ALBERT

Description: The application of plagioclase geothermometry to plagioclase-bearing volcanic ash layers and to the glassy margins of pillow basalts from the fast-spreading East Pacific rise, the moderately spreading Gorda and Juan de Fuca ridges, and the slow-spreading mid-Atlantic ridge has shown that magma temperatures, as well as average An contents of plagioclases, are negatively correlated with spreading rates. A detailed investigation of the major element chemistry of volcanic glasses from each of these areas suggests that the observed consistent element-element covariances among individual populations of samples have been caused by fractional crystallization of the magmas. The regularity of chemical variation and the similarity of magma temperatures within each population of samples suggest that magmas ascending from beneath each ridge have had similar evolutionary histories. Vector analysis of the chemical data of all samples of volcanic glasses indicate that each population of samples from each of the spreading centers is chemically distinct, even though all samples have been subjected to similar amounts of fractional crystallization. The compositional distinctiveness of each population of oceanic tholeiites probably reflects differences in the depths at which the magmas were generated. Calculated magma temperatures and geothermal gradients calculated from published heat flow measurements can be used to estimate depths of magma generation of about 16 km beneath the East Pacific rise and about 23 km beneath the mid-Atlantic ridge.

Description: Recent developments in side-scan sonar technology have increased the potential for fundamental changes in our understanding of ocean basins. Developed in the late 1960s, “side looking” sonars have been widely used for the last two decades to obtain qualitative estimates of the acoustic properties of the materials of the seafloor. Modern developments in the ability to obtain spatially correct digital data from side-scan sonar systems have resulted in images that can be subsequently processed, enhanced, and quantified. With appropriate processing, these acoustic images can be made to resemble easily recognizable optical photographs. Any geological interpretation of these images requires an understanding of the inherent limitations of the data acquisition system. When imagery is collected, these limitations are largely centered on the concept of resolution. In side-scan sonar images, there are several different types of resolution, including along- and across-track resolution, display resolution, and absolute instrumental resolution. All of these parameters play a critical role in our ability to calibrate and ultimately to interpret the new pictures of the ocean floor. Acoustic image processing is a new application of an old and well-established technique. Digital optical images have benefited from several decades of development in processing techniques. These relatively sophisticated techniques have been applied to photographic images from satellites and spacecraft, images which are “noisy” and difficult to obtain but extremely valuable. Side-scan sonar systems, on the other hand, have only recently been able to produce spatially correct, digital images of the seafloor. The application of digital signal-processing techniques to side-scan sonar data will now allow us to quantify what had been previously very subjective and qualitative interpretations of images of the seafloor. The goal of all this processing of acoustic images remains clear: the development of an interpretable map of the geology of the seafloor.

Description: Accretionary prisms are composed of initially saturated sediments caught in subduction zone tectonism. As sediments deform, fluid pressures rise and fluid is expelled, resembling a saturated sponge being tectonically squeezed. Fluid flow from the accretionary prism feeds surface biological cases, precipitates and dissolves minerals, and causes temperature and geochemical anomalies. Structural and metamorphic features are affected at all scales by fluid pressures or fluid flow in accretionary prisms. Accordingly, this dynamic tectonic environment provides an accessible model for fluid/rock interactions occurring at greater crustal depths. Porosity reduction and to a lesser degree mineral dehydration and the breakdown of sedimentary organic matter provide the fluids expelled from accretionary prisms. Mature hydrocarbons expulsed along prism faults indicate deep sources and many tens of kilometers of lateral transport of fluids. Many faults cutting accretionary prisms expel fluids fresher than seawater, presumably generated by dehydration of clay minerals at depth. Models of fluid flow from accretionary prisms use Darcy's law with matrix and fractures/faults being assigned different permeabilities. Fluid pressures in accretionary prisms are commonly high but range from hydrostatic to lithostatic. Matrix or intergranular permeability ranges from less than 10−20 m² to 10−13 m². Fracture permeability probably exceeds 10−12 m². A global estimate of fluid flux into accretionary prisms suggests they recycle the oceans every 500 m.y. Fluid flow out of accretionary prisms occurs by distributed flow through intergranular permeability and along zones of focused flow, typically faults. Focused fluid flow is 3 to 4 orders of magnitude faster than distributed flow, probably representing the mean differences in permeability along these respective expulsion paths. During the geological evolution of accretionary prisms, distributed flow through pore spaces decreases as a result of consolidation and cementation, whereas flow along fracture systems becomes dominant. Although thrust faults are most common in the compressional environment of accretionary prisms, normal and strike-slip faults are efficient fluid drains, because they are easier to dilate. Observations from both modern and ancient prisms suggest episodic fluid flow which is probably coupled to episodic fault displacement and ultimately to the earthquake cycle.

