Dredge Hauls Research Articles

Neogene Hydrothermal Fe‐ and Mn‐Oxide Mineralization of Paleozoic Continental Rocks, Amerasia Basin, Arctic Ocean

AbstractRocks dredged from water depths of 1,605, 2,500, 3,300, and 3,400 m in the Arctic Ocean included Paleozoic continental rocks pervasively mineralized during the Neogene by hydrothermal Fe and Mn oxides. Samples were recovered in three dredge hauls from the Chukchi Borderland and one from Mendeleev Ridge north of Alaska and eastern Siberia, respectively. Many of the rocks were so pervasively altered that the protolith could not be identified, while others had volcanic, plutonic, and metamorphic protoliths. The mineralized rocks were cemented and partly to wholly replaced by the hydrothermal oxides. The Amerasia Basin, where the Chukchi Borderland and Mendeleev Ridge occur, supports a series of faults and fractures that serve as major zones of crustal weakness. We propose that the stratabound hydrothermal deposits formed through the flux of hydrothermal fluids along Paleozoic and Mesozoic faults related to block faulting along a rifted margin during minor episodes of Neogene tectonism and were later exposed at the seafloor through slumping or other gravity processes. Tectonically driven hydrothermal circulation most likely facilitated the pervasive mineralization along fault surfaces via frictional heating, hydrofracturing brecciation, and low‐ to moderate temperature Fe‐ and Mn‐rich hydrothermal fluids, which mineralized the crushed, altered, and brecciated rocks.

Skerki Bank (Strait of Sicily) is a hotspot of molluscan biodiversity: multiple new records for Tunisian waters and description of three new species

Sea bottom sampling of the Skerki Bank in the Strait of Sicily updates the census of marine molluscs inhabiting a biogeographically key sector of the Mediterranean Basin. One single dredge haul at 112-m depth yielded 110 molluscan species belonging to Gastropoda, Bivalvia, and Scaphopoda, of which 41 are new records for Tunisian jurisdictional waters, including three gastropod species new to science. Nearly all the species are shared with the fauna inhabiting the adjacent Italian waters, and most of them are reported also from Malta. We stress that sites characterized by a relevant species richness, diagnosed by hosting more than 100 species, inclusive of rare taxa, are important areas for conservation in need of adequate management.

The unknown bathyal of the Canaries: new species and new records of deep-sea Mollusca

A set of seven dredge hauls, between 195-215 m and 655-660 m deep on the NW slope of Gran Canaria (Canary Islands, Spain), recovered 15 000 specimens belonging to 295 species of molluscs. Of these, 254 are identified at the species level. Only 47 species, totalling 867 specimens, were live collected, which amounts to 84% of the species and 94% of the specimens represented only by shells. The dredges DW 133 (shallowest) and DW 130 (deepest) hold the highest number of species and abundance, representing about 90% of the material. Fifty-one species are new records for Canarian waters; of these, 23 are new for Spanish waters overall and three are the first reference in eastern Atlantic waters. Another 13 species, in the genera MikroWaren, 1996, DiscaclisMoolenbeek & Waren, 1987, Mucronalia A. Adams, 1860, Marginella Lamarck, 1799, Dentimargo Cossmann, 1899, Prunum Herrmannsen, 1852, Microvoluta Angas, 1877, Spirotropis G.O. Sars, 1878, Gymnobela Verrill, 1884, Mitromorpha Carpenter, 1865, Orbitestella Iredale, 1917 and Liostomia G.O. Sars, 1878, are described as new, most of them from the deepest haul at 655-660 m. Anatoma richardi (Dautzenberg & Fischer, 1896) is restored as valid species and the identification of Canarian specimens as Anatoma tenuis (Jeffreys, 1877) is disputed. Ancistrobasis lavaleyeiHoffman & Freiwald, 2017 is synonymized with A. reticulata (Philippi, 1844). Pleurotomella megalembryon (Dautzenberg & Fischer, 1896), type species of Azorilla Nordsieck, 1968, is assigned to Teretia Norman, 1888; Teretia strongyla (Dall, 1927) is synonymized with T. megalembryon, and Azorilla with Teretia. Pleurotoma teres Reeve, 1844 is selected under ICZN Art. 70.3.2 as type species of Teres Bucquoy, Dautzenberg & Dollfus 1883 and of its substitute name Teretia, discarding therefore Pleurotoma anceps Eichwald, 1830.

