The Plastocene – Plastic in the sedimentary record

The University of Hull was privileged to host the annual British Science Festival in 2018. One of the key events was the Huxley Debate, which brings together world-leading experts to discuss a pressing issue facing society. The theme in Hull was “what do we do about ocean plastics?”. As part of the discussion, Professor Dan Parsons, Director of the Energy and Environment Institute, suggested that in the future we will see the evidence of our plastic waste in the sedimentary record, and that the emergence of this in the record could mark the boundary between Holocene and Anthropocene.

Professor Parson’s suggestion prompted one journalist to remark, in jest, that a better name for the new epoch would be “the plastocene”. It may have been a moment of mirth, but the feasibility of the emergence of the plastocene should not be discounted.

Plastic pollution is a huge issue. You will have undoubtedly heard the key statistics over and over by now – 12 million tonnes of the stuff enters the world’s oceans every year, and at that rate there will be more plastic than fish in them by 2050. Plastic is designed to be virtually indestructible and never really disappears, instead breaking down into smaller and smaller pieces. It is possible that the only way we will ever get it out of the oceans is to wait for it to settle on the ocean floor and be buried – something we might be waiting a long time for – it floats.

Plastic deposits are already being found – some have useful (but unwelcome) side-effects such as marking the extents of flood events by forming wrack lines. For example, rubbish deposited by storms and tsunamis has been used to date the events (using packaging and labels) and determine the flood extents. On a recent visit to the south of Spain as part of an undergraduate field trip I observed wrack lines formed of tiny fragments of multi-coloured plastic. Such deposits can get buried, preserving the records with them.

Plastic wrack lines left behind in a dry river in the south of Spain (Authors own photos). Simon Reeve’s documentary ‘Mediterranean’ highlighted the issue of microplastics in this region of Spain, showing plastic sheeting buried within a dried river bed.

Plastics have already been found being formed into rocks. Corcoran et al (2015) observed a new type of rock dubbed ‘plastiglomerates’ around the beaches of Hawaii. They formed when plastics melted and fused with natural materials, such as beach deposits and volcanic rocks. They found no evidence of recent lava flows so suggested that the plastiglomerates were formed by the deliberate burning of the plastics.

The lithification processes behind beach and marine carbonate cementation can be extremely rapid, forming rocks in as little as a year, which means evidence of human activity is preserved and can be used for dating – examples include glass drink bottles and World War II debris. In the future we could end up seeing plastics preserved in these carbonate cements.

However, despite all of this it is unlikely our ancestors, or whatever might inherit the planet from us, will observe this period of the sedimentary record and refer to it as the plastocene. Plastic pollution is not the only pressure we are exerting on the planet, it is not even the most significant. We have also been making our mark on the sedimentary record for millennia, with the earliest mining and smelting activities resulting in spikes of lead in deposits to the nuclear tests of the 1950s leaving an imprint of Plutonium 239. We cannot isolate plastics from our impact on what truly is the Anthropocene.