Sunday, March 27, 2011

Harold Urey and Cesare Emiliani

The pause in global warming research continued a surprisingly long time. World War II accounts for part of this--scientists were working on problems directly related to the war effort and not more esoteric research. Afterward, there was the Cold War, in which although funding for scientific research expanded enormously, much continued to be directed towards military applications. This was the case until the International Geophysical Year of 1957-1958, which stimulated much research relevant to anthropogenic global warming, triggering investigations some of which continue to this day. But that's for another entry.

However, not all was quiet on the anthropogenic global warming front. Harold Urey (1893-1981) is one of the towering giants of 20th century science. He won the Nobel Prize for chemistry in 1934 for research in isotopes. He discovered deuterium in 1931, and was the first to isolate pure liquid deuterium from liquid hydrogen. He did much research on the isotopes of uranium, being a member of the brain trust that helped develop the atom (fission) bomb. He came up with a model for the early atmosphere of the Earth in 1952, speculating it was composed of ammonia, methane, and hydrogen, which he published in his book The Planets: Their Origin and Development. Urey's hypothesis for the composition of the atmosphere of the early Earth has since been shown to be wrong, but it was a good kind of wrong that stimulated a lot of research. Harold Urey was Stanley Miller's professor and adviser, and together they crafted the Miller-Urey experiment, one of the most famous experiments of 20th century science. This experiment showed that complex organic compounds, including many amino acids, could be generated easily and in large quantities by natural processes.

Harold Urey was also Isaac Asimov's chemistry professor at Columbia University.

After World War II, Harold Urey turned his attention to isotopes of oxygen. It was a natural question for him to explore. Urey had used centrifuges to separate deuterium from ordinary hydrogen, and had helped work out how to separate uranium-235 from uranium-238 by creating uranium hexafluoride which could be spun in centrifuges to separate the lighter uranium-235 from the heavier uranium-238 (and helped created the more exciting and dangerous world of today)

Harold Urey realized that evaporation of water and condensing it into glaciers could act as a natural way to separate isotopes. Oxygen-18 is a rare but stable isotope of oxygen. A water molecule containing an atom of oxygen-18 is heavier than water molecules containing oxygen-16, and does not evaporate as easily. This means that during ice ages, when more and more evaporated water is trapped in ice sheets, the remaining water in the oceans is enriched in oxygen-18. Therefore, the more oxygen-18 is concentrated in the oceans, the greater the volume of ice sheets and (presumably) the colder the Earth was!

Urey wrote in 1947 that we should check coring samples of the ocean for deposits of foraminifera (forams) shells in the sediments, hypothesizing that those living in times of past ice ages would have enriched levels of oxygen-18 in their shells. This was the first time that nuclear science and biology had been combined to solve a scientific problem!

The problem was taken up by Cesare Emiliani (1922-1995), a geologist from Italy who was one of Urey's students after the war (Many of Urey's students became scientific giants of their own working on problems suggested by Urey).

There were many difficult problems. Sediment coring up until that time was not very sophisticated---it was simply dropping a very heavy and dense metal tube into oceanic sediments. This had been done since the 1870s Challenger expedition, discussed in a previous blog entry, but it blurred and mixed the core samples too much to provide reliable samples of foram shells inside. This is not to say that these primitive core samples were useless---they did provide much information on sediment layers but they were too crude for the sort of research Emiliani was doing.

Borge Kullenberg saved the day. Working on the Swedish Deep Sea Expedition of 1947, he developed a new coring device that used a piston which deployed when the coring tube hit the ocean floor, enabling core sample tubes to be wider and penetrate far deeper into the ocean floor sediments. The coring samples were much clearer, and were 15 meters long, instead of a couple meters. By 1951 he developed a 20 meter coring apparatus.

Back in the lab, people could take precise samples of each layer, tease out a few hundred foram shells, which were then ground and roasted in the presence of pure oxygen-16 gas to form carbon dioxide. The oxygen-18 could then be measured by spinning the carbon dioxide in centrifuges, separating the heavy molecules containing oxygen-18 out.

As Kullenberg's new coring apparatus replaced older coring techniques in a few years, lots of samples became available for inspection. Emiliani used the new technique of carbon dating on the top layers to determine an average rate of sediment deposition on the ocean floor. (beyond about 40,000 years carbon dating does not work as carbon-14 decays). With the new sediment cores of 20 meters, he was able to get samples as much as 300,000 years old.

Emiliani found several pieces of the climate puzzle.

He found that the signature of ice ages could be clearly and consistently seen from ocean-floor sediment samples around the globe.

He also found that the temperature curves generated from these samples matched the Milanković theory very well.

Milanković had not achieved much scientific recognition up until this point---his chronology for ice ages differed from the scientific consensus developed in the late 19th century. But he lived long enough to see vindication through Emiliani's work.

Emiliani also found that there were sharp changes in the temperature of the earth---lots of evidence that ice ages were not smooth curves of cooler and warmer temperatures, but lots of sharp jagged swings in temperature, in periods of hundreds of years. Sharp advances and retreats.

When Emiliani published his research in 1955, it was recognized immediately as groundbreaking. The conclusion that ice ages were driven by Milanković cycles was accepted.

But the sharp temperature swings were not. The coring technique was new--perhaps it was affecting the sampling. The idea that the climate could change by large magnitudes in hundreds of years was against the scientific consensus---there was no way that Milanković cycles could explain that. Sampling errors seemed more likely, and the idea of rapid climate change was unsettling.

Today we know from ice core samples and more sophisticated ocean sediment sampling techniques that rapid changes in the climate have indeed occurred in the past. And not just in hundreds of years. But in a decade. Or less. Emiliani's work was a big clue that climate was not a stable beast. That climate could turn on a dime. But that realization only came much later.

Harold Urey, 1963:

Cesare Emiliani 1952 (?)

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