Shipboard Characterization of Tuvalu, Samoa, and Lau Dredge Samples Using Laser-Induced Breakdown Spectroscopy (LIBS)

Chemical analysis using laser-induced breakdown spectroscopy (LIBS) is well suited for field applications and was applied here for shipboard characterization of a large sample set during the RR1310 rock dredging expedition to the Tuvalu Seamounts. Although recently the most common data treatment for LIBS has consisted of a partial least squares approach to define sample groupings, we show that quantitative data of useful quality can be obtained with a univariate approach. Here, our analysis goal was a quantitative comparison of the total alkali (Na2O + K2O) versus silica (SiO2) contents of 586 representative dredge samples with known ranges in common rock types. Out of those samples, >400 form a single large group of alkalic basalts with minor basanites/tephrites (SiO2: 43-48 wt%, Na2O + K2O: 3-5 wt%), similar to known shield-stage compositions of the Rurutu and Samoa hotspots in the sampling area. In contrast, several dredge hauls contain samples with compositions that do not overlap with the majority of samples. This includes three dredges performed on the northern boundary of the Lau Basin that contain similar SiO2 compositions, but slightly higher total alkali (Na2O+K2O) content. Despite this difference, they classify as basanite/tephrite, similar to a subset of the main group. More importantly, similar compositions were previously reported from the same tectonic boundary, ascribed to hotspot mantle source material mixed into the Lau Basin back-arc. Although the quality of the compositional data suffices to enable sample selection for time-intensive analyses, higher precision is required for more in-depth petrogenetic interpretation. Error analysis based on repeat standard measurements suggests averaging 100 spectra per sample is optimal here, while use of a higher resolution spectrometer, together with better laser control, would improve results and interpretations.

Widespread Neogene volcanism on Central Kerguelen Plateau, Southern Indian Ocean

ABSTRACTWe report new age determinations and compositions for rocks from 18 dredge hauls collected from eight submarine areas across Central Kerguelen Plateau (CKP). Sea knolls and volcanic fields with multiple small cones were targeted over a ∼125 000 km2 region that includes Heard and McDonald Islands. Large early Miocene (22–16 Ma) sea knolls rise from the western margin of the CKP and are part of a NNW–SSE line of volcanic centres that lie between Îles Kerguelen and Heard and McDonald Islands. These are probably related to hotspot activity now underlying the Heard Island area. We see evidence of much younger activity (5 Ma to present) in volcanic fields to the north of, and up to 300 km NE of, Heard Island. Compositions include basanite, basalt and trachybasalt, which are broadly similar to plateau lava flows from nearby Ocean Drilling Program Site 1138, lower Miocene lavas at Îles Kerguelen, dredged rocks from the early Miocene sea knolls, and Big Ben lavas from Heard Island. Geochemical data indicate decreasing fractions of mantle source melting with time. We propose that a broad region of the CKP became volcanically active in Neogene time owing to incubation of plume material at the base of the relatively stationary overlying plateau. The presence of pre-existing crustal faults gave access for melts from the Heard mantle plume to rise to the surface.

