The rock record is our key to understanding the past. When we try to reconstruct the habitat and lifestyles of ancient creatures we turn to the rock record to find their fossilised bodies, their footprints, their burrows and the preserved sediments of the environments they lived in. It’s interesting then to think about how we, humans, will be preserved in the rock record.
We’ll certainly leave more than just bones behind us. Humans have an enormous effect on the environment – physically, chemically and biologically – to the extent that a new informal name has been suggested for the current geological epoch: the Anthropocene, the age of man. If geologists in millions of years’ time were to use the rock record to look back at the Anthropocene, what would they see?
Let’s start with plastic. Plastic crops up everywhere in our surroundings today,and under the right conditions (heat from forest- or campfires combined with abundant plastic debris) can form what could be described as man-made rocks, ‘plastiglomerates’. If plastiglomerates sink to the bottom of the ocean where they can be buried they stand a decent chance of making it into the rock record. In the long term, rocks are often subjected to intense heats and pressures which may cause this buried plastic to revert back to oil, but in rare cases plastics may be preserved as carbon films. Future geologists may find carbon imprints of plastic bottles in the same way as we find imprints of fossil leaves.
Our houses and cities (perhaps things that we can be prouder of making than plastiglomerate) may also be preserved, provided that they are buried rather than eroded. In fact, we are indirectly boosting the chances of such preservation – projected sea level rise will flood low lying and coastal cities, moving them from an erosional regime (on land) to a depositional one (underwater). Underground structures such as mines may be thought of as human-made burrows, and would be seen by future geologists cutting through pre-existing strata.
If sea level rise is large enough it too may leave a ‘transgressional’ signal in the rock record as sediments originating in deeper water are deposited over those of shallower origin. This projected sea level rise is a result of global warming, which is itself largely due to greenhouse gases pumped into the atmosphere by the burning of fossil fuels. The classic graphs of skyrocketing carbon dioxide levels come from data either collected directly from the atmosphere or from bubbles in ice cores – although neither of these will be around in millions of years’ time, evidence of our fossil fuel use will remain. The carbon contained in fossil fuels is isotopically light; when this light carbon enters the ocean it is incorporated into the shells of organisms such as plankton making them isotopically light too. Nuclear bombs also leave a geochemical trace in the rock record, with artificial nuclear fission products found globally in post-1945 strata.
So much for the physical and chemical records of the Anthropocene, but how will the results on the biosphere appear? Unfortunately the answer to that question may well be as a mass extinction. Though our current loss of species does not yet constitute a mass extinction (a loss of over 75% of estimated species) the extinction of creatures now defined as ‘critically endangered’ over the next few thousand years would tip us into that category. There have only been five previous mass extinctions in life’s history, and the record of the Anthropocene may contain the sixth.
Geologists looking back in millions of years’ time will find no lack of evidence that we existed.
“Rocks Made of Plastic Found on Hawaiian Beach.” http://news.sciencemag.org/earth/2014/06/rocks-made-plastic-found-hawaiian-beach.
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