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?

Age over Point?

Geologists are set on finding a golden spike (GSSP) to mark the onset of the Anthropocene Epoch, as has been necessary for (almost) every other geological time unit in history. But it seems this marker needn't be golden, nor a spike; recently there have been arguments that the Anthropocene could instead be defined by a numerical age boundary (GSSA). A GSSA is essentially a point of time in the human calendar, a chronological reference point if you wish, assigned as a starting date for a unit of geological time. At present, the ICS have only defined periods of time older than ~500 million years ago (Ma) using GSSAs and chronometric dating (aka most of the Precambrian). This is because there is an insufficient fossil record/preservation level to identify key events needed for a GSSP this far back in history (Lewis and Maslin 2015).

Does a GSSP offer a practical advantage over a GSSA? Currently, all units within the Phanerozoic Eon are defined (or planned to be) by golden spikes (Zalasiewicz et al 2015). This wasn't always the case, though. The inception of the Holocene used to be 10,000 radiocarbon years BP (before present). This officially changed 6 years ago to 11,700 years b2k (before 2000), marked by a GSSP in a Greenland ice core (see here; Walker et al 2009). Prior to the switch from GSSA to GSSP, scientists were studying an enormous range of aspects within the Holocene without major issues in its age-based boundary. Does this show that age-based or stratotype-and-point-based boundaries are irrelevant to the deeper context and events within that time period?

The Trinity atomic bomb, pictured 16 milliseconds after
detonation, could be the defining marker for the Anthropocene.
(Source)

Zalasiewicz et al (2015) (note, the majority of the AWG...) recently proposed the Anthropocene should start when the Trinity atomic bomb was detonated in Alamogordo, New Mexico, at 05:29:21 Mountain War Time (+-2s), July 16, 1945. The authors liken this assignment of the age-based boundary to that of the Cretaceous-Palaeogene boundary: marked by the moment a meteorite struck the planet, leading to the demise of the non-avian dinosaurs. Though radiogenic fallout became more prominent on a global scale in the years to succeed, this GSSA is based on the first ever nuclear test, representing a clear historical turning point (Zalasiewicz et al 2015). Finney (2014) also recently considered the use of a GSSA for the Anthropocene, arguing that a GSSA would be more accurate than a calibrated onset formulated from a GSSP, with a date in millisecond precision. 

As it stands, some people disagree that any time period should be defined geologically by a numerical age. Bleeker (2004) argues that the Precambrian needs redefining from the present chronometric divisions to a more 'natural' timescale. By this, Bleeker means the use of GSSPs in the rock record to split the Precambrian into eons accurately reflecting stages of planetary evolution. If you're a bit of a geology/Earth history nerd like me (*waves*), I highly recommend giving Bleeker's paper a read...

...I digress. By raising this point, I'm asking you: if we are still contesting the use of GSSAs for periods of time where fossils do not exist (and thus necessitate an alternative boundary method), then how likely is the use of a GSSA where fossils do and will soon exist? Lewis and Maslin (2015) argue that because candidate GSSP markers exist, there is no need for a GSSA. But are those candidate GSSPs the best suited for this epoch? If the answer is no, then a GSSA could be more beneficial. It would also allow the fossil record to develop more fully, better representing the epoch of time than if we rush into using some proposed GSSPs, which, in hindsight might not depict the Anthropocene accurately (see this post for more!). The age of the Anthropocene is still very much in its development phase, and thus selecting a numerical age for its onset may prove a solution for defining the time whilst fossils and potential GSSPs continue to develop. 

Key take-home questions

We know that anthropogenic effects on Earth systems are now being recorded in sediment and rock, but will a significant enough stratigraphic record be produced for the future? Do we know for certain what changes humanity will bring to the planet? Would the main proposed GSSA (Zalasiewicz et al 2015) be superseded by future signals (e.g. a mass extinction event or meteorite)? Is the use of a GSSA practical and able to be used effectively by scientists? Additionally, would the use of a GSSA help engage other academic circles (besides geology) into the emerging Anthropocene literature?

