Published 23 August 2017
What can the geological record tell us about the Anthropocene biosphere?
The biosphere is some 4 billion years old on Earth, and the fossil record, though very incomplete for the first 3.5 billion years of that evolution, tells us about the major step changes in its structure over time. For example, for the first 1.5 billion years the biosphere was anaerobic (because the atmosphere contained no free oxygen) and comprised only of prokaryotic microbial cells (those without membrane-bound organelles and nucleus) that were utilising energy pathways that did not involve free oxygen in water and air.
About 2.5 billion years ago (and possibly earlier), oxygenic photosynthesis evolved, releasing free oxygen to the atmosphere and oceans, and changing – forever – how life would evolve on Earth. This step change freed the biosphere from localised energy resources (like volcanic centres) and allowed it to become global (as it still is today).
If the first cells were prokaryotes, then also in very deep time – at least as early as 1.8 billion years ago – and possibly much earlier, we also have evidence for the development of single celled eukaryotic organisms (with cells like those in our bodies, with membrane-bound organelles and nucleus). This complexity probably arose from a collaboration between prokaryotes. It ultimately led to more complex lifestyles in single-celled organisms, and eventually, perhaps as early as about 800 million years ago, to another massive step change, with the development of animal life, wherein these complex eukaryote cells collaborated to make bodies.
The earliest of these animals may have been sponges, though this is still contentious. This changed the Earth forever and led to the development of the complex animal-rich marine ecosystems that characterise the Earth today (burgeoning during the so called ‘Cambrian explosion’, some 540 million years ago). Later, about 470 million years ago, complex plants evolved to colonise the land – followed by animals, and ushering in the development of a complex terrestrial biosphere too. Changes to the biosphere induced by humans over the past few millennia, and accelerating today, need to be assessed against such deep-time changes.
Are the changes occurring to the biosphere today significant when viewed in this geological context? There are at least 4 ways in which the modern biosphere appears to be unique from all past states: a large component of the energy resources in the biosphere are appropriated by one species – humans – (perhaps more than 25% of net primary production of terrestrial vegetation, a huge amount of biomass from the oceans, and all of the other energy resources we use, such as gas, oil and coal); the translocation of organisms across the globe by humans – the neobiota – that include everything from maize to rabbits, and which have reset the organism component of ecosystems almost worldwide; the direct intervention of humans in changing the morphology and genetics of organisms to make ‘human-useful’ animals – the domestic chicken is a classic example – to wholesale modification of ecosystems that we utilise for food; and finally, the synergy developing between the technosphere* and the biosphere, the former essentially growing out of the former.
Looking back over time, it is difficult to think of one organism controlling the terrestrial and marine biosphere in the way that humans do. Indeed, it is probably unique from the 4 billion year history of life on Earth. This is perhaps the value that geology can bring to the debate about human impacts on planet Earth. It shows us just how huge our impact is.
Are the human-driven changes to the biosphere similar to other periods of evolutionary transitions?
The changes happening at present are unique. How we rank them, relative to past changes depends on how sustainable these changes are. At the moment, the technosphere is consuming materials at a colossal rate (oil and concrete for example), and unlike the biosphere it does not recycle those components it needs to survive long-term, so that could lead to its collapse (and much of the biosphere that relies on it – human populations and their ‘commensal’ organisms – might collapse too). If that were to happen soon, the biosphere might recover quite quickly, and the human impact would be recorded essentially by that wholesale relocation of species that would leave its own particular fossil record.
The degree of biosphere change unfolding at present might be comparable with deep time though if it is sustained. In this scenario, it would represent a wholesale reconfiguration going forward (just as the development of oxygenic photosynthesis reconfigured the biosphere 2.5 billion years ago). Such a biosphere would be intimately connected to the technosphere, and its most pressing problem in the near future would be to devise a strategy for preserving biodiversity.
The Anthropocene changes to the biosphere are, of course, not directly comparable with any of the deep time geological changes noted above, because each step-change in the structure of the biosphere possessed its own forcing mechanisms and evolutionary trajectory. It is just that at present, humans are the forcing mechanism.
What can the geological record tell us about the anthropogenic activities that marked the new epoch?
An Anthropocene Epoch is not yet formally defined and is a work in progress. This is the work that the Anthropocene Working Group (AWG) undertakes, to make a scientific case to the Subcommission on Quaternary Stratigraphy of the International Commission on Stratigraphy (a component part of the International Union of Geological Sciences). It is the latter bodies that examine the evidence presented by the AWG and determine whether an Anthropocene is to form a part of the Geological Time Scale [http://www.stratigraphy.org/index.php/ics-chart-timescale].
