Pathways of manganese in an open estuarine system     

Bjorn Sundby  Norman Silverberg  Roger Chesselet 《Geochimica et cosmochimica acta》1981,45(3):293-307

The distribution of Mn was examined in the bottom sediments and water column (suspended paniculate matter) of the Laurentian Trough. Gulf of St. Lawrence. A characteristic profile of Mn with depth in the sediment consisted of a Mn-enriched surface oxidized zone, less than 20 mm thick, and a Mn-depleted subsurface reducing zone. A subsurface Mn maximum occurred within the oxidized zone. Below this maximum the concentration dropped sharply to nearly constant residual levels in the reducing zone. The accumulating estuarine sediments are deficient in Mn compared to the river input of suspended matter and are definitely not the ultimate sink for manganese. Manganese escapes from the sediment by diffusion and resuspension, forming Mn-enriched, fine-grained particles which are flushed out in the estuarine circulation. 5.0 × 10⁹gyr^?1 of Mn, or 50% more than the river input of dissolved Mn. are exported to the open ocean. In spite of the efficient mobilization and export of Mn, the quantity exported is a small fraction (0.2%) of the total flux to the deep-sea sediments. This is related to the low levels of paniculate matter transported by the St. Lawrence River. The export phénomenon, however, is probably true of many coastal regions of muddy sediments and thus has interesting implications for the oceanic budget of Mn.  相似文献   

Speciation of reduced C-O-H volatiles in coexisting fluids and silicate melts determined in-situ to ∼1.4 GPa and 800 °C     

Bjorn O. Mysen  Shigeru Yamashita 《Geochimica et cosmochimica acta》2010,74(15):4577-155

The structure of silicate melts in the system Na₂O·4SiO₂ saturated with reduced C-O-H volatile components and of coexisting silicate-saturated C-O-H solutions has been determined in a hydrothermal diamond anvil cell (HDAC) by using confocal microRaman and FTIR spectroscopy as structural probes. The experiments were conducted in-situ with the melt and fluid at high temperature (up to 800 °C) and pressure (up to 1435 MPa). Redox conditions in the HDAC were controlled with the reaction, Mo + H₂O = MoO₂ + H₂, which is slightly more reducing than the Fe + H₂O = FeO + H₂ buffer at 800 °C and less.The dominant species in the fluid are CH₄ + H₂O together with minor amounts of molecular H₂ and an undersaturated hydrocarbon species. In coexisting melt, CH₃ - groups linked to the silicate melt structure via Si-O-CH₃ bonding may dominate and possibly coexists with molecular CH₄. The abundance ratio of CH₃ - groups in melts relative to CH₄ in fluids increases from 0.01 to 0.07 between 500 and 800 °C. Carbon-bearing species in melts were not detected at temperatures and pressures below 400 °C and 730 MPa, respectively. A schematic solution mechanism is, Si-O-Si + CH₄?Si-O-CH₃+H-O-Si. This mechanism causes depolymerization of silicate melts. Solution of reduced (C-O-H) components will, therefore, affect melt properties in a manner resembling dissolved H₂O.  相似文献   

Acidification of Lower St. Lawrence Estuary Bottom Waters     

Alfonso Mucci  Michel Starr  Denis Gilbert  Bjorn Sundby 《大气与海洋》2013,51(3):206-218

Accumulation of metabolic CO₂ can acidify marine waters above and beyond the ongoing acidification of the ocean by anthropogenic CO₂. The impact of respiration on carbonate chemistry and pH is most acute in hypoxic and anoxic basins, where metabolic CO₂ accumulates to high concentrations. The bottom waters of the Lower St. Lawrence Estuary (LSLE), where persistently severe hypoxia has developed over the last 80 years, is one such case. We have reconstructed the evolution of pH in the bottom waters from historical and recent data, and from first principles relating the stoichiometry of CO₂ produced to oxygen consumed during microbial degradation of organic matter. Based on the value of the atmospheric partial pressure of CO₂ that best reproduces the preformed dissolved inorganic carbon concentration in the bottom waters, we estimate the average ventilation age of the bottom waters to be 16?±?3 years. The pH of the bottom waters has decreased by 0.2 to 0.3 over the last 75 years, which is four to six times greater than can be attributed to the uptake of anthropogenic CO₂. The pH decrease is accompanied by a decline in the saturation state with respect to both calcite and aragonite. As of 2007, bottom waters in the LSLE are slightly supersaturated with respect to calcite (Ω_c?≈?1.06?±?0.04) but are strongly undersaturated with respect to aragonite (Ω_a?≈?0.67?±?0.03).

