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.     

Control of deep convection by sub-cloud lifting processes: the ALP closure in the LMDZ5B general circulation model

Recently, a new conceptual framework for deep convection scheme triggering and closure has been developed and implemented in the LMDZ5B general circulation model, based on the idea that deep convection is controlled by sub-cloud lifting processes. Such processes include boundary-layer thermals and evaporatively-driven cold pools (wakes), which provide an available lifting energy that is compared to the convective inhibition to trigger deep convection, and an available lifting power (ALP) at cloud base, which is used to compute the convective mass flux assuming the updraft vertical velocity at the level of free convection. While the ALP closure was shown to delay the local hour of maximum precipitation over land in better agreement with observations, it results in an underestimation of the convection intensity over the tropical ocean both in the 1D and 3D configurations of the model. The specification of the updraft vertical velocity at the level of free convection appears to be a key aspect of the closure formulation, as it is weaker over tropical ocean than over land and weaker in moist mid-latitudes than semi-arid regions. We propose a formulation making this velocity increase with the level of free convection, so that the ALP closure is adapted to various environments. Cloud-resolving model simulations of observed oceanic and continental case studies are used to evaluate the representation of lifting processes and test the assumptions at the basis of the ALP closure formulation. Results favor closures based on the lifting power of sub-grid sub-cloud processes rather than those involving quasi-equilibrium with the large-scale environment. The new version of the model including boundary-layer thermals and cold pools coupled together with the deep convection scheme via the ALP closure significantly improves the representation of various observed case studies in 1D mode. It also substantially modifies precipitation patterns in the full 3D version of the model, including seasonal means, diurnal cycle and intraseasonal variability.     

喀喇昆仑断裂带走滑过程中伴随的快速隆升作用:热年代学证据

喀喇昆仑断裂带是青藏高原西部的一条大型右旋走滑断裂带,它是喜马拉雅山脉西段北侧重要的地质边界.本文在岩石学、变形构造的研究基础上,对喀喇昆仑断裂带东南段阿伊拉日居山-噶尔盆地地区的喀喇昆仑韧性剪切带中变质岩石的同构造矿物进行了40Ar/39Ar热年代学研究.显微构造研究表明,剪切带中的矿物记录了从高温(>600℃)到低温(<250℃)条件下的连续变形,表现为近水平的右旋剪切运动转变成斜向的右旋正滑,使绿片岩相的变形作用叠加在中-高温变形之上.暗示出走滑过程中存在隆升作用,热年代学结果显示其连续剪切变形作用从早中新世以来至少持续到4Ma,并且出现三个快速冷却阶段:第一个快速冷却阶段为从25～22Ma到21～18Ma期间,可能代表的是浅部高温剪切过程中变形局部停止或减慢的过程;第二个快速冷却时期为从15Ma到12～10Ma,是喀喇昆仑断裂带走滑过程中,阿伊拉日居山的快速隆升、噶尔盆地开始形成以及主要河流深切过程阶段;9Ma以来是第三个快速冷却过程,使阿伊拉日居山脉进一步快速隆升、噶尔盆地定形过程.根据不同年代地表地貌特征的右旋错位距离以及不同层次变形特征,估算出喀喇昆仑断裂带长期滑移速率为8～10mm/a,伴随的隆升速率为1mm/a.从显微构造和热年代学证据表明,晚第四纪以来该断裂经历了强烈的右旋走滑运动的同时伴随强烈的隆升作用.     

The outgassing and composition of Comet 19P/Borrelly from radio observations

Radio spectroscopic observations of Comet 19P/Borrelly were performed during the 1994 apparition and at, and near, the time of the Deep Space 1 flyby in 2001. HCN, CS, CH₃OH, and H₂CO were detected using the 30-m telescope of the Institut de Radioastronomie Millimétrique and the James Clerk Maxwell Telescope, and their production rates relative to water are estimated to be 0.06-0.11, 0.07, 1.7, and 0.4%, respectively. Only upper limits are derived for H₂S and CO. The upper limit for CO/H₂O (<15%) is not very constraining, while the upper limit for the H₂S/H₂O ratio of 0.45% is near the bottom of the range of values measured for other comets. Observations of the OH radical at the Nançay radio telescope provide water production rates a few weeks before the 1994 and 2001 perihelia. Observations of the 1₁₀-1₀₁ water line at 557 GHz with the Odin satellite yield a water production rate of (2.5±0.5)×10²⁸ s⁻¹ on September 22, 2001, at the time of the Deep Space 1 encounter, and (3.3±0.6)×10²⁸ s⁻¹ averaged over the September 22-24, 2001 period. The line shapes are asymmetric and blueshifted by V₀∼−0.18 km s⁻¹ for the best observed HCN lines recorded one week after perihelion. The HCN line shapes, and the similar OH and HCN velocity shifts over the September-November 1994 and August-September 2001 periods, favor anisotropic outgassing towards the Sun. Strong outgassing directed along the primary dust jet seen on visible images is not excluded by the HCN line shapes, but unrealistically high gas expansion velocities are required to explain the line shapes in that case.     