Description: Zero-age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6-10 wt. %), strongly depleted in incompatible elements. Fractionation-corrected Na2O contents ('Na(sub 8)') are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid-ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible-element ratios (e.g., K2O/TiO2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small-degree partial melting as their cause, rather than long-lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K2O/TiO2. Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller-degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na(sub 8) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occuring beneath the ridge, nor do they require a heterogeneous source.

Description: The best place to seek evidence of the style of past magma flow through a conduit is in the country rock. Heat flow has been studied in country rock adjacent to two Tertiary dolerite sills intruding the Caledonian schists and quartzites, on the Isle of Mull, Scotland. Radiogenic 40Ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the Ultra-Violet Laser Ablation Micro-Probe, to discriminate between a history of prolonged magma flow, a history of conductive cooling following laminar flow, and instantaneous emplacement of the intrusions. The 40Ar/39Ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3–5 months resulted in extensive argon loss from the micas, country rock melting, and mineral breakdown adjacent to a 6-m sill. These features were absent from the wall rocks of a smaller 2.7-m-thick sill, which exhibited even less argon loss than might have been predicted for an instantaneous intrusion. If the heat loss from the 6-m sill observed in one locality had been repeated along its length, it would have formed an important magma conduit to the Mull volcano, but dolerite is not a common flow composition on Mull. If on the other hand, the heat loss from the sill varies along strike, it constitutes strong evidence for channelling and heterogeneous flow within the sill.

Description: Monthly climatologies of near-surface phytoplankton pigment concentration and sea surface temperature (SST) were derived for the Gulf of Mexico from multiyear series of coastal zone color scanner (CZCS) (November 1978 to November 1985) and advanced very high resolution radiometer (AVHRR) (January 1983 to December 1987) images. We complement these series with SST from the comprehensive ocean-atmosphere data set (1946–1987) and Climate Analysis Center (1982–1990), and hydrographic profile data from the NOAA National Oceanographic Data Center (1914–1985). The CZCS ocean color satellite data provide the first climatological time series of phytoplankton concentration for the region. The CZCS images show that seasonal variation in pigment concentration seaward of the shelf is synchronous throughout the gulf, with highest values (〉0.18 mg m−3) in December to February and lowest values (∼0.06 mg m−3) in May to July. Variation in SST is also synchronous throughout the gulf, with maxima in July to September and minima in February to March, The amplitude of the SST variation in the western gulf is about twice that observed in the eastern gulf, and SST maxima and minima persist longer in the west. Larger amplitudes in SST variation are also observed toward the margins. While annual cycles of SST and pigment concentrations are out of phase relative to each other, the phases of mixed layer depth change and pigment concentration change are similar. Model simulations suggest that the single most important factor controlling the seasonal cycle in surface pigment concentration is the depth of the mixed layer. The combined use of ocean color and infrared images permits year-round observation of spatial structure of the surface circulation in the gulf and the pattern of dispersal of the Mississippi River plume. Infrared images are most useful between November and mid-May, when strong SST gradients occur. During this time, pigment concentrations are high and can be horizontally homogeneous. In contrast, between late May and October, SST fields are uniform, but the Loop Current and large anticyclonic eddies could be traced with the CZCS. Three anticyclonic eddies were observed in 1979, and at least two were observed in 1980. No eddies were observed during summers of subsequent years in the CZCS time series, but this may be a result of the dramatic decrease in the satellite sampling rate. The series of color images showed that small parcels of Mississippi River water were frequently (2–4 times a year) entrained in the cyclonic edge of the Loop Current, stretched along the Current, and carried to the southeast along the western Florida shelf. However, most of the Mississippi River water flowed to the west, following the Louisiana-Texas coast as far south as the Mexico-United States border. Here, a persistent cyclone may reside, exporting shelf constituents to deeper regions of the gulf.

