Edicara sandstones in the Flinders Ranges of South Australia (credit: Wikipedia).
The first macroscopic life forms were the enigmatic bag-like and quilted fossils in sedimentary rocks dating back to 635 Ma in Australia, eastern Canada and NW Europe. They are grouped as the Ediacaran Fauna named after the Ediacara Hills in South Australia where they are most common and diverse. Generally they are not body fossils but impressions of soft-bodied organisms, often in sandstones rather than muds. Some are believed to be animals that absorbed nutrients through their skin, whereas others are subjects of speculation. One thing seems clear; these first metazoans arose because of some kind of trigger provided by the global glacial conditions that preceded their appearance. It has always been assumed that, whatever they were, Ediacaran organisms lived on the sea floor, probably in shallow water. New sedimentological evidence found in the type locality by Gregory Retallack of the University of Oregon seems set to force a complete rethink about these hugely important life forms (Retallack, G.J 2012. Ediacaran life on land. Nature (online), doi:10.1038/nature11777). So momentous are his conclusions that they form the subject of a Nature editorial in the 13 December 2012 issue.
Retallack, a specialist on ancient soils of the Precambrian, examined reddish facies of the Ediacara Member of the Rawnsley Quartzite of South Australia, whose previous interpretation have a somewhat odd background. Originally regarded as non-marine, before their fossils were discovered, when traces of jellyfish-like organisms turned up this view was reversed to marine, the red coloration being ascribed to deep Cretaceous weathering. A range of features, such as clasts of red facies in grey Ediacaran rocks, the presence of feldspar in the red facies – unlikely to have survived deep weathering – bedding surfaces with textures very like those formed by subaerial biofilms, and desiccation cracks, suggest to Retallack that the red facies represents palaeosols in the sedimentary sequence. Moreover, some features indicate a land surface prone to freezing from time to time. The key observation is that this facies contains Ediacaran trace fossils representing many of the forms previously regarded as marine animals of some kind, including Spriggina, Dickinsonia and Charnia on which most palaeontologists would bet good money that they were animals, albeit enigmatic ones.
Specimen of Edicaran Dickinsonia (credit: Wikipedia)
If Retallack’s sedimentological observations are confirmed then organisms found in the palaeosols cannot have been animals but perhaps akin to lichens or colonial microbes, and survived freezing conditions. As they occur in other facies more likely to be subaqueous, then they were ‘at home’ in a variety of ecosystems. As the Nature editorial reminds us, from the near-certainty that early macroscopic life was marine there is a chance that views will have to revert to a terrestrial emergence first suggested in the 1950s by Jane Grey. Uncomfortable times lie ahead for the palaeontological world.
The Earth 640 million years ago during the Marinoan ‘Snowball’ event (credit: Cornell University via Flickr)
Palaeobiologists generally believe that without a significant boost to oxygen levels in the oceans macroscopic eukaryotes, animals in particular, could not have evolved. Although the first signs of a rise in atmospheric oxygen enter the stratigraphic record some 2.4 billion years ago and eukaryote microfossils appeared at around 2 Ga, traces of bulky creatures suddenly show up much later at ~610 Ma with possible fossil bilaterian embryos preserved in 630 Ma old sediments. An intriguing feature of this Ediacaran fauna is that it appeared shortly after one of the Neoproterozoic global glaciations, the Marinoan ‘Snowball’ event: a coincidence or was there some connection? It has looked very like happenstance because few if any signs of a tangible post-Marinoan rise in environmental oxygen have been detected. Perhaps the sluggish two billion-year accumulation of free oxygen simply passed the threshold needed for metazoan metabolism. But there are other, proxy means of assessing the oxidation-reduction balance, one of which depends on trace metals whose chemistry hinges on their variable valency. The balance between soluble iron-2 and iron-3 that readily forms insoluble compounds is a model, although iron itself is so common in sediments that its concentration is not much of a guide. Molybdenum, vanadium and uranium, being quite rare, are more likely to chart subtle changes in the redox conditions under which marine sediments were deposited.
Dickinsonia; a typical Ediacaran animal. Scale in cm (credit: Wikipedia)
Swapan Sahoo of the University of Nevada and colleagues from the USA, China and Canada detected a marked increase in the variability of Mo, V and U content of the basal black shales of the Doushantuo Formation of southern China, which contain the possible eukaryote embryos (Sahoo, S.K and 8 others 2012. Ocean oxygenation in the wake of the Marinoan glaciation. Nature, v. 489, p. 546-549). These rocks occur just above the last member of the Marinoan glacial to post-glacial sedimentary package and are around 632 Ma old. Since the black shales accumulated at depths well below those affected by surface waves that might have permitted local changes in the oxygen content of sea water the geochemistry of their formative environment ought not to have changed if global chemical conditions had been stable: the observed fluctuations may represent secular changes in global redox conditions. The earlier variability settles down to low levels towards the top of the analysed sequence, suggesting stabilised global chemistry.
