There’s more to the picture than meets the eye: Sampling microphytobenthos in a heterogeneous environment

Distributions of microphytobenthos are highly heterogeneous at scales as small as a few centimetres. However, the sampling protocols currently used for the absolute determination of microphytobenthos biomass through chlorophyll a concentration measurements in surface sediments are too limited to take this variability into account, typically relying on 3-5 samples taken within a randomly located 1 m² quadrat in a given environment. We address this issue by objectively and quantitatively inferring the minimum number of samples required to obtain reliable estimates of microphytobenthos biomass on the basis of high-resolution sub-sampling (225 regularly spaced samples) within each of nine 1 m² quadrats at an unvegetated sheltered intertidal sandbank of the Gold Coast (Queensland, Australia). The results were generalised using data obtained in previous studies on an exposed sandy shore and on sheltered estuarine sandy muds of the Eastern English Channel. Estimates of chlorophyll a concentration exhibited a high degree of heterogeneity, both between and within quadrats. The number of samples needed to estimate the average chlorophyll a concentration, and hence mean microphytobenthos biomass with 95% confidence intervals, ranged from 15 to 115, and mainly depended on the presence of global and local gradients within the quadrats. These results have major implications for intertidal ecology by implying a possible systematic bias in the measurement of both microphytobenthos biomass and production of up to 40%. Finally, we emphasise that this issue can be circumvented using field spectrometry or PAM fluorescence measurements coupled with traditional sediment sampling techniques, and urge for unified protocols to be adopted for the routine use of these combined methods.     

The impact of soil moisture–atmosphere coupling on daily maximum surface temperatures in Southeastern South America

Based on a series of experiments conducted by two regional climate models (RCA4 and LMDZ) with and without soil moisture-atmosphere coupling, we investigate the role of soil moisture on the occurrence of surface air temperature extremes and its persistence in Southeastern South America. Our analysis reveals that both factors, soil moisture-atmosphere coupling and relatively drier soil conditions, enhance the temperature extremes. In addition, the existence of soil-atmosphere coupling and the associated soil moisture variability is crucial for the development of the extremes in SESA. The key role of soil-atmosphere coupling is also reflected in the intrinsic persistence of hot days, which is greater in simulations with interactive soil moisture than in those with prescribed soil conditions. In the absence of soil-atmosphere coupling, the imprint of the anomalous dry (and also wet) soil conditions on the intensity and persistence of hot days is weaker.

     

Variation of surface temperature during the last millennium in a simulation with the FGOALS-gl climate system model

A reasonable past millennial climate simulation relies heavily on the specified external forcings, including both natural and anthropogenic forcing agents. In this paper, we examine the surface temperature responses to specified external forcing agents in a millennium-scale transient climate simulation with the fast version of LASG IAP Flexible Global Ocean-Atmosphere-Land System model (FGOALS-gl) developed in the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics (LASG/IAP). The model presents a reasonable performance in comparison with reconstructions of surface temperature. Differentiated from significant changes in the 20th century at the global scale, changes during the natural-forcing-dominant period are mainly manifested in the Northern Hemisphere. Seasonally, modeled significant changes are more pronounced during the wintertime at higher latitudes. This may be a manifestation of polar amplification associated with sea-ice-temperature positive feedback. The climate responses to total external forcings can explain about half of the climate variance during the whole millennium period, especially at decadal timescales. Surface temperature in the Antarctic shows heterogeneous and insignificant changes during the preindustrial period and the climate response to external forcings is undetectable due to the strong internal variability. The model response to specified external forcings is modulated by cloud radiative forcing (CRF). The CRF acts against the fluctuations of external forcings. Effects of clouds are manifested in shortwave radiation by changes in cloud water during the natural-forcing-dominant period, but mainly in longwave radiation by a decrease in cloud amount in the anthropogenic-forcing-dominant period.     