Controls on ferromanganese crust composition and reconnaissance resource potential, Ninetyeast Ridge, Indian Ocean

A reconnaissance survey of Fe–Mn crusts from the 5000km long (~31°S to 10°N) Ninetyeast Ridge (NER) in the Indian Ocean shows their widespread occurrence along the ridge as well as with water depth on the ridge flanks. The crusts are hydrogenetic based in growth rates and discrimination plots. Twenty samples from 12 crusts from 9 locations along the ridge were analyzed for chemical and mineralogical compositions, growth rates, and statistical relationships (Q-mode factor analysis, correlation coefficients) were calculated. The crusts collected are relatively thin (maximum 40mm), and those analyzed varied from 4mm to 32mm. However, crusts as thick as 80mm can be expected to occur based on the age of rocks that comprise the NER and the growth rates calculated here. Growth rates of the crusts increase to the north along the NER and with water depth. The increase to the north resulted from an increased supply of Mn from the oxygen minimum zone (OMZ) to depths below the OMZ combined with an increased supply of Fe at depth from the dissolution of biogenic carbonate and from deep-sourced hydrothermal Fe. These increased supplies of Fe increased growth rates of the deeper-water crusts along the entire NER. Because of the huge terrigenous (rivers, eolian, pyroclastic) and hydrothermal (three spreading centers) inputs to the Indian Ocean, and the history of primary productivity, Fe–Mn crust compositions vary from those analyzed from open-ocean locations in the Pacific.The sources of detrital material in the crusts change along the NER and reflect, from north to south, the decreasing influence of the Ganga River system and volcanic arcs located to the east, with increasing influence of sediment derived from Australia to the south. In addition, weathering of NER basalt likely contributed to the aluminosilicate fraction of the crusts. The southernmost sample has a relatively large detrital component compared to other southern NER crust samples, which was probably derived predominantly from weathering of local volcanic outcrops.Fe–Mn crusts from a dredge haul at 3412m water depth, 2°S latitude, are pervasively phosphatized along with the substrate rocks (site D7). Phosphatization took place through replacement of carbonate, preferential replacement of Fe oxyhydroxide relative to Mn oxide in the crusts, preferential replacement of silica-rich phases relative to Al-rich phases in the crusts, and precipitation of carbonate fluorapatite in pore space. The preferentially replaced silica may have been Si adsorbed on the Fe oxyhydroxide. The enrichment of Ni, Zn, and Cu in the phosphatized crust reflects preferential adsorption into the tunnel structure of todorokite. The rare earth element plus yttrium (REY) patterns indicate a lower oxidation potential during phosphatization of the NER crusts compared to Pacific phosphatized crusts. NER phosphatization occurred in a deeper-water environment than typical for phosphatization of Pacific crusts, occurred post-middle Miocene, a younger age than phosphatization the Pacific crusts, and had in part a different set of chemical changes produced by the phosphatization than did the Pacific crusts.The southern third of NER has Fe–Mn crusts with the highest Co (0.91%), Ni (0.43%), ΣREY (0.33%), Cu (0.22%), Te (146ppm), Pt (1.5ppm), Ru (52ppb), and Rh (99ppb) contents. These are among the highest Pt, Ru, and Rh concentrations measured in marine Fe–Mn deposits. Because of these high metal concentrations, exploration is warranted for the southern sector of the NER, especially at shallower-water sites where the platinum group elements (PGE) and Co are likely to be even more enriched.

An abandoned rift in the southwestern part of the South Scotia Ridge (Antarctica): Implications for the genesis of the Bransfield Strait

AbstractThe western segment of the South Scotia Ridge is a narrow submarine ridge composed of continental crustal blocks, which hosts the left‐lateral boundary between the Scotia and Antarctic plates. This plate boundary segment gradually merges westward with the Bransfield Strait, a young (<4 Ma) basin located between the South Shetland Islands and the northern segment of the Antarctic Peninsula. Here we present a series of multichannel seismic profiles that reveal the presence of a linear chain of submarine volcanic edifices running parallel to the present‐day Scotia‐Antarctic plate boundary. These edifices consist of alkali basalts (~4 Ma), as shown by published data from a dredge haul. The occurrence, distribution, and shape of these magmatic manifestations suggest that they were emplaced in a rift‐related environment, associated with the activity of the former western segment of the Scotia‐Antarctic plate boundary. When spreading centers of the Phoenix‐Antarctic ridge NW of the South Shetland Islands became inactive at about 3.3 Ma, the former western Scotia‐Antarctic plate boundary was abandoned, and it moved to the present‐day location of the Bransfield Strait, where the motion was accommodated, and where active extension is ongoing. The proposed mechanism for the development of the Bransfield Strait, consistent with the temporal sequence of tectonic events, reconciles discrepancies in tectonic evolutionary models presented till now and does not require the presence of slab‐pull and/or slab retreat forces. The Bransfield Strait is not a classic back‐arc basin, but rather an actively extending marginal basin at the transition from mature rifting to incipient, punctiform spreading.

