Carbon
Explorer floats follow ocean currents, yo-yoing back and forth in the
first kilometer below the surface of the sea, then resurfacing to report
their data and receive new instructions via satellite. Since the early
2000s a dozen Carbon Explorers have produced detailed information on the
carbon cycle in the Atlantic, Pacific, and Southern Oceans?information
that would be unaffordable and in some cases impossible to obtain from
shipboard. Working 24/7 for voyages of up to a year or more, they’ve
compiled an average of 350 kilometers (217 miles) of up-and-down ocean
profiling per float, and they continue to rack up impressive results.
Carbon
Explorers were developed at Lawrence Berkeley National Laboratory
(Berkeley Lab) by Jim Bishop of the Earth Sciences Division, who is also
a professor of Earth and Planetary Science at the University of
California at Berkeley. Carbon Explorers are based on the
temperature-and-salinity floats called SOLOs, pioneered by the Scripps
Institution of Oceanography. On July 29 Bishop’s newest generation of
Carbon Explorers, called Carbon Flux Explorers, set out from Monterey
Bay aboard the Research Vessel Point Sur, operated by Moss Landing
Marine Laboratories.
This
test of three Carbon Flux Explorers follows close on the heels of a
strenuous and storm-tossed trial of the first of the new breed in the
Channel Islands in June, but these three will be launched two hundred
kilometers out in the open ocean, in the heart of the California Current
– the same strong current that swept many a Spanish galleon right past
the Golden Gate in the two centuries before San Francisco Bay was
discovered by land.
Tough autonomous robots to measure ocean carbon
Basic
Carbon Explorers work by collecting and measuring the concentration of
particulate organic carbon, charting how these concentrations follow the
day-and-night rhythms of primary production of oceanic plant life and
feeding by oceanic animals. Much of the variation is due to
phytoplankton, plants that absorb carbon dioxide from the atmosphere.
In
2001, at Ocean Station PAPA in the North Pacific, Carbon Explorers made
the first observation that iron-rich windblown dust (in this instance
from the Ghobi Desert in Asia) stimulates phytoplankton blooms in ocean
areas otherwise rich in nutrients but lacking essential iron. Contrary
to expectations, however, the Carbon Explorers showed that these
iron-stimulated blooms last only a short time.
The first new Carbon Flux Explorer test float was deployed on June 27, 2011, in a fast-rising gale that soon forced the ship to head into the wind and leave the float on its own. |
In
2002 and 2003, a 14-month deployment in the Southern Ocean saw two
Carbon Explorers riding out an Antarctic winter in the latitudes of the
“howling 50’s” while a third spent the winter in the “screaming 60s,”
three months of that time under the ice. The Carbon Explorers helped
establish that plankton blooms can be fertilized by adding iron
artificially?even in waters poor in silica, which many plant species
need to form their cell walls. Over the next year, however, two of the
floats compiled data revealing that not much of the carbon from the
plankton blooms made it to the deep ocean for permanent storage. So far,
artificially fertilizing the ocean with iron does not look like a good
plan for soaking up excess carbon dioxide in the atmosphere.
Basic
Carbon Explorers depend on transmissometers to estimate how much
carbon-containing material is in the water, by measuring how much light
can get through a glass plate on which the material collects. This past
June, Bishop and his team set sail from San Diego aboard the Scripps
Institution’s Research Vessel New Horizon to test the new breed of
Carbon Flux Explorer and its new instrument system.
A
Carbon Flux Explorer not only measures overall carbon sedimentation, it
can determine exactly what’s in each sample, for example how much of
the carbon is locked up in calcium carbonate (so-called particulate
inorganic carbon), which is heavy enough to act as ballast to carry
carbon into the deep.
Says
Bishop, “Carbon Flux Explorers will do for the mechanisms of carbon
sedimentation what Carbon Explorers did for biomass variability. We’ll
be able to see the arms, legs, bodies, fecal pellets, and shapeless
aggregates that make up the material falling into the deep sea.”
The
camera system will measure sedimentation on hourly time scales for
entire seasons, at different locations around the globe, making it
possible to calculate how much carbon in particle form is being
delivered to the deep sea. It can also be used to measure how increasing
acidification (because carbon dissolved in sea water forms carbonic
acid) affects the formation of calcium-carbonate phytoplankton shells
and skeletons. Knowing what’s in the sediment, including what’s eating
the detritus, adds essential knowledge for understanding the ocean
carbon cycle.
“Every
day organisms swim up from the deep to feed,” says Bishop. “The process
results in a huge amount of organic carbon that rides sinking particles
from the sunlit layer into deeper water”?some 10 quadrillion grams of
carbon globally each year?“and no one has sampled this variability.”
Bishop
adds, “Until we understand bioresponses to waves and light and the
elements, all the physics and chemistry, we’re flying without a flight
plan, unable to predict whether the future will bring a strengthening,
which would be good, or weakening, which would be bad, of this so-called
ocean biological carbon pump.”
The
new Carbon Flux Explorer not only carries a new instrument, it’s
simpler, more robust, and weighs substantially less than earlier
versions; it has an all-new pressure case and uses much less energy to
power its more sophisticated computational, data storage, and
communications electronics – technological improvements that mesh with
Berkeley Lab’s research into better batteries and more efficient
lighting.
Into the storm
Launched
from the RV New Horizon on June 27th into the waters of the Channel
Islands, the test float was programmed to descend to depth and resurface
on schedule. After each collection and recording the optical system was
flushed out and new sedimentation was allowed to accumulate.
After three stormy days the weather abated and the unperturbed Carbon Flux Explorer was lifted from the sea. Throughout the storm the float had continued to surface and send its position and data to the ship via satellite. |
At
the same time, similar instruments were deployed from a surface buoy,
hung 250 meters down; after each of the buoy’s “photography sessions,”
the actual particles were collected in bottles for control and
comparison with the results from the float.
Even
as the free float and the control buoy were being launched, the wind
was rising; by that afternoon the gale had increased to 45 knots. To
ride out the storm, the ship could only point into the wind.
“We
couldn’t do any work for a day and a half,” Bishop says. “At one point
the ship took a roll that sent stuff flying – even the gear that was
tied down to a table in the laboratory went across the room when the
table ripped out of the wall.”
For
the next three days the Carbon Flux Explorer float was subjected to a
“torture test,” but, says Bishop, “no data was lost. Every time it
surfaced, within minutes it sent us its location and data via satellite –
the major hurdle it had to pass.”
When
the gale finally abated, the float and the instrumented surface buoy
were recovered, and the RV New Horizon made its way back to port,
concluding a successful cruise.
The
more severe test of the three Carbon Flux Explorers that began on July
29 is much longer; after launch from the RV Point Sur, the floats are
left to themselves for a month before the ship returns to pick them up.
“We’re
teaching the technology to crawl and then walk and then run,” says
Bishop. “Next year we’ll return to Ocean Station PAPA in the North
Pacific, where the first pair of Carbon Explorers was deployed 10 years
ago, and kick a couple of the new Carbon Flux Explorers over the side.”
The floats will transmit their high-frequency sedimentation data to
shore in real time via satellite. “This promises a new, revolutionary
view of the largely unobserved process of ocean carbon sedimentation. If
we’re lucky, we’ll get them back in six months for a complete
assessment of their performance.”