GEOLOGICAL TIME AND THE ANTHROPOCENE
The Hall at Malvern Cube was full, as it always is for the u3a Geology Group. After the “Rock of the Month”, brought by Kay Hughes, Professor Fairchild began his lecture with different depictions of Geological Time, from a layered cake to a spiral and a pie diagram. The latter demonstrated that the Phanerozoic section of time when complex life appeared and evolved makes up a very small part of the geological time scale.
It has been suggested that there should be a new geological unit representing the present time, called the Anthropocene. It has not been fully defined yet, and there is a group of people from different fields working on this.
Professor Fairchild then went through the different ways of dividing up geological time:
· Geochronology, the accurate dating of rocks by studying the decay of uranium 238 in zircon crystals. It is possible to date the boundaries in the Precambrian, where fossils cannot be used, but improved dating means that the times of some geological boundaries have changed, not greatly, but enough to be significant.
· Lithostratigraphy, the dividing of rocks by sediment type.
· Biostratigraphy, the assemblages of fossils which change over time. The problem with this is that organisms which have died out in one part of the world might still exist in other areas. For example, marine transgression occurred in the early Silurian across Wales as far as Malvern, and different species evolved at different depths in different places.
· Chronostratigraphy – subdivision based on changes which can be assumed to have occurred at the same time, for example
o Global evolutionary shifts (Biostratigraphy)
o Alterations in the polarity of the Earth’s magnetic field ( Magneto-stratigraphy)
o Global climatic changes (Climatostratigraphy)
o Deposits from major volcanic eruptions (Tephrostratigraphy)
Some examples of global changes were then discussed, though most of these are major unconformities which represent time gaps. The Precambrian – Cambrian boundary is one where fossils with skeletons appear suddenly. Trilobites could be used, but instead trace fossils of the burrows of penis worms have been used. Debate has continued as to where the exact boundary should lie, but groups of geologists have decided the official boundary, or Global Stratotype Section and Point (GSSP), marked by a “golden spike”. This shows this boundary at Fortune head, Newfoundland, ratified a few years ago. However, things change and an index fossil has been found a few metres below the GSSP.
The position of this GSSP has not been changed, but it could be, as more evidence has come to light.
GSSPs or Golden Spikes have been ratified at many important stratigraphic boundaries, using different criteria, but there are still gaps, particularly in the Jurassic. However, Professor Fairchild was involved in the definition of a new period, the Ediacaran, in the Precambrian rocks of South Australia, marking the end of an ice age and the beginning of a marine transgression. The GSSP and Golden Spike are in the Flinders Range, and one radiometric date put it at 635 Ma, which was fortunately confirmed by further dating.
The audience was amused to see how seriously the Chinese take the occurrence of GSSPs in their territory, with a sculpture of a conodont on a column in a park, to mark the base of the Mesozoic.
The asteroid impact at the Cretaceous to Paleocene and Mesozoic to Cenozoic boundary was discussed, and it was pointed out that, although the asteroid struck the Yucatan Peninsula, evidence is difficult to find in Mexico, which is too close to the action. Changes in foraminifera can be seen in deep sea cores from the Atlantic, but these are not often used for defining boundaries. The Cretaceous and Tertiary sediments in Oregon show microtektites and an Iridium layer, but this gives two possible positions for the boundary. There is a much clearer boundary with an Iridium spike in Tunisia, used to date the GSSP at 66.04Ma.
Up until recently the Quaternary boundary was put at 1.8 million years ago, coinciding with a magnetic reversal, but big changes in temperature occurred much earlier. The boundary was changed to 2.6 Ma in 2009.
The start of the post-glacial epoch, the Holocene is recognised as beginning about 11,670 years ago, shown by ice cores which fall within the definition of a rock. Stalagmites show divisions within the Holocene and other boundaries have been proposed. These have to go through various international committees before they can be officially recognised.
The Holocene was originally defined as the Recent Epoch, by Charles Lyell in 1833, and since then it has changed its name and got bigger. But at some point a new geological period could begin, and this is what geologists are calling the Anthropocene. The final slide before the break showed the impact of human activity in Cyprus, with an insole in chalky deposits on Governor’s Beach, seen on a recent u3a field trip.
Introducing the Anthropocene, by Ian Fairchild
After the break a few members of the u3a Geology Group took part in a theatrical “debate”, an original way of explaining the concept of the Anthropocene. The script was developed with members of the Theatre Studies department at Birmingham University.
By means of questions and answers, with slides to illustrate, the audience learnt that the Anthropocene represents not just the time when humans have occupied the planet, but the changes they have brought about; not only climate change caused by the huge rise in the burning of fossil fuels but also rising sea levels, habitat changes and mass extinctions. The “Great Acceleration” since the Second World War, with nuclear tests, the manufacture of plastic and concrete, increasing uses of pesticides and fertilisers as well as cars has led to much greater pollution of the planet.
The “geological police”, the International Commission on Stratigraphy (ICS) has set up a panel called the Anthropocene Working Group in which earth system scientists have been joined by archaeologists, geographers, ecologists, a philosopher and a lawyer. First they have to convince the ICS that changes in geological deposits show that we are in a new episode of geological time. Corals, stalagmites, snow and ice, trees and sediments in lakes and oceans can all be used to find chemical signals indicating major change. These chemical signals include pesticide residues, sulphates from coal burning and radioactive isotopes created by nuclear tests, all of which can be found in the remotest parts of the earth.
After some questions and answers about when the changes began, and whether they have always been taking place, the debate continued with questions about whether the Anthropocene should be an Age, an Epoch, a Period, an Era or an Eon! The only conclusion which could be made was that we are a few decades into changes that we are powerless to reverse, but we are still responsible for the planet’s future.
Questions
1. What is the length of time which will be needed by the working group?
ANS: the GSSP dates from the mid-20th century, but defining it precisely will take 2 to 5 years.
2. What are the practical consequences of establishing the Anthropocene?
ANS: It has been an academic exercise but the word is beginning to be used in other fields as a reality, but not all politicians recognise this.
3. How was the dividing line established, when the Industrial Revolution started the increase of CO2 emissions?
ANS: the signals must be world-wide, and the Industrial revolution had no impact in Africa. Around 1950 there was an inflection in the graphs of CO2 and other human changes, showing a marked increase in the rate of change.
4. Does the evidence answer the points made by climate change deniers?
ANS: the carbon cycle can be quantified with a greater degree of certainty. Atmospheric CO2 should have increased more than it has, because trees are absorbing it, but this is not permanent. Industrialists as well as environmentalists will put pressure on the deniers to develop alternative sources of energy.
5. Will the Anthropocene be an epoch, an era …?
ANS: probably an epoch.
The Chairman, Richard Harris, thanked Ian Fairchild for his most interesting lecture.