ALBERT

Description: To study the resolving power of teleseismic P waveforms for receiver structure, we model synthetic waveforms using a time domain waveform inversion scheme beginning with a range of initial models to estimate the range of acceptable velocity structures. To speed up the waveform inversions, we implement Randall's (1989) efficient algorithms for calculating differential seismograms and include a smoothness constraint on all the resulting velocity models utilizing the “jumping” inversion technique of Shaw and Orcutt (1985). We present the results of more than 235 waveform inversions for one‐dimensional velocity structures that indicate that the primary sensitivity of a receiver function is to high wavenumber velocity changes, and a depth‐velocity product, not simply velocity. The range of slownesses in a typical receiver function study does not appear to be broad enough to remove the depth‐velocity ambiguity; the inclusion of a priori information is necessary. We also present inversion results for station RSCP, located in the Cumberland Plateau, Tennessee. Our results are similar to those from a previous study by Owens et al. (1984) and demonstrate the uncertainties in the resulting velocity estimate more clearly.

Description: High concentrations of gold and visible gold-bearing phases have been found in sulfides from the Central Valu Fa Ridge, a spreading center in the Lau back-arc basin west of the Tonga Trench, southwest Pacific. This is believed to be the first known occurrence of visible gold in sulfides from active vents on the seafloor, according to Peter Herzig of the Aachen University of Technology, Federal Republic of Germany, and Mark D. Hannington of the Geological Survey of Canada, Ottawa. Samples were collected during the NAUTILAU Cruise April 17–May 10, 1989 (see Eos, May 1, 1990, p. 678), which consisted of a scientific team from France, Germany, and Tonga that studied the processes of seafloor ore-mineral formation associated with hydrothermal circulation along the volcanic Valu Fa Ridge. The cruise recorded the first observation of an active black smoker hydrothermal field in a back-arc environment.

Description: A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near‐fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America‐Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n = 0.002–0.05); depletion of Ti (300–1600 ppm), Zr (0.1–10 ppm), and Sr (0.1–10 ppm); and fractionation of Zr relative to Ti (Ti/Zr = 250–4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in ≤0.1% increments to a total of 5–25% melting, a process very close to Rayleigh (fractional) melting; batch melting of a LREE‐depleted source cannot account for the observed trace element concentrations in abyssal peridotites. The shapes of some REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Trace element concentrations in calculated integrated fractional liquids approximate the composition of primitive ocean floor basalts, consistent with postsegregation aggregation of small increment melts produced over a depth and melting interval.

Description: The simple theoretical model of Alpers and Hennings describing the radar imaging of submarine bottom topography in coastal waters with strong unidirectional tidal currents is analytically extended to show the influence of advection. The theory applies for L band radar, where second‐order terms in the hydrodynamic interaction can be neglected as a first approximation. If future imaging radars from satellites and space platforms as the ERS‐I (First European Remote Sensing Satellite), the JERS‐I (First Japanese Earth Remote Sensing Satellite), and the EOS (Earth Observing System) are to be used for cartographic applications, it is necessary to include the effect of advection to improve accuracy. This extension of the model simulates the position of the radar cross‐section modulation relative to coastal geomorphological bedforms. By application of that theory it is possible to map features such as the crests of sandbanks and sand waves.

Description: Variations in carbonate flux and dissolution, which occurred in the equatorial Atlantic during the last 24,000 years, have been estimated by a new approach that allows the point‐by‐point determination of paleofluxes to the seafloor. An unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation. The method is based on observations that the flux of unsupported 230Th into deep‐sea sediments is nearly independent of the total mass flux and is close to the production rate. Thus excess 230Th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated. The study was conducted at two sites (Ceará Rise; western equatorial Atlantic, and Sierra Leone Rise; eastern equatorial Atlantic) in cores raised from three different depths at each site. From measurements of 230Th and CaCO3, changes in carbonate flux with time and depth were obtained. A rapid increase in carbonate production, starting at the onset of deglaciation, was found in both areas. This event may have important implications for the postglacial increase in atmospheric CO2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters (and possibly the North Atlantic Deep Water). Increased carbonate dissolution occurred in the two regions during deglaciation, followed by a minimum during mid‐Holocene and renewed intensification of dissolution in late Holocene. During the last 16,000 years, carbonate dissolution was consistently more pronounced in the western than in the eastern basin, reflecting the influence of Antarctic Bottom Water in the west. This trend was reversed during stage 2, possibly due to the accumulation of metabolic CO2 below the level of the Romanche Fracture Zone in the eastern basin.

