nepheloid layer or nepheloid zone is a layer of water in the deep sea basin, above the seabed, that contains significant amounts of suspended sediment.[Glossary of Geology, 5th Edition (American Geological Institute)] It is from 200 to 1000 m thick. The name comes from Greek language: nephos, "cloud". The particles in the layer may come from the upper ocean layers and from stripping the sediments from the ocean floor by currents.
Formation mechanisms
A
surface nepheloid layer (
SNL) may be created, due to particle flotation, while
intermediate nepheloid layers (
INL) may be formed at the slopes of the ocean bed due to the dynamics of
. These intermediate nepheloid layers are derived from
bottom nepheloid layers (BNL) after the layers become detached and spread along
isopycnal surfaces.
Open ocean convection has a prominent effect on the distribution of nepheloid layers and their ability to form in certain areas of the ocean, such as the northern Atlantic Ocean and the northwestern Mediterranean Sea.
Impacts
Bathymetry
The existence of the nepheloid layer complicates
Bathymetry measurements: one has to take into account the reflections of
lidar or
Ultrasound pulses from the upper interface of this layer, as well as their absorption within the layer.
Interference from the thick layers of suspended sediments can ultimately produce inaccurate results concerning submarine
topography.
Marine life
Depending on the characteristics of a particular nepheloid layer, they can have a significant impact on marine life in the area.
The layers of sediments can block natural light, making it difficult for
Photosynthesis organisms to survive. In addition, the suspended particulates can harm
Filter feeder organisms and
plankton by blocking their gills or weighing them down.
Examples
Gulf of Mexico
A prominent nepheloid layer exists in the Gulf of Mexico extending from the delta of the
Brazos River to South Padre Island.
The layer of
turbid water can begin as shallow as 20 meters and is caused mostly by clay run-off from multiple rivers. The silty bottom of the gulf also contributes to the high
turbidity. Due to the blockage of light by this nepheloid layer,
algae and
coral are sparse, resulting in an animal-dominated community. This community is largely composed of
infauna and consists of a
detrital-based food chain.
Many species of
polychaete worms,
Amphipod, and
Brittle star inhabit the
benthic surface and can also be accompanied by some secondary consumers such as
flounders,
shrimp,
Crab, and
Starfish.
Porcupine Bank
A prominent nepheloid layer exists in the
Porcupine Bank.
[Dickson RR, McCave IN (1986) Nepheloid layers on the continental slope west of Porcupine Bank. Deep Sea Res 33:791–818] Geographically, the nepheloid layers are more detectable and prominent along the Porcupine Bank's western slope.
Both the bottom and intermediate nepheloid layers form due to a myriad of factors such as
Internal tide, waves, and subsequent bottom
erosion. The intermediate nepheloid layer can also manifest by breaking off from the bottom layer, and the water column above the area in which the bottom nepheloid layer forms is marked by significant differences in temperature, density, and
salinity.
