Editor's Note: Science Scan is a roundup of recently published biotechnology-relevant research.

Radiocarbon dating is currently accurate to about 26,000 years before the present. Marine scientists at Woods Hole Oceanographic Institution claim a technique that doubles that limit to a neat 50,000 years. That expanded window in time embraces crucial moments in human evolution, geology and climate change. Such carbon dating is based on the premise that living beings take up atmospheric carbon-14, (C-14), which then decays at a constant rate. Because concentrations of atmospheric C-14 have varied over time, carbon dates for some time periods can be off by several thousand years.

To compensate for discrepancies, researchers try to link radiocarbon-dated artifacts with items of known absolute ages, such as tree rings or annual layers in sediment or ice cores. Konrad Hughen and his co-authors did just that by matching shells from tropical marine sediment with annual layers of a Greenland ice core during a 50,000-year stretch. Comparing age determinations from the two methods should help scientists "correct" the radiocarbon clock.

Willard Libby, an American chemist at the University of Illinois, was awarded the Nobel Prize in chemistry in 1960 for inventing carbon-14 dating in 1948. One of its first applications was to measure C-14 radioactivity in a wooden beam across the tomb of an Egyptian pharaoh, King Snefru. It checked out at 26,000 years of age precisely. In the half-century since Libby's 1948 contribution, carbon-14 dating has become an essential and commonplace item in the kit of tools for alkaloid synthesis and nuclear medicine, as well as a myriad of other chores of C-14 isotope dating.

French chemists have contributed a brief editorial to Science, printed Dec. 19, 2003, and titled "A better radiocarbon clock." It leads off with a commentary stating, "An accurate chronometer that covers the past 50,000 years is a fundamental tool for geology and archeology. For example, some prehistorians believe that Neanderthals overlapped chronologically with modern humans, whereas others claim that they predated modern humans by several millennia. Accurate dating is essential to understand this crucial time in human evolution. The main chronometer for this time range is radiocarbon (C-14) but its use is not free of perils. This is because the atmospheric C-14/C-12 ratio - the starting point of the radioactive clock - varies over time. Fossil trees have been used to produce a high-resolution tree-ring calibration, which has been extended with data from layered sediments and tropical corals.

"The latest official' calibration curve," the commentary continued, "during the 18th International Radiocarbon Conference in Wellington, New Zealand, stops at 26,000 years before 1950 A.D. Several groups have tried to extend this curve from 26,000 to 50,000 calendar years B.P. (before the present). The records used for this purpose include annually laminated sediments from Lake Suigetsu in Japan, corals from the uplifted terraces of New Guinea, sediments of the former Lake Lisan in Israel, carbonate deposits from a submerged cave of the Bahamas, and deep-sea sediments whose stratigraphy can be tied to the Greenland Summit ice cores.

"New results based on the stratigraphy of ocean sediments were also presented during the conference," the French scientists continued. "Most notable is the work by Konrad Hughen, et al., reported in this issue of Science." Hughen is lead author of the full-length Science paper, titled: "14C activity and global carbon cycle changes over the past 50,000 years."

Hydrogen, Futuristic Fuel For Tomorrow's Autos Gets M.O. For Safe, Efficient Storage, Production

The technology of using hydrogen (H2) as an environmentally clean and efficient fuel is an active research area worldwide. Liquid hydrogen, which carries a high density of the element (70 grams per liter) and is currently the most commonly used form in prototype automobiles, is very energy intensive - up to 40 percent of the energy content must be spent to liquefy hydrogen at its very low condensation temperature (T). That low T cannot be provided by practical and inexpensive cooling agents, such as liquid nitrogen; hence, the continuous boil-off of liquid hydrogen poses problems to on-board storage.

Researchers have developed a new method of storing significant amounts of hydrogen gas in crystalline compounds. Their results might facilitate the development of technologies related to the use of hydrogen as a clean and efficient fuel. Currently, most hydrogen power systems use liquid hydrogen, a very cold (20 K) liquid that requires a lot of energy to produce.

Wendy and Ho-kwang Mao, a wife and husband team of geophysicists at the University of Chicago, are co-authors of a paper in the Proceedings of the National Academy of Sciences (PNAS) titled "Hydrogen storage in molecular hydrides." Their paper reports that hydrogen clathrate hydrate, a crystal containing one hydrogen molecule and two water molecules, is comparatively easy to prepare and store. To create the crystals, the co-authors compressed hydrogen gas and ice with a pressure of 2,000 to 3,000 atmospheres. Using liquid nitrogen to cool the crystals to 77 K, they showed that the hydrogen was retained even after the pressure was reduced to ambient levels. When the temperature was raised to 140 K, the stored hydrogen was released - theoretically, to a generator.

"At 50 grams of hydrogen per liter of crystal," the paper suggested, "the clathrate does not hold as much hydrogen by weight as liquid hydrogen, and may be difficult to produce in large quantities."

Novel AIDS/HIV Vaccine Measures Ideal Timing Of Sterilizing Protection In Macaque Monkey Models

Pig-tailed macaque monkeys (Macaca nemestrina) at the National Institute of Allergy and Infectious Diseases modeled an experimental vaccine against AIDS to measure the timing for optimum efficacy of the HIV-immunization process. A paper in the PNAS, released online Nov. 17, 2003, is titled "Passive transfer of neutralizing IgG at 6 h but not 24 h postinfection confers sterilizing protection to S[imian]-HIV-challenged macaques: Implications for HIV-1 vaccine development."