* Astronomy

Determining the concentration of carbon dioxide in the ancient atmosphere remains a critical hurdle to understanding Earth surface temperatures, compositional changes in atmospheric chemistry, and the evolution of Earths earliest biosphere.

Kah and Riding report the finding of petrographic fabrics in 1.2-billion-year-old carbonate strata that suggest the process of cyanobacterial calcification. When ambient atmospheric partial pressure of CO2 (pCO2) concentrations fall below roughly 10 times present atmospheric levels (PAL), cyanobacteria begin to use a combination of dissolved carbonate species (CO32- and HCO3-) in the photosynthetic production of organic matter. The cyanobacteria, however, must perform a series of biochemical gymnastics to utilise HCO3- in this process. As a byproduct of these biochemical changes, the pH of the microbial sheath rises dramatically and induces the precipitation of calcium carbonate minerals. Identification of these calcified sheaths in the geologic record thus place an upper limit on pCO2. Atmospheric concentrations of CO2 <10 times PAL in the Mesoproterozoic are significantly lower than previous estimates, and indicate the requirement of additional greenhouse gasses, such as methane, in the atmosphere to keep the oceans from freezing.

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The other day I was talking to a geologist friend of mine who told me about a pebble she found that held a clue to the amount of carbon dioxide in Earth's atmosphere 3.2 billion years ago.
The walnut-sized pebble, she explained, was found in a slender core sample drilled from a gold mine in South Africa its timeworn surface long ago weathered to a rind by running water. My geologist friend discovered that the pebble's crusty skin is made of quartz (like white beach sand) and iron-rich carbonate minerals.
The iron-rich carbonate is the clue. Carbonate contains a lot of carbon.
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Oxygen in Ancient Atmosphere Rose Gradually to Modern Levels:

The history of life on Earth is closely linked to the appearance of oxygen in the atmosphere. The current scientific consensus holds that significant amounts of oxygen first appeared in Earth's atmosphere some 2.4 billion years ago, with a second large increase in atmospheric oxygen occurring much later, perhaps around 600 million years ago.

However, new findings by University of Maryland geologists suggest that the second jump in atmospheric oxygen actually may have begun much earlier and occurred more gradually than previously thought. The findings were made possible using a new tool for tracking microbial life in ancient environments developed at Maryland. Funded by the National Science Foundation and NASA, the work appears in the December 2 issue of Science.

Graduate researcher David Johnston, research scientist Boswell Wing and colleagues in the University of Maryland's department of geology and Earth System Science Interdisciplinary Centre led an international team of researchers that used high-precision measurements of a rare sulphur isotope, 33S, to establish that ancient marine microbes known as sulphur disproportionating prokaryotes were widely active almost 500 million years earlier than previously thought.

The intermediate sulphur compounds used by these sulphur-disproportionating bacteria are formed by the exposure of sulphide minerals to oxygen gas. Thus, evidence of widespread activity by this type of bacteria has been interpreted by scientists as evidence of increased atmospheric oxygen content.

"These measurements imply that sulphur compound disproportionation was an active part of the sulphur cycle by [1.3 million years ago], and that progressive Earth surface oxygenation may have characterized the (middle Proterozoic)" .

The Proterozoic is the period in Earth's history from about 2.4 billion years ago to 545 million years ago.

"The findings also demonstrate that the new 33S-based research method can be used to uniquely track the presence and character of microbial life in ancient environments and provide a glimpse of evolution in action. This approach provides a significant new tool in the astrobiological search for early life on Earth and beyond" - David Johnston

When our planet formed some 4.5 billion years ago, virtually all the oxygen on Earth was chemically bound to other elements. It was in solid compounds like quartz and other silicate minerals, in liquid compounds like water, and in gaseous compounds like sulphur dioxide and carbon dioxide. Free oxygen -- the gas that allows us to breath, and which is essential to all advanced life -- was practically non-existent.

Scientists have long thought that appearance of oxygen in the atmosphere was marked by two distinct jumps in oxygen levels. In recent years, researchers have used a method developed by University of Maryland geologist James Farquhar and Maryland colleagues to conclusively determine that significant amounts of oxygen first appeared in Earth's atmosphere some 2.4 billion years ago. Sometimes referred to as the "Great Oxidation Event," this increase marks the beginning of the Proterozoic period.

A general scientific consensus has also held that the second major rise in atmospheric oxygen occurred some 600 million years ago, with oxygen rising to near modern levels at that time. Evidence of multicellular animals first appears in the geologic around this time.

"There has been a lot of discussion about whether the second major increase in atmospheric oxygen was quick and stepwise, or slow and progressive. Our results support the idea that the second rise was progressive and began around 1.3 billion years ago, rather than 0.6 billion years ago" - Boswell Wing.

In addition to Johnston, Wing's Maryland co-authors on the December 2 paper are geology colleagues James Farquhar and Jay Kaufman. Their group works to document links between sulphur isotopes and the evolution of Earth's atmosphere using a combination of field research, laboratory analysis of rock samples, geochemical models, photochemical experiments with sulphur-bearing gases and microbial experiments.

"Active microbial sulphur disproportionation in the Mesoproterozoic" by David T. Johnston, Boswell A. Wing, James Farquhar and Alan J. Kaufman, University of Maryland; Harald Strauss, Universität Münster; Timothy W. Lyons, University of California, Riverside; Linda C. Kah, University of Tennessee; Donald E. Canfield, Southern Denmark University: Science, Dec. 2, 2005.

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