In order to investigate the mechanism of arsenic release to anoxic ground water in alluvial aquifers, we sampled ground waters from three piezometer nests, 79 shallow (< 45m) wells, and 6 deep (> 80m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of arsenic (200 -1180 µg l -1 ) are accompanied by high concentrations of iron (3 -13.7 mg l -1 ) and phosphate (1 -6.5 mg l -1 ). Ground water that is rich in manganese (1 -5.3 mg l -1 ) contains < 50 µg l -1 of arsenic. The composition of shallow ground water varies at the 100m-scale laterally and the metre-scale vertically, with vertical gradients in arsenic concentration reaching 200 µg l -1 m -1 . The arsenic is supplied by reductive dissolution of FeOOH and release of the sorbed arsenic to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (< 10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release arsenic to ground water.Arsenic concentrations are high (>> 50 µg l -1 ) where reduction of FeOOH oxide is complete and its entire load of sorbed arsenic is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction incomplete, the sediments are brown in colour and resorption of arsenic to residual FeOOH keeps arsenic concentrations below 10 µg l -1 in the presence of dissolved iron. Sorbed arsenic released by reduction of manganese oxides does not release arsenic to ground water because the arsenic resorbs to FeOOH. Arsenic pollution of ground water is common in the alluvial aquifers of the Bengal Basin because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.

[1] Mapping, dating, and modeling of paleoshorelines uplifted in the footwall of the 1981 Gulf of Corinth earthquake fault, Greece (Ms 6.9-6.7), are used to assess its slip rate history relative to other normal faults in the area and study strain localization. The 234 U-230 Th coral ages from Cladocora caespitosa date uplifted shoreface sediments, and paleoshorelines from glacioeustatic sea level highstands at 76, (possibly) 100, 125, 175, 200, 216, 240, and 340 ka. Uplifted Quaternary and Holocene paleoshorelines decrease in elevation toward the western tip of the fault, exhibiting larger tilt angles with age, showing that uplift is due to progressive fault slip. Since 125 ka, uplift rates varied from 0.25 to 0.52 mm/yr over a distance of 5 km away from the fault tip. Tilting was also occurring prior to 125 ka, but uplift rates were lower because the 125 ka paleoshoreline is at 77% of the elevation of the 240 ka paleoshoreline despite being nearly half its age. Comparison of paleoshoreline elevations and sedimentology with the Quaternary sea level curve shows that slip rates increased by a factor of 3.2 ± 0.2 at 175 ± 75 ka, synchronous with cessation of activity on a neighboring normal fault at 382-112 ka. We suggest that the rapid localization of up to 10-15 mm/yr of extension into the narrow gulf ($30 km wide) resulted from synchronous fault activity on neighboring faults followed by localization rather than sequential faulting, with consequences for the mechanism controlling localization of extension.

No abstract

Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete "ground truth" now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m 2 predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165øC. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative "seafloor weathering"; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100-to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900-1000 m below seafloor, within the transition between extrasives andsheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends--that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never-:, been sampled in situ...

