Great Meteor Seamount Research Articles

The upper ocean circulation at Great Meteor Seamount

Recent observations of hydrography and currents at the Great Meteor Seamount are combined with a numerical model to investigate the flow regime at this seamount. Periodic tidal forcing is the dominant process, leading to trapped waves, flow rectification, internal wave generation and a system of closed circulation cells. Steep slopes and a flat summit plain lead to a previously unreported mixed-layer thickness anomaly along the edge of the seamount. The seamount vicinity is characterized by high levels of variability in the tidal band, on spatial scales set by the topography – and the data from even relatively dense observational grids alone may lead to serious misinterpretations of the nature of the seamount-induced circulation and mass structure. The model will be used in Part II of this study to further investigate biologically relevant questions.

Harpacticoida (crustacea, copepoda) of the great meteor seamount, with first conclusions as to the origin of the plateau fauna

Harpacticoida (crustacea, copepoda) of the great meteor seamount, with first conclusions as to the origin of the plateau fauna

Biodiversity and species-environment relationships of the demersal fish assemblage at the Great Meteor Seamount (subtropical NE Atlantic), sampled by different trawls

Quantitative data collected with different bottom trawls at the Great Meteor Seamount (subtropical NE Atlantic, 30°N; 28.5°W) in 1967, 1970 and 1998 are compared. Bootstrap estimates of total catch per unit effort increased from 6.96 and 10.8 ind. m–1 h–1 in 1967 and 1970, respectively, to 583.98 ind. m–1 h–1 in 1998. Gear effects and an effect of gear over time accounted for 47.1% and 20% of species variability. Further significant factors were time of day and habitat, while season was not significant. A total of 43 species was collected. Including supplementary species information, a grand total of 46 species was found associated with the Great Meteor Seamount. Diversity was higher in 1967 and 1970 (Shannon's diversity: H′=2.5 and 1.6) than in 1998 (H′=0.9). Species–environment relationships are discussed in terms of a sound-scattering layer–interception hypothesis, i.e. utilisation of prey from a diurnally moving sound-scattering layer for the bentho-pelagic community. This is probably augmented by concentration effects in a circular current around the seamount (Taylor-column). Long-term changes are discussed with respect to a decrease in biodiversity due to considerable increases in Macroramphosus scolopax and Capros aper. In 1998, the increase of abundance of Trachurus picturatus and the respective decreases for genuine benthic species were likely to have been caused by a change of gear.

Diel and habitat-dependent resource utilisation of deep-sea fishes at the Great Meteor seamount (subtropical NE Atlantic): niche overlap and support for the sound-scattering layer-interception hypothesis

Interspecific relationships of 4 dominant fish species of the Great Meteor seamount fish community (subtropical NE Atlantic, 30°N, 28.5°W), Macroramphosus spp. Lacepede 1803, Capros aper (L. 1758), Antigonia capros Lowe 1843 and Zenopsis conchifer (Lowe 1852) were analysed with respect to diet composition, habitat-dependent resource utilisation and niche overlap. For all 4 species, planktonic and micronektonic prey prevailed. In terms of the Relative Importance index (RI), the benthic share was 18.25% RI for Macroramphosus spp., 19% RI for C. aper and 20.38% RI for A. capros. Prey of Z. conchifer consisted of 48.57 % RI mesopelagic and pelagic fishes and of 47.7% RI bentho-pelagic fishes. For all fishes, a permutation test revealed significant selection of prey in plateau margins of the seamount. Unweighted and novel-weighted overlap indices combining prey composition, habitat use and prey utilisation within habitats revealed high overlap between the boarfishes A. capros and C. aper and smaller overlap between other pairs. The results are in support of the sound scattering layer interception hypothesis (Isaacs & Schwartzlose 1965), which implies: (1) primarily pelagic food utilisation for bentho-pelagic fishes; (2) increased habitat-dependent utilisation rates at locations of interception with the sound-scattering layer; (3) diel changes in utilisation rates due to availability of prey; (4) sufficient resource partitioning among species in order to avoid competitive exclusion.

The relation between habitus and habitat structure as evidenced by a new species of Glochinema (Nematoda, Epsilonematidae) from the plateau of the Great Meteor Seamount1

Glochinema kentrosaurides sp. n. is described from coarse biogenic sands on the plateau of the Great Meteor Seamount at 455 m depth. It is characterized by two rows of 12 long thorns between the caudal body enlargement and the tail base, by two bands of hair-like spines dorsally on the body enlargement, by a diverse waffle-like ornamentation of the cuticle, by a pair of small dorsal pharyngeal thorns, by five to six rows of ambulatory setae adding up to a total of 28 setae in the male and 24 in the female, and by a sexual dimorphism in the form of the amphids. It is the first record of Glochinematinae from the Atlantic, all others being known from the Pacific so far. The distinguishing features of all species of Glochinema Lorenzen, 1974 are summarized. The new species blurs the distinction between the genera Glochinema and Metaglochinema Gourbault & Decraemer, 1996. It is discussed that some of the salient external features may have less systematic importance than hitherto believed because of their variation in response to habitat structure.