The AWG is currently looking at a range of geological signals that might define the Anthropocene. These include records of human activity in such disparate settings as lake, estuary and ocean sediments, ice, cave deposits (speleothems), and marine skeletons (of corals and molluscs). The signals we look for include biological signals (such as changes in the fauna and flora recorded in sedimentary successions as a result of human-introduced species, or of local extirpations). We also look for chemical signals (pollutants in lakes, rivers and seas; signals of climate change from the changed chemistry of marine shells), radioactive markers (from A-bomb explosions in the mid and late 20th century), and novel materials (such as concrete and plastics).
For example, we might look at the sedimentary record of human-induced invasive species into San Francisco Bay, California. The date of these invasions is often well documented (simply from human observations) and includes species from across the Pacific such as the Amur Clam (which arrived in SF Bay in 1986, from coastal East Asia). To establish whether such invasive species might provide widespread markers of the Anthropocene, we have to look for them in the sediments accumulating in the Bay, record their precise distribution (especially the order in which they appear through the sedimentary deposits) and work out if their signal will be preservable in the long-term. This is one of the next jobs of the AWG, as well as investigating all of the other signals noted above – and much more.
What the recent geological record will show, from its various stratigraphical signals, is: when we began to make concrete and plastic (novel human materials), when we learned how to split the atom to make catastrophic explosions, when we began to change the climate system, and when we began to reconfigure the biosphere on a global scale.
Where should the Golden Spike go to mark the end of the Holocene and the beginning of the Anthropocene?
It is not possible to say where the Golden Spike should be placed yet, as this is the work that the AWG is undertaking now. But, to be useful, the Golden Spike needs to define what geologists call an isochronous surface – essentially the same time plane everywhere – that can be recognised across the Earth in many different settings (from lake to sea, to land). And, to do that, the signal needs to be consistent and widespread. The signal that has found much favour amongst the AWG is the radioactive signature of A-bomb tests. This can be recognised in many different archives (both biological and sedimentary), and it happened quickly, the signal beginning in 1945, and strengthening into the early 1950s. In this sense, it’s similar in terms of its ‘sudden-ness’ to the bollide impact that marks the Cretaceous-Tertiary boundary 66 million years ago. But it is not the only stratigraphical signal being mooted to define the boundary of the Anthropocene.
What can the geological record tell us about our future? Is there space for hope in the Anthropocene biosphere?
The record of life on Earth records five mass extinctions over the past 500 million years, with many other extinction events of a lower order. These events were precipitated by many different environmental factors, most of which are still not fully understood. The current driving force of major environmental change is humans. We are the successors to the ‘crown’ formerly held by the motley crew of climate change, massive volcanic activity, and a large bollide impact. Past major extinction events tell us that the biosphere takes millions of years to recover, but it is a resilient structure, and it can recover. The problem here – for us – is the timeframe. Millions of years might be a small time for the Earth, but it’s a huge expanse of time viewed from a human perspective, so if we ruin it now, we literally cost the Earth for future human generations.
There are at least two (and then some) trajectories that might be mooted here. The first is that a rampant technosphere uses up all of the natural resources and eventually collapses, taking much of its human component with it. The second is that the technosphere – with its human component – develops to a stage of sustainability. For example, we could act collectively as a species to prevent the demise of coral reef systems in the oceans – by reducing the impact of ocean acidification – but we have to act fast to do this. If not, the demise of coral reef systems worldwide will simply be another clearly recorded signal of human impacts in the 20th and 21st century. A geologically permanent and rather sad one.
*used here in the sense of that great thinker Peter Haff, to designate the globally emergent system that includes humans, their technology, and associated political, scientific, religious and artistic institutions
Mark Williams is a Professor of Palaeobiology at the University of Leicester. He is interested in the evolution of the biosphere over geological timescales, with an emphasis on understanding the rate and degree of current biological change. He is a founding member of The Anthropocene Working Group, and with Jan Zalasiewicz, is the author of the popular science books ‘The Goldilocks Planet’ (2012), ‘Ocean Worlds’ (2014) and ‘Skeletons: the frame of life’ (late 2017) (Oxford University Press).
Mark Williams presented a lecture on the geological record and the Anthropocene at How Humans made the Anthropocene Biosphere – a public seminar presented by the Sydney Environment Insitute and Sydney Ideas. Click here for details on this past event.