The new Holocene Ages
NOTE: This post assumes familiarity with a few concepts that I have explained in my previous post: The new 'Meghalayan Age' of Earth's history.
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On 14th July, 2018, the IUGS-International Commission of Stratigraphy released the latest version of the Geologic Time Scale. There were three new Ages defined: the Greenlandian, the Northgrippian and the Meghalayan (older to younger). The Meghalayan emerged as the cool kid on the block (ironic, because it's the other two that are defined in ice cores). It made news headlines because:
- it's apparently the Age we're living in now. (The kind of headlines that attract non-Earth-scientist Earthlings..)
- it's defined by the first GSSP from India, and the first GSSP in a stalagmite.
- there is a debate around the necessity for this definition, the efficacy of this definition, and if it cuts into the significance of the proposed Anthropocene Epoch.
But I see no reason for neglecting the other two Ages. This post is about the Holocene Ages and their associated GSSPs.
The base of the Holocene and the Greenlandian Age
The term Holocene had been formally adopted at the Third International Geological Congress (a big conference) in 1885. So technically, the question whether the last interglacial in the Earth's history really deserves a distinct name and status is quite old. The GSSP for the Holocene was, however, designated well over a century later in 2009 by Mike Walker et al. in the Journal of Quaternary Science ( Link 1 | Link 2 ). This GSSP was the first to be placed in an ice core, and the authors have explained in their paper why they chose this ice core over marine / terrestrial (conventional) sedimentary sequences. In short, the ice core afforded very well resolved chronological constraints as well as a range of climatic signatures to define the boundary. The ice core was extracted from a site in Greenland, and hence the first division of the Holocene is called the Greenlandian Age.
Apart from the Jurassic Period ( 201.3 - 145 Ma) , the Ice Ages are probably the most famous of episodes from the Earth's past. The Quaternary Period ( 2.58 - 0 Ma), divided into the Pleistocene and the Holocene epochs, is characterized by the climatic oscillations between the glacial and interglacial periods. It is easy to confuse these glacials with the "Ice Ages", but in reality, the entire Quaternary is actually an Ice Age (yes, a single one) and we are currently simply in an interglacial period of it.
I suggest you neglect the text on the graph, but focus only on the shape at first. The graph shows the atmospheric concentrations of carbon dioxide (CO2) through time, with left → right indicating older → younger (more recent time). The interglacials or the warm periods were all characterized by high CO2 concentrations and are marked in grey in the top half of the graph, filled with yellow. The glacials or the cool periods, with low values of CO2 in the atmosphere, are marked in grey in the bottom half and filled with cyan.
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Now, deglaciation in the last glacial-interglacial cycle began around 14500 years Before Present (BP). However, that's not where the Holocene starts. Instead of starting at the termination of the last glacial, the Holocene begins after the termination of a cold event called the Younger Dryas. The timeline basically goes this way:
If you have access to Walker et al. (2009) ( Link 1 | Link 2 ), I suggest you take a look at Figure 5(a) and note the abrupt jump the graph takes 11700 years BP. The jumps are most noticeable in the values of the isotope ratios of oxygen and hydrogen.
- Last Glacial Maximum: 26500 years BP (cold peak of last glacial)
- End of last glacial: 14500 years BP (warming begins)
- Younger Dryas: 12900 - 11700 years BP (yep, this one's the culprit if you're confused)
- Holocene Epoch : 11700 - 0 years BP ("our" Epoch which was recently divided)
If you have access to Walker et al. (2009) ( Link 1 | Link 2 ), I suggest you take a look at Figure 5(a) and note the abrupt jump the graph takes 11700 years BP. The jumps are most noticeable in the values of the isotope ratios of oxygen and hydrogen.
The "8.2 event" and the Northgrippian Age
So far, we know that as the Holocene began 11700 years BP, the Earth eased into an interglacial and the climate adopted a warming trend. This warming trend was interrupted roughly 8186 years BP, when the Earth witnessed an abrupt and short-lived cooling episode. Since geologists are used to thinking in terms of thousand-years (kilo-annum, or "ka") when talking about the Holocene, this event has earned the nickname the "8.2 event". The 8.2 event shows up as a distinct chemical change in the ice-core records, particularly in the oxygen isotope ratio. Walker et al. published a discussion paper in the Journal of Quaternary Science in 2012, and proposed that the GSSP that marks the 8.2 event be defined in an ice core extracted from a North GReenland Ice core Project (NGRIP) site. Hence the Age that commences after the event is called the Northgrippian Age.
If you have access to the paper ( Link 1 | Link 2 ) I suggest you take a look at the sharp signature of the 8.2 event in Figure 1.
If you have access to the paper ( Link 1 | Link 2 ) I suggest you take a look at the sharp signature of the 8.2 event in Figure 1.
