Spatial variation in fluid flow and geochemical fluxes across the sediment-seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Numerous mud volcanoes (MVs) have been discovered along oceanic plate margins, and integrated elemental fluxes are potentially significant for oceanic chemical budgets. Here, we present the first detailed study of the spatial variation in fluid and chemical fluxes at the Carlos Ribeiro MV in the Gulf of Cadiz. To this end, we combine analyses of the chemical composition of pore fluids with a 1-D transport-reaction model to quantify fluid fluxes, and fluxes of boron, lithium and methane, across the sediment-seawater interface. The pore fluids are significantly depleted in chloride, but enriched in lithium, boron and hydrocarbons, relative to seawater. Pore water profiles of sulphate, hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34-180 cm depth below seafloor. Clay mineral dehydration, and in particular the transformation of smectite to illite, produces pore fluids that are depleted in chloride and potassium. Profiles of boron, lithium and potassium are closely related, which suggests that lithium and boron are released from the sediments during this transformation. Pore fluids are expelled into the water column by advection; fluid flow velocities are 4 cm yr⁻¹ at the apex of the MV but they rapidly decrease to 0.4 cm yr⁻¹ at the periphery. The associated fluxes of boron, lithium and methane vary between 7-301, 0.5-6 and 0-806 mmol m⁻² yr⁻¹, respectively. We demonstrate that fluxes of Li and B due to mud volcanism may be important on a global scale, however, release of methane into the overlying water column is suppressed by microbial methanotrophy.     

Reconnaissance de la structure géologique du bassin de saïss occidental, Maroc, par sondages électriques

A geophysical study, based on 96 electrical resistivity measurements with a line length up to 4 km, was performed in the southern and southwestern parts of the Meknes Plateau, Morocco, which is a part of the Saiss Basin, located between the Rif Range to the north and the Middle Atlas Range to the south. This basin, whose maximum depth is 1.5km in the north, is filled with Triassic to Quaternary deposits overlying the Palæozoic basement and includes two main aquifers. The interpretation of the resistivity measurements, calibrated from deep boreholes, made it possible to obtain a new hydrogeological model for the Saiss Basin. The understanding of the basin structure is of primary importance for the water supply of this area, which has been affected by severe droughts in recent years.     

Inter‐laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis

Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca₁₀(PO₄)₆(F,Cl,OH)₂ solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for ³⁷Cl/³⁵Cl homogeneity using SIMS followed by δ³⁷Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10‰ (1s) for the five samples with > 0.5 % m/m Cl and ± 0.19‰ (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38‰ between the mean ³⁷Cl/³⁵Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ³⁷Cl_SMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 10³δ³⁷Cl_SMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25.     

Chevron Ridges and Runup Deposits in the Bahamas from Storms Late in Oxygen-Isotope Substage 5e

Landward-pointing V-shaped sand ridges several kilometers long are common along the windward margin of the Bahama Islands. Their axes share a northeast–southwest trend. Internally, the ridges contain low-angle oolitic beds with few erosional truncations. Commonly interbedded are tabular, fenestrae-rich beds such as those formed by the sheet flow of water over dry sand. Defined here as “chevron ridges,” these landforms appear to have originated in the rapid remobilization of bank margin ooid bodies by the action of long-period waves from a northeasterly source. Deposits along adjacent coastlines also preserve evidence of the impact of large waves. Reworked eolian sand bodies preserve beach fenestrae and hydraulic scour traces up to +40 m on older ridges. On cliffed coasts, 1000-ton boulders have been thrown well inland, recording the impact of large waves. Amino acid ratios confirm a correlation of the ridges across the archipelago, while stratigraphy, spacing, and cross-cutting relationships indicate emplacement as sea level fell rapidly from the substage 5e maximum at or above +6 m.     

TRANSIT,an experimental archaeological program in periglacial environment: Problem,methodology, first results

The aim of the experimental archaeological program TRANSIT is to improve the scientific study of Paleolithic sites. This program is based on studies and experiments carried out at high altitudes in the French Alps. One of its goals is to assess the effects of periglacial processes on spatial distributions within archaeological assemblages and on artifacts and bones. Research on sedimentary environments elucidates stratogenesis in periglacial contexts. In particular, sedimentary models that are useful for interpreting fossil deposits were identified. The first results obtained from experimental artifact test plots emphasize the importance and speed of changes that occur in such a climatic and morphodynamic context. Mean lateral displacements from 1.66 to 4.75 cm yr⁻¹ were measured. Moreover, analysis of osseous and dental pieces showed that they experienced important damage. Works on pollen assemblages have shown that most of the pollen deposited on the ground surface is quickly removed and that, owing to the very uneven preservation of pollen taxa, an important distortion of the initial spectrum occurs. A model of spatial arrangement of artifacts linked to the action of solifluction processes is also proposed. Finally, application of these results to some Paleolithic sites in the Massif Central and southwest France are discussed. © 1998 John Wiley & Sons, Inc.     

