Matching Earth Time deposits around the world

At present a vast range of deposits are being laid down in many different environments over the Earth's surface. So there are river muds, now being deposited in the Thames estuary in southern England, that are the same age as glacial debris generated in polar and alpine regions and muds accumulating on the deep ocean floor. The latter are full of the remains of minute planktonic organisms and the odd whale bone, not to mention the occasional shipwreck. They, in turn, are the same age as sediment derived from the Grand Canyon and carried by the Colorado River into the Gulf of California. However, there is no direct means of measuring the exact age and contemporaneity of these deposits by any kind of geological date stamp.

Nevertheless, there is a chance that they can be closely correlated by a variety of indirect measures and especially from their organic content. 'Closely' is the critical word here. I mean close in the geological sense, which means that you have to read the small print before laying any serious bets on the correlation. This problem of correlation of sedimentary deposits and rock strata has consumed the professional lives of countless geologists since the days when

James Hutton, 1726-97, Scottish Enlightenment natural philosopher who studied medicine in Leiden, Holland, pioneered agricultural improvement and geological studies of volcanic and stratified rocks. His Theory of the Earth, 1788, was highly influential but mostly through Playfair's 1802 version.

John Playfair, 1748-1819, Scottish professor of mathematics and geology at Edinburgh who was a close friend of Hutton's and wrote Illustrations of the Huttonian Theory of the Earth, 1802, a more 'user-friendly' version of Hutton's ideas, especially those about stratification and the 'depth' of time.

James Hutton and friends, such as John Playfair, first stared into the abyss of geological time at the end of the eighteenth century.

So how will future geologists know that a layer of Thames river mud was laid down at the same time as a layer of Colorado river mud nearly 9000 km away and bordering different oceans (the Atlantic and Pacific respectively)? What would you actually find and see if you were to take a handful of sediment from both locations and compare them? For most people a mud is a mud is a mud, with not much to choose between them, nor anything much to distinguish one particle of mud from another even if you could see an individual particle, which you cannot without a high-powered microscope. Mind you, muds do vary enormously. If they did not many criminal prosecutions that depend on tracing and matching the origin of mud on the victim, suspect and so on would fail.

The composition of mud can vary because there are many different kinds of clay minerals that comprise mud and their chemistry is wonderfully complex. If it were not so we would not have the range of pottery and ceramics and many other clay-based products that we do. Even so, neither the layperson nor many geologists can distinguish one mud from another, although with the right kind of analytical equipment and know-how a great deal can be distinguished. It is possible to tell what kind of parent rocks the mud was originally derived from, what kind of climate weathered the parent rock and what kind of transport mechanism carried the mud to its final resting place.

Muds themselves cannot be directly dated, but they often contain pieces of wood or other carbonaceous organic material that can be radiocarbon dated, provided the dating is done within some 40,000 years of the life of the organism from which the carbon was derived. But even carbon dates are calculated estimates, with margins of error that amount to tens or hundreds of years, so no very precise date can be obtained by this method. Nevertheless, they are often good enough for most archaeological or geological purposes. The muds will contain a variety of microfossils, some of which belong to small organisms, especially single-celled plants (such as diatoms) and animals (such as foraminiferans) that live within the environment of deposition, plus others that have been transported by river waters, especially pollen from land-living plants.

Because our two sites - the Thames and the Grand Canyon - have a difference of more than 20 degrees in latitude and thus belong to very different climate zones and indeed continents, there will be no plant or animal species in common between them. So there is no way that the sediments can be directly matched on the basis of their contained fossils. However, indirect correlation is possible.

All species occupy finite spaces in space and time. Species' geographical range can vary from a single lake to virtually global, such as the barn owl and modern humans. Such a global distribution for a single species is unusual. Originally (around 200,000 years ago) humans were confined to Africa, so our distribution has expanded enormously since then. We have also changed over this time (evolved) and will continue to do so. The life of a species ranges from hundreds of thousands of years to several million years. Those with the widest geographical distribution but the shortest duration in time are the most useful for correlation, always providing they have preservable and identifiable fossil remains.

Thames mud is full of pollen derived from vegetation typical of the south of England, but today that includes many species that are not native and have been cultivated. Such sudden appearances can still be very useful for correlation because within a few years their pollen will be found over a wide region. Our future geologist may be able to pinpoint species in the fossil assemblage that only became common around the turn of the twentieth to twenty-first centuries, but it will still not provide a precision greater than + or — several years and will not help direct correlation with our Colorado muds in the north of the Gulf of California. The latter will also contain a great diversity of pollen typical of the vegetation found throughout the river's huge drainage basin.

A great deal is known about these plant species and their chronological ranges in North America. Consequently, there is a good chance that it will be possible to match the pollen with a particular pollen assemblage that has a restricted range in time (a so-called pollen biozone). But again, such zones are difficult to pin down to intervals much shorter than a few thousand years. Then again, matching of pollen biozones between North America and Europe is not simple because the two continents have different native species.

Estuaries and deltas are stressful environments for life because they are subject to marked changes, often on a daily basis. The presence of tides introduces changes in water salinity, temperature, clarity, level, flow direction and speed. Only tough creatures and plants can survive under such conditions. Indeed, these environments are well known for their special biotas, which include various oyster and mussel species and strange small crustaceans called ostracodes, shrimplike creatures just a millimetre or so in size whose bodies are enclosed within tiny clam-like bivalved shells. They can be very abundant and their shells are commonly fossilised. Ostracode biozones are well developed and correlated around the world. Although they evolve quite rapidly by Earth Time standards, the relative degree of resolution that they provide is no finer than hundreds of thousands of years.

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