Iron is vital to plant, animal and microbial life. Nearly all organisms ingest it in the course of daily life. If scientists can devise a method to distinguish between iron that has been processed by a living organism and iron that has not been metabolized, they will have a way to measure the distribution of microbes on Earth billions of years ago. Because iron is common on the moon, planets and other objects in space, the technique could be used to detect signs of past life beyond our own planet. Sedimentary rock reflects the accumulation of sediments, including organic material and trace elements such as iron.
Igneous rock is forged deep in the Earth at very high temperatures where life is absent.
It also can contain iron. Working in collaboration with scientists from NASA's Jet Propulsion Laboratory and the Institute for Great Lakes Study at UW-Milwaukee, the Wisconsin team sampled the isotopic composition of iron from the two sources by incinerating samples of iron and measuring charged particles from the reaction is a mass spectrometer, a device that sorts and counts ionized particles. The mass differences are small, but large enough that a microorganism could have made the difference. We wondered if inorganic processes might have the same effects, but we found that the isotopic composition of iron in igneous rocks is constant. Knowing this, it may now be possible for scientists to look at sedimentary rock and gain a sense of the worldwide ebb and flow of microbial populations in the distant past, perhaps as far back as 2 billion years ago, when the Earth's oceans were full of soluble iron.