3 May 2009

An ancient tsunami in New York

Posted by Dave Petley

The BBC are running a slightly strange story today about the possibility that the News York area was hit by a tsunami about 2,300 years ago. Given that by far the most likely source of a tsunami is a submarine landslide, this is of some interest. The odd part of this is the timing of the story – I cannot quite work out why it has popped up again now. A very similar story made the news in November last year – see here – and again in December (see here), so I can’t quite see what has changed to bring this back to the forefront again. Perhaps a paper regarding this is about to be published – but there is no reference to this in the news report on the BBC.

The BBC news report focuses upon work being undertaken by Steve Goodbred and colleagues at Vanderbilt University in Tennessee. They have identified what appears to be a tsunami deposit from cores across New York and New Jersey. Tsunami deposits are interesting as the wave can transport coarse-grained particles (typically gravels) and even marine fossils from the near continental shelf to deposit them some way inland. Thus, they tend to be quite distinctive. Unfortunately, a major problem is that storm surges associated with very large storms can also bring in coarse shallow marine materials, which means that determining for sure that a deposit was generated by a tsunami is difficult.

In the case of the New York area, a quite extensive deposit has been found. Dr Goodbred and his colleagues suggest in the BBC article that this is probably a tsunami created by a submarine landslide, much like the 1929 Grand Banks (Newfoundland) event.

My sense is that considerable further work is needed here. First, we should all be distinctly pleased that there is no suggestion being made that this tsunami deposit was generated by the ludicrous hypothesis postulated a few years ago by Ward and Day that occasional flank collapses on the volcanoes of the Canary Islands can generate tsunamis that devastate the whole North Atlantic basin. However, the November 2008 news stories centred around a suggestion that a research team from Harvard and Colombia University had found evidence of an impact event origin for the tsunami deposit – i.e. that the wave was generated by a meteorite impact into the ocean. They claim that the deposit contains the types of particles that such an event would generate. I note that there does appear to be some difference in the size of the wave that the teams believe occurred – Goodbred suggests c.5 m, whilst the Harvard team suggest 20 m.

Personally I would like to see some peer reviewed publications on this before we make too much of it. There are a whole series of strands here that really need to go through peer review:
1. The structure of the deposit and its distribution;
2. Its age;
3. Analyses of its origin, looking in particular at its composition (particle mineralogies and sizes, fossil materials; etc), variations in its thickness, the presence of nano-diamonds that are supposedly indicative of an impact event, etc.

Of course the media interest in this is that the potential destruction of New York by a tsunami is rather exciting. We do need to understand Atlantic tsunamis rather better, but there is little doubt that they are really pretty rare and have rather localised impacts in terms of the tsunamis that they generate.