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).
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:
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.
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).
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| 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). |
- A principal correlation event (the global stratigraphic marker)
- Other additional secondary markers (auxiliary stratotypes)
- Correlation on both regional and global scales
- Complete continuous sedimentation above AND below the marker
- An exact location at 1 point on Earth: longitude, latitude, depth/height
- Accessible
- 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.
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