Ghost forest

Ghost forests occur in areas where sea level has changed. When there is a change in sea level, coastal regions may become inundated with sea water, altering the coastline and sometimes killing large areas of trees, leaving behind what is called a “ghost forest.” Ghost forests may develop in a variety of environments. And while ghost forests can occur in many different places, the physical coastal features are what dictate why they occur. For example, in the southern US coastal marshes are expanding into dry wooded areas, killing trees and leaving behind areas of dead trees called snags. This region of the US is at or below sea level, making the area more susceptible to tides. The coastal features that dictated the creation of these ghost forests are directly affected by changing sea levels and therefore indirectly affected by climate change. With global sea level rise, the coastlines in the southern US are being altered and leaving behind salt marshes filled with dead and dying trees.

In other parts of the world, ghost forests can be the result of tectonic activity and not necessarily global sea level rise. In the Pacific northwest, there is a large, active subduction zone called the Cascadia Subduction Zone. Here, we have a convergent plate boundary where the Gorda plate, the Juan de Fuca plate, and the Explorer plate are being subducted underneath the North American plate. As these plates attempt to slide past one another, they often become stuck. For several hundred years the plates will be locked in place and the tension builds. As a result of this tension we get what is called orogenic uplift. This is where the tension building between two converging plates gets translated into vertical uplift of the mountains on the coast. Orogenic uplift is usually associated with earthquakes and mountain building. But then, every 500+ years, there is large earthquake in the Cascadia Subduction Zone and all that built up tension is released. The release of this tension results in what is called subsidence. And with subsidence the once elevated coastline will drop down several meters to below sea level. Here, sea level has not changed, but the coastline has been deformed, making it susceptible to tides. Areas of the coastline are now inundated with sea water, creating marshes and leaving behind ghost forests.

In addition to subsidence, these large earthquakes will also cause tsunamis. It is possible to determine that ghost forests in the Pacific northwest were created by earthquakes and subsidence by looking at the stratigraphic record. Digging down into the earth, adjacent to a ghost forest, we will see different layers of sediment. These different layers represent different environments. When observing the stratigraphy in a ghost forest, one will see a layer of material that is filled with organic material. This layer represents the old forest floor, prior to subsidence. Then, on top of the layer that represents the old forest floor, there will often be a large sandy deposit. This layer represents the tsunami event, where the coast was flooded with sea water that is filled with sandy sediment. Superimposed on top of the tsunami deposit will be a muddy deposit, representative of an area subjected to ocean tides.

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Wikipedia