Abundance and size distribution of transparent exopolymer particles (TEP) in a coccolithophorid bloom in the northern Bay of Biscay     

Jérôme Harlay  Caroline De Bodt  Anja Engel  Sandra Jansen  Quentin d’Hoop  Judith Piontek  Nicolas Van Oostende  Steve Groom  Koen Sabbe  Lei Chou 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(8):1251-1265

The distribution of transparent exopolymer particles (TEP) was investigated during a coccolithophorid bloom in the northern Bay of Biscay (North Atlantic Ocean) in early June 2006. MODIS chlorophyll-a (Chl-a) and reflectance images before and during the cruise were used to localize areas of important biological activity and high reflectance (HR). TEP profiles along the continental margin, determined using microscopic (TEP_micro) and colorimetric (TEP_color) methods, showed abundant (6.1×10⁶–4.4×10⁷ L^?1) and relatively small (0.5–20 μm) particles, leading to a low total volume fraction (0.05–2.2 ppm) of TEP_micro and similar vertical profiles of TEP_color. Estimates of carbon content in TEP (TEP-C) derived from the microscopic approach yielded surface concentration of 1.50 μmol C L^?1. The contribution of TEP-C to particulate organic carbon (POC) was estimated to be 12% (molar C ratio) during this survey. Our results suggest that TEP formation is a probable first step to rapid and efficient export of C during declining coccolithophorid blooms.  相似文献   

Nitrogen, phosphorus, silica, and carbon in Moreton Bay, Queensland, Australia: Differential limitation of phytoplankton biomass and production     

Patricia M. Glibert  Cynthia A. Heil  Judith M. O'Neil  William C. Dennison  Mark J. H. O'Donohue 《Estuaries and Coasts》2006,29(2):209-221

Subtropical estuaries have received comparatively little attention in the study of nutrient loading and subsequent nutrient processing relative to temperate estuaries. Australian estuaries are particularly susceptible to increased nutrient loading and eutrophication, as 75% of the population resides within 200 km of the coastline. We assessed the factors potentially limiting both biomass and production in one Australian estuary, Moreton Bay, through stoichiometric comparisons of nitrogen (N), phosphorus (P), silicon (Si), and carbon (C) concentrations, particulate compositions, and rates of uptake. Samples were collected over 3 seasons in 1997–1998 at stations located throughout the bay system, including one riverine endmember site. Concentrations of all dissolved nutrients, as well as particulate nutrients and chlorophyll, declined 10-fold to 100-fold from the impacted western embayments to the eastern, more oceanic-influenced regions of the bay during all seasons. For all seasons and all regions, both the dissolved nutrients and particulate biomass yielded N:P ratios <6 and N:Si ratios <1. Both relationships suggest strong limitation of biomass by N throughout the bay. Limitation of rates of nutrient uptake and productivity were more complex. Low C:N and C:P uptake ratios at the riverine site suggested light limitation at all seasons, low N:P ratios suggested some degree of N limitation and high N:Si uptake ratios in austral winter suggested Si limitation of uptake during that season only. No evidence of P limitation of biomass or productivity was evident.  相似文献   

Stable carbon and oxygen isotopic evidence for late Pleistocene to middle Holocene climatic fluctuations in the interior of southern Africa     

Jeannette M. Smith  Julia A. Lee‐Thorp  Judith C. Sealy 《第四纪科学杂志》2002,17(7):683-695

Stable carbon and oxygen isotope analyses of ungulate grazers from four archaeological sites located in different environs within the Caledon River Valley have provided a relatively well‐dated proxy palaeoenvironmental and palaeoclimatic sequence for the period between 16 000 and 6000 calendar (cal.) yr BP. Within the overall trend towards hot mid‐Holocene temperatures and a summer rainfall pattern, stable carbon isotope results show that there were three periods when growth season temperatures were cool enough for C₃ grasses to be present: 16 000–14 000; 10 200–9600, and 8400–8000 cal. yr BP. Similar trends were recorded in stable oxygen isotope values, reflecting shifts in either temperature or available moisture. Although having a similar pattern to that of the lower altitude site, sites situated in foothills and montane portions of the valley consistently maintained lower temperatures until the mid‐Holocene altithermal. At this time growth season temperatures warmed sufficiently for a 100% C₄ grassland to expand in altitude from the warmer low lying localities. In relation to present understanding of synoptic and global climatic patterning, these findings suggest that the early to middle Holocene transition was not a gradual warming trend, but rather it was marked by a series of climatic fluctuations. Of particular note is the possible global, rather than regional, occurrence of the 8200 cal. yr BP ‘event’. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

