Nepheloid layer
Anepheloid layerornepheloid zoneis a layer of water in thedeep oceanbasin, above theocean floor,that contains significant amounts of suspendedsediment.[1]It is from 200 to 1000 m thick. The name comes fromGreek: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.[2]Its thickness depends on bottom current velocity and is a result of balance betweengravitational settlingof particles and turbulence of the current. The formation mechanisms of nepheloid layers may vary, but primarily depend on deep ocean convection. Nepheloid layers can impact the accuracy of instruments when measuringbathymetryas well as affect the types of marine life in an area. There are several significant examples of nepheloid layers across the globe, including within theGulf of Mexicoand thePorcupine Bank.
Formation mechanisms
[edit]Asurface nepheloid layer(SNL) may be created, due to particle flotation, whileintermediate nepheloid layers(INL) may be formed at the slopes of the ocean bed due to the dynamics ofinternal waves.These intermediate nepheloid layers are derived frombottom nepheloid layers (BNL)after the layers become detached and spread alongisopycnalsurfaces.[3]
Open ocean convectionhas a prominent effect on the distribution of nepheloid layers and their ability to form in certain areas of the ocean, such as the northernAtlantic Oceanand the northwesternMediterranean Sea.[4]Nepheloid layers are more likely to form based on patterns of deep ocean circulation that directly affect theabyssal plain.[5]This is largely through the disruption of accumulated sediments in areas that deep ocean currents interact with. Convection currents that disturb areas of the ocean floor such as those that circulate viaocean gyresalso affect the concentration and relative sizes of the suspended sediments, and by extension the area's correspondingbioticactivity.
Impacts
[edit]Bathymetry
[edit]The existence of the nepheloid layer complicatesbathymetricmeasurements: one has to take into account the reflections oflidarorultrasonicpulses from the upper interface of this layer, as well as their absorption within the layer.[3]Interference from the thick layers of suspended sediments can ultimately produce inaccurate results concerning submarinetopography.
Marine life
[edit]Depending on the characteristics of a particular nepheloid layer, they can have a significant impact on marine life in the area.[6]The layers of sediments can block natural light, making it difficult forphotosyntheticorganisms to survive. In addition, the suspended particulates can harmfilter feedingorganisms andplanktonby blocking their gills or weighing them down.
Examples
[edit]Gulf of Mexico
[edit]A prominent nepheloid layer exists in theGulf of Mexicoextending from the delta of theBrazos RivertoSouth Padre Island.[2]The layer ofturbidwater 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 highturbidity.Due to the blockage of light by this nepheloid layer,algaeandcoralare sparse, resulting in an animal-dominated community. This community is largely composed ofinfaunaand consists of adetrital-based food chain.[7]Many species ofpolychaeteworms,amphipods,andbrittle starsinhabit thebenthic surfaceand can also be accompanied by some secondary consumers such asflounders,shrimp,crabs,andstarfishes.
Porcupine Bank
[edit]A prominent nepheloid layer exists in thePorcupine Bank.[8]Geographically, the nepheloid layers are more detectable and prominent along the Porcupine Bank's western slope.[6]Both the bottom and intermediate nepheloid layers form due to a myriad of factors such asinternal tides,waves, and subsequent bottomerosion.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, andsalinity.
References
[edit]- ^Glossary of Geology, 5th Edition (American Geological Institute)
- ^abHunkins, Kenneth; Thorndike, Edward; Mathieu, Guy (1969). "Nepheloid layers and bottom currents in the Arctic Ocean".Journal of Geophysical Research.74(28): 6995–7008.Bibcode:1969JGR....74.6995H.doi:10.1029/JC074i028p06995.
- ^abPak, Hasong; Zaneveld, J. Ronald V.; Kitchen, J. (1980). "Intermediate nepheloid layers observed off Oregon and Washington".Journal of Geophysical Research.85(C11): 6697.Bibcode:1980JGR....85.6697P.doi:10.1029/jc085ic11p06697.ISSN2156-2202.
- ^Madron, X. Durrieu de; Ramondenc, S.; Berline, L.; Houpert, L.; Bosse, A.; Martini, S.; Guidi, L.; Conan, P.; Curtil, C.; Delsaut, N.; Kunesch, S. (2017)."Deep sediment resuspension and thick nepheloid layer generation by open-ocean convection"(PDF).Journal of Geophysical Research: Oceans.122(3): 2291–2318.Bibcode:2017JGRC..122.2291D.doi:10.1002/2016JC012062.ISSN2169-9291.
- ^Biscaye, Pierre E.; Eittreim, Stephen L. (1977). "Suspended particulate loads and transports in the nepheloid layer of the abyssal Atlantic Ocean".Marine Geology.23(1): 155–172.Bibcode:1977MGeol..23..155B.doi:10.1016/0025-3227(77)90087-1.ISSN0025-3227.
- ^abMcCave, I. N. (1986). "Local and global aspects of the bottom nepheloid layers in the world ocean".Netherlands Journal of Sea Research.20(2): 167–181.Bibcode:1986NJSR...20..167M.doi:10.1016/0077-7579(86)90040-2.ISSN0077-7579.
- ^Diercks, Arne-R.; Dike, Clayton; Asper, Vernon L.; DiMarco, Steven F.; Chanton, Jeffrey P.; Passow, Uta (2018)."Scales of seafloor sediment resuspension in the northern Gulf of Mexico".Elem Sci Anth.6(1): 32.doi:10.1525/elementa.285.ISSN2325-1026.
- ^Dickson RR, McCave IN (1986)Nepheloid layers on the continental slope west of Porcupine Bank.Deep Sea Res 33:791–818