Carbon fluxes through major phytoplankton groups during the spring bloom and post-bloom in the Northwestern Mediterranean Sea

The carbon flux through major phytoplankton groups, defined by their pigment markers, was estimated in two contrasting conditions of the Northwestern Mediterranean open ocean ecosystem: the spring bloom and post-bloom situations (hereafter Bloom and Post-bloom, respectively). During Bloom, surface chlorophyll a (Chl a) concentration was higher and dominated by diatoms (53% of Chl a), while during Post-bloom Synechococcus (42%) and Prymnesiophyceae (29%) became dominant. The seawater dilution technique, coupled to high pressure liquid chromatography (HPLC) analysis of pigments and flow cytometry (FCM), was used to estimate growth and grazing rates of major phytoplankton groups in surface waters. Estimated growth rates were corrected for photoacclimation based on FCM-detected changes in red fluorescence per cell. Given the 30% average decrease in the pigment content per cell between the beginning and the end of the incubations, overlooking photoacclimation would have resulted in a 0.40 d^?1 underestimation of phytoplankton growth rates. Corrected average growth rates (μ_o) were 0.90±0.20 (SD) and 0.40±0.14 d^?1 for Bloom and Post-bloom phytoplankton, respectively. Diatoms, Cryptophyceae and Synechococcus were identified as fast-growing groups and Prymnesiophyceae and Prasinophyceae as slow-growing groups across Bloom and Post-bloom conditions. The higher growth rate during Bloom was due to dominance of phytoplankton groups with higher growth rates than those dominating in Post-bloom. Average grazing rates (m) were 0.58±0.20 d^?1 (SD) and 0.31±0.07 d^?1. The proportion of phytoplankton growth consumed by microzooplankton grazing (m/μ_o) tended to be lower in Bloom (0.69±0.34) than in Post-bloom (0.80±0.08). The intensity of nutrient limitation experienced by phytoplankton indicated by μ_o/μ_n (where μ_n is the nutrient-amended growth rate), was similar during Bloom (0.78) and Post-bloom (0.73). Primary production from surface water (PP) was estimated with ¹⁴C incubations. A combination of PP and Chl a synthesis rate yielded C/Chl a ratios of 34±21 and 168±75 (g:g) for Bloom and Post-bloom, respectively. Transformation of group-specific Chl a fluxes into carbon equivalents confirmed the dominant role of diatoms during Bloom and Synechococcus and Prymnesiophyceae during Post-bloom.     

地下闭式循环地热交换发电系统简介

为解决能源问题而提出了地下闭式循环地热交换发电系统.该系统地下部分通过全井下套管形成一个封闭系统,克服了传统地热发电和干热岩发电受地质条件制约、对环境造成危害等缺点,是一种新型的不依赖位置、环境友好的地热发电系统.采用大位移技术进行钻井,应用悬链线技术对井身剖面结构进行了优化,提出了地下连接技术要求,应用分支井和膨胀套管技术固井,设计了水泥配方并进行了相关试验,优化了载热流体,计算了有机工质朗肯循环(ORC)的发电效率.     

Revision of the Conchostracan Genus Tenuestheria from the Upper Cretaceous Lanxi Formation in Zhejiang and Its Biostratigraphic Significance in Southeast China

The diagnosis of the conchostracan genus Tenuestheria from the Lanxi Formation in Jinhua, Zhejiang Province in Southeast China is revised following an examination of the type species under a scanning electron microscope, which revealed some morphological features on the carapace that had not been recognized previously. The importance of the Tenuestheria Fauna is considered in the context of correlation of six Turonian–Santonian formations in this region of China. The value of the Cenomanian Nemestheria and Turonian Linhaiella faunas in correlating the underlying early Late Cretaceous formations is also noted.     

C-Isotope stratigraphy,a monitor of paleoenvironmental change: A case study from the early cretaceous

