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Free-drifting icebergs as proliferating dispersion sites of iron
enrichment, organic carbon production and export in
the Southern Ocean

Reports from the field
Written by Alison Kelley

March 02-08 | March 09-15 | March 16-22 | March 23-31 | Early April | Mid-April

Mid-April 2009 Research activities and Iceberg Alley

Iceberg and ocean

Iceberg and ocean

Weeks before, we'd explored this area in search of another suitable large solitary tabular iceberg for our study. As you see here, the aptly named Iceberg Alley offers few views free of icebergs – though not of the specs we were initially after, later in the expedition this region provided a unique sampling opportunity that we describe in this post. (Photo by Alison Murray)

Iceberg Alley is a labyrinth of blue, white, and grey, floating islands of ice tucked along the western Weddell sea/Antarctic Peninsula and Powell Basin, just north of the Peninsula. In many ways, this city of ice resembles the southwestern desert of the United States. Imagine the famous photos of Antelope Canyon, but replace the desert's terrestrial palette of red, gold, orange, and tan with Antarctica's spectrum of aquatic blues.

Map of sampling area

Map of general sampling area.

Iceberg

Iceberg

Many of the processes that weather the fragments of ice in Iceberg Alley are similar to those that create the exotic arched and curved sandstone canyons of the southwestern United States. Do you see familiar shapes and patterns in the ice as those you might see in the desert? What common element is responsible for these intricate shapes? (Photo by Alison Murray)

We thought first it would be important to describe the research tools we have used to characterize the bacterioplankton community while we are at sea. Epifluorescence microscopy provides a way to look at the morphologies and abundance of bacterioplankton (note: Murray field team member Alison Kelley has perfected the art of shipboard microscopy – taking pictures on a rocking and rolling ship is an absolute art form!) – but given that bacteria don't have many distinguishing features that provide clues as to their metabolism or identity we need to rely on other types of measures as well. For this project, we've used two activity assays – leucine incorporation and ectoenzyme activity (EEA) – that provide quantitative assessments of microbial activity for both environmental and experimental samples. In the SEEx experiments, these real-time data allowed us to measure the effects of various nutrient additions, and observe how they compare to control samples, or seawater samples from different sources (for example, near-field vs. far-field seawater).

Alison Kelly preparing slides

Alison Kelly prepares a series of slides for epifluorescence microscopy

Vivian Peng performing ecotenzyme assay series

Vivian Peng performs a series of ectoenzyme activity assays
on seawater samples. (Photo by Alison Murray)

Alison Murray conducting assays

Alison Murray conducts leucine incorporation assays in the "RadVan"
located on the stern deck of the ship.

One of the primary goals of this project as a whole is to extrapolate what we learn by studying large icebergs to larger geographic iceberg-influenced regions. Now that we've had the opportunity to explore the influence that a large, tabular iceberg (C18a) might have on the surrounding environment, as opposed to the environment outside its influence (the reference site), what might we learn from an area populated by a large number of small icebergs of various ages and in various stages of degradation? Iceberg Alley offered an interesting opportunity to explore this concept.

Radar screen

The ship's radar display confirms our suspicion that we are, indeed, in the heart of Iceberg Alley. Each bright yellow-orange spot on the screen is an iceberg in the sea around us. (Photo by Alison Murray)

As at the other sites, everyone pitched in to support the grueling task of mapping the physical, biological, and chemical properties of the surface waters and underlying water column. How would you expect the water here to differ from the water at our reference site? Or at C18a? Thinking in terms of salinity and temperature, what trends might you expect? Dig a little deeper into the water column. Would you expect to see changes in the bacterial community in this area? Why or why not?

We had an extra opportunity in iceberg alley – a fortuitous gap in the schedule allowed us to deploy the CTD for a truly deep cast. Prior to this event, we had sampled no deeper than 600-meters, but on April 9, we sent the CTD and Nisking bottle rossette down to 1250-m, 2250-m, 3000-m, and ultimately a mere 8-m from the bottom of the ocean, at 3337-m deep. We collected samples for DNA, 3H-leucine incorporation, and microscopy. Take a look at the microscope slides from our 1250-m and 3337-m samples. Do they surprise you?

Seawater sample under microscope

This slide shows 200-ml of water we filtered to select cells larger than 0.2-µm
from seawater collected at 1250-m deep. (Photo by Alison Kelley)

Seawater sample under microscope

This slide shows 200-ml of seawater we collected at our deepest cast, 3337-m. Compare the cells and cell environment in this slide with those we see at 1250-m. There appear to be more cells in the deepest sample – is this what you would expect? If you look closely, you can see translucent patches of particulate matter. If so, perhaps the abundance of particulate carbon in the deep sea (food) supports the relatively high cell abundance and activity we observed in the deepest water. (Photo by Alison Kelley)

Other activities this week: The Shaw group, which studies rare earth elements as indicators of terrestrial sources in the seawater, finally found their dream "dirty" iceberg. While the rest of us were awestruck by the pristine beauty of brilliant blue ice sculptures, Dr. Tim Shaw sought grimy grey and black ice chunks that might harbor the terrestrial wind-blown dust and glacial debris. He, Dr. Cole Hexel, and Scott Kindleberger boarded the Zodiac to collect dirty ice fragments for radium analysis (See log).

Shaw group rides off in boat

The quest for "dirty" ice. The Shaw group rides off in search of ice littered with dust and sediment, to study its radium signature, and indicator of the ice source and age. (Photo by Vivian Peng)

Though good weather has been in short supply, none of us can complain about the daily wildlife visitations - we've even managed a few Adelie penguin sightings! Flocks of cape petrels, Antarctic petrels, and storm petrels were our constant companions as we traveled up and down the iceberg alley. For most of us, however, it was the large marine mammals that stole our attention. What is it about the site of a spout in the distance that causes each of us to stop, for just a moment, step back, and take a moment to enjoy their presence? An oil-slick back, the steady slice of a dorsal fin at the water's surface, an arched spine as she begins her dive, and the silent wave of a fluke before it glides, splash-free into the sea...even those who've been at sea many years line the deck with their cameras poised to capture the moment.

Adelie penguins

Adelie penguins settle comfortably on a floating mass
of ice and dust, in Iceberg Alley. (Photo by Alison Murray)

Cape petrels in flight

Cape petrels take flight from a resting spot on
top of an iceberg we found near sunset. (Photo by Alison Murray)

A pair of Adelie penguins drifts in brash ice

A pair of Adelie penguins drifts in brash ice alongside
the RVIB Nathaniel B Palmer. (Photo by Alison Murray)

The grandeur of iceberg alley and its numerous architectural forms provided stunning photographic material over thee last week of the expedition. To top the last full day in iceberg alley, we were treated to the first proper sunset of the trip.

Iceberg

floating ice sculptures

natural ice sculpture

A small sampling of the variety of ice sculptures adrift in Iceberg Alley. (Photo by Alison Murray)

Sun dog

Sun dogs over Iceberg Alley encircle the Nathaniel B Palmer's bridge. (Photo by Alison Murray)

Sunset over a tabular iceberg

A rare sunset casts the RVIB Nathaniel B Palmer's silhouette against a tabular iceberg.
(Photo by Alison Murray)