Icebergs such as this one carry iron-rich sediment from Antarctica out into the Southern Ocean. The darker parts of the ice contain higher concentrations of sediment. Photo: Debbie Nail Meyer © 2009 MBARI |
The first
comprehensive study of the biological effects of Antarctic icebergs shows that
they fertilize the Southern Ocean, enhancing the growth of algae that take up
carbon dioxide from the atmosphere and then, through marine food chains,
transfer carbon into the deep sea. This process is detailed in 19 new research
papers published electronically in a special issue of Deep Sea Research Part
II: Topical Studies in Oceanography.
The research team was led by MBARI marine biologist Ken Smith and funded by
the National Science Foundation. Smith, along with researchers from more than a
dozen other institutions, conducted three month-long cruises to the Weddell Sea in 2005, 2008, and 2009. By tracking
individual icebergs and deploying remotely controlled aircraft and
submersibles, as well as robotic drifters, the team was able to document a
process that had previously been suspected, but never proven.
Global climate change is causing Antarctic ice shelves to shrink and split
apart, releasing thousands of free-drifting icebergs that are carried by
currents into the nearby Weddell Sea. The new
research suggests that these icebergs carry iron-rich sediment from the land
out into the ocean. As these icebergs melt and drift across the ocean, some of
the iron dissolves in the seawater, creating a trail of iron-rich meltwater
that can be up to 19 km (12 miles) long. The iron in this water helps fertilize
the growth of microscopic algae.
During their three cruises, the team studied an area they called “iceberg
alley” in the inhospitable and sometimes dangerous Southern Ocean. Instead of
avoiding icebergs, they directed their research vessel to approach and follow drifting
icebergs the size of small towns. To follow individual icebergs, they used
satellites and GPS tracking devices that were dropped on the icebergs using a
radio-controlled airplane. They also used three different robotic submersibles
to study life on the undersides of the icebergs.
MBARI engineers, led by Alana Sherman, developed a new robotic instrument
that was programmed to sink 600 m (about 2,000 ft) below the ocean surface
while a large iceberg drifted overhead, then rise back to the sea surface after
the iceberg had passed. This instrument, called a “Lagrangian sediment trap,”
was used to collect particles of sediment, bits of dead algae, and other debris
that drifted down from the waters under and around the iceberg. This device
allowed scientists to measure, for the first time, the amount of organic carbon
sinking into the deep sea beneath a large (6-km wide, 35-km long, and 28-m
tall), free-floating iceberg.
Ken Smith (left), Alana Sherman (right), and other members of the research team examine a Lagrangian sediment trap after it was brought back on board the research vessel. Photo: Debbie Nail Meyer © 2009 MBARI |
The researchers
compared the amount of carbon sinking down to 600 m beneath the iceberg with
the amount of carbon sinking in the open ocean nearby. They found that about
twice as much carbon sank into the deep sea within a 30-km (18.6-mile) radius
of the iceberg, compared with an open-ocean “control” area.
Extrapolating their findings to the rest of the Weddell Sea, the researchers
concluded that the icebergs (both large and small) were playing an important
role in controlling how much carbon from the atmosphere was taken up by algae
and ultimately transported into the deep sea. “The role of icebergs in removing
carbon from the atmosphere may have implications for global climate models that
need to be further studied,” said Smith.
In addition to the direct measurements of material sinking beneath the
icebergs, the multidisciplinary research team performed a wide variety of
physical, chemical, and biological studies around the icebergs. Many of these
studies are detailed in the special issue of Deep-Sea Research. The 19
papers in this special issue describe:
- New methods and tools for tracking icebergs from a moving ship at
sea - Water-sampling and video surveys by remotely operated vehicles
- Aerial surveys and tracking of icebergs using radio-controlled
aircraft - The fresh water, iron, and sediment released by melting icebergs
- The design and operation of the Lagrangian sediment traps, and the
amounts of debris, organic carbon, and other materials collected by these traps - Populations of marine microbes, algae, marine animals, and
seabirds around the icebergs
After overcoming many challenges to study a few
icebergs, Smith and Sherman are working on strategies and automated tools for
measuring the effects of the many icebergs in “iceberg alley” over periods of
months to years. This would provide a much better estimate of how icebergs are
affecting the chemistry and biology of the Southern Ocean.