Perotrochus caledonicus (Gastropoda: Pleurotomariidae) revisited: descriptions of new species from the South-West Pacific

Morphological (shell) and molecular examination of a large suite of specimens of pleurotomariids from around New Caledonia and the Coral Sea reveals the existence of four species in the complex of Perotrochus caledonicus: Perotrochus deforgesi Métivier, 1990 and P. pseudogranulosus sp. nov. live allopatrically on the plateaus and guyots of the Coral Sea; Perotrochus caledonicus Bouchet & Métivier, 1982 and Perotrochus wareni sp. nov. live sympatrically - but essentially not syntopically - on the slopes of New Caledonia, Norfolk Ridge and the Loyalty Ridge. All species live in the 300–500 m interval, and together form a significant component of the mollusc fauna living on hard bottoms in the SW Pacific, with individual dredge hauls containing up to 25 specimens of Perotrochus.

Scientific Results of theHasslerExpedition. Bryozoa. No. 2. Brazil

Fifty species of bryozoans from three Brazilian stations dredged by L. Agassiz and L. F. Pourtalès during the Hassler Expedition (1871–1872) are described and illustrated here.The U.S.S. Hassler was built in New Jersey for use by the U.S. Coast Survey off the west coast of North America. Benjamin Peirce, then superintendent of the Survey, invited L. Agassiz to form a scientific party to collect specimens and make scientific observations during the course of repositioning the Hassler from Boston to San Francisco. The bryozoan samples were discovered in the Invertebrate Paleontology collection at the Museum of Comparative Zoology and are now incorporated into the collections of the Department of Marine Invertebrates of the Museum of Comparative Zoology, Harvard University. The first paper on the collection, consisting of the bryozoan material from Barbados, was published in 2009 (Winston and Woollacott 2009).Taxonomic results from the Brazilian segment of the expedition include 24 new cheilostome species, Antropora hastata, Retevirgula multipunctata, Parellisina lunatirostris, Discoporella salvadorensis, Canda alsia, Micropora angustiscapulis, Floridina proterva, Labioporella tuberculata, Cellaria brasiliensis, Puellina octospinosa, Trypostega tropicalis, Poricella frigorosa, Exochella frigidula, Metrarabdotos jani, Parasmittina simpulata, Parasmittina loxoides, Stylopoma rotundum, Stylopoma carioca, Fenestrulina constellata, Buffonellaria variavicularis, Turbicellepora brasiliensis, Fodinella atlantica, Plesiocleidochasma foliosum, and Cigclisula tuberculata; one new combination; and three new cyclostome species, Plagioecia bugei, Nevianopora arcuata, and Disporella brasiliana. Two new families, Marcusadoreidae and Vitrimurellidae, and one new genus, Vitrimurella, are also described.The bryozoan material comes from three of the Brazilian locations where the Hassler Expedition dredged. Eleven species were collected from a station north of Salvador, Bahia state, in 15–17 fathoms (fms) (27.4–31 m). The other two stations were both off Cabo Frio, Rio de Janeiro state. Eleven species were found in the dredge haul from 35 fms (56.7 m), while 32 species occurred in the second haul from 35–45 fms (56.7–82 m). Species from the Bahia station have more tropical affinities; those collected from off Cabo Frio are subtropical–temperate. There was no overlap between the stations from the two regions. Most striking is the fact that 142 years after the collections were made, and on the basis of this very limited material, more than half the species found were new to science, or were species that had previously been misidentified, so needed new names and descriptions: Canda alsia, Labioporella tuberculata, Metrarabdotos jani, Nevianipora arcuata, Parasmittina loxoides, Parasmittina simpulata, and Plagioecia bugei.

