Recent sediment transport and accumulation on the NW Iberian margin

Five transects across the NW Iberian margin were studied in the framework of the EU-funded Ocean Margin EXchange II (OMEX II) project, to determine and establish recent sediment and organic carbon transport and accumulation processes and fluxes.On the Galician shelf and shelf edge, resuspension of sediments resulting in well-developed bottom nepheloid layers was observed at all stations, but transport of suspended sediment appears largely confined to the shelf. On the continental slope, only very dilute bottom nepheloid layers were present, and intermediate nepheloid layers were only occasionally seen. This suggests that cross-slope transfer of particles is limited by the prevailing northerly directed shelf and slope currents.Optical backscatter and ADCP current measurements by the BOBO lander, deployed at 2152 m depth on the Galician slope, indicated that particles in the bottom boundary layer were kept in suspension by tidal currents with highest speeds between 15–25 cm s⁻¹. Net currents during the recording period August 6th–September 10th 1998, were initially directed along-slope toward the NNW, but later turned off-slope toward the SW.The separation of the water masses on the slope from the sediment-laden shelf water by the along-slope current regime is reflected in the recent sedimentary deposits of the Galician shelf and slope. Apart from compositional differences, shelf deposits differ from those on the slope by their higher flux of excess ²¹⁰Pb (0.57–5.37 dpm cm⁻²y⁻¹ versus 0.11–3.00 dpm cm⁻²y⁻¹), a much higher sediment accumulation rate (315.6–2295.9 g m⁻²y⁻¹ versus 10.9–124.7 g m⁻²y⁻¹) and organic carbon burial rate (1.01–34.30 g m⁻²y⁻¹ versus 0.01–0.69 g m⁻²y⁻¹).In contrast to the observations on the Galician margin, pronounced nepheloid layers occurred in the Nazaré Canyon, which extended to considerably greater water depths. This indicates that significantly greater transport of fine-grained particles in both the INL and the BNL was occurring within the canyon, as reflected in the exceptionally high ²¹⁰Pb excess flux (up to 34.09 dpm cm⁻²y⁻¹), mass accumulation rates (maximum 9623.1 g m⁻²y⁻¹) and carbon burial fluxes (up to 180.91 g m⁻²y⁻¹) in the sediment. However, radioisotope fluxes in the lower canyon were only slightly higher than at comparable depths on the Galician margin. This suggests that transport and rapid accumulation is focused on the upper and middle part of the canyon, from where it is episodically released to the deep sea. Compared to the Galician margin, the Nazaré Canyon may be considered as an important organic carbon depocenter on short time-scales, and a major conduit for particulate matter transport to the deep sea on >100 y time-scales.     

Quaternary high-Nb basalts: existence of young oceanic crust under the Sanandaj–Sirjan Zone,NW Iran

Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj–Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO₂, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65–70) and high contents of Cr (>300 ppm), Ni (>177 ppm), and TiO₂ (>1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their ⁸⁷Sr/⁸⁶Sr values range from 0.7049 to 0.7053 and their ?⁰_Nd values lie between –0.2 and 1.1. The small negative values of ?⁰_Nd indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere; furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.     

Soil moisture potential and water content in the unsaturated zone within the arid Ejina Oasis in Northwest China

Three soil profiles were selected in the Ejina Oasis, northwest China, to determine water content profiles and evolution of soil moisture potentials in the unsaturated zone within the arid area. The total soil moisture potentials have been monitored for about 3 months in 2001 at different depths in the soil profiles. The occurrence and movement of water in the unsaturated zone was analyzed using the zero flux plane (ZFP) method. It is shown that convergent ZFPs and divergent ZFPs may occur at depths between 0.5 and 3.0 m, and that the depth of the ZFPs was controlled by the root zone of plants growing on the land surface. Profiles of the total soil moisture potentials were observed to be coincident with those of the water contents at the three experimental sites. The total soil moisture potential showed a slight increasing trend and the ZFPs tend to vanish from summer to winter as the water extraction by roots decreased. Evapotranspiration through vegetation has an important bearing on the water content and the total potential in the unsaturated zone.     