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Application of Dynamic Subgrid-scale Models for Large-eddy Simulation of the Daytime Convective Boundary Layer over Heterogeneous Surfaces   总被引：1，自引：0，他引：1  

Hsin-Yuan Huang  Bjorn Stevens  Steven A. Margulis 《Boundary-Layer Meteorology》2008,126(3):327-348

The sensitivity of large-eddy simulation (LES) to the representation of subgrid-scale (SGS) processes is explored for the case of the convective boundary layer (CBL) developing over surfaces with varying degrees of spatial heterogeneity. Three representations of SGS processes are explored: the traditional constant Smagorinsky–Lilly model and two other dynamic models with Lagrangian averaging approaches to calculate the Smagorinsky coefficient (C _S ) and SGS Prandtl number (Pr). With initial data based roughly on the observed meteorology, simulations of daytime CBL growth are performed over surfaces with characteristics (i.e. fluxes and roughness) ranging from homogeneous, to striped heterogeneity, to a realistic representation of heterogeneity as derived from a recent field study. In both idealized tests and the realistic case, SGS sensitivities are mostly manifest near the surface and entrainment zone. However, unlike simulations over complex domains or under neutral or stable conditions, these differences for the CBL simulation, where large eddies dominate, are not significant enough to distinguish the performance of the different SGS models, irrespective of surface heterogeneity.  相似文献   

Solution mechanisms of silicate in aqueous fluid and H₂O in coexisting silicate melts determined in-situ at high pressure and high temperature     

Bjorn O.Mysen   《Geochimica et cosmochimica acta》2009,73(19):5748-5763

The structure of H₂O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na₂O·4SiO₂–H₂O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H₂O at nominal densities, ρ_fluid, (from EOS of pure H₂O) of 0.90 and 0.78 g/cm³. With ρ_fluid = 0.78 g/cm³, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρ_fluid = 0.90 g/cm³, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na₂O·4SiO₂ starting material.Molecular H₂O (H₂O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H₂O)-ratio is positively correlated with temperature. The OH/(H₂O)° in melts is greater than in coexisting fluids. Structural units of Q³, Q², Q¹, and Q⁰ type are observed in all phases under all conditions. An expression of the form, 12Q³ + 13H₂O2Q² + 6Q¹ + 4Q⁰, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.  相似文献   

Observing and Modeling Earth’s Energy Flows   总被引：1，自引：0，他引：1  

Bjorn Stevens  Stephen E. Schwartz 《Surveys in Geophysics》2012,33(3-4):779-816

This article reviews, from the authors’ perspective, progress in observing and modeling energy flows in Earth’s climate system. Emphasis is placed on the state of understanding of Earth’s energy flows and their susceptibility to perturbations, with particular emphasis on the roles of clouds and aerosols. More accurate measurements of the total solar irradiance and the rate of change of ocean enthalpy help constrain individual components of the energy budget at the top of the atmosphere to within ±2 W m^?2. The measurements demonstrate that Earth reflects substantially less solar radiation and emits more terrestrial radiation than was believed even a decade ago. Active remote sensing is helping to constrain the surface energy budget, but new estimates of downwelling surface irradiance that benefit from such methods are proving difficult to reconcile with existing precipitation climatologies. Overall, the energy budget at the surface is much more uncertain than at the top of the atmosphere. A decade of high-precision measurements of the energy budget at the top of the atmosphere is providing new opportunities to track Earth’s energy flows on timescales ranging from days to years, and at very high spatial resolution. The measurements show that the principal limitation in the estimate of secular trends now lies in the natural variability of the Earth system itself. The forcing-feedback-response framework, which has developed to understand how changes in Earth’s energy flows affect surface temperature, is reviewed in light of recent work that shows fast responses (adjustments) of the system are central to the definition of the effective forcing that results from a change in atmospheric composition. In many cases, the adjustment, rather than the characterization of the compositional perturbation (associated, for instance, with changing greenhouse gas concentrations, or aerosol burdens), limits accurate determination of the radiative forcing. Changes in clouds contribute importantly to this adjustment and thus contribute both to uncertainty in estimates of radiative forcing and to uncertainty in the response. Models are indispensable to calculation of the adjustment of the system to a compositional change but are known to be flawed in their representation of clouds. Advances in tracking Earth’s energy flows and compositional changes on daily through decadal timescales are shown to provide both a critical and constructive framework for advancing model development and evaluation.  相似文献   

Multi-proxy paleolimnological assessment of biogeochemical versus food web controls on the trophic states of two shallow,mesotrophic lakes     

Bjorn A. Lake  Courtney R. Wigdahl  Kristin E. Strock  Jasmine E. Saros  Aria Amirbahman 《Journal of Paleolimnology》2011,46(1):45-57

