CHARLESTON — This city was born from the ocean and to the ocean it will one day return. About 70 million years ago, South Carolina was the easternmost part of a relatively shallow, nutrient-dense sea that ran through the southeast, passed through the western states, and sliced up through Canada to the North Pole. Over the next six months, the South Carolina Aquarium’s new “Jurassic Seas” exhibition aims to take visitors back to this sunken landscape.