Biomization and quantitative climate reconstruction techniques in northwestern Mexico—With an application to four Holocene pollen sequences

New paleovegetation and paleoclimatic reconstructions from the Sierra Madre Occidental (SMO) in northwestern Mexico are presented. This work involves climate and biome reconstruction using Plant Functional Types (PFT) assigned to pollen taxa. We used fossil pollen data from four Holocene peat bogs located at different altitudes (1500‑2000 m) at the border region of Sonora and Chihuahua at around 28° N latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.; Ortega-Rosas, C.I., Peñalba, M.C., Guiot, J. Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico. Submitted for publication of Palaeobotany and Palynology). The closest modern pollen data come from pollen analysis across an altitudinal transect from the Sonoran Desert towards the highlands of the temperate SMO at the same latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.). An additional modern pollen dataset of 400 sites across NW Mexico and the SW United States was compiled from different sources (Davis, O.K., 1995. Climate and vegetation pattern in surface samples from arid western U.S.A.: application to Holocene climatic reconstruction. Palynology 19, 95–119, North American Pollen Database, Latin-American Pollen Database, personal data, and different scientific papers). For the biomization method (Prentice, I.C., Guiot, J., Huntley, B., Jolly, D., Cheddadi, R., 1996. Reconstructing biomes from paleoecological data: a general method and its application to European pollen data at 0 and 6 ka. Climate Dynamics 12, 185–194), we modified the pollen-PFT and PFT-biomes assignation of Thompson and Anderson (Thompson, R.S., Anderson, K.H., 2000. Biomes of western North America at 18,000; 6000 and 0 14C yr BP reconstructed from pollen and packrat midden data. Journal of Biogeography 27, 555–584) for a better representation of the modern vegetation of NW Mexico. The biome reconstruction method was validated with the modern pollen sites and applied to the fossil sites. Our results show that, during the early Holocene, a cool conifer forest extended at least down to 1700 m, while today this biome is present above 2000 m in the Chihuahua state. The Younger Dryas event was recorded in one site with cold and dry conditions. The reconstructed annual temperature for this period was 3°–6 °C colder than today, and annual precipitation was 250 mm lower than at present (900 mm/yr). The middle Holocene after 9200 cal yr BP was marked by a warming trend, reaching temperatures 2 °C warmer than today at 7000 cal yr BP, and by the installation of a warm mixed forest, the present day biome, at 1700 m elevation, while at higher elevations (1900 m) the cool conifer forest was still present. Summer precipitation was 200 mm/yr above the early Holocene values, suggesting that monsoon-like conditions strengthened since 9200 cal yr BP at this region. During the last 4000 yr, the same warm mixed forest was reconstructed below 1700 m and a conifer forest above 1700 m. A great variability of vegetation and climate patterns was recorded for the last 3000 yr particularly at high elevation sites, where warming and cooling trends would be coeval of the Medieval warm period and Little Ice Age, likely related to ENSO variability.     

The Gibraltar Arc seismogenic zone (part 2): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity

The Great Lisbon earthquake has the largest documented felt area of any shallow earthquake and an estimated magnitude of 8.5–9.0. The associated tsunami ravaged the coast of SW Portugal and the Gulf of Cadiz, with run-up heights reported to have reached 5–15 m. While several source regions offshore SW Portugal have been proposed (e.g.— Gorringe Bank, Marquis de Pombal fault), no single source appears to be able to account for the great seismic moment as well as all the historical tsunami amplitude and travel time observations. A shallow east dipping fault plane beneath the Gulf of Cadiz associated with active subduction beneath Gibraltar, represents a candidate source for the Lisbon earthquake of 1755.Here we consider the fault parameters implied by this hypothesis, with respect to total slip, seismic moment, and recurrence interval to test the viability of this source. The geometry of the seismogenic zone is obtained from deep crustal studies and can be represented by an east dipping fault plane with mean dimensions of 180 km (N–S) × 210 km (E–W). For 10 m of co-seismic slip an Mw 8.64 event results and for 20 m of slip an Mw 8.8 earthquake is generated. Thus, for convergence rates of about 1 cm/yr, an event of this magnitude could occur every 1000–2000 years. Available kinematic and sedimentological data are in general agreement with such a recurrence interval. Tsunami wave form modeling indicates a subduction source in the Gulf of Cadiz can partly satisfy the historical observations with respect to wave amplitudes and arrival times, though discrepancies remain for some stations. A macroseismic analysis is performed using site effect functions calculated from isoseismals observed during instrumentally recorded strong earthquakes in the region (M7.9 1969 and M6.8 1964). The resulting synthetic isoseismals for the 1755 event suggest a subduction source, possibly in combination with an additional source at the NW corner of the Gulf of Cadiz can satisfactorily explain the historically observed seismic intensities. Further studies are needed to sample the turbidites in the adjacent abyssal plains to better document the source region and more precisely calibrate the chronology of great earthquakes in this region.     

Are red tides affecting economically the commercialization of the Galician (NW Spain) mussel farming?

