It was as if the Holocene never existed...

Cast your minds back to one of my first ever posts (here's the link if you can't), where I summarised the main proposals for the onset of the Anthropocene - unsurprisingly, one of the hottest debates in the discourse to date. Today I'll be focusing on one key idea: what if the Anthropocene and the Holocene are coeval? In other words, what if the Holocene was replaced entirely by the Anthropocene?

Smith and Zeder (2013) are the main proponents of this argument, though Certini and Scalenghe (2015) have recently moved away from their proposal of an onset with 'Anthropogenic soils' in 2,000 BP (Certini and Scalenghe 2011), and instead towards one which agrees with Smith and Zeder (2013), arguing that the onset of the Holocene should instead be renamed and reconfigured to become the Anthropocene. Certini and Scalenghe (2015) do, however, still believe that anthropogenic soils (e.g. those affected by repetitive ploughing, application of fertilisers, contamination, enrichment etc) could provide an additional auxiliary stratotype to the 5 existing stratotypes recognised for the Pleistocene-Holocene boundary (Walker et al 2009). The basis of the arguments for regarding the Holocene and Anthropocene as coeval are as follows:

• Origins of agriculture approximately coincide with the onset of the Holocene (11,700 years BP) (Balter 2013; Certini and Scalenghe 2015). 
• Niche Construction Theory (NCT) - the wide-spread domestication of plants and animals during the early Holocene allowed human societies to significantly modify ecosystems (Smith and Zeder 2013). This is different from other species as humans pass these behaviours on throughout generations through social learning (Smith and Zeder 2013).
• No new golden spike (GSSP) would be necessary as we could continue to use the Holocene's existing spike in the NGRIP Ice Core (Certini and Scalenghe 2015).
• Might be more useful to merge the Anthropocene with the Holocene, as there is abundant evidence of human societies developing tools (domesticates) to be used in the subsequent 10,000 years to reshape ecosystems, as opposed to 'limiting it to the last two centuries on the basis of extant geological standards' (Smith and Zeder 2013).
• The coeval proposal provides a solution to the challenge of satisfying formal geological standards needed to establish the Anthropocene as a new epoch (Smith and Zeder 2013).
• The transitions between previous geologic time intervals (e.g. P-T extinction event, lasted 61±48 thousand years (Burgess et al 2014)) makes the 12,000 years between the onset of the Anthropocene and Holocene seem a 'ridiculous range of time', thus it is better to see them as one combined epoch (Certini and Scalenghe 2015).

Well, one might argue, why bother changing the name of the epoch we are in from the 'Holocene' to the 'Anthropocene'? Smith and Zeder (2013) propose that the ICS may consider using 'Holocene' in scientific contexts, and the 'Anthropocene' in media and popular discourse. Essentially this would mean keeping the Anthropocene as an informal term for the time we live in, continuing its use to encourage action and focus attention on human impact on the planet. Certini and Scalenghe (2015) also argue the term 'Anthropocene' is more 'durable' than the Holocene. "Durability?" you ask? Let me explain what they mean. Their argument is that a new glacial inception (ice age) would end the Holocene (as the basis of the Holocene is that it is an interglacial and marks the end of the last ice age), but that the same ice age would not stop human impact on Earth (as the basis of the Anthropocene is the focus of humanity's effects on the planet, and not it being an interglacial) (Certini and Scalenghe 2015). I'm not entirely convinced by this argument or the example of an ice age; a new ice age will only occur once atmospheric CO2 levels are low enough (~280ppmv) and albedo high enough to allow ice growth and expansion (see an earlier post of mine). Surely allowing this to occur would signify the end of human alteration of Earth Systems through fossil fuel combustion, deforestation etc? UNLESS humanity makes a deliberate attempt to modify Earth Systems in a way which permits a glacial inception (something I personally can't see happening any time soon...).

Should the focus of the last 11,700 years, and (hopefully) many millennia to follow be entirely on humans? Are we bold enough to conclude that this epoch in Earth History will always be characterised by human impact on Earth Systems, even far into the distant future? Not everyone agrees with the idea that the Anthropocene and Holocene should be coeval. Clive Hamilton (2014) heavily criticises Smith and Zeder's (2013) approach of focusing on ecosystems and the biosphere, as opposed to the Earth in totality in a guest blogpost on the Anthropocene Review Blog. Hamilton (2014) is openly frustrated by Smith and Zeder's paper, as according to him, they have entirely misunderstood the concept of the Anthropocene. He argues the Anthropocene must focus on when humans first started altering the Earth as a whole, and not just ecosystems. To summarise his post, he basically tells ecologists to "butt out" (his words, not mine!) in trying to redefine the Anthropocene. Ouch. 

