摘要:

The feasibility of deriving photosynthetically active radiation (PAR) from satellite observations on global scale was previously demonstrated (Pinker and Laszlo, 1992). The methodology used was evaluated against available ground truth (Pinker et al., 1993). The objective of this study was to provide, for a first time, consistent information on the temporal and spatial variability in PAR on a continental scale during a growing season; such information can be used to develop new parameterizations of net primary productivity and evapotranspiration, that till now were not possible.

ISSN:0886-6236    

DOI:10.1029/93GB03395

年代:1994

数据来源: WILEY

摘要:

Methane emission from Philippine rice paddies was monitored with a closed chamber technique during the 1991 and 1992 wet season. The methane emission from plots amended with 6.66 tons.ha−1gypsum was reduced by 55–70% compared to non‐amended plots. Although CH4emission from fields with a high input of fresh organic matter was strongly enhanced, the experiments showed that the relative reduction in CH4emission upon gypsum application was independent of organic matter addition. The reduced CH4emission upon gypsum application was most likely due to inhibition of methanogenesis by sulfate‐reducing bacteria. Observed SO42−concentrations in the soil solution of gypsum‐amended plots were well above minimum concentrations reported in the literature for successful competition of sulfate‐reducing bacteria with methanogens. The data provide a base for reducing the estimates of CH4emissions from rice grown on high‐sulfate containing soils or gypsu

ISSN:0886-6236    

DOI:10.1029/94GB00386

年代:1994

数据来源: WILEY

摘要:

Flooded rice fields emit methane and are important contributors to the increasing atmospheric methane concentration. Various estimates of global release rates of methane from rice paddies range from a low of 20 Tg per year to a high of 200 Tg per year. Global estimates of methane emissions from rice fields depend upon obtaining reliable data from a variety of soil types. We have compared a variety of methane emission data sets obtained over a four‐year period from three different soil types found at the Texas Agricultural Experiment Station near Beaumont, Texas, with several physical and chemical properties of the soils. We find that seasonal methane emissions directly correlate with the percent sand in the soils. Along a transect with soil sand content ranging from 18.8% to 32.5%, seasonal methane emissions ranged from 15.1 g m−2to 36.3 g

ISSN:0886-6236    

DOI:10.1029/94GB00588

年代:1994

数据来源: WILEY

摘要:

A model is used to evaluate the relative importance of temperature, soilpCO2, and organic acidity on silicate weathering rates in an average soil of warm temperate climate. The model has a structure similar to the Model of Acidification of Groundwater in Catchments, (Cosby et al., 1985a, b) used to predict catchment responses to acid deposition, but it is modified to account for the effects of temperature, the partial pressure of carbon dioxide in the soil, and organic acidity on dissolution of silicate minerals. The model is run with a mean soil temperature of 15°C, and when equilibrium is achieved it is raised to 19°C. SoilpCO2and organic acids adjust accordingly. It is found that temperature is the main control of the weathering rate; the other two factors have a very minor forcing effect. The transient after the step function disturbance occurs on a century to millennial timescale. Consequently, it is not possible to study weathering rates through either purposeful soil manipulations lasting several years or by examining the effect on soils of land use changes because steady state may have not been achieved ye

ISSN:0886-6236    

DOI:10.1029/94GB00491

年代:1994

数据来源: WILEY

摘要:

The role of the ocean as a sink for anthropogenic carbon dioxide is a subject of intensive investigation and debate. Interest in this process is driven by the need to predict the rate of future increase of atmospheric carbon dioxide and subsequent global climatic change. Although estimates of the magnitude of the oceanic sink for carbon dioxide appear to be converging on a value of ∼2 (Gt) C yr−1for the 1980s, a detailed understanding of the temporal and spatial variability in the rate of exchange of carbon dioxide between the ocean and the atmosphere is not available. For example, recent modeling work and direct measurements of air‐sea carbon dioxide flux produce very different estimates of the air‐sea flux in the northern hemisphere. As a consequence, it has been suggested that a large unidentified oceanic carbon dioxide sink may exist in the North Pacific. As a part of our time series observations in the North Pacific Subtropical Gyre, we have measured dissolved inorganic carbon and titration alkalinity over a four‐year period. These measurements constitute the most extensive set of observations of carbon system parameters in the surface waters of the central Pacific Ocean. Our results show that the ocean in the vicinity of the time series site is a sink for atmospheric carbon dioxide. On the basis of these observations, we present a mechanism by which the North Pacific Subtropical Gyre can be a potential sink for ∼0.2 Gt C yr−1of atmospheric carbon dioxide. Although our observations indicate that the North Pacific Subtropical Gyre is a sink for atmospheric carbon dioxide, the magnitude of this oceanic sink is relatively small. Our data and interpretations are therefore consistent with the argument for a relatively large sink during the 1980s in northern hemisphere terrestrial biomass. Another possibility is that the net release of carbon dioxide to the atmosphere owing to land use activities in tropical regions has been