The quasi-stationary feature of nocturnal precipitation in the Sichuan Basin and the role of the Tibetan Plateau

The nocturnal precipitation in the Sichuan Basin in summer has been studied in many previous works. This paper expands the study on the diurnal cycle of precipitation in the Sichuan Basin to the whole year. Results show that the nocturnal precipitation has a specific quasi-stationary feature in the basin. It occurs not only in summer but also in other three seasons, even more remarkable in spring and autumn than in summer. There is a prominent eastward timing delay in the nocturnal precipitation, that is, the diurnal peak of precipitation occurs at early-night in the western basin whereas at late-night in the center and east of the basin. The Tibetan Plateau plays an essential role in the formation of this quasi-stationary nocturnal precipitation. The early-night peak of precipitation in the western basin is largely due to strong ascending over the plateau and its eastern lee side. In the central and eastern basin, three coexisting factors contribute to the late-night peak of precipitation. One is the lower-tropospheric southwesterly flow around the southeastern edge of the Tibetan Plateau, which creates a strong cyclonic rotation and ascendance in the basin at late-night, as well as brings abundant water vapor. The second is the descending motion downslope along the eastern lee side of the plateau, together with an air mass accumulation caused by the warmer air mass transport from the southeast of the Yunnan-Guizhou Plateau, creating a diabatic warming at low level of the troposphere in the central basin. The third is a cold advection from the plateau to the basin at late-night, which leads to a cooling in the middle troposphere over the central basin. All these factors are responsible for precipitation to occur at late-night in the central to eastern basin.     

Interannual variability of deep convection in the Northwestern Mediterranean simulated with a coupled AORCM

A hindcast experiment of the Mediterranean present-day climate is performed using a fully-coupled Atmosphere–Ocean Regional Climate Model (AORCM) for the Mediterranean basin. The new model, called LMDz-NEMO-Med, is composed of LMDz4-regional as atmospheric component and of NEMOMED8 as oceanic component. This AORCM equilibrates freely, without any flux adjustment, neither in fresh water nor in heat. At its atmospheric lateral boundary conditions, it is driven by ERA-40 data from 1958 to 2001, after a spin-up of 40 years in coupled configuration. The model performance is assessed and compared with available observational datasets. The model skill in reproducing mean state and inter-annual variability of main atmospheric and oceanic surface fields is in line with that of state-of-the-art AORCMs. Considering the ocean behaviour, the inter-annual variations of the basin-scale heat content are in very good agreement with the observations. The model results concerning salt content could not be adequately validated. High inter-annual variability of deep convection in the Gulf of Lion is simulated, with 53 % of convective winters, representative of the present climate state. The role of different factors influencing the deep convection and its inter-annual variability is examined, including dynamic and hydrostatic ocean preconditioning and atmospheric surface forcing. A conceptual framework is outlined and validated in linking the occurrence of deep convection to the efficiency of the integrated surface buoyancy fluxes along the winter season to mix the initially stratified averaged water column down to the convective threshold depth. This simple framework (based only on 2 independent variables) is able to explain 60 % (resp. 69 %) of inter-annual variability of the deep water formation rate (resp. maximum mixed layer depth) for the West Mediterranean Deep Water (WMDW) formation process.     

The Mid-Infrared View of Star Formation in Collisional Ring Galaxies

We present mid-infrared images and spectra of Arp 10 and Arp 118, two collisional ring galaxies observed as part of the ISOCAM GT program CAMACTIV (P.I.: I.F. Mirabel). The observations reveal the distribution of hot dust in the galaxies and enable us to probe the mechanisms responsible for the heating of the ISM. Unlike the peculiar mid-infrared colours recently discovered in the Cartwheel, the prime example of a collisional ring, Arp 10 has colours typical of those found inspiral arms of late type spirals. Similarly for Arp 118 (NGC 1144),the mid-infrared emission is associated with the regions of star formation in the ring. Moreover, a hot continuum in the 5.1–6.7 μmrange, which is a typical mid-infrared signature of an AGN, is clearly detected from the Seyfert 2 nucleus of the galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gas‐melt interactions and their bearing on chondrule formation

