DENDROCHRONOLOGICAL DATING, RESULTS AND OPEN PROBLEMS

The availability of long, year-by-year, tree-ring chronologies from a spread of locations has allowed the exploration of several abrupt, apparently catastrophic, environmental events in recent millennia.  This paper will explore the ways in which data are accumulating relevant to these events.  It will also explore the chronological pitfalls encountered when evidence from sources of  differing dating quality are encountered. In many cases there are definite limits to the precision with which environmental parameters can be dated.  This applies particularly to palynological and sedimentary records; even varve records and ice-cores
have their limitations when it comes to highly refined dating questions. Currently it is fair to say that, for anything earlier than the present millennium, there is no good understanding of the causes of the various environmental downturns noted in the tree-ring records.  The paper will also explore some of the possibilities which have been tentatively addressed so far, namely volcanism, extraterrestrial bombardment and undersea outgassing.

In 1984 Val LaMarche and Kathy Hirschboeck pointed out a severe frost ring in their Californian bristlecone pine tree-ring record relating to the calendar year 1627 BC.  Their suggestion that this frost event might have been due to the eruption of the Santorini volcano in the Aegean is still a source of active debate.  Their work stimulated the observation of a series of narrowest-ring events in an Irish oak chronology at dates 3195 BC, 2345 BC, 1159 BC, 207 BC and AD 540.  These dates, it turns out, fall in the vicinity of several possibly traumatic environmental events marked in human records by such phenomena as dynastic changes, Dark Ages and plagues (Baillie 1995).  Circumstantial cases can be erected which would allow the environmental downturns to explain some of the human effects.  Curiously, mythology hints that several of the events may have had cometary associations.  Such cases then allow the formulation of research programs aimed at uncovering physical evidence from suitable deposits. Detailed examination of oak specimens in which the narrowest-ring events were observed, and consideration of the detailed responses of trees in various geographical areas to the events hint at complex reactions over periods of years and it may be some considerable time before definitive solutions are available.  In the meantime active speculation in the literature is aimed at flushing out relevant information from specialists in a wide variety of fields.