Science Literature

Paul Knauth and Martin Kennedy have been studying isotopic signatures in carbonate sediments. Indeed, they have "examined the chemical composition of all known limestones dating from the Neoproterozoic era, which stretched from 1 billion years ago up to the start of the Cambrian." The ratio of carbon-13 to carbon-12 is a frequently measured parameter, because plants preferentially absorb carbon-12. Similarly, freshwater is depleted in oxygen-18, oxygen isotope ratios are also interesting. "So sediments deposited in these conditions have a recognisable carbon-12 to oxygen-18 ratio." Significantly, the authors report that the 'modern' signature for land-derived sediments goes back - not to the Ordovician (where the record of land plants begins) - but to 850 Ma (which is Late Precambrian).

"Knauth says the balance of carbon-12 to oxygen-18 in the limestones is "screaming" that they were laid down in shallow seas that received extensive rainwater run-off from a land surface thick with vegetation."

"Screaming" is a strong word, but data itself does not scream. An interpretation placed on that data, however, can provide a compelling argument. The question to be addressed is whether that argument is strong, or whether the researchers are just shouting.

Was a moss and liverwort filled world responsible for oxygenating the atmosphere? (Source here)

There are several strands for analysis in this research. The first is whether the isotope ratios are best interpreted in the terms suggested. As well as the positive evidence from isotope fractionation, the researchers report finding a sparsely populated area of their data plots - which they named the "forbidden zone."

"If previous interpretations of carbon isotope data were correct, there would be no forbidden zone on these cross plots," Knauth said. "The forbidden zone would be full of Neoproterozoic data."
"These zones show that the isotopic fingerprints in limestone we see today started in the late Precambrian and must have involved the simultaneous influx of rain water that fell on vegetated areas, infiltrated into coastal ground waters and mixed with marine pore fluids. During sea level drops, these coastal mixing zones are dragged over vast geographic regions of the flooded continents of the Neoproterozoic," Knauth said. "Vast areas of limestone can form in these mixed pore fluids."

It will be interesting to find out what others make of this interpretation, but my initial reaction is that the argument is strong. The main objection thus far appears to be the lack of fossil evidence for vegetated land surfaces. This blog proceeds on the basis that the researchers have documented isotopic evidence strongly suggesting vegetated areas in the Neoproterozoic.

The second strand of analysis is concerned with the relationship between the evidence for vegetated land and the Cambrian explosion. The abstract includes the comment: "This facilitated a rise in O2 necessary for the expansion of multicellular life". The paper expands slightly with the words: "The terrestrial expansion of an extensive, simple land biota indicated by the isotope data may thus have been a critical step in the transition from the Precambrian to the Phanerozoic world". The press release accompanying advance publication says that the researchers "believe they have found the trigger for the Cambrian explosion". This, and the Science Daily reports suggest that the problem of the Cambrian Explosion may have been solved. "It was a massive greening of the planet [that] virtually set the table for the later explosion of life through the development of early soil that sequestered carbon, led to the build up of oxygen and allowed higher life forms to evolve". The New Scientist story says that the plants "turned the hitherto barren Earth green, created the first soils and pumped oxygen into the atmosphere, laying the foundations for animals to evolve in the Cambrian explosion that started 542 million years ago".

It has to be said that the Cambrian Explosion needs to be addressed on two fronts: biological and environmental. The new work adds nothing to the conundrums faced by biological science: the emergence of new body plans, complex organs and organelles. What it does do is contribute to the environmental story. If marine animals are to thrive, they need oxygenated waters. Accompanying this is the need for conducive water chemistry, suitable temperatures, appropriate food supplies, etc. Nevertheless, with these elements all present, life will not simply emerge to fill the space! To think this is to make the same mistake as the abiogenesis researchers who seem to think that if the building blocks are assembled, life just happens! The same problem is faced by astrobiologists: they get excited about finding a planet in the habitable zone with water - but this is just the environmental dimension. It provides a necessary but not sufficient condition. Real solutions are only obtained by the generation of complex specified information - which is the hallmark of intelligent design.

If it is the case that biological diversification is triggered by environmental factors, it can be argued that this is an indication of design in the workings of the natural world. Create a suitable environment, and it is filled with living things! Maybe the story of life on Earth is governed by ecology: where conditions are appropriate, the animals and plants that can live in those environments move in to colonise them. Ecology, rather than evolution from primitive ancestors, may be the key to understanding the development of living things.

The late Precambrian greening of the Earth
L. Paul Knauth & Martin J. Kennedy
Nature, 460, 728-732 (6 August 2009) | doi:10.1038/nature08213 (pdf here)

Many aspects of the carbon cycle can be assessed from temporal changes in the 13C/12C ratio of oceanic bicarbonate. [. . .] Here we compile all published oxygen and carbon isotope data for Neoproterozoic marine carbonates, and consider them in terms of processes known to alter the isotopic composition during transformation of the initial precipitate into limestone/dolostone. We show that the combined oxygen and carbon isotope systematics are identical to those of well-understood Phanerozoic examples that lithified in coastal pore fluids, receiving a large groundwater influx of photosynthetic carbon from terrestrial phytomass. Rather than being perturbations to the carbon cycle, widely reported decreases in 13C/12C in Neoproterozoic carbonates are more easily interpreted in the same way as is done for Phanerozoic examples. This influx of terrestrial carbon is not apparent in carbonates older than ~850 Myr, so we infer an explosion of photosynthesizing communities on late Precambrian land surfaces. As a result, biotically enhanced weathering generated carbon-bearing soils on a large scale and their detrital sedimentation sequestered carbon. This facilitated a rise in O2 necessary for the expansion of multicellular life.

When Earth greened over
Explosion of animal life could have been triggered by blanket of vegetation.
Eric Hand
Nature 460, 161 (8 July 2009) | doi:10.1038/460161a

Abstract: A thick, green carpet of photosynthetic life, on the scale of that seen today, exploded across Earth 850 million years ago - much earlier than thought - a new study suggests.