Harmful algal blooms (HABs), more specifically red tides, are among the most critical environmental factors affecting mussel cultivation in Galicia (NW Spain), and they often have been blamed for economic losses for producers. This statement is based on the correlation between days of closure of the production areas and unsold product. The present article shows that such a statement is not always correct, at least in the case of Galician mussel farming, because red tides only cause losses to producers under specific circumstances that arise from the impossibility of placing their product in the market. In addition, this article reveals the importance of finding organizational solutions within the framework of the production sector to counteract the impact of this type of phenomenon.     

Salt tectonics in the Cap Boujdour Area,Aaiun Basin,NW Africa

Seismic reflection data indicate the Moroccan salt basin extends to the Cap Boujdour area in the Aaiun Basin. Two salt diapir structures have been identified and several areas of collapsed strata indicate probable salt removal at the shelf edge. The presence of salt in this area correlates to the conjugate southern George's Bank Basin and the Baltimore Canyon area, and it is suggested that the salt extends southward from the known salt diapir province in the George's Bank Basin southward to the Great Stone Dome. The paucity of salt diapirs is attributed to the thick carbonate and anhydrite sequence, which was deposited soon after salt deposition that inhibited halokinesis. The presence of salt along this large segment of the Atlantic margin should increase its hydrocarbon potential with traps created around salt diapirs and provision of migration pathways from deep potential source rocks in the early Cretaceous and Jurassic strata to shallower levels.     

Correlation of the major late Jurassic —early Tertiary low- and highstand cycles of south-west Egypt and north-west Sudan

The mainly continental deposits of northwest Sudan and south-west Egypt have been correlated with coeval shallow marine and marine deposits in northern Egypt along a north-south running cross-section, based on surface and subsurface data. The palaeodepth curve of northern Egypt illustrates the gradual seal-level rise, reaching its maximum during the Late Cretaceous with conspicuous advances during the Aptian and late Cenomanian. A general highstand is also recorded during the Campanian-Maastrichtian in north-west Sudan. A detailed facies correlation is given for the Aptian and late Cenomanian highstand in western Egypt. The correlation of the Cenomanian Bahariya and Maghrabi formations displays short-term relative sealevel fluctuations. The interpretation illustrates the extensiveness of related erosional processes in the hinterland, partly intensified by temporarily uplift of the Uweinat-Aswan High in the south. Regional uplift and constant erosion took place in south-west Egypt during Coniacian and Santonian times. The regional stratigraphic gaps and uncertain interpretation of the Bahariya Uplift are induced by the influence of the Trans-African Lineament, especially during the Late Cretaceous. Low-stand fluvial sheet sandstones characterized by non-cyclic sequence development and high facies stability occur, especially in the Neocomian and early Turonian. During the Barremian and Albian, fluvial architecture changes to more cyclic fluvial sequences and increasing soil formation, due to increasing subsidence, more humid climatic conditions and the generally rising sea level, culminating in the extensive shallow marine Abu Ballas and Maghrabi formations.     

Early Ordovician (Floian) ostracods from the Cordillera Oriental,Northwest Argentina

The ostracod fauna from the Floian (Lower Ordovician) strata of the Cordillera Oriental, Argentina (Acoite Formation) are documented. One new genus Acoitella and four species are recognized, two of which are new (Nanopsis victoria sp. nov. and Conchoprimitia frequens sp. nov.). The diversity of the Early Ordovician ostracod faunas of the Central Andean Basin is evaluated. Compared with other regions, the ostracod diversity, at generic and specific level, of the Cordillera Oriental was comparatively high during the Tremadocian (nine species recorded) and comparatively low during the Floian (four species recorded). The taxonomic composition of the fauna is similar during the entire Early Ordovician of the Cordillera Oriental, where it displays a high percentage of soanellids. The Floian assemblages are characterized by the dominance of the eridostracan C. frequens sp. nov. with a high percentage of nondimorphic palaeocopids: Nanopsis and two genera of the Soanellidae family (Saltite and Acoitella gen nov.). An ostracod shell bed is described and interpreted to be a parautochthonous assemblage, as a result of transport within the same habitat. The composition of the fauna, with the presence of Saltite and Nanopsis, shows significant affinities with East Gondwana. Copyright © 2011 John Wiley & Sons, Ltd.     

浙西北地区反转构造初步研究

通过浙西北地区金衢、乌马和杭嘉湖3个地震解译剖面及地表地质的综合分析,指出研究区在以T₂为主变形期的逆冲变形体制之前存在同方向的引张体制,并随后发生反转。通过计算反转比等综合分析,发现其总体反转程度自南东往北西方向逐渐变小,反映出反转后的褶皱冲断变形程度呈现自南东往北西方向变弱之趋势。