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 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 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 , 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

134 © 2006 Nature Publishing Group

             NATURE averaging dates the from zircons. several datedbe accurately, reducing error the from dating analyses, but now asingle zircon can 1990s,multiplethe zircons often were for used found— microscopic insediments. In crystals eter to produce uranium/lead dates on zircons zona combines with amassspectrom- alaser LaserChron Center at University the of Ari- have improved. also For instance, Arizona the from Bowring’s lab at MIT. EARTHTIME investigators now obtain them Bowring bought up available allthe spikes, and of error. In abid to enforce standardization, labs, between increasingto vary margin the material lostinanalysis. But ‘spikes’ these tend cally argon, uranium or lead, to correct for Center inCalifornia. ogy Geochronol- tor of Berkeley the precision, says Paul Renne,direc- EARTHTIME’s desired degree of obtain to able be will five about but of United 20inthe States, only wide doing argon/argon dating, from about 10million years ago. nique’s ranges usefulness backwards intime 4.5 billion years ago. Theuranium/lead tech- from 2,000years ago back to Earth’s some birth, lead. Argon/argon dating can date materials same with the isotopes ofdoes uranium and and argon-39, and uranium/lead dating, which compares amount the of radioactive argon-40 The techniques for uranium/lead dating Researchers radioactive reagents, use typi- There are some 50labs world- But most important the development in | Vol 444 | 9 November 2006 highest level.” debates tothe to resolve dating of EARTHTIME is “The philosophy . Gradstein, F. M.,Ogg,J. G.&Smith,A. 8. 7. Mundil, R. 6. Mundil, R. 3. Cyranoski, D. says, and an earlier date for P/Tr the boundary “We have should toMundil listened more,” he prevailingthe isthat view Mundil wascorrect. ana, Now, co-edited who book. the says, Ogg much,not seem but sidesclung both on to their or minus 200,000years mass extinction at 252.6million years ago, plus batches zircon multiple of analysis its had not accounted sufficiently for lossin lead Bowring’s date, suggesting that MITgroup the questioned lab geochronology Berkeley the 300,000 years of 251.4millionary years ago, plus or minus uranium/lead zircon dates for P/Tr the bound- forbe to them jettison published their views. have dates, intheir and shows how hard it can — shows personal investment the researchers — which involves key EARTHTIME members years ago. Theebband of dispute flow this marine life that some 250million occurred (P/Tr),ary amassextinction of terrestrial and in time, such Permian–Triassic asthe bound- reanalyse material from hotly debated points Mattinson’s to prompted others technique The datedebate overlead introducing geologic time, errors. for removing zircon that surfaces may have lost with ahigh-temperature field the technique sity of California, Santa Barbara, revolutionized from radiation. James Mattinson of Univer- the treat zircons to compensate for lossof the lead . Bowring, S.A. 5. 4. Mattinson, J. D. Erwin, H. 1. debates level,” to highest the hesays. of EARTHTIME isto resolve kindsof these plus or minus philosophy 100,000years. “The latest date for same the event: 252million years, bourne, Australia, reported Bowring histeam’s scheduled forbook, 2008. will probably nextedition inthe used of be the uranium/lead dating ability the hasbeen to applied it to P/Tr zircons —and placed the of Mattinson’slearning Mundil technique, 2. Clarke, T. Scale 2004 In 1998,Bowring and colleagues reported At aconference two months ago inMel- The difference two the between figures might ©

2 Nature 0 et al. et al. (Cambridge University Press, 2004). 0 Annu. Rev. Planet.Sci. Earth 6 Chem. Geol. Nature University in West Lafayette, Indi- James ageologist Ogg, at Purdue ing “a great of deal criticism”, says MIT’s used book dates, prompt- cal Time Scale 2004 a major refer ence text, to rollset for edition new the of —just presses asthe view were et al.

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