Showing papers in "Global Biogeochemical Cycles in 1998"

Terrestrial sedimentation and the carbon cycle: Coupling weathering and erosion to carbon burial

Abstract: This paper examines the linkages between the carbon cycle and sedimentary processes on land. Available data suggest that sedimentation on land can bury vast quantities of organic carbon, roughly 1015 g C yr−1. To evaluate the relative roles of various classes of processes in the burial of carbon on land, terrestrial sedimentation was modeled as a series of 864 scenarios. Each scenario represents a unique choice of intensities for seven classes of processes and two different global wetland distributions. Comparison was made with presumed preagricultural conditions. The classes of processes were divided into two major component parts: clastic sedimentation of soil-derived carbon and organic sedimentation of autochthonous carbon. For clastic sedimentation, masses of sediment were considered for burial as reservoir sediment, lake sediment, and combined colluvium, alluvium, and aeolian deposits. When the ensemble of models is examined, the human-induced burial of 0.6-1.5·1015 g yr−1 of carbon on land is entirely plausible. This sink reaches its maximum strength between 30° and 50° N. Paddy lands stand out as a type of land use that warrants future study, but the many faces of rice agriculture limit generalization. In an extreme scenario, paddy lands alone could be made to bury about 1·1015 g C yr−1. Arguing that terrestrial sedimentation processes could be much of the sink for the so called “missing carbon” is reasonable. Such a hypothesis, however, requires major redesign of how the carbon cycle is modeled. Unlike ecosystem processes that are amenable to satellite monitoring and parallel modeling, many aspects of terrestrial sedimentation are hidden from space.

The decoupling of production and particulate export in the surface ocean

Abstract: The relationship between primary production in the surface ocean and export of particulate organic carbon (POC) on sinking particles is examined in studies that have utilized 234Th as a tracer of upper ocean export. Comparisons between production and export are made in a wide range of open ocean settings and seasons. The results indicate that much of the ocean is characterized by low POC export relative to primary production (export/production = ThE < 5–10%). Exceptions to this pattern are found during blooms at high latitudes, accompanying spring blooms at midlatitudes, and perhaps in association with more episodic export pulses. These sites of high export are most often characterized by food webs dominated by large phytoplankton, in particular diatoms. These results can be used to better parameterize surface export in biogeochemical models in order to provide a more accurate prediction of the flow of C and associated nutrients in the oceans.

Global distribution of nitrous oxide production and N inputs in freshwater and coastal marine ecosystems

Abstract: This study examines N2O emissions from aquatic environments globally, particularly as they are affected by anthropogenic activity. The global distribution of N