Rates of transformation, recycling and burial of nitrogen and their temporal and spatial variability were investigated in deep-sea sediments of the Porcupine Abyssal Plain (PAP), NE Atlantic during eight cruises from 1996 to 2000. Benthic fluxes of ammonium (NH4) and nitrate (NO3) were measured in situ using a benthic lander. Fluxes of dissolved organic nitrogen (DON) and denitrification rates were calculated from pore water profiles of DON and NO3, respectively. Burial of nitrogen was calculated from down core profiles of nitrogen in the solid phase together with 14C-based sediment accumulation rates and dry bulk density. Average NH4 and NO3-effluxes were 7.4 ± 19 μmol m−2 d−1 (n = 7) and 52 ± 30 μmol m−2 d−1 (n = 14), respectively, during the period 1996–2000. During the same period, the DON-flux was 11 ± 5.6 μmol m−2 d−1 (n = 5) and the denitrification rate was 5.1 ± 3.0 μmol m−2 d−1 (n = 22). Temporal and spatial variations were only found in the benthic NO3 fluxes. The average burial rate was 4.6 ± 0.9 μmol m−2 d−1. On average over the sampling period, the recycling efficiency of the PON input to the sediment was 94% and the burial efficiency hence 6%. The DON flux constituted 14% of the nitrogen recycled, and it was of similar magnitude as the sum of burial and denitrification. By assuming the PAP is representative of all deep-sea areas, rates of denitrification, burial and DON efflux were extrapolated to the total area of the deep-sea floor (>2000 m) and integrated values of denitrification and burial of 8 ± 5 and 7 ± 1 Tg N year−1, respectively, were obtained. This value of total deep-sea sediment denitrification corresponds to 3–12% of the global ocean benthic denitrification. Burial in deep-sea sediments makes up at least 25% of the global ocean nitrogen burial. The integrated DON flux from the deep-sea floor is comparable in magnitude to a reported global riverine input of DON suggesting that deep-sea sediments constitute an important source of DON to the world ocean. 相似文献
The Stockholm archipelago spans roughly a semicircular area with a radius of approximately 60 km, traditionally partitioned into three parts: the inner, the middle and the outer archipelago. This subdivision coincides with differing water exchange regimes. The inner and middle archipelagos are characterised by comparatively larger basins which are interconnected by a limited number of straits. This configuration is well suited for a discrete basin (DB-) model approach by partitioning the area into a set of sub-basins that are only resolved vertically. The advantage of this approach over 3D-models is the possibility for enhanced vertical resolution and improved strait exchange formulation, outweighing the disadvantage of neglected horizontal gradients within the basins. In the inner archipelago the dominating exchange process is estuarine circulation, induced by the marked freshwater discharge and the vertical mixing. In the outer and middle archipelagos the density fluctuations due to Ekman pumping along the Baltic boundary interface produce another type of baroclinic process that clearly dominates. Measurements to adequately resolve these density variations do not exist. Missing forcing data are provided by linking the middle archipelago's boundary straits to a 3D-model of the Baltic with a grid resolution of 0.5 nautical miles (n.m.). This fine resolution model (FR-domain) is in turn driven by the atmospheric forcing and the density variation at the rectangular boundary of the FR-domain which acceptably resolves both the interfacial straits and the outer archipelago's complex hypsography. Massive computing resources would be demanded if the FR-domain were extended to comprise the entire Baltic. The FR-domain is thus interfaced with an existing coarse resolution model of the entire Baltic (CR-domain) with a grid size of 5 n.m., the open boundary of which is located in the Kattegat. This 3-fold model set-up has been run for one whole year (1992) with a one-year spin-up time to make up for the lack of initial data. The model concept is at this stage to be regarded as a framework for further development in anticipation of improved formulations, particularly for the strait exchange formulation. Therefore only primary validation experiments and a few sensitivity analyses have been performed. 相似文献