Small-scale spatial structuring of burrowed sandeels and the catching properties of the dredge

Abstract Johnsen, E., and Harbitz, A. 2013. Small-scale spatial structuring of burrowed sandeels and the catching properties of the dredge – ICES Journal of Marine Science, 70: 379–386. The lesser sandeel (Ammodytes marinus) remains burrowed for long periods in the seabed in sandy areas, and its habitat preference is reflected in a large-scale (10 km) patchy distribution in the North Sea. The small-scale spatial structure of sandeels in the seabed is less known. In this study, it was found that sandeel catches in grabs and dredges fit gamma distributions with a common shape parameter on both a 10 m and a 100 m scale. These spatial models were used to examine the catching properties of the dredge, which, in the new ICES sandeel assessment, is used to estimate recruitment and maturity curves. Analyses of the empirical data and simulations of the dredge and grab catch rates show very low dredge catchability (∼5%), but no length-dependent catchability in the dredge was found. However, analyses of the catch rates of parallel dredge hauls indicated increasing dredge catchability with sandeel density. The observed small-scale patchiness and the low and seemingly density-dependent dredge catchability result in biased abundance estimates with low precision. Therefore, we recommend a joint international effort to develop a more catch-efficient sampling dredge, where the effect of density on catchability is further examined.

Geochemistry and petrology of three granitoid rock cores from the Nicaraguan Rise, Caribbean Sea: implications for its composition, structure and tectonic evolution

The Nicaraguan Rise is a major submarine structure of poorly known origin. Its lithologies have been studied from dredge hauls and land outcrops on the Greater Antilles and Central America and its structure from geophysical data. In this paper we present the first geochemical analyses for granitoids that were recovered during the 1970s from cores drilled on the Nicaragua Rise for oil prospecting. The three Nicaraguan Rise rocks are calk-alkaline granitoids, and lie in the high-K field for Caribbean granitoids similar to the Above Rocks, Jamaica and Terre Neuve, Haiti intrusions. All of these intrusions are considered to be of Late Cretaceous – Paleocene age. Key elements abundances - K, La, Ce, Nd, Hf, Zr and Sm - indicate that the three Nicaraguan Rise rocks present more affinity with mature oceanic arc rocks similar to other granitoids from the Greater Antilles rather than mature continental arcs. The Pb, Nd and Sr isotope data show no evidence of a continental component, thus indicating that the more eastern and northern submarine area of the Northern Nicaraguan Rise is not underlain by continental crust of the Chortis block. Although of similar age, the Nicaraguan Rise samples are different from the more depleted Cuban granitoids of the Sierra Maestra, though both show strong similarities in their 207Pb/204Pb composition. We postulate that the Northern Nicaraguan Rise was most likely a Caribbean oceanic arc system that may have interacted only at its margin with the continental blocks bounding the region to the west in the area of the Northern Honduran borderland

Xandarovula patula (Gastropoda: Ovulidae) new to Scandinavia

In August 2009 six specimens of the ovulid gastropod Xandarovula patula (Pennant, 1777) (formerly known as Simnia patula Pennant, 1777), were found in dredge samples from a locality west of Smögen in western Sweden (58°22′N 11°05′E). In June and November 2010 a total of three specimens of the same species were found in dredge samples from near Svelgen Bridge, Øygarden, Hordaland, western Norway (60°27 ′N 04°57 ′E). Several small colonies of the presumed prey species, Alcyonium digitatum Linnaeus, 1758 and Tubularia indivisa Linnaeus, 1758, were found in the same dredge hauls. Xandarovula patula has been recorded from the Atlantic coast of southern Spain to the western end of the English Channel, with scattered records from the west coasts of Ireland and Britain, as far north as the Orkneys. More recently it has been reported from most Irish coasts, several parts of the Scottish coast and also from some places in the North Sea. Until now there have been no confirmed records from Scandinavian waters. The specimens recorded here may indicate recent immigration of a southern species due to warmer water temperatures.

