What Is Geologic Time And How Does It Work?

I’m sure you all remember where you were when you found out that the statistical correlation of magneto-biostratigraphic calcareous nannofossils with M-sequence magnetic anomalies approximated new boundaries for Tethyan Kimmeridgian of Sardinia (Muttoni et al. 2018). I was on my laptop at the time. I’ll confess, I struggle even to begin to understand what this new paper is about, beyond the broad principle that a chunk of the rock record as it relates to geological time is being slightly tweaked. The Kimmeridgian is a stage of the Upper Jurassic series – the last series in the more-famous-than-most Jurassic System. We are talking, of course, about the geological time scale, the ridiculously complex scale we use to describe and divide the history of the Earth into different sections. Like the periodic table, the Geologic time scale is one of those amazing human endeavours to turn a vast amount of evidence from astronomy, rocks, fossils, history and politics into a nifty little diagram which makes the whole thing appear nice, neat and resolved. Most children’s books or entry texts on aspects of geology will include a geologic time scale in the introduction with the representative organisms from each major event on the way illustrated alongside a neatly stacked pile of coloured rocks: like a cake.

Unlike the periodic table, despite the fancy sounding magneto-biostratigraphic techniques used to create the geologic time scale, it doesn’t mean a great deal. If maths and physics are the nearest, we have to a universal language to communicate with visiting extra-terrestrials, handing them a geologic time scale would be about as useful as a Jackson Pollock painting. It looks hella scientific, and if you don’t know your Cenomanian from your Valanginian, it sounds scientific, but essentially it’s a very fancy way of telling the story about the history of the Earth. Periods, eras and aeons are some of the units used to describe sections of the Earth’s history but where these begin, and end are based on a very subjective reading of the rock record. The names of these units record a bit about the history of their description too. Dividing geologic time has also been the cause of some professional falling outs between geologists including the Great Devonian controversy and subsequently the Devonian period is named after the face-saving rocks from Devon. Whether or not the Anthropocene can be defined causes similar disagreements today albeit between social scientists and earth scientists (as well as between earth scientists). If you were to hop over to an Earth in a Universe parallel to ours, their geologic time scale (if they have one) would almost definitely be different. I’d wager up to £30 on it.

Our subjective division of time-based on interpreting an incomplete fossil record may be irreproducible as a whole but the geologic time scale does provide a useful device for plotting the order of major events in the history of life and earth. I’ve written before about the difficulty of defining certainty when it comes to major facts about when animals lived. We readily band about terms like Jurassic, Cretaceous and Miocene here at Lost World’s Revisited as a shorthand to refer to different times in earth’s history but if you start to look for the hard boundaries between these different units, you’ll find it varies from place to place and also through time itself. For example, the Cambrian period was once thought to have begun 470 million years ago and has been as old as 620 million years, now defined as just over 540 million years old (Gradstein et al. 2004). The Jurassic Period once extended between 145 million years ago and ended 108 million years ago, now it ends 145 million years ago and begins just over 200 million years ago. Some of the early attempts at putting the time in order were based on isotope decay and measuring the thicknesses of rock. Stable isotopes, statistical and geo-mathematical, magnetostratigraphy and biostratigraphy methods have refined and expanded the accuracy of timing the formation of rocks and dividing time into units.

Now obviously, the organisms alive during the Silurian period didn’t know they were living in the Silurian. They didn’t hold a “welcome to the Devonian party” when the clock struck midnight 419.2 give-or-take million years ago, and the global events that we’ve used to mark the end of different units may have taken millions of years not just a moment. However, a calibrated and co-ordinated geologic time scale is needed to try to piece together Earth’s history. Without the geologic time scale, it would be impossible to compare rocks made at the same time in Wales and the Czech Republic or North America to Russia. That earth scientists have achieved this, with perpetual tweaking, and can condense 4.6bn years’ worth of history into a pretty wall chart or bookmark is an impressive feat it’s all too easy to take for granted.

The keepers of deep time are the International Commission on Stratigraphy (ICS) an international non-governmental body with the very humble mission of “setting global standards for the fundamental scale for expressing the history of the Earth”. The ICS is responsible for agreeing upon Global Boundary Stratotype Section and Points (GSSP), that is rock exposures thought to mark the lowest point of a geological stage, normally defined by faunal events in the fossil record. These GSSPs can then be calibrated with other rock sections and then used to define periods in time. GSSPs, once agreed, are then marked with a ‘golden spike’ (normally a plaque or marker not an actual golden spike) to indicate their importance as an international boundary marker. The ICS enforce various rules for nominating, agreeing and marking boundaries.

Let’s take probably one of the most famous boundaries in the geological time scale as an example. The well-known boundary between the Cretaceous and Palaeogene, A.K.A the K-T or K-Pg boundary. This is the one where the non-avian dinosaurs, and many other groups of organisms, bit the dust. The GSSP for the end of the Cretaceous and the beginning of the Palaeogene is at Oued Djerfane, west of El Kef, Tunisia and is marked with a “golden spike” (Molina et al. 2006). The rock marking the beginning of the Palaeogene is a humble 50cm thick layer of reddish clay, and the boundary is defined by an Iridium geochemical anomaly and the major extinction of “dinosaurs, ammonites, foraminifers, etc.” Extinctions, as well as appearances of species in the fossil record, mark many of the boundaries, almost comically so in some instances. So although the Devonian period is often called “the age of fishes” due to the explosion in the diversity of fish between 420 and 360m years ago it’s beginning is technically marked by the appearance of the graptolite species Monograptus uniformis, hemichordate animals whose fossils look like little drawings on rocks. The Triassic, Jurassic and Cretaceous are best known as “the age of the reptiles”, specifically when dinosaurs ruled the Earth. The appearance of tiny teeth elements of an eel-like jawless animal called a conodont. Specifically, Hindeodus parvus, is the technical herald of the age of the dinosaurs according to the ICS. The appearance of various species of ammonites, belemnites, trilobites, foraminifera, nannofossils and magnetic events mark the boundaries of many of the defined GSSPs although there are still some boundaries without a golden spike or defined beginnings and endings. So it may be the charismatic giants and the catastrophic events that grab the attention when it comes to palaeontology and the headlines when it comes to reporting but it’s the less celebrated but more useful fossil workhorses and the diligent work as the paper I mentioned at the beginning that add the essential punctuation when telling the story of Earth.

Credit: Mark Carnall for The Guardian, 2 May 2018.