Dissolved and particulate organic carbon in hydrothermal plumes from the East Pacific Rise, 9°50′N

Chemoautotrophic production in seafloor hydrothermal systems has the potential to provide an important source of organic carbon that is exported to the surrounding deep-ocean. While hydrothermal plumes may export carbon, entrained from chimney walls and biologically rich diffuse flow areas, away from sites of venting they also have the potential to provide an environment for in-situ carbon fixation. In this study, we have followed the fate of dissolved and particulate organic carbon (DOC and POC) as it is dispersed through and settles beneath a hydrothermal plume system at 9°50′N on the East Pacific Rise. Concentrations of both DOC and POC are elevated in buoyant plume samples that were collected directly above sites of active venting using both DSV Alvin and a CTD-rosette. Similar levels of POC enrichment are also observed in the dispersing non-buoyant plume, ∼500 m downstream from the vent-site. Further, sediment-trap samples collected beneath the same dispersing plume system, show evidence for a close coupling between organic carbon and Fe oxyhydroxide fluxes. We propose, therefore, a process that concentrates POC into hydrothermal plumes as they disperse through the deep-ocean. This is most probably the result of some combination of preferential adsorption of organic carbon onto Fe-oxyhydroxides and/or microbial activity that preferentially concentrates organic carbon in association with Fe-oxyhydroxides (e.g. through the microbial oxidation of Fe(II) and Fe sulfides). This potential for biological production and consumption within hydrothermal plumes highlights the importance of a multidisciplinary approach to understanding the role of the carbon cycle in deep-sea hydrothermal systems as well as the role that hydrothermal systems may play in regulating global deep-ocean carbon budgets.     

Multistage reaction textures in xenolithic high-MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration-driven metasomatism

High‐MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh‐temperature (UHT) metamorphism and near‐isothermal decompression, the rocks were intruded in a lit‐par‐lit manner by felsic melts (charnockite), which caused local‐scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions. The sequence of multiphase reaction textures in the high‐MgAl xenoliths reflects the changes of physico‐chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, > 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)₁ + garnet (grt)₁ + sapphirine (spr)₁ + spinel (spl)₁ + rutile (rt). A subsequent phase of near‐isothermal decompression in the order of 1–2 kbar (stage 2) resulted in extensive replacement of grt₁ and opx₁ megacrysts by lamellar (opx₂ + spr₂) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg₃ + opx_2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx₂ + spr₂) symplectites by biotite and sillimanite and/or plg₃, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain‐size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4–7 kbar) as indicated by local growth of late garnet (grt₅) at the expense of (opx + spr + plg) domains at static conditions. Recently published isotope data suggest that the multistage evolution of the high‐MgAl granulites at Anakapalle followed a discontinuous P–T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep‐crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid‐crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high–T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.     

High pressure phase relations of primitive high-alumina basalts from Medicine Lake volcano,northern California

Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79–35g and 82–72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine+augite+plagioclase+spinel+/-orthopyroxene near the liquidus at about 11 kbar. Experiments on mixtures of sample 79–35g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine+augite+orthopyroxene+plagioclase at 10 kbar and olivine+augite+orthopyroxene+spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine+augite+ orthopyroxene+spinel+plagioclase at 11 kbar. Orthopyroxene observed as a near liquidus phase in an 11 kbar experiment on sample 82–72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K₂O (0.07 wt.%), high in MgO (>10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.     

