When is it high tide?
Do we have a spring tide?
These questions are asked by tourists getting ready for a day at the beach and by sailors arriving at port. The tide makes a difference to their day. But tides may have an influence on far larger timescales, too.
At the recent General Assembly of the European Geosciences Union in Vienna, Austria, two presentations suggested that tides may have changed the history of our planet on more than one occasion.
Tides may have helped initiate a “snowball Earth” phase by becoming exceptionally weak, according to Mattias Green of the School of Ocean Sciences at Bangor University in the United Kingdom, along with researchers from the University of Lisbon in Portugal, Monash University in Australia, and Northwestern University in the United States. They reached this conclusion from model calculations of tides the world over, taking into account the location of continents, ocean depth, and ice cover.
Originally, Green told Eos, he and his colleagues were looking for arguments against the snowball Earth hypothesis, which posits that during the Cryogenian, the geologic period from 720 million to 635 million years ago, two extreme glaciations covered the planet with ice, except possibly for a band along the equator.
There is no consensus, however, that glaciation really was that extreme. Green thought that if tides turned out to be strong during the Cryogenian, it might not be possible for so much ice to exist.
“We know that the tides on Earth today are important around the Antarctic and in Greenland,” Green said. “They lift the ice, which sets up stress fractures. But most importantly, the tide brings warm water in, which melts the ice on the underside. The cold, fresh meltwater would then insulate the ice, but the tide gets rid of that. There are studies that suggest that if you don’t have tides in Antarctica, the melt rate would be a quarter of what it is now.”
After the calculations were complete, Green had to conclude that the opposite was true in the Cryogenian: Tides were exceptionally weak, about 10% of their present amplitude.
“They were doing nothing,” Green said.
But that doesn’t mean that tides played no role during snowball Earth. On the contrary, their weakness supported the stable cold phase of the planet by starving ice sheets of any warm water that could have kept them in check.
“All the other processes that the tides are contributing to, like vertical transport, ocean circulation, [they aren’t] there. So it’s possible that along with a lot of other things, this is one potential mechanism that helped bring along the snowball stage,” said Green.
One initial reason for weak tides during the Cryogenian was that the familiar continents of Earth were gathered into one supercontinent, surrounded by one ocean. According to an earlier study by Green, when there are more continents and smaller oceans, the resonance of tidal flows is more likely to be amplified, just like water will slosh higher and higher in a bathtub if you move your body at just the right beat. Right now, this is the reason tides in the Atlantic are particularly high.
Once Earth entered the snowball stage, for reasons that are still debated, conditions on the planet further dampened the tides, according to Green. Water turning into ice means shallower oceans and thus less water for the Moon and the Sun to pull around. Friction on the underside of the ice means tidal currents are weakened.
The feedback would work in the other direction once the supercontinent started breaking up and the ice lost its grip on Earth and its tides. The Cryogenian ended.
Tidal Change in Evolution
In a warmer world, life flourished in the sea and eventually on land. And in that transition, too, tides may have played a pivotal role, according to another study by Green, together with Steven Balbus of the University of Oxford in the United Kingdom and Per Ahlberg and Hannah Byrne of Uppsala University in Sweden.
The researchers argue that during the Devonian, 419 million to 359 million years ago, tides forced certain lobe-finned fish to make the move onto land. These organisms were the ancestors of modern tetrapods, or four-limbed vertebrates, including humans.
An explanation for that move onto land is needed, Balbus told Eos.
“That’s not something that naturally occurs. Fish don’t come out of the ocean regularly and make themselves happy on land. The other way is very common. Lots of species love to go back to the ocean; they evolve, and they become very fishlike: whales, porpoises, and before them ichthyosaurs. They can swim, and if they need oxygen, they go up. If you’re a fish on the land and you need to have water over your gills, what are you going to do? So it requires some kind of special impetus for that to happen.”
That impetus, he proposed in a 2014 paper in Proceedings of the Royal Society A, could have come from the phenomenon of spring tides and their opposite, neap tides. These tidal variances occur because the gravitational pulls of the Sun and the Moon, comparable in size, sometimes reinforce and sometimes work against each other.
“For the spring tides, the ocean comes up very far onto the land. The next time there’s a high tide, it doesn’t come as far,” Balbus explained. “That means there’s an isolated tidal pool. If you’re a fish, it’s not good.”
That’s because with Earth tides being what they are, a fish will have to wait roughly half a month for a splash of water and a chance to escape the pool—unless it can call on some extra capabilities.
“If the fish can somehow flail out, and wouldn’t die immediately, there would be another place reasonably nearby to get to, that was replenished a little more often, because it’s closer to the sea,” Balbus said.
In other words, on a coast with high tides, even rudimentary locomotion would be advantageous for a fish, making the development of fleshy, lobed fins into weight-bearing limbs a good evolutionary adaptation.
Balbus and his colleagues investigated tidal patterns and evolution by analyzing the geography of 400 million years ago. “The idea is to take reconstructions of what the continents during the Devonian looked like, work out what the Moon’s orbit was back then, and ask: What tidal patterns were there?”
His team was the perfect multidisciplinary group, Balbus said: he himself is an astrophysicist, Green is an oceanographer, and Ahlberg and Byrne are paleontologists. They did the calculations and at the Vienna conference presented two regions in what are now southern China and the Baltics that seem particularly promising to establish a correlation between tidal range and the emergence of early tetrapods.
“There are bays with very high tidal responses,” Balbus said. “And, in fact, they are associated with very interesting fossil records that tell us that a large radiation of key transitional fish species occurred. And the early tetrapods were located there.”
An article on the findings has been submitted to the Proceedings of the National Academy of Sciences of the United States of America, but Balbus admits that “we are a long way from proving it. People argue about the reasons why these fish developed limbs; you can think of all kinds of reasons why that would be good.”
One way to clinch the debate would be for the group to point to locations that had very high tides in the Devonian and then find transitional or early tetrapod fossils there. “That would be a very important vindication, if we can calculate where to look for fossils,” said Balbus.
Tides on Earth are nowadays a bit stronger than they were in the Devonian, and all over the world, tidal pools are a common feature of the coastal landscape. So the same evolutionary pressure for fish to adapt to live on land still exists, Balbus agrees.
Modern fish, however, have a problem that the lobe-finned fish of the Devonian did not: “God help the fish that can’t do very well on land. There are a lot more creatures now that will have them for breakfast than when this first happened. But there are fish that live in these intertidal regions, and you can see what kind of biological adaptations they’ve developed. Air breathing capacity and also some sort of mobility. In Asia, perches actually climb trees, and they inhabit areas that I guess are similar to the sort of areas we are talking about here, extensive swamps.”
Such behavior shows these areas to be, he said, the most likely places for vertebrates to have come on shore for the first time. “And it seems to have happened.”
—Bas den Hond (firstname.lastname@example.org), Science Writer
den Hond, B. (2019), The tides they are a-changing, Eos, 100, https://doi.org/10.1029/2019EO124073. Published on 19 June 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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