January Editorial      


Dating Sedimentary Rocks


Calcium carbonate (the same material which clogs up kettles and makes water 'hard') is the most abundant mineral precipitate on the surface of Earth. So abundant, in fact, that it accounts for about 4% by weight of the Earth's crust. Calcium carbonate takes several different geological forms. As well as chalk and marble, one of the most common of these forms is the sedimentary rock known as limestone. Because calcium carbonate is so common, many microbial organisms use it as the main chemical building block of their shells. When these creatures die, their shells are incorporated as part of the sedimentary rock. Therefore, as calcium carbonate is deposited and turns into sedimentary rock, it leaves a chemical fingerprint of the animals and their environment, and of the plants and bacteria that were there at the time.

It has always been difficult to distinguish between microbial and non-biological contributions to calcium carbonate rock formations. The extent to which each source has contributed to the composition of the rock has been a controversial subject. But now recently concluded research on the growth of sedimentary rock at the Mammoth Hot Springs in Yellowstone National Park has given a much clear picture of the microbial contribution. Bruce Fouke, a professor of geology and molecular and cellular biology, led a team from the University of Illinois which has spent the last 10 years quantifying the physical, chemical and biological aspects of the environment of hot springs. The team chose Yellowstone National Park because of the high precipitation rates of calcium carbonate in the hot springs there. In fact, in the springs which they researched, the calcium carbonate grows by millimetres per day and is highly enriched with microbial life.

One of the crucial experiments which the group performed was to draw water from the hot springs vent andmeausre how much calcium carbonate was deposited with the microbes present. This was then compared with what happened after the microbes were removed from the water. They found that the rate of calcium precipitation was much higher in intact spring water. The difference was startling. The researchers found that the the rate of precipitation sometimes dropped by more than half when the microbes were not present. This finding is very important since the rate of precipitation controls the chemistry and shape of calcium carbonate crystals. The findings from Yellowstone park allow researchers to look at the chemistry of different sedimentary rocks and determine the precipitation rate. This in turn will allow them to determine the extent of the contribution of microbes to the rocks being studied.

Scientists can now go back to previous records and fingerprint samples which will tell them if microbes were present at the time the rock was formed, and calculate the rate at which the rock formation happened. This can not only be done with sedimentary rocks collected from the Earth's surface but also with rocks from other planets. The discoveries from Yellowstone park could affect how certain sequences of sedimentary rock are dated, and and also provide a means by which scientists can search for evidence of life on other planets.

Professor Fouke's research was funded by the US National Science Foundation and has been accepted for publication in the Journal of Sedimentary Research.


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