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Exactly when did that happen? Explorationists spend a lot of time correlating events. Over the last few decades, our ability to determine geological time frames has become much more refined, because we have better tools, such as the ion microprobe and the refinement of radionuclide parent-daughter relationships. Precision still requires two things to match up: the isotopic evidence (geochronology) and the depositional history and relative location of the sample (chronostratigraphy). Most folks know that the end of the Cretaceous Period (K) and the beginning of the Tertiary Period, known as the K-T boundary, occurred about 66 million years ago, accompanied by the mass extinction of dinosaurs. The prevailing theory is that a massive asteroid slammed into the Gulf of Mexico, just offshore Yucatan, and triggered the extinction event. However, recent study of melted rock ejected far from the Chicxulub impact crater suggests that the impact was too early to have caused the mass extinction, about 300,000 years too early. Using an array of electron microscopy techniques, Markus Harting of the University of Utrecht in the Netherlands says that glass spherules found in several layers resulted from the meteor impact, but were eroded to create later layers of sediments displaced from where they originally hit the ground. The displaced spherules are misleadingly close to the K-T boundary, says Harting, based on the fossils in the same sediments. The original spherule layer is not hard to see, since its spherules are not as abraded as those which were moved around and re-deposited in later, higher sediments. Above these, and younger still, Harting says he found the famous layer of extraterrestrial iridium, which is found worldwide, and was originally touted as the smoking gun for an impact somewhere on Earth at the K-T boundary. “In most of the sections we found spherules we also found the iridium layer at or near the K-T boundary,” said Harting. “That makes the 300,000-year mismatch with Chicxulub even more obvious...The K-T iridium layer is a totally different event.” Samuel A. Bowring is a professor of geology in MIT’s Department of Earth, Atmospheric and Planetary Sciences, and he wants to refine the timing of the event. Bowring thinks the K-T boundary and dinosaur extinction are related, but he believes that it is possible to more precisely date the sequence of events before, during and after the extinction. “There are questions that need to be addressed that are preserved in the rock record, questions about rates of evolution, and sudden climate change in the past,” said Bowering. In 2003, Bowring launched the Earthtime initiative to bring together scientists from all over the world to work together to calibrate and sequence Earth’s history. This will be accomplished through integration of high-precision geochronology and quantitative chronostratigraphy. “It’s creating a lot of new interactions that weren’t there before, paleontologists talking to geochronologists,” Bowring said. “It brings together evolutionary biology, developmental biology, paleontology and oceanography.” Earthtime’s official goal is to bring dating accuracy to better than 0.1%. That means in dating a 250-million-year-old fossil, results would be plus or minus 250,000 years. Bowring thinks it is possible to improve precision to half of that error, to within 0.05%. Two main dating techniques are applied to volcanic ash layers that contain rock fossil records. One technique measures the decay of uranium-to-lead inside zircon crystals, which are often found in volcanic ash beds. The other technique measures the decay of potassium-to-argon gas produced in minerals like feldspar. The two methods produce slightly different results, but with correct correlation and using data from fossil records from all over the world, they are beginning to generate a more accurate time scale. “We are working on the age of the K-T boundary right now using zircon, and we think we can constrain it to within about 50,000 years,” Bowring said. Other issues that a more precise Earth time scale could address include the mystery behind a massive extinction about 252 million years ago at the end of the Permian period, in which 90% of all sea life became extinct. A postulated cause might be a gigantic outpouring of lava, which occurred about 251 to 252 million years ago from volcanoes in the Siberian Traps. Better timing of that eruption – which is not well known – relative to the extinction could answer the question whether the two events are connected. The Cambrian “explosion” is another mystery. This refers to the huge proliferation of life about 530 million years ago; every order and class of animals that we see today appears in the fossil record within a relatively short time. There are unanswered questions as to whether this is a “...record of sudden appearance or a record of sudden preservation, because it is also when animals developed hard parts,” Bowring said. Better dating could also be useful in determining cause and effect for climate changes. The organization is in its third year and has more than 200 members hailing from a variety of disciplines, including the oilpatch. Plans are being set for work to be done in the coming years. The schedule includes investigation into other dating techniques, including absolute calibrating of the 40Ar-39Ar, absolute calibration of the U-Pb system, and other radioisotopic systems (e.g., Lu-Hf, Re-Os). There is a lot of good information at Earthtime’s website: www.earth-time.org. Interested parties can get zircons from ash beds in the Permian/ Triassic Meishan section of southern China, as well as zircon and baddeleyite from the Duluth complex. There will be a full-day Earthtime session at the annual GSA (Geological Society) meeting in Philadelphia (Oct. 22 – 25, 2006). In a related development, the High Precision Isotope Ratio (HiPiR) Consortium, an international group of earth scientists and analytical chemists working jointly to advance ultra-high precision elemental isotope ratio determinations, has a new website: http://spot.colorado.edu/~hildebrr/ There are two up-to-date geologic timescales at the Earthtime website: The International Commission on Stratigraphy Timescale, and the GSA timescale. There is also a third timeline that shows the history of timeline changes. The ability to precisely time geologic events will lead to a much better understanding of geologic cause and
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