Visitors to the Grand Canyon, and especially those who hike to Plateau Point at the end of Bright Angel Trail, will see a major change in rock type when looking into the inner canyon. The steep walls reveal metamorphosed basement rocks, but resting on these are the horizontally-bedded fresher looking Tapeats Sandstones. The linear boundary between them is known as the "Great Unconformity". There are many other unconformities to be found in the Grand Canyon, but this one is by far the most dramatic. It can be traced as far as the eye can see - and beyond. It is found on most continents:
"The Great Unconformity is well exposed in the Grand Canyon, but this geomorphic surface, which records the erosion and weathering of continental crust followed by sediment accumulation, can be traced across Laurentia and globally, including Gondwana, Baltica, Avalonia and Siberia, making it the most widely recognized and distinctive stratigraphic surface in the rock record."
The trail to Plateau Point on the Tapeats Sandstone (above the Great Unconformity) is in the foreground. A cross-section through the rocks, showing the Great Unconformity, can be seen on the other side of the inner gorge. (Source here)
Not only is the unconformity visually striking, but also it occurs at a special horizon in the rock record: above it are fossils of hard-bodied animals - the unconformity marks their first appearance all over the world. So, this feature is much more than a lithological discontinuity: it is also a faunal discontinuity known affectionately as the "Cambrian Explosion". This distinctive feature of the fossil record has engaged the minds of geologists since it was first recognised. The debate has been whether the "explosion" of life forms was an abrupt punctuation of Earth history, or whether the Great Unconformity hides an inferred record of gradual transformation. Geologists in the 19th Century were unduly influenced by James Hutton and Charles Lyell who set out a view of Earth history that involved endless cycles ("we find no vestige of a beginning, - no prospect of an end"). However, as Gould has admirably explained, geological time is more accurately represented by an arrow, the view that was advanced by some of Lyell's contemporaries.
"Lyell and the catastrophists were locked in a fascinating debate of substance about the way of our world, not a wrangle about methodological aspects of uniformity. Their struggle pitted a directional view of history as a vector leading toward cooler climates and more complex life, and fueled by occasional catastrophes, against Lyell's vision of a world in constant motion, but always the same in substance and state, changing bit by bit in a stately dance toward nowhere. This real debate, so lost at our peril in the success of Lyell's rhetoric, was the grandest battle ever fought between the visions of time's arrow and time's cycle." (page 132)
In a recent paper, two geologists have suggested that the processes involved in forming the Great Unconformity provided a trigger for the Cambrian Explosion.
"The magnitude of the unconformity is without rival in the rock record," Gaines says. "When we pieced that together, we realized that its formation must have had profound implications for ocean chemistry at the time when complex life was just proliferating." "We're proposing a triggering mechanism for the Cambrian explosion," says Peters. "Our hypothesis is that biomineralization evolved as a biogeochemical response to an increased influx of continental weathering products during the last stages in the formation of the Great Unconformity." Peters and Gaines looked at data from more than 20,000 rock samples from across North America and found multiple clues, such as unusual mineral deposits with distinct geochemistry, that point to a link between the physical, chemical, and biological effects." (Source here)
The research has examined sediments in North America overlying the Great Unconformity. These are known collectively as the Sauk sequence. The impact of erosion to produce these rocks on seawater chemistry has been assessed comprehensively. The researchers have consider all the major ionic products of weathering and different depositional environments. Carbonate sedimentation is distinctive:
"The signal of enhanced continental crustal weathering is perhaps most conspicuously expressed by precipitation of carbonate sediments, which reached a Phanerozoic peak in shelf burial flux during the Sauk transgression. In Laurentia, the large quantity of Cambrian-Early Ordovician carbonates is known as the 'Great American Bank'. Precipitation of carbonates is an important sink for alkalinity that is derived from chemical weathering." [. . .] "[Their] results are consistent with a recent model of the Cambrian carbon cycle, which demonstrated that unusually large absolute rates of carbon throughput are required to explain global carbon isotopic excursions."
Alongside the carbonate data are analyses of deposits of glauconite, a potassium-, silica-, and iron-rich mineral that is rarely formed today. These findings confirm the same narrative of extensive continental weathering and have led to the proposal of a "trigger" for the Cambrian Explosion.
