Reading Earth’s magnetic history
In order to date environmental events from Earth's history — such as meteorite impacts or climate change — geologists have long studied variations in slow-growing seafloor sedimentary rocks called ferromanganese crusts that build up in layers over the eons. The layers can be dated by various means, such as by analyzing radioactive isotopes, but those methods don't provide accurate dating on small scales: a millimeter of rock, for example, can include information that spans as much as hundreds of thousands of years. Now, a new technique pioneered by researchers at MIT and in Japan provides a reliable way to date these “archives” of environmental changes with much finer precision. Because the Earth’s magnetic field flips to an opposite (north or south) orientation at random intervals, the layers of rock — which record the magnetic orientation at the time they formed — produce a kind of natural barcode that can be...