Vol 444|9 November 2006 SPECIAL REPORT Telling the time Geochronologists can pin down dates in deep time more accurately than ever before. Rex Dalton talks to the researchers who are rewriting the details of Earth’s history.
y fine-tuning their techniques, research- Smithsonian Institution in Washington DC ers are refining their ability to measure and co-organizer of EARTHTIME, notes that Bever more precisely the ticking of Earth’s major extinctions are often followed by a burst geological clock. of speciation. “I think these new dating meth- For decades, geologists and palaeontolo- ods will be a powerful spur to new work on gists have had only ball-park estimates for evolutionary rates,” he says. when major events happened in the history of One of EARTHTIME’s major thrusts life on Earth. Now a series of new methods has will involve ‘astronomical tuning’, as Earth’s radically improved their understanding of time orbital motion affects the geological record. long gone1. With unprecedented precision, Earth’s changing movements — the angle of researchers are now arguing over whether date the globe’s spin axis, the path of its orbit and estimates are off by as little as 100,000 years — the orientation of its axis relative to the Sun remarkably accurate for events that may have — change cyclically, and in turn influence occurred hundreds of millions of years ago. climate. These climatic changes appear in the Leading the quest for increasing accuracy sedimentary record as changes in the thick ness is the international EARTH- of sediment layers, in the ratio TIME project, the brainchild “In a decade, we of carbon or oxygen isotopes, At the rock face: researchers study sediments of Sam Bowring, a geologist hope to radically and in the abundance of tiny dating from the Cretaceous–Tertiary boundary. at the Massachusetts Insti- fossils, for example. recalibrate Earth’s K. JOHNSON tute of Technology (MIT) in Scientists have already calcu- But although such methods can estimate the Cambridge. Its goal is nothing history back to the lated this astronomical record age and conditions under which a particular less than a complete restruc- origin of animals.” as far back as nearly 40 million rock was formed, they cannot reveal how long turing of the chronology of years. Now they plan to push it those conditions lasted. That’s where astro- deep time2. “In a decade, we hope to radically back even further. nomical tuning comes in, by providing a long- recalibrate Earth’s history back to the origin of Traditional techniques to date rocks include running sequence of climatic events, such as animals,” Bowring says. identifying fossils that were known to exist ice ages, that are recorded in seafloor cores. Researchers involved in the effort gathered during particular eras; counting the ratios of “You see variations of colour and lightness in Philadelphia last month at a meeting of the different isotopes of radioactive elements, such in the cores,” says Heiko Palike, a geologist at Geological Society of America to plan out their as uranium, argon or lead, as these change over the National Oceanography Centre in South- strategy. Yet as they try to standardize their time; or relying on the intermittent reversals ampton, UK. “It is like counting tree rings, complex dating procedures, they must accept of Earth’s magnetic field, recorded in rocks. then analysing the isotopic composition of the a re-evaluation of dates that they themselves microfossil shells in the layers.” may have determined. Last year, Bowring won a US$1-million Deep time grant for research with about 225 collabora- To expand this astronomical record, research- tors for three years from the National Science ers are designing ocean drilling expeditions Foundation. In January, team members plan to primarily to examine these time-specific sedi- apply for another grant to undertake ‘a proof mentary cores3. The drill ship JOIDES Reso- of concept’ experiment — seeing whether they lution is set to travel to the Pacific Ocean in can standardize geological dates for the Cre- November 2007, to drill a core dating between taceous–Tertiary (K/T) mass extinction about 35 million to 55 million years ago, to help pin
65 million years ago. down the chronology of events during that MUSEUM GEHLING/SOUTH AUSTRALIAN J. The group hopes to date the K/T boundary time period. in a series of sediments in eastern Colorado, But ocean cores only can reveal sediments and use that date as a benchmark for studies going back about 180 million years; any older in other geographical regions. They will also sediments have been recycled into Earth’s target another geologic interval from the Cre- interior. When it comes to exploring further taceous period, about 90 million to 100 mil- back in time, geochemical analyses are the lion years ago, to further cross-check dates and gold standard — specifically, isotopic analyses develop a precise chronology. Fossils known to exist at a certain time can help of radioactive elements as they decay. Douglas Erwin, a palaeobiologist at the researchers put a date to rocks. Methods include argon/argon dating, which
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