Abstract— Interactions between nebular gas and molten silicates or oxides could have played a major role in the formation and differentiation of the first solids formed in the solar system. In order to simulate such interactions, we set up a new experimental device in which isothermal condensation experiments have been conducted. Partially molten chondrule‐like samples have been exposed to high SiO(g) partial pressures, for intervals between 80 and 300 s and at temperatures ranging from 1600 to 1750 K. Results show that silica entering from the gas phase could be responsible for several textural and mineralogical features observed in natural chondrules. For instance, these experiments reproduce not only the mineralogical zonation of porphyritic olivine‐pyroxene chondrules with the peripheral location of pyroxenes, but also olivine resorption textures and the common poikilitical enclosure of olivines in pyroxenes. In the light of these similarities, we advocate that gas‐melt interactions through condensation are viable mechanisms for chondrule formation and hence may place severe constraints on the history of these primitive objects. In the nebula, high SiO(_g) partial pressures could have been established by the volatilization of regions with high dust/gas ratio. A possible scenario for this stochastic thermal activity is the intense activity of the protosun in its young stellar object phase.     

Helioseismology of Time-Varying Flows Through The Solar Cycle

Flows in the upper convection zone are measured by helioseismology on a wide variety of scales. These include differential rotation and meridional circulation, local flows around complexes of magnetic activity and sunspots, and convective flows. The temporal evolution of flows through cycle 23 reveals connections between mass motions in the solar interior and the large-scale characteristics of the magnetic cycle. Here I summarize the latest observations and their implications. Observations from local helioseismology suggest that subsurface flows around active regions introduce a solar-cycle variation in the meridional circulation.     

Styles of back-arc extension in the Central Mediterranean

In the Central Mediterranean two back-arc basins, the Liguro-Provençal (LPb) and the Tyrrhenian basin (Tb), opened progressively and consecutively from the late Eocene–Oligocene to the present. Evolution in space and time of rifting and drifting processes, along three different transects across these basins, shows differences in the style of extension: LPb opened with the formation of a narrow, single rift, while in the Tb deformation and magmatism is spread over a wide area. Moreover at the Northern end of the Tb the locus of extension progressively migrated towards the east whereas in the Southern Tb the locus of extension and magmatism migrated inside the basin, inducing continental break-up and drifting of the previously formed older conjugate basins. We propose that these different styles of back-arc extension depend upon internal conditions, such as prerift rheology linked with its geological heritage, and external conditions, e.g. the style of subduction.     

Reconstructing 4000 years of mass movement and tsunami history in a deep peri‐Alpine lake (Lake Geneva,France‐Switzerland)

The study of mass movements in lake sediments provides insights into past natural hazards at historic and prehistoric timescales. Sediments from the deep basin of Lake Geneva reveal a succession of six large‐scale (volumes of 22 × 10⁶ to 250 × 10⁶ m³) mass‐transport deposits, associated with five mass‐movement events within 2600 years (4000 cal bp to 563 ad ). The mass‐transport deposits result from: (i) lateral slope failures (mass‐transport deposit B at 3895 ± 225 cal bp and mass‐transport deposits A and C at 3683 ± 128 cal bp ); and (ii) Rhône delta collapses (mass‐transport deposits D to G dated at 2650 ± 150 cal bp , 2185 ± 85 cal bp , 1920 ± 120 cal bp and 563 ad , respectively). Mass‐transport deposits A and C were most probably triggered by an earthquake, whereas the Rhône delta collapses were likely to be due to sediment overload with a rockfall as the external trigger (mass‐transport deposit G, the Tauredunum event in 563 ad known from historical records), an earthquake (mass‐transport deposit E) or unknown external triggers (mass‐transport deposits D and F). Independent of their origin and trigger mechanisms, numerical simulations show that all of these recorded mass‐transport deposits are large enough to have generated at least metre‐scale tsunamis during mass movement initiation. Since the Tauredunum event in 563 ad , two small‐scale (volumes of 1 to 2 × 10⁶ m³) mass‐transport deposits (H and I) are present in the seismic record, both of which are associated with small lateral slope failures. Mass‐transport deposits H and I might be related to earthquakes in Lausanne/Geneva (possibly) 1322 ad and Aigle 1584 ad , respectively. The sedimentary record of the deep basin of Lake Geneva, in combination with the historical record, show that during the past 3695 years, at least six tsunamis were generated by mass movements, indicating that the tsunami hazard in the Lake Geneva region should not be neglected, although such events are not frequent with a recurrence time of 0·0016 yr^?1.