In the eastern extremity of the Isle of Wight, near Bembridge, marine interglacial deposits occur at a variety of different elevations. The highest of these, the Steyne Wood Clay, is an estuarine deposit that lies between 38 and 40 m o.d . and rests on Bembridge Marls (Lower Oligocene). The Steyne Wood Clay, which had previously been assigned to the post-temperate substage of a Middle Pleistocene interglacial, has now yielded a diverse coccolith assemblage dominated by Gephyrocapsa oceanica and G. caribbeanica . The absence of both Pseudoemiliania lacunosa , with a last occurrence datum at ca. 0.475 M a B P. and Emiliania huxleyi , with a first occurrence datum at ca. 0.275 Ma B P, suggests deposition during this time interval. The dating of the Steyne Wood Clay is further constrained by palaeomagnetic data, indicating normal geomagnetic polarity, and by amino acid ratios consistent with an early M iddle Pleistocene age. An extended and revised list of Foraminifera and Ostracoda is given, including the description of Leptocythere steynewoodensis sp.nov. The low-level interglacial deposits make up the Bembridge Raised Beach, here formally defined as consisting of high-energy beach gravels, intertidal sands and organic muds, which represent a single fining-upwards sequence. Pollen analysis of the organic muds indicates that these accumulated during the early and late-temperate substages of the Ipswichian interglacial (Ip IIb - III) . Therm oluminescence dates of ca . 115 Ka BP have been obtained from sand lenses within the Raised Beach itself, which also support correlation with the Ipswichian. The Bembridge Raised Beach occupies an altitudinal range of 5-18 M O.D . and thickens rapidly in a westerly direction where it abuts a cliff cut into the Bembridge Marls. Details are given of the composition, morphology and sedimentology of the gravels constituting the Beach, and similarities to recent cuspate foreland and split accumulations are highlighted. A similar origin is proposed for this feature. The upper surface of the Beach has been soliflucted and deposits of matrixsupported gravel, rich in clay, thicken downslope in an easterly direction. This solifluction has been disturbed by cryoturbation. Both the in situ and soliflucted beach are mantled by brickearth, a reworked aeolian silt, which reaches a maximum thickness of 10 m. A Late Devensian age for this unit has been established by thermoluminescence dates in the range 16.0+1.5 to 21.5 + 2 K a BP. Near Lane End, a sedge-peat rich in plant macrofossils and insects occurs between two gravel units. These appear to post-date the Raised Beach and are interpreted as of fluvial origin. The gravel capping the cliffs at Priory Bay, the richest source of Palaeolithic artefacts on the Isle of Wight, occurs between 29 and 33 M O.D. and is also thought to be a fluvial aggradation unrelated to the Raised Beach. The relationship of these marine deposits to those occurring on the adjacent mainland are considered. The Steyne Wood Clay is correlated with the Slindon Sands at Boxgrove, part of the Goodwood-Slindon Raised Beach, which occur at an identical elevation and have produced a similarly diverse coccolith assemblage. Additional palaeontological evidence from Boxgrove suggests that the interglacial deposits should be assigned to a temperate stage falling in the latter part of the ‘Cromerian Complex’. Correlation of the Steyne Wood Clay and Slindon Sands with oxygen isotope stage 9, 11 or 13 seems very probable. Reasons for the occurrence of marine deposits of this age at ca. 40 M O.D.. are considered and it is thought that neotectonic activity is at least partly responsible. Mean rates of uplift of between 5.3 and 15.5 mm ka -1 have been calculated from age estimates for stages 9, 11 and 13 derived from the deep-sea record. However, it is unlikely that the uplift was uniform in either rate or direction. The diverse coccolith assemblages preserved in the Steyne Wood Clay and in the Slindon Sands indicate a full open connection with the marine waters of the central English Channel, and suggests that a thermocline was then present in the Channel at a time when the Straits of Dover were probably closed. The interglacial channel deposits on the modern foreshore of Bracklesham Bay near Earnley have produced a limited coccolith assemblage. Because the altitudinal and palynological differences between these deposits and the Steyne Wood Clay are so great, they are thought to belong to different interglacial stages. The Bembridge Raised Beach is thought to equate with similar deposits on the northern shore of the Solent at Selsey, Stone and West Wittering, which has also now yielded pollen, reported here.

[1] We derive rates of uplift of $0.7-0.8 mm/yr for the western end of the Gulf of Corinth, Greece, using geomorphic paleaoshoreline modeling. We calibrate the modeling with new 234 U-230 Th dates on the coral Cladocora caespitosa collected from raised marine terraces uplifted in the footwall of the active Psathopyrgos fault, the only major active normal fault, reported on published maps controlling the downthrown Rio Straits at the western end of the Gulf of Corinth. In this area of high (15 -22 mm/yr) extension rates measured with GPS, the ratio of uplift-rate to extensional velocity is 0.025-0.035, much lower than values of 0.15 -0.25 found further east in the gulf. These low values imply that if GPS extension rates are correct then mechanical/kinematic models developed for the eastern and central gulf may not be applicable to the western gulf.

No abstract

No abstract