The researchers looked at the different types, or isotopes, of oxygen atoms trapped in the samples and, using a well-established technique, were able to infer water temperatures at the time the organisms built their shelly parts.

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To find out how warm the earth was before that, climatologists have had to resort to indirect methods, such as measuring tree-rings, or counting the different isotopes of oxygen in layers of polar ice.

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Thus, it reasonably bars the export of HEU for use as reactor fuel or as targets to produce medical isotopes except on an interim basis to recipients who provide assurances that an alternative nuclear fuel or target will be used in their facilities whenever possible.

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But the pieces need to be found quickly because the isotopes last for only days or weeks.

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Finding certain isotopes - xenon gases in particular - would help experts determine whether a plutonium or uranium-based device was used.

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Radioactive isotopes accumulate in milk after they spread through the atmosphere, fall to earth in rain or dust, and settle on vegetation, where they are ingested by grazing cattle.

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