Sarah Gabbott and Jan Zalasiewicz
As an eternal testament of humanity, plastic bags, cheap clothes and chicken bones are not a glorious legacy. But two scientists exploring which items from our technological civilisation are most likely to survive for many millions of years as fossils have reached an ironic but instructive conclusion: fast food and fast fashion will be our everlasting geological signature…….Continue reading….
Source: The Guardian
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Critics:
Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better.
Increases in erosion due to farming and other operations will be reflected by changes in sediment composition and increases in deposition rates elsewhere. In land areas with a depositional regime, engineered structures will tend to be buried and preserved, along with litter and debris. Litter and debris thrown from boats or carried by rivers and creeks will accumulate in the marine environment, particularly in coastal areas, but also in mid-ocean garbage patches. Such human-created artifacts preserved in stratigraphy are known as “technofossils”.
Changes in biodiversity will also be reflected in the fossil record, as will species introductions. An example cited is the domestic chicken, originally the red junglefowl Gallus gallus, native to south-east Asia but has since become the world’s most common bird through human breeding and consumption, with over 60 billion consumed annually and whose bones would become fossilised in landfill sites. Hence, landfills are important resources to find “technofossils”.
Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to the element into which it decays shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers.
Consequently, paleontologists must usually rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the “layer-cake” that is the sedimentary record, and has been compared to a jigsaw puzzle. Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil’s age must lie between the two known ages.
Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is very difficult to match up rock beds that are not directly next to one another. However, fossils of species that survived for a relatively short time can be used to link up isolated rocks: this technique is called biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period.
If rocks of unknown age are found to have traces of E. pseudoplanus, they must have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought. Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying evolution.
However, this is difficult for some time periods, because of the problems involved in matching up rocks of the same age across different continents. Family-tree relationships may also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated “family tree” says A was an ancestor of B and C, then A must have evolved more than X million years ago.
It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived – by assuming that DNA mutations accumulate at a constant rate. These “molecular clocks”, however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two.
Life on earth has suffered occasional mass extinctions at least since 542 million years ago. Despite their disastrous effects, mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of an ecological niche passes from one group of organisms to another, this is rarely because the new dominant group outcompetes the old, but usually because an extinction event allows a new group, which may possess an advantageous trait, to outlive the old and move into its niche.
The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between mass extinctions becoming longer and the average and background rates of extinction decreasing. However, it is not certain whether the actual rate of extinction has altered, since both of these observations could be explained in several ways: The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions:
Dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; marine ecosystems became more diversified so that food chains were less likely to be disrupted. Reasonably complete fossils are very rare: most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks.
So paleontologists have mistakenly assigned parts of the same organism to different genera, which were often defined solely to accommodate these finds – the story of Anomalocaris is an example of this. The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many “superfluous” genera are represented by fragments that are not found again, and these “superfluous” genera are interpreted as becoming extinct very quickly.
Biodiversity in the fossil record, which is “the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time” shows a different trend: a fairly swift rise from 542 to 400 million years ago, a slight decline from 400 to 200 million years ago, in which the devastating Permian–Triassic extinction event is an important factor, and a swift rise from 200 million years ago to the present.
Changes in drainage patterns traceable to human activity will persist over geologic time in large parts of the continents where the geologic regime is erosional. This involves, for example, the paths of roads and highways defined by their grading and drainage control. Direct changes to the form of the Earth’s surface by human activities (quarrying and landscaping, for example) also record human impacts.
It has been suggested that the deposition of calthemite formations exemplify a natural process which has not previously occurred prior to the human modification of the Earth’s surface, and which therefore represents a unique process of the Anthropocene.Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment. Calthemites grow on or under man-made structures (including mines and tunnels) and mimic the shapes and forms of cave speleothems, such as stalactites, stalagmites, flowstone etc.
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