Non-linear altimetric geoid inversion for lithospheric elastic thickness and crustal density

SUMMARY The inversion of high-resolution geoid anomaly maps derived from satellite altimetry should allow one to retrieve the lithospheric elastic thickness, T e , and crustal density, r c . Indeed, the bending of a lithospheric plate under the load of a seamount depends on both parameters, and the associated geoid anomaly is correspondingly dependent on the two parameters. The diVerence between the observed and modelled geoid signatures is estimated by a cost function, J, of the two variables, T e and r c . We show that this cost function forms a valley structure along which many local minima appear, the global minimum of J corresponding to the true values of the lithospheric parameters. Classical gradient methods fail to find this global minimum because they converge to the first local minimum of J encountered, so that the final parameter estimate strongly depends on the starting pair of values (T e , r c ). We here implement a non-linear optimization algorithm to recover these two parameters from altimetry data. We demonstrate from the inversion of synthetic data that this approach ensures robust estimates of T e and r c by activating two search phases alternately: a gradient phase to find a local minimum of J, and a tunnelling phase through high values of the cost function. The accuracy of the solution can be improved by a search in an iteratively restricted parameter subspace. Applying our non-linear inversion to the Great Meteor Seamount geoid data, we further show that the inverse problem is intrinsically illposed. As a consequence, minute geoid (or gravity) data errors can induce large changes in any recovery of lithospheric elastic thickness and crustal density.

The Great Meteor seamount: seismic structure of a submerged intraplate volcano

During R/V METEOR cruise No. 12/2 in summer 1990, an OBH airgun refraction line was shot across the Great Meteor seamount, Northeast Atlantic. Velocity structure was modeled using 1-dimensional velocity analysis and 2-dimensional ray tracing of P-wave travel times. The results represent a seismic image of a submerged intraplate volcano. Upper crustal and apron P-velocities of the seamount are significantly lower than those observed in the oceanic layer 2. We interpreted these low velocities in terms of extrusive lavas and of debris flows with high average porosities. At 3 km beneath the seamount summit, the core of the edifice appears to be dominantly intrusive, with P-velocities >6.0 km/s. A distinct high-velocity body (P-velocities >6.5 km/s) suggests a central conduit or a plutonic suite. Weak evidence was found for the existence of a subcrustal plutonic complex with P-velocities intermediate between those of layer 3 and the upper mantle as it is typical for many volcanic islands.

Isostatic geoid anomalies over mid-plate swells in the Central North Atlantic

The relation of geoid height data from Geosat/ERM altimeter measurements to seafloor topography from recent shipborne data is investigated for eight igneous provinces located in the Central North Atlantic. The long wavelength undulations of the geoid, reflecting deep-seated density anomalies, were removed by subtracting a low degree and order spherical harmonic representation of the geoid. After converting residual geoid heights and topography to anomalies related to the thermal plate model, both maps were low-pass filtered to isolate the signal associated with local compensation from surface loading. Finally, the ratio of geoid height to topography was determined by fitting a straight line to the data. Cape Verde, Bermuda, Canary and Madeira swells exhibit high geoid/topography ratios, which signify reheating of the lower lithosphere. These features were classified as thermal swells. Geoid/topography ratios occurring over the New England, Corner, Azores and Great Meteor seamount chains can be explained by Airy compensation model of crustal thickening. This requires non-hotspot processes to be active within the Azores and Great Meteor seamounts.

Evidence for age and evolution of Corner Seamounts and Great Meteor Seamount Chain from multibeam bathymetry

The Comer seamounts in the western North Atlantic and Great Meteor seamount “chain” in the eastern North Atlantic are thought to progress in age from Late Cretaceous through late Cenozoic. They both presumably formed by volcanism above the New England hotspot when first the North American plate, and then the Mid‐Atlantic Ridge axis and African plate, moved over the hotspot. High‐resolution, multibeam bathymetry of the seamounts shows geomorphic features such as guyots, terraces, and a base level plateau (Cruiser plateau) that we interpret to have formed at sea level. We have backtracked these features to sea level along the North Atlantic crustal age‐depth curve in order to estimate their ages. The derived age pattern of volcanism indicates formation of the Comer seamounts at ca. 80 Ma to 76 Ma, with migration of the Mid‐Atlantic Ridge plate boundary over the hotspot and formation of the Cruiser plateau about 76 Ma. Seamount ages suggest that subsequent volcanism on the African plate moved first northward, in the Late Cretaceous to early Cenozoic (Plato, Tyro, and Atlantis seamount groups), then southward to Great Meteor Seamount in the late Cenozoic. Recurrent volcanism appears to have occurred at some seamounts up to 20–30 m.y. after their initial passage over the hotspot. It would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away.

