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Evolution of the Fundy Tides
From: Trevor Kenchington
Date: 2004 Jan 3, 17:25 +0000
From: Trevor Kenchington
Date: 2004 Jan 3, 17:25 +0000
With apologies for straying very far off topic: A few days ago, Frank Reed queried a statement of mine about the changing sea levels in Atlantic Canada and I warned that my reply should not be considered as fully reliable. I have now checked the latest published research and discovered that things are a whole lot more complicated than either Frank or I had suggested (no surprise there!). Shaw et al. (2002, Quaternary Science Reviews vol. 21, pages 1861-1878) have summarized the (limited and scattered) available data, with a large dose of interpolation added in, to produce maps of both the shorelines and the height of contemporary sea levels (relative to modern sea level) for most 1,000-year intervals between 13,000 and 6,000 years ago. [Trends from 6,000 to now seem to have been fairly steady.] Their maps cover southern Labrador to Cape Cod. {There is a very brief, semi-popular account of the same research on the Web at: http://www.mar.dfo-mpo.gc.ca/science/review/e/html/2001/BIO-English.html That account is also available in hard copy in the Bedford Institute of Oceanography's "2001 In Review".} In the past 13 millennia, most coastal and offshore areas in this region have first moved up and then down, though some have dipped consistently. The zero line, where contemporary sea level was the same as modern sea level, has moved over time and is by no means straight. Hence, this is _not_ a matter of the continent tilting around a simple hinge line. In overly-brief summary: The Atlantic coast of Nova Scotia has been dipping downwards continuously for about 11,000 years. However, New Brunswick and the New England coast rose quite considerably at first -- the current shoreline rising out of the water. (That included my half-remembered case of southern New Brunswick rising, though I mistakenly supposed that the uplift was continuing.) What is now Georges Bank rose out of the sea, forming a large peninsular attached to Cape Cod, before dipping again. Through the last 10,000 years or so, the zero line (contemporary sea level equal to modern sea level) has roughly followed the coast of Maine and extended southwest across Massachusetts. Nova Scotia, southeastern Massachusetts and the offshore areas have been dipping. Inland parts of New England have been rising. However, somewhere in New Brunswick, the zero line turns northwest to near the St.Lawrence river, then loops round eastwards to Newfoundland, before running north up the Labrador Sea. Thus, much of New Brunswick, most of the Gulf of St.Lawrence and southern Newfoundland are all dipping, whereas most of Quebec, Labrador and northern-most Newfoundland are rising. (Where I am typing this has dived some 40 metres into the sea in the past 10,000 or so years -- or very roughly 5mm per year on average. The rate may have slowed somewhat over the millennia but it is still fast enough for the effects to be seen along the shore.) The Bay of Fundy reached something like its present surface shape by 12,000 years ago. However, it is continuing to deepen, particularly on its Nova Scotian side -- getting perhaps 10 metres deeper in the past 10,000 years. Perhaps more to the point, Georges Bank reached its maximum emergence some 11,000 years ago. It then began to dip but did not reach anything close to its present state until around 8,000. Now that Shaw et al. have illustrated the change in basin shape and now that numerical modelling of the Fundy tides is quite advanced, it would be interesting to see how the resonant amplification of the semi-diurnal lunar tide has developed over time. This really isn't a topic for guesswork but I would not be surprised if there were no exaggerated tides in the Bay until something in the bracket of 5,000 to 10,000 years ago. Nor would I be surprised if the continued slow deepening is gradually bringing the Bay closer to perfect resonance -- thus accounting for the erosional features mentioned in earlier e-mails. Hopefully, this is food for thought, even though not information of practical value for navigators. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus