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For a story I am writing, I need a planet covered in a shallow ocean. What I mean is that I want the majority of the planet's surface (=> 60%) to be in the photic zone of the ocean.

This planet must sustain life, with the most intelligent being some smalamanders. Ideally this planet can remain habitable until my smalamanders evolve into smalamen, but I am not opposed to benevolent aliens changing the conditions to be more suited to life. (And if necessary, the topography of the planet.)

What kind of environmental or planetary factors could achieve this?

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Were the Earth purely sperical with no mountains and valleys, a 2.7 km deep ocean would cover it all.

However, Earth is not a smooth sphere. While admittedly nowhere near 60%, I would say that a reasonably large part of Earth's ocean lies in the photic zone, except in those unusual times when Earth has ice sheets at the poles.

Normally (with a very imprecise meaning of "normally") Earth does not have permanent ice; periods with permanent ice at the poles are somewhat rare and somwhat short (on the geological scale, that is). When there is no ice at the poles the seas are much higher than in our present ice age and cover a lot of low-lying land.

North America and Central Europe were covered with shallow seas for quite a long time:

  • The Western Interior Seaway which, from the Cretacic to the Paleogene, used to separate North America into Laramidia to the west and Appalachia to the east. "The Western Interior Seaway was a shallow sea, filled with abundant marine life." (Wikipedia)

  • The Paratethys Sea "was a large shallow sea that stretched from the region north of the Alps over Central Europe to the Aral Sea in Central Asia" from the Jurassic to the Pliocene. "Today's Black Sea, Caspian Sea, Aral Sea, Lake Urmia and others are remnants of the Paratethys Sea." (Wikipedia)

At various times during the Mesozoic, shallow seas invaded continental interiors and then drained away. During Middle Triassic time, a marine incursion -- the Muschelkalk Sea -- covered the continental interior of Europe. Seas again transgressed upon the continents between the Early and Late Jurassic and in the early Cretaceous, leaving extensive beds of sandstone, ironstone, clays, and limestone. A last major transgression of marine waters flooded large sediments later in the Cretaceous. Those sharp rises in sea level and resultant worldwide flooding are thought to have had two causes. The first was warm global temperatures, which prevented large volumes of water from being sequestered on land in the form of ice sheets. The second was related to accelerated seafloor spreading; the attendant enlargement of ocean ridges displaced enormous amounts of ocean water onto the landmasses. Marine transgression was so extensive that in North America, for example, a seaway spread all the way from the Arctic to the Gulf of Mexico in the Cretaceous Period.

John P. Rafferty, The Mesozoic Era: Age of Dinosaurs, The Rosen Publishing Group, 2010 (link goes to Google Books)

Remeber that the Cretaceous is so named after "the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of western Europe" (Wikipedia).

Even during more recent times large parts of western Siberia were covered with shallow seas:

The seaway extended from North Pakistan and India to North Siberia through a system of inland seas and straits. [...] During the Bartonian and Priabonian the West Siberian inland sea was isolated completely from the Arctic Basin during the last phase of marine sedimentation. It was connected to the Turan sea through the Turgai strait.

Mikhail A. Akhmetiev, Nina I. Zaporozhets et al., "The paleogene history of the Western Siberian Seaway", Austrian Journal of Earth Sciences, vol. 105/1, 2012.

Conclusion: Just imagine that the sea is 60 meters above present levels: a significantly large part of low-lying continental land will be covered with water, including a large-ish inland sea in Australia and a greatly expanded Caspian linked to the ocean through the Manych strait and the Black Sea.

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The principle challenge here is that we get our oceans from our volcanic activity which is also responsible for the plate tectonics that give us our high mountains and low oceans. In theory, the earlier in an Earth like world's development you are, the flatter it will be and the shallower it's oceans. But, this makes being around long enough for intelligent life to evolve difficult, another option might be a smaller planet like mars. It will lose it's volcanic activity earlier in its life cycle making it remain flatter while also putting out an ocean much more slowly, but then there is the significant risk of what happens to the atmosphere when the core cools down, because then you might end up with... well, Mars. I'm not really sure what solution would work better, but in either case, you are looking at a race against time.

If your story allows for it, it might make the most since if your intelligent species actually came from another world in their forgotten past. That would make the evolutionary timeline less restrictive.

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If you literally mean that 60% of the surface is in the photic zone, then you already have an example of earth. The photic zone is the first 80m of water, so any water that has any depth will immediately be in the photic zone.

If you mean that 60% of the planetary surface is 0-80 meters below sea level (i.e., 60% of the surface is only in the photic zone), then you might have to watch out for the temperature and CO2 Levels. The deep ocean is currently the biggest repository of CO2 in the world, stored at the very bottom where it's trapped by pressure and being really cold. You could make the rest of your ocean deeper to compensate, or you could have more efficient photosynthesis in your plants, or any number of factors. The oceans also act as a heat sink, but any body of water large enough, regardless of depth, should do the trick.

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  • $\begingroup$ If you've got shallow oceans, you've got the potential for an enormous carbon sink in the form of shells turning into limestone. $\endgroup$ – Mark May 25 at 0:53

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