The bivalve Lembulus bicuspidatus may enhance denitrification in shelf sediment at the Angola–Benguela Frontal Zone     

A Neumann  A Flohr 《African Journal of Marine Science》2018,40(1):91-96

We collected living individuals of the bivalve Lembulus bicuspidatus, which shows an unusual preference for the oxygen-deficient habitat found at the Angola–Benguela Frontal Zone of the southeastern Atlantic. With a series of incubation experiments with ¹⁵N-labelled nitrate as a tracer in combination with membrane-inlet mass spectrometry, we studied the potential contribution of L. bicuspidatus to nitrate reduction in the upper sediment layer. Our preliminary results suggest that L. bicuspidatus enhances nitrate reduction if the oxygen concentration is sufficiently low. The Lembulus-mediated nitrate reduction rate is then similar to the rate of microbial nitrate reduction in the surrounding sediment.  相似文献   

Probing the (in)compatibilities of social theory and policy relevance in Piers Blaikie’s political ecology     

Roderick P. Neumann 《Geoforum》2008,39(2):728-735

One of Piers Blaikie’s most important contributions to the development of political ecology is his critique of land and resource conservation policy in the global South. In this paper I trace the development of Blaikie’s ideas about the policy relevance of political ecology, focusing particularly on the challenges posed by the introduction of poststructural social theory into the field. I begin by revisiting Blaikie’s earlier critiques of environment and development policy. This will provide the departure point to explore how his thinking on the relationship of theory and policy and of academic and development practices has evolved in subsequent writings. I have invented two personas, “early Blaikie” and “late Blaikie”, to facilitate this task. Second, I want to probe some of the challenges that late Blaikie presents for doing political ecology research, to some extent by pitting early Blaikie against late Blaikie and letting them hash it out. Third, I turn to my own and others’ research and consultation experiences as a way to examine the possibilities for reconciling theoretically driven critiques with policy relevant research.  相似文献   

Trace element diagenesis in pyrite-rich sediments of the Achterwasser lagoon, SW Baltic Sea     

Florian Scholz  Thomas Neumann 《Marine Chemistry》2007,107(4):516-532

The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O₂ within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer.The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.  相似文献   

Origin of ore fluids in the Muruntau gold system: Constraints from noble gas, carbon isotope and halogen data   总被引：6，自引：0，他引：6  

Torsten Graupner  Samuel Niedermann  Reiner Klemd  Achim Bechtel 《Geochimica et cosmochimica acta》2006,70(21):5356-5370

Hydrothermal vein minerals directly associated with native gold mineralization in the Muruntau vein system (Uzbekistan) have been studied for noble gas, carbon isotope and halogen chemistry of the trapped ore-related fluids. Helium trapped in early arsenopyrite 1, which has preserved the original fluid signature better than associated scheelite and quartz, indicates a small input from a mantle source (?5% of total He). However, the overwhelming majority of the He in the fluid (∼95%) is from crustal sources. The noble gases Ne, Kr and Xe in the sample fluids are dominated by gases of atmospheric origin. The carbon isotope (δ¹³C: −2.1‰ to −5.3‰) and halogen characteristics of the fluids (log Br/Cl: −2.64 to −3.23) lend further support for the activity of juvenile fluids during the main ore stage. The high proportion of crustal components in the ore-forming fluids may be explained by intense fluid-rock interaction and is also supported by previous Nd and Sr isotope studies. The involvement of a juvenile fluid component during the main stage of hydrothermal activity at Muruntau (∼275 Ma) can be linked to the emplacement of lamprophyric dikes at Muruntau, due to apparently overlapping ages for high-temperature alteration, related ore vein formation and intrusion of the dikes. The input of mantle-derived fluids, possibly related to the Hercynian collisional event in the western Tien Shan, stimulated intense fluid-rock interaction in the crust. In this context, the mantle-derived fluid should be considered as one possible carrier of metals. Significant amounts of external meteoric fluids circulating in fracture systems are interpreted to have modified the noble gas signature of fluid in quartz, mostly during late, low temperature fluid circulation.  相似文献   

Observation of gravity waves in a boreal forest     

X. Lee  H.H. Neumann  G. Hartog  R.E. Mickle  J.D. Fuentes  T.A. Black  P.C. Yang  P.D. Blanken 《Boundary-Layer Meteorology》1997,84(3):383-398

In this paper we report the results of the analysis of two 60-min wave events that occurred in a boreal aspen forest during the 1994 BOREAS (Boreal Ecosystems-Atmosphere Study) field experiment. High frequency wind and temperature data were provided by three 3-D sonic anemometer/thermometers and fourteen fine-wire thermocouples positioned within and above the forest. Wave phase speeds, estimated from information revealed by spectral analysis and linear plane wave equations, are 2.2 and 1.3 m s^-1 for the two events. The wavelengths are 130 m and 65 m respectively and are much larger than the vertical wave displacements. There is strong evidence from the present analysis and from the literature supporting our postulate that these waves are generated by shear instability. We propose that wind shear near the top of the stand is often large enough to reduce the gradient Richardson number below the critical value of 0.25 and thus is able to trigger the instability. When external conditions are favorable, the instability will grow into waves.  相似文献   

IGGOS as a potential partner in IGOS     

Philip L. Woodworth  Thorkild Aarup  Reiner Rummel 《Journal of Geodynamics》2005,40(4-5):432