The relative abundance of the isotopes of Li,Be and B and the age of cosmic rays     

W. R. Webber  J. A. Lezniak  J. Kish  S. V. Damle 《Astrophysics and Space Science》1973,24(1):17-29

Using a balloon borne double dE/dx x total energy telescope we have determined the isotopic composition of cosmic ray Li, Be and B nuclei in the energy range 100–250 MeV nuc.^–1. The measured mass resolution, for these nuclei is 0.3 AMU. The observed isotopic composition is in agreement with that predicted on the basis of interstellar fragmentation with the exception of a deficiency of Be¹⁰. If the low abundance of Be¹⁰ is attributed to the decay of this radioactive isotope we obtain a mean cosmic ray lifetime of (3.4 _–1.3 ^+3.4 )×10⁶ yr.A recent measurement which we have used in this paper gives this lifetime to be (1.5±0.3)×10⁶ yr (Yiou and Raisbeck, 1972).  相似文献   

A comparison of the energy spectra of cosmic ray helium and heavy nuclei     

T. T. von Rosenvinge  W. R. Webber  J. F. Ormes 《Astrophysics and Space Science》1969,5(3):342-359

Recent observations of the spectra of cosmic ray helium, M, LH and VH nuclei in the energy range from 200 MeV/nuc to>22 GeV/nuc are reported. The differential spectra of all of these groups of nuclei are found to have a maximum at 300–400 MeV/nuc at sunspot minimum. The average exponents on the integral rigidity spectra in the range 5 to 50 GV are 1.54±0.03 for He nuclei, 1.50±0.04 for M nuclei, 1.47±0.06 for LH nuclei and 1.40±0.08 for VH nuclei. The spectra of these groups of nuclei are compared and it is found that the average He/M, He/LH and He/VH ratios are 16±1, 70±3 and 200±15 respectively. These values are reasonably constant from the highest down to the lowest energies measured although some evidence for a dip is present in the 500–1000 MeV/nuc range for both the He/LH and He/VH ratios. Solar modulation effects on these ratios are discussed and it is concluded that the ratios measured at earth are representative of those existing in interstellar space only if energy loss processes in interplanetary space are unimportant. The influence of interstellar propagation on the spectra and charge ratios at low energies arising from ionization energy loss and nuclear spallation during matter traversal is examined. It is found that propagation models that contain a large number of relatively short path lengths significantly modify the expected effects of ionization energy loss at low energies. Specifically it is suggested that the presently measured charge ratios are consistent with the passage of the average cosmic radiation through enough matter to reproduce the abundances of the so-called fragmentation nuclei, Li, Be, B and He³. Two component models are not required to explain our data. Rather we feel that a better representation of the situation results from considering a continuous spatial distribution of sources which, along with the actual interstellar propagation conditions, leads to a particular distribution of matter path lengths. It is pointed out that large differences exist in the approaches used in the literature to calculate the effects of matter traversal in interstellar space at low energies. These differences play an important role in the interpretation of the experimental results. Significant modifications of the charge ratios at low energies can also be obtained by requiring that some of the matter traversal occur in the cosmic ray sources themselves during the cosmic ray acceleration process. This may be sufficient to produce charge ratios that are essentially flat at low energies even in the presence of interstellar ionization loss.  相似文献   

Evidence for the existence of adiabatic energy loss in interplanetary space from observations of the decay of the February 25–March 2, 1969 series of solar cosmic ray events     

J. A. Lezniak  W. R. Webber 《Solar physics》1972,26(2):474-483

The integral flux of low energy protons (> 10 MeV) observed by the University of New Hampshire cosmic ray detector aboard the Pioneer 9 spaceprobe has been compared with similar measurements of the near-Earth spacecraft Explorer 34 during the decay phase of the February 25–March 2, 1969 series of solar cosmic ray events. At this time the Pioneer 9 spaceprobe was 0.8 AU from the Sun and close to the Sun-Earth radial line. The ratio of integral fluxes as measured by the separated spacecraft can be calculated theoretically during the convective phase of the decay of these events and will depend on whether energy loss processes are operative. A comparison of the observed and theoretically calculated ratios suggests that the adiabatic energy loss process is operative.  相似文献   