My thoughts on the matter

I can empathise with wanting to make the Anthropocene and Holocene coeval purely for the simplicity and ease it would have over debating new onsets which must obey strict geologic standards. However, the reasoning behind this combination of epochs should not be for ease or simplicity, it should be because the evidence is overwhelming, which in this instance, I think it is not. There are many other proposals for the onset of the Anthropocene which have far more convincing and stand-out examples of humans altering Earth Systems, such as the Trinity atomic detonation, producing global radioactive fallout as well as representing a momentous change in humanity's ability to both create and destroy at the same time with a new nuclear age (Zalasiewicz et al 2015). Furthermore, is there any point in changing the Geologic Time Scale if we are just to continue using the Anthropocene informally and the Holocene formally? The only difference is that we would be discussing the same length of geologic time, and not using the Anthropocene for interpreting the last few centuries (or decades, depending on your view).

Also, with reference to NCT (Smith and Zeder 2013), should the example of humans first altering ecosystems with plant and animal domestication not be something more paramount? For example, the Haber-Bosch process of the early 20th century provides a much more significant instance of humans artificially producing nitrogen, completely modifying the nitrogen cycle even now in our present day. Or, the instance of when human creation and use of chlorofluorocarbons (CFCs) led to a large hole developing in the ozone-layer above Antarctica? There are many other, perhaps more convincing examples of humanity altering ecosystems as well as the Earth System as a whole, and not just the terrestrial biosphere. I do, however, agree with Certini and Scalenghe (2015) that ending the Holocene and starting the Anthropocene after a mere ~12,000 years seems like too short an amount of time to end an epoch and begin another. Although, this is not to say that the best way over that hurdle is to merge two epochs with separate identities and characteristics into one with confusing formal and informal terminology. I'm still of the opinion that if the Anthropocene is to become formally defined, it should not simply replace the Holocene epoch.

A human concept, lost in stratigraphy?

"As stratigraphers, we require criteria to map the Anthropocene with relevant and consistent meaning. Presently, we are left to map a unit conceptually rather than conceptualizing a mappable stratigraphic unit."

The above quote (emphasis added), written by Autin and Holbrook (2012) in an article in GSA Today, forced me to question some fundamental arguments in this debate. Is the Anthropocene a term rooted in pop culture, based more on conceptual meaning in search of a stratigraphic signal, as opposed to starting with a distinct stratigraphic record? Finney (2014) asked the very same question, and I find his opinion akin to mine. The more I become involved with the Anthropocene debate in this blogging journey, the more I am questioning everything I once believed at face-value without a second thought. The idea behind this post is rather like the famous question of what came first, the chicken or the egg? In this instance, should concept come before stratigraphic evidence?

A defining feature of the Anthropocene, distinct from other units of geologic time, is that the proposed epoch is in the present and continually evolving. Therefore, instead of looking back in retrospect at fossils and stratigraphic evidence, the conceptualising step must come first, with stratigraphic evidence following later. Whether this is the 'correct' way to classify a new unit of geologic time is dubious. Historically, changes in stratigraphy have been identified, and then conceptual units of time have been assigned to these clear shifts and events in the strata. In our present day, the world has identified a new concept, one which views humanity as a centrally dominant geologic force, and now we're searching for a stratigraphic signal to fit that concept. Perhaps this signifies a time for change in how the Geologic Time Scale (GTS) should be constructed in a present-day mindset, recognising that fossils and stratigraphic evidence can become secondary to conceptual assignments. 