What this might indicate is quite simple to work out. When the overall chemistry of the oceans is reducing Mo, V and U are more likely to enter sulfides in sediments, thereby forcing down their dissolved concentration in sea water. With a steady supply of those elements, probably by solution from basalt lavas at ocean ridges, sedimentary concentrations should stabilise at high levels in balance with low concentrations in solution. If seawater becomes more oxidising it holds more Mo, V and U in solution and sediment levels decline. So the high concentrations in sediments mark periods of global reducing conditions, whereas low values signal a more oxidising marine environment. Sahoo et al.’s observations suggest that marine geochemistry became unstable immediately after the Marinoan glaciation but settled to a fundamentally more oxidising state than it had been in earlier times, perhaps by tenfold increase in atmospheric oxygen content. So what might have caused this and the attendant potential for animals to get larger in the aftermath of the Snowball Earth event? One possibility is that the long period of glaciers’ grinding down continental crust added nutrients to the oceans. Once warmed and lit by the sun they hosted huge blooms of single-celled phytoplankton whose photosynthesis became an oxygen factory and whose burial in pervasive reducing conditions on the sea bed formed a permanent repository of organic carbon. The outcome an at-first hesitant oxygenation of the planet and then a permanent fixture opening a window of opportunity for the Ediacarans and ultimately life as we know it.
Artist's impression of the Neoproterozoic Earth during a Snowball episode. Image by guano via Flickr
The combination of glaciogenic sediments with palaeomagnetic evidence for their formation at low-latitudes, together with dates that show glacial events were coeval in just two or three Neoproterozoic episodes are the linchpins for the Snowball Earth hypothesis. There is little doubt that the latest Precambrian Era did witness such extraordinary climatic events. Evidence is also accumulating that, in some way, they were instrumental in that stage of biological evolution from which metazoan eukaryotes emerged: the spectacular Ediacaran fossil assemblages follow on the heels of the last such event (see Bigging-up the Ediacaran in Earth Pages for March 2011). One of the difficulties with the ‘hard’ Snowball Earth hypothesis is how the middle-aged planet was able to emerge from a condition of pole-to-pole ice cover; hugely increased reflectivity of that surface should have driven mean global temperature down and down. Clearly the Earth did warm up on each occasion, and the leading model for how that was possible is massive release of greenhouse gases from sea-floor sediments or deep-ocean waters to increase the heat-retaining powers of the atmosphere; sufficiently voluminous release from volcanic action seems less likely as there is little evidence of upsurges in magmatism coinciding with the events. Almost all glaciogenic units from the Neoproterozoic have an overlying cap of carbonate rocks, indicating that hydrogen carbonate (formerly bicarbonate) ions together with those of calcium and magnesium suddenly exceeded their solubilities in the oceans.
Modern sea-floor hydrothermal vent. Image via Wikipedia
To seek out a possible source for sufficient carbon release in gaseous form geochemists have turned to C-isotopes in the cap carbonates. Early studies revealed large deficits in the heavier stable isotope of carbon (13C) that seemed to suggest that the releases were from large reservoirs of carbon formed by burial of dead organisms: photosynthesis and other kinds of autotrophy at the base of the trophic pyramid selectively take up lighter 12C in forming organic tissues compared with inorganic chemical processes). As in the case of the sharp warming event at the Palaeocene-Eocene boundary around 55.8 Ma ago (See The gas-hydrate ‘gun’ in June 2003 Earth Pages), these negative d13C spikes have been interpreted as due to destabilisation of gas hydrates in sea-floor sediments to release organically formed methane gas. This powerful greenhouse gas would have quickly oxidised to CO2 thus acidifying the oceans by jacking up hydrogen carbonate ion concentrations. Detailed carbon-, oxygen- and strontium-isotope work in conjunction with petrographic textures in a Chinese cap carbonate (Bristow, T.F. et al. 2011. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China. Nature, v. 274, p. 68-71) suggests an alternative mechanism to produce the isotopically light carbon signature at the end of Snowball events. The greatest 13C depletion occurs in carbonate veins that cut through the cap rock and formed at temperatures up to 378°C and even the early-formed fine grained carbonate sediment records anomalously high temperatures. So, it seems as if the cap-rock was thoroughly permeated by hydrothermal fluids, more than 1.6 Ma after it formed on the sea floor. This triggered oxidation of methane within the sediments themselves, with little if any need for an atmospheric origin through massive methane release from destabilised gas hydrates elsewhere.