"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
less 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. For instance,
the Antikythera Mechanism (ca. 205 BCE) is a unique object until the Renaissance. Despite
impressive recent gains in the ability to detect the wider impacts of civilization on landscapes
and ecosystems (Kidwell, 2015), we conclude that for potential civilizations older than about 4
Ma, the chances of finding direct evidence of their existence via objects or fossilized examples of
their population is small."

"There is an interesting paradox in considering the Anthropogenic footprint on a geological
timescale. The longer human civilization lasts, the larger the signal one would expect in the
record. However, the longer a civilization lasts, the more sustainable its practices would need
to have become in order to survive. The more sustainable a society (e.g. in energy generation,
manufacturing, or agriculture) the smaller the footprint on the rest of the planet. But the smaller
the footprint, the less of a signal will be embedded in the geological record. Thus the footprint of
civilization might be self-limiting on a relatively short time-scale."

"The Anthropocene layer in ocean sediment will be abrupt and multi-variate, consisting of seemingly
concurrent specific peaks in multiple geochemical proxies, biomarkers, elemental composition, and
mineralogy. It will likely demarcate a clear transition of faunal taxa prior to the event compared
to afterwards. Most of the individual markers will not be unique in the context of Earth history
as we demonstrate below, but the combination of tracers may be. However, we speculate that
some specific tracers that would be unique, specifically persistent synthetic molecules, plastics, and
(potentially) very long-lived radioactive fallout in the event of nuclear catastrophe. Absent those
markers, the uniqueness of the event may well be seen in the multitude of relatively independent
fingerprints as opposed to a coherent set of changes associated with a single geophysical cause."

"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 δ13C 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, for instance, the coincidence of crustal formation events with
the climate changes suggest that the intrusion of basaltic magmas into organic-rich shales and/or
petroleum-bearing evaporites (Storey et al., 2007; Svensen et al., 2009; Kravchinsky, 2012) may
have released large quantities of CO2 or CH4 to the atmosphere. 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.
Current changes appear to be significantly faster than the paleoclimatic events (figure 1), but
this may be partly due to limitations of chronology in the geological record. Attempts to time the
length of prior events have used constant sedimentation estimates, or constant-flux markers (e.g.
3He (McGee & Mukhopadhyay, 2012)), or orbital chronologies, or supposed annual or seasonal
banding in the sediment (Wright & Schaller, 2013). The accuracy of these methods suffer when
there are large changes in sedimentation or hiatuses across these events (which is common), or
rely on the imperfect identification of regularities with specific astronomical features (Pearson &
Nicholas, 2014; Pearson & Thomas, 2015). Additionally, bioturbation will often smooth an abrupt
event even in a perfectly preserved sedimentary setting. Thus the ability to detect an event onset
of a few centuries (or less) in the record is questionable, and so direct isolation of an industrial
cause based only on apparent timing is also not conclusive."

"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 δ13C 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 (as it would ours).
However any large release of biogenic carbon whether from methane hydrate pools or volcanic
intrusions into organic rich sediments, will have a similar signal. We therefore have a situation
where the known unique markers might not be indicative, while the (perhaps) more expected
markers are not sufficient."