Walvis Ridge
TheWalvis Ridge(walvismeanswhaleinDutchandAfrikaans) is anaseismicocean ridgein the southernAtlantic Ocean.More than 3,000 km (1,900 mi) in length, it extends from theMid-Atlantic Ridge,nearTristan da Cunhaand theGough Islands,to the African coast (at 18°S in northernNamibia).[1]The Walvis Ridge is one of few examples of a hotspot seamount chain that links aflood basalt provinceto an active hotspot. It is also considered one of the most important hotspot tracks because theTristan Hotspotis one of few primary or deep mantle hotspots.[2]
Geology
[edit]Apart from the Mid-Atlantic Ridge, the Walvis Ridge and theRio Grande Riseare the most distinctive feature of the South Atlantic sea floor. They originated fromhotspotvolcanismand together they form a mirrored symmetry across the Mid-Atlantic Ridge, with theTristan Hotspotat its centre. Two of the distinct sections in the Walvis Ridge have similar mirrored regions in the Rio Grande Rise; for example, the eastern section of the Walvis Ridge evolved in conjunction with the Torres Arch (the western end of the Rio Grande Rise, off the Brazilian coast) and, as the South Atlantic gradually opened, these structures became separated. The complex of seamounts in the western end of the Walvis Ridge, however, does not have a similar structure on the American side, but there is a Zapiola Seamount Complex south of the eastern end of the Rio Grande Rise.[3]The formation of this mirrored structure is the result of the opening of the South Atlantic some 120 Mya and theParaná and Etendekacontinental flood basalts,the lateral-most parts of the structure, formed at the beginning of this process in areas that are now located inBrazilandNamibia.[2]
The Walvis Ridge is divided into three main sections:[1]
- A first 600 km (370 mi) long segment stretching from Africa to approximately longitude 6°E and varying in width between 90–200 km (56–124 mi).
- A second section, 500 km (310 mi) long, stretching north–south, and narrower than the first section.
- A third more discontinuous section, which is marked byseamountsand connects the Walvis Ridge to the Mid-Atlantic Ridge.
Cretaceouskimberlitesin the central Democratic Republic of Congo and Angola align with the Walvis Ridge.[4]
The Tristan-Gough hotspot track first formed over the mantle plume that formed the Etendeka-Paraná continental flood basalts some135 to 132Ma.[5]The eastern section of the ridge is thought to have been created in the MiddleCretaceousperiod, between120 to 80Ma.[6][7]While the mantle plume remained large and stable, the eastern Walvis Ridge formed along with the Rio Grande Rise over the Mid-Atlantic Ridge.[5]During theMaastrichtian60million years ago,the orientation of spreading changed, which is still visible in the orientation of the various sections of the Walvis Ridge.[2]The mantle plume then gradually became unstable and bifurcated60 to 70Mato produce the two separate Tristan and Gough hotspot tracks. It finally disintegrated35 to 45Maand formed the guyot province in the western end of the ridge.[5]
Hundreds of volcanic explosions were recorded on the Walvis Ridge in 2001 and 2002. These explosions seemed to come from an unnamed seamount on the northern side of the ridge and are thought to be unrelated to the Tristan hotspot.[8]
TheEwing Seamountis part of the ridge.
Palaeoclimatic role
[edit]TheEocene Layer of Mysterious Origin(Elmo) is a period ofglobal warmingthat occurred53.7Ma,about two million years after thePaleocene–Eocene Thermal Maximum.This period manifests as a carbonate-poorred claylayer unique to the Walvis Ridge and is similar to the PETM, but of smaller magnitude.[9][10]
Oceanography
[edit]The Walvis Ridge is a natural obstacle for theAgulhas rings,mesoscalewarm core ringsthat are shed from theAgulhas Currentsouth of theAgulhas Bank.In average, five such rings are shed each year, a number that varies considerably between years.[11]The rings tend cross the Walvis Ridge at its deepest part, but they still lose transitional speed and many rings decay rapidly.[12] Their transitional speed drops from 5.2±3.6 km/day to 4.6±3.1 km/day, but it is not clear how much the Walvis Ridge is responsible for this drop, since the rings' speed drops to 4.3±2.2 km/day between the Walvis Ridge and the Mid-Atlantic Ridge.[13] The rings can cross the South Atlantic in 2.5–3 years but only two thirds make it farther than the Walvis Ridge.[11] When the rings pass over the Cap Basin south of the Walvis Ridge they are frequently disturbed by theBenguela Current,interaction between rings, and bottom topography such as theVema Seamount,but there are fewer obstacles and disturbances west of the Walvis Ridge were the rings tend stabilise. [14] The Agulhas rings transport an estimated 1-5Sv(millions m3/s) of water from the Indian Ocean to the South Atlantic.[15]
Originating around Antarctica,Antarctic Bottom Water(AABW) enters theCape Basinbetween the Agulhas Bank and theAgulhas Ridgeafter which it flows west north of the Agulhas Ridge. AABW then retroflects at the south-western end of the Walvis Ridge, flows north-east along the ridge before being retroflected south byNorth Atlantic Deep Water,with which it exits the Cape Basin and flows into the Indian Ocean.[16]
References
[edit]Notes
[edit]- ^abGoslin et al. 1974,Introduction, p. 469
- ^abcSager 2014,pp. 2–5
- ^O'Connor & Duncan 1990,Introduction, p. 17475
- ^de Wit 2007,Fig. 7, p. 380; Fig. 9, p. 385
- ^abcRohde et al. 2013,Conclusions, pp. 69-70
- ^Pastouret & Goslin 1974
- ^Müller, Royer & Lawver 1993
- ^Haxel & Dziak 2005,Abstract
- ^Lourens et al. 2005,Abstract
- ^"Eocene Layer of Mysterious Origin".JOIDES Resolution.RetrievedMay 17,2015.