Site preservation along an active meandering and avulsing river: The Red River,Arkansas

The low-gradient Red River is a rapidly migrating, sinuous stream with easily erodible banks. Avulsion is common at many scales, from individual meander bends that are cut off to major sections of the river that form multiple, complex meander belts. The present meander belt can be subdivided into mappable landforms—termed phases—that are associated with river courses of different ages and thus associated with archeological sites of different ages. Within the study area two phases are present. The younger Modern meander belt phase has formed within the past 0.2–0.3 ky, precluding preservation of prehistoric archaeological sites. Any protohistoric artifacts that may have been preserved in this meander belt phase would be deeply buried because as much as 2 m of the vertical accretion sediment has accumulated between artificial levees in <0.1 ky and 1–2 m of sediment has accumulated beyond the artificial levees in <0.2 ky. Archeological site preservation in this highly mobile fluvial end member can be used as a predictor for other, similar streams. A large prehistoric site is preserved on an older (0.5–1 kya) Late Prehistoric meander belt phase associated with an abandoned river course. In the study area a Fourche Maline 7 period (A.D. 800–900) through Caddo IV period (ca. A.D. 1500–1700) archeological site (3MI3/30) is preserved on this slightly higher altitude portion of the flood plain. At locations proximal to the river, the site may be buried by overbank sediment 0.4 m thick, but at more distant locations the site is at the surface or only buried by thin overbank sediment because of low sedimentation rates (0.04 cm yr⁻¹) over the span of a millennium. Sites, such as 3MI3/30, that are occupied contemporaneous with overbank sedimentation may be stratified; however, localized erosion and removal of some archeological material may occur where channelized flow crosses the natural levee. © 1998 John Wiley & Sons, Inc.     

Challenges of an integrated water resource management for the Distrito Federal, Western Central Brazil: climate, land-use and water resources

Land-use/cover change (LUCC) and climate change are major controlling factors for water resources in the Distrito Federal in Western Central Brazil. Dynamic LUCC in the region has severe impacts on water resources, while climate changes during the last three decades is thought to have only moderate effects. LUCC affects water quantity mostly during base flow conditions. River basins with substantial expansion of agriculture since the end 1970s show a dramatic decrease of base flow discharge by 40?C70%, presumably due to irrigation. In contrast, the effects of urbanization on runoff are less distinct, since factors controlling runoff generation might be more variable. For water quality, we found urban areas to have a strong influence on the parameters CSB, NH₄ ⁺, and suspended solids. In addition, we assume emerging pollutants, e.g. organic (micro)pollutants, might play a major role in the future. The project IWAS-áGUA DF focuses on creating the scientific base to face these problems in frame of an IWRM concept for the region. Results of our study will be a contribution to an IRWM concept for the Distrito Federal and will help to maintain high standards in water supply for the region.     

Palynology suggests hydrocarbon mobilization in Ordovician of Kasba Tadla Basin,Morocco

Palynological analysis of twelve wells shows that the metamorphic gradient of the pre-Mesozoic in the Kasba Tadla Basin of Morocco increases rapidly northwest-, southeast-, and eastward, from the center of the Basin. In the center, the Ordovician may still be releasing mobile hydrocarbons.

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Zusammenfassung Eine palynologische Untersuchung von zwölf Bohrungen zeigt, daß der thermale Gradient sich im Premesozoikum des Kasba-Tadla-Beckens in Marokko vom Zentrum des Beckens aus nach Nordwesten, Südosten und Osten schnell vergrößert. Es ist sehr wohl möglich, daß das Ordovizium des Beckenzentrums noch flüssige und gasförmige Kohlenwasserstoffverbindungen abgibt.

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Résumé L'analyse palynologique de 12 sondages montre que le gradient métamorphique du pré-Mésozoïque dans le Basin du Kasba Tadla, au Maroc, augmente à partir du centre du Bassin vers le nordouest, le sudest et vers l'est. Il est possible que l'Ordovicien de la partie centrale du Bassin produise encore des hydrocarbures liquides et gaseux.

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12 , Kasba Tadla -, - . , .

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The authors gratefully acknowledge support of this project through Grant GF-32510-X from the National Science Foundation, Washington, D.C., U.S.A.     

Isotopic evidence on the origin of the Shamrocke copper mine,Rhodesia

The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide ³⁴S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar ³⁴S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734     

Man-made turbidity currents in Lake Superior

The discharge of taconite tailings into Lake Superior at Silver Bay, Minnesota, produces turbidity current flow. The silty fine-sand tailings fraction transported to the deepest part of the lake has formed a small fan with valleys similar in gross morphology to a submarine fan. Current meters anchored 5 m above the lake floor over the wall and over the levee of a distributary valley on the fan recorded intermittent turbidity current flows during 30 weeks in 1972–73. At least twenty-five discrete periods of observation of turbidity current flow were obtained; single episodes lasted 4?328+ h. Only flows thick enough to overflow the eastern levee of the valley could be observed, and this accounts for the intermittent nature of our observations, as flow within the valleys is expected to be continuous as long as tailings are discharged. Flow velocities were higher near the valley axis where the flow is thicker. Velocities measured over the valley wall averaged 10.8 cm/s for eleven episodes; velocities measured over the levee, more than 1/2 km from the valley axis, only 3.3 cm/s. The maximum velocity during 1300 h of observation did not exceed 31 cm/s. This agrees reasonably well with velocities calculated from channel properties, as commonly done for turbidity currents on deep-sea fans. Current meters tethered above the bottom meters indicate that lake currents normally parallel the shore throughout the water column. With the onset of a turbidity current, currents higher in the water column remain unchanged but velocities near the bottom go to zero, currents then change azimuth by 90° to parallel the downslope (down-valley) direction of the fan, then increase in velocity. During a turbidity current episode, the direction of bottom flow stays relatively constant (± 20° of the down-valley trend) but the velocity oscillates (commonly with 10 cm/s amplitude), periods being of 1/2 h or less to several hours. Turbidity currents generated on Reserve Mining Company's delta are effective in carrying essentially all tailings discharged into the lake into deeper water, where they are deposited.