Dr. Richard Gaschnig of UMD Geology gave an interesting talk today about inferring the composition of the Earth’s ancient crust from glacial sediment deposits.
Banded iron formation with tiger-eye, Mount Brockman, Australia. From http://www.pbase.com/image/94666060.
Gaschnig described detailed analyses of transition metals (TMs) to understand how the composition of minerals at the Earth’s surface might have changed over billions of years. These elements can be especially sensitive to their chemical environment, and so the TM content of ancient rocks tells us what things were like chemically as the rocks were formed.
A particularly interesting result emerged from Gaschnig’s work: the TM signatures he’s measured for some ancient rocks show evidence for initially low levels of oxygen immediately following the beginning of the Great Oxygenation Event (GOE), the phase in Earth’s history when ancient cyanobacteria became so prominent that they produced vast quantities of oxygen.
All this oxygen dramatically affected the chemical environment on the Earth, and its relatively sudden appearance in the atmosphere is reflected the mineralogy of some rocks, such as the banded iron formations shown at the right. Of course, without oxygen, most life as we know it would be impossible, but when it first appeared, the oxygen probably devastated the biosphere.
Some of Gaschnig’s rock samples dated to before and after the GOE show chemical changes in the TM content that may be consistent with the presence of oxygen but only at low levels (at least immediately after the GOE began) and so can be used to fill in some of the missing details about this dramatic chapter in Earth’s history.