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!