摘要:

We have made measurements of the methane (CH4) flux and δ13C value in CH4from rice paddies in Ryugasaki, Japan. This study is the first we are aware of in which a significant change in the δ13C signature of emitted CH4has been documented over the rice growing season. Nutrient treatments studied were of two kinds: compound mineral fertilizer either with or without rice straw from the previous growing season incorporated into the inorganic fertilizer. The calculated annual emission rates during the 1990 growing season were 43.1 g/m2(straw) and 40.6 g/m2(no straw) for the two treatments. In both treatments, CH4started out relatively enriched in13C, became lighter in13C, and then became more enriched again during the latter part of rice growth. The 1991 growing season showed a lower integrated flux in both nutrient treatments than for 1990 but plots of the fluxes versus time had the same general shape as the flux curves in 1990 and a similar although less pronounced trend in δ13CH4signal. Seasonal changes in δ13C are probably related to changes in CH4production and oxidation and plant‐mediated transport. The likelihood of each process occurring and its effect on δ13C values is discussed. The range of δ13CH4values from seasonal effects was ∼12‰ in 1991 for both nutrient treatments. The δ13CH4range for 1991 was ∼10‰ (straw) and ∼5‰ (no straw). Our data indicate that when using flux‐weighted isotopic signatures to put constraints on the tropospheric CH4budget, attention should be paid to seasonal changes in isotopic signatures from rice paddy CH4in a manner similar to that previously suggested from measuremen

ISSN:0886-6236    

DOI:10.1029/93GB03123

年代:1994

数据来源: WILEY

摘要:

The carbon‐isotope composition of both forest and grassland soils from Papua New Guinea exhibit predictable trends with increasing altitude. Soils under pure C3vegetation (forests and alpine grasslands above 4000 m) show an increase in δ13C value with altitude paralleling the increase in δ13C value observed in plant leaves byKörner et al.[1988]. Grassland soils show a decrease in δ13C value above about 1000 m, from maximum values which are close to pure C4values (−12 to −13‰ vs. PDB) to minimum values which are indistinguishable from pure C3values at 3500–4000 m (∼−26‰). Within this general framework, several factors can influence the soil δ13C value at individual locations. In local forest settings, soil δ13C values will be influenced by the degree to which respired CO2is re‐utilized during photosynthesis, the proportions of leaf and wood litter, and the degree of decomposition. In grassland settings the primary factor controlling the observed δ13C variability at any specific altitude is the amount of nongrass C3carbon present in the sample. It is also possible that other factors, such as moisture availability, may play some role in determining the proportions of C3and C4grasses at any given altitude, although further work would be required to substantiate such a link. The results provide a framework within which to more accurately constrain the carbon‐isotope composition of terrestrial carbon pools and to interpret variations in the isotopic composition of riverine

ISSN:0886-6236    

DOI:10.1029/93GB03487

年代:1994

数据来源: WILEY

摘要:

We explore the effects of a changing terrestrial biosphere on the atmospheric residence time of CO2using three simple ocean carbon cycle models and a model of global terrestrial carbon cycling. We find differences in model behavior associated with the assumption of an active terrestrial biosphere (forest regrowth) and significant differences if we assume a donor‐dependent flux from the atmosphere to the terrestrial component (e.g., a hypothetical terrestrial fertilization flux). To avoid numerical difficulties associated with treating the atmospheric CO2decay (relaxation) curve as being well approximated by a weighted sum of exponential functions, we define the single half‐life as the time it takes for a model atmosphere to relax from its present‐day value half way to its equilibrium pCO2value. This scenario‐based approach also avoids the use of unit pulse (Dirac Delta) functions which can prove troublesome or unrealistic in the context of a terrestrial fertilization assumption. We also discuss some of the numerical problems associated with a conventional lifetime calculation which is based on an exponential model. We connect our analysis of the residence time of CO2and the concept of single half‐life to the residence time calculations which are based on using weighted sums of exponentials. We note that the single half‐life concept focuses upon the early decline of CO2under a cutoff/decay scenario. If one assumes a terrestrial biosphere with a fertilization flux, then our best estimate is that the single half‐life for excess CO2lies within the range of 19 to 49 years, with a reasonable average being 31 years. If we assume only regrowth, then the average value for the single half‐life for excess CO2increases to 72 years, and if we remove the terrestrial component completely, then it increases furt

ISSN:0886-6236    

DOI:10.1029/93GB03392

年代:1994

数据来源: WILEY

摘要:

We investigate the response of a three‐dimensional ocean circulation model (Hamburg LSG) coupled on‐line with an oceanic carbon cycle model (HAMOCC‐3) to El Niño‐Southern Oscillation (ENSO) induced fluctuations of the wind field. During El Niño 1982/1983, when upwelling and biological productivity in the equatorial Pacific were strongly reduced and sea surface temperatures were increased, the oceanic CO2partial pressure in this region decreased significantly. Consequently, in 1982/1983 the CO2flux from the tropical ocean into the atmosphere was reduced. However, in 1983 the interannual deviations from the long‐term trend in atmospheric CO2showed in January low and in December high values with a total shift by more then 1.4 GtC. The model simulation supports the oceanic measurements and predicts a temporary uptake of 0.6 GtC during the ENSO year 1983. We conclude that the concurrent release of CO2from the land biosphere must have been

ISSN:0886-6236    

DOI:10.1029/93GB03134

年代:1994

数据来源: WILEY

摘要:

A nonlinear inverse method is applied to nutrient data upon approximately 20 neutral surfaces in each of the South Atlantic, Indian, and Pacific basins, between 400 and 4000 m depth. By accounting for the gradients in nutrients due to the mixing of “preformed” concentrations of the major water masses, the nutrient changes due to biological activity are examined, and the time‐mean, basin‐wide Redfield ratios calculated. It is found that the P/N/Corg/−O2ratios of nutrient regeneration between 400 and 4000 m (corrected for the effect of denitrification) are approximately constant with depth and basin, at a value of 1/16 ± 1/117 ± 14/170 ± 10. These ratios agree with those of fresh organic matter, suggesting that the flux of organic material to the deep ocean may be dominated by fast‐sinking matter produced by sporadic, high‐productivity events. Sedimentary denitrification reduces the N/P utilization ratio to 12 ± 2 between 1000 and 3000 m. In the Indian and Pacific basins the Corg/Cinorgregeneration ratio decreases from approximately 7 ± 3 at 400 m to 3 ± 1 at 1000 m and to 1 ± 0.5 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in the India

ISSN:0886-6236    

DOI:10.1029/93GB03318

年代:1994

数据来源: WILEY

摘要:

Published experiments are reevaluated regarding the uptake ratio of dissolved inorganic carbon (DIC) and nitrate in two plankton communities and the resulting elemental ratio of particulate organic matter (POM). The ratios are lower than Redfield values of 6.6 or 7.6 (by atoms); the uptake ratios of dissolved moieties are not measures for the composition of the newly formed POM; and uptake of DIC and nitrate may be entirely uncoupled. The use of closed systems of several 100 L to 1 m−3content is suggested for studies of the underlying mechanism

ISSN:0886-6236    

DOI:10.1029/93GB02865

年代:1994

数据来源: WILEY

摘要:

Variations of the13C content of marine participate organic carbon (δ13CPOC) in the modern ocean were studied using literature data to test the assumptions underlying the calculation of atmospheric pCO2through geological time from the δ13C of sedimentary organic matter. These assumptions are that (1) concentrations of CO2in the atmosphere and the surface ocean are at equilibrium at all times and latitudes and that (2) carbon isotopic fractionation of phytoplankton (ϵp) covaries primarily with concentrations of dissolved molecular CO2([CO2]aq). Previous studies and compilations have shown that the first assumption does not strictly hold, although [CO2]aqmay be predicted with a reasonable degree of accuracy from sea surface temperature for specific regions of the world ocean. The second assumption is shown to be questionable due to the weak covariation of ϵpand [CO2]aqin the modern ocean. The large residual variance for regressions of ϵp against [CO2]aqsuggests that factors other than [CO2]aqstrongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO2]aqcannot be easily calibrated using δ13CPOCdata from the modern

ISSN:0886-6236    

DOI:10.1029/93GB03272

年代:1994

数据来源: WILEY

摘要:

Closed cultures of marine phytoplankton were established under variable conditions of CO2concentration, temperature, growth rate (by light limitation), andpH (but with nearly identical [CO2aq]) in order to assess the relative influence of these variables on the extent of carbon isotope fractionation relative to dissolved inorganic carbon sources. Culture biomass was not allowed to increase beyond levels that would significantly affect the dissolved carbon system in the closed cultures. In experiments withSkeletonema costatumandEmiliania huxleyi, increasing CO2concentrations led to increased carbon isotope discrimination (resulting in organic matter progressively depleted in δ13C, i.e., a greater, more negative ϵp). ϵpvalues forE.huxleyiwere 8–10‰ less than forS. costatumunder identical conditions. For theS. costatumcultures, there was nearly a 20 ‰ range in [CO2aq]‐dependent ϵp. The effect was nonlinear with a leveling off at high [CO2aq]. Over apH range of 7.5–8.3 but at a constant [CO2aq] there was a variation in carbon isotope fractionation byS. costatumof about 9 ‰ with a minimum atpH 7.8–7.9. There was a temperature effect of ∼8‰ on fractionation even after equilibrium temperature dependency of δ13C of CO2aq was taken into account. No growth rate effect was found forS. costatumover a modest range of growth rates. Culture experiments used to determine the carbon isotope fractionation by phytoplankton species must be conducted under well‐defined conditions of temperature,pH, and CO2concentrations. Hindcasts of ancient atmosphericpCO2from measurements of δ13C of organic carbon in marine sediments will require careful calibration because of the variety of possible facto

ISSN:0886-6236    

DOI:10.1029/93GB03393

年代:1994

数据来源: WILEY

摘要:

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the δ15N of core top sediments were strongly related to [NO3−] in surface waters. With distance from the equator in the equatorial Pacific, δ15N increased from 7‰ to 16‰ as [NO3−] decreased from 8μM to<0.1 μM. Going from 60° to 30° S in the SE Indian Ocean, core top δ15N increased from 5‰ to 11‰ as surface [NO3−] decreased from 25μM to<0.1 μM. These results are strong evidence that sedimentary δ15N in these regions is recording the increasing isotopic enrichment of near‐surface NO3−with its depletion by phytoplankton. In the case of the equatorial Pacific, δ15N values for sinking particles collected at 150 m matched well the core top sediment values, demonstrating little diagenetic alteration of the near‐surface generated isotopic signal. These equatorial Pacific data sets have variations with near‐surface [NO3−] consistent with Rayleigh fractionation kinetics for a fractionation factor (ϵu) of 2.5‰. This value is substantially lower than previously found for temperate or polar regions, perhaps as a result of differences in phytoplankton species assemblage or growth condition. In the southern ocean south of the polar front, comparison of δ15N values for opal‐rich sediments south and sinking particles indicates an apparent +5‰ diagenetic enrichment relative to the surface‐generated signal that requires further investigation. This exception aside, our observations show that the surface‐water relationship of increasing δ15N with increasing NO3−depletion is generally transmitted to and preserved in the sediments, an important requirement for further development and app

ISSN:0886-6236    

DOI:10.1029/93GB03396

年代:1994

数据来源: WILEY