"The influx of ions to the oceans also likely posed a challenge to the organisms living there. "Your body has to keep a balance of these ions in order to function properly," Peters explains. "If you have too much of one you have to get rid of it, and one way to get rid of it is to make a mineral."
The fossil record shows that the three major biominerals -- calcium phosphate, now found in bones and teeth; calcium carbonate, in invertebrate shells; and silicon dioxide, in radiolarians -- appeared more or less simultaneously around this time and in a diverse array of distantly related organisms."
Some cautionary words are now worth making. The authors have demonstrated a convincing association of changing seawater chemistry and the first appearance of a great diversity of hard-bodied animals. It is reasonable to propose connections between these observational data. The word "trigger" can be used to describe this association. However, the researchers are proposing more than this. They are claiming that the changing environment drove adaptive change in organisms such that they constructed biominerals: bones and teeth, shells and tests. This additional proposal is unsupported by evidence. Whilst we know that organisms today can extract ions from seawater and make minerals, we do not know that organisms lacking the internal systems to make minerals can adapt to changing environments and somehow develop the necessary internal systems. This is not supported by empirical work today, and any talk of it happening in the past is no more than story-telling. This approach brings the same problems as some other proposed "triggers": the higher levels of oxygenation of seawater and the evolution of eyes.
The "adaption to biomineralization" hypothesis can be tested using the data known to us. By studying the way organisms adapt to environmental change, we can assess the potential for the evolution of systems to build hard body parts. The evidence we have points to extinction rather than adaptive change: natural selection can tweak existing parameters affecting morphology and physiology, but biomineralization requires complex specified information - something that natural selection has not been able to provide. Furthermore, we know something of the life forms living before the Cambrian Explosion, and palaeontologists have found it very difficult to show any direct links between these organisms and the Cambrian animals. In 2007, Adolf Seilacher wrote: "The notion that the majority of Ediacaran fossils do not represent stem groups to modern metazoan phyla is now increasingly accepted." (source here). The Ediacaran fauna is still an enigma. A more viable hypothesis for the proposed "trigger" is that changing seawater chemistry was the reason for most of the Ediacaran species becoming extinct.
In a situation like this, where a convincing correlation has been documented but causation is in great need of critical discussion, there is a strong case for multiple working hypotheses. This is needed to remind researchers and students that proposed causation mechanisms are tentative, if not speculative. Alternatives to that suggested in this paper are urgently required. This blog has advanced an alternative perspective on understanding faunal and floral changes found in the fossil record: the concept of ecological succession constrained by environmental factors. This theme has been discussed here in the context of the Cambrian Explosion, and has appeared in numerous other blogs addressing fossilised animals and plants. The research paper discussed above developed the arguments previously advanced. The implication of pursuing the ecology hypothesis is that the fossil record is not so much a record of evolutionary transformation, but a record of colonisation of Earth environments by the diversity of living things.
Formation of the 'Great Unconformity' as a trigger for the Cambrian explosion
Shanan E. Peters & Robert R. Gaines
Nature, 484, 363-366 (19 April 2012) | doi:10.1038/nature10969
The transition between the Proterozoic and Phanerozoic eons, beginning 542Ã¢â‚¬â€°million years (Myr) ago, is distinguished by the diversification of multicellular animals and by their acquisition of mineralized skeletons during the Cambrian period. Considerable progress has been made in documenting and more precisely correlating biotic patterns in the Neoproterozoic-Cambrian fossil record with geochemical and physical environmental perturbations, but the mechanisms responsible for those perturbations remain uncertain. Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540-480 Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust. These geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by extensive physical reworking of soil, regolith and basement rock during the first continental-scale marine transgression of the Phanerozoic. The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity. Although Darwin and others have interpreted this widespread hiatus in sedimentation on the continents as a failure of the geologic record, this palaeogeomorphic surface represents a unique physical environmental boundary condition that affected seawater chemistry during a time of profound expansion of shallow marine habitats. Thus, the formation of the Great Unconformity may have been an environmental trigger for the evolution of biomineralization and the 'Cambrian explosion' of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals.
Evidence for a Geologic Trigger of the Cambrian Explosion, ScienceDaily (18 April 2012)
Luskin, C., Does Lots of Sediment in the Ocean Solve the "Mystery" of the Cambrian Explosion? (Evolution News & Views, 27 April 2012)
Tyler, D. Mapping the appearances of Cambrian animals, (ARN Literature blog, 23 December 2010).
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