ALBERT

Beschreibung: Alteration patterns in the lavas and dykes of the Troodos Ophiolite, Cyprus, record a complex history of axial hydrothermal alteration, crustal aging, and subsequent uplift and emplacement of the ophiolite. Field mapping shows that distribution of five alteration zones, each with distinct mineralogical, geochemical, and hydrologie characteristics, is influenced by igneous stratigraphy, structure, and the nature and thickness of the overlying sediments. Paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off-axis. Along spreading axes, low temperatures (≤50°C) were maintained by the rapid flow of seawater in and out of the lavas, and only minimal alteration took place. In contrast, lower water/rock ratios and higher temperatures (〉200°C) in the dykes promoted extensive seawater-rock interaction. Although the sharp rise in temperature between the two regimes generally coincides with the lava-dyke transition, late-stage intrusions or hydrothermal upwelling zones locally cause high-temperature alteration to extend upward into the lavas. As a segment of crust moved off-axis, temperatures remained low in the lavas and progressively decreased, from 〉250° to 〈80°C, in the dykes. High permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and, thus, the original seafloor morphology. Although field relations show that alteration has a consistent vertical pattern in Troodos, the alteration zones are not laterally continuous, and the stratigraphie depth of their boundaries varies considerably.

Beschreibung: In the western equatorial Pacific, the El Niño/Southern Oscillation (ENSO) phenomenon is characterized by precipitation variability associated with the migration of the Indonesian low pressure cell to the region of the date line and the equator. During ENSO events, Tarawa Atoll (1°N, 172°E) experiences heavy rainfall which has an estimated δ18O of about −8 to −10‰ δ18OSMOW. At Tarawa, sufficient precipitation of this composition falls during ENSO events to alter the δ18O and the salinity of the surface waters. Oxygen isotope records from two corals collected off the reef crest of Tarawa reflect rainfall variations associated with both weak and strong ENSO conditions, with approximately monthly resolution. Coral skeletal δ18O variations due to small sea surface temperature (SST) changes are secondary. These records demonstrate the remarkable ability of this technique to reconstruct variations in the position of the Indonesian Low from coral δ18O records in the western equatorial Pacific, a region which has few paleoclimatic records. The coral isotopic data correctly resolve the relative magnitudes of recent variations in the Southern Oscillation Index. Combining the Tarawa record with an oxygen isotopic history from a Galápagos Islands coral demonstrates the ability to distinguish the meteorologic (precipitation) and oceanographic (SST) anomalies that characterize ENSO events across the Pacific Basin over the period of common record (1960–1979). Comparison of the intensity of climatic anomalies at these two sites yields insight into the spatial variability of ENSO events. Isotope records from older corals can provide high-resolution, Pacific-wide reconstructions of ENSO behavior during periods of different climate boundary conditions.

Beschreibung: As shown by the work of Dansgaard and his colleagues, climate oscillations of one or so millennia duration punctuate much of glacial section of the Greenland ice cores. These oscillations are characterized by 5°C air temperature changes, severalfold dust content changes and 50 ppm CO2 changes. Both the temperature and CO2 change are best explained by changes in the mode of operation of the ocean. In this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50°N. Based on this finding, we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.

Beschreibung: In accretionary wedges, often morphologically similar sedimentary intrusions, when observed by remote geophysical means, may have one of two quite different driving mechanisms and a highly variable significance for the regional hydrogeologic picture. For example, mud diapirs are driven by buoyancy forces that arise from bulk density contrasts. In them, mud and pore fluid upwell en masse and fluid migration is a related (sometimes important) but generally subsidiary process. In contrast, diatremes contain sediments fluidized during rapid fluid advection and are forcibly and directly driven by the hydrogeologic system. The nature of fluid input from local and exotic source regions can, therefore, strongly affect sedimentary intrusive processes and vice versa. This complicates the process of defining the main features of the hydrogeological systems operating in accretionary wedges. Focused vertical advection through steep sided (piercement) mud diapirs requires conduit systems, otherwise flow will be diffuse and directed more horizontally out of the low-permeability mud mass. However, where the permeability of the overburden is less than that of the diapir, the whole diapir may act as a conduit. Apart from this special case, conduits will be associated with highly anisotropic scaly fabrics that can sometimes develop in the marginal shear zone of diapirs. Scaly fabrics form during deformation and compaction of a mud matrix under conditions of constant or increasing effective stress. However, the effective stress path can be complex as it is both controlled by the relative rates of upward intrusion and burial (by sedimentation and/or structural thickening) and the hydrogeologic system. Due to this, it appears likely that even in a geographically related group of diapirs, effective stress histories will vary widely between intrusions so that some can form advective pathways for fluids and some cannot. Mixed systems of behavior may also be present with local diatremes developing within diapirs above the terminations of conduit systems and rapidly expanding methane gas pockets. The potentially heterogeneous near-surface behavior may be why the surface manifestations of sedimentary intrusions are so variable when observed in the field. Diatremes can also form separately as large primary features above any structural or stratigraphic conduit that rapidly expels water or gas into the base of an unlithified sediment column. When active, large diatremes require enormous quantities of fluid (water or gas) to drive them, particularly if they are long lived features and hence are a direct indication of at least an episodically vigorous hydrogeologic system.