Finney (2014) continues this philosophical debate by questioning the conceptual basis of the GTS, asking whether it is suitable to be applied to 'chronometers of recorded and future human history'. As so much of what constitutes the Anthropocene is in the present and near-future, should it formally become part of the GTS, considering that all units currently recognised are events which occurred deep in the past (Finney 2014)? I agree with Finney that this is a very important question that needs addressing by the AWG, but I do not agree that the Anthropocene (or any other future time intervals) should be written off from formalisation just because it has not occurred in the deep past. In my eyes, the GTS should modernise to accommodate for new periods of time which will likely come to fruition during humanity's existence on the planet. This may be as simple as providing alternative time resolution intervals which acknowledge shorter events and ages (e.g. centuries to millennia) within larger chronostratigraphic units (e.g. epochs and periods). Clearly the time resolution currently used on the GTS (millions to billions of years) would scarcely recognise the Anthropocene, a blip of time in Earth's 4.5 billion year history. So, should the GTS downscale to additionally accommodate smaller, yet important, intervals of time, extending the use of GSSAs? Or, should the GTS refuse to formalise these smaller events and continue formalising only those longer time intervals with robust GSSPs and a clear stratigraphic record?

Crucially, the Anthropocene is the first proposed geologic interval whereby the focus itself (humanity) is able to influence and change the very nature of the time wished to classify. This brings me onto my next big question to address: is the Anthropocene a unit of human history rather than Earth history?

Waiting Ages for Eons...

BOO! I hope you are all having a terrifyingly frightful Halloween!

The aim of this evening's post is to clarify the different geological time periods based upon how the IUGS International Commission on Stratigraphy (ICS) define them. We know the Anthropocene has been proposed as an epoch, but what does that actually mean?

Chronostratigraphy refers to the layers of rock (strata) which correspond to geochronologic time units. The official units in geochronology and stratigraphy (used by the ICS chart) are as follows, from largest to smallest in length:

EON
Chronostratigraphic unit equivalent: Eonothem
Examples: Hadean, Archean & Proterozoic (taken together as Precambrian), and Phanerozoic (current Eon)
Time Span: Indefinite length of time, but roughly billions of years +

ERA
Chronostratigraphic unit equivalent: Erathem
Examples: Paleozoic (old life), Mesozoic (intermediate life), and our current Cenozoic (recent life)
Time Span: Several hundred million years

PERIOD
Chronostratigraphic unit equivalent: System
Examples: Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary (current period)
Time Span: 30-80 million years

EPOCH
Chronostratigraphic unit equivalent: Series
Examples: e.g. Ogliocene, Miocene, Pliocene, Pleistocene, and Holocene (official present epoch). Does the Anthropocene belong here, too?
Time Span: Tens of millions of years

AGE
Chronostratigraphic unit equivalent: Stage
Examples: e.g. Campanian, Danian, Tortonian, Gelasian, Calabrian... maybe the Anthropocene?
Time Span: Lowest ranking unit, usually a few million years

(all information from the IUGS Stratigraphic Guide)

The ICS official geologic time scale as of 2015. If you can't read the writing (sorry), the original image can be downloaded here.

The Anthropocene is proposed as a potential geologic epoch, which would consequently terminate our current Holocene epoch. The Anthropocene would remain within the present Quaternary Period (which is in the Cenozoic Era, within the Phanerozoic Eon). From looking at other epochs in Earth History, they are pretty hefty chunks of time, so it is questionable whether the Anthropocene would suit this hierarchical level, or if perhaps instead it should become an Age within the Holocene...

Now you know what the different names for intervals of time actually mean, let's look at how long humans have been around for...

History of our planet Earth condensed into 12 hours. Though the start of the Anthropocene Epoch is subjective here to the author, the point remains that humans are just a blip in Earth History. (Source)

It's quite eye opening to realise that on this huge time scale, humans appear to be quite insignificant. If Earth's 4.5 billion years of existence were condensed into a 12 hour time period, humanity wouldn't exist until the last TWO SECONDS before midnight (almost like Cinderella..). However, in terms of effect on the planet in this short interval of time, humans have had a hugely disproportionate influence on the Earth. How can we already determine if humanity warrant an entire epoch, which might potentially last millions of years? Say, for example, that in the next century humans make home on our neighbouring planet, Mars. This (hypothetical, yet possible) planetary exploration might make a much greater defining signature for a human-dominated period of time. Or, perhaps the harnessing of nuclear fusion in a few decades might make a far more suited landmark for the start of a human-dominated age?

In my next post, I'll be exploring the evidence for the Holocene as our current (official) epoch, and discussing further whether the Anthropocene should be considered on the same hierarchical level. Hopefully this has given you some perspective on humans in Earth History and clarified the different terminology for geologic time intervals. And just because videos are far more entertaining than reading all this, especially on Halloween, check out the one below:

So catchy.