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The Persian Gulf, today, looks like this.

enter image description here

But about 4,000 years ago, it looked like this.

enter image description here

This extended to the point where Ur, currently an inland city, was a coastal city. During this era, no other coastlines that I am aware of have had such a dramatic change. The research I've done has not yielded many results. I do believe that a while ago, I read something saying a flood of the Tigris and Euphrates rivers deposited sediment lower down. I was unable to find this again.

So my question is, what were the cause(s) of the change of coastline?

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There has been lots of works trying to reconstruct the evolution of the Gulf shoreline through time. They do not agree on the exact same dates (see this review by Sissakian et al. 2020), but they generally agree that there was first a transgression (sea level rise) episode following the last ice age, then a regression (sea level drop). Reasons invoked to explain the regression are (not mutually exclusive):

  • Change in climate. The sea level peaked around the post-glacial climatic optimum. After that, the climate got cooler, which means more ice at the poles and less water in the seas.
  • Sediment filling of the Mesopotamian delta. The Euphrates and the Tigris carry a lot a sediments, which progressively accumulate in the lowlands, making the Gulf shoreline advance (seaward).
  • Neotectonics. There are evidence for anticline growth under the area, which could have contributed to its rise above sea level. This activity also changed river paths, which could have promoted point 2.
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    $\begingroup$ "shoreline retreat" I'm confused - did you mean "advance" ? $\endgroup$
    – Keith McClary
    Mar 31, 2021 at 15:59
  • $\begingroup$ @KeithMcClary retreats from where it was from the ocean or beach's (or any set point on land's) perspective, advance from where it was from the land's perfective as a whole? $\endgroup$
    – JeopardyTempest
    Mar 31, 2021 at 16:20
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    $\begingroup$ It seems to be defined terminology: Shoreline retreat: Multi-annual average landward displacement of the high-water line. $\endgroup$
    – Keith McClary
    Mar 31, 2021 at 19:07
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    $\begingroup$ Anyway, are you saying that sedimentation makes the shoreline move seaward? $\endgroup$
    – Keith McClary
    Mar 31, 2021 at 19:55
  • $\begingroup$ @KeithMcClary Yes, I've edited the answer accordingly, thanks! $\endgroup$
    – Jean-Marie Prival
    Apr 1, 2021 at 7:10

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