Anaerobic propane oxidation in marine hydrocarbon seep sediments     

Steven D. Quistad  David L. Valentine 《Geochimica et cosmochimica acta》2011,75(8):2159-2169

Propane (C₃H₈) is an abundant hydrocarbon in subsurface reservoirs with significance to atmospheric chemistry and to marine biogeochemistry. The anaerobic oxidation of propane coupled to sulfate reduction may prevent sub-seafloor accumulations of propane from entering the ocean and atmosphere. Anaerobic oxidation of propane has recently been demonstrated in cultures of novel sulfate-reducing bacteria, but has not been directly demonstrated or quantified in nature. In this work we describe a method involving incubation with ¹³C-propane to quantify rates of anaerobic oxidation of propane in anoxic sediment, and we conclusively demonstrate the oxidation of propane under sulfidic conditions in fresh sediments of a marine hydrocarbon seep. Observed rates of anaerobic oxidation of propane adhere to first-order kinetic behavior, enabling the modification of this method for whole core rate determinations. Whole core rates in nine cores from two hydrocarbon seeps measured 0.04-2100 nmoles C₃H₈ cm⁻³ day⁻¹ by this method. The seep persistently supplied with more propane displayed substantially higher rates of anaerobic oxidation of propane, by 1-2 orders of magnitude when averaged over the top 10-cm, suggesting the development of the microbial community is strongly modulated by the availability of propane. This work is the first to estimate rates for anaerobic oxidation of propane in any environment, and demonstrates the potential importance of the process as a filter for preventing propane from entering the ocean and atmosphere.  相似文献   

Erratum to: Crystal growth during dike injection of MOR basaltic melts: evidence from preservation of local Sr disequilibria in plagioclase     

Georg F. Zellmer  Kenneth H. Rubin  Peter Dulski  Yoshiyuki Iizuka  Steven L. Goldstein  Michael R. Perfit 《Contributions to Mineralogy and Petrology》2011,161(1):175-176

Profiles of a total of 23 plagioclase crystals erupted within the 1982–1991 and 1993 flows of the Coaxial segment of the Juan de Fuca ridge, the 1996 flow of the North Gorda ridge, and from the Western Volcanic Zone of the ultra-slow spreading Gakkel Ridge, have been studied for variations in major and trace element concentrations. We derive equilibration times for the relatively rapidly diffusing Sr in mid-ocean ridge basalt (MORB) plagioclase crystals of the order of months to a few years in each case. All crystals preserve diffusive disequilibria of strontium and barium. Crystal residence times at MORB magmatic temperatures are thus significantly shorter, of the order of days to a few months at most, precluding prolonged crystal storage in axial magma chambers and instead pointing to rapid crystal growth (up to ~10⁻⁸ cm s⁻¹) and cooling (up to ~1°C h⁻¹) shortly prior to eruption of these samples. Growth of these crystals is therefore inferred to occur almost entirely within oceanic layer 2 during dike injection. Crystals that grew at lower crustal levels or earlier in the differentiation sequence appear to have been excluded from the erupted magmas, as might occur if most of the gabbroic rocks in oceanic layer 3 formed an interlocking crystal framework, with viscosities that are too high to carry earlier formed crystals with the melt. The vertical extent of eruptible, crystal-poor melt lenses within the gabbroic zone is constrained to ~1 m or less by considering the width of local equilibrium growth zones, equilibration times, and crystal settling velocities. This lengthscale is consistent with field evidence from ophiolites. Finally, crystal aggregates within the Gakkel ridge sample studied here are the result of synneusis within the propagating dike during melt ascent.  相似文献   

Soil phytoliths from miombo woodlands in Mozambique     

Julio Mercader  Tim Bennett  Steven Simpson 《Quaternary Research》2011,75(1):138-150

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The monsoon imprint during the ‘atypical’ MIS 13 as seen through north and equatorial Indian Ocean records     

Thibaut Caley  Bruno Malaizé  Franck Bassinot  Steven C. Clemens  Nicolas Caillon  Rossignol Linda  Karine Charlier  Helene Rebaubier 《Quaternary Research》2011,76(2):285-293

