The latest newly discovered meteor impact that did not cause the Younger Dryas

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There is little doubt among archaeologists that the Younger Dryas, a cold snap following the initial retreat of Ice Age conditions some 11,000 years ago, had a major impact on human history. It seems that humans are highly motivated to return the impact to the Younger Dryas. Two times in recent years, evidence of an impact, a celestial object whacking into the Earth, has been suggested as the cause of the famous climatic “two step.” As sexy as impacts are, however, it is very unlikely that the Younger Dryas was caused by one.

I cut my climate teeth on the Younger Dryas. I was studying under Glynn Isaac when a number of palaeoclimate related data and observations rather suddenly came together, and the likely relevance of Milankovitch orbital geometry emerged. This is the subtle and regular pattern of the way the Earth goes around the Sun, resulting in periods of time — thousands of years long — when the Summer Solstice (in Late June, these days) is also a time of year when the total amount of Sun’s energy falling on the Earth is at a periodic minimum. In other words, the potential for having a really cold summer is high. Or a few of them in a row. This in and of itself does not cause an Ice Age. It probably takes a handful of different things being true all at once for the planet to plunge into a cold phase, including the position of continents and mountains, behavior of sea currents, and atmospheric dust caused by large volcanic eruptions or meteor impacts. But during these Milankovitch set-ups, an Ice Age, or more properly termed, a glacial period, is reasonably likely all else being slightly colder than equal.

The periodic and orbit-determined nature of glacial periods was proven from work done during the 1960s, 1970s, and then assembled into something that made sense in the 1980s. Fort this to happen, oceanography had to be invented. Stable isotope chemistry had to develop. The ability to raise cores from the deep sea needed to develop, and then, a bunch of such cores had to be raised and studied. Then it all had to be put together. The key “moment” was the development of deep-time core sequences, first from deep sea sediments, then ice cores, covering over 100,000 years (and eventually, 800,000 years) of time, showing changes in the isotopic composition of sea water at a scale approaching year-to-year variation.

I got to see that happen, and I was enthralled, while being Glynn’s student. He knew all the coring and isotope people, and they were in and out of Isaac’s “Stone Age Lab.” Not long into this, however, Glynn died, and the Stone Age Lab was taken over by Ofer Bar-Yosef. Ofer was working on the origin of plant agriculture in the Near East, and was very interested in the fine tuned time scales provided by dipsy cores, er, deep sea cores. (Ofer’s Israeli accent confused many an undergrad. “I love this course, but he keeps showing all these ocean cores but he’s talking about dipsy this and dipsy that!!!”) These cores, with their detailed isotopic records, showed the coming and going of major ice ages very clearly.

The cores contain the remains of the tiny skeletal bits of microscopic organisms that made part of their hard parts using Oxygen from the sea in which they lived. Short lived organisms, they contributed on their deaths to the sediment at the bottom of the sea, leaving an ongoing record of that Oxygen’s isotopic makeup. Specifically, you can measure, with some fancy machines and some necessary and logical adjustments to the data, the ratio of heavier vs. lighter isotopes of Oxygen in the ocean at the time the organisms lived. Water made with the lighter oxygen literally jumps out of the ocean, in the process of evaporation, at a slightly higher rate than the heavy Oxygen based water.

Therefore, rainwater is isotopically light.

Therefore, glacial ice is isotopically light.

Therefore, when a lot of the Earth’s rainwater is trapped as glacial ice, the oceans are isotopically heavy.

Therefore the organisms that use that oxygen are isotopcialy heavy or light depending on global climate, and this signal is preserved in the dipsy, er, deep sea, cores.

And so, we were able to see the “Younger Dryas” pretty clearly.

But what is the Younger Dryas? What is a Dryas? And what ever happened to the Older Dryas?

A Dryas is a cold loving cute little flowering plant that is abundant enough, when it is abundant, to produce sufficient pollen that is readily identifiable as to be counted in ancient pollen records, taken from fresh water swamps and lakes and such.

