摘要:

In a recent letter to this journal, Smith and Mackenzie [1991] emphasized the role of near‐shore environments, such as bays and estuaries, as being the principal locus for the enhanced burial of organic matter due to anthropogenically derived nutrient inputs to the oceans. I commend them for this because of the relative lack of attention given to near‐shore environments by much of the biogeochemical and ocanographic community. It is along continental margins and not in the pelagic ocean where most organic matter, as well as most sediment, is buried. From an extensive review of the sediment literature I have shown [Berner, 1982]that principal organic carbonburial on a global basis occurs in deltas and other continental margin environments. (A large proportion of the organic C may be of terrestrial origin.) Although the average carbon content of the surficial portions of deltaic and continental shelf muds is only about 0.75%C, the large mass of sediment carrying this small carbon percentage adds up to a very large burial rate. The riverine flux of suspended sediment to the oceans has been estimated [e.g., Milliman and Meade, 1983; Milliman and Syvitski, 1992] to be between 12,000 and 25,000 Tg (102grams) per year, and most of this sediment is deposited at present on or near the continental shelves. At 0.75 % C this amounts to anannual carbon burial rate of 90–190 Tg C. Burial rates in other environments such as offshore upwelling zones of high productivity or the vast pelagic realms of the oceans do not come close to matching these rates. If one is to look for a locus for the burial of excess carbon from fossil fuel burning and deforestation, the place to look is near the conti

ISSN:0886-6236    

DOI:10.1029/91GB02990

年代:1992

数据来源: WILEY

摘要:

Methane production, transport and emission in a floodplain lake in central Amazonia were investigated by isotopic studies and gas exchange measurements. Samples of sediment free gas were depleted in δ13CCH4, δ13DCH4,and δ13CCO2values. The isotopic composition of the sediment free methane clearly demonstrated a methane production by methyl fermentation. This finding was strengthened by the coexisting δ13CCO2and δ13CCO2values in the sediment free gas. The flux rates of methane ebullition and diffusion were measured during a complete annual cycle using the static chamber method. Significant differences were observed in the release of methane from individual vegetation types, i.e., phytoplankton, floating grass mats, and flooded forest. Each vegetation type showed a distinct seasonal pattern. The highest ebullition rates (mean value, 69 mg CH4m−2d−1) were recorded in the flooded forest, covering the higher areas of the floodplains with a long subaerial period. Significantly lower averages of the gas bubble flux were recorded in the permanently aquatic areas of the lake (mean value, 29 mg CH4m−2d−1) and in the intermediate area with floating grass mats (mean value, 23 mg CH44m−2d−1. Ebullition was the predominant mechanism for the methane transport from the varzea sediment into the atmosphere with maximum values of up to 200 mg CH4m−2d−1. The diffusive flux remained below 29 mg CH4m−2d−1at all sites throughout the entire annual cycle. The variation of the ebullutive flux was found to determine the spatial and temporal variation of the total methane flux in the varzea. We estimate that ebullition accounts for 80% of the total methane

ISSN:0886-6236    

DOI:10.1029/91GB01767

年代:1992

数据来源: WILEY

摘要:

Methane flux from plots ofPeltandra virginicain a bottomland hardwood swamp in southeast Virginia varied from 270 to 670 mg CH4m−2d−1from May to August and was 6 times greater than the flux from unvegetated adjacent open waters. Variations in chamber temperature, light, and CO2levels failed to produce systematic changes in CH4flux rate or its isotopic composition from vegetated plots, suggesting that CH4flux fromPeltandrais independent of stomatal aperture or hourly variations in photosynthetic rate. Comparisons of different chamber techniques for the collection of CH4for isotopic analysis and for flux measurements fromPeltandrasuggest that the effect of using simple uncontrolled chambers is small relative to temporal and spatial variations in methane emission and its isotopic composition. These results may be extended to other aquatic macrophytes which also transport gas primarily by molecular diffusion such asOryza, Carex, Pontederia, Sagittaria, and Cladium.The δ13C of CH4released fromPeltandravaried between −61 and −71 °/oo. Isotopic variations in CH4associated withPeltandraappeared to be controlled by transport effects. Methane withdrawn fromPeltandrastems was considerably13C enriched relative to sedimentary CH4and was not an indicator of the isotopic composition of CH4emitted by these plants. Hence emergent aquatic macrophytes rooted in organic‐rich sediments do not appear to be a source of13C enriched methane to the atmosphere. Emitted CH4was13C depleted relative both to the CH4within stems (−49 to −55 °/oo) and within sediments (‐56 to ‐58 °/oo), suggesting isotopic fractionation associated with the release of CH4by the plants. The preferential release of12CH4by the plants is partly compensated for by the enrichment of13C in CH4contained within the plant stem. Although the presence of methanotrophic bacteria within the rhizosphere ofPeltandrawas demonstrated, the stable isotopic composition of methane emitted from these plants or collected from sedimentary bubbles and plant stems presents no evidence for extensive oxidation of methane withinPeltandraor its rhizosphere. Either stable isotopes are not a good indicator of rhizopheric methane oxidation or the microbes' in situ respiration is limited by competition for oxygen with the demands of root respiration, oxidation of complex organic matter, ferrous iron oxidati

