On these pages please find background informations about the following topics:
- Plate tectonics and mantle dynamics
- Methods and specific goals of the MANIHIKI research project related to geochemical studies and age dating on magmatic rocks
- Evolution and Biogeography of benthic deep-sea communities of the Manihiki Plateau
- Further Reading (Geology)
- Further Reading (Biology)
Home of SO193 MANIHIKI
The Earth consists of three major compositional shells: 1) the metallic core, 2) the peridotitic mantle extending to depths of 2,900 km, and 3) oceanic (basaltic) and continental (granitic) crust forming the outermost 5-70 km. Rigid lithospheric plates (5-300 km thick) on the Earth’s surface, consisting of the crust and varying thicknesses of the uppermost mantle, move at rates of centimeters per year relative to the mantle. Three types of boundaries characterize plate margins. Divergent plate margins where plates are being pulled apart and new oceanic crust is being formed. As the plates are pulled apart, deeper, hotter mantle rises into the void and melts as a result of pressure release at shallow depth. The melts either erupt on the ocean floor at mid ocean ridges or intrude into the ocean crust, thus forming new crust. However, this process may also start beneath a continent and may cause the break-up of continents. For example, the East African graben is considered as such a continental rift zone. At convergent plate margins, an older oceanic plate is subducted (pulled) beneath a more buoyant continental or younger oceanic plate into the Earth’s mantle, as it is occuring for example at the western edge of South America. Large earthquakes and explosive volcanic activity are associated with subduction zones. Transform plate margins form the third type of plate boundary, where two plates move laterally relative to each other. Large earthquakes also occur along transform faults such as the San Andreas fault in California, U.S.A.
In addition to divergent and convergent plate boundaries, volcanism on the Earth also occurs within the lithospheric plates at hotspots. The Hawaiian, Galápagos and Canary Islands and Iceland were formed above hotspots or mantle plumes, cylindrical or pipe-like regions of rising mantle rock several hundred kilometers in diameter. These regions of upwelling mantle, which rise at rates of tens of centimeters per year, extend from a thermal boundary layer within the Earth, such as the core/mantle boundary, to the base of a lithospheric plate. As a result of dynamic uplift of the rising plume material and thinning and heating of the lithospheric plate, a swell or upward bulge is formed in the lithosphere above the hotspot. Melts from the plume ascend to the surface forming submarine and island volcanoes. In contrast to the drifting plates, a mantle plume or hotspot is relatively stationary and will remain at the same longitude and latitude through time. As plate motion moves the volcanic islands and seamount volcanoes away from the hotspot, the volcanoes are removed from their source of magma and will become extinct. As the plate cools, thickens and subsides, the extinct volcano will also subside and be eroded. Eventually the former island will dip beneath sea level and become a seamount. New volcanoes and ocean islands, however, are continuously being built above the stationary hotspot. The hotspot track is a chain of extinct volcanoes stretching away from the hotspot in the direction of plate motion (e.g., Hawaii and the Emperor Seamount Chain). Magma production rates above a mantle plume are escpecially high during its initial stage. In this stage many million cubic kilometers of volcanic rocks may form within a few million years. This leads to the formation of huge lava plateaus or Large Igneous Provinces as, for example, the Dekkan Trapp in India, the Columbia River Basalts, the Caribbean Plate or the Ontong Java, Manihiki and Hikurangi Plateaus.
In addition to divergent and convergent plate boundaries, volcanism on the Earth also occurs within the lithospheric plates at hotspots. The Hawaiian, Galápagos and Canary Islands and Iceland were formed above hotspots or mantle plumes, cylindrical or pipe-like regions of rising mantle rock several hundred kilometers in diameter. These regions of upwelling mantle, which rise at rates of tens of centimeters per year, extend from a thermal boundary layer within the Earth, such as the core/mantle boundary, to the base of a lithospheric plate. As a result of dynamic uplift of the rising plume material and thinning and heating of the lithospheric plate, a swell or upward bulge is formed in the lithosphere above the hotspot. Melts from the plume ascend to the surface forming submarine and island volcanoes. In contrast to the drifting plates, a mantle plume or hotspot is relatively stationary and will remain at the same longitude and latitude through time. As plate motion moves the volcanic islands and seamount volcanoes away from the hotspot, the volcanoes are removed from their source of magma and will become extinct. As the plate cools, thickens and subsides, the extinct volcano will also subside and be eroded. Eventually the former island will dip beneath sea level and become a seamount. New volcanoes and ocean islands, however, are continuously being built above the stationary hotspot. The hotspot track is a chain of extinct volcanoes stretching away from the hotspot in the direction of plate motion (e.g., Hawaii and the Emperor Seamount Chain). Magma production rates above a mantle plume are escpecially high during its initial stage. In this stage many million cubic kilometers of volcanic rocks may form within a few million years. This leads to the formation of huge lava plateaus or Large Igneous Provinces as, for example, the Dekkan Trapp in India, the Columbia River Basalts, the Caribbean Plate or the Ontong Java, Manihiki and Hikurangi Plateaus.
