Oceans are natural barriers because

Subtle and short-lived differences in ocean salinity or temperature function as physical barriers for phytoplankton, and result in a patchy distribution of the oceans' most important food resource. The new research may help explain the large biodiversity in the sea. Hope it helps!

a mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. this uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.a geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.