Zircons hint at early tectonic activity, life
Two analyses of tiny mineral bits that crystallized during the Earth’s formative years have provided new insights into the planet’s earliest days.
One study of mineral inclusions in zircons from the Jack Hills of Western Australia hints that the crystals formed at depths of around 25 kilometers and at temperatures of about 700° Celsius. Those findings, in turn, suggest that plate tectonics had already begun on Earth between 4.19 billion and 4.02 billion years ago, less than 600 million years after the planet first coalesced (SN: 1/3/09, p. 10).
Another study, this one of the carbon-isotope ratios of microdiamond inclusions in zircons that ranged between 3.05 billion and 4.25 billion years of age, found a much-lower-than-average concentration of the carbon-13 isotope in those inclusions, a possible sign of biological processes (SN: 8/2/08, p. 13). Metabolic processes that take place in an organism’s cells, and especially in microorganisms, produce isotopically light carbon. A much-higher-than-average concentration of carbon-12, the lightest of carbon’s stable isotopes, often is a sign that the carbon was generated by biologic activity.
The ratio of carbon-13 to carbon-12 was far below that found in other diamonds and even in other reservoirs of isotopically light carbon, including carbon-rich meteorites and interplanetary dust. If the ratio is indeed a sign of life, it pushes back the presence of life on Earth at least 400 million years.
Fire under ice Thick layers of volcanic ash (above, ash samples shown in inset) blanketing a patch of Arctic seafloor point to explosive volcanism at a depth greater than 4,000 meters, a phenomenon scientists long thought impossible because of the immense pressures at such depths (SN Online: 6/25/08).
Dates in doubt Chemical biomarkers in 2.7-billion-year-old Australian rocks, once thought to be the oldest known evidence of complex life on Earth, may have infiltrated long after the sediments were laid down, new analyses suggest (SN: 11/22/08, p. 5).
Antarctic trees Trees that grew in Antarctica millions of years ago, when its climate was more mild, had a growth pattern much different from modern trees (SN: 11/8/08, p. 16).
Minerals evolve too In a recounting of the history of mineral evolution, researchers find the number of minerals in the solar system has increased through time, and some minerals on Earth exist because of life (SN: 12/6/08, p. 10).
Firm evidence A ground motion recorded in Japan provides the strongest, most direct evidence that Earth’s inner core (illustrated below) is solid (SN: 9/13/08, p. 14).
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Sea-to-land shift New fossils of an ancient four-limbed creature help fill in the blanks of the evolutionary transition between fish and the first land-adapted vertebrates (SN Online: 6/25/08).
Off the ice A comprehensive record of sea level variations between 542 million and 251 million years ago, compiled from rock strata (one shown right), reveals fluctuations that could have been caused by geological processes other than the formation of massive, land-based ice sheets (SN Online: 10/2/08).Ancient rocks Scientists may have found the world’s oldest intact rocks in a patch of bedrock on the eastern shore of Canada’s Hudson Bay. Analyses suggest the rocks are about 4.28 billion years old ( SN: 10/11/08, p. 12 ).
Forget bird-brained Paleontologists discover a new species of carnivorous dinosaur, Aerosteon riocoloradensis, that breathed like a bird (SN: 10/25/08, p. 14).
Life down deep As much as 70 percent of the Earth’s microbial life resides on and just below the ocean floor, two new studies suggest (SN: 6/21/08, p. 7).
Inorganic building blocks Hydrocarbons in the fluids spewing from a set of hydrothermal vents on the Atlantic seafloor were produced by inorganic chemical reactions within the ocean crust, a finding with implications for the origins of life (SN: 2/2/08, p. 67).