Arxiv – If an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today? Researchers summarize the likely geological fingerprint of the Anthropocene, and demonstrate that while clear, it will not differ greatly in many respects from other known events in the geological record. They propose tests that could plausibly distinguish an industrial cause from an otherwise naturally occurring climate event.
The researchers define industrial civilizations here as the ability to harness external energy sources at global scales.
One of the key questions in assessing the likelihood of finding such a civilization is an understanding of how often, given that life has arisen and that some species are intelligent, does an industrial civilization develop? Humans are the only example we know of, and our industrial civilization has lasted about 300 years so far. This is a small fraction of the time we have existed as a species, and a tiny fraction of the time that complex life has existed on the Earth’s land surface. Complex life has existed for about ∼400 million years ago. This short time period raises the obvious question as to whether this could have happened before. They call this the “Silurian Hypothesis”. It is named after a story and species in Dr Who.
The fraction of life that gets fossilized is always extremely small and varies widely as a function of time, habitat and degree of soft tissue versus hard shells or bones. Fossilization rates are very low in tropical, forested environments, but are higher in arid environments and fluvial systems. There are only a few thousand near- complete specimens of dinosaurs, or equivalently only a handful of individual animals across thousands of taxa per 100,000 years. Given the rate of new discovery of taxa of this age, it is clear that species as short-lived as Homo Sapiens might not be represented in the existing fossil record at all.
The chance for finding direct evidence of a civilization older than 4 million years via objects or fossilized examples of their population is small.
Stable isotope anomalies of carbon, oxygen, hydrogen and nitrogen
Since the mid-18th Century, humans have released over 0.5 trillion tons of fossil carbon via the
burning of coal, oil and natural gas at a rate orders of magnitude faster than natural long-term sources or sinks. In addition, there has been widespread deforestation and addition of carbon dioxide into the air via biomass burning. All of this carbon is biological in origin and is thus depleted in 13C compared to the much larger pool of inorganic carbon. Thus the ratio of 13C to 12C in the atmosphere, ocean and soils is decreasing with a current change of around -1h δ 13C since the pre-industrial in the surface ocean and atmosphere.
The combustion of fossil fuel, the invention of the Haber-Bosch process, the large-scale application of nitrogenous fertilizers, and the enhanced nitrogen fixation associated with cultivated plants, have caused a profound impact on nitrogen cycling such that δ 15N anomalies are already detectable in sediments remote from civilization.
Abrupt Paleozoic, Mesozoic and Cenozoic Events
The summary for the Anthropocene fingerprint above suggests that similarities might be found
in (geologically) abrupt events with a multi-variate signature.
They reviewed a partial selection of known events in the paleo-record that have some similarities to the hypothesized eventual anthropogenic signature.
The clearest class of event with such similarities are the hyperthermals, most notably the Paleocene-Eocene Thermal Maximum (56 Ma), but this also includes smaller hyperthermal events, ocean anoxic events in the Cretaceous and Jurassic, and significant (if less well characterized) events of the Paleozoic. We don’t consider of events (such as the K-T extinction event, or the Eocene-Oligocene boundary) where there are very clear and distinct causes (asteroid impact combined with massive volcanism, and the onset of Antarctic glaciation (likely linked to the opening of Drake Passage respectively). There may be more such events in the record but that are not included here simply because they may not have been examined in detail, particularly
in the pre-Cenozoic.
Industrial Civilization Markers
The specific markers of human industrial activity discussed above (plastics, synthetic pollutants, increased metal concentrations etc.) are however a consequence of the specific path human society and technology has taken, and the generality of that pathway for other industrial species is totally unknown. Large-scale energy harnessing is potentially a more universal indicator, and given the large energy density in carbon-based fossil fuel, one might postulate that a light δ 13C signal might be a common signal. Conceivably, solar, hydro or geothermal energy sources could have been tapped preferentially, and that would greatly reduce any geological footprint.
1. Despite copious existing work on the likely Anthropocene signature, they recommend further synthesis and study on the persistence of uniquely industrial byproducts in ocean sediment environments. Are there other classes of compounds that will leave unique traces in the sediment geochemistry on multi-million year timescales? In particular, will the byproducts of common plastics, or organic long-chain synthetics, be detectable?
2. A deeper exploration of elemental and compositional anomalies in extant sediments spanning previous events be performed (although we expect that far more information has been obtained about these sections than has been referenced here). Oddities in these sections have been looked for previously as potential signals of impact events (successfully for the K-T boundary event, not so for any of the events mentioned above), ranging from iridium layers, shocked quartz, micro-tectites, magnetites etc. But it may be that a new search and new analyses with the Silurian hypothesis in mind might reveal more. Anomalous behaviour in the past might be more clearly detectable in proxies normalized by weathering fluxes or other constant flux proxies in order to highlight times when productivity or metal production might have been artificially enhanced.
3. Should any unexplained anomalies be found, the question of whether there are candidate species in the fossil record may become more relevant, as might questions about their ultimate fate.
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