The dissipation, , of turbulent kinetic energy (TKE) is a key parameter in atmospheric boundary-layer (ABL) models. Besides being a sink for momentum, it is often used together with the TKE to define an internal turbulence time scale for closure relations. A prognostic formulation for the dissipation of TKE is formulated, based on isotropic tensor modeling methods. The formulation is coupled to a level 2.5 second-order closure model and evaluated against measurements taken in horizontally homogeneous conditions, as well as against a tailored length-scale formulation. A formulation suitable for convective as well as neutral and stable ABLs is suggested.On leave from Department of Meteorology, Uppsala University, P.O. Box 516, S-751 20 Uppsala, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
Hyporheic exchange flow (HEF) at the streambed–water interface (SWI) has been shown to impact the pattern and rate of discharging groundwater flow (GWF) and the consequential transport of heat, solutes and contaminants from the subsurface into streams. However, the control of geographic and hydromorphological catchment characteristics on GWF–HEF interactions is still not fully understood. Here, the spatial variability in flow characteristics in discharge zones was investigated and averaged over three spatial scales in five geographically different catchments in Sweden. Specifically, the deep GWF discharge velocity at the SWI was estimated using steady-state numerical models, accounting for the real multiscale topography and heterogeneous geology, while an analytical model, based on power spectral analysis of the streambed topography and statistical assessments of the stream hydraulics, was used to estimate the HEF. The modeling resulted in large variability in deep GWF and HEF velocities, both within and between catchments, and a regression analysis was performed to explain this observed variability by using a set of independent variables representing catchment topography and geology as well as local stream hydromorphology. Moreover, the HEF velocity was approximately two orders of magnitude larger than the deep GWF velocity in most of the investigated stream reaches, indicating significant potential to accelerate the deep GWF velocity and reduce the discharge areas. The greatest impact occurred in catchments with low average slope and in reaches close to the catchment outlet, where the deep GWF discharge velocity was generally low.
Comet 9P/Tempel 1 was the target of a multi-wavelength worldwide investigation in 2005. The NASA Deep Impact mission reached the comet on 4.24 July 2005, delivering a 370-kg impactor which hit the comet at 10.3 km s−1. Following this impact, a cloud of gas and dust was excavated from the comet nucleus. The comet was observed in 2005 prior to and after the impact, at 18-cm wavelength with the Nançay radio telescope, in the millimeter range with the IRAM and CSO radio telescopes, and at 557 GHz with the Odin satellite. OH observations at Nançay provided a 4-month monitoring of the outgassing of the comet from March to June, followed by the observation of H2O with Odin from June to August 2005. The peak of outgassing was found to be around between May and July. Observations conducted with the IRAM 30-m radio telescope in May and July 2005 resulted in detections of HCN, CH3OH and H2S with classical abundances relative to water (0.12, 2.7 and 0.5%, respectively). In addition, a variation of the HCN production rate with a period of 1.73±0.10 days was observed in May 2005, consistent with the 1.7-day rotation period of the nucleus. The phase of these variations, as well as those of CN seen in July by Jehin et al. [Jehin, E., Manfroid, J., Hutsemékers, D., Cochran, A.L., Arpigny, C., Jackson, W.M., Rauer, H., Schulz, R., Zucconi, J.-M., 2006. Astrophys. J. 641, L145-L148], is consistent with a rotation period of the nucleus of 1.715 days and a strong variation of the outgassing activity by a factor 3 from minimum to maximum. This also implies that the impact took place on the rising phase of the “natural” outgassing which reached its maximum ≈4 h after the impact. Post-impact observations at IRAM and CSO did not reveal a significant change of the outgassing rates and relative abundances, with the exception of CH3OH which may have been more abundant by up to one order of magnitude in the ejecta. Most other variations are linked to the intrinsic variability of the comet. The Odin satellite monitored nearly continuously the H2O line at 557 GHz during the 38 h following the impact on the 4th of July, in addition to weekly monitoring. Once the periodic variations related to the nucleus rotation are removed, a small increase of outgassing related to the impact is present, which corresponds to the release of ≈5000±2000 tons of water. Two other bursts of activity, also observed at other wavelengths, were seen on 23 June and 7 July; they correspond to even larger releases of gas. 相似文献