Previous studies have suggested that Marine Isotope Stage (MIS) 13, recognized as atypical in many paleoclimate records, is marked by the development of anomalously strong summer monsoons in the northern tropical areas. To test this hypothesis, we performed a multi-proxy study on three marine records from the tropical Indian Ocean in order to reconstruct and analyse changes in the summer Indian monsoon winds and precipitations during MIS 13. Our data confirm the existence of a low-salinity event during MIS 13 in the equatorial Indian Ocean but we argue that this event should not be considered as “atypical”. Taking only into account a smaller precession does not make it possible to explain such precipitation episode. However, when considering also the larger obliquity in a more complete orbitally driven monsoon “model,” one can successfully explain this event. In addition, our data suggest that intense summer monsoon winds, although not atypical in strength, prevailed during MIS 13 in the western Arabian Sea. These strong monsoon winds, transporting important moisture, together with the effect of insolation and Eurasian ice sheet, are likely one of the factors responsible for the intense monsoon precipitation signal recorded in China loess, as suggested by model simulations.  相似文献   

Assessment of MTBE biodegradation in contaminated groundwater using C and C analysis: Field and laboratory microcosm studies     

Steven F. Thornton  Simon H. Bottrell  Keith H. Spence  Roger Pickup  Michael J. Spence  Nadeem Shah  Helen E.H. Mallinson  Hans H. Richnow 《Applied Geochemistry》2011,26(5):828-837

Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The ¹⁴C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, δ¹³C compositions of MTBE in groundwater samples from the plume showed no significant ¹³C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O₂ was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O₂. The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7‰ in the MTBE δ¹³C composition and an isotope enrichment factor (ε) of −1.53‰ when dissolved O₂ was not limiting. However, for the low dissolved O₂ concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5-0.7‰, corresponding to an ε value of −0.22‰ to −0.24‰. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O₂. Under low O₂ conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O₂ concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE δ¹³C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes.  相似文献   

Simulation of the Hurricane Dennis storm surge and considerations for vertical resolution     

Dmitry S. Dukhovskoy  Steven L. Morey 《Natural Hazards》2011,58(1):511-540

A high-resolution storm surge model of Apalachee Bay in the northeastern Gulf of Mexico is developed using an unstructured grid finite-volume coastal ocean model (FVCOM). The model is applied to the case of Hurricane Dennis (July 2005). This storm caused underpredicted severe flooding of the Apalachee Bay coastal area and upriver inland communities. Accurate resolution of complicated geometry of the coastal region and waterways in the model reveals processes responsible for the unanticipated high storm tide in the area. Model results are validated with available observations of the storm tide. Model experiments suggest that during Dennis, excessive flooding in the coastal zone and the town of St. Marks, located up the St. Marks River, was caused by additive effects of coincident high tides (~10–15% of the total sea-level rise) and a propagating shelf wave (~30%) that added to the locally wind-generated surge. Wave setup, the biggest uncertainty, is estimated on the basis of empirical and analytical relations. The Dennis case is then used to test the sensitivity of the model solution to vertical discretization. A suite of model experiments is performed with varying numbers of vertical sigma (σ) levels, with different distribution of σ-levels within the water column and a varying bottom drag coefficient. The major finding is that the storm surge solution is more sensitive to resolution within the velocity shear zone at mid-depths compared to resolution of the upper and bottom layer or values of the bottom drag coefficient.  相似文献   

The impact of natural versus anthropogenic aerosols on atmospheric circulation in the Community Atmosphere Model     

