7. desember 2011

Grunnfjell avslører oksygengåte

Jordas atmosfære ble ikke rik på oksygen ved en enkelt hendelse, snarere gjennom en serie episoder over flere hundre millioner år. Det mener forskere som har gransket borekjerner fra grunnfjellet i nordvest-Russland.

Core SamplesCORE SAMPLES: Victor Melezhik, Aivo Lepland and Melanie Mesli examining drill cores at the NGU laboratory. Photo: Gudmund LøvøThe discoveries were made by an international research group and have been published jointly in Science by, among others, NGU geoscientists Victor Melezhik, Aivo Lepland and Alenka Crne. The work forms part of an international project, the Fennoscandia Arctic Russia - Drilling Early Earth Project (FAR DEEP).

The fieldwork took place over five months on the Kola Peninsula and in Karelia in 2007. Cores were drilled into basement rocks spanning an unbelievably long period from 2440 million to 2000 million years.

“In the NGU laboratory, we’ve investigated and described 3.6 kilometres of core samples. All the information from the rock cores has been fed into a database on the Internet. Scientists from 15 nations have so far performed a variety of studies of this material. The samples are available and are very interesting for understanding this eventful period in the Precambrian,” the NGU researchers, Melezhik and Lepland, said.

A long history

Imadra-VarzugaGREENSTONE BELT: Panorama from Russia’s Imadra-Varzuga Greenstone Belt, where some of the FAR DEEP project drilling took place. Photo: Victor MelezhikOne of the results has now been presented in the 1 December number of Science. “We’ve always thought that oxygen came into the atmosphere really quickly during an event,” the lead author of the paper in Science, Lee Kump from Penn State University, said in a press report. “We are no longer looking for an event. Now we’re looking for when and why oxygen became a stable part of the Earth’s atmosphere.”

“This timing depends on which threshold you’re looking for. The gradual spread of oxygen in the atmosphere took hundreds of millions of years,” the NGU researchers said.

Results of carbon isotope studies of FAR DEEP cores were compared with equally old samples from Gabon to find out whether oxygen appeared simultaneously elsewhere in the world. The samples from northwestern Russia and Gabon show comparable changes in carbon isotopes through time, and the changes occurred gradually and globally.

In organic material

About 2500 million years ago, oxygen levels finally crossed the threshold at which the mineral pyrite could be oxidized. A couple of hundred million years later, red sedimentary rocks containing iron oxides were deposited around the world under a gradually increasing amount of free oxygen. The sparse amounts of oxygen were produced by photosynthesis of single-celled organisms.

By 2000 million years ago, the oxygen in the atmosphere had reached one per cent of the present-day level, enough for it to seep into the groundwater and oxidize buried organic material.

The scientists have thus been able to shed light on the entire, never-ending carbon cycle involving water, air and soil during the Precambrian.

Time and mechanism

“Insights into Earth’s carbon cycle offer tantalizing clues to the history of atmospheric oxygen levels, and Kump and others have revealed unrecognized details of the timing and mechanism of the Great Oxidation Event,” said Enriqueta Barrera, program director in the American National Science Foundation’s Division of Earth Sciences.

Lee Kump’s co-authors are Michael Arthur at Penn State University, Christopher Junium at Syracuse University, Alex Brasier and Anthony Fallick at the Scottish Universities Environmental Research Centre, Victor Melezhik, Aivo Lepland and Alenka Crne at NGU, and Genming Luo at the China University of Geosciences.

The Norwegian part of the research has been supported by the Research Council of Norway.

Fieldtrip2007: Victor Melezhik and Aivo Lepland preparing their vehicle for the five-month spell of fieldwork in northwestern Russia in 2007. Photo: Gudmund Løvø

Kump, L. R., Junium, C., Arthur, M. A., Brasier, A., Fallick, A., Melezhik, V., Lepland, A., Crne, A. E., Luo, G. Isotopic Evidence for Massive Oxidation of Organic Matter Following the Great Oxidation Event. Science. 1 December 2011.