Marine regression

A marine regression is a geological process where sea levels drop. This process exposes parts of the seafloor that were once submerged. These newly exposed areas become dry land instead of ocean. This event is the opposite of a marine transgression. A transgression occurs when the sea rises and floods previously dry land.

These shifts in water levels change the shape of our continents and oceans.

Scientists have different ideas about why these regressions happen. One hypothesis involves the movement of tectonic plates. If the rate of seafloor spreading slows down, the mid-ocean ridges change. These ridges might take up less space in the ocean basins. This reduction in space could lead to a generalized drop in sea level. Such changes in plate tectonic activity might also lead to major episodes of global volcanism. Examples of this include the Siberian Traps and the Deccan Traps.

Another common cause of marine regression is the presence of ice ages. This involves the balance between the cryosphere and the hydrosphere. The cryosphere includes all the frozen water on Earth. The hydrosphere includes all the liquid water in our oceans and lakes. During an ice age, more water is stored in massive ice sheets on land. Because this water is trapped as ice, there is less liquid water available for the oceans.

This shift causes the sea level to fall significantly.

We can see evidence of these changes throughout the fossil record. Fluctuations in sea levels are thought to be linked to several mass extinctions. A mass extinction is an event where many different species die out. One major example is the Permian-Triassic extinction event. This occurred 250 million years ago (Ma). It remains the largest extinction event in Earth's history. During this specific event, the global sea level fell by 250 meters (820 feet).

Another example of a regression linked to ice is found in the Pleistocene epoch. During the height of the last ice age, about 18,000 years ago, sea levels were much lower. The ocean was 120 to 130 meters (390 to 425 feet) lower than it is today. We also see a regression linked to a cold spell around 6 million years ago. This event was connected to an advance in glaciation. It also coincided with the start of the Messinian salinity crisis in the Mediterranean basin.

Some regressions do not seem to be caused by ice at all. For instance, a major regression happened at the end of the Cretaceous period. This regression accompanied a mass extinction event. Another theory involves the formation of the supercontinent Pangaea. As all the major landmasses joined into one large body, the ocean basins changed. This coalescence might have slightly enlarged the ocean basins. This enlargement could have helped facilitate a marine regression.

It is important to understand how these events affect life. A major regression can cause marine organisms in shallow seas to go extinct. This happens because their habitat disappears as the water retreats. However, mass extinctions usually involve both terrestrial and aquatic species. Because of this, scientists see regressions as correlates or symptoms of extinctions. They are often seen as part of a larger pattern rather than the single primary cause of all life loss.