962 resultados para dust proof
Resumo:
The concentration and isotopic composition of Nd in water and particles collected in the western Mediterranean Sea are studied by two complementary approaches. The first examines local vertical profiles and time series; the second considers the global Nd budget of the whole western Mediterranean Sea. These two approaches are used to quantify the Nd inputs and the dissolved/particulate exchange processes in the water column. Two profiles of Nd in seawater in the Ligurian Sea taken in May and October 1992 show an average epsilon-Nd(0) = -9.6 ± 0.5. Seawater from the Strait of Sicily, representative of the eastern waters flowing into the western basin, is more radiogenic [epsilon-Nd(0) = -7.7 ± 0.6]. Profiles of particulate matter collected in sediment traps in coastal (Gulf of Lions) and offshore (Ligurian Sea) environments are also shown. Particles are enriched in Nd and are more radiogenic near the coast than offshore. Measurements of Nd concentration and epsilon-Nd(0) of external sources to the western Mediterranean Sea compared with the literature data demonstrate that particulate flux of atmospheric Saharan origin are more rich ([Nd] = 38 ± 10 µg/g) and less radiogenic [epsilon-Nd(0) = -13.0 ± 1.0] than riverine particulate discharge ([Nd] = 21.5 ± 4.4 µg/g; epsilon-Nd(0) = -10.1 ± 0.5), allowing to trace Nd particulate inputs in the water column. Nd atmospheric flux appears to be the major source into the whole western basin, although lateral advection of riverine material is the prevailing process in the coastal environment. Offshore, the vertical propagation of an important Saharan dust event has been recorded for two months in sediment traps at 80, 200 and 1000 m. The evolution of the resulting negative epsilon-Nd(0) peak along depth and time shows that the particles reach 200 m on a time scale of one week. For the first time, the Nd budget in the western Mediterranean basin is constrained by both concentrations and isotopic compositions measured in particles and seawater. Surface budget requires a remobilization of 30 ± 20% of particulate Nd input. In deep water, dissolved Nd concentrations are balanced by a scavenging of 10 ± 20% of the sinking particulate flux. On the other hand, the deep isotopic compositions suggest an exchange between 30 ± 20% of the sinking particles and the deep waters. The hypothesis of a non-stationary regime for the surface waters in the Ligurian Sea is also considered.
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The global aerosol/climate model ECHAM5-HAM is used in order to investigate the dust cycle for four interglacial and one glacial climate conditions. The 20-year time-slices are the pre-industrial control (CTRL), mid-Holocene (6000 years BP), last glacial inception (115000 years BP), Eemian (126000 years BP) and Last Glacial Maximum (LGM) (21000 years BP) time intervals. The study is focused on the Antarctic region. The model is able to reproduce the magnitude order of dust deposition globally for the pre-industial and LGM climates. Correlation coefficient of the natural logarithm of the observed and modeled values is 0.78 for the CTRL and 0.81 for the LGM. For the pre-industrial simulation the model overestimates observed values in Antarctica by a factor of about 2-3 due to overestimation of the Australian dust source and too high wet deposition in the Antarctica interior. In the LGM, the model underestimates dust deposition in eastern Antarctica by a factor of about 4-5 due to underestimation of the South American dust source. More records are needed to validate dust deposition for the past interglacial time-slices. The modeled results show that dust deposition in Antarctica in the past interglacial time-slices is higher than in the CTRL simulation. The largest increase of dust deposition in Antarctica is simulated for the LGM, showing about 10-fold increase compared to CTRL.
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Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids produced by soil bacteria and occur in near coastal marine sediments as a result of soil organic matter input. Their abundance relative to marine-derived crenarchaeol, quantified in the BIT index, generally decreases offshore. However, in distal marine sediments, low relative amounts of brGDGTs can often still be observed. Sedimentary in situ production as well as dust input have been suggested as potential, though as yet not well constrained, sources. In this study brGDGT distributions in dust were examined and compared with those in distal marine sediments. Dust was sampled along the equatorial West African coast and brGDGTs were detected in most of the samples, albeit in low abundance. Their degree of methylation and cyclisation, expressed in the MBT' (methylation index of branched tetraethers) and DC (degree of cyclisation) indices, respectively, were comparable with those for African soils, their presumed source. Comparison of DC index values for brGDGTS in global soils, Congo deep-sea river fan sediments and dust with those of distal marine sediments clearly showed, however, that distal marine sediments had significantly higher values. This distinctive distribution is suggestive of sedimentary in situ production as a source of brGDGTs in marine sediments, rather than dust input. The presence of in situ produced brGDGTs in marine sediments means that caution should be exercised when applying the MBT'-CBT palaeothermometer to sediments with low BIT index values, i.e. < 0.1, based on our dataset.