There are two fairly recent periods when Dryas, always rare, peaks, in pollen profiles observed in the Middle East and parts of Europe. An older time (27K to 24K years ago) and a more recent time (12,900 to 11,700 years ago). The more recent time is the Younger Dryas. Since the plant shows up during cold periods, the Younger Dryas was considered to be a brief return of ice age conditions following the initial de-glaciation from the last major Ice Age.

People figured out that the Younger Dryas was actually visible in other places, not just where those pollen profiles showed up. In fact, everywhere where there is a record of ice age glacial activity that is sufficiently detailed, it is there. In North America, it is called the “Lexingtonian Readvance” because of a lobe of glacial ice re-grew near Lexington Massachusetts, indicating a rapid and short turn return to glacial conditions. Every major region glaciated at the end of the last Ice Age has these re-advance lobed, generally locally named. It is pretty clear that the Younger Dryas was global, and significant enough to leave a mark.

The Younger Dryas is absolutely clear in the ancient record of ocean Oxygen isotopes. It looks like this:

This is a portion of a graph you can see in Wikipedia. The squiggly lines represent Oxygen isotopes, but in ice rather than deep sea sediments. You can see a trend from lower (colder) to higher (warmer) that is followed by a long period of warm conditions (that’s us, now, on the far left). The arrow points to a clear reversal in the warming trend, showing this as the Younger Dryas.

Ofer Bar-Yosef has proposed that the Younger Dryas was linked to the origin of cereal agriculture in the Middle East. He was probably right. How did that work? People living in the Levant at the time were pretty successful foragers. They relied on a wide range of foods, including hunted gazelle and a range of plant products. But they were increasingly exploiting cereal grains, likely wild barley and wheat. Foraging was so successful that many settlements formed that became permanently occupied. People may have moved around a lot, but somebody was staying in these small villages year round.

As climate got better and better, over a period of just a few human generations, this pattern developed. But then the cold shift occurred. Bar-Yosef suggests that this was the trigger to increase exploitation of certain resources, shift away from some resources that became more rare, but mainly, to start or accelerate an ongoing process of tending the grains that were already being harvested. The morphological indicators that distinguish wild from domestic grain show up in various species of edible grasses starting at this time. Humans invent agriculture in the Middle East.

I hasten to add that humans invented agriculture (horticulture and/or animal husbandry) in many locations around the world, starting at or near the beginning of the warm period shown in the graph above. But the situation in the middle east was a bit different, probably earlier than all or most other cases, and very sudden owing to the invention, or as Bar-Yosef puts it, revolution, of agriculture.

So, the Younger Dryas, which you now know all about, was very impactful for humans, in a way you now also know about. But what about the reverse? Did impacts cause the Younger Dryas?

I ask this now because there are news stories everywhere (see this) about a giant crater found in Greenland, suggesting that this meteor strike caused the Younger Dryas.

A meteor impact could cause a cooling trend. If the meteor hit a major Greenland glacier, it could cause melt-water to alter sea currents, thus causing the Younger Dryas. Indeed, the idea of glacial melt-water associated with the simple melting of glaciers following an ice age was at one time thought to be sufficient to trigger a re-glaciation, short lived.

Unfortunately, the dating of this impact is very very iffy, and the chances of it having actually happened at the exact right time to be implicated with the Younger Dryas is approximately zero. Also, any melting of ice causing fresh water to alter Atlantic sea currents would possibly cause a climate shift like the Younger Dryas; there is evidence of melt water changing currents around that time; that evidence has already been examined closely and the timing of that event does not fit with the Younger Dryas.

So, no.

