The term ‘Anthropocene’ has entered scientific literature as an expression of the fundamental environmental change caused to planet Earth by humankind, despite not being a formally defined geological unit within the geological time scale. The hunt is on for the “golden spike” – a marker for future researchers to point to in millions of years and identify as the geological start of the Anthropocene epoch.
A Golden Spike
The ‘Anthropocene’ is currently being considered by the Anthropocene Working Group as a potential geological epoch, at the same hierarchical level as the Pleistocene and Holocene epochs, along with the implication that it is within the Quaternary period, but that the Holocene has terminated. Alternatively, it might also be considered at a lower hierarchical level – an age that would imply it is a subdivision of the ongoing Holocene Epoch.
For Earth scientists, the “Golden Spike” is a Global Boundary Stratotype Section and Point (GSSP) – an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. Almost all GSSPs are based on paleontological changes. Hence GSSPs are described in terms of transitions between different faunal stages.
The distinction between these periods is largely based upon the geological record.
Written in Stone
The geological record comprises the set of solid evidence that has remained with us over the ages, and tells us about the history of our planet.
It is often based upon distinctive features such as:
- mineralogy or
- chemical signatures.
For instance, graptolites are common fossils and they have a worldwide distribution. The preservation, quantity and gradual change over a geologic time scale of graptolites allows the fossils to be used to date strata of rocks throughout the World.
Graptolite fossils are an important index for dating Palaeozoic rocks, as they evolved rapidly with time and formed many different species. Graptolites are also useful for estimating the local water depth and temperature over their lifetimes.
Or it is based upon cycles in paleo-climate such as:
- ice ages and
- interglacial periods.
Recently, a newer agent of geological change was suggested to be at work. Humankind. Us…
We ARE the Agent of Change!!
For the most part, the influence of humans on geology has largely been considered insignificant. However this influence has now become too great to ignore. Actually, human beings have done so much to alter the “geology” of our World that some scientists suggested that we have warranted a new geological epoch: the Anthropocene.
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time. It relies upon isotope geology and geo-chronology to derive hard dating of known and well defined rock units, which contain the specific fossil assemblages defined by the stratigraphic system.
Chronostratigraphic units are the following:
eonothem – Phanerozoic
erathem – Paleozoic
system – Ordovician
series – Upper Ordovician
stage – Ashgill.
The methodology of Chronostratigraphy derives from the Law of Superposition and the principles of cross-cutting relationships.
The Law of Superposition – So Obvious It Hurts!
The Law of Superposition was first proposed in the 17th century by Danish scientist Nicolas Steno (1638 – 1686). It forms one of the bases of the sciences of Geology, Archaeology, and other fields dealing with geological stratigraphy – the study of rock layers.
The Law of Superposition states that in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence. This is important to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.
As it is very difficult in practice to date isotopically most fossils and sedimentary rocks directly, inferences are made to arrive at an age date which reflects the beginning of a geological interval.
Igneous rock may form with or without crystallisation, either below the surface as intrusive plutonic rocks, or on the surface as extrusive volcanic rocks. This magma can be derived from partial melts of existing rocks in either a planet’s mantle or crust.
The melting is caused by one or more of three physical processes:
- an increase in temperature,
- a decrease in pressure, or
- a change in composition.
Because igneous rocks occur at specific intervals in time and are essentially instantaneous on a geologic time scale, and because they contain mineral assemblages, which may be dated more accurately and precisely by isotopic methods, the construction of a chronostratigraphic column relies heavily upon intrusive and extrusive igneous rocks.
Metamorphism is often associated with faulting. It may also be used to bracket depositional intervals in a chronostratigraphic column. Metamorphic rocks can occasionally be dated. This provides some limits to the age at which a bed could have been laid down.
If a bed containing graptolites overlies crystalline basement at some point, dating the crystalline basement will give a maximum age of that fossil assemblage.
This process requires a considerable degree of effort and checking of field relationships and age dates. For instance, there may be many millions of years between a bed being laid down and an intrusive rock cutting it; the estimate of age must necessarily be between the oldest cross-cutting intrusive rock in the fossil assemblage and the youngest rock upon which the fossil assemblage rests.
Most metamorphic rocks can be distinguished from igneous rocks because they show a characteristic banding or alignment of minerals.
The Anthropocene Epoch
For an Anthropocene Epoch to be added to the official timeline of Earth history – the Chronostratigraphic Chart – the final decision will need to win the backing of the International Commission on Stratigraphy (ICS), then be further ratified by the executive committee of the International Union of Geological Sciences (IUGS).
Ten panel members from the 35-strong group believe that the best spike will probably be that of the plutonium fallout from bomb tests in the 1950s, to be found in marine or lake sediments, ice layers or perhaps even speleothems (stalagmites and stalactites). Others think there could be better spikes than the radionuclide, and a few of the counter-proposals include remnant plastics or some kind of carbon signature that signifies the rapid rise in CO2 emissions.
However, there is broad agreement that whichever marker is chosen, it should reflect the events on Earth around the 1950s.
The Hole Story…
One day, geologists of the distant future may conceivably look back at the Earth’s geological record. They will see the Jurassic period dominated by the large fossils of dinosaurs, and the Anthropocene epoch with its gargantuan pits dug for mines, its spikes in radiation recorded by rock layers following nuclear explosions, human fossils and the buried ruins of our cities.
The Anthropocene Working Group expects it will take two or three years to settle on the best golden spike (or spikes).