[Stevie Bain (OP)]

Geoff would like know:

Was the Earth ever covered 100% by water?

Can you help?

[jeffreyH]

Dr Anthony Burnham and team have found zircon crystal in western Australia that are the in the oldest rocks yet found at 4.4 billion years old. His research indicates that at that time the earth was almost completely flat and covered with water. This matches with zircon over the subsequent 1.5 billion years showing that the evolution of earth from a water world to the one we see today was slow.
https://m.phys.org/news/2017-05-earth-barren-flat-billion-years.html

[chris]

at that time the earth was almost completely flat and covered with water

Just to be clear, lest any flat-Earth conspiracy people get excited by that statement, it's saying that the surface of the Earth was flat - the planet was still a ball...

Also to answer the enquiry about the planet being completely water-covered in the past, this has occurred, we think, within the last 60 million years.

The period is referred to as the palaeocene-eocene thermal maximum; global temperatures rose by up to 10 degrees C and there were no icecaps. Massive release of carbon into the atmosphere is believed to have caused it:

https://en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Thermal_Maximum

[chris]

The quoted temperature changes referred to above are based, in part, on sediment analysis. I discussed some of the publications on this, in Nature (Nature 441, 601–605 (1 June 2006) ; Nature 441, 606–609 (1 June 2006) ; Nature 441, 610–613 (1 June 2006) ; Nature 441, 579–581 (1 June 2006)), with the authors:

"Speaking to Chris Smith, here's Rhode Island University's Kate Moran.

Kate Moran: We've recovered for the very first time a very long climate record from the Arctic Ocean. Previous to this there is only about half a million year's worth of climate record, so we were able to basically extend the information about the climate at the top of the planet from, essentially, the present day to 57 million years ago.

Chris Smith: How did you do that because 57 million years is a long time?

Kate Moran: This is actually a technique we've used in other oceans. It turns out that sediments in the deep parts of the world's oceans get deposited at very slow rates, about a centimetre to two per thousand years, so that means a pile of sediment can represent a very long record, basically like a textbook going back in time, turning the pages back in time. And so what we did was we went back and we cored, we recovered a continuous core section of about 420 metres of sediment below the central Arctic Ocean in about 1,100 to 1,300 metres of water. It was a tough thing to do but we were able to do it.

Chris Smith: Once you got that back to the lab, what fruit has it borne, if you like?

Kate Moran: First, on board we actually had a few scientists on board and we were looking at little bits of each piece of core that came up. We carefully protected most of the core when we were off-shore and while we were out there we found that we had recovered a very important time interval called the Palaeo-Eocene thermal maximum, which was about 55 million years ago. And that was a time where the planet warmed significantly from greenhouse gases. And it seemingly warmed very rapidly, much like today. So this particular time period is important for us to study because then we can see how the Earth responded to these warmings. Now, previous to this, climate modellers had suggested that the difference between the climate at the Equator and the Poles would be quite large during warming periods like today. What we found is that the Arctic Ocean at that time was quite warm as well, surprisingly warm.

Chris Smith: So how warm was it? One of the researchers involved in trying to pinpoint the temperature at that time was Appy Sluijs of the University of Utrecht. He told me what he found.

Appy Sluijs: The key finding is that about 55 million years ago when we know there was an episode of very high greenhouse gas concentrations in the atmosphere, temperatures at the North Pole actually reached 24 degrees and there were also tropical algae swimming in the Arctic Ocean.

Chris Smith: 24 degrees, that actually sounds warm enough for a swim, which seems quite remarkable compared to the temperature, what is the temperature of the Arctic Ocean now?

Appy Sluijs: Well, the temperature of the Arctic Ocean at present is basically zero or below zero, we have an icecap, we have sea ice drifting on the Arctic Ocean at the moment, so lots of difference.

Chris Smith: OK, and I presume, does that mean that there was just no ice there at that time then?

Appy Sluijs: Exactly, yes, there was no ice and there was, as you said, it was actually nice for a swim, it was warmer than the present North Sea, for example.

Chris Smith: Indeed, so how did you make this finding?

Appy Sluijs: We know also for this time interval what kind of species were living in the Tropics and what kind of species were living on the higher latitudes and now it appears that this specific tropical dinoflagellate, this algae, migrated all the way up to the North Pole and to the Arctic Ocean during this time interval. So also on the basis of algae we could say there is actually tropical algae swimming in the Arctic Ocean 55 million years ago. And the other method we use is an organic palaeo thermometer, you could say, basically a way to measure temperature in the Arctic Ocean 55 million years ago, and it's based on fossil molecules made by Archaea and basically on the ratio of various of these molecules there is a way to actually quantify temperature so that way we can actually say it's 24 degrees.

Chris Smith: Appy Sluijs speaking there. So that was one of the major findings, that the Arctic Sea was once very warm. But the expedition also found that this warm period, sometimes called the greenhouse world, may have come to a rather abrupt end. And this might mean our understanding of how greenhouse gases like carbon dioxide affect the world isn't as good as we thought. Here's Kate Moran again.

Kate Moran: If we go younger in our sediment history, to approximately 45 million years ago, we found something that we visually saw in the core and it was a pebble, quite a large pebble, about a centimetre, and we're in the middle of the Arctic Ocean, in very deep water, and in order to move something like that from land to that part of the ocean you need to transport it with something that can float and move, and the only way that we think it could have got there was from ice. And so what we're suggesting, we think that the ocean basin started to freeze at that time, forming sea ice, perhaps icebergs, and that is much, much earlier than anyone ever thought that the northern hemisphere actually began to cool.

Chris Smith: And we know this is a greenhouse-mediated effect?

Kate Moran: If you look at the Earth in that time period, zero to 57 million years ago, we've got two big periods of time, one is the greenhouse world, one is the icehouse world. So we hadn't thought that we would have any evidence of ice during the so-called greenhouse world in the northern hemisphere. Now, at about 42 million years there are suggestions from other studies that that's when cooling began in antarctica. The fact that we have seen cooling now means that it could be that the planet is cooling at the same time at the North and the South Pole. And this again is important because climate changes, either from some kind of atmospheric change, CO2, or from some major change in ocean circulation due to tectonics, like closing of some kind of seaway, and it's those seaway closings that cause slow response, so one part of the planet could cool first and then ocean circulation changes and the other part of the planet could cool later. That's what we thought had happened, but this study suggests that that may not be the case. So I think of us will have to rethink how we moved from the greenhouse world to the icehouse world now."

[evan_au]

Was the Earth ever covered completely by water?

to answer the enquiry about the planet being completely water-covered in the past, this has occurred, we think, within the last 60 million years.

I think this may be answering a different question, such as "Was there a period where the Earth's icecaps were converted entirely into water?".

The Paleo-Eocene thermal maximum would have raised sea levels by perhaps 100m-200m, drowning many coastal areas. However, with a somewhat thick crust and active continental drift, there would be many mountain areas higher than this.

The ancestors of whales and dolphins appeared around this time, but if the Earth were completely covered by water in the past 60 million years, there would be far less variety of land-based plants, insects, mammals, reptiles and birds alive today. 

See: https://en.wikipedia.org/wiki/Past_sea_level#Changes_through_geologic_time

[chris]

You're right; I peaked too soon on that one!

[Bored chemist]

The earth is currently covered entirely by water.
Much of it is in the vapour phase.