The frequency of nuisance algal blooms has been increasing during the last two decades in the shallow, headwater East Pond (Smithfield, Maine, USA). Meanwhile, the hydrologically linked North Pond has not experienced an increase in algal blooms, despite similar morphometry and higher external nutrient loads. Possible explanations for this difference include stronger trophic cascade effects from planktivorous white perch (Morone americana) in East Pond as well as differences in phosphorus (P) release from the sediments of these two lakes. We conducted a paleolimnological investigation of these two lakes to assess whether sedimentary evidence supported trophic cascade effects based on cladoceran ephippia size, diatom fossils, and fossil pigments or biogeochemical controls based on potential sedimentary P release as the primary driver of these increased algal blooms in East Pond. At the time of white perch introduction (~1930–1950), ephippia size increases in East Pond, although no changes are observed in either diatom abundance or trends in the algal pigments. Instead, algal pigments increase in recent decades (~1980 to present) along with an increase in diatom taxa with higher TP optima These results suggest that predation by white perch is not resulting in top-down effects on algal abundance in East Pond, as predicted by the trophic cascade hypothesis. While the P content of sediments from both lakes is relatively equal, the releasable P in the top 10 cm of sediment in East Pond constitutes a greater percentage of the P extracted. Also, North Pond sediments exhibit a greater capacity to permanently bury P via the mechanisms of sorption to Al(OH)_3(s) and a slower mineralization of organic P compounds. The results of this investigation suggest that the ultimate driver of the recent algal blooms in East Pond is internal P release from the sediments instead of trophic cascade effects.  相似文献   

Olivine/melt transition metal partitioning, melt composition, and melt structure—Melt polymerization and Q-speciation in alkaline earth silicate systems     

Bjorn O. Mysen 《Geochimica et cosmochimica acta》2008,72(19):4796-4812

The two most abundant network-modifying cations in magmatic liquids are Ca²⁺ and Mg²⁺. To evaluate the influence of melt structure on exchange of Ca²⁺ and Mg²⁺ with other geochemically important divalent cations (m-cations) between coexisting minerals and melts, high-temperature (1470-1650 °C), ambient-pressure (0.1 MPa) forsterite/melt partitioning experiments were carried out in the system Mg₂SiO₄-CaMgSi₂O₆-SiO₂ with ?1 wt% m-cations (Mn²⁺, Co²⁺, and Ni²⁺) substituting for Ca²⁺ and Mg²⁺. The bulk melt NBO/Si-range (NBO/Si: nonbridging oxygen per silicon) of melt in equilibrium with forsterite was between 1.89 and 2.74. In this NBO/Si-range, the NBO/Si(Ca) (fraction of nonbridging oxygens, NBO, that form bonds with Ca²⁺, Ca²⁺-NBO) is linearly related to NBO/Si, whereas fraction of Mg²⁺-NBO bonds is essentially independent of NBO/Si. For individual m-cations, rate of change of K_D(m−Mg) with NBO/Si(Ca) for the exchange equilibrium, m^melt + Mg^olivine ? m^olivine + Mg^melt, is linear. K_D(m−Mg) decreases as an exponential function of increasing ionic potential, Z/r² (Z: formal electrical charge, r: ionic radius—here calculated with oxygen in sixfold coordination around the divalent cations) of the m-cation. The enthalpy change of the exchange equilibrium, ΔH, decreases linearly with increasing Z/r² [ΔH = 261(9)-81(3)·Z/r² (Å⁻²)]. From existing information on (Ca,Mg)O-SiO₂ melt structure at ambient pressure, these relationships are understood by considering the exchange of divalent cations that form bonds with nonbridging oxygen in individual Qⁿ-species in the melts. The negative ∂K_D(m−Mg)/∂(Z/r²) and ∂(ΔH)/∂(Z/r²) is because increasing Z/r² is because the cations forming bonds with nonbridging oxygen in increasingly depolymerized Qⁿ-species where steric hindrance is decreasingly important. In other words, principles of ionic size/site mismatch commonly observed for trace and minor elements in crystals, also govern their solubility behavior in silicate melts.  相似文献   

Single-target echo detections of jellyfish   总被引：2，自引：1，他引：2  

Brierley  Andrew S; Axelsen  Bjorn Eric; Boyer  David C; Lynam  Christopher P; Didcock  Carol A; Boyer  Helen J; Sparks  Conrad A.J; Purcell  Jennifer E; Gibbons  Mark J 《ICES Journal of Marine Science》2004,61(3):383-393

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Growth and population parameters of ringed seals (Pusa hispida) from Svalbard, Norway, 2002-2004     

Krafft  Bjorn A.; Kovacs  Kit M.; Frie  Anne Kirstine; Haug  Tore; Lydersen  Christian 《ICES Journal of Marine Science》2006,63(6):1136-1144

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