Heat flow evidence for hydrothermal convection in Cretaceous crust of the Madeira Abyssal Plain

36 measurements of heat flow have been made along two transects in an area to the east of Great Meteor Seamount, Madeira Abyssal Plain. Values ranging from 51 to 74 mW m −2 correlate partly with the basement topography. These data, together with results from a previous heat flow survey, have been compared to a numerical heat flux model which incorporates a depth-related conductivity structure for both sediment and basement. The model suggests that the remaining heat flux variability probably relates to basement hydrothermal circulation which is being driven by topographic relief. No new evidence, in terms of non-linear temperature profiles, was found for sediment porewater advection.

A regional compensation model for the Great Meteor Seamount using the response function technique

The first-order correlation between gravity and topography in the region of the Great Meteor Seamount is explained by a regional compensation model of an elastic plate floating on a liquid substratum. The response function of this model can be inverted to yield estimates for the densities and the flexural rigidity of the plate if seismic results are additionally taken into account. The isostatic response function has been computed for a 162 × 160 km 2 area including the Great and Small Meteor Seamounts. The inversion of the response function, including seismic results, yields an average density ρ 0 = 3020 kg/m 3 for the Great Meteor Seamount and a flexural rigidity in the range of 9 to 11 × 10 20 Nm for the plate. The maximum depth of the crust-mantle boundary below the center of the Great Meteor Seamount is 26 km according to the model calculations.

Geochemical oxidation fronts in NE Atlantic distal turbidites and their effects in the sedimentary record

Summary Overall sediment accumulation on the abyssal plain E of Great Meteor Seamount is dominated by distal turbidite deposition. A colour contrast is seen at the tops of many individual turbidites, so that a single turbidite has two distinct colour tones. Porewater data from the uppermost turbidite are used to demonstrate that these colour contrasts are consequences of an oxidation-front phenomenon. Distributions of organic carbon, uranium, vanadium and copper are proposed as diagnostic solid-phase indicators in both active and fossil types. A generalized description of the features of such turbidites is presented.

Recurrent uranium relocations in distal turbidites emplaced in pelagic conditions

The sediments of the Madeira Abyssal Plain, east of Great Meteor Seamount, are dominated by distal turbidite deposition. While the turbidites exhibit a wide compositional range (25–80% CaCO 3), individual examples can be correlated over a wide area and are relatively homogenous. Organic C oxidation, by bottom water oxygen, proceeds from the turbidite tops downwards after emplacement in pelagic conditions, and the progress of this oxidation front is marked by a sharp colour contrast in the sediments ( Wilson et al., 1985). In turbidites with C org ⪢ 0.5%, redistribution of authigenic U occurs to form a concentration peak (4–9 ppm U), just below the oxidation front or colour change. Several tens μg U/cm 2 may be mobilised, and in all examples studied ⪢60% of the remobilised U is relocated into the peak. Following burial by subsequent turbidites, such U concentration peaks are persistent as relict indicators of their extinct oxidation fronts for at least 2 × 10 5 years. In the case of thin turbidites where labile C org is almost exhausted, the U peaks may be located in underlying sedimentary units because of their relationship to the oxidation front. A redox mechanism for U peak formation is suggested from these data rather than a complexation with organic matter.

Heat flow, sediment faulting and porewater advection in the Madeira Abyssal Plain

This report describes 21 closely spaced heat flow measurements which were made along two transects in an area of faulted sediments east of Great Meteor Seamount in the Madeira Abyssal Plain. The heat flow was found to be correlated with basement topography as mapped by seismic reflection profiling. Data modelling suggests that this is due both to the thermal conductivity contrast between sediments and basement rocks and to the presence of hydrothermal circulation within basement highs. The existence of non-linear temperature profiles in sediments covering basement highs suggests that the underlying circulation is causing an upward movement of porewater. There is no firm evidence to show that sediment faults act as preferred pathways for porewater advection.