Origin of the Aves Ridge and Dutch–Venezuelan Antilles: interaction of the Cretaceous ‘Great Arc' and Caribbean–Colombian Oceanic Plateau?

Abstract: In this paper we reassess the geochronology and geochemistry of three dredge hauls from the SE corner of the Aves Ridge (Caribbean Sea) originally sampled in 1968 by Duke University's R.V. Eastward . Two hauls consist of light rare earth element-enriched granitoids with a U–Pb zircon emplacement age of 75.9 ± 0.7 Ma. A further haul contains mostly calc-alkaline island arc basaltic andesites of uncertain age. Petrological, trace element and isotopic constraints indicate that the granitoids have an oceanic crustal source and were formed by melting of the lower arc, oceanic or oceanic plateau crust. The mafic rocks formed by partial melting of an incompatible trace element-enriched mantle wedge, which was probably composed of mantle plume material. Both the dredged rocks and data from the Dutch–Venezuelan Antilles indicate a period of west-dipping underthrusting and subduction beneath, or close to, the Caribbean–Colombian Oceanic Plateau between c . 88 and c . 59 Ma, concurrent with collision of part of the plateau with northwestern South America. Constraints from the geochemistry and geochronology of offshore southern Caribbean arc and plateau rocks suggest that in the southern Caribbean there was no pre-existing west-dipping subduction system during formation of the Caribbean–Colombian Oceanic Plateau, whereas long-lived SW-dipping subduction in the northern Greater Antilles is more probable. Supplementary material: Sample details, major and trace element data (file 1), cathodoluminescence images of analysed zircons (file 2) and whole-rock standards (file 3) are available at http://www.geolsoc.org.uk/SUP18438 .

Mineralogical and geochemical constraints on the origin of ferromanganese crusts from the Rivera Plate (western margin of Mexico)

Marine hydrogenetically precipitated ferromanganese crusts are widespread in the Pacific Ocean. They occur as pavements coating volcanic or sedimentary hard-rock substrates, mainly on the slopes of seamounts, plateaus and hills in ocean basins and continental margins. We studied three ferromanganese crusts from one dredge haul from the Rivera Plate (western margin of Mexico), which are up to 15 mm thick and grow directly on a substrate of pillow basalt fragments. They consist of laminated, botryoidal, porous aggregates mostly of Fe and Mn oxyhydroxides and up to 10% silicates. XRD analysis showed the predominance of poorly crystallized mineral phases in the crusts that include Mn-feroxyhyte and vernadite, and an authigenic smectite-like clay. Detrital minerals probably derive from granodiorites of the eastern wall of the neighboring Middle America Trench. Scattered barite grains occur on the crust surface and suggest plume fall-out derived from hydrothermal vents, although a possible pelagic source cannot be dismissed. Ratios between major (Fe, Mn, Si) and trace (Co, Ni, Cu) elements reveal that such crusts are predominantly hydrogenetic in origin, although they show a hydrothermal influence that increases in the outer layers. Iron contents range from 16.2 to 25.2 wt.%, and manganese from 3.4 to 14.5 wt.%. The Fe/Mn ratio ranges from 1.6 to 7.0. The SiO 2/Al 2O 3 ratio ranges from 4.3 to 6.6, indicating the presence of biogenic silica (radiolarians). The concentrations of copper (up to 383 ppm) and cobalt (up to 534 ppm) are significantly below those of the typical hydrogenetic crusts, whereas Ni (up to 1320 ppm) is about the same or slightly lower. REE normalized profiles and ΣREE values (486 to 732 ppm) match those reported for hydrogenetic crusts, but suggest a hydrothermal contribution for the later crust layers. The inferred crust growth rates using the Co-chronometer (44 and 229 mm/Myr) are higher than those reported for pure hydrogenetic crusts (mostly 1 to 6 mm/Myr), thus indicating an increasing hydrothermal influence. The highest growth rate (229 mm/Myr, corresponding to the outer crust layers) suggests a regional hydrothermal input of iron and manganese to seawater. The intensification of regional submarine hydrothermal activity began about 13,000 yr ago, and may be related to the tectonic activity in the complex junction of the Rivera, Cocos, Pacific and North America plates.