ISSN:0886-6236    

DOI:10.1029/94GB00387

年代:1994

数据来源: WILEY

摘要:

The nutrient cycles of coastal upwelling zones are studied with simple mass balance models of Ekman transport, longshore transport, surface productivity, and dissolved phosphorous. The models are constrained with data from the Peru, northwest Africa, and Oregon upwelling systems. The onshore‐offshore mass balance model agrees with published Ekman transport, surface productivity, and nutrient data as well as hypothesized nutrient f‐ratios for highly productive coastal settings. The onshore‐offshore model suggests that increased primary productivity in glacial‐era coastal upwelling zones was not a linear function of Ekman transport, but instead was probably dependent on the physical and chemical dynamics of a specific setting. In the Peru upwelling system, longshore equatorward surface currents and poleward undercurrents produce positive surface nutrient gradients in the equatorward direction and relatively constant gradients in subsurface waters. Longshore nutrient gradients off northwest Africa are positive in the equatorward direction for both surface and subsurface waters. These observations are consistent with the conceptual model of surface and subsurface currents which are moving toward the equator and continually being upgraded by the offshore flux of nutrients. The northwest Africa and Peru data are not consistent with the longshore nutrient model of Redfield et al.

ISSN:0886-6236    

DOI:10.1029/94GB00097

年代:1994

数据来源: WILEY

摘要:

We conducted an experiment to test explicitly the accuracy of upper ocean sediment trap fluxes using the particle‐reactive radionuclide234Th (t1/2= 24.1 days). Two independent VERTEX‐style multitrap arrays were used for collection of sinking particles at 95 m and 97 m depths over a four‐day period in May 1992 at the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time‐series Study (BATS) site. Samples for total234Th were collected every 8 m between the surface and 96 m and immediately combined for analysis to obtain the vertically integrated activity of234Th. We collected a total of 27 samples over the four‐day period. The234Th samples were collected daily at each of the two traps and every other day on a 6 × 6 km grid to characterize the entire source region for particles collected in the traps. In situ flow sensors at one trap array indicated low horizontal shear at the trap mouth (5‐10 cm/s) compared to normal values at BATS. The predicted234Th flux from the watercolumn profiles was not significantly different from zero (−30 ± 140 disintegrations per minute/m2/d). The measured trap234Th flux at both arrays was significantly higher (290 ± 15 dpm/m2/d). We hypothesize that upper ocean traps at Bermuda may overcollect during low‐flux periods and undercollect during high‐flux periods, thus recording a biased signal of th

ISSN:0886-6236    

DOI:10.1029/94GB00207

年代:1994

数据来源: WILEY

摘要:

