Kudos for the humble clay

Article for 7 May 2007:

Kennedy M, Droser M, Mayer LM, Pevear D, Mrofka D. 2006. Late Precambrian oxygenation: inception of the clay mineral factory. Science 311, 1446-1449.

Quote: "...the advent of soils sufficiently biotic for clay formation likely predated complex terrestrial ecosystems." 

Many articles emphasise the importance of soils, but perhaps not in quite as spectacular terms as this one, which suggests that the intimately linked co-evolution of early life and soils on Earth allowed the subsequent development of complex organisms and their ecosystems. 

The mechanism for this begins with the intense weathering of rock materials, facilitated by [micro]organisms, to form phyllosilicates - the "clay" minerals. Clays have been well-known for some time to stabilise organic compounds in soils and sediments against decomposition, due to adsorption of organic compounds, creation of microporosity, and so on. If organic matter does not decompose aerobically, then oxygen is not consumed and the net global effect of these processes was to allow oxygen concentrations in the Earth's atmosphere to increase to values allowing the development and survival of higher organisms.

The novelty of the ideas in this paper are those of scale - taking phenomena which have been well-established in disciplines like soil science and considering their implication for the evolution of the Earth system, over the last billion years or so. Their ideas are supported by their data - phyllosilicate contents of sedimentary rocks (normalized to quartz) certainly increase from near-zero at 800 Ma towards the 500 Ma mark, in three stratigraphic sequences from Australia, Baltica and China. Marine sulfate concentrations and strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) also increase over this time frame, consistent with the assumed increase in atmospheric O₂.

Further novelty, and perhaps controversy, is in the authors' own admission that this represents a non-uniformitarian approach to Earth system development. Our group is mainly composed of non-geologists, and presumably such approaches exist elsewhere in contemporary geology (not surprisingly, we also had some trouble understanding the significance of strontium isotope ratios).

Some unanswered questions:
- What was consuming O₂ prior to the stabilisation of organic matter by clays, if significant life had not yet developed on earth (and presumably, organic carbon stocks were low) ?
- Are three stratigraphic sequences enough?

Transmission electron microscope image by Dr Jian Li, CSIRO Minerals