Because I have a child in preschool, we watch a lot of documentaries…many of them about dinosaurs. All of them seem to contain the phrase, “Millions of years ago, the earth was covered by warm, shallow seas…” or “Dividing the North American continent was a warm, shallow sea…” The question I have is, what do they mean by shallow, and what do they mean by warm?
The sea that divided the North American continent during the Cretaceous was an epicontinental one, that is, covering part of the continental mass itself. It probably ranged from a few tens to a few hundreds of feet deep over most of its extent. This is in contrast to the deep ocean basins like the Atlantic and Pacific that are thousands of feet deep.
There is no precisely similar sea today that covers a large area of the interior of a continental land mass. However, the Sunda Sea in South East Asia covers part of the Asian continental mass. The large islands of Sumatra, Borneo, and Java are parts of the continental mass that remain above water.
In the Cretaceous concentrations of carbon dioxide in the atmosphere were much higher than they are now. It was a “greenhouse” world, with significantly higher temperatures than today worldwide. (Offhand I forget the range, but probably at least on the order of 10 degrees F.) Sea level was unusually high, partially because no water was tied up in ice, partially because of thermal expansion of the oceans, and partially because of the distribution of land between the existing continental masses. Most of the margins of continents were drowned, and there were epicontinental seas in North America and I believe a few other places.
By shallow, they mean a few thousand feet or less. When sea level rose it covered much of the continental land masses. The seas were shallow only by comparison with the roughly 2 depth mile of most of the world’s oceans.
According to this, the Jurassic climate was roughly 10°C warmer than present day. Exactly how warm the seas were would depend on where they were located. The link provides maps with rough temperature zones included.
I find all sorts of mentions of ‘warm, shallow seas’ but no specific definition of just how shallow or how warm they were.
At a guess I’d say the sea was never more than several hundred feet deep. Why that number? No reason but it somehow feels reasonable even if wrong. Several hundred feet may not seem very shallow to most of us but compared with the oceans that can get 30,000+ feet deep a few hundred feet is peanuts.
As for warm that’s an even harder guess. Sea/ocean temperatures vary widely depending on currents, depth, geographic location and so on. On the whole I suppose temperature would also be a relative thing as compared to ‘typical’ ocean temperatures but whether you or I would describe the sea as being warm is debatable (you’d probably find 72[sup]O[/sup] water chilly even though that is a generally comfortable air temperature). I imagine anything above 70[sup]O[/sup] F might be considered a ‘warm’ sea but again that is a complete guess on my part.
Here are two sites you might find interesting:
http://www.miwg.freeserve.co.uk/geology.htm
The second link has a nifty map of the US covered by a ‘warm, shallow sea’. Most of the East coast, Florida, Texas and on up into Canada was covered by water at one time. Of course, such things could be commonly found all over the world throughout earth’s history as continents shifted and mountains rose and fell.
NOTE: Squink is right on this one that the average depth of the world’s oceans is around 2 miles. My note of 30,000+ feet related to the very deepest spots with the deepest being the Marianas Trench at 35,813 feet (or some 7 miles). As an interesting aside the pressure at that depth is some 16,000 pounds/sq. inch! (That info is all in the link I provided and there’s plenty more to be found with a Google search)
Well, not that commonly. As I said, the Cretaceous was rather unusual in this regard. A greater percentage of continental areas were submerged at this time than at almost any other time in the Earth’s history. There were a few other times when there was a relatively high sea stand, but the Cretaceous was exceptional.
One Period when the seas were relatively low was the Permian, when all the continents coalesced into a supercontinent called Pangaea. Very little of the continental area was below water at this time, which may have contributed to a massive extinction event of shallow water organisms.
Some checking shows that global sea level was at least 150 m (almost 500 ft) higher throughout the Cretaceous than it is now, with a maximum high stand at 260 m (more than 850 ft). Therefore much more of the Earth’s land area would have been submerged than now.
Presently the continental margins are covered to a depth of about 200 meters (660 ft), where the continental slope begins, and these may be regarded as the world’s “shallow seas” (as opposed to the deep waters of the ocean basins themselves). In the Cretaceous, these epicontinental seas could have thus been up to 460 m deep (or over 1500 ft), but most would have been shallower.
Thanks…it doesn’t sound that shallow to me, but I guess compared to two miles it’s shallow. I suppose we’ll eventually find out for ourselves thanks to global warming…
Mom2Boyz