I'm writing a fantasy world that takes Earth's Prehistorical landforms as basis for its geography, and adding a twist to it. I read about the Messinian Salinity Crisis and the Zanclean flood, and how low the oceans got during the Last Glacial Maximum, with sea levels lowering about 125m. This drop doesn't ever comes close to how deep the Gibraltar Strait is, which surpasses 900m.
However, what if The Gibraltar Strait wasn't that deep, and it was instead 100m deep instead on its shallowest parts on the Camarinal Sill, so that in every Last Glacial Maximum, the sill acted like a Dam? Let's also consider no matter how powerful they became, no Outburst floods would ever erode the Sill enough to stop it acting as a dam when the sea levels dropped again, at least during some tens of millions of years. What would be the effects of these periodical deluges, at least in geological timescales, to the European and the World's ecosystems as a whole?