Researchers have determined the isotope composition of the rare trace elements Hafnium and Neodymium in 2.7 billion year-old seawater using high purity chemical sediments from Temagami Banded Iron Formation (Canada) and concluded that large landmasses must have existed then.
The Temagami Banded Iron Formation was formed 2.7 billion years ago during the Neoarchean period and can be used as an archive because the isotopic composition of many chemical elements such as Hafnium and Neodymium directly mirrors the composition of Neoarchean seawater. These two very rare elements allow many valuable conclusions about weathering processes to be drawn. Earlier work has shown that these Canadian rocks only contain chemical elements that directly precipitated from ocean water.
During their investigations, they found that this ancient seawater contained an unusually high abundance of the radioactive isotope Hafnium 176 but a comparably low abundance of the radioactive isotope Neodymium 143, similar to what can be observed in present day seawater. Conclusion: 2.7 billion years ago, relatively large landmasses emerged from the oceans that were exposed to weathering and erosion by the sun, wind and rain.
"If in the Neoarchean period 97% of the Earth's surface had been, as estimated from computer models, covered by water, these geochemical signals would not have been found for Neoarchean seawater," says Dr. Elis Hoffmann from the University of Cologne. "The isotope Hafnium 176 in contrast to its counterpart Neodymium 143 was transported by means of weathering into the oceans and became part of iron-rich sediments on the sea floor 2,700 million years ago."
"We are able to carry out these isotope measurements for very rare elements, the concentrations of which are in the ppb range, i.e. only a few parts per billion," added co-author Professor Carsten Münker.
The examinations were carried out in the joint clean laboratory of the Universities of Cologne and Bonn.