ALBERT

Description: Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Description: In their article, Geyer and Martí (2010) propose that the evolution and origin of the volcanic islands which constitute the Canarian archipelago are strongly controlled by regional tectonic “Atlantic” and “African” structures. In their Fig. 1a they sketch the geometry of the Iberian and Moroccan microplates and the respective boundary zones with respect to Africa (Nubia) and Eurasia (Mantovani et al., 2007). Dashed lines indicating presumed plate boundaries cross the Canarian archipelago, which will therefore be located along a lithospheric fracture, the boundary between the Moroccan and African (Nubia) microplates. This regional fracture extends from the Atlas to the Atlantis fracture zone, coinciding in parts with the propagating fracture postulated by Anguita and Hernán (1975). In Fig. 1b of Geyer and Martí (2010), dashed lines indicate the orientation of the most evident tectonic structures visible on the ocean floor. As we show in this comment, all of these mapped “faults” are artifacts.

Description: The Logatchev hydrothermal field at 14°45′N on the MAR is characterized by gas plumes that are enriched in methane and helium compared to the oceanic background. We investigated CH4 concentration and δ13C together with δ3He in the water column of that region. These data and turbidity measurements indicate that apart from the known vent fields, another vent site exists northeast of the vent field Logatchev 1. The distribution of methane and 3He concentrations along two sections were used in combination with current measurements from lowered acoustic Doppler current profilers (LADCP) to calculate the horizontal plume fluxes of these gases. According to these examinations 0.02 μmol s−1 of 3He and 0.21 mol s−1 of methane are transported in a plume that flows into a southward direction in the central part of the valley. Based on 3He measurements of vent fluid (22 ± 6 pM), we estimate a total vent flux in this region of about 900 L s−1 and a total flux of CH4 of 3.2 mol s−1.

Description: The water column imprint of the hydrothermal plume observed at the Nibelungen field (8 18'S 13 degrees 30'W) is highly variable in space and time. The off-axis location of the site, along the southern boundary of a non-transform ridge offset at the joint between two segments of the southern Mid-Atlantic Ridge, is characterized by complex, rugged topography, and thus favorable for the generation of internal tides, subsequent internal wave breaking, and associated vertical mixing in the water column. We have used towed transects and vertical profiles of stratification, turbidity, and direct current measurements to investigate the strength of turbulent mixing in the vicinity of the vent site and the adjacent rift valley, and its temporal and spatial variability in relation to the plume dispersal. Turbulent diffusivities K(rho) were calculated from temperature inversions via Thorpe scales. Heightened mixing (compared to open ocean values) was observed in the whole rift valley within an order of K(rho) around 10(-3) m(2) s(-1). The mixing close to the vent site was even more elevated, with an average of K(rho) = 4 x 10(-2) m(2) s(-1). The mixing, as well as the flow field, exhibited a strong tidal cycle, with strong currents and mixing at the non-buoyant plume level during ebb flow. Periods of strong mixing were associated with increased internal wave activity and frequent occurrence of turbulent overturns. Additional effects of mixing on plume dispersal include bifurcation of the particle plume, likely as a result of the interplay between the modulated mixing strength and current speed, as well as high frequency internal waves in the effluent plume layer, possibly triggered by the buoyant plume via nonlinear interaction with the elevated background turbulence or penetrative convection. (C) 2010 Elsevier Ltd. All rights reserved.

Description: Tharsis Tholus, a more than 3.9 Ga old composite shield volcano to the east of the major Tharsis Montes, has experienced a complex history of growth and destruction. On the basis of new high resolution images we analysed the morphology as well as the tectonic structures of the Tharsis Tholus volcano in detail. From morphological data, cross-cutting relations of the surface structures, and crater modelling ages we propose a chronostratigraphy for the volcano-tectonic history of Tharsis Tholus. The strongly faulted volcano reveals two large-scale landslide events followed by two subsequent shield re-growth phases between 3.8 and 1.7 Ga and two caldera collapses. Tharsis Tholus was also affected by regional extensional tectonics between 1.7 Ga and 0.4 Ga recorded by sub-parallel sets of NE trending graben structures. The steep and up to 5.4 km high landslide scarps on Tharsis Tholus suggest deep faulting of the edifice. In order to confirm this hypothesis we used analogue sand box models in which we demonstrated that gravitational flank movement on top of weak basal substrata may have produced the deformation structures as observed on Tharsis Tholus. (C) 2011 Elsevier B.V. All rights reserved.

Description: During the Joint Air-Sea Interaction (JASIN) experiment conducted in the northern Rockall Trough in the summer of 1978, oceanographic moorings with surface buoys carrying wind recorders were deployed in an array designed to investigate the variability of the near-surface wind field at scales of from 2 to 200 km. The wind records together with observations taken on board the research vessels participating in JASIN have provided ground truth measurements for the sea surface wind velocity sensors on the Seasat satellite. During most of the experiment the wind field was characterized by spatial scales large in comparison with the separations between the buoys. On several occasions, spatial differences associated with cold fronts were identified, and it was possible to track the passage of the front through the array. However, quantitative analysis of the variability of the wind field was complicated both by a lack of data due to mechanical failures of some instruments and by significant differences in the performance of the diverse types of wind recorders. Reevaluation of the instruments used in JASIN and recent comparison of some of these instruments with more conventional sets of wind sensors confirm the possibility that there is significant error in the JASIN wind measurements made from the buoys. In particular, the vector-averaging wind recorder on W2, which was one of the few instruments to recover a full length record and which was chosen during a Seasat-JASIN workshop as the JASIN standard, had performance characteristics that were among the most difficult to explain.

Description: Two examples of high wave number (one cycle per 5 m to one cycle per kilometer) temperature spectra from constant depth tow segments are presented. The first is from observations in the center of the main thermocline of the northwest Atlantic at a depth of about 700 m. Four independent estimates of the spectrum are statistically similar to one another. The result is continuous with previous results at lower wave numbers and compares favorably with the 1975 Garrett and Munk internal wave model prediction. The second example is from a tow through a surface mixed layer, at a depth of 26 m, in a nearby area. In contrast to the above, it describes the lack of stationarity of the near-surface spectrum; a result reminiscent of the frequency spectra observed by Sabinin.

Description: The Vema Channel represents the only major conduit through which the deepest and coldest (〈0.2 °C potential temperature) Antarctic Bottom Water (AABW) flows from the Argentine into the Brazil Basin. From 2003 to 2007 two current meter moorings were present on each side of the Vema Sill, close to the narrowest spot of the Vema Channel. The data from the moorings are compared with earlier current and temperature observations. On average the maximum current core lies ∼100 m above the bottom of the sill with a mean northward speed of 0.3 m s−1. Farther up in the water column where Lower Circumpolar Deep Water and North Atlantic Deep Water prevail, one finds a level of sluggish currents with a southward tendency in the sub-centimeter-per-second range. The lower boundary of a layer of ‘no’ motion was observed at ∼3700 m depth where the mean potential temperature amounts to 1.5 °C. The evolution of the abyssal warming phenomenon over the last decades with notable fluctuations at the choke point between the Argentine and the Brazil Basin differs from the more stable attitude of deep horizontal currents. Starting with CTD observations in 1972 we find a steady increase of temperatures of the coldest AABW in the Vema Channel. This general trend of rising abyssal potential temperatures of almost 2 mKelvin per year is based on mostly annual CTD observations. The overall warming trend is fully compatible with our three-year moored temperature series in agreement with earlier records with high temporal resolution. Distinct frequently fluctuating horizontal current shear between the western and eastern sides of the Vema Sill may be explained by two different catchment areas for AABW at the mouth of the Vema Channel. One pathway originates at the American continental rise and advects bottom water in form of the deep western boundary current. A second pathway is supplied by an eastern boundary current along the Mid Atlantic Ridge in the Argentine Basin. Both source waters merge at the channel entrance, mix, and their respective strengths can alternate within the sill area.

Description: Highlights: • We present results from a current meter array and hydrographic observations on the eastern flank of Reykjanes Ridge, Iceland Basin. • The June 2000–August 2002 average volume transport for the Iceland Scotland Overflow Water (ISOW) plume is 3.8±0.6 Sv. • Our flux estimate compares favorably with historical observations and recent model results. • Downstream drainage of ISOW through Charlie–Gibbs Fracture Zone only accounts for 60% of our ISOW transport estimate • Periods of stronger flow coincide with a higher fraction of ambient water in the ISOW plume. Abstract: Here we present results from a combined moored current meter/hydrography array deployed within the Iceland Scotland Overflow Water (ISOW) plume on the eastern flank of Reykjanes Ridge approximately 1000 km downstream of Faroe Bank Channel (FBC) between June 2000 and August 2002. Based on the array measurements during this period the ISOW plume exhibited a time mean volume transport of 3.8±0.6 Sv (standard error, 1 Sv=106 m3/s). The transport estimate favorably compares with other recent estimates obtained by different methods, confirming that the fate of the ISOW plume downstream of the array is far from being fully understood. Historical observations show that drainage of ISOW through Charlie–Gibbs Fracture Zone (CGFZ) only amounts to 60% of our upstream transport estimate. To date, no reliable transport estimates of the fractions of ISOW recirculating within the Iceland Basin or being drained through fracture zones other than CGFZ do exist. Our observed 2-years-long transport time series show pronounced subseasonal variability with a standard deviation of 1.3 Sv. Simultaneous hydrographic observations reveal, that temporal changes in the strength of the flow go along with changes in the water mass properties. Periods of stronger flow within the ISOW plume coincide with a reduction in salinity.

Description: Highlights: • Open-access online scholarly biodiversity databases are threatened by a lack of funding and institutional support. • Strategic approaches to aid sustainability are summarised. • Issues include database coverage, quality, uniqueness; clarity of Intellectual Property Rights, ownership and governance. • Long-term support from institutions and scientists is easier for high-quality, comprehensive, prestigious global databases. • Larger multi-partner governed databases are more sustainable; i.e. ‘bigger (multi-partner) databases are better’. Abstract: Scientists should ensure that high quality research information is readily available on the Internet so society is not dependant on less authoritative sources. Many scientific projects and initiatives publish information on species and biodiversity on the World Wide Web without users needing to pay for it. However, these resources often stagnate when project funding expired. Based on a large pool of experiences worldwide, this article discusses what measures will help such data resources develop beyond the project lifetime. Biodiversity data, just as data in many other disciplines, are often not generated automatically by machines or sensors. Data on for example species are based on human observations and interpretation. This requires continuous data curation to keep these up to date. Creators of online biodiversity databases should consider whether they have the resources to make their database of such value that other scientists and/or institutions would continue to finance its existence. To that end, it may be prudent to engage such partners in the development of the resource from an early stage. Managers of existing biodiversity databases should reflect on the factors being important for sustainability. These include the extent, scope, quality and uniqueness of database content; track record of development; support from scientists; support from institutions, and clarity of Intellectual Property Rights. Science funders should give special attention to the development of scholarly databases with expert-validated content. The science community has to appreciate the efforts of scientists in contributing to open-access databases, including by citing these resources in the Reference lists of publications that use them. Science culture must thus adapt its practices to support online databases as scholarly publications. To sustain such databases, we recommend they should (a) become integrated into larger collaborative databases or information systems with a consequently larger user community and pool of funding opportunities, and (b) be owned and curated by a science organisation, society, or institution with a suitable mandate. Good governance and proactive communication with contributors is important to maintain the team enthusiasm that launched the resource. Experience shows that ‘bigger is better’ in terms of database size because the resource will have more content, more potential and known uses and users of its content, more contributors, be more prestigious to contribute to, and have more funding options. Furthermore, most successful biodiversity databases are managed by a partnership of individuals and organisations.