Mark Boslough, on Twitter, and Stefan Rahmstorf, in several publications, agree. Rahmstorf has documented a 1470 year period abrupt glacial event phenomenon, of which the Younger Dryas looks like just another one. During these glacial periods, either full on glacial events or as the planet is pulling out of or falling into one, there is a lot of wild swinging around of the climate. The Younger Dryas is probably just one of those swings, and may well be one that fits into a periodic schedule unrelated to meteors, as sexy as meteors are.

Expect more on this over the next few weeks. Keep an eye on RealClimate blog, I’ll bet they discuss it there.

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11 thoughts on “The latest newly discovered meteor impact that did not cause the Younger Dryas

  1. “Unfortunately, the dating of this impact is very very iffy, and the chances of it having actually happened at the exact right time to be implicated with the Younger Dryas is approximately zero.” -Greg Laden above.

    Approximately? So you’re saying there’s a chance? 😉

    Seriously though, can you elaborate a bit more on this here, please ?

    Just how iffy is the dating and why and what are the error bars and ranges for it and why is it so outside matching the right date for causing the Younger Dryas?

    Good and interesting article here though I’d really like more info here on those points, thanks.

    1. There is a chance, but there remains the problem that the mechanism proposed for the Meteor turning on the Younger Dryas is meltwater, and meltwater (though from a different source) has been looked at and found lacking with respect to timing. The meltwater and conveyor changes show up at a different time.

  2. Unfortunately, the dating of this impact is very very iffy, and the chances of it having actually happened at the exact right time to be implicated with the Younger Dryas is approximately zero.

    It does seem a stretch.

    Also, any melting of ice causing fresh water to alter Atlantic sea currents would possibly cause a climate shift like the Younger Dryas; there is evidence of melt water changing currents around that time; that evidence has already been examined closely and the timing of that event does not fit with the Younger Dryas.

    I was under the impression that current thinking still favoured proglacial Lake Agassiz as the source of freshwater flux that triggered an AMOC slowdown, but that the original hypothesis of Broecker et al. (1989) was revised. The drainage route is now proposed to have been northwest via the Mackenzie Valley into the Arctic ocean rather than east along the St Lawrence Valley into the N Atlantic (Tarasov & Peltier 2005; Condron & Windsor 2010).

    1. Maybe, but the fact still remains that whatever meltwater was pouring into the Atlantic, leaving its various traces in various records (foram die-off, dropstones, etc) from whatever source, don’t form a compelling case for causing the YD.

    2. Why not, exactly? I can see why a freshwater flux *would* inhibit NADW formation and so reduce the AMOC and so produce an antiphased climatic effect exactly as see in the YD: NH cooling centred on the NA and warming in the SH.

  3. BBD, it could. If any abrupt melt-water initiated the YD, it was probably the aforementioned St Lawrence/Sea of Champlain event. Do note, however, there was one Younger Dryas, and the Sea of Champlain events are two. That somewhat weakens the cause and effect, if the cause happens twice but the effect happens one. In fact, there are four or more discharge events overall during the late Pleistocene (https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/91JC01624).

    These events are probably not related to a meteor impact, and it is especially not related to this particular meteor impact.

    Then there is the external yet non meteoric (probably) mysterious but powerful explanation here:

    https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2003GL017115

    Meltwater events are common during a deglaciation. The Younger Dryas is associated with a moment in time at which a different mechanism may be more likley, but it is mysterious (and not a meteor).

    This isn’t the kind of mystery where we might find a likely suspect on the train and then slowly connect the dots between a murder and that suspect. This is the kind of mystery where we have a train full of suspects but one of them is very suspicious looking and thus gets blamed all the time, but is no more likely than any other to have committed the deed.

    I didn’t go into detail on this part of the story too much because I really am anticipating something from at lease one of the authors of at least one of the above cited studies some time soon.

    1. These events are probably not related to a meteor impact, and it is especially not related to this particular meteor impact.

      Sure, and I’m absolutely not in the bolide-wot-dunnit camp. In fact I think it’s amusing that we have a whopping ~3Ma date range for the Greenland impact and suddenly it just has to be the trigger for the YD.