The "4.2 event" and the Meghalayan Age
Much about the Meghalayan Age and its GSSP has already been discussed in the previous blog post. However, to recapitulate briefly, the the Meghalayan Age begins at what is informally called the "4.2 event", which was a brief, widespread drought (more formally called an "aridification event"). The GSSP for the Meghalayan Age has been placed in a stalagmite from a cave in the Indian state of Meghalaya. The 4.2 event can be seen as an abrupt change in the oxygen isotope composition, in Fig. 1 of Berkelhammer et al. (2012) or also in Fig. 5 in Walker et al. (2012).
Interestingly, while the 4.2 event registers as a drying event in the middle and low latitudes, the high latitudes experienced a cooling and high rainfall scenario at the same time. It is considered primarily a mid/low latitude phenomenon, as opposed to the 8.2 event which is considered primarily a high/mid latitude phenomenon. The choices of the GSSPs are hence, in accordance with these phenomena. The 8.2 event GSSP is placed in an ice core from the high latitudes, whereas the 4.2 event GSSP is placed in a speleothem from the mid-latitude region.
Interestingly, while the 4.2 event registers as a drying event in the middle and low latitudes, the high latitudes experienced a cooling and high rainfall scenario at the same time. It is considered primarily a mid/low latitude phenomenon, as opposed to the 8.2 event which is considered primarily a high/mid latitude phenomenon. The choices of the GSSPs are hence, in accordance with these phenomena. The 8.2 event GSSP is placed in an ice core from the high latitudes, whereas the 4.2 event GSSP is placed in a speleothem from the mid-latitude region.
Do we really need new names?
The "Holocene" was defined in 1885. If you think about it, there's really nothing special about the Holocene as an interglacial. It may be argued that the significance arises out of the evolution and widespread existence of modern humans, that the Holocene is special because it's our Epoch, the Epoch we live in. But in my opinion, we only lend the distinction of being recorders of the Earth's history in this case. Yes, we have altered the Earth's history, but that's the basis of the Anthropocene, not the Holocene.
I think the more relevant point is that younger strata are better resolvable in time. It is easier to date the Quaternary Period than it is to date the Jurassic or the Cambrian. In fact, the GSSP for the base of the Jurassic is placed not at the absolute value of 201.3 Ma, but at 201.3 ± 0.2 Ma. This means that there is an uncertainty of 200,000 years associated with that GSSP, which is about 17 times the entire span of the Holocene. It is simply easier to make all these calculations for more recent samples, and the mysteries get harder to solve as one goes back in time.
Walker et al. (2012) explained that phrases such as early Holocene / middle Holocene / late Holocene were already in popular use, and formally defining these boundaries only served to add consistency to their usage. Combine this with the fact that younger divisions are more amenable to being subdivided, and we begin to see why stratigraphers spent time and effort defining these recent, relatively tiny time intervals.
However ... .. ..
I think the more relevant point is that younger strata are better resolvable in time. It is easier to date the Quaternary Period than it is to date the Jurassic or the Cambrian. In fact, the GSSP for the base of the Jurassic is placed not at the absolute value of 201.3 Ma, but at 201.3 ± 0.2 Ma. This means that there is an uncertainty of 200,000 years associated with that GSSP, which is about 17 times the entire span of the Holocene. It is simply easier to make all these calculations for more recent samples, and the mysteries get harder to solve as one goes back in time.
Walker et al. (2012) explained that phrases such as early Holocene / middle Holocene / late Holocene were already in popular use, and formally defining these boundaries only served to add consistency to their usage. Combine this with the fact that younger divisions are more amenable to being subdivided, and we begin to see why stratigraphers spent time and effort defining these recent, relatively tiny time intervals.
However ... .. ..
Do we really need THESE new names?
I currently do not have the insight to deliver an informed opinion on- the Holocene / Meghalayan vs. Anthropocene debate (I am loath to using the word 'controversy').
Related reading: "Geology's Timekeepers Are Feuding". Please note that the illustrations use the BC-AD terminology to denote age, which I personally think is confusing. It may have been to reach out to the wider masses, but apart from the tabloid start, the article doesn't really seem for laymen. - if the 8.2 and 4.2 events offer the best boundaries to subdivide the Holocene.
Relevant tweet:
4.2ka is no one thing - it doesn't exist in all Holocene records, doesn't occur at the same time in those records, doesn't manifest itself as the same climate anomaly, and takes place during at least one other large climate shift (end of the AHP). Not a good stratigraphic marker.— Kevin Anchukaitis (@thirstygecko) July 18, 2018
Still doing my homework on this. Next post should focus on Point no. 2, ETA within a week.
"whether the last interglacial in the Earth's history really deserves a distinct name and status is quite old."
ReplyDeleteWhat's 'interglacial' here?
During an Ice age (such as the one we're living in right now), the Earth goes through relatively colder and warmer phases. The warmer phases are called interglacials. The glacials and interglacials can also be seen in the figure above.
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