Nutrient irrigation of the North Atlantic

The North Atlantic, as all major oceans, has a remarkable duality in primary production, manifested by the existence of well-defined high and low mean primary production regions. The largest region is the North Atlantic Subtropical Gyre (NASTG), an anticyclone characterized by bowl shaped isopycnals and low production. The NASTG is surrounded at its margins by smaller cyclonic high-production regions, where these isopycnals approach the sea surface. The most extensive cyclonic regions are those at the latitudinal extremes, i.e. the subpolar and tropical oceans, though smaller ones do occur at the zonal boundaries. In this article we review historical data and present new analyses of climatological data and a selected number of hydrographic cruises in the western/northwestern and eastern/southeastern boundaries of the NASTG, with the objective of investigating the importance of upward epipycnal advection of nutrient-rich subsurface layers (irrigation) in maintaining high primary production in the euphotic layers. In the North Atlantic Subpolar Gyre (NASPG) irrigation implies intergyre exchange caused by the outcropping extension of the Gulf Stream (GS), following the formation of the deep winter mixed-layer. In the eastern boundary of the NASTG irrigation is attained through a permanent upwelling cell, which feeds the Canary Upwelling Current (CUC). In the southeastern corner irrigation occurs in fall, when the Guinea Dome (GD) is reinforced, and in winter, when the CUC reaches its southernmost extension. Other characteristics of the north/south extension of the GS/CUC are the seasonal nutrient replenishing of subsurface layers (spring restratification of NASPG and winter relaxation of the GD) and the maintenance of high levels of diapycnal mixing during the last phase of nutrient transfer to the euphotic layers. Off the Mid-Atlantic Bight the GS transports a total of about 700 kmol s⁻¹ of nitrate, with almost 100 kmol s⁻¹ carried in the surface (σ_θ < 26.8) layers and some 350 kmol s⁻¹ in the intermediate (26.8 < σ_θ < 27.5) layers. A box model suggests that north of Cape Hatteras most surface and upper-thermocline nitrates are used to sustain the high levels of primary production in the NASPG. Off Cape Blanc there is winter along-shore convergence of order 10 kmol s⁻¹ of nitrate in the near-surface layers (possibly larger in summer), with only a small fraction used to sustain local primary production in the coastal upwelling band and the remainder carried to the interior ocean. Nutrients and biomass exported from these cyclonic regions may account for the concentration levels observed within the NASTG.     

Fluctuations and Variations in Coastal Marine Environments

Abstract. The manifold aspects of variability of coastal marine ecosystems are reviewed, with special reference to the response of species and communities. Fluctuations are defined as recurrent modifications in community structure, for instance due to seasonality, whereas variations are defined as changes permanently leading from a given community to a different one. Both patterns are described and linked to the interaction between life history traits and environmental factors. Resting stages are proposed to play a still underestimated role in the dynamics of coastal communities, especially for plankton. Rare species are considered as an essential constant of communities, being the source of potential variation and alternance in the contribution to the standing biomass.     

Photoacclimation of the invasive alga Caulerpa racemosa var. cylindracea to depth and daylight patterns and a putative new role for siphonaxanthin

The introduced green alga Caulerpa racemosa var. cylindracea grows along a broad depth range (from very shallow to 60 m depth) in the Mediterranean basin. In the present work, the photoacclimation capacity of this invasive variety was investigated in summer, the season of its maximum spread. Natural populations from the Gulf of Naples (Italy) were analyzed for photoresponse on two scales of light variability: a spatial scale (at three stations along a depth gradient, from 0.3 to 20 m depth) and a temporal scale (on the shallowest meadow, from sunrise to sunset). These responses were studied through pigment analysis (with HPLC), and in situ measurements of photosynthetic parameters (with a Diving‐PAM fluorometer). Electron transport rate (ETR)–irradiance curve parameters showed acclimation along environmental gradients dominated by variation in irradiance. In the shallowest plants, the lack of a midday depression in both the maximum relative ETRs and the photosynthetic efficiency at sub‐saturating irradiance (α) pointed to a maintenance of energy conversion levels despite the protective lowering of light‐harvesting efficiency revealed by the trend in F_v/F_m. On the other side, variation of photosynthetic efficiency occurred with depth and buffered the effect of decreasing light on maximum photosynthetic rates. A previously undescribed xanthophyll cycle centred on lutein‐siphonaxanthin interconversion appeared to operate in the shallowest populations in addition to the violaxanthin/antheraxanthin/zeaxanthin cycle commonly occurring in Chlorophyta; this would further enhance phototolerance of the alga. A further role of siphonaxanthin is in the acclimation to low light of deep environments as indicated by its stronger increase from the surface to the deepest station with respect to siphonein and chlorophyll b.     