However, in response to increasing problems in explaining intraplate volcanism in many areas (in particular in the southern Pacific region), a global debate has developed on the origin of intraplate volcanism, in particular the role of mantle plumes or hotspots in creating intraplate volcanism (e.g., „Great Plume Debate“). Furthermore recent studies suggest that some oceanic plateaus being considered as LIPs have not formed within a few million years but over much longer time scales (e.g., over some 10 mill. years). Therefore there are many open questions concerning intraplate volcanism and LIP formation which should also be addressed by the research project SO193 MANIHIKI.
Magmatic rocks sampled by the RV Sonne from the ocean floor (see upper photographs) will be analyzed with different methods in several geochemical laboratories. Major element geochemistry will constrain magma chamber processes within the crust, and also yield information on the average depth of melting, temperature and source composition to a first approximation. Further analytical effort will concentrate on methods that constrain deep seated mantle processes. For example, trace element data help to define the degree of mantle melting and help to characterize the chemical composition of the source. Radiogenic isotopic ratios such as 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and 176Hf/177Hf are independent of the melting process and reflect the long term evolution of a source region and thus serve as tracers to identify mantle and recycled crustal sources. 3He/4He-isotopic ratios trace the depth from which the mantle material originates. Elevated 3He-sigantures are commonly thought to reflect an origin from the less degassed lower mantle.
The ages of whole rocks, volcanic glasses and minerals will be determined by 40Ar/39Ar laser dating. The dating will constrain in which time intervals volcanic activity occurred within the various geomorphological units of the Manihiki Plateaua and if the plateau and associated seamounts have been formed by a single event or in several phases. Furthermore age dating of the edges of the plateau and scarps, of seamounts on the plateau edges and of seamounts on the adjacent oceanic crust will narrow the time frame of possible rift events, which may have formed the scarps at the south-western and eastern edges of the plateau as well as the Danger Islands Troughs. The combination of these data with those from the Hikurangi Plateau will help to test the hypothesis that the Hikurangi and Manihiki Plateaus may have once formed a combined Hikurangi/Manihiki plateau. Additionally combined age, volcanological, and morphological data will be used to reconstruct the subsidence history of the Hikurangi Plateau.
Through integration of the age data, the various geochemical parameters, volcanological, and morphological data, the origin and the temporal, spatial, and geochemical evolution of the Manihiki Plateau can be reconstructed.
Upper left photograph: Electron microprobe for analyses of the composition of minerals and volcanic glasses. Upper right photograph: Clean lab for preparation of rock samples for isotope analyses. Lower left photograph: Mass spectrometer for analyses of radiogenic isotope ratios. Lower right photograph: Ar/Ar-Laser-dating lab.
Only few biodiversity data exist for the Manihiki Plateau, mainly based on the results of the New Zealand TUI cruises in 1986. According to these results, the plateau's margins turn into deep slopes reaching 3000 m depth and consisting of debris of reef-building corals and other invertebrates. Sea level low stands during the ice ages may have allowed corals to grow on top of the seamounts. Global warming 10.000 years ago caused the sea level to rise again, thereby killing the light-dependent reefs, which subsequently eroded down the slopes. Such biogenetically formed sediments represent an ideal habitat for benthic macrofaunal communities, as has been shown by the coral mounds investigated during SO168. The plateau itself is covered with carbonate sediment layers mainly consisting of foraminiferan shells or clay-like muds with high rates of bioturbation. A deep trench divides the Manihiki Plateau in an Eastern and a Western part. This trench may serve as a dispersal barrier, especially for meiofaunal organisms with a low dispersal ability. However, some meiofaunal species are able to drift with the water currents so that they can possibly be found on either side of the trench. The overall morphology of the plateau promises a highly diverse benthic fauna, which may serve as a centre of origin for other Pacific regions.
Previous results from SO144-3, SO158 and SO168 show that dispersal of developmental stages of sessile invertebrates in the deep sea is supported by currents, which run parallel to the mid ocean ridges. These ridges form a network of hard substrate in all oceans and therefore may serve as underwater highways for pelagic invertebrate larvae. But also vertical exchange between shallow and deep water may be possible: During SO168 we discovered a new brachiopod species, Kakanuiella chathamensis, in more than 1.000 m depth (upper figure). Its closest relative, the Oligocene fossil Kakanuiella hedleyi, lived in subtropical shallow waters.
The dispersal of meiofaunal organisms like kinorhynchs is supposed to be hampered by the mid ocean ridges, because they are holobenthic animals without any pelagic stage during their life cycle. Amongst kinorhynchs certain species are definitely deep-sea endemics (lower figure), but in other meiofaunal groups like loriciferans the same species can occur in shallow waters as well as in the deep. Massive abundances of meiofaunal organisms in sediments of transform faults show that these faults may act as dispersal corridors for microscopic infaunal communities.
