Younger Dry-as dust?

Clarity started to emerge when new techniques for lining up the ice cores in Antarctica and Greenland were developed. One technique used the very rapid changes in methane (which could be measured in both poles) to synchronise the chronologies. The thought being that methane changes are well mixed and so large changes in one hemisphere get transmitted very quickly to the other. With this came a big surprise – the Antarctic Cold Reversal started hundreds of years before the Younger Dryas! In fact, Antarctica stopped cooling just as the YD was getting started. This was evidence of a bi-polar see-saw in the ocean – something the models did seem happy to show.

But what about the New Zealand glaciers? How did they fit in? There had been some loosely constrained pollen data that didn’t show much cooling reported in 1999, but the result was still ambiguous. This is where the first of the new papers comes in. In it, Barrows et al show with improved dating that the New Zealand peak glacial advances actually were significantly younger than the YD. These dates seem more solid that the previous estimates and are supported by nearby ocean sediment evidence for a continued warming through the YD.

So now that the southern hemisphere oddities have left the scene, does that mean we now have a full understanding of the event? Not quite. The ocean circulation theory has indeed been strengthened in recent years, but the search for a trigger continues – why did it happen when it did? As always, many ideas have been put forward – a shift in drainage pathways for Lake Agassiz from the Mississippi to the St. Lawrence, a solar trigger or a tropical Pacific trigger and now we have a brand new idea – a cometary impact.

This has been suggested by a large group of researchers who have collectively been working on archaeological sites (Clovis) in North America and who noticed a layer of charcoal at about the same time as the YD at a number of disparate sites. They claim too that within this charcoal there is significant evidence of impact ejecta, and from this they suggest that the trigger for the YD was in fact an extraterrestrial impact. This doesn’t really undermine the ocean mechanism – the comet is hypothesised to have caused significant meltwater to flow into the Atlantic and the ocean circulation changed as would be expected in the standard view. Some suggestions were made at meetings that the direct impact due to dust and smoke forcing from fires actually caused the initial YD cooling, but this doesn’t seem quite as plausible (dust falls out of the air quickly).

The researchers have however tried to link the impact with everything that was previously linked in time to the Younger Dryas – mammoth extinctions, the disappearance of the Clovis culture etc. – but it is very difficult to disentangle a direct consequence of an impact from the indirect consequence of the subsequent climate change. But these ideas are quite intriguing and they made quite a splash when announced in a coordinated session at AGU in the spring.

There are three aspects of this work that will require independent confirmation to determine whether or not this is a viable explanation. Firstly, it should be possible to find the ejecta layer almost anywhere – peat bogs, lake sediment, ice cores etc. – wherever there is material of the right age. If that is indeed found (big if), then the first part of the hypothesis might be confirmed – that there was an impact at this time. The subsequent parts are much harder: depending on where any object landed or was centered, how can one show that it produced the meltwater that presumably caused the ocean circulation change? The source to the ocean of the meltwater (be it the Arctic, St Lawrence or Hudson Bay) has been unclear for many years. Finally, how can you show that the direct effect of the hypothesised comet was responsible for any impacts, rather than the indirect effect of the ocean change? These issues will, I suspect, take a long time to resolve.

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