ISSN:0886-6236    

DOI:10.1029/91GB02969

年代:1992

数据来源: WILEY

摘要:

Efflux rates and oxidation rates of methane (CH4) were measured in a northernSphagnumbog, Thoreau's Bog in Concord, Massachusetts, by using a gradient methodology which does not change in situ conditions. A remote‐sampling technique was devised to obtain undisturbed CH4profiles as a function of depth in the peat; effective diffusion coefficients in peat were estimated both physically and by using propane as a tracer. By combining these techniques we estimated the average late summer CH44 flux from deep, anaerobic methanogenic sediments to the unsaturated zone to be 3.5 × 10−11mol cm−2s−1(± 1.0 × 10−10mol cm−2s−1), while the CH4flux from this unsaturated zone to the atmosphere was 3.7 × 10−12mol cm−2s−1(± 5.0 × 10−12mol cm−2s−1). Therefore a large fraction of the CH4flux was consumed before it reached the atmosphere. Most CH4consumption, presumably by oxidation, occurred between the water table, located 12 to 15 cm below the bog surface, and about 6 cm below the bog surface. In this region, CH4concentrations and oxidation rates were unevenly distributed, probably following patterns of upward transport of CH4by bubbles via fissures and tubes in the saturated zone. Between the surface of the bog and 6‐cm depth, CH4concentrations were more uniformly distributed, most likely because of greater horizontal mixing in this depth range. Analysis of CH4distributions in unsaturated peat is a straightforward and practical technique to measure both net CH4efflux and CH4oxidation with minimal disturbance to the peat stru

ISSN:0886-6236    

DOI:10.1029/91GB02989

年代:1992

数据来源: WILEY

摘要:

A phosphorus‐based model of nutrient cycling has been developed and used in conjunction with a general circulation model to evaluate the roles of the dissolved and sinking particulate phases in the downward transport of organic matter in the ocean. If sinking particles dominate the downward transport and remineralize in accord with observations made primarily with sediment traps, we find in equatorial upwelling regions that particle fluxes and thermocline nutrient concentrations are higher than observed. These enhanced fluxes and concentrations are a result of what we term “nutrient trapping,” a positive feedback whereby high upwelling produces high new production that results in remineralization and enhanced nutrient concentrations in the upwelling water, which further increases new production. Nutrient trapping in shallow upwelling zones can be eliminated by increasing the particle flux length scale, which suggests that if sinking particles dominate the downward transport of organic matter then the flux length scale is longer than observed. Even with a longer particle flux length scale, we find that nutrients are trapped in some deep convective regions of the southern ocean, where new production is predicted to be much higher than the observed primary production. In simulations where the downward transport of organic matter takes place primarily in a dissolved phase, nutrient trapping is completely eliminated, in both upwelling and convective regions. The models with dissolved organic matter also agree fairly well with nutrient transports in the north Atlantic Ocean calculated from observed nutrient and hydrographic data (Rintoul and Wunsch, 1991). Our results therefore support the dissolved organic nitrogen and carbon measurements made with the high‐temperature combustion technique of Suzuki et al. (1985) and Sugimura and Suzuki (1988) and suggest that there exists an as‐yet undiscovered pool of dissolved organic phosphorus in the ocean. We also use the various models to make an estimate of global new production of 2.9 to 3.6 mol C/m2/yr (12 to 15

ISSN:0886-6236    

DOI:10.1029/91GB02718

年代:1992

数据来源: WILEY