The study of young star cluster (YSC) systems, preferentially in starburst and merging galaxies, has seen great interest in the recent past, as it provides important input to models of star formation. However, even some basic properties (such as the luminosity function; LF) of YSC systems are still being debated. Here, we study the photometric properties of the YSC system in the nearest major merger system, the Antennae galaxies. We find evidence for the existence of a statistically significant turnover in the LF. 相似文献
Two European temperature reconstructions for the past half-millennium, January-to-April air temperature for Stockholm (Sweden) and seasonal temperature for a Central European region, both derived from the analysis of documentary sources and long instrumental records, are compared with the output of climate simulations with the model ECHO-G. The analysis is complemented by comparisons with the long (early)-instrumental record of Central England Temperature (CET). Both approaches to study past climates (simulations and reconstructions) are burdened with uncertainties. The main objective of this comparative analysis is to identify robust features and weaknesses in each method which may help to improve models and reconstruction methods. The results indicate a general agreement between simulations obtained with temporally changing external forcings and the reconstructed Stockholm and CET records for the multi-centennial temperature trend over the recent centuries, which is not reproduced in a control simulation. This trend is likely due to the long-term change in external forcing. Additionally, the Stockholm reconstruction and the CET record also show a clear multi-decadal warm episode peaking around AD 1730, which is absent in the simulations. Neither the reconstruction uncertainties nor the model internal climate variability can easily explain this difference. Regarding the interannual variability, the Stockholm series displays, in some periods, higher amplitudes than the simulations but these differences are within the statistical uncertainty and further decrease if output from a regional model driven by the global model is used. The long-term trend of the CET series agrees less well with the simulations. The reconstructed temperature displays, for all seasons, a smaller difference between the present climate and past centuries than is seen in the simulations. Possible reasons for these differences may be related to a limitation of the traditional ‘indexing’ technique for converting documentary evidence to temperature values to capture long-term climate changes, because the documents often reflect temperatures relative to the contemporary authors’ own perception of what constituted ‘normal’ conditions. By contrast, the amplitude of the simulated and reconstructed inter-annual variability agrees rather well. 相似文献
Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used for fast high-resolution, three-dimensional resistivity mapping. Standard interpretation tools are often based on layered earth inversion procedures which, in general, explain the HEM data sufficiently. As a HEM system is moved while measuring, noise on the data is a common problem. Generally, noisy data will be smoothed prior to inversion using appropriate low-pass filters and consequently information may be lost.For the first time the laterally constrained inversion (LCI) technique has been applied to HEM data combined with the automatic generation of dynamic starting models. The latter is important because it takes the penetration depth of the electromagnetic fields, which can heavily vary in survey areas with different geological settings, into account. The LCI technique, which has been applied to diverse airborne and ground geophysical data sets, has proven to be able to improve the HEM inversion results of layered earth structures. Although single-site 1-D inversion is generally faster and — in case of strong lateral resistivity variations — more flexible, LCI produces resistivity — depth sections which are nearly identical to those derived from noise-free data.The LCI results are compared with standard single-site Marquardt–Levenberg inversion procedures on the basis of synthetic data as well as field data. The model chosen for the generation of synthetic data represents a layered earth structure having an inhomogeneous top layer in order to study the influence of shallow resistivity variations on the resolution of deep horizontal conductors in one-dimensional inversion results. The field data example comprises a wide resistivity range in a sedimentary as well as hard-rock environment.If a sufficient resistivity contrast between air and subsurface exists, the LCI technique is also very useful in correcting for incorrect system altitude measurements by using the altitude as a constrained inversion parameter. 相似文献