My thoughts on the matter

For this recent chapter in human and Earth history, I believe there are many changes to be made to the way we visualise geologic time units. The Anthropocene is unique in that much of its content is in the present and near-future, and only the relatively recent past. In a time where everything is changing at rapid pace, the ICS need to adopt new ways to represent different ages within the Holocene (or Anthropocene, depending on your view) with techniques best suited to each time period, be that a GSSA or GSSP. I agree with Zalasiewicz et al (2015) that a GSSA may be a simpler way of defining the Anthropocene in the present day, as within the many proposed onsets, few have a definite and suitable GSSP (see a previous post of mine for GSSPs). As long as the GSSA chosen represents the moment we wish to characterise the nature of this epoch, and that there are various global stratigraphic signals relating to and following that moment, I see no issue in choosing an age over point. 

Do you have an opinion on this debate? Don't hesitate to voice it in the comments below!

Generation Anthropocene: Are we doomed?

I found this really interesting video on YouTube earlier, and wanted to share it with you all. If you're not already familiar with TED talks, I suggest you watch a few! They're very concise, intellectual, yet accessible talks on a whole spectrum of important topics.

As Mike Osborne and Miles Traer say, lets put on our "Anthropocene goggles" and begin to appreciate that the more we study the world, the more we realise humanity is an integral part of it. According to these two fairly optimistic scientists, the Anthropocene doesn't mean the end of anything, but it does mean that everything will change. Can humanity, the cause and defining feature of the Anthropocene, adapt to these changes?

What are your opinions on the matter? Don't hesitate to comment below!

Misconceptions, misunderstandings, and mistakes?

Before moving onto my next post about GSSA's in the Anthropocene (an extremely hot debate), I came across a paper in the Anthropocene Review by Hamilton (2015) which I couldn't resist blogging about. The paper is different from most academic articles I've read lately, with a rather opinionated, somewhat negative review of Lewis and Maslin's (2015) paper. To recap, Lewis and Maslin (2015) conclude AD 1610 and the atomic bomb spike in 1964 are the only two viable proposals for the onset of the Anthropocene, due to having GSSPs that meet geological criteria (see my last post for the criteria). Hamilton (2015) argues that the Anthropocene's inception in 1610 is not a credible suggestion, and that Lewis and Maslin are trying to find a marker where there is no real event, and consequently ignoring key events where there is no (traditional) marker. 

To briefly summarise the main argument of the paper, Hamilton proclaims that Lewis and Maslin made a fundamental error by failing to recognise that a paradigm shift has occurred, whereby the global environment has been replaced by Earth System Science. In most definitions of the Anthropocene, humans are recognised as a 'force of nature', able to modify the functioning of the Earth System, not just changing parts of ecology, or the landscape (Hamilton 2015). Hamilton, alongside other authors who also responded to Lewis and Maslin's paper (e.g. Waters et al 2015), argue the dip in CO2 of 7-10ppm is not actually proven to be caused by human activity, and is more likely to be natural variability in the Earth System. Hamilton also argues Lewis and Maslin have become too heavily preoccupied with finding a GSSP, adopting a 'spike fetish' which has unfortunately meant they have overlooked the real key concepts of the Anthropocene. 

Though this is clearly a satirical cartoon, it is interesting
to wonder what will be found millions of years
into the future to define the age of humanity... (Source)

Related to my post yesterday about GSSPs and their strict geological criteria, Hamilton goes on to conclude that traditional stratigraphy is no longer suited for making judgements about the Anthropocene. Many other, older geologic time units have been classified by assessing fossil species in rock strata, however this is NOT a method suitable for identifying changes in Earth System functioning (Hamilton 2015). Despite this, a clear datable marker is still necessary if the AWG (Anthropocene Working Group) are to persuade the ICS to formally ratify the new epoch. The difference that Hamilton outlines is that the AWG are satisfied with finding a marker that will be apparent in the future, but Lewis and Maslin are unsatisfied unless they find a suitable GSSP now. 

