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April 30, 2004
Going Deep to Study Ocean Carbon

Currently a third of the carbon humans put into the atmosphere — most of it from fossil fuels — goes into the world's oceans. But this situation may not last. A research plan recently issued by a group of scientists representing seven U.S. government agencies, titled Ocean Carbon and Climate Change, warns that "feedbacks among climate change, ocean circulation, marine biota, and ocean carbon dynamics" make the future of the ocean's carbon sink impossible to predict.

Or as oceanographer Jim Bishop of the Earth Sciences Division (ESD) puts it, "We don't know how the carbon cycle will respond to CO2 in the atmosphere. We don't even know if the feedback will be positive or negative."

That's because, while some pieces of the puzzle are relatively clear-cut — physics and chemistry, for example, can explain why cold ocean waters absorb atmospheric CO2 and warm waters emit it — "we lack a clear understanding of the solar-powered biology of the oceans," Bishop says. "Without rules for carbon transformation and sedimentation by marine life, we're flying in the dark."

If Bishop has his way, we won't be in the dark for long. Because of continual satellite monitoring and the remote sensing capabilities of autonomous instruments like Carbon Explorer SOLO floats, "we're on the cusp of a huge explosion in our ability to follow important and very fast processes in the ocean carbon cycle."

SOLO floats were originally developed by Russ Davis at the Scripps Institution of Oceanography to measure and report on salinity, temperature, and currents at depths of up to two kilometers. Bishop has led the effort to instrument SOLOs for monitoring carbon biomass and carbon flux as well. "Since 2001 we've deployed 12 Carbon Explorers around the world, racking up a total of eight float-years so far, and several are still going strong."

In the North Atlantic west of Iceland, Phoebe Lam and Jim Bishop launch a SOLO Carbon Explorer from the National Oceanic and Atmospheric Agency's research vessel Ronald H. Brown. [Photo courtesy Alexey Mishonov, Texas A&M University]

The research payoff has been momentous. In the spring of 2001, Carbon Explorer floats observed the first natural fertilization of a plankton bloom in the North Pacific by iron-rich, wind-blown dust from a storm in Central Asia — a phenomenon predicted but never seen before.

For several months early in 2002, Carbon Explorers tracked a plankton bloom created by artificial iron fertilization during SOFeX, the 17-institution Southern Ocean Iron Experiment. Operating far from any manned vessel, the floats recorded the first evidence of carbon exported to the ocean depths by artificially fertilized plankton — and kept reporting data for over a year, long after the plankton bloom had dissipated.

In 2003 Bishop and UC Berkeley graduate student Phoebe Lam launched Carbon Explorers from the National Oceanographic and Atmospheric Administration's Research Vessel Ronald H. Brown in the North Atlantic. One was instrumented with a new sensor equipped with polarizing filters to identify the phytoplankton known as coccolithophores (from the birefringent optical signatures of their microscopic calcite plates), the Carbon Explorers are still operating. The Berkeley Lab group in addition collected an unprecedented set of optical observations along an 8,000-kilometer track, from west of Iceland to the tropical island of Madeira, and on to Natal, Brazil.

Coccolithophores form milky blooms tens of thousands of square kilometers in area. "Such prolific blooms can change the upper ocean heat balance," says Bishop, "by reflecting sunlight and shading organisms lower down. No one has observed such a bloom form or dissipate — they've only been seen by satellites when the clouds part," so more information is badly needed. In 2004, in the South Atlantic, Berkeley Lab researchers are scheduled for another 8,000-kilometer shipboard journey, timed to pass right through the southern coccolithophores' spring bloom (autumn in the north).

Seen from a SeaWiFS satellite, coccolithophores bloom near Newfoundland. [Photo NASA]

Thus squadrons of Carbon Explorers have repeatedly gathered data inaccessible to other techniques. Unlike shipboard expeditions, which are very efficient at sampling spatially, the floats are relatively inexpensive, can be widely deployed, and operate continuously for long periods of time to provide unprecedented temporal information. And unlike satellites, they see beneath clouds and plunge deep beneath the surface of the water.

Development and field validation of autonomous instruments is a key element in the research strategy outlined by Ocean Carbon and Climate Change. Says Bishop, "We're working as hard as we can on this."

Having dramatically demonstrated the worth of Carbon Explorers, Bishop and his colleagues are hard at work improving them. "When I came to Berkeley Lab I said, 'Wow, this place has infrastructure,'" Bishop remarks. "'How can I apply the same talent that built detectors like the SNO neutrino detector to ocean carbon measurement?'"

Carbon Explorers were a first step, adding two-way communications to SOLO floats and equipping them with instruments to measure how much light is blocked by accumulated carbon particles. But these simple measurements of carbon flux say nothing about which particles are involved, whether "dead plankton or the leavings of larger animals," as Bishop puts it.

Jim Bishop, Derek Yegian, and Todd Wood test the Optical Sediment Trap on land before sending it to Hawaii for sea trials.

With funding from the Office of Science, Bishop has recently enlisted the Engineering Division's Zach Radding and Derek Yegian in the design, construction, and sea-testing of a sophisticated Optical Sediment Trap that makes use of "better optics, a little CPU, and a four-gigabyte flash memory — the kind of components you can find in hand-held computers and consumer digital cameras these days." The sample chamber was fabricated at the Design Center's Rapid Prototyping facility, which uses computer drawings to turn out precisely crafted, one-of-kind, free-form parts.

The new optical system, which Bishop says is designed to "bolt onto a SOLO after we've lopped its head off," concentrates naturally settling particles using a funnel and photographs the collected plants, animals, and waste particles on a clear flat surface as they accumulate — then reports the results every 20 minutes. Periodically the system cleans out the sample chamber, and repeats the process of watching the stepwise accumulation of particles. "This will help us understand the food web in the deep sea — the missing piece of the carbon-export process — in real time and on a better-than-hourly time scale."

Earlier this year, Yegian and ESD's Todd Wood tested the Optical Sediment Trap during a 10-day voyage to Station ALOHA, 100 kilometers north of Oahu, aboard the University of Hawaii's Research Vessel Kilo Moana. On the last of its eight deployments the float spent 36 hours at depth with no problems, heralding a new generation of sophisticated Carbon (Flux) Explorers.

The research vessel Kilo Moana tested a Carbon Explorer float equipped with the new Optical Sediment Trap at sea. The float is now helping to study vertical transport of carbon particles in the Pacific Ocean near Hawaii.

Bishop confesses "I love to build instruments — but I'm driven by the fact that this is the only way to learn the answers to some fundamental questions." These include the consequences of various carbon sequestration schemes "or the consequences of doing nothing."

Bishop adds, "I'm optimistic that the oceans are resilient. But we'd better find out fast, or we may be in for some nasty surprises."

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