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Nutrient supply in the area off Northwest Africa is mainly regulated by two processes, coastal upwelling and deposition of Saharan dust. In the present study, both processes were analyzed and evaluated by different methods, including cross-correlation, multiple correlation, and event statistics, using remotely sensed proxies of the period from 2000 to 2008 to investigate their influence on the marine environment. The remotely sensed chlorophyll-a concentration was used as a proxy for the phytoplankton biomass stimulated by nutrient supply into the euphotic zone from deeper water layers and from the atmosphere. Satellite-derived alongshore wind stress and sea-surface temperature were applied as proxies for the strength and reflection of coastal upwelling processes. The westward wind and the dust component of the aerosol optical depth describe the transport direction of atmospheric dust and the atmospheric dust column load. Alongshore wind stress and induced upwelling processes were most significantly responsible for the surface chlorophyll-a variability, accounting for about 24% of the total variance, mainly in the winter and spring due to the strong north-easterly trade winds. The remotely sensed proxies allowed determination of time lags between biological response and its forcing processes. A delay of up to 16 days in the surface chlorophyll-a concentration due to the alongshore wind stress was determined in the northern winter and spring. Although input of atmospheric iron by dust storms can stimulate new phytoplankton production in the study area, only 5% of the surface chlorophyll-a variability could be ascribed to the dust component in the aerosol optical depth. All strong desert storms were identified by an event statistics in the time period from 2000 to 2008. The 57 strong storms were studied in relation to their biological response. Six events were clearly detected in which an increase of chlorophyll-a was caused by Saharan dust input and not by coastal upwelling processes. Time lags of <8 days, 8 days, and 16 days were determined. An increase in surface chlorophyll-a concentration of up to 2.4 mg m**3 after dust storms in which the dust component of the aerosol optical depth was up to 0.9 was observed.
Resumo:
Corresponding millennial-scale climate changes have been reported from the North Atlantic region and from east Asia for the last glacial period on independent timescales only. To assess their degree of synchrony we suggest interpreting Greenland ice core dust parameters as proxies for the east Asian monsoon systems. This allows comparing North Atlantic and east Asian climate on the same timescale in high resolution ice core data without relative dating uncertainties. We find that during Dansgaard-Oeschger events North Atlantic region temperature and east Asian storminess were tightly coupled and changed synchronously within 5-10 years with no systematic lead or lag, thus providing instantaneous climatic feedback. The tight link between North Atlantic and east Asian glacial climate could have amplified changes in the northern polar cell to larger scales. We further find evidence for an early onset of a Younger Dryas-like event in continental Asia, which gives evidence for heterogeneous climate change within east Asia during the last deglaciation.
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Diamond dust (DD) refers to tiny ice crystals that form frequently in the Polar troposphere under clear sky conditions. They provide surfaces for chemical reactions and scatter light. We have measured the specific surface area (SSA) of DD at Barrow in March-April 2009. We have also measured its chemical composition in mineral and organic ions, dissolved organic carbon (DOC), aldehydes, H2O2, and the absorption spectra of water-soluble chromophores. Mercury concentrations were also measured in spring 2006, when conditions were similar. The SSA of DD ranges from 79.9 to 223 m**2/kg . The calculated ice surface area in the atmosphere reaches 11000 (±70%) µm**2/cm**3, much higher than the aerosol surface area. However, the impact of DD on the downwelling and upwelling light fluxes in the UV and visible is negligible. The composition of DD is markedly different from that of snow on the surface. Its concentrations in mineral ions are much lower, and its overall composition is acidic. Its concentrations in aldehydes, DOC, H2O2 and mercury are much higher than in surface snows. Our interpretation is that DOC from the oceanic surface microlayer, coming from open leads in the ice off of Barrow, is taken up by DD. Active chemistry in the atmosphere takes place on DD crystal surfaces, explaining its high concentrations in aldehydes and mercury. After deposition, active photochemistry modifies DD composition, as seen from the modifications in its absorption spectra and aldehyde and H2O2 content. This probably leads to the emissions of reactive species to the atmosphere.
Resumo:
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Resumo:
African dust outbreaks are the result of complex interactions between the land, atmosphere, and oceans, and only recently has a large body of work begun to emerge that aims to understand the controls on-and impacts of-African dust. At the same time, long-term records of dust outbreaks are either inferred from visibility data from weather stations or confined to a few in situ observational sites. Satellites provide the best opportunity for studying the large-scale characteristics of dust storms, but reliable records of dust are generally on the scale of a decade or less. Here the authors develop a simple model for using modern and historical data from meteorological satellites, in conjunction with a proxy record for atmospheric dust, to extend satellite-retrieved dust optical depth over the northern tropical Atlantic Ocean from 1955 to 2008. The resultant 54-yr record of dust has a spatial resolution of 1° and a monthly temporal resolution. From analysis of the historical dust data, monthly tropical northern Atlantic dust cover is bimodal, has a strong annual cycle, peaked in the early 1980s, and shows minimums in dustiness during the beginning and end of the record. These dust optical depth estimates are used to calculate radiative forcing and heating rates from the surface through the top of the atmosphere over the last half century. Radiative transfer simulations show a large net negative dust forcing from the surface through the top of the atmosphere, also with a distinct annual cycle, and mean tropical Atlantic monthly values of the surface forcing range from -3 to -9 W/m**2. Since the surface forcing is roughly a factor of 3 larger in magnitude than the top-of-the-atmosphere forcing, there is also a positive heating rate of the midtroposphere by dust.