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11.
Klaas R. Timmermans Martha Gledhill Rob F. Nolting Marcel J. W. Veldhuis Hein J. W. de Baar Constant M. G.van den Berg 《Marine Chemistry》1998,61(3-4)
Short-term iron enrichment experiments were carried out with samples collected in areas with different phytoplankton activity in the northern North Sea and northeast Atlantic Ocean in the summer of 1993. The research area was dominated by high numbers of pico-phytoplankton, up to 70,000 ml−1. Maximum chlorophyll a concentrations varied from about 1.0 μg l−1 in a high-reflectance zone (caused by loose coccoliths, remnants from a bloom of Emiliania huxleyi) and about 3.5 μg l−1 in a zone in which the phytoplankton were growing, to about 0.5 μg l−1 in the northeast Atlantic Ocean. From the high-reflectance zone to the northeast Atlantic Ocean, nitrate concentrations increased from 0.5 μM to 6.0 μM. Concentrations of reactive iron in surface water showed an opposite trend and decreased from about 2.6 nM in the high-reflectance zone to <1.0 nM in the northeast Atlantic Ocean. In the research area, no signs of true iron deficiency were found, but iron enrichments in the high-reflectance zone, numerically dominated by Synechococcus sp., resulted in increased nitrate uptake. Ammonium uptake was hardly affected. Strong support for the effect of Fe on cell physiology is given by the increase in the f-ratio. Net growth rates of the phytoplankton (changes in cell numbers over 24 h) were almost unchanged. Phytoplankton collected from the northeast Atlantic Ocean, did not show changes in the nitrogen metabolism upon addition of iron. Net growth rates in these incubations were low or negative, with only slightly higher values with additional iron. 相似文献
12.
Ramon D. Wolstencroft S. M. Scarrott T. M. Gledhill 《Astrophysics and Space Science》1995,224(1-2):395-398
The asymmetries in the brightness structure and levels of polarization between the two sides of the Pic disc are discussed. The possibility of these differences being caused by a planet, or planets, within the circumstellar disc is investigated. 相似文献
13.
Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl2–MgCl2 solutions at 1 bar (0.1 MPa) pCO2 and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca2+:Mg2+= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca2+:Mg2+= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 ? Omega;) n , where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10?2 mol m?2 hr?1. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10?2 mol m?2 hr?1). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca2+:Mg2+ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO2 sequestration. 相似文献
14.
15.
T. M. Gledhill A. Chrysostomou J. H. Hough J. A. Yates 《Monthly notices of the Royal Astronomical Society》2001,322(2):321-342
We use ground-based imaging polarimetry to detect and image the dusty circumstellar envelopes of a sample of protoplanetary nebulae (PPNe) at near-infrared wavelengths. This technique allows the scattered light from the faint envelope to be separated from the glare of the bright central star, and is particularly well suited to this class of object. We detect extended (up to 9-arcsec diameter) circumstellar envelopes around 15 out of 16 sources with a range of morphologies including bipolars and shells. The distribution of scattered light in combination with its polarization (up to 40 per cent) provides unambiguous evidence for axisymmetry in 14 objects, showing this to be a common trait of PPNe. We suggest that the range of observed envelope morphologies results from the development of an axisymmetric dust distribution during the superwind phase at the end of the AGB. We identify shells seen in polarized light with scattering from these superwind dust distributions, which allows us to provide constraints on the duration of the superwind phase. In one object (IRAS 19475+3119) the circumstellar envelope has a two-armed spiral structure, which we suggest results from the interaction of the mass-losing star with a binary companion. 相似文献
16.
Hα and HeI emission in the jets of pre-main sequence stars – implications for the heating mechanisms
M. Takami A. Chrysostomou J. Bailey T.M. Gledhill M. Tamura H. Terada 《Astrophysics and Space Science》2003,287(1-4):143-146
Jets and winds in pre-main sequence (PMS) stars are often heated and ionized, and understanding their heating mechanism is desired to compare UV-to-IR spectral signatures with models and thus investigate their driving mechanism. We review our results of long-slit spectroscopy and spectro-astrometry for He I 1.083 μm and Hα emission, and investigate the heating mechanisms in the jet in the outer (>10 AU) and inner (<10 AU) regions, respectively. 相似文献
17.
