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 golden spike?

The Holocene is our current official interglacial epoch, beginning 11,700 calendar years b2k (before AD 2000), marking the end of the Pleistocene Epoch (Walker et al 2009). This current interval is formally characterised by human activity, climate change, sea-level rise and more (Walker et al 2009), but most importantly marks the termination of the Last Glacial period, when glacial ice began rapidly retreating at the end of the Younger Dryas. Despite long being recognised at Series status, the Holocene was only formally ratified in 2008 by the IUGS as our current Epoch with a distinct stratigraphic marker (Walker et al 2009). 

This marker, a Global Stratotype Section and Point (GSSP), aka "Golden Spike" marks the lower boundary/beginning of a geological time unit (Lewis and Maslin 2015). The "spike" identified in strata of rock, ice, or sediment, for example, reflects a global-change phenomenon for a new unit of time. A really distinct example of a GSSP is the Cretaceous-Paleogene boundary, marked by mass extinction of dinosaurs 65.5 million years ago (Zalasiewicz et al 2011). Such a significant change in the Earth system like this is recorded clearly in rock records, providing a clean-cut boundary for a new interval of time. But what about more recently? The base of the Holocene is now officially defined by the NorthGRIP (NGRIP) ice-core from the Greenland ice sheet, with boundary level at 1492.45m depth in Borehole NGRIP2 (Walker et al 2009). 

The Holocene-Pleistocene boundary is identifiable here by measuring δ18O, marked by a shift to 'heavier' oxygen isotope values at the end of the Younger Dryas/Greenland Stadial 1 (GS-1) (Walker et al 2009).

Woah, woah, woah...I should slow down. Here I am talking about GSSP's and I haven't even told you about their (very!) strict criteria. Not just any old bit of rock, sediment, or ice will suffice. To have a geologic time unit approved by the big guys in the IUGS, Gradstein et al (2012) identify 7 key features for an appropriate GSSP:

1. A principal correlation event (the global stratigraphic marker)
2. Other additional secondary markers (auxiliary stratotypes)
3. Correlation on both regional and global scales
4. Complete continuous sedimentation above AND below the marker
5. An exact location at 1 point on Earth: longitude, latitude, depth/height
6. Accessible
7. The ability to be protected and conserved

We know the Holocene's GSSP meets these requirements, despite slightly differing from other markers in Earth History, due to being defined by a climatic signal in ice, as opposed to fossil evidence in rocks (biostratigraphic marker) (Walker et al 2009). But how does the Anthropocene stand up against this strict criteria? The Anthropocene Working Group are yet to come to a consensus for the onset of the epoch, let alone define a GSSP which meets the IUGS criteria.

From all the different suggestions of onsets for the Anthropocene (see this post for my quick summary), Lewis and Maslin (2015) suggest only two measure up with suitable accompanying GSSPs: the Orbis spike with a CO2 dip and minima in AD 1610, and the radioactive bomb spike in 1964 correlating with peak 14C. Rises in CO2 personally strike me as an obvious marker for the Anthropocene, though Zalasiewicz et al (2011) recognise that on longer timescales the changes, for example at the start of the Industrial Revolution, are too gradual to be recognised on an annual-decadal level.

With such difficulty in finding and agreeing on a robust GSSP for the Anthropocene, it has been suggested that a GSSA (Global Standard Stratigraphic Age) should be used instead. This would involve simply selecting a numerical age e.g. 1945, which marks a historical turning point for the Anthropocene-Holocene boundary: the Alamogordo test explosion (Zalasiewicz et al 2015). Though this may seem like an obvious alternate and simpler answer for the Anthropocene, many people have disagreed with the use of a GSSA for such a modern and developing time interval...

That's all for now, folks. Next time I'll be further discussing why there has been resistance for the use of a GSSA-based boundary for the Anthropocene.

Further to last week...

Hello everyone! Hope you're all having a good start to your reading week.

At the end of last week's post I asked whether you thought we'd already played the "human card" in defining the Holocene. Now, what I meant by this is that in the very first paragraph of the formal ratification paper for the Holocene (Walker et al 2009), human evolution and activity are recognised as key features of the epoch, alongside climate change, sea-level rise, and faunal migrations. If we've already acknowledged that humans are a key characteristic of the Holocene, why do we need to re-recognise this in the Anthropocene? Isn't the recognition given in the Holocene's ratification enough? Gibbard and Walker (2013) argue without including humanity's impact in the Holocene, the Holocene is simply another interglacial (warm period) alongside many others in the Pleistocene. Human influence is one of the defining features of the last 11,700 years. Therefore, they conclude that during the Quaternary Period (last 2.8 million years), on a climatic basis there is no justification for a new epoch (Gibbard and Walker 2013). Gibbard and Walker (2013) insist this "human card" cannot be used twice over for the Anthropocene...

However, one could argue that the "human card" played in the Anthropocene is very different from the first use in the Holocene. This time around, we're not just recognising human activity, but instead recognising human dominance of Earth Systems and the mass geological forcing that humanity holds. As someone commented on my last post, perhaps a modified version of this "human card" will be played many more times over into the future as human influence on the planet develops and changes.