USC Robot Explorers Find Enigmas On The Ocean Floor

Dr. Berelson, an associate research professor of earth sciences in the USC College of Letters, Arts and Sciences, is now pondering twin puzzles posed by his latest undersea explorer missions, both with significant implications for humans:

* Why does the bottom of Los Angeles Harbor process wastewater nitrogen at only a fraction of the efficiency of the sea bottom off Melbourne, Australia, which is otherwise extremely similar in measurable characteristics?  (The answer to this may help clean up Los Angeles harbor, as well as protect Australian waters.)

* Why did the deep waters of the Pacific Ocean suddenly become significantly more acidic approximately 3,500 years ago?  (The answer may be tied to the carbon cycles linked to global warming.)

Berelson began designing his submersible landers, technically called "benthic flux chambers" ("benthic" means "of or referring to the ocean floor environment"), in 1981.  He first successfully deployed a lander in 1983, in the ocean off Los Angeles.  USC technicians have built many others since then. The landers are all designed to descend to the ocean floor and form a seal, trapping a volume of ocean water in contact with a small patch -- a little less than a square yard -- of submarine soil.

The devices contain instrumentation to probe biochemically, on site, the way the soil, and the complex community of animals, plants and micro-organisms living in this soil, process various important elements and minerals.  They also can inject measured quantities of known substances into the sealed area to study what happens to them at the sea bottom.

A good example of the kind of analysis Berelson's landers can provide is a recent study the scientist conducted in Port Phillip Bay, Australia, off the city of Melbourne.

Human activity on land sends the nitrogen compound ammonia (NH3) into offshore waters in many forms -- treated sewage, agricultural and landscaping runoff containing fertilizers, and others.  Such waste nitrogen can produce dense, foul-smelling algae blooms and other undesirable consequences in the water. 

The ocean bottom can play an important role in nitrogen cycling in the ocean, because the community of micro-organisms living there can -- in some places at least -- convert ammonia and other nitrogen compounds into environmentally benign nitrogen gas.

But Berelson found that ocean floors vary widely in their ability to convert nitrogen compounds.  The sea floor of Port Phillip Bay was extremely efficient at carrying out this conversion.  In many bay sites tested by the landers, all or almost all of the ammonia that entered the benthic sediments was being recycled.

Berelson is able to say that the benthic reactions in Port Phillip Bay are unusually efficient because he has measured the same reaction in other areas, including the Adriatic Sea, San Francisco Bay and Los Angeles Harbor.

"In Los Angeles Harbor, lander measurements showed a much lower efficiency.  Instead of 80% to 100% of the ammonia entering the sea bottom being recycled, as we found in Port Phillip Bay, we found zero to 40% being recycled in Los Angeles Harbor," Berelson explains.

Berelson says the huge difference is mysterious because the two areas are otherwise quite similar.  The sea bottoms in both places receive about the same amount of carbon, in the form of a "rain" of dead plant and animal material.  The populations of marine animals "irrigating" the sea bottom are similar.  The size and type of sediments are similar.  So, too, are the waters' temperature, their salinity, and the amount of dissolved oxygen available at the sea bottom.

Port Phillip Bay is a much larger area than Los Angeles Harbor, and the parts of the bay receiving the most pollution were found to have the least ability to recycle nitrogen waste.  "This finding still leaves the question of what's causing the difference," Berelson says.  "If we knew that, we might be able to improve conditions in both Los Angeles Harbor and Port Phillip Bay."

Berelson's Port Phillip Bay research was part of a study funded by Australian authorities.  A report based on his research is under review by the Estuarine, Coastal and Shelf Science Journal.

Berelson has uncovered another submarine enigma in the deep waters of the central Pacific.  In a recently completed study, to be published in the next edition of the journal Deep Sea Research, Berelson and a team of collaborators examined the fate of the element carbon on the ocean floor.

Carbon falls to the bottom in various forms.  Most arrives as organic material, in the form of the soft tissues in the dead bodies of plants, animals and micro-organisms, and as calcium carbonate, the limy material in animal bones and mollusk shells.  Calcium carbonate, also known as calcite, slowly dissolves in the waters of the deep ocean, "like an Alka-Seltzer? tablet in a glass of water," as Berelson puts it.

But in many places on the ocean floor, calcite accumulates because the rate of "rain" of new particles is greater than the rate at which old particles dissolve.

In a series of  measurements based on lander data, Berelson and his team established that in recent geologic times, beginning 3,000 to 4,000 years ago, the rate of accumulation abruptly slowed.

Using sophisticated measurements of trace levels of radioisotopes in bottom sediments, the group established that the slowing wasn't due to fewer new calcite particles raining down:  that amount has remained constant over at least the last 10,000 years or so.

Another analysis ruled out another possible explanation -- that the slowing resulted from an increase in the amount of soft-tissue organic, non-carbonate carbon falling in the rain of particles from above, and that the greater quantity of carbon would speed the dissolution of calcite.

Elimination of those two possibilities implied that the composition of the bottom water itself changed markedly about 3,500 years ago, becoming significantly more acidic and, hence, better able to dissolve calcite.

Berelson is looking now for an explanation of what might account for such a change.  That date, 3,500 years before the present, is not associated with any obvious geological change, such as the end of an ice age.  Possibilities include a slowdown in the transport mechanism that circulates deep ocean water around the world, or  some stimulus that might have led to greatly increased production of organic carbon somewhere else in the world, "upstream" of the Pacific equatorial study zone.  But why either change might suddenly have taken place 3,500 years ago remains a mystery.

The answer could prove important.  Knowing how such a large-scale change in the ocean's metabolism of the key element carbon could have occurred may help us to understand how the greatly increased introduction of fossil-fuel carbon in recent times will affect the air and oceans.

Berelson's team on the carbon experiments included researchers from Lamont-Doherty Earth Observatory, Oregon State University, North Carolina State University, the Woods Hole Oceanographic Institution, the University of Hawaii, the University of Rhode Island, and the University of Miami (Fla.).&nbsp