      The first study you link offers some support for the proglacial lake drainage hypothesis (my bold):

      High?resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,000 14C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at about 14,500 (and probably older), 13,500, 12,000 and 10,500 years B.P. Each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean (inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land). In addition, each of these times may be associated with brief episodes of cooler climate in the North Atlantic region, the best example of which is the Younger Dryas cooling of 10,500 years ago. These results support models linking meltwater discharge, decreased NADW production, and decreased North Atlantic heat flux.

      As for Rahmstorf’s argument that there really is a regular 1500y DO cycle with an external driver, IMO he’s pushed his analysis quite hard to justify his conclusions. Many other researchers see rather less evidence for such a degree of regularity in DO events. They also propose internal mechanisms for DO events, eg. MacAyeal’s original binge-purge hypothesis (MacAyeal 1993), Wunsch’s argument that DO events are triggered by interaction between windfields and continental ice sheets (Wunsch 2006), and the suggestion that DO events are triggered by changes in the height of the continental ice sheets Zhang et al. 2014).

      Another important thing here is that many researchers disagree with Rahmstorf that the YD was a DO event at all, arguing that DO events are essentially regional, expressed most strongly in the N Atlantic, and that the magnitude of the YD and its global effect mark it out as a different phenomenon.

      Anyway, whatever the case may be, I think we’re all agreed that the bolide impact trigger for the YD is a bit of a stretch. Not least because there’s evidence that a YD-type event occurred during termination III.

  4. Cheers for the elaboration and further explanations there Greg Laden – and BBD.

    I’d still love a bit more info on how the dating is so “iffy”and what the error bars around it and ranges of it are but those answers are appreciated.

    “The Younger Dryas is associated with a moment in time at which a different mechanism may be more likely, but it is mysterious (and not a meteor).” -Greg Laden

    Why do we rule out a meteor there – is it just that timing factor again or is there more to it that says it wouldn’t have been a bolide impact causing it?

    1. At this stage, the impact date range is “sometime during the Pleistocene” 🙂 (~3Ma – 12.9ka). Not exactly precise.

      Another problem is that none of the markers for a bolide impact really stand up to scrutiny. There’s a couple of links that discuss this here and here.

      The latter goes into the work done by Melott et al. (2010), which examined the atmospheric nitrate and ammonium signature of the Tunguska bolide in the GISP2 Greenland ice core and compared it with nitrite and ammonium anomalies during the YD. An impactor of sufficient size to be a possible cause of the YD would have left a whopping fingerprint and it’s just not there.

  5. Everybody knows we are headed towards one serious kick-ass ice age from the Super Grand Solar Minimum 2020-2053 (THE ZHARKOVA MINIMUM as it shall be called, henceforth, with accolades, fanfare and Grand Proclimations). ALL the REAL scientists agree. IF you disagree then obviously you are not a REAL scientist, just some dumbass blogger. Truth is, scientifically, speaking, millions will die because of this “Zharkova Minimum”. Crops will fail because of it, causing people to starve to death. Millions will die in the ensuing economic collapses leading to wars for scarcity of resources. Global Warming Alarmist Dumbasses will no longer be able to blog because of infrustructure collapses in power grids, internet, and banking systems. Neighbor will eat neighbor and the neighbor’s children. All this BEFORE 2075! PREPARE FOR ICE 2020-2053. GROW YOUR FOOD INDOORS NOW OR YOU WILL DIE!

    1. What does this have to do with the causes of the YD?

      * * *

      Anyway, same question as asked on the other thread where you were wittering about solar minima:

      1/ What is the change in average solar flux during the supposed ‘super grand solar minimum’, in W/m^2? Take the solar constant to be ~1366 W/m^2 at TOA.

      2/ Given the net increase in anthropogenic forcing since 1750 is >2W/m^2 and rising, what sign is the sum of the two forcing changes?

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