摘要:

In April 1990, forty‐two scientists from eight countries attended a workshop at the Bermuda Biological Station for Research to compare field measurements with model estimates of the distribution and cycling of sulfur and nitrogen species in the North Atlantic Ocean's atmosphere. Data sets on horizontal and vertical distributions of sulfur and nitrogen species and their rates of deposition were available from ships' tracks and island stations. These data were compared with estimates produced by several climatological and event models for two case studies: (1) sulfate surface distributions and deposition and (2) nitrate surface distributions and deposition. Highlights of the conclusions of the case studies were that the measured concentrations and model results of nitrate and non‐sea‐salt sulfate depositions appeared to be in good agreement at some locations but in poor agreement for some months at other locations. The case studies illustrated the need for the measurement and modeling communities to interact not only to compare results but also to cooperate in improving the designs of the models and the field experi

ISSN:0886-6236    

DOI:10.1029/91GB02977

年代:1992

数据来源: WILEY

摘要:

We use the terrestrial ecosystem model (TEM), a process‐based model, to investigate how interactions between carbon (C) and nitrogen (N) dynamics affect predictions of net primary productivity (NPP) for potential vegetation in North America. Data on pool sizes and fluxes of C and N from intensively studied field sites are used to calibrate the model for each of 17 non‐wetland vegetation types. We use information on climate, soils, and vegetation to make estimates for each of 11,299 non‐wetland, 0.5° latitude × 0.5° longitude, grid cells in North America. The potential annual NPP and net N mineralization (NETNMIN) of North America are estimated to be 7.032 × 1015g C yr−1and 104.6 × 1012g N yr−1, respectively. Both NPP and NETNMIN increase along gradients of increasing temperature and moisture in northern and temperate regions of the continent, respectively. Nitrogen limitation of productivity is weak in tropical forests, increasingly stronger in temperate and boreal forests, and very strong in tundra ecosystems. The degree to which productivity is limited by the availability of N also varies within ecosystems. Thus spatial resolution in estimating exchanges of C between the atmosphere and the terrestrial biosphere is improved by modeling the linkage between C and N dynamics. We also perform a factorial experiment with TEM on temperate mixed forest in North America to evaluate the importance of considering interactions between C and N dynamics in the response of NPP to an elevated temperature of 2°C. With the C cycle uncoupled from the N cycle, NPP decreases primarily because of higher plant respiration. However, with the C and N cycles coupled, NPP increases because productivity that is due to increased N availability more than offsets the higher costs of plant respiration. Thus, to investigate how global change will affect biosphere‐atmosphere interactions, process‐based models need to consider linkages between

ISSN:0886-6236    

DOI:10.1029/92GB00219

年代:1992

数据来源: WILEY

摘要:

Abstract. Stable hydrogen (δD) and carbon (δ13C) isotope ratios of sedimentary methane from five subtropical Florida freshwater sites exhibited smaller, less distinct seasonal variations than previously observed in temperate sediments, apparently due to the smaller range of temperatures forcing the subtropical environments. Negative correlations observed between δD‐CH4and δ13C‐CH4at these sites probably resulted from mixing methane produced via acetate fermentation with methane formed from CO2and H2. The inverse δD/δ13C trends, high sedimentary gas methane concentrations (up to 80 mole%), and visual observations imply that ebullition dominates methane transport from the sediments during at least part of the seasonal cycle. Many values deviated from the main inverse δD/δ13C trend such that bacterial oxidation was not indicated. The deviation may result from the reduction of CO2with varying δ13C under conditions of elevated H2concentrations or enhanced interspecies hydrogen transfer, which may cause relatively greater hydrogen isotopic fractionation. Alternatively, the deviation may result from fermentation of autotrophically formed,13C‐depleted acetate. Differences in the δ13C of bulk organic matter from two of the sites potentially account for about half of the difference in mean δ13C‐CH4between the two sites. In other cases, however, the immediate methanogenic precursor (e.g., acetate versus CO2/H2) appeared to be the major determinant of methane stable is

ISSN:0886-6236    

DOI:10.1029/92GB00155

年代:1992

数据来源: WILEY

摘要:

Abstract. This paper summarizes 4 years (1987‐1990) of weekly net CH4flux measurements at permanent sites representing important plant components of Arctic tundra. The data coincide with variations in precipitation and temperature of interest in regional and global modeling efforts and are useful in placing bounds on the role of tundra in the global CH4budget. Precipitation in the study area during the summer emission period ranged from twice to half the long‐term mean, and air temperature anomalies were about +2 °C. This data set also permits consideration of temporal (seasonal to interannual) and spatial variability in CH4flux. We studied the relationship between the net CH4flux and subsurface properties (water table depth, thaw depth, soil temperature, /pCH4distributions) at these permanent sites during the 1988 and 1989 emission periods. Net CH4emission and subsurface properties are largely unrelated. Relationships between soil temperature (or any single variable) and emission are site specific and are of little value as flux predictors. Parameters that integrate conditions influencing flux appear to be the best flux predictors over the emission period. We estimate that Arctic wet meadow and tussock:shrub tundra presently emit about 42 ± 26 Tg CH4yr−1to the atmosphere. This estimate has a North American bias, but it is supported by measurements in a range of locations, transect studies, and model calcu

ISSN:0886-6236    

DOI:10.1029/92GB00430

年代:1992

数据来源: WILEY

摘要:

Abstract. Remote marine aerosols collected at four Pacific island stations (Midway, Oahu, Enewetak, and Fanning) and during four Pacific cruises were used in determining the chemical form of iron in marine atmospheric aerosols. Aerosols collected from an urban area, Xian, near the Chinese loess plateau were also studied using the same procedures. Marine and urban Chinese aerosol samples and Chinese loess samples were leached by acidic deionized water over a pH range of 2.0–5.6 to determine the extent of dissolution of iron in atmospheric waters. The results indicated that (1) more than 90% of the atmospheric iron in remote marine aerosols over the North Pacific is released in 2 N acidic solutions, and in the mean ~50% of the atmospheric iron in remote marine aerosols was in the form of Fe(II); and (2) the solubility of iron in the marine aerosols was 5–17 times greater than that of iron in Chinese loess at the same pH. This may be largely due to the photoreduction of Fe(III) to Fe(II) during its long‐range transport. At a total iron concentration of less than 20 nmol/kg, ∼55% of the atmospheric iron dissolves in solutions with a pH of 3.8 and 25–30% dissolves at a pH of 4.8. We postulate that (1) the mixing of mineral dust with sulfate/sulfuric acid aerosols as a result of coalescence processes within clouds might be a mechanism to increase the solubility of atmospheric iron in cloud water or rainwater; and (2) photochemical reduction of Fe(III) to Fe(II) may be an important mechanism increasing the solubility of iron, which would be rapidly released to rainwater, fog, or cloud droplets when the aerosol particles contact them during long‐range transport over the open ocean. The total atmospheric iron input was estimated to be ∼15 ‐ 25 × 1012g/yr for the entire Nor

ISSN:0886-6236    

DOI:10.1029/92GB00756

年代:1992

数据来源: WILEY

摘要:

Carbonyl sulfide (COS) measurements in the surface waters of the temperate and subtropical North Atlantic and the Gulf of Mexico showed COS to be supersaturated with respect to atmospheric equilibrium at almost all locations and times investigated. Its concentration follows a pronounced diel cycle which can be modeled using a simple equation which includes hydrolysis removal and photo‐chemical production. The concentration and ocean/atmosphere flux of COS depends strongly on marine productivity, the highest levels being found in coastal and shelf waters. Analysis of presently available data yields an estimate of 13 Gmol yr‐1(0.41 Tg S(COS) yr−1) for the total flux of COS from the oceans to the atmos

ISSN:0886-6236    

DOI:10.1029/91GB02809

年代:1992

数据来源: WILEY

摘要:

Reports of the13C content of marine particulate organic carbon are compiled and on the basis of GEOSECS data and temperatures, concentrations, and isotopic compositions of dissolved CO2in the waters in which the related phytoplankton grew are estimated. In this way, the fractionation of carbon isotopes during photosynthetic fixation of CO2is found to be significantly correlated with concentrations of dissolved CO2. Because ancient carbon isotopic fractionations have been determined from analyses of sedimentary porphyrins [Popp et al., 1989], the relationship between isotopic fractionation and concentrations of dissolved CO2developed here can be employed to estimate concentrations of CO2dissolved in ancient oceans and, in turn, partial pressures of CO2in ancient atmospheres. The calculations take into account the temperature dependence of chemical and isotopic equilibria in the dissolved‐inorganic‐carbon system and of air‐sea equilibria. Paleoenvironmental temperatures for each sample are estimated from re‐constructions of paleogeography, latitudinal temper‐ ature gradients, and secular changes in low‐latitude sea surface temperature. It is estimated that atmospheric partial pressures of CO2were over 1000 μatm 160–100 Ma ago, then declined to values near 300 μatm during the next 100 Ma. Analysis of a high‐resolution record of carbon isotopic fractionation at the Cenomanian‐Turonian boundary suggests that the partial pressure of CO2in the atmosphere was drawn down from values near 840 μatm to values near 700 μatm du

ISSN:0886-6236    

DOI:10.1029/92GB00190

年代:1992

数据来源: WILEY

摘要:

In situ microelectrode, box‐core pore water gradient, and in situ benthic chamber estimates of organic carbon degradation and CaCO3dissolution are combined with organic‐C and carbonate‐C accumulation rates to approximate the total carbon flux to the seafloor along two transects of the continental slope and rise off central California. Microelectrode profiles of dissolved O2demonstrate that sediments at 13 sites, ranging in water depth from 580 to 4080 m, become anoxic below the uppermost 0.4–3 cm of the sediment column. If a current‐swept area of nondeposition on the upper slope is excluded, we find total organic‐C and carbonate‐C fluxes to the seafloor vary from 40 to 100 μmol C cm−2yr−1and from 32 to 91 μmol C cm−2yr−1, respectively. From the distribution of these fluxes there is no indication that total fluxes or remineralization rates of organic or carbonate carbon are influenced markedly by conditions in the oxygen minimum zone. Instead, the upper continental rise with its system of submarine valleys and fans stands out as the most important locus for carbon deposition and remineralization. When benthic fluxes and burial rates are extrapolated over the whole slope and rise of the region, aerobic respiration is the major mechanism of organic matter oxidation, and organic‐C and carbonate‐C recycling are on average 87% and 98% efficient, respectively. These results suggest that modern sediments on the outer regions of continental margins are important sources of CO2that is injected directly into ocean deep water. However, if benthic carbon fluxes on the central California margin are typical of margins globally, this injection rate is less than 0.7 Gt C yr−1, which does not indicate a significant anthropogenic enhancement of carbon export to

ISSN:0886-6236    

DOI:10.1029/92GB00105

年代:1992

数据来源: WILEY