On Stranger Tides

Ancient tides were greatly different than today's.  An article from my alma mater on how tides in the recent geological past were significantly different than today's, with some being much higher, and others being much lower:

Due to phenomena such as ice ages, plate tectonics, land uplift, erosion and sedimentation, tides have changed dramatically over thousands of years and may change again in the future, a new study concludes.

Some tides on the East Coast of the United States, for instance, may at times in the past have been enormously higher than they are today – a difference between low and high tide of 10-20 feet, instead of the current 3-6 foot range.

And tides in the Bay of Fundy, which today are among the most extreme in the world and have a range up to 55 feet, didn’t amount to much at all about 5,000 years ago. But around that same time, tides on the southern U.S. Atlantic coast, from North Carolina to Florida, were about 75 percent higher.

The findings were just published in the Journal of Geophysical Research. The work was done with computer simulations at a high resolution, and supported by the National Science Foundation and other agencies.

“Scientists study past sea levels for a range of things, to learn about climate changes, geology, marine biology,” said David Hill, an associate professor in the School of Civil and Construction Engineering at Oregon State University. “In most of this research it was assumed that prehistoric tidal patterns were about the same as they are today. But they weren’t, and we need to do a better job of accounting for this.”

One of the most interesting findings of the study, Hill said, was that around 9,000 years ago, as the Earth was emerging from its most recent ice age, there was a huge amplification in tides of the western Atlantic Ocean. The tidal ranges were up to three times more extreme than those that exist today, and water would have surged up and down on the East Coast.

One of the major variables in ancient tides, of course, was sea level changes that were caused by previous ice ages. When massive amounts of ice piled miles thick in the Northern Hemisphere 15,000 to 20,000 years ago, for instance, sea levels were more than 300 feet lower.

Actually, this makes a lot of sense. The height of the tide at a given location is determined by a complicated interplay of the astronomical phenomena related to the moon and sun motions relative to the earth, and the coastline of that the water interacts with.  With a huge change in water level between the last ice age and now, tremendous differences in the coastline virtually guarantee that the resulting tides might be quite different.  The paper gives an excellent example:

“Part of what we found was that there are certain places on Earth where tidal energy gets dissipated at a disproportionately high rate, real hot spots of tidal action,” Hill said. “One of these today is Hudson Bay, and it’s helping to reduce tidal energies all over the rest of the Atlantic Ocean. But during the last ice age Hudson Bay was closed down and buried in ice, and that caused more extreme tides elsewhere.”

I work in a place in Maine with extreme tides, up to 15 ft.  Below are a couple of pictures from the Penobscot River showing the effects of the tide on the river:

High Tide

Low Tide

I also have a somewhat amusing tide story.  A few years ago I went to visit a colleague in Georgia, near Savannah (where they have 9-10 ft tides), to write a joint proposal.  We took a day off to do some sightseeing, and rented a small skiff to do some exploring on one of the outer islands.  As we picked the boat up, his wife said "High Tide is about 11:00 right?

"Uh, I think so."  That should have been a clue...

Anyway, we took the boat out around 9:00, got to the island around 10:00, anchored the boat in a tidal creek that was brimming with water, and proceeded to take a 3 hour walk.  We came back to the boat a little after noon to find:

That little bit of water in back was all that was left.  We had to wait until nearly dark for the water to get back into the creek to get the boat out.  However, all was not lost.  We brought ample food and drink, and one of the pools left in the bottom of the tidal creek had these...

So I do have a deeper appreciation of the phrase "Time and tide wait for no man."