华南泥盆-石炭系界线剖面旋回地层学研究

泥盆-石炭纪过渡期是地球演化过程中的一个重要阶段,发生了地球气候系统从温室向冰室转型和生物灭绝等重大事件。准确且高分辨率的年代格架是理解这些事件的基础。本文利用高分辨率磁化率作为古气候替代指标,对贵州省长顺县睦化Ⅱ剖面和大坡上剖面以及广西壮族自治区南丹县度里剖面的泥盆-石炭系界线附近深水沉积序列进行旋回地层学研究。识别出受到短偏心率、斜率和岁差控制的沉积旋回厚度分别为1.092.39、0.290.73、0.180.51 m。利用轨道短偏心率进行天文校准,结果证实泥盆纪晚期短偏心率、斜率、岁差周期分别为约100、32.227.2和20.915 ka。利用大坡上剖面已知的U-Pb放射性同位素年龄和天文校准结果建立高精度浮动天文年代标尺并进行旋回地层对比。确定了泥盆-石炭系界线附近牙形类生物带和Hangenberg生物灭绝事件的持续时间,并计算出泥盆-石炭系界线年龄为(359.58±1.9) Ma。     

The response of lake margin sedimentary systems to climatically driven lake level fluctuations: Middle Devonian,Orcadian Basin,Scotland

Lake margin sedimentary systems can provide highly sensitive records of sedimentary response to climate change. The Middle Old Red Sandstone of Northern Scotland comprises a thick succession of cyclic lacustrine sediments. Within this succession the deepest lake phase, the Achanarras fish bed, allows bed‐scale correlation over 160 km across the basin. This provides a unique opportunity to examine the character of synchronous lake margin deposits, and their response to climatically driven lake level fluctuations, across a large continental basin. Detailed characterization of two separate lake margin systems was carried out utilizing multiple sections in western Orkney, in the north, and Easter Ross, in the south. Seven facies have been recognized, which include upper and lower shoreface, deep lake, shallow lake, playa, turbidite and fluvial facies. Differences in vertical and lateral facies stacking patterns reflect the response of these systems to climatically driven fluctuations in lake level. Comparison of the northern and southern systems examined highlights the variable response of lake margin systems to the same climatic change and related lake level fluctuations. In the south, a greater fluvial influence is recognized on the development of the lake margin successions, whereas in the northern example, which lay on the downwind margin of the lake, shore zone facies are more commonly developed. The variability recognized can be accounted for by regional variations in sediment supply, coastal physiography, lake size, bathymetry and potential fetch. Lake level stability is also recognized as a major control on the development of lake margin sedimentary systems, as is the linked or unlinked relationship of the catchment and the lake basin climate for which a conceptual model is proposed.     

温度改性水玻璃固化黄土冻融特性试验研究

随着各种复杂黄土湿陷问题的呈现,不断有新的地基处理方法得到推广应用,硅化法是主要化学方法之一。为了探究温度改性水玻璃固化黄土在冻融环境下的加固效果,对其进行了冻融特性试验研究。通过控制不同温度的水玻璃溶液配制黄土试样,研究其经过冻融循环的无侧限抗压强度、质量损失以及微观结构的变化特征,分析温度改性水玻璃对加固黄土工程特性的影响和冻融循环对试样的破坏作用机理。试验结果表明：固化黄土试样随着冻融循环次数的增加,质量损失率增加;冻融循环次数相同时,质量损失率随着改性温度的增加而减小;固化黄土样的无侧限抗压强度在冻融初期存在补强作用,以后随着冻融循环次数的增加而减小;改性温度越高,20°Bé水玻璃固化黄土样的冻融循环次数相对值越大。     

Finite element analysis on deformation of highembankment in heavy-haul railway subjected to freeze-thaw cycles

Finiteelement simulations are increasingly providing a versatile environment for this topic. In this study, a two-dimensional finite element analysis is conducted to predict the deformation of highembankment in Bazhun heavy-haul railway, China. A recently developed nonlinear softening-type constitutive model is utilized to model the behavior of subgrade filling materials subjected to freeze-thaw cycles. For the convenience of practical application, the dynamic loading induced by a vehicle is treated as a quasi-static axle load. The deformation of this embankmentwith different moisture content under freeze-thaw cycles is compared. The results show that when subjected to the first freeze-thaw cycle, the embankmentexperienced significant deformation variations. Maximum deformation was usually achieved after the embankment with optimum moisture content experienced six freeze-thaw cycles, however, the embankment with moisture content of 8.0% and 9.5% deforms continuously even after experiencing almost ten freeze-thaw cycles. Overall, this study provides a simple nonlinear finite element approach for calculating the deformation of the embankmentinchanging climate conditions.     

不同养生龄期下水泥土经冻融循环后力学性能试验探究

针对季节性冻土区域,对不同养护期龄的水泥改良土在不同冻融循环次数条件下进行单轴抗压强度试验。探究养护期龄、冻融循环次数对抗压强度、质量损失率以及变形模量E₅₀的影响。结果表明：受冻融循环与水泥水化反应的影响,龄期28 d时,二者共同作用,使其结构重塑,导致三种掺量下的抗压强度随冻融循环次数增加均先减小后增大;质量损失率均先下降后上升,且小于0;变形模量E₅₀数据随循环次数增加整体呈收敛趋势,即掺量10%时呈下降趋势,6%时呈上升趋势。而龄期90 d后,水化反应基本完全,结构近于稳定,经冻融循环造成不可修复的损伤,导致抗压强度均随循环次数增加呈下降趋势;掺量6%时的质量损失率也逐渐增长大于0;E₅₀数据整体呈发散状。