Centrifugal modelling of dynamic soil-structure interaction

This paper presents a centrifuge model that is capable of realistically representing soil-structure systems subjected to earthquake-like excitation. The model is validated by performing (i) free field soil tests, (ii) dynamic soil-structure interaction tests and (iii) a numerical analysis of the experimental results. The free field experiments show that the simulated earthquake, which is generated by the hammer-exciter plate method, is similar in amplitude and frequency content to a real earthquake. The experiments also demonstrate that a confined soil sample can satisfactorily model a horizontal soil stratum of infinite lateral extent when the containment walls are lined with an absorptive material to attenuate wave reflections that would otherwise occur. Measurements of the acceleration at different locations on the free soil surface indicate that the surface motion is fairly uniform over a relatively large area. This is further confirmed by a comparison made between the measured free and scattered field motions for a surface foundation. Next, a series of soil-structure interaction tests are performed which examine the dependence of radiation damping on the natural frequencies of the structure relative to the fundamental frequency of the soil stratum. The experimental results are shown to be consistent with established theories. Finally, the experimental results are used to compute the stiffness and damping parameters of a two degree of freedom numerical model of the soil-structure system. The experimental parameters are shown to be in good agreement with calssical text book formulae. This study demonstrates that the centrifuge model consistently behaves as expected for simple, but realistic, dynamic soil and soil-structure systems, and can, therefore, be used with confidence to examine more complicated systems that are not yet fully understood.     

A comparison of lead-isotope measurements on exploration-type samples using inductively coupled plasma and thermal ionization mass spectrometry

Thermal ionization mass spectrometry (TI-MS) has long been the method of choice for Pb-isotope determinations. More recently, however, inductively coupled plasma mass spectrometry (ICP-MS) has been used to determine Pb-isotope ratios for mineral exploration. The ICP-MS technique, although not as precise as TI-MS, may promote a wider application of Ph-isotope ratio methods because it allows individual isotopes to be determined more rapidly, generally without need for chemical separation (e.g., Smith et al., 1984; Hinners et al., 1987). To demonstrate the utility of the ICP-MS method, we have conducted a series of Pb-isotope measurements on several suites of samples using both TI-MS and ICP-MS.     

Nutrient limitation of the macroalga,Penicillus capitatus,associated with subtropical seagrass meadows in Bermuda

Nutrient limitation of the rhizophytic macroalgaPenicillus capitatus found associated with subtropical seagrass meadows in Bermuda was determined from enrichment assays and subsequent tissue analyses. The photosynthetic response ofP. capitatus to additions of inorganic nitrogen (N) or phosphorus (P), measured as oxygen evolution in closed incubation chambers, increased significantly in both the 16 h and 6 d experiments only with nitrogen enrichment. The average photosynthetic response for all treatments was virtually identical in the two experiments, indicating that there was not a significant time lag in nutrient uptake and that the short term (16 h) assay accurately reflected the longer term (6 d) photosynthetic response to nutrient enrichment. Average tissue nitrogen levels for the nitrogen-treated algae were 29% higher than the phosphorus-treated algae and 18% greater than the controls, corroborating the results from the photosynthesis assay.P. capitatus may acquire nutrients directly from sediment sources via rhizoid holdfasts. Ratios of total dissolved nitrogen (TN) to total dissolved phosphorus (TP) in pore water at 10 and 20 cm depths (6.1 and 4.5, respectively) indicate a nitrogen-limited nutrient pool. These low pore water TN:TP ratios may be a function of a limited sorptive capacity of the calcium carbonate sediments for phosphate, anthropogenic nutrient inputs, or high rates of denitrification, all of which would induce N rather than P limitation in these carbonate-rich sediments.     

Distribution and abundance of large floating plastic in the north-central Gulf of Mexico

Aerial surveys were conducted in the north-central Gulf of Mexico from June 1988 to May 1989. Sightings of marine debris and specifically large floating plastics were recorded during these surveys. Five study areas off the Louisiana coast were monitored and seasonal distribution and densities were estimated. Each study area contained plastic throughout the year. The estimated densities of plastic were largest in the offshore areas (4 & 5) and smallest in the inshore areas (1 & 3). Seasonally, density of plastic was smallest in the summer and largest in the fall. The reasons for the differences in seasons and study areas are not apparent but the amount of plastic in each area and season could be affected by variations in currents, winds, discharge from rivers, and human activity.     

