The Silurian Hypothesis: Would it be possible to detect an industrial civilisation in the geological record?
The answer is apparently "maybe". And yes, they named this after Doctor Who, like all sensible people.
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 (Behrensmeyer et al., 2000). Fossilization rates are very low in tropical, forested environments, but are higher in arid environments and fluvial systems. As an example, for all the dinosaurs that ever lived, there are only a few thousand near- complete specimens, 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 (so far) might not be represented in the existing fossil record at all.
The likelihood of objects surviving and being discovered is similarly unlikely. Zalasiewicz (2009) speculates about preservation of objects or their forms, but the current area of urbanization is ess than 1% of the Earth’s surface (Schneider et al., 2009), and exposed sections and drilling sites for pre-Quaternary surfaces are orders of magnitude less as fractions of the original surface. Note that even for early human technology, complex objects are very rarely found.
There are undoubted similarities between previous abrupt events in the geological record and the likely Anthropocene signature in the geological record to come. Negative, abrupt δ 13C excursions, warmings, and disruptions of the nitrogen cycle are ubiquitous. More complex changes in biota, sedimentation and mineralogy are also common. Specifically, compared to the hypothesized Anthropocene signature, almost all changes found so far for the PETM are of the same sign and comparable magnitude. Some similarities would be expected if the main effect during any event
was a significant global warming, however caused. Furthermore, there is evidence at many of these events that warming was driven by a massive input of exogeneous (biogenic) carbon, either as CO2 or CH4.
At least since the Carboniferous (300–350 Ma), there has been sufficient fossil carbon to fuel an industrial civilization comparable to our own and any of these sources could provide the light carbon input. However, in many cases this input is contemporaneous to significant episodes of tectonic and/or volcanic activity... Impacts to warming and/or carbon influx (such as increased runoff, erosion etc.) appear to be qualitatively similar whenever in the geological period they occur. These changes are thus not sufficient evidence for prior industrial civilizations.
While we strongly doubt that any previous industrial civilization existed before our own, asking the question in a formal way that articulates explicitly what evidence for such a civilization might look like raises its own useful questions related both to astrobiology and to Anthropocene studies. Thus we hope that this paper will serve as motivation to improve the constraints on the hypothesis so that in future we may be better placed to answer our title question.
I have a hard time believing that of the ~100 billion humans who have ever existed, none would be detectable as a fossil millions of years from now. And though the urbanisation area is still small, the area accessible and affected by humans is much larger. So, I dunno, I'm skeptical that it would be so hard to detect an earlier civilisation in the fossil record.
https://arxiv.org/pdf/1804.03748.pdf
I heard a lecture from a garbologist. Yes, they do exist, archaeologists of the present. Future inhabitants of earth will date us precisely as just after the Age of Clorox. For in the 1950s and 60s, Clorox bleach came in massive glass bottles which will last until the earth is consumed by the red giant our star will become.
ReplyDeleteHumans have a habit of disposing of their remains in places other than mudpits. We basically ensure that our remains turn to compost, and if not, burn them or serve them up as dinner - even turning them into decorations. We also tend to dig up our own fossils and store them in museums, ready to be nuked out of existence. So we actively reduce our fossil footprint.
ReplyDeleteAs for our buildings surviving, reinforced concrete structures only last at most 200 years then rot into nothing as oxidation reaches the steel rebar.
Any civilization that lasted more than a few centuries would leave quite a mark, in the chemical composition of the strata if not in artifacts.
ReplyDeleteI think it's pretty much beyond doubt that we'd leave a detectable signal of some sort behind for many millions of years. The interesting question I take from this is whether or not this would actually be detected, after all the perishables are gone and everything that can be fossilised is left under several hundred metres of rock. The question boils down to what fraction of the Earth would contain the detectable signal versus what fraction of the Earth we've actually investigated.
ReplyDeleteIt's not simply a matter of lying under the dirt waiting for us to find it. Things are buried at different rates, uncovered, eroded, buried again, hydrothermally altered, shoved around by glaciers, folded, roasted by a rising dolomite dyke, covered by seawater etc. etc. And not everything happens at the same rate in the same place.
ReplyDeleteTo illustrate, when we geophysically prospect for uranium, we're not actually looking for uranium but for gamma emissions from the decay products, and that should lead us to the uranium but often the stuff has buggered off somewhere else through leaching.
Now the kicker is, that if there is a chemical signature, it is likely to be very thinly salted as well as chopped up and separated. We don't bother investigating anything that doesn't have a certain %age ore grade. Whilst there's iron, copper etc in a building, there is far less platinum than you would find even in a crappy low-yield platinum reef.
We could find it if we looked for a dinosaur industrial civilisation, but we'd also need to know when, where to look for it and also what to look for. Chicxulub might have been the Ultimate Weapon in a dinosaur war and that's the evidence staring at us in the face. Extremely unlikely but who knows? Nature seems to like to evolve intelligence.