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Younger Dryas watershed

26 August 2012

This is really basic geology but I'm placing it under the catastrophism banner as it has a bearing on the recent hypothesis of a comet impact at the Younger Dryas boundary – now, in light of a paper by Bill Napier, regarded as the Earth encountering a Taurid stream of material rather than an impact event as such. At http://geology.gsapubs.org/content/38/4/383.short?rss=1&amp%3bssource=mfr … 'What Caused the Younger Dryas Cold Event?' doi:10.1130/focus042012.1v38no4page383-4 and actually explores the route of the meltwater during the Younger Dryas. The consensus view is that the Younger Dryas was caused by an increase in fresh water discharge into the North Atlantic which triggered the cold stadial. When the Ice Age ended, 17.5 thousand years ago, the melting ice sheets formed into a huge lake at the bottom of the ice sheet, known as Lake Agassiz (the modern Great Lakes formation is the residue of this water). As the ice retreated further from the lake fresh melt water was diverted along what is now the St Lawrence river system – and a particularly large spill over was responsible for the Younger Dryas stadial. This theory, going back to 1976 (Johnson and McClure), 1982 (Rooth), and 2006 (Broecker) was one of those 'common sense' breakthroughs, or so it seemed at the time, a neat explanation. However, it was subsequently found there were a series of similar stadials (known as Heinrich events) littering the last Ice Age – so what was going on? In addition, there was a massive stadial that had occurred at the end of the Ice Age, the so called Oldest Dryas event (or Heinrich One). This all required an explanation and the idea there were bursts of fresh water discharge on all those occasions came to appear a somewhat thin opinion. It was necessary for Broecker to think in terms of the Younger Dryas as a rather special event, unlike the other stadials, and this special status allowed the comet impact theory to also take the same high ground (or is it low ground). It seems pretty certain that something similar sparked all seven Heinrich events, and the Younger Dryas, and probably the shorter stadial around 6200BC, and perhaps even that conjectured at 3200BC. Whilst all this may fit nicely into the Clube and Napier theory of the orbit of the Earth coming into contact with streams of dust and debris from the break-up of former comets, and taking various lengths of time in passing through those dusty regions of space, it doesn't militate too well with a succession of fresh water injections or a protracted number of impacts.

Stadials as a result of resonant orbits is one way of looking at this, and in this paper, we may note (going back to the thrust of the post), actual evidence of outflow via the St Lawrence has been found, but more importantly, at the outset of the Younger Dryas the outlet remained closed – which was true of much of the Younger Dryas period. It was after all a stadial. Hence, no evidence of meltwater via the Mississippi or the St Lawrence during the Younger Dryas = no sudden discharge into the North Atlantic. Where did it go? Did it have to go anywhere – it was a cold period and presumably glaciation advanced (in winter if not in summer). However, that factor is not the main thrust of the paper. It is an exploration of the Firestone et al theory and assumes an impact, or atmospheric explosion, is still the favoured option, ignoring the Napier paper. Its therefore a bit clever as it is able to be negative without fully dismissing the idea, using the Tunguska impact/ explosion as a guide and measuring the amount of nitrates and ammonium etc it produced and comparing it with what is known from ice cores at the time of the Younger Dryas. However, in the course of disproving the impact theory they have at the same time placed a big question mark around the rival consensus model of Broecker.

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