The Anthropocene is as much of a debate about geology, climate science, Earth System functioning and ecology, as it is about the socio-economic conceptual consequences of its proposal. Though I will discuss GSSA's more in my next post, Zalasiewicz et al (2015) recently proposed that 1945 should be the start date for the Anthropocene due to the first nuclear bomb tests in Alamogordo, New Mexico. Inevitably, the radionuclides from the blasts have created a visible layer in rock strata. The key argument Hamilton makes about this proposal, in comparison to Lewis and Maslin's assessment, is that Zalasiewicz et al (2015) understand that the marker is just that: a marker. Zalasiewicz et al recognise that the Anthropocene isn't defined by nuclear bombs, but instead by human-induced alteration of the way the Earth System functions through fossil fuel combustion and resultant climate change. The nuclear signal, does, however signify the USA's global economic dominance and the post-war 'boom years', and thus the resultant ability for Great Acceleration to take place with rapidly increasing GHGs and consequential warming (Hamilton 2015). The Alamogordo blasts simply act as a signal for geologists in millions of years to detect as the marker of the Anthropocene - an age which has much bigger and wider global socio-economic and environmental consequences than one golden spike could wholly represent.

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.

Are we jumping the gun?

"It is not yet clear what the character of the fully developed Anthropocene will be, and it might be wise to let future generations decide, with hindsight, when the Anthropocene started, acknowledging only that we are in the transition towards it." 

The above quote from Wolff's (2014) paper strikes a chord with my own personal feelings. As I briefly touched on in my last post, how can we already determine (albeit with much contention...) a start date for a time period at epochal scale, which may only be in its early developmental stages? Unlike other geologic time periods, we have no idea what the Anthropocene will look like in 20, 200, 20,000 or 2,000,000 years time. As Wolff (2014) argues, atmospheric CO2 concentrations have risen significantly, but they are still rising and could easily reach somewhere between 500-1000 ppmv in the foreseeable future. Then again, if we mitigate emissions and impacts urgently, the current perturbation of Earth Systems might be the defining signature of the Anthropocene (Wolff 2014). I personally don't think mitigation of that magnitude would be likely, unless politicians could look back in retrospect at the 'human environment' and see the enormity of the changes caused to Earth because of it.

Zalasiewicz et al (2015) agree with Wolff in his conclusions that greater changes lie ahead, and that the Anthropocene's stratigraphic signature will likely differ from today when looking back in hindsight. However, Zalasiewicz et al (2015) argue that the scale of these future predicted changes (e.g. perturbations of the biosphere - see Barnosky et al 2012) may actually start to correspond more to period/era level changes in Earth Systems. One issue I have with this argument is that the Anthropocene epoch may barely come into fruition before it is argued that human impacts are instead on a period/era scale.

Do we argue that the onset of the Anthropocene is marked by the first instance of human alteration of Earth Systems, or by the time of greatest magnitude of change? How do we identify the moment when human impact on the Earth becomes so substantial that it warrants the beginning of a new interval of geologic time? Millions of years from now, what event would a geologist pinpoint as the start of the Anthropocene? Perhaps the Industrial Revolution? The Great Acceleration? Harnessing the Sun's power with nuclear fusion? Or colonising other planets, even solar systems? These are all big philosophical questions which I cannot answer objectively. I can, and will, however, weave these thoughts into posts throughout my remaining blogging journey.

I agree with Wolff (2014) that we need to acknowledge humanity's dominance over Earth Systems. BUT, were this proposed epoch to last tens of millions of years (as other epochs have done), we ought not to be too hasty in trying to pinpoint its starting point now. The current pace of change in technology, atmospheric chemistry, ocean dynamics, and biodiversity etc is so rapid that enormous change is inevitable in the near future. Perhaps, some future unknown-as-of-yet event or discovery would be a better signature of a human-dominated age than anything that has occurred to date. 

Some satirical humour for y'all on this rainy day. Does the Anthropocene mark the end of the Holocene? Should we hold off and let generations of the future decide? (Source)

In my next post, I'll be reviewing whether proposals for the start date of this epoch (see my previous post for the summary of onsets) meet GSSP criteria standards, or whether perhaps instead the Anthropocene should be defined by a GSSA. To keep your brains ticking over until then, sit down with a cuppa and consider this: have we already played the "human card" in the Holocene? Are we trying to play the same card again in the Anthropocene?

Let me hear your thoughts in the comments below!