Formation of todorokite from vernadite in Ni-rich hemipelagic sediments

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of . The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg²⁺ cations. The average structural formula of 10Å-vernadite is if the layers contain vacancy sites, or alternately , if they contain Mn³⁺. The average formula of todorokite is .A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg²⁺ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg²⁺ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg²⁺ (i.e., high layer charge), or Mn³⁺ and Mn⁴⁺ cations ordered following the Mn³⁺-Mn⁴⁺-Mn⁴⁺ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.     

The wider context of the Lower Jurassic Toarcian oceanic anoxic event in Yorkshire coastal outcrops,UK

The Toarcian Oceanic Anoxic Event (T-OAE, ～183 Ma) was characterized by enhanced carbon burial, a prominent negative carbon-isotope excursion (CIE) in marine carbonate and organic matter, and numerous geochemical anomalies. A precursor excursion has also been documented at the Pliensbachian/Toarcian boundary, but its possible causes are less constrained. The T-OAE is intensively studied in the Cleveland Basin, Yorkshire, UK, whose sedimentary deposits have been litho-, bio- and chemostratigraphically characterised. Here, we present new elemental data produced by hand-held X-ray fluorescence analysis to test the expression of redox-sensitive trace metals and detrital elements across the upper Pliensbachian to mid-Toarcian of the Cleveland Basin. Detrital elemental concentrations (Al, Si, Ti, Zr) are used as proxies for siliciclastic grain content and thus, sea-level change, which match previous sequence stratigraphic interpretations from the Cleveland Basin. The timescale of the event is debated, though our new elemental proxies of relative sea level change show evidence for a cyclicity of 350 cm that may be indicative of ～405 kyr eccentricity cycles in Yorkshire. Trends in total organic carbon and redox-sensitive elements (S, Fe, Mo, As) confirm scenarios of widespread ocean deoxygenation across the T-OAE. The correlation of comparable trends in Mo across the T-OAE in Yorkshire and the Paris Basin suggests a similar oceanic drawdown of this element accompanying widespread anoxia in the two basins. Data from Yorkshire point to a transgressive trend at the time of the Mo drawdown, which contradicts the “basin restriction” model for the euxinic conditions that characterise the CIE interval.     

The Characterization of a Former World War I Battlefield by Integrating Multiple Signals from a Multireceiver EMI Soil Sensor

World War I (WWI) battlefields contain a large diversity of buried features that are situated just below the plough layer. A noninvasive interdisciplinary approach is preferred to characterize the buried heritage embedded within its soil landscape. In this study, the potential of integrating both electrical conductivity and magnetic susceptibility measurements from a multireceiver electromagnetic induction (EMI) instrument at a heavily shelled WWI battlefield was evaluated. Primarily, both measurements allowed the attribution of approximate depth interval to the buried trench remains. These remains proved dominantly present within the 0.4–0.8 m depth interval. Subsequently, both apparent electrical conductivity and magnetic susceptibility measurements were separately integrated to improve the contrast between buried features and their surrounding background. Trenches that were ditches filled with topsoil material could be distinguished from trenches that were continuously or partially filled with metal reinforcements or debris. To conclude, integrating multiple signals from a multireceiver EMI system allowed for a more straightforward discrimination of the buried WWI relicts.     

Observations of Saturn's outer ring and new satellites during the 1980 edge-on presentation

Observations of Saturn's satellites and external rings during the 1980 edge-on presentation were obtained with a focal coronograph. A faint satellite traveling in the orbit of Dione and leading it by 72° has been detected, together with the two inner satellites already suspected (cf. J. W. Fountain and S. M. Larson, 1978,Icarus36, 92–106). The external ring has been observed on both east and west sides; it may extend up to $\text{?8.3}$ Saturn radii, and appears structured.