Robert J. Allen  Steven C. Sherwood 《Climate Dynamics》2011,36(9-10):1959-1978

The equilibrium response of atmospheric circulation to the direct radiative effects of natural or anthropogenic aerosols is investigated using the Community Atmosphere Model (CAM3) coupled to two different ocean boundary conditions: prescribed climatological sea surface temperatures (SSTs) and a slab ocean model. Anthropogenic and natural aerosols significantly affect the circulation but in nearly opposite ways, because anthropogenic aerosols tend to have a net local warming effect and natural aerosols a net cooling. Aerosol forcings shift the Intertropical Convergence Zone and alter the strength of the Hadley circulation as found in previous studies, but also affect the Hadley cell width. These effects are due to meridional gradients in warming caused by heterogeneous net heating, and are stronger with interactive SST. Aerosols also drive model responses at high latitudes, including polar near-surface warming by anthropogenic aerosols in summer and an Arctic Oscillation (AO)-type responses in winter: anthropogenic aerosols strengthen wintertime zonal wind near 60°N, weaken it near 30°N, warm the troposphere, cool the stratosphere, and reduce Arctic surface pressure, while natural aerosols produce nearly opposite changes. These responses are shown to be due to modulation of stratospheric wave-driving consistent with meridional forcing gradients in midlatitudes. They are more pronounced when SST is fixed, apparently because the contrast in land-ocean heating drives a predominantly wavenumber-2 response in the northern hemisphere which is more efficient in reaching the stratosphere, showing that zonal heating variations also affect this particular response. The results suggest that recent shifts from reflecting to absorbing aerosol types probably contributed to the observed decadal variations in tropical width and AO, although studies with more realistic temporal variations in forcing would be needed to quantify this contribution.  相似文献   

Modeling the time-dependent response of the Asian summer monsoon to obliquity forcing in a coupled GCM: a PHASEMAP sensitivity experiment   总被引：1，自引：0，他引：1  

Guang-Shan Chen  Zhengyu Liu  Steven C. Clemens  Warren L. Prell  Xiaodong Liu 《Climate Dynamics》2011,36(3-4):695-710

To study the time-dependent response of the Asian summer monsoon to obliquity forcing, we analyze a 284,000-year long transient simulation produced by a fully coupled global climate model (GCM) using a new phase mapping (PHASEMAP) approach. Here we focus on understanding the phase response of monsoonal circulation to insolation forcing at the Earth-orbital obliquity band (41 Kyr). Our results show that the East Asian summer monsoon (EASM) can be divided into two geographic regions: the North East Asian summer monsoon (NEASM) and the South East Asian summer monsoon (SEASM). The Indian summer monsoon (ISM) and the SEASM are in phase at the obliquity band, strengthened with an increase in obliquity from Obliquity minima (Omin) to Obliquity maxima (Omax). The NEASM is out of phase with the ISM and SEASM, weakened with an increase in obliquity from Omin to Omax. We hypothesize that the inverse phase between the NEASM and the ISM at the obliquity band results from an ISM–NEASM teleconnection linked to the formation mechanism of the Bonin High.  相似文献   

Consistent scale-dependency of future increases in hourly extreme precipitation in two convection-permitting climate models     

Helsen  Samuel  van Lipzig  Nicole P. M.  Demuzere  Matthias  Vanden Broucke  Sam  Caluwaerts  Steven  De Cruz  Lesley  De Troch  Rozemien  Hamdi  Rafiq  Termonia  Piet  Van Schaeybroeck  Bert  Wouters  Hendrik 《Climate Dynamics》2020,54(3):1267-1280

Climate Dynamics - Convection-permitting models (CPMs) have been proven successful in simulating extreme precipitation statistics. However, when such models are used to study climate change,...  相似文献   

Modeling Use of Space from Social Media Data Using a Biased Random Walker     

Steven D. Prager  R. Paul Wiegand 《Transactions in GIS》2014,18(6):817-833

Individuals and other entities move through space as a function of local characteristics of place, their internal behavioral models, and the topological structure of the underlying space. When a collection of locations (i.e. geotagged photos or other geotagged social media information) from a large number of individuals is assembled, it becomes possible to understand the interrelationship between the individuals and the space they occupy. This research systematically considers this interrelationship through an examination of the effect of the intersection of behavioral and spatial characteristics on individuals moving on street networks. The research illustrates how social media data, in combination with a biased random walker, can be used to understand and model the interaction of spatial structure and social‐environmental factors on influencing individuals' use of their environment. The biased walker offers a flexible approach to incorporate consideration of both social‐environmental and structural factors into a model and we demonstrate this through a case study wherein we are able to use the random walker to model the characteristics of Flickr users in New York City.  相似文献