Description: We conducted a seismic refraction experiment across Flemish Cap and into the deep basin east of Newfoundland, Canada, and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots. The continental crust at Flemish Cap is 30 km thick and is divided into three layers with P wave velocities of 6.0–6.7 km/s. Across the southeast Flemish Cap margin, the continental crust thins over a 90-km-wide zone to only 1.2 km. The ocean-continent boundary is near the base of Flemish Cap and is marked by a fault between thinned continental crust and 3-km-thick crust with velocities of 4.7–7.0 km/s interpreted as crust from magma-starved oceanic accretion. This thin crust continues seaward for 55 km and thins locally to ~1.5 km. Below a sediment cover (1.9–3.1 km/s), oceanic layer 2 (4.7–4.9 km/s) is ~1.5 km thick, while layer 3 (6.9 km/s) seems to disappear in the thinnest segment of the oceanic crust. At the seawardmost end of the line the crust thickens to ~6 km. Mantle with velocities of 7.6–8.0 km/s underlies both the thin continental and thin oceanic crust in an 80-km-wide zone. A gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth. Normal mantle velocities of 8.0 km/s are observed ~6 km below basement. There are major differences compared to the conjugate Galicia Bank margin, which has a wide zone of extended continental crust, more faulting, and prominent detachment faults. Crust formed by seafloor spreading appears symmetric, however, with 30-km-wide zones of oceanic crust accreted on both margins beginning about 4.5 m.y. before formation of magnetic anomaly M0 (~118 Ma).

Description: Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near-to-average oceanic crust over a spatial scale of 400 km. The SIGMA III transect, located ∼600 km south of the Greenland-Iceland Ridge and the presumed track of the Iceland hot spot, shows that the continent-ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup. Initially, 18.3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8–10 km thick crust. Changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system, which was close to sea level for at least 1 m.y. of subaerial spreading despite a reduction in productivity from 17 to 13.5 km thick crust over this time interval. This could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system. The thermal anomaly must be exhaustible, consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet. Exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial, to shallow marine, and finally to deep marine extrusion in ∼2 m.y. is shown by the morphological changes. In addition, comparison to the conjugate Hatton Bank shows a clear asymmetry in the early accretion history of North Atlantic oceanic crust. Nearly double the volume of material was emplaced on the Greenland margin compared to Hatton Bank and may indicate east directed ridge migration during initial opening.

Description: Microseism recordings from four European broadband stations and from three seismic arrays in Scotland, Norway, and Germany are compared with model wave data of the oceanic wave field in the North Atlantic and local ocean wave data from the Norwegian coast at 60�N, both measured during February–March 2000. Two approaches have been tested to locate generation areas of microseismic energy: a new amplitude correlation technique and beam backprojection from the three seismic arrays. Both techniques reveal that the main generation areas are located in specific regions off the coast of Southwest Norway and North Scotland. Seismic stations distant from these generation areas record a superposition of seismic energy from different source regions. Those close to a specific source region also show a high correlation with it. Both techniques give upper limits for the extent of the generation area of the strongest storm on 6/7 March at the southwest Norwegian coast of about 500 km. By using marine X-band radar measurements of the two-dimensional wave height spectrum, we estimate that the relative change of the extension of the generation area off the coast of southwest Norway during several storms is less than a factor of 3. This indicates that the size of the generation area is controlled by static features as coastline or bathymetry, and not by the extent of the storms. Microseism energy appears to be mainly controlled by the wave height in distinct and identifiable generation regions, so that the wave climate in these regions can be studied using historical records of microseisms.

Description: The radiogenic isotope composition of dissolved trace metals in the ocean represents a set of relatively new and not yet fully exploited tracers with a large potential for oceanographic and paleoceanographic research on timescales from the present back to at least 60 Ma. The main topic of this review are those trace metals with oceanic residence times on the order of or shorter than the global mixing time of the ocean (Nd, Pb, Hf, and, in addition, Be). Their isotopic composition in the ocean has varied as a function of changes in paleocirculation, source provenances, style and intensity of weathering on the continents, as well as orogenic processes. The relative importance of these processes for each trace metal is evaluated, which is a prerequisite for reliable interpretation of their time series in terms of changes in paleocirculation or weathering inputs. This analysis of processes includes a discussion of the long-term isotopic evolution of Sr and Os, which are well mixed in the ocean and have thus not been influenced by circulation changes. The radiogenic isotope evolution of those trace metals with intermediate oceanic residence times can be used as paleoceanographic proxies to reconstruct paleocirculation and weathering inputs into the ocean. This is demonstrated by studies from different ocean basins, mainly carried out on ferromanganese crusts, which show that radiogenic trace metal isotopes provide important new insights and can complement results obtained by other well-established paleoceanographic tracers such as carbon isotopes.