Dating prehistoric artifacts

The contents of the droppings give more than a window into the giant bird's eating habits—they preserve a record of what the long-gone moa's ecosystem was like. The arid conditions of New Zealand caves provide the perfect place for poo preservation. Australia should, too, the researchers say, but the droppings of ancient marsupials just haven't turned up.

As professor Alan Cooper says, "A key question for us is 'where has all the Australian poo gone? If you think your metal detector has uncovered some treasures, try finding vintage plutonium in the backyard. Jon Schwantes of the Pacific Northwest National Laboratory was called in to analyze a sample of plutonium accidentally discovered in a safe during the cleanup of the Hanford nuclear site in Washington. One clue was the "signature" left by the reactor—every reactor's is different.

The fingerprint of this discarded material led him to a reactor not in Hanford, but in Oak Ridge, TN. It also led him to the conclusion that it was created in , meaning it was created during the Manhattan Project, making it one of the world's oldest-known samples of enriched plutonium. A pile of skeletons probably wouldn't tell us much more than the obvious.

But University of Leicester archaeologist Simon James sees evidence that, to him, dates the first known chemical warfare attack back to A. In that year, Persians attacked a Roman garrison at Dura-Europos in Syria; when they tried to mine under the walls, Romans tried to counter by mining under the Persian tunnels.

Archaeologists found the pile of Roman bodies in one of the tunnels, but no cause of death. James thinks it was asphyxiation. In the tunnels, he says, there was bitumen and sulfur—materials that, when burned, give off toxic gas. So, he says, the Persians probably used chemical warfare to do in their rivals. One classical way to date objects is to take note of what strata of rock they occupy—rocks come in layers, with the oldest at the bottom.

Chronological dating

But those rocks also carry less obvious information—their magnetic signatures. The Earth's magnetic field varies all the time, by both strength and orientation. At the time rocks form, however, their magnetic materials acquire the particular orientation of the planet's magnetism at the time, giving geologists a window into the Earth's magnetic past. You've probably heard about ice cores, but what are they exactly?