Results and analysis of soil-structure interaction experiments performed in the centrifuge

In this paper a centrifuge model that is capable of realistically representing soil-structure systems subjected to earthquake-like excitation is used to create a data pool which demonstrates the influence of (i) the frequencies of the structure, (ii) the foundation embedment and (iii) the foundation shape on radiation damping and soil-structure interaction effects for a structure on a semi-infinite soil layer over bedrock. The centrifuge model used in this study was developed and validated by the authors in an earlier publication,¹ and employs an internal method of earthquake simulation, and the clay-like material, Duxseal, to absorb wave reflections at the boundary of the soil sample. The results of the experimental study are used to compute damping and stiffness values of a two-degree-of-freedom piecewise-linear numerical model of the soil-structure systems. The experimental parameter values are then compared to the values computed using classical text book formulae. The analysis demonstrates the value of the experimental data in validating and developing soil-structure interaction theory, and confirms the accuracy of classical text book formulae in the linear range.     

Caldera formation at Volcán Colima, Mexico, by a large large holocene volcanic debris avalanche

About 4,300 years ago, 10 km³ of the upper cone of ancestral Volcán Colima collapsed to the southwest leaving a horseshoe-shaped caldera 4 km in diameter. The collapse produced a massive volcanic debris avalanche deposit covering over 1550 km² on the southern flanks of the volcano and extending at least 70 km from the former summit. The avalanche followed a steep topographic gradient unobstructed by barriers, resulting in an unusually high area/volume ratio for the Colima deposit. The apparent coefficient of friction (fall height/distance traveled) for the Colima avalanche is 0.06, a low value similar to those of other large-volume deposits. The debris avalanche deposit contains 40–75% angular volcanic clasts from the ancestral cone, a small proportion of vesicular blocks that may be juvenile, and in distal exposures, rare carbonate clasts plucked from the underlying surface by the moving avalanche. Clasts range in size to over 20 m in diameter and are brecciated to different degrees, pulverized, and surrounded by a rock-flour matrix. The upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders. A thick, coarse-grained, compositionally zoned scoria-fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse. A fine-grained surge layer is present beneath the avalanche deposit at one locality, apparently representing an initial blast event. Most of the missing volume of the ancestral volcano has since been restored at an average rate of 0.002 km³/yr through repeated eruptions from the post-caldera cone. As a result, the southern slope of Volcán Colima may again be susceptible to collapse. Over 200,000 people are now living on primary or secondary deposits of the debris avalanche, and a repetition of this event would constitute a volcanic disaster of great magnitude.Ancestral Volcán Colima grew on the southern, trenchward flank of the earlier and larger volcano Nevado de Colima. Trenchward collapse was favored by the buttressing effect of Nevado, the rapid elevation drop to the south, and the intrusion of magma into the southern flank of the ancestral volcano. Other such trenchward-younging, paired volcanoes are known from Mexico, Guatemala, El Salvador, Chile, and Japan. The trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits, as occurred at Colima and Popocatepetl in Mexico and at San Pedro Volcano in Chile.     

The relationship of smooth cordgrass (Spartina alterniflora) to tidal datums: A review

An analysis of data relatingSpartina alterniflora Loisel. to tidal elevations along the Atlantic and Gulf coasts demonstrated that although this species is primarily confined to the intertidal zone, its elevational limits. of occurrence do not correspond to a consistent elevation relative to a tidal datum in all marsh locations. The variation in the vertical distribution of this species reported among marsh studies was attributed primarily to differences in mean tide range (MTR). A positive correlation between MTR and elevational growth range (r=0.91) demonstrated that theSpartina alterniflora zone expands with increasing tidal amplitude. Differences in MTR among marsh locations accounted for 70 and 68% of the statistical variation in the upper and lower limits, respectively, ofS. alterniflora growth. Among marshes of similar tidal amplitudes, the upper limit of occurrence ofS. alterniflora in northern marshes was significantly lower than that in marshes at lower latitudes. These results, in combination with regional differences in plant species distribution across the upper intertidal zone, suggested that some of the variation in the upper limit was due to latitudinal differences in growth conditions and/or differences in interspecific competition. Local and regional differences in other factors such as salinity, nutrients, or physical disturbance may have also contributed to the variation in the limits of growth relative to a tidal plane within and among marshes.