Assessing modern deep-water ages in the New Zealand region using deep-water corals

The radiocarbon levels of 19 deep-sea corals from seamounts and plateaus in the New Zealand region were determined to assess their ability to record contemporaneous radiocarbon levels in surrounding (in situ) waters. Four species of subthermocline-dwelling corals from 366 to 1680 m water depth were collected between 1964 and 2000 using dredge hauls. Carbonate directly underlying living polyps was analyzed for radiocarbon content. Corals underlying subantarctic waters to the south of the South Island and subtropical waters to the north of New Zealand were collected from 426 to 1680 m water depth and had Δ 14C levels ranging from −11‰ to −146‰ (45–1200 14C years). These radiocarbon levels are in good agreement with the radiocarbon levels of the regional subthermocline waters, as determined for the ∑CO 2 in the waters by GEOSECS in 1974 and WOCE in 1992. The differences in Δ 14C levels in corals sampled in the 1960s and 1990s trace a pattern of changing bomb radiocarbon influence on the carbon in subthermocline waters. The subtropical frontal zone, an area known for significant eddies and mesoscale mixing, sits astride the Chatham Rise to the east of New Zealand, and radiocarbon profiles there are correspondingly more complex. Corals collected on the Chatham Rise were retrieved from 366 to1100 m and document Δ 14C levels ranging from −116‰ to +12‰ (945 14C years before present to significantly bomb-influenced). Corals sampled from depths shallower than ∼500 m tended to have higher and more consistent levels of bomb-carbon input, whereas corals sampled from depths deeper than ∼800 m show little or no bomb-carbon influence. Published measurements of the radiocarbon levels of the dissolved inorganic carbon from a transect taken in 1988 across the Chatham Rise indicate localized injection of surface waters containing bomb radiocarbon to depths as great as 600 m, which appears to be faithfully recorded in the corals. These results indicate that deep corals record even decadal-scale changes in the radiocarbon signal of the water in which they grew, making them excellent recorders of deep-water radiocarbon past and present.

Variable microstructure of peridotite samples from the southern Mariana Trench: Evidence of a complex tectonic evolution

We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594–7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region.

