Thursday Morning: Useful Uranium, and Footprints of a Flood

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Candace Major speaks at press conference
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Candace Major explains her research on ocean circulation to curious reporters at a press conference Wednesday. (photo by Hugh Powell, WHOI)

Map of an ancient submarine delta
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Map of an ancient submarine delta under the Black Sea by Giosan and colleagues showing ridges, some more than 12 meters (40 feet) high, and channels formed by erosion of the seafloor

Related Links

» Geochemical Archives Encoded in Deep-Sea Sediments
More on the uranium decay products protactinium and thorium in Oceanus magazine

» The Once and Future Danube River Delta
More of Liviu Giosan's work on deltas in the Black Sea, from Oceanus magazine

» The Moving Shoreline
A broad selection of web resources on coastal erosion, from the WHOI topic pages

Ocean Currents Recorded by Decaying Uranium

When uranium hits the news, it’s usually as the bad guy: an element that can fuel bombs and emit dangerous radiation over millennia. But oceanographers like Candace Major and Jerry McManus are putting the slow decay rate of this rare but widespread element to a good use: understanding ocean circulation, especially how the ocean-climate system worked in the past. Major described some of her work at a press conference Wednesday morning and in a scientific session that afternoon.

At issue is the way that water flows through the Atlantic Ocean, bringing heat from the tropics north to warm the climate over Europe. As uranium in that water decays, it forms two different products that settle onto the sea bottom. By analyzing differences in the buildup of the two products, ocean scientists can gauge how much water was flowing through the ocean currents at particular times in the past.

Ever since the end of the last ice age (about 23,000 years ago) stronger Atlantic currents have corresponded with warmer temperatures, a study found last year. Major and her colleagues did more work to see if the ocean-climate system worked the same way 30,000 to 60,000 years ago, during the last ice age.

It’s a valid question considering how different the oceans must have been when glaciers covered much of the Earth. Sea levels were 120 m (almost 400 feet) lower, meaning ocean currents might have flowed differently and exerted a much different effect on the climate.

Major analyzed uranium decay products in Atlantic sediments and matched them against estimates of temperature for the period she studied. Her results showed that during warm periods in the last ice age, Atlantic currents were carrying about as much water as they do today. Cooler climate periods were accompanied by weaker currents. The results are a further suggestion that watching ocean circulation -- particularly in the North Atlantic -- is a prudent way to keep tabs on Earth’s climate.

Rising Sea-Level Caused Sudden Flood?

Could the Black Sea have been the site of Noah’s flood many centuries ago? The idea has been intriguing but the evidence scanty ever since William Ryan, of Lamont-Doherty Earth Observatory, and colleagues proposed the idea in 1997. On Wednesday, Liviu Giosan spoke about new evidence he had collected with Ryan and Mark Siddall of the University of Bern, Switzerland.

Today, the Black Sea flows into the Mediterranean through a narrow strait called the Bosporus. Ryan suggested that about 9,200 years ago, as ice sheets retreated across northern Europe, that strait was actually a dam that kept out salty Mediterranean waters. As glaciers melted worldwide, sea level rose until water on the Mediterranean side of the dam was up to 70 m (230 feet) higher than in the Black Sea. When the dam broke, it would have unleashed a deluge that, if not precisely Biblical in proportion, would have lasted at least 30 years and flooded early human settlements around the Black Sea shore.

To look for evidence, Giosan, Ryan, and their colleagues boarded their own ark (actually a research vessel) and used bathymetric and seismic instruments to make detailed maps of the seafloor around the Bosporus. They uncovered a pattern of erosion and sedimentation that had the unmistakable shape of a river delta: a broad main channel fanning out into a network of branches. Between the channels were rows of low, muddy rises in the sea bottom that resembled the way sediments collect when a river floods its banks.

Earlier investigations of Ryan’s theory looked for evidence of ancient settlements submerged along an old shoreline. But Giosan, a geologist who specializes in river deltas, said he thought it made more sense to look for clues at the scene of the flood.


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Last updated December 9, 2005
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