By Dr Jonathan Dean, Prof Melanie Leng and Prof Anson Mackay
The Anthropocene is a term that is increasingly being used to refer to the current interval in geological time in which humans have become a dominant force of global environmental change. It was coined by Prof Eugene Stoermer, a biologist, in the 1980s and popularised in the early 2000s by Prof Paul Crutzen, an atmospheric chemist. It is now indisputable that humans are leaving their mark on the planet (see the recent Climatica summary of the ‘Climate change: evidence and causes’ report here: http://climatica.org.uk/royal-society-national-academy-sciences-climate-change-evidence-causes). For instance, over the last century or so, atmospheric CO2 levels have risen dramatically, by around 40%, to the highest seen in nearly a million years. Human activities are also influencing the rate and extent of erosion – we transport much more soil and rock around the surface of the planet than natural processes do; and our impacts on the environment are also causing species extinction rates to rise well beyond background levels.
In 2016, a working group of the International Commission on Stratigraphy (the group of scientists who have the final say on where geological boundaries are set) will report on whether they consider the Anthropocene to be distinctive enough from the current geological epoch (the Holocene) to warrant its designation as a new time period. If so, they will need to decide when to set the end of the Holocene and the beginning of the Anthropocene (the geological boundary). This is being hotly debated, with suggestions ranging from:
- Several thousands of years ago when humans started modifying the environment by deforestation and farming.
- AD1950 when dramatic changes in the concentration of atmospheric gases are seen in the records.
- Sometime in the future when large temperature rises are predicted to kick in due to the release of greenhouse gases into the atmosphere.
Here, we consider the role that isotope analysis or ‘isotope geochemistry’ can play in informing this debate. Isotopes are different types of an element: they have the same number of protons but a different number of neutrons. Changes in the ratio of one isotope to another can be used to trace natural and human impacts on the environment. Let’s use carbon as an example…
Changes in the global carbon cycle
It is well known that the concentration of CO2 in the atmosphere has been increasing over the last century. We can use isotope geochemistry to help us to identify the sources of this carbon. This is because different sources of carbon have contrasting isotopic signatures. Isotopes have shown this is likely due to human activity, in particular the burning of fossil fuels. There are three main isotopes of carbon:
- carbon-12 (12C) with 12 protons and neutrons
- carbon-13 (13C) with 13 protons and neutrons
- carbon-14 (14C) with 14 protons and neutrons
Carbon-14 is unstable and radioactive, whereas carbon-12 and carbon-13 are not (they are ‘stable’). As CO2 concentrations have risen, there has been a decline in the ratio of carbon-13 to carbon-12 in atmospheric CO2. This is because when plants grow, they take up carbon as a source of food. They prefer to use carbon-12 more than carbon-13, so more carbon-12 gets incorporated into the plant matter. Over geological time, plant matter can turn into fossil fuels (e.g. coal). When humans burn these fossil fuels, CO2 rich in carbon-12 is released into the atmosphere, diluting the concentrations of carbon-13.
This trend in carbon isotope ratios in atmospheric CO2 is recorded in natural archives such as tree rings and corals, since they incorporate CO2 as they grow. When we analyse the carbon from these natural archives, we can see that there has been a decrease in the amount of carbon-13 relative to carbon-12 in the atmosphere from the start of the Industrial Revolution. This trend accelerated after AD1950 when the burning of fossil fuels began to increase.
Changes in the global nitrogen cycle
There have also been recent increases in certain compounds of nitrogen such as nitrates and ammonia in the atmosphere. There are two stable isotopes of nitrogen: nitrogen-14 and nitrogen-15. Human sources of nitrogen, especially fossil fuel burning and the emissions from the production of fertilisers, generally have more nitrogen-14 compared to natural sources of nitrogen. So the fact the ratio of nitrogen-15 to nitrogen-14 has been declining as the concentration of these nitrogen compounds has increased suggests this trend has mainly been caused by emissions from human sources. The trend of declining nitrogen isotope ratios started in the nineteenth century but really accelerated after AD1950.
Isotopes from nuclear bombs
At the same time as humans have been modifying the carbon and nitrogen cycles, there have been other human impacts on the planet such as deforestation and species extinction. For this reason, the period from AD1950 is sometimes referred to as the ‘Great Acceleration’ in human impacts on the global environment. Usefully, from around AD1952, isotopes produced from nuclear weapons testing, such as plutonium-239 and plutonium-240, were released into the atmosphere and deposited as ‘fallout’ on land and in lakes and oceans across the world. In geological archives such as lake sediments, we can see a peak in these isotopes in around AD1963 just after an atmospheric nuclear test ban treaty came into effect. We can use this layer of isotopes as a useful time indicator, because we know that it was deposited at roughly the same time across the world. So, thousands of years from now, assuming bomb testing does not resume, geologists will be able to use this unique radioactive marker in the geological record to pinpoint the time of the ‘Great Acceleration’.
Humans have caused heavy metal pollution for millennia. During mining for lead, tiny fragments are thrown up into the atmosphere. These fragments can be transported long distances in the air before falling to Earth. Different lead mines have different lead isotopes ratios, and these can be used like a fingerprint to identify the source of the pollution. It has been shown that some of the earliest lead pollution recorded in the Greenland ice cores was from lead found in rocks in Spain, which it is assumed came from early Roman mining and smelting two millennia ago. Over the past few decades, the analysis of lead isotopes from environmental records has highlighted the decline in lead pollution released from human sources. This is, in large part, due to the fact that leaded petrol is no longer widely used.
So, when should the boundary be set?
Recently, Dr Jan Zalasiewicz revealed that the working group of the International Commission on Stratigraphy may be leaning towards AD1950 as a date for the start of the Anthropocene. This is because multiple lines of evidence from environmental records point towards this as being the time of the ‘Great Acceleration’ in human impacts on the Earth. Different isotopes can be used as a record of human impacts on the environment at different times in the past. However, while we have shown that lead isotopes show humans were polluting the Earth millennia ago, there is a clear acceleration in the rate of human impact, especially seen in carbon and nitrogen isotopes, from approximately AD1950 onwards, so isotope geochemistry could be used to support the designation of the Holocene-Anthropocene boundary at this time.
For a more detailed review of how isotope geochemistry can contribute to the Anthropocene debate, our paper can be accessed free of charge here.
Jonathan Dean is an Isotope Geochemist within the Stable Isotope Facility at the British Geological Survey. He recently finished his PhD at the University of Nottingham, where he used oxygen isotopes from carbonates and diatoms to reconstruct the hydroclimate of the Near East, with a particular focus on palaeoseasonality. Twitter @jrdean_uk
Melanie Leng is a Professor of Isotope Geosciences at The University of Nottingham, Director of the Centre of Environmental Geochemistry and a Science Director at the British Geological Survey where she manages the Stable Isotope Facility. Her research concentrates on using isotopes to assess climate and environmental change in the Southern Ocean, Northern Europe, the Mediterranean and East Africa. Twitter @MelJLeng
Anson Mackay is a Professor in the Environmental Change Research Centre at UCL and Visiting Research Associate at the British Geological Survey. He researches human and climate impacts on large freshwater ecosystems such as Lake Baikal in Russia and the Okavango Delta in Botswana, using biological organisms and stable isotopes. Twitter @AnsonMackay