Sedimentologic and geochemical studies of box and gravity cores recovered from the Black Sea during the first leg of a multileg international Black Sea expedition in 1988 allow reconstruction of the basinwide Holocene environmental history of the Black Sea. In the deeper parts of the basin, box cores typically recovered a flocculent surface layer (“fluff”), laminated coccolith marls of Unit I (25–45 cm thick), and the upper 5–10 cm of finely laminated, dark‐colored sapropels of Unit II. Fine‐grained, homogeneous mud turbidites are interbedded with Units I and II over much of the basin, but the stratigraphie position of these turbidites differs, from site to site. The deposition of individual turbidites up to 15 cm thick does not appear to have significantly disturbed underlying laminae. Sediment trap deployments in the Black Sea suggest that light and dark laminae couplets represent annual increments of sedimentation (i.e., varves); we have therefore constructed a varve chronology for the sequence in order to correlate and date distinctive sedimentation and paleoenvironmental events. Distinctive groups of laminae in Unit I can be correlated across the entire deeper basin (a distance of more than 1000 km). This implies a remarkable homogeneity in production, accumulation, and preservation of biogenic material over much of the Black Sea during deposition of Unit I. The change from deposition of finely laminated, organic carbon‐rich sapropels (Unit II) to laminated, more calcareous, coccolith‐rich marls (Unit I) is thought to represent the crossing of a salinity threshold forEmiliania huxleyi.The varve chronology sets this change at about 1.63 ka (1633±100 yr B.P.), but the record of magnetic secular variation measured in several cores produces an age estimate of about 2.0 ka for the base of Unit I, or about 1.2 times the varve age. The average of six calibrated accelerator mass spectrometry radiocarbon ages for the base of Unit I is 2.7 ka, or about 1.7 times the varve age. Following the initial change to coccolith‐dominated sedimentation, deposition of sapropel resumed for at least one significant period, 1.56–1.25 ka. Since 1.25 ka, cycles of carbonate deposition with quasi‐decadal periodicities have produced characteristic darker and lighter assemblages of laminae. These cycles may have been climatically driven. Geochemical analyses coupled with the varve ages adopted herein indicate that accumulation rates of carbonate are nearly an order of magnitude higher in Unit I (averaging 35–45 g m−2yr−1) than in sapropelic Unit II. which contains primarily detrital carbonate. The accumulation of lithogenic components in parts of Unit I is only 1.5 times the rate in Unit II. Deepwater organic carbon accumulation rates are somewhat higher in Unit I (3.5–4.5 g m−2yr−1) than in the upper part of Unit II. Organic carbon accumulation rates in Unit I are somewhat antithetic to those of carbonate, and on the basis of this and additional constraints placed by pyrolysis and carbon isotopic analyses of organic material, it appears that terrestrial organic matter is an important component (perhaps>25%) of total organic carbon burial in the basin. Unit I in the western part of the Black Sea has a higher terrestrial organic component and higher accumulation rates of terrigenous clastic material than Unit I in the eastern part. This difference between eastern and western Black Sea is to be expected because of the major rivers that empty into the western Black Sea from eastern Europe, Ukraine, and Russia. Shallow slope sites, but still within euxinic bottom waters, have lower organic carbon accumulation rates and lower pyrolysis hydrogen indices than deepwater basinal sites, suggesting selective resuspension and oxidation of organic matter at basin margins and focusing of organic matter deposition toward the basin center. A comparison of the Black Sea data with those from several open ocean sites with similar water depths showed no significant difference between organic carbon accumulation rates under oxic and anoxic conditions. For a given bulk accumulation rate the organic carbon accumulation rates, normalized to primary productivity, a

ISSN:0886-6236    

DOI:10.1029/94GB00297

年代:1994

数据来源: WILEY

摘要:

Benthic chambers were deployed in the equatorial eastern Pacific Ocean on a transect along the equator between 103°W and 140°W and on a transect across the equator at 140°W in order to establish the rate of calcium carbonate dissolution on the seafloor. Dissolution was determined from the rate of alkalinity increase within an incubation chamber, measured over an 80–120 hour incubation period. Dissolution rates were lowest at eastern Pacific sites (0.2‐0.4 mmol CaCO3/m2/d) and highest at the equatorial, 140°W sites (0.5‐0.7 mmol/m2/d). Both oxygen consumption rates and the degree of bottom water saturation govern dissolution rates. Measured dissolution and oxygen consumption rates are used with a numerical model to constrain the value of the dissolution rate constantk, formulated according to the equation developed byKeir[1980]: dissolution rate =kγ(1‐Ω)n. The observed dissolution fluxes are predicted by the model whenk= 5 to 100%/d andn= 4.5. This range ofkvalues has important implications regarding the type of carbonate dissolving and its location within the sediment column. At low values ofk, organic carbon rain rates to the seafloor become the dominant driving force of carbonate dissolution. At higher values ofk, the degree of bottom water undersaturation becomes more important. Dissolution of carbonate within equatorial Pacific sediments can be adequately described withk= 20 ± 10%/d, a rate constant much lower than some previously used values. Dissolution rates do not vary significantly over chamber boundary layer thicknesses between 200 and 800 μm, indicating that dissolution is not controlled by hydrodynamic conditions. Chambers acidified with HCl yield very large dissolution rates, but for a given degree of acidification the dissolution rate was constant for sites ranging from water depths of 3300–4400 m. This implies that there are not more and less easily dissolved forms of CaCO3arriving on the seafloor between these depths. A budget for alkalinity in the deep Pacific, predicted by the dissolution model and based on the assumption that all carbonate dissolution takes place in the sediments, is within 85% of the input required by a published box model alkalinity budget based on the distribution of nutrients and the water mas

ISSN:0886-6236    

DOI:10.1029/93GB03394

年代:1994

数据来源: WILEY