Today's disturbance of the global carbon cycle induced by anthropogenic processes has raised new interest in the history of the global carbon cycle and its relationship to climate and other geochemical cycles. Carbon-isotope stratigraphy proves to be most useful as a monitor of the history of the carbon-cycle during the last 200 million years. In the introductory paragraphs of this review the mode of functioning of the global carbon-cycle is summarized and the connection between carbon-cycle and carbon isotope geochemistry is documented. A case study on the disturbance of the global carbon cycle during the Aptian-Albian is presented. The disturbance of the carbon cycle lasting up to millions of years is recorded in the carbon-isotope stratigraphy of pelagic sediments. It is superimposed on high frequency sedimentological cycles, related to climate and oceanographic cycles of 20, 40 or 100 ky duration. The data reviewed suggest that the change in the global carbon system was linked to a global acceleration of geochemical cycles triggered by a long-term change in atmospheric CO₂ controlled by the rate of sea-floor formation and by volcanic activity. Increased accumulation rates of terrestrial material and terrestrial organic matter in marine sediments may be used as an indicator of an intensified hydrological cycling resulting in higher water-discharge rates. An intensification of the Aptian-Albian water cycle is further reflected in continental sediments monitoring a period of elevated humidity. An increase in water discharge rates should have affected the transfer rate of dissolved nutrients from continents to oceans. Elevated concentrations of phosphorus may have led to an increase in Aptian-Albian oceanic productivity enhancing the transfer of marine organic matter from the oceanic into the sedimentary reservoir. Increased productivity, increased bulk sedimentation rates and poorly oxygenated deep-water led to increased preservation of marine and terrestrial organic matter in marine sediments. The accelerated output of marine organic carbon from the oceanic reservoir is ultimately registered in the positive carbon-isotope excursion of the marine carbonate carbon-isotope stratigraphy.     

Continental crust formation seen through the Sr and Nd isotope systematics of S-type granites in the Hercynian belt of western France

The isotopic compositions of Sr, Nd and Pb in leucogranites which are intercorrelated (Bernard-Griffiths et al., 1985 [1]) may be explained by the mixing of ancient basement (1800 Ma) with juvenile crust (late Precambrian or early Palaeozoic). This hypothesis does not involve the existence of Mid-Proterozoic crust, as apparently indicated by theT_DM model ages of the leucogranites (ranging between 1600 and 1100 Ma). The Nd isotopes reveal the crustal reworking while Sr isotopes mainly record juvenile crust formation. This paradox is explained by the geochemical heterogeneity of the sources involved.     

The role of rural-rural migration as a survival strategy in the Sahelian zone of the Sudan —

Taking the example of Burush, N Darfur, Sudan, the authors analyse the role of migration as a survival strategy in the desertification-stricken Sahelian zone. It is being argued that the rual-rual migration plays a greater role than the rual-urban migration among the Berti ethnic group of northern Darfur. Their high mobility within their own area is part of their traditional economic pattern for coping with aridity and high rainfall variability     

Evaluation and application of dialysis porewater samplers for microbiological studies at sediment-water interfaces

The equilibrium diffusion technique has become a valuable tool for ecological and biogeochemical studies in aquatic environments. In sediment ecosystems, changes in concentration of microbial metabolites with increasing depth can be determined dependably and reproducibly with this technique. Since the permeation characteristics of the membranes employed are crucial, selecting the proper membrane requires knowledge about its behavior under conditions which prevail in the natural environment. Thirteen polymer sheets were evaluated comparing permeation terms for biogeochemically relevant solutes, biodegradability, and mechanical strength. Cellulose-based dialysis membranes are most satisfactory when employed in low temperature anoxic environments. For this membrane, correction terms were calculated to account for diffusion losses during retrieval and sampling. Optimal incubation times can now be predicted from experimentally determined permeation coefficients for several porewater solutes. Dialysis porewater samplers (DPS) have been successfully applied during more than 100 independent experiments for the collection of interstitial water from surface sediments. DPS were used for water depths as deep as 290 meters.     

Late variscan tectonic evolution by thinning of earlier thickened crust. An Ar---Ar study of the Montagne Noire, southern Massif Central, France

⁴⁰Ar---³⁹Ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the Montagne Noire (the southern part of the Massif Central, France).

This range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high T/medium P conditions. The basement is structurally overlain by epimetamorphic Paleozoic sediments in the north. In the south, equivalent rocks form a well-known pile of recumbent nappes. The Variscan history of this area comprises early shortening and thickening of the continental crust. The climax of this event took place around 320 Ma ago, as is shown by previous Rb---Sr data and by ⁴⁰Ar---³⁹Ar measurements. New developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core. South-southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts. This movement was accompanied and followed by dextral extensional wrenching of the Paleozoic cover along the northern and southern margins of the axial zone. Attendantly, ductile shear zones were formed.

In order to decipher the complex history, three structural levels have been dated with the ⁴⁰Ar---³⁹Ar method. These levels are: (1) The migmatitic and gneissic series in the axial core, where micas yield ages of c. 315 Ma; (2) Mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c. 310 Ma; (3) Muscovites and biotites from the epimetamorphic nappe structures yielding ages of c. 297 Ma. The later ages may represent a younger thermal pulse. These data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints. A detailed geodynamic evolution of the studied segment is proposed.     

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region,Atlantic Ocean

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb${}^{206}\text{Pb}{}^{204}\text{Pb}$ up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and${}^{143}\text{Nd}{}^{144}\text{Nd}$ increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas.Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.