- ^abSchouten et al. 2000,Discussion and Conclusions, p. 21933
- ^Schouten et al. 2000,Abstract, Introduction, pp. 21913-21914
- ^Schouten et al. 2000,Rings paths, pp. 21916-21918
- ^Schouten et al. 2000,Ring Decay, pp. 21918-21919
- ^Ruijter et al. 2003,p. 46
- ^Gruetzner & Uenzelmann-Neben 2014,Fig 1.A
Sources
[edit]- de Wit, M. (2007)."The Kalahari Epeirogeny and climate change: differentiating cause and effect from core to space"(PDF).South African Journal of Geology.110(2–3): 367–392.doi:10.2113/gssajg.110.2-3.367.RetrievedJune 20,2015.
- Goslin, J.; Mascle, J.; Sibuet, J.; Hoskins, H. (1974)."Geophysical Study of the Easternmost Walvis Ridge, South Atlantic: Morphology and Shallow Structure".Geological Society of America Bulletin.85(4): 619–632.Bibcode:1974GSAB...85..619G.doi:10.1130/0016-7606(1974)85<619:gsotew>2.0.co;2.RetrievedMay 17,2015.
- Gruetzner, J.; Uenzelmann-Neben, G. (2014)."Contourites at the eastern Agulhas Ridge and Cape Rise seamount shaped by Southern Ocean derived water masses"(PDF).2nd Deep-Water Circulation Congress, 10-12 Sept. 2014, Ghent, Belgium.RetrievedJuly 5,2015.
- Haxel, J. H.; Dziak, R. P. (2005)."Evidence of explosive seafloor volcanic activity from the Walvis Ridge, South Atlantic Ocean"(PDF).Geophysical Research Letters.21(13): L13609.Bibcode:2005GeoRL..3213609H.doi:10.1029/2005GL023205.RetrievedMay 17,2015.
- Lourens, L. J.; Sluijs, A.; Kroon, D.; Zachos, J. C.; Thomas, E.; Röhl, U.; Bowles, J.; Raffi, I. (2005). "Astronomical pacing of late Palaeocene to early Eocene global warming events".Nature.435(7045): 1083–1087.Bibcode:2005Natur.435.1083L.doi:10.1038/nature03814.hdl:1874/11299.PMID15944716.S2CID2139892.
- Müller, D.; Royer, J.-Y.; Lawver, A. (1993)."Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks"(PDF).Geology.21(3): 275–278.Bibcode:1993Geo....21..275D.doi:10.1130/0091-7613(1993)021<0275:rpmrtt>2.3.co;2.
- O'Connor, J. M.; Duncan, R. A. (1990)."Evolution of the Walvis Ridge‐Rio Grande Rise Hot Spot System: Implications for African and South American Plate motions over plumes"(PDF).Journal of Geophysical Research: Solid Earth.95(B11): 17475–17502.Bibcode:1990JGR....9517475O.doi:10.1029/jb095ib11p17475.RetrievedMay 17,2015.
- Pastouret, L.; Goslin, J. (1974)."Middle Cretaceous sediments from the eastern part of Walvis Ridge".Nature.248(5448): 495–496.Bibcode:1974Natur.248..495P.doi:10.1038/248495a0.S2CID4176945.
- Rohde, J. K.; van den Bogaard, P.; Hoernle, K.; Hauff, F.; Werner, R. (2013)."Evidence for an age progression along the Tristan-Gough volcanic track from new40Ar/39Ar ages on phenocryst phases ".Tectonophysics.604:60–71.Bibcode:2013Tectp.604...60R.doi:10.1016/j.tecto.2012.08.026.RetrievedJune 20,2015.
- Ruijter, W. P. M., de; Cunningham, S. A.; Gordon, A. L.; Lutjeharms, J. R. E.; Matano, R. P.; Piola, A. R. (2003)."On the South Atlantic Climate Observing System (SACOS)"(PDF).Report of the CLIVAR/OOPC/IAI Workshop.NOAA.RetrievedJanuary 20,2015.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - Sager, W. W.(2014)."Scientific Drilling in the South Atlantic: Rio Grande Rise, Walvis Ridge and surrounding areas"(PDF).U.S. Science Support Program Workshop Report.RetrievedMay 17,2015.
- Schouten, M. W.; Ruijter, W. P. M., de; Leeuwen, P. J., van;Lutjeharms, J. R. E.(2000)."Translation, decay and splitting of Agulhas rings in the southeastern Atlantic Ocean".Journal of Geophysical Research: Oceans.105(C9): 913–921.Bibcode:2000JGR...10521913S.doi:10.1029/1999jc000046.
{{cite journal}}:CS1 maint: multiple names: authors list (link)