‘Evolution of a Storm-driven Cloudy Boundary Layer in the Arctic’     

Jun?InoueEmail author  Branko?Kosovi?  Judith?A.?Curry 《Boundary-Layer Meteorology》2005,117(2):213-230

To investigate the processes of development and maintenance of low-level clouds during major synoptic events, the cloudy boundary layer under stormy conditions during the summertime Arctic has been studied using observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and large-eddy simulations (LES). On 29 July 1998, a stable Arctic cloudy boundary-layer event was observed after the passage of a synoptic low pressure system. The local dynamic and thermodynamic structure of the boundary layer was determined from aircraft measurements including the analysis of turbulence, cloud microphysics and radiative properties. After the upper cloud layer advected over the existing cloud layer, the turbulent kinetic energy (TKE) budget indicated that the cloud layer below 200 m was maintained predominantly by shear production. Observations of longwave radiation showed that cloud-top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our LES results demonstrate the importance of the combination of shear mixing near the surface and radiative cooling at the cloud top in the storm-driven cloudy boundary layer. Once the low-level cloud reaches a certain height, depending on the amount of cloud-top cooling, the two sources of TKE production begin to separate in space under continuous stormy conditions, suggesting one possible mechanism for the cloud layering. The sensitivity tests suggest that the storm-driven cloudy boundary layer is possibly switched to the shear-driven system due to the advection of upper clouds or to the buoyantly driven system due to the lack of wind shear. A comparison is made of this storm-driven boundary layer with the buoyantly driven boundary layer previously described in the literature.  相似文献   

Lacustrine oxygen isotope record of 20th-century climate change in central EuropeL evaluation of climatic controls on oxygen isotopes in precipitation     

Jane L. Teranes  Judith A. McKenzie 《Journal of Paleolimnology》2001,26(2):131-146

We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (¹⁸O_p) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (¹⁸O_c). Comparison of the calcite oxygen isotope proxy for ¹⁸O_p in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This ¹⁸O_c/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite ¹⁸O values since the 1960s. Possible explanations for this recent ¹⁸O_c shift, as it is not related to mean annual air temperature, include changes in ¹⁸O_p due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in ¹⁸O_c data.  相似文献   

Dolomite formation and biogeochemical cycles in the Phanerozoic   总被引：7，自引：0，他引：7  

Stephen J. Burns  Judith A. Mckenzie†  Crisogono Vasconcelos† 《Sedimentology》2000,47(S1):49-61

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Micromechanical modeling for rate‐dependent behavior of salt rock under cyclic loading     

Xianda Shen  Jihui Ding  Chlo Arson  Judith S. Chester  Frederick M. Chester 《国际地质力学数值与分析法杂志》2021,45(1):28-44

The dependence of rock behavior on the deformation rate is still not well understood. In salt rock, the fundamental mechanisms that drive the accumulation of irreversible deformation, the reduction of stiffness, and the development of hysteresis during cyclic loading are usually attributed to intracrystalline plasticity and diffusion. We hypothesize that at low pressure and low temperature, the rate‐dependent behavior of salt rock is governed by water‐assisted diffusion along grain boundaries. Accordingly, a chemo‐mechanical homogenization framework is proposed in which the representative elementary volume (REV) is viewed as a homogeneous polycrystalline matrix that contains sliding grain‐boundary cracks. The slip is related to the mass of salt ions that diffuse along the crack surface. The relationship between fluid inclusion‐scale and REV‐scale stresses and strains is established by using the Mori–Tanaka homogenization scheme. It is noted from the model that a lower strain rate and a larger number of sliding cracks enhance stiffness reduction and hysteresis. Thinner sliding cracks (i.e., thinner brine films) promote stiffness reduction and accelerate stress redistributions. The larger the volume fraction of the crack inclusions, the larger the REV deformation and the larger the hysteresis. Results presented in this study shed light on the mechanical behavior of salt rock that is pertinent to the design of geological storage facilities that undergo cyclic unloading, which could help optimize the energy production cycle with low carbon emissions.  相似文献