Beschreibung: The isotopic composition of dissolved O2 in seawater, expressed as the δ18O of O2, is unique among the bioactive tracers of the aphotic zone in that it is not linearly related to oxygen utilization via the stoichiometry of organic matter decomposition. In fact, δ18O of O2 depends on the history of water mixing and O2 consumption in the sample studied (Craig and Kroopnick, 1970; Kroopnick and Craig, 1976). For this reason, the variation of δ18O of O2 with O2 concentration depends on regional circulation patterns and oxygen utilization rates. The δ18O of O2 can be used to chartacterize these processes by decoupling their effects. As an example of this assertion, we interpret the covariation between the concentration of O2 and its isotopic composition in the Pacific Ocean as reported by Kroopnick (1987), using four simple representations of seawater mixing and respiration. Kroopnick's data are in general accord with an elementary model of isopycnal mixing represented by diffusive exchange and oxygen utilization in the ocean's interior, coupled with atmospheric equilibrium at the point where the isopycnals outcrop at the sea surface. This specific result illustrates the general point that δ18O of O2 in seawater can serve as an important constraint on more extensive and sophisticated physical models used to estimate rates of oxygen utilization in the deep sea.

Beschreibung: Statistical heterogeneity of abyssal hill properties is often evident in seafloor topography, even under periods of relatively constant spreading direction and rate. In this paper we relate the statistics of topographic slopes computed on finite spatial scales to the autocovariance function and investigate the practicality of using these functions in describing such heterogeneous abyssal hill terrains. For a two-dimensional homogeneous surface, a direct relation exists between the sample autocovariance and the slope distributions at different spatial scales. However, for a heterogeneous field characterized by large transient signals, the computed autocovariance estimate no longer has a clear statistical interpretation and becomes dominated by the transients. In contrast, the family of slope distributions can still be used to derive stable descriptors of the field. Slope statistics are thus useful in deriving a more robust estimate of the autocovariance than the usual sample autocovariance. Moreover, slope statistics may also be used to derive stable estimates of quantities not measurable with the autocovariance function or power spectra, such as the statistical asymmetry of features. Examples of the use of slope statistics and a comparison with autocovariance methods are presented. We document and quantify evidence of statistical asymmetry in a region of abyssal hills in the northeast Pacific and, in a second example, the presence of multiple lineations in a region where a fracture zone cuts through abyssal hill terrain.