Collecting will be focused on certain macro- and meiofaunal key groups, which promise high abundances and, therefore, good comparability with samples from previous expeditions (SO 144-3, SO158, SO168). Macrofaunal key groups are Brachiopoda, Bryozoa and Porifera, whereas collection of meiofauna will be focused on Kinorhyncha and Loricifera. If possible, all animals found will be determined to species level; those species, which are new to science, will be decribed according to current taxonomic rules. Specimens belonging to other groups will be sorted and sent to international specialists for identification. All biological specimens will be conserved either for morphological (histology, SEM, TEM) or genetical studies to offer as many ways as possible for subsequent examination. Morphological and molecular characteristics will be used for phylogenetic analyses, which are a prerequisite for further hypotheses concerning the biogeography of the benthic communities of the Manihiki Plateau.
CO-OPERATING PARTNERS
Prof. Dr. Francisco Javier Cristobo - Spanish Institute of Oceanography, Gijón - fjcristobo(at)yahoo.es
PD Dr. Ingrid Kröncke - Forschungsinstitut Senckenberg, Wilhelmshaven - ingrid.kroencke(at)senckenberg.de
Prof. Dr. Reinhardt M. Kristensen, Dipl.-Biol. Iben Heiner - Zoologisk Museum, University of Copenhagen - rmkristensen(at)snm.ku.dk, Iheiner(at)zmuc.ku.dk
(A) Manihiki, Hikurangi and Ontong Java Plateaus, Large Igneous Provinces, Mantle plumes and „Great Plume Debate“
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Beiersdorf H, Bickert T, Cepek P, Fenner J, Petersen N, Schönfeld J, Weiss W and Won MZ (1995b) High-resolution stratigraphy and the response of biota to Late Cenozoic environmental changes in the central equatorial Pacific Ocean (Manihiki Plateau). Marine Geol 125: 29-59.
Beiersdorf H, Erzinger J (1989) Observations on the bathymetry and geology of the northeastern Manihiki Plateau, Southwestern Pacific Ocean. CCOP/SOPAC South Pacific Mar. Geol. Notes 3 (4): 33-46.
Clague DA (1976) Petrology of basaltic and gabbroic rocks dredged from the Danger Island Troughs, Manihiki Plateau. Init. Rep. DSDP 33: 891-911.
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Geldmacher J, Hoernle K, Bogaard Pvd, Duggen S, Werner R (2005) New 40Ar/39Ar age and geochemical data from seamounts in the Canary and Madeira Volcanic Provinces: A contribution to the “Great Plume Debate”. Earth and Planetary Science Letters 237: 85-101.
Geldmacher J, Hoernle K, Klügel A, Bogaard Pvd, Duggen S (2006) A geochemical transect across a heterogeneous mantle upwelling: implications for the evolution of the Madeira hotspot in space and time. Lithos 90: 131-144.
Gladczenko TP, Coffin MF, Eldholm O (1997) Crustal structure of the Ontong Java Plateau: modeling of new gravity and existing seismic data. J Geophys Res 102: 22710-22729.
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(B) Biology
Heiner, I. & B. Neuhaus (2007): Loricifera from the deep sea at the Galápagos Spreading Center, with a description of Spinoloricus turbatio gen. et sp. nov. (Nanaloricidae). Helgol. Mar. Res. (in press; DOI: 10.1007/s10152-007-0064-9)
Lüter, C. (2005): The first Recent species of the unusual brachiopod Kakanuiella (Thecideidae) from New Zealand deep waters. Systematics and Biodiversity 3, 105-111.
Lüter, C. (in press) Anatomy. In: Kaesler, R.L. (ed.) Treatise on Invertebrate Paleontology, part H: Brachiopoda, revised, Vol. 6 Supplement. The Geological Society of America and University of Kansas Press, Boulder and Lawrence.
Lüter, C. (in press) Embryology and Development. In: Kaesler, R.L. (ed.) Treatise on Invertebrate Paleontology, part H: Brachiopoda, revised, Vol. 6 Supplement. The Geological Society of America and University of Kansas Press, Boulder and Lawrence.
Lüter, C. (in press): Recent brachiopods collected during the deep-sea cruise SO 168 ZEALANDIA with the research vessel FS SONNE between Mt. Spong (Tasman Sea) and the Chatham Islands (Pacific) in 2002/2003. Fossils and Strata, Supplement.
Lüter, C. (in press) New record of Annuloplatidia annulata (Atkins, 1959) (Brachiopoda, Platidiidae) from deep water at the Cocos Ridge, East Pacific. Mitteilungen aus dem Museum für Naturkunde in Berlin, Zoologische Reihe.
Neuhaus, B. & R. P. Higgins (2002): Ultrastructure, biology, and phylogenetic relationships of Kinorhyncha. - Integ. Comp. Biol 42: 619-632.
Neuhaus, B. (2004): Description of Campyloderes cf. vanhoeffeni (Kinorhyncha, Cyclorhagida) from the central American East Pacific deep sea with a review of the genus. - Meiofauna Mar. 13: 3-20.
