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

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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?

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.

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.

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?

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.

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.

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.

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.

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).