Beschreibung: Some basic characteristics of ridge axis topography are related to spreading rate and distance from neighboring transform faults. For example, the presence of an axial depression coincides in most cases with slow spreading rates, and the overall depth of the ridge axis increases toward ridge-transform intersections (RTIs). On the other hand, it is also well known that the relief and width of the axial valley on, say, the Mid-Atlantic Ridge (MAR) vary along strike in an unpredictable manner. The purpose of the present study is to quantify how much of the observed variation in the first-order topography at the axis is related to changes in other parameters, such as spreading rate and distance from RTIs. To carry out this test, the zero-age depth and the relief and width of the axial valley have been estimated on 46 profiles that cross the axis of the MAR between the equator and 50°N (full spreading rates 22–36 km/Ma). Zero-age depth is here defined to be the depth at age zero of the best fit thermal subsidence trend. Axial valley relief and width have been measured with respect to the ridge flanks by the least squares fit of a Gaussian bell. The measured axial valley relief varies between 600 and 2100 m (average ∼1300 m), while the valley width varies between 16 and 62 km (average ∼35 km). The correlation between zero-age depth, axial valley relief and width, latitude of axial crossing, spreading rate, distance from nearest RTT, and offset on the nearby transforms has been investigated using linear regression techniques. The main results of the present study are that (1) zero-age depth significantly correlates with latitude of crossing, distance from nearest RTI, and offset on the nearby transforms; and (2) the variation in axial valley relief and width is essentially uncorrelated with spreading rate, zero-age depth, distance from nearest RTT, and offset on nearby transforms. The preferred explanation for the observed spatial variation in axial valley geometry is that it reflects a temporal variation. In fact, if the rough abyssal hill topography typical of the MAR flanks is created within the axial valley, the shape of the axial valley cannot be steady state (although the existence of an axial valley may be a steady state phenomenon). This hypothesis is supported by the observation that the variability in axial valley relief is similar to the overall amplitude of abyssal hill topography, measured as the residual on the thermal subsidence trend.

Beschreibung: Early diagenesis of the coupled dissolved silica-opal system in bioturbated sediments may be explained by one of three possible models of increasing mathematical complexity, i.e. the simple, but unused constant-opal model (abbreviated C.O.), the Schink et al. (1975) model (abbreviated SG&F) for which Wong and Grosch (1978) have supplied an analytical solution (designated as W&G), and the improved but much more sophisticated model proposed by Schink and Guinasso (1980) that must be solved numerically (S&G). Scaling analysis and computational comparisons show that the C.O. model and the SG&F model, as calculated via the W&G solution, are asymptotically valid forms of more complete S&G model for the limits of “large” and “small” opal concentrations, respectively. Specifically, the C.O. model is found to provide an excellent approximation to the vastly more complicated S&G model if the amount of opal preserved at depth in the sediment, b(∞), satisfies the inequality, b(∞) ≥ {[0.1(1 - φ)DB][φγDS(CS - CW)]−1 + ρb−1}−1, where φ is the porosity, DB is the mixing coefficient, DS is the tortuosity-corrected molecular diffusivity of silica, CS is the solubility of opal, CW is the silica concentration in the overlying waters, ρb is the intrinsic density of opal and γ is a unit conversion constant if b and C are in different units. Schink and Guinasso (1980) have criticized Wong and Grosch (1978) for utilizing their solution of the earlier SG&F model to describe opal accumulation, a situation for which they believed the SG&F model was invalid. This study has found, however, that for conditions characteristic of the deep sea, the SG&F model and so the W&G solution remain reasonably accurate even if small amounts of opal escape dissolution and collect, but that radical divergence from the S&G model can be expected if the flux of silica is sufficient to create a siliceous ooze. This reflects the minor role played by the divergence of the advective flux in the balance of terms in the diagenetic conservation equations when little opal is preserved. These findings should resolve any uncertainty and controversy over the use of the W&G solution. Opal diagenesis in bioturbated shelf-like sediments appears to be adequately described by the C.O. model alone. The C.O. model, coupled where necessary to the W&G solution, constitutes an attractive alternative to the S&G model because relatively simple analytical methods of solution may be employed rather than advanced numerical techniques.

Beschreibung: 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.

Beschreibung: The Lau Basin, a back arc spreading center, is one of the most active hydrothermal areas in the ocean. A scientific team from France, Germany, and Tonga investigated the southern Lau Basin near Tonga in 1989 to study the processes of seafloor ore-mineral formation associated with hydrothermal circulation along the volcanic Valu Fa ridge (Ride de Valu Fa in Figure 1), which lies in back of the Tonga-Kermadec trench. Between April 17 and May 10 scientists on the R/V Nadir used the submersible Nautile to make 22 dives in the southern Lau Basin. The cruise was called NAUTILAU, for Nautile in Lau Basin. In addition to the standard equipment of the submersible (video and photo cameras, and temperature probe), a CTD (conductivity-temperature-depth) instrument was integrated with a “mini rosette” water sampling device used for the first time on the Nautile to obtain correlations between the geological observations and the physical and chemical anomalies measured in the seawater.