The Ocean

The global ocean covers 71% of the Earth’s surface, and only 5% of it has been explored. Oceans are on average 3000m deep, but at subduction zones can reach upwards of 10,000 meters below the sea surface. For example, the Philippine and Tonga trenches. The Marianna trench has the greatest depth of all at around 11,000m. Shockingly, the deepest manned dive ever recorded in 2019 found plastics at the bottom of this trench, as well as persistent organic pollutants (POPs) such as PCBs and PBDEs. These were used as fire retardants and electrical insulators from 1930s to 1970s until they were banned. They're highly toxic. 

Trenches form mainly at destructive tectonic boundaries where convection currents cause the heavier oceanic plate to subduct below the lighter continental plate causing a dip in land. This is an oceanic trench. 

Friction between the sliding plates builds up creating lots of pressure. This is released in seismic waves at the focus and can sometimes lead to earthquakes and tsunamis. The subducted plate is heated by the mantle where it is recycled. The convection currents force up new seafloor at constructive plate boundaries forming a mid ocean ridge. This process is incredibly slow, forming and destroying around 2cm of the margin per year. 

Epipelagic Zone

The first 200m below the surface of the ocean is known as the epipelagic zone, sunlight or euphotic zone. Sunlight is able to penetrate this layer which means photosynthesis can occur here. 

Cyanobacteria were the first organisms to photosynthesise and are responsible for the increase in oxygen levels in the modern atmosphere. This is known as the Great Oxidation Event. 

It is estimated that, of all marine life, 90% live in the epipelagic zone.

Mesopelagic Zone

The depth 200m to 1000m below the surface is labelled the mesopelagic zone, AKA the twighlight zone or dysphotic zone.  Some light can still penetrate this layer, but not enough for photosynthesis to be possible. This means no algae is found here. 

The temperature ranges from 20 to 4 degrees Celsius from the epi-mesopelagic boundary to the meso-bathypelagic boundary. This rapid change is known as a thermocline. 

The sperm whale can dive no deeper than the meso-bathypelagic boundary. At this point the pressure is equivalent to ten elephants standing on your head. 

Bathypelagic Zone

1000m to 4000m below the surface is the bathypelagic Zone, AKA the midnight or aphotic zone. No light reaches this layer. In some parts of the world, the ocean floor can be reached at this depth, and the Titanic rests here at approximately 3800m below the surface.

Most organisms living 1000m or below will feed on detritus since food is very scarce.

Abyssopelagic Zone

Stretching 4000m to 6000m down, the temperature steadies at 2 degrees Celsius. The abyssopelagic zone, also called the abyssal or aphotic zone, covers 80% of the benthic environment. 

Oxygen concentrations can reach low levels and without any light reaching this layer, producers use a process called chemosynthesis to make their food. This uses energy stored in the bonds of hydrogen sulphide and methane to produce glucose, water and carbon dioxide. Sulfur containing compounds are formed as by-products. 

Hadalpelagic Zone

This layer, also called the hadal zone, is named after the Greek God, Hades of the underworld. This is the zone of oceanic trenches and the deepest part of the ocean, Challenger Deep, which is located in the Marianna trench. 

REFERENCES 

'The Brilliant Abyss' by Helen Scales

2021

https://www.worldatlas.com/oceans/ocean-zones.html

World Atlas - Ocean Zones

Diptarka Ghosh

2023

Date Published: 22nd December 2023