column chemistry.

A fluorometer on the CTD measured phytoplankton concentration.

A rosette of twenty-four Niskin bottles on the CTD recovered water samples from depth, allowing us to perform more elaborate water column measurements (e.g. nutrient concentrations) and identify species of phytoplankton living at different depths.

A yo-yo camera captured high-resolution images of a 1 m2 block of seafloor, allowing us to quantify benthic diversity.

ROV Genesis aka Suzee recorded video transects of seafloor life and collected biological samples of special interest.

A megacore gently collected twelve 4″ x 20″ tubes of sediment that allowed us to quantify diversity of “infauna” (invertebrates living in the sediment).

All in all, most of the work we had originally planned remains to be completed (see map above). Notably absent from our list of accomplishments is any mention of cold seeps, sampling fauna from which is the primary reason I am aboard (more about this in a later entry). Luckily, our patience may have paid off: the rotten fast ice is softer and laced with floes of pack ice (floating ice chunks)—that the Palmer may now be able to navigate. Surrounded by the deafening roar of pack ice scraping against the hull as the Palmer plods southward, we are keeping our fingers crossed that the Larsen B will be navigable and our planned fieldwork can be completed.