Parabolic equation modeling of bathymetry: Sensitivity to bathymetric irregularity and rough bottom scattering

The split‐step Fourier parabolic wave equation model has been used to study the effects of bathymetric irregularity on transmission loss (TL) for CW acoustic sources. In particular, we have examined the variability in predicted TL curves when bathymetry is imperfectly known. Conversely, requirements on the accuracy of bathymetric measurements in order to reliably predict TL have been obtained. Realistic bottom profiles for the Straits of Florida and for the Great Meteor sea mount have been used. TL curves have been computed for various stochastic realizations of random perturbations superimposed on the deterministic bathymetric profile, and statistical analyses of the results have been made.

Asymmetric sedimentation around Great Meteor Seamount (North Atlantic)

Sediments around Great Meteor Seamount (about 30°N 28°W) are pelagic calcareous oozes with an unusually high proportion of silt. The upper “A-interval” comprising Holocene and Late Pleistocene (i.e. the last 130,000 years) is yellowish-brown and exhibits strong bioturbation. The lower “B-interval” (Late to Middle Pleistocene) is white and extremely carbonate-rich with a silt maximum formed mainly by coccoliths. These coccoliths are the main part of a considerable proportion of allochthonous carbonate material which was reworked from the upper slope and summit of the seamount. On the eastern flank the sediments of the B-interval show the strongest input of displaced material, the highest sedimentation rates, distinct lamination, and lack of bioturbation. We explain the asymmetric distribution of the sediments east and west of Great Meteor Seamount by different bottom current velocities. The velocity of the northeastward-flowing Antarctic Bottom Water (AABW) is enhanced at the western flank of the obstacle and decreased east of it, mainly as a consequence of the Coriolis force. Because the ocean circulation and AABW intensity were stronger during the period earlier than 130,000 years ago, asymmetric sedimentation was more pronounced at that time.

K–Ar age of basalts from Great Meteor and Josephine seamounts (eastern North Atlantic)

Six basalt samples from Josephine Seamount were dated by the K–Ar method. Their age ranged from 8 to 13 m.y. with increasing water depth. The Quaternary (and possibly Neogene) age of sediments overlying the basalt does not contradict those apparent ages.Two basalt samples from the flank of the Great Meteor Seamount yielded ages of 11 and 16 m.y. Surf-rounded basalt pebbles with pocket-fillings of shallow-water limestone show that after the end of volcanic activity the basalt presently found at a water depth of about 600 m was only a few meters below sea level. Those data and assumptions about the age of underlying oceanic layer 2 (80 m.y.) allow inferences on the minimum volcanic production of Great Meteor Seamount (3.5 × 105m3yr−1). The volcanic activity of many eastern Atlantic seamounts ended about 10 m.y. ago.

Correction [to “‘Gravity anomalies and flexure of the lithosphere: A three-dimensional study of the Great Meteor Seamount, northeast Atlantic’ by A. B. Watts, J. R. Cochran, and G. Selzer”

Correction [to “‘Gravity anomalies and flexure of the lithosphere: A three-dimensional study of the Great Meteor Seamount, northeast Atlantic’ by A. B. Watts, J. R. Cochran, and G. Selzer”

Gravity anomalies and flexure of the lithosphere: A three-dimensional study of the Great Meteor Seamount, northeast Atlantic

Simple models for the flexure of the lithosphere caused by the load of the Great Meteor seamount have been determined for different assumed values of the effective flexural rigidity of the lithosphere. The models utilize a new method for determining the flexure of the lithosphere caused by a three-dimensional load. The gravity effect of the models has been computed and compared with observed free-air anomalies in the vicinity of the seamount. Computations show that the observed free-air anomalies can be most satisfactorily explained for an assumed effective flexural rigidity of the lithosphere of about 6 x 1029 dyn cm. This value, which is similar to other values determined for loads of different ages, suggests that the oceanic lithosphere is rigid enough to support applied loads for periods of time of at least several tens of millions of years. A basic postulate of the modern concept of plate tectonics is that a strong rigid outer layer of the earth (the lithosphere) overlies a weaker layer (the asthenosphere). The assumption that lithospheric plates act as rigid units for up to a few hundreds of millions of years is implicit in the reconstruction of

Evolution of the sea floor in the Corner Seamounts Area

A bathymetric and magnetic survey was conducted in the Corner seamounts area, northwest North Atlantic. From the magnetic data the area is dated as Late Cretaceous, and the bathymetric data reveal two different trend directions which indicate a major change in the pole of rotation for the North Atlantic at this time. Several models are proposed for the generation of the seamounts and the tectonic evolution of the sea floor for this area. A hot spot model is preferred with the Kelvin-New England seamount chain and the Corner seamounts forming as the North American plate moved over the hot spot in the mantle. Great Meteor, Cruiser, and Plato seamounts were formed in the eastern basin when the migrating mid-Atlantic ridge reached the hot spot in Late Cretaceous time.