Macrobenthic community structure of the northeast Indian shelf, Bay of Bengal

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 341:59-73 (2007) - doi:10.3354/meps341059 Macrobenthic community structure of the northeast Indian shelf, Bay of Bengal T. Ganesh, A. V. Raman* Marine Biological Laboratory, Department of Zoology, Andhra University, Waltair, Visakhapatnam 530003, India *Corresponding author. Email: akkur.v.raman@gmail.com ABSTRACT: Knowledge of tropical benthic fauna is limited. Two cruises (January 1999 and July 2000) were made covering 24 stations along 5 transects between 16° and 20°N in shelf waters (depth 30 to 200 m) in the Bay of Bengal off northeast India. Altogether 62 infaunal samples (Smith-McIntyre grab 0.1 m2) and 32 epibenthos (through a naturalist dredge 40 × 40 cm) were collected that revealed 169 species representing 15 diverse groups. Gastropods, bivalves, polychaetes and decapods constituted the bulk of the population. Polychaetes were by far the most dominant group (64.98%) among the infauna followed by amphipods (25.23%), which were numerically more abundant (mean 1080 ± 292 ind. m–2) at 51 to 75 m than at greater depths (345 ± 185 ind. m–2 at >150 m). Biomass was higher (wet weight 6.94 ± 4.8 g m–2) at 51 to 75 m than at 101 to 150 m (1.08 ± 1.23 g m–2). There was a preponderance of decapods (26%), gastropods (20.7%), bivalves (8.3%) and several (28.3%) large-sized polychaetes (e.g. Diopatra neapolitana, Eunice indica, Pista sp.) in the dredge hauls (mean abundance 42 ± 8 ind. haul–1). Infaunal diversity (Shannon-Wiener H’) was higher (H’: 2.455 ± 0.18) at 51 to 75 m relative to the sites at >150 m depth (H’: 0.981 ± 0.17). Multivariate analyses were used to define assemblages named after the most important (determining) taxon. Three epifaunal associations could be recognised off the northeast Indian shelf, namely the Charybdis Assemblage (30 to 50 m), the Liagore Assemblage (51 to 75 m), and the Amygdalum watsoni –Tibia delicatula Assemblage (>100 m). Infaunal associations consisted of Ampelisca (30 to 75 m), Nephtys (76 to 100 m) and Cossura coasta (>100 m). Canonical correspondence analysis (CCA) showed that depth, sand, sediment organic matter and sediment mean size influenced epifauna distribution, whereas for infauna, salinity, temperature, mean particle diameter, sand and depth proved important. KEY WORDS: Macrobenthic assemblages · Shelf sediments · Canonical correspondence analysis · Epifauna · Infauna · Marine ecosystems · Bay of Bengal Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 341. Online publication date: July 04, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Nature of the continent–ocean transition zone along the southern Australian continental margin: a comparison of the Naturaliste Plateau, SW Australia, and the central Great Australian Bight sectors

Abstract We document the interpretation of three crustal sections from coincident deep seismic reflection, gravity and magnetic data acquired on Australia's southern margin: one section from the Naturaliste Plateau and the Diamantina Zone; and two in the Great Australian Bight (GAB). Interpretations are based on an integrated study of deep multichannel seismic, gravity and magnetic data, together with sparse sonobuoy and dredging information. All interpreted sections of the margin show a transition from thinned continental crust, through a wide continent ocean transition zone (COTZ). In the GAB the transition is to slow sea-floor spreading oceanic crust that dates from breakup in the Campanian ( c . 83 Ma); in the Naturaliste–Diamantina margin the earliest oceanic crust is undated. The COTZ on these margins is geologically and geophysically complex, but interpretation of all data, including dredge hauls, is consistent with the presence of a mixture of modified continental lower crust, breakup related volcanics and exhumed continental mantle. Serpentinized detachment faults are not well imaged, but have been inferred from high-amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites. Alternative models for the structure of the COTZ, involving either mafic underplating or aborted sea-floor spreading, have been explored, but are considered unlikely on this margin. Similarity in the final architecture of these margins has major implications for the nature of rifting in the Southern Rift System, and may point to the entire 4000 km-long system being non-volcanic in character. Second-order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate. Instead, they probably reflect complexities owing to the multiple tectonic events that occurred during final Gondwanide fragmentation. The most dramatic of these is the impact of hotspot activity in the Kerguelen Plateau, which commenced some 50 Ma prior to final breakup in that sector.

Late Silurian or Early Devonian corals from the continental slope off southern New South Wales

Latest Silurian or Early Devonian tabulate corals have been recovered in a dredge haul from submarine bedrock at a water depth of 2620 – 2155 m on the continental slope off southern New South Wales 45 km east of Tuross Heads from the submerged and unexplored part of the eastern Lachlan Orogen. The corals have been identified as latest Silurian to Early Devonian taxa, Favosites gothlandicus and Squameofavosites squamuliferus forma nitidus. The presence of shallow water deposits containing these forms indicates that a crustal thickening event or events took place near the end of Silurian or early in Devonian time in the eastern Lachlan Orogen east of the present coastline. This occurred well after the end of subduction in Early Silurian time which is marked by the welding of the Cambrian — Late Ordovician Wagonga Group accretionary wedge or collided terrain on to the eastern margin of the orogen and significantly earlier than the emplacement of the Moruya Suite granites in Middle Devonian time.

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES