The marine barite saturation state of the world's oceans

This paper addresses the question of the eventual control of barium concentration in seawater by an equilibrium with barite. For this, we have used a new thermodynamic model to compute the barite saturation index of ocean waters, mainly from GEOSECS data. Our results show that equilibrium between barite and seawater is reached in a number of places: cold surface waters of the Southern Ocean, waters at intermediate depths (2000–3500 m) in the Pacific, deep waters (2000–3500 m) of the Gulf of Bengal. The only samples for which a slight barite supersaturation is found are the surface waters at GEOSECS station G89 in the Weddell Gyre. Besides these locations, the rest of the world's oceans is undersaturated, as was established by Church and Wolgemuth [Church, T.M., Wolgemuth, K., 1972. Marine barite saturation, Earth Planet. Sci. Lett. 15 35–44.]. There is a return to undersaturation of the water column at depths of about 3500 m in the Pacific and of about 2500 m in the Southern Ocean. The reverse is found for GEOSECS station 446 in the Gulf of Bengal for which the highest Ba concentrations can be found at depth: surface waters are undersaturated and equilibrium is reached below 2000 m. Finally, we briefly discuss the role of biogenic and inorganic processes on barite formation in the ocean as well as the influence of strontium substitution in marine barites.     

Characterizing uncertainties for quantifying bathymetry change between time-separated multibeam echo-sounder surveys

Changes of bathymetry derived from multibeam sonars are useful for quantifying the effects of many sedimentary, tectonic and volcanic processes, but depth changes also require an assessment of their uncertainty. Here, we outline and illustrate a simple technique that aims both to quantify uncertainties and to help reveal the spatial character of errors. An area of immobile seafloor is mapped in each survey, providing a common ‘benchmark’. Each survey dataset over the benchmark is filtered with a simple moving-averaging window and depth differences between the two surveys are collated to derive a difference histogram. The procedure is repeated using different length-scales of filtering. By plotting the variability of the differences versus the length-scale of the filter, the different effects of spatially uncorrelated and correlated noise can be deduced. The former causes variability to decrease systematically as predicted by the Central Limit Theorem, whereas the remaining variability not predicted by the Central Limit Theorem then represents the effect of spatially correlated noise. Calculations made separately for different beams can reveal whether problems are due to heave, roll, etc., which affect inner and outer beams differently. We show how the results can be applied to create a map of uncertainties, which can be used to remove insignificant data from the bathymetric change map. We illustrate the technique by characterizing changes in nearshore bed morphology over one annual cycle using data from a subtidal bay, bedrock headland and a banner sand bank in the Bristol Channel UK.     

Does the Phaeocystis bloom affect the diel migration of the suprabenthos community?

The suprabenthos comprises all bottom-dependent animals, mainly crustaceans (including decapods and peracarids), which perform--with varying amplitude, intensity and regularity--seasonal or daily vertical migrations above the sea floor. The presence of organisms in the Benthic Boundary Layer is determined by two general factors: (1) organism behaviour, which depends on the light penetration in the water column and (2) boundary-layer hydrodynamics. In the coastal zone of the eastern English Channel, during the spring Phaeocystis bloom, the presence of gelatinous colonies modifies the penetration of light in the water column, which may seriously affect the abundance and/or the behaviour of the suprabenthos community. To clarify this point, 19 suprabenthic hauls were taken with a modified Macer-GIROQ sledge both during the day and during the night, from March to June 2002 (i.e., before, during and after the bloom). Two sites, located in the coastal and offshore areas of the Ophelia medium sand macrobenthic community were investigated. The bloom had no effect on species richness and abundance in either site. However, the diel migrations of some dominant species--such as the cumaceans Pseudocuma longicornis and Pseudocuma similis, the mysid Gastrosaccus spinifer and the amphipod Stenothoe marina--were modified. During the bloom, diurnal and nocturnal suprabenthic abundances were similar, and in the absence of bloom, species remained benthic during the day. The permanent presence of suprabenthic species in the Benthic Boundary Layer could have a consequence on their predation by fish (mainly juveniles which preferentially consume small crustaceans in their diet), unless fish behaviour and predation efficiency--especially for visual predators--are also disturbed by changes in light intensity.     

Simulated responses of Pinus halepensis forest productivity to climatic change and CO2 increase using a statistical model

Tree ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate impacts of climatic change and CO₂ increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east of France). A chronology of net primary productivity (NPP) both for the 20th century and for each stand was estimated using tree ring data (width and density). The response of each stand to climate in terms of NPP was statistically modelled using response functions. Anomalies between estimated NPP and NPP reconstructed by response functions were calculated to evaluate the fertilising effect of CO₂ increase on tree growth. The changes in anomalies during the 20th century were attributed to the effect of CO₂ increase. A multiplying factor (β) linking CO₂ concentration and stand productivity was then calculated, on the basis of the trend observed during the 20th century. In this study, the value of the β factor obtained under natural conditions (β=0.50) is consistent with those from controlled CO₂ enrichment experiments. Both response functions and the β factor were used to predict NPP changes for a 2×CO₂ scenario. The 2×CO₂ climate was obtained using predictions from Météo France's ARPEGE atmospheric general circulation model (AGCM) downscaled to Marseilles meteorological station. NPP increased significantly for nine stands solely when the climatic effect was taken into account. The main factors responsible for this enhancement were increased winter and early spring temperatures. When the fertilising effect of the CO₂ increase was added, NPP was significantly enhanced for 14 stands (i.e. NPP enhancement ranged from 8% to 55%). Although the effects of global change were slightly detectable during the 20th century, their acceleration is likely to lead to great changes in the future productivity of P. halepensis forests.     

Pleistocene northwards fold propagation of the Jura within the southern Upper Rhine Graben: seismotectonic implications

On the basis of the geophysical (seismic profiles and electric tomography), geomorphic and geological data, we re-evaluate the post-Pliocene structural interpretation of the southern Upper Rhine graben (Basel–Mulhouse area): we demonstrate a Plio–Pleistocene northward propagation of the Jura thrust and fold belt up to Mulhouse proceeding from a succession of four 10 km apart ramps (from north to south Ferrette, Muespach, Magstatt and Rixheim) rooted within the late Triassic evaporitic marls acting as a decollement. This domain was previously considered as having undergone an on-going continuous extension (horst of Mulhouse bounded by the Quaternary Sirentz and Dannemarie grabens).The Quaternary activity of this thin-skinned tectonics induces the growth of a sedimentary wedge whose regional slope, which comprised between 1.4° and 1° to the north, also attests to a low friction basal detachment. More into details, these ramps correspond to 40–50-m high jumps within the forward topographic slope. Pleistocene activity is suggested just above the Muespach ramp by the presence of a 5–10-m north-facing scarp corresponding in depth to a 3-m vertical offset of early Pleistocene alluvial deposits. Farther to the north, a stronger incision of the Rhine Würm terrace can be interpreted as the result of the growth of the Mulhouse–Rixheim frontal ramp.This northward propagation of the Jura thrust and fold belt is strongly controlled by the Oligocene structural inheritage. The development of the frontal ramp in Mulhouse has to be related to the Oligocene significant vertical offset of the Triassic evaporite along the Mulhouse Railway Station fault preventing a propagation of the decollement farther to the north. In the same way, the fold propagation is laterally segmented by the N20°E trending Oligocene fabrics (from East to West, Rhine Valley flexure fault, Allschwil–Istein fault system and Illfurth fault) which acts above the decollement as lateral ramps. To the west, the development of a shallow anticline along the Illfurth fault suggests that the thin-skinned propagation is oblique with respect to the Oligocene fabrics. It results in spacial contrast between a left-lateral-reverse and a right-lateral–normal shallow kinematics along the western and eastern lateral ramps, respectively. In depth to the east, it also induces a vertical contrast between shallow (right-lateral–normal) and deep (left-lateral given by fault plane solutions) kinematics along the Istein–Allschwill–Rhine Valley fault system.Few arguments supporting a nucleation of the Basel-1356 earthquake, the strongest event in NW Europe in the last thousand years, onto the Rhine Valley fault system beneath the decollement have been given. However, we emphasize that the above mentioned coeval thin (aseismic)- and thick (seismic)-skinned tectonics along the Istein–Allschwill–Rhine Valley fault system would make difficult both the identification and the interpretation of the surface rupture of the Basel-1356 earthquake.     

Virioplankton distribution and activity in a tropical eutrophicated bay

The study of lysogeny in aquatic systems is an often overlooked aspect of microbial ecology, especially in tropical environments. Herein, the fraction of lysogenized cells (FLC) was detected in the surface waters of 20 coastal stations distributed from the eutrophicated shoreline to seaward waters of Hann Bay (Senegal). Concurrently, viral lytic infection rates were extrapolated from the frequency of visibly infected bacterial cells (FVIC), as determined from transmission electron microscopy observations. The experimental induction of prophage was observed in less than 3% of indigenous marine bacteria, suggesting that lysogenic stages of infection are rare in Hann Bay. Similarly, only 0.5–4.7% of bacteria showed visible signs of lytic infection. However, the positive correlation between the fraction of lysogenic and lytic cells (r = 0.67, p < 0.05, n = 20) may actually indicate that the coexistence of both lifestyles may be due to the massive and rapid induction of lysogens, potentially from the high levels of local UV radiation. Overall, we suggest that the determination of FVIC and FLC to examine the predominance of one type of cycle versus the other may be a source of misinterpretation in some particular aquatic environments.     

Influence of orographic precipitation on the topographic and erosional evolution of mountain ranges

The influence of climate on mountain denudation has been the topic of an intense debate for two decades. This debate partly arises from the covariation of rainfall and topography during the growth of mountain ranges, both of which influence denudation. However, the denudational response of this co-evolution is poorly understood. Here, we use a landscape evolution model where the rainfall evolves according to a prescribed rainfall–elevation relationship. This relationship is a bell curve defined by a rainfall base level, a rainfall maximum and a width around the rainfall peak elevation. This is a first-order model that fits a large range of orographic rainfall data at the ca. 1-km spatial scale. We carried out simulations of an uplifting block with an alluvial apron, starting from an initially horizontal surface, and testing different rainfall–elevation relationships. We find that the denudation history is different from that with constant rainfall models. The results essentially depend on the ratio between the final steady-state summit elevation H_ss and the prescribed rainfall peak elevation H_p. This ratio is hard to predict because it depends on the transient coupling of rainfall and elevation. We identified three types of results according to H_ss/H_p. If H_ss/H_p > 4 (Type I), the denudation rates peak when the summits reach values close to H_p. If H_ss/H_p > 1.5 and < 4 (Type II), the denudation is strongly accelerated when the elevation of the summits approaches H_p, and then the denudation increases slowly towards the uplift rate. If H_ss/H_p < 1.5 (Type III), the denudation evolution is similar to situations with constant and homogeneous rainfall. In the Type I and II experiments, the mountain top is subjected to aridification once the summits have passed through H_p. To adapt to this reduced rainfall, the slopes increase. This can lead to a paradoxical situation where the mountain relief increases faster, whereas the denudation increases more slowly. The development of orographic precipitation may thus favour the stability of the mean denudation rate in a rising mountain. Despite the model limitations, including a constant rainfall–elevation relationship, our study suggests that the “classical” exponential increase in the denudation rate predicted by constant rainfall models is not the common case. Instead, the common case involves pulses and acceleration of the denudation even in the absence of uplift or global climate variations.     

Decadal oscillations in a simplified coupled model due to unstable interactions between zonal winds and ocean gyres

A simplified coupled ocean–atmosphere model, consisting of a one-layer bidimensional ocean model and a one-layer unidimensional energy balance atmospheric model [J. Clim. 13 (2000) 232] is used to study the unstable interactions between zonal winds and ocean gyres. In a specific range of parameters, decadal variability is found. Anomalies, quite homogeneous zonally, show small-scale wavelength in latitude: perturbations emerge and grow at the southern limb of the intergyre boundary and propagate southward before decaying. The wind stress anomalies are proportional to the meridional gradient of the atmospheric temperature anomalies: this ratio acts as a positive amplification factor, as confirmed by a parameter sensitivity analysis. Assuming zonally-averaged anomalies harmonic in the meridional direction, a very simple analytical model for the perturbations is derived, based on forced Rossby wave adjustment of the western boundary current and its associated anomalous heat transport: it accounts for the scale selection, the growth and the southward propagation of sea surface temperature anomalies in the subtropical gyre. The latter is not only due to the slow advection by the mean current, but to a prevailing mechanism of self-advecting coupled oceanic and atmospheric waves, out of phase in latitude. Relevance to the observational record is discussed.     

Simulating the Holocene climate evolution at northern high latitudes using a coupled atmosphere-sea ice-ocean-vegetation model

The response of the climate at high northern latitudes to slowly changing external forcings was studied in a 9,000-year long simulation with the coupled atmosphere-sea ice-ocean-vegetation model ECBilt-CLIO-VECODE. Only long-term changes in insolation and atmospheric CO₂ and CH₄ content were prescribed. The experiment reveals an early optimum (9–8 kyr BP) in most regions, followed by a 1–3°C decrease in mean annual temperatures, a reduction in summer precipitation and an expansion of sea-ice cover. These results are in general agreement with proxy data. Over the continents, the timing of the largest temperature response in summer coincides with the maximum insolation difference, while over the oceans, the maximum response is delayed by a few months due to the thermal inertia of the oceans, placing the strongest cooling in the winter half year. Sea ice is involved in two positive feedbacks (ice-albedo and sea-ice insulation) that lead regionally to an amplification of the thermal response in our model (7°C cooling in Canadian Arctic). In some areas, the tundra-taiga feedback results in intensified cooling during summer, most notably in northern North America. The simulated sea-ice expansion leads in the Nordic Seas to less deep convection and local weakening of the overturning circulation, producing a maximum winter temperature reduction of 7°C. The enhanced interaction between sea ice and deep convection is accompanied by increasing interannual variability, including two marked decadal-scale cooling events. Deep convection intensifies in the Labrador Sea, keeping the overall strength of the thermohaline circulation stable throughout the experiment.