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Ocean Research


September 2012 Issue

Scientists Discover How Carbon Is Stored in the Southern Ocean
British and Australian scientists have determined how carbon is drawn down from the surface of the Southern Ocean to the deep waters beneath.

Rather than carbon being absorbed uniformly into the deep ocean in vast areas, it is drawn down and locked away from the atmosphere by currents 1,000 kilometers wide, scientists from the British Antarctic Survey (BAS) and Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) reported in Nature Geoscience in July. These winds, currents and ocean eddies create localized pathways or funnels for carbon to be stored.

“Until now we didn’t know exactly the physical processes of how carbon ends up being stored deep in the ocean,” lead author Jean-Baptiste Sallée, of BAS, said. “Now that we have an improved understanding of the mechanisms for carbon drawdown, we are better placed to understand the effects of changing climate and future carbon absorption by the ocean.”

In 2002, 80 Argo floats were deployed in the Southern Ocean to collect data on temperature and salinity, creating 10 years worth of observations. Due to the size and remote location of the Southern Ocean, scientists have only recently been able to explore the workings of the ocean with Argo floats, which today number 3,000 worldwide. The team also analyzed temperature, salinity and pressure data collected from ship-based CTD profiler observations since the 1990s. Around 40 percent of the annual global CO2 emissions absorbed by the world’s oceans enter through this region.

The rate-limiting step in the anthropogenic carbon uptake by the ocean is the physical transport from the surface into the ocean interior, CSIRO co-author Richard Matear said. “Our study identifies these pathways for the first time, and this matches well with observationally derived estimates of carbon storage in the ocean interior.”

Iron-Fertilized Algae Takes Absorbed CO2 Down to Seabed
Scientists from the Alfred Wegener Institute for Polar and Marine Research published in July the results of an ocean iron fertilization study that demonstrates a substantial proportion of carbon from an iron-induced algal bloom sank to the deep seafloor.

Fertilization of the ocean by adding iron compounds into an eddy of the Antarctic Circumpolar Current induced diatom-dominated phytoplankton blooms accompanied by considerable CO2 drawdown in the ocean surface layer, the scientists reported. The bloom developed in a 100-meter-deep mixed layer, which is much deeper than believed to be the lower limit for bloom development.

The team used 7 tonnes of iron sulfate to fertilize a patch of 150 square kilometers within an vertically coherent, mesoscale eddy in the Southern Ocean. The iron stimulated the growth of phytoplankton, and the team followed the development of the resulting bloom for five weeks.

“We were able to prove that over 50 percent of the plankton bloom sank below 1,000 meters depth, indicating that their carbon content can be stored in the deep ocean and in the underlying seafloor sediments for timescales of well over a century,” said study author Victor Smetacek.

However, because the fate of bloom biomass could not be adequately resolved in these experiments, the time­scales of carbon sequestration from the atmosphere are uncertain, the study notes. Controversy surrounding iron fertilization experiments led to a lengthy evaluation of the institute’s experiment results before publication, Smetacek added to explain the eight-year delay between the 2004 fertilization experiment and the results being published in Nature.

Team Defines Limits of Microbial Life In an Undersea Volcano
Microbiologists reported in August in Proceedings of the National Academy of Sciences the first detailed data about the biological and chemical factors that determine microbial community structure and growth rates of a group of methane-exhaling microbes that live deep in the cracks of hot undersea volcanoes.

The team used a combination of cultivation, molecular and geochemical tools to verify pure culture hydrogen threshold measurements for hyperthermophilic methanogenesis in low-temperature hydrothermal fluids from the Axial Volcano and Endeavour Segment in the northeastern Pacific Ocean. Using the research submarine Alvin, the team collected samples of hydrothermal fluids flowing from black smokers up to 350° C at the study sites.

In a 2-liter bioreactor where hydrogen levels could be controlled, pure cultures of hyperthermophilic methanogens from the study site were grown alongside a commercially available hyperthermophilic methanogen species as a control. All three organisms grew at the same rate when given equal amounts of hydrogen and had the same minimum growth requirements, establishing for the first time that these methanogens need at least 17 micromolar of hydrogen to grow.

At the Axial site, the team found hydrogen above their methanogen threshold for growth and molecular, organismal and geochemical evidence of ongoing methanogenesis, while at Endeavour, hydrogen levels were below their threshold and evidence for methanogenesis was largely absent. At the low-hydrogen Endeavour site, the scientists found that hyperthermophilic methanogens can eke out a living by feeding on the hydrogen waste produced by other hyperthermophiles.

“We hypothesized that the methanogens grow syntrophically with the hydrogen-producing microbes, and it worked out that way in the lab with a strain from the site,” said study author and microbiologist James Holden.

Survey to Decide NOAA’s Future Arctic Charting Plans
The NOAA Ship Fairweather began in August a 30-day survey mission in the Arctic to check a 1,500-nautical-mile coastal corridor from Dutch Harbor, Alaska, north through the Bering Strait and east to the Canadian border.

The mission will collect information to determine NOAA’s future charting survey projects in the Arctic and will cover sea lanes that were last measured by Capt. James Cook in 1778. Existing Alaskan coastal nautical charts, created by NOAA’s Office of Coast Survey, use sporadic depth readings reported by private vessels, some decades or centuries old.


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