Friday, December 08, 2006
Geologists have uncovered evidence in the oil fields of Oman that explains how Earth could suddenly have changed 540 million years ago to favor the evolution of the single-celled life forms to the multicellular forms we know today.
Reporting in the December 7 issue of the journal Nature, researchers from MIT, the California Institute of Technology, and Indiana University show that there was a sudden change in the oxygenation of the world's oceans at the time just before the 'Cambrian explosion,'* one of the most significant adaptative radiations in the history of life. With a increased availability of oxygen, the team speculates, single-celled life forms that had dominated the planet for the previous three billion years were able to evolve into the diverse metazoan phyla that still characterize life on Earth.
'The presence of oxygen on Earth is the best indicator of life,' says coauthor John Grotzinger (homepage), the Fletcher Jones Professor of Geology at Caltech and an authority on sedimentary geology. 'But it wasn't always that way. The history of oxygen begins about two and a half billion years ago and occurs in a series of steps. The last step is the subject of this paper.'
The key insight was derived when Grotzinger's student Dave Fike, who is lead author of the paper, analyzed core samples and drillings taken at a depth of about three kilometers from oil wells in Oman, which are known to have the oldest commercially viable oil on the planet. The results of carbon and sulfur isotopic analyses from the material led the team to the conclusion that the oceanic conditions that laid down the deposits originally in Oman were quite different from conditions of today.
Continued at "Geologists Provide New Evidence for Reason Behind Rise of Life in Cambrian Period" (Fossils)
Based on the Letter to Nature "Oxidation of the Ediacaran Ocean"
First Paragraph (Full text is also currently available via this link)
Oxygenation of the Earth's surface is increasingly thought to have occurred in two steps. The first step, which occurred approx 2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period (approx 800 - 542 Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates and may have led to oxygenation of the deep ocean. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (approx 635 to approx 548 Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon and sulphur cycles that occurred during this significant period of Earth's history.
* Info on the Cambrian Explosion:
"The Cambrian explosion of species refers to the geologically sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). In addition, a similar pattern of diversification is seen in other organisms such as phytoplankton and the various colonial calcareous micro-fossils grouped together as calcimicrobes. The base of the Cambrian is also marked by strong geochemical perturbations, including excursions in carbon and sulfur isotopes." (Continued here)
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