Circumstellar structure of RU Lupi down to au scales 总被引：1，自引：0，他引：1
M. Takami J. Bailey T. M. Gledhill A. Chrysostomou J. H. Hough 《Monthly notices of the Royal Astronomical Society》2001,323(1):177-187
We have used the technique of spectro-astrometry to study the milliarcsecond scale structure of the emission lines in the T Tauri star RU Lupi. The wings of the H α emission are found to be displaced from the star towards the south-west (blue wing) and north-east (red wing) with angular scales of 20–30 mas. This structure is consistent with a bipolar outflow from the star. From a study of the variability of the intensity and position spectra, we argue that a combination of magnetically driven bipolar outflow and accreting gas contributes to the H α emission. On the other hand, the [O i ] and [S ii ] emission are displaced from the star to the south-west but at much larger distances than the H α , hundreds of milliarcseconds for the high-velocity component (HVC) and down to 30 mas for the low-velocity components (LVCs). The presence of both redshifted and blueshifted outflows in H α but only a blueshifted outflow in the forbidden lines can be explained if the disc obscures the redshifted forbidden line outflow, but a disc gap with outer radius 3–4 au allows the redshifted H α to be seen. This gap could be induced by an unseen companion. 相似文献
18.
T. M. Gledhill I. Bains J. A. Yates 《Monthly notices of the Royal Astronomical Society》2002,332(3):L55-L58
Stars in the post-asymptotic giant branch (post-AGB) phase of evolution are surrounded by detached circumstellar envelopes containing dust which emits thermally in the mid- and far-infrared. Here we present 850-μm SCUBA photometry of nine candidate post-AGB stars. All targets are detected at 850 μm and we use these fluxes to estimate the envelope dust masses and, by comparison with the 100-μm IRAS fluxes, the dust emissivity index. 相似文献
19.
Nina Keul John W. Morse Rik Wanninkhof Dwight K. Gledhill Thomas S. Bianchi 《Aquatic Geochemistry》2010,16(3):337-351
This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term
factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ωaragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on
the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs
on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO2 fugacity (fCO2), total alkalinity (TA) and Ωaragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity.
However, inner shelf waters (<50 m depth) exhibited large variations in fCO2, TA, and Ωaragonite that were not directly related to salinity or distance from the Mississippi River plume. Changes in TA values were not the
result of simple mixing of end-member freshwater and seawater TA concentrations but exhibited a minimum in values near salinity
of 25. This minimum could be the result of microbial recycling across salinity gradients, biological removal of alkalinity
by formation of calcium carbonate or mixing of a third end-member with a low alkalinity such as Terrebonne Bay. All waters
were supersaturated with respect to aragonite. Offshore waters had an average Ωaragonite of 3.86 with a standard deviation of only ±0.06 and inner shelf waters had a range in Ωaragonite values from 3.9 to 9.7 with a median of 4.3. Shelf water Ωaragonite values were elevated relative to the offshore as a consequence of both high TA input from Mississippi River and biological
drawdown of CO2. A dominant factor controlling Ωaragonite distribution in offshore waters with relatively constant temperatures was fCO2, with higher supersaturation occurring in areas with low fCO2. 相似文献
20.
I. Bains J. A. Yates T. M. Gledhill A. M. S. Richards 《Monthly notices of the Royal Astronomical Society》2003,346(3):L46-L50
We report the diminution of the 1667-MHz OH maser in the post-asymptotic giant branch star IRAS 17436+5003, by a factor of ≳17 over a period of ≲12 yr, from observations with MERLIN. This circumstellar maser was detected by Likkel in 1987, at the 13σ level of her observations with the Green Bank Telescope. We discuss a number of possible reasons for this phenomenon and conclude that it is most likely due to turbulence arising from interacting stellar winds. 相似文献