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2004 Field Season: Day 39
July 31, 2004
I want to use today's dispatch to describe to you Warren Sharp's research. He joined our group a few days ago, and his work is going along very well. His work is part of a new study to help us find better ways of dating our sites. Like Al Deino, he's collecting samples of rocks from all over the basin. But the kinds of rocks he collects are very different. Al collects bits of volcanic tuff for dating using the argon-argon technique or for chemical correlation. Warren collects rocks in which a different radioactive element, uranium 238, is found. U-238 collects in many different kinds of rocks, but here in the Olorgesailie basin, the main places it collects are in rocks where groundwater has leached through the soil, leaving behind the minerals that were dissolved in it. Around here, the minerals sometimes gather around the stems and roots of plants. Sometimes the minerals just form white chunks in the soil. Sometimes this process happens at the mouth of a spring, and the minerals form a large pile of rock, called a tufa.
Uranium is easily dissolved in water, so it is carried into the soil by the ground water. When the water evaporates, the uranium gets incorporated into the rocks just like the other minerals, where it gets trapped. Just like argon, uranium decays over time, forming several daughter products. Many of these daughter elements are also radioactive, so they also decay. Eventually, after eight intermediate daughter elements, uranium decays into lead, which is no longer radioactive. One of the intermediate daughter elements, thorium 230, is what Warren uses to tell the age of the rocks. He compares the ratio of thorium to uranium left in the rock, and is able to tell the age. This kind of dating is called "uranium-series dating."
U-series dating can be very useful, especially if we want to get a date for a place that has no volcanic tuff. Unfortunately, this kind of dating has several problems with it that make it less reliable than argon-argon dating. (Because it's so reliable, argon-argon dating is considered the best way to date anything here in the African Rift Valley, where there's been a lot of volcanic activity.) One of the problems is contamination. If there are bits of thorium-rich sand or clay in the sample, they can disturb the ratio and thus give you an incorrect date. The ease with which uranium dissolves is another problem - if water has leached back into the rock and has taken away some of the uranium without removing the thorium (thorium does not dissolve easily), that would also throw off the ratio and give you an incorrect date. Third, because thorium reaches saturation in about 650,000 years, U-series cannot be used to date anything older than that. Finally, it's difficult to know exactly if the age Warren gets from the samples is the age of the event he's actually trying to measure. For example, once a soil surface is formed and eventually enough groundwater leaches through it, uranium-rich chunks of rock may develop in the soil. But it's possible that there was a lot of time between the forming of the soil and the development of the chunks of rock.
Part of Warren's project here, then, is to get samples of rock very near to places where we have argon-argon dates. He can then compare the dates he gets from U-series dating with the argon-argon results, to see how reliable U-series is. This is an approach that's never been tried before. But because of the superb series of argon-argon dates here at Olorgesailie, and also due to the presence of samples for U-series dating, we've got an ideal scientific setup! Warren and Al will compare their results, and we'll find out the kinds of samples and the time period when U-series dating works really well.