This short note reviews our thinking on how IGGOS can best achieve a high status within the set of global monitoring programmes. If such a high status can be obtained, then the importance of geodetic networks and services will be recognized more widely, and their activities will consequently be better resourced in the long term. One particular aspect concerns how IGGOS can complement the roles of the various IGOS partners within global monitoring. The different ways in which IGGOS can contribute to IGOS are outlined.  相似文献   

High-resolution fluvial records of Holocene environmental changes in the Sahel: the Yamé River at Ounjougou (Mali,West Africa)     

L. Lespez  Y. Le Drezen  A. Garnier  M. Rasse  B. Eichhorn  S. Ozainne  A. Ballouche  K. Neumann  E. Huysecom 《Quaternary Science Reviews》2011,30(5-6):737-756

The Yamé river, in the Bandiagara Plateau, Dogon Country, Mali, is characterised by extensive alluvial sedimentary records, particularly in the 1 km long Ounjougou reach where Holocene floodplain pockets are inset in the Pleistocene formations. These alluvial records have been investigated via geomorphologic fieldwork and sedimentologic and micromorphologic analyses and are supported by 79 radiocarbon dates. The alluvial deposits of the valley floor correspond to a vertical accretion of 3–10 m. The reconstruction of fluvial style changes provides evidence of four main aggradation periods. From 11,500 to 8760 cal. BP, the alluvial architecture and grain-size parameters indicate a wandering river. This period included phases of pulsed high-energy floods and avulsion related to a northward shift of the summer monsoon to around 14°N after 11,500 cal. BP. From 7800 to 5300 cal. BP, a swampy floodplain environment with standing water pools within a Sudanian savanna/woodland mosaic corresponds to the culmination of the Holocene humid period. From 3800 cal. BP onwards, rhythmic sedimentation attests to an increase in the duration and/or intensity of the dry season, giving a precise date for the local termination of the Holocene Optimum period. During the last two millennia and for the first time during the Holocene, the alluvial formations are progressively restricted whereas the colluvial deposits increase, indicating strong soil erosion and redeposition within the watershed related to an increase in human impact. Four major periods are characterised by incision (I1: ante 11,500, I2: 8760–7800; I3: 6790–6500 cal. BP; I4; 2400–1700 cal. BP) pointing to dramatic changes in fluvial style. They result from high-energy flood flows during dry spells and confirm the capacity of the floodplain pocket in the upstream reach of the Sahelian belt to record rapid Holocene climatic change.  相似文献   

Natural fracking and the genesis of five-element veins     

Gregor Markl  Mathias Burisch  Udo Neumann 《Mineralium Deposita》2016,51(6):703-712

  相似文献   

The occurrence of nocturnal low‐level jets In new England and the Canadian maritimes     

R.B.B. Dickison  H.H. Neumann 《大气与海洋》2013,51(4):287-300

Abstract

Detailed low‐level wind data collected in southern New Brunswick each summer since 1973 have revealed the existence of a significant low‐level jet during the evening on many occasions. The major contributing factor is considered to be an opposing thermal wind generated by the land/sea surface temperature difference when the low‐level airflow parallels the coastline. The maximum wind is characteristically located between 300 and 500 m above msl. Intensities have been found as great as 15 m s^‐1 in excess of the 1000‐m wind speed. A detailed study is presented for a well‐documented case on 8 July 1975, using hourly information from three stations near Fredericton, N.B., and Doppler measurements of the drift of a specially equipped research aircraft. There is clear evidence of an undulation in the altitude of the “nose” of the low‐level jet and of a progressive increase in intensity from early evening until midnight.  相似文献   

Spatial patterns of groundwater‐lake exchange – implications for acid neutralization processes in an acid mine lake     

Christiane Neumann  Julia Beer  Christian Blodau  Stefan Peiffer  Jan H. Fleckenstein 《水文研究》2013,27(22):3240-3253

Exchange of groundwater and lake water with typically quite different chemical composition is an important driver for biogeochemical processes at the groundwater‐lake interface, which can affect the water quality of lakes. This is of particular relevance in mine lakes where anoxic and slightly acidic groundwater mixes with oxic and acidic lake water (pH < 3). To identify links between groundwater‐lake exchange rates and acid neutralization processes in the sediments, exchange rates were quantified and related to pore‐water pH, sulfate and iron concentrations as well as sulfate reduction rates within the sediment. Seepage rates measured with seepage meters (?2.5 to 5.8 L m^‐2 d^‐1) were in reasonable agreement with rates inverted from modeled chloride profiles (?1.8 to 8.1 L m^‐2 d^‐1). Large‐scale exchange patterns were defined by the (hydro)geologic setting but superimposed by smaller scale variations caused by variability in sediment texture. Sites characterized by groundwater upwelling (flow into the lake) and sites where flow alternated between upwelling and downwelling were identified. Observed chloride profiles at the alternating sites reflected the transient flow regime. Seepage direction, as well as seepage rate, were found to influence pH, sulfate and iron profiles and the associated sulfate reduction rates. Under alternating conditions proton‐consuming processes, for example, sulfate reduction, were slowed. In the uppermost layer of the sediment (max. 5 cm), sulfate reduction rates were significantly higher at upwelling (>330 nmol g^‐1 d^‐1) compared to alternating sites (<220 nmol g^‐1 d^‐1). Although differences in sulfate reduction rates could not be explained solely by different flux rates, they were clearly related to the prevailing groundwater‐lake exchange patterns and the associated pH conditions. Our findings strongly suggest that groundwater‐lake exchange has significant effects on the biogeochemical processes that are coupled to sulfate reduction such as acidity retention and precipitation of iron sulfides. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献