Description: A 30 km2 diapiric field has been identified near 13°50′N up to 12 km seaward of the deformation front of the Barbados accretionary complex. Using a deep‐towed side scan sonar with a 3.5‐kHz profiler, we identified 31 different diapiric structures. Based on seismic stratigraphy, we show that this field has been active for 200,000 years and that it is a transient feature triggered by the seaward propagation of the high pore fluid pressure associated with the décollement beneath the accretionary complex. Both basement and décollement in this area are anomalously shallow due to the presence of a N110° basement ridge. The height of the diapirs above seafloor does not exceed 40–50 m and can be related to the pressure head of the mud below the décollement. Two types of structures are identified: mud volcanoes and massively emplaced diapirs and ridges on the one hand, enigmatic circular very flat mud pies and conical mounds on the other hand. The second type of structures has steeper slopes and appears to be associated with very active venting, as confirmed by a submersible exploration reported in a companion paper (Le Pichon et al., this issue (b)). The venting results in the formation of a stiff carbonate crust and of large subsiding basins around the mud pies. Continuous active fluid expulsion through these structures indicates that an efficient piping system still connects them to the zone of anomalously high pore pressure below the protodécollement.

Description: We describe a 0.55 km2 semicircular mud pie and three adjacent diapiric mounds explored using the submersible Nautile near 13°50′N, 12 km seaward of the Barbados accretionary complex deformation front. This diapiric field had been previously explored with an acoustically navigated deep‐towed side scan sonar, thus providing an accurate base map. The mud pie is situated at a depth of 4938 m. In addition to visual observations, water, rock, sediment, and biological sampling, we measured temperature gradients and made geochemical analyses of the samples. Fluid venting associated with chemosynthetic animal communities is widespread on all four structures but is maximal in the central third of the mud pie. The chemosynthetic life distribution on the mud pie displays a concentric zonation which we relate to an increase in venting activity toward the center, as evidenced by thermal gradients. Recent mud flows are present in the centralmost part of the mud pie. The simplest interpretation of the structure is that it is a mud lake covered by a mechanically resistant carbonate crust. We determine that approximately 106 m3 of fluid are advected upward through the crust of the mud lake every year. This large amount of fluid implies large‐scale lateral transport.

Description: The present work deals with the petrography and geochemistry of lavas dredged from five active submarine volcanoes (named Mehetia, Moua Pihaa, Rocard, Teahitia, and Cyana) from the southeast end of the Society Islands hotspot trace. Most samples are basic and alkaline, ranging from 16 to 5 wt % MgO, with about 5% normative nepheline. Fractionation modelling based on major and minor compatible element variations suggests that olivine and minor clinopyroxene were the major fractionating phases and implies a maximum range of fractionation of 30–35%. Rocard and Cyana have yielded more evolved, trachy-phonolitic, glassy samples. These evolved samples are thought to be derived by removal of 70% cumulate from the basalts. Both basaltic and phonolitic samples are incompatible-element enriched, with La/YbN ≈ 15 in most of the basalts. The trachy-phonolite patterns show middle rare earth element (REE) depletion and negative Eu anomalies. The Moua Pihaa basalts have flatter patterns than the other basalts (La/YbN = 7.5–12.4). All samples, with the exception of a sample from Moua Pihaa which has elevated 206Pb/204Pb, fall on linear Sr-Nd-Pb isotopic arrays, suggesting two end-member mixing. The most depleted end-member is shown to be a pristine ocean island basalt magma with no detectable contribution from a depleted, mid-ocean ridge basalt (MORB) upper mantle. The flatter REE patterns and higher 206Pb/204Pb of the Moua Pihaa sample are taken to indicate a more depleted, U-enriched (high μ) component in its source. This component may be altered oceanic crust. The Sr isotopic variations in the samples excluding Moua Pihaa correlate positively with Rb/Nb, Pb/Ce, and SiO2 variations, indicating a component of mantle enriched by injection of material from a subducted oceanic slab. Correlation of 207Pb/204Pb with 87Sr/86Sr suggests that the subducted material is geochemically old. Mapping the geochemical variations shows that the contribution to the lavas from the subduction component is greater over the north of the hotspot than in the south. The absence of a MORB component in the Society magmatism, the small volumes of the Polynesian hotspot volcanoes, and the lack of more intense volcanic activity near the center of the Pacific Supers well, all lead us to conclude that the latter is unlikely to be caused by a large convective plume. The Superswell is more probably located above a region in the asthenospheric mantle which, due to its higher content of recycled continental debris, is anomalously hot.

Description: Active fluid venting and its surface manifestations (unique animals and carbonates) occur over the accretionary prism in the Cascadia subduction zone located off central Oregon. A large variety of authigenic carbonate deposits and unique carbonate structures have been observed from submersibles and remotely operated vehicles and recovered with aid of submersibles and bottom trawls from the outermost continental shelf and lower continental slope. The carbonate deposits range from relatively thin crusts and slabs to irregular edifices and well-formed circular chimneys that rise from 1 to 2 m above the seafloor. Mineralogically, the carbonate cement consists of aragonite, calcite, Mg-calcite, or dolomite with varying amounts of detrital constituents. Stable carbon and oxygen isotope data identify four distinct subgroups of methane-derived carbonates from several different vent sites and different fluid source zones. Subgroup I represents one vent site on the lower slope and is characterized by oxygen isotope values ranging from +6.8‰ to +4.7‰ PDB. Subgroup II represents another vent site about 1 km away and exhibits oxygen values of +3.4‰ to +4.9‰ PDB. Carbon isotopic values range from −40.96 to −30.23‰ versus −44.26 to −53.44‰ PDB, respectively, for the two vents. An irregular edifice from the outer shelf has the same isotopic composition as subgroup II. A companion study shows that the expelled fluids contain largely biogenic methane and methane-derived dissolved carbonate; a shallow fluid source zone (〈1 km) is indicated. The isotopic carbon values of the subgroup I and II carbonates are consistent with the carbon composition of the expelled fluids and apparently represent a historical record of the composition of these fluids. In subgroup III, strong 18O enrichment and heavier carbon values characterize the dolomitic chimneys from the outer continental shelf. Cemented sandstones from a “window” in the accretionary complex of the lower slope (subgroup IV) are characterized by extreme δ18O (−5.9 to −5.98 ‰) and moderate δ13C (−18.7 to −12.67‰)-depleted carbonates. This “light” oxygen isotope composition most likely originated from the upward migration of warm hydrothermal fluids along the main décollement, which tapped the warm subducting basaltic slab, during the early stages of formation of the accreted complex. Well-defined plumbing tubes within some carbonate chimneys on the shelf infer a single well-defined subsurface conduit with a fairly energetic fluid flow. The majority of the chimneys probably formed above the seafloor as long as the rate of carbonate precipitation exceeded the rate of detrital input during their formation. We calculate a minimum of one conduit for each 35 m2 at one vent site on the shelf. A less energetic flow is suggested by the chaotic plumbing network of an irregular edifice and by the widespread occurrence of the carbonate slabs and crusts at numerous vent sites.

Description: Fluid venting from the toe of the accretionary prism off Oregon was measured in situ during a series of dives with DSRV Alvin in 1987 and 1988. A benthic chamber was placed over active vent sites to sequentially collect samples of venting fluids and to make direct measurements of discharge rates. Calibrated flow meter measurements and flow rates determined from dissolved methane transfer indicate that discharge from two vent sites, Alvin 1428 and Alvin 1900, ranges roughly between 100 and 500 L m−2 d−1. with the most reliable estimates falling in the range of 125–150 L m−2d−1. These rates imply subsurface advective flow on the order of 100 m yr−1. Comparison of observed discharge rates with rates calculated for steady state expulsion supported by accretion-related compaction indicates that the observed flow is greater than predicted flow by several orders of magnitude. The disparity dictates that fluids are not derived locally, but are transported laterally within the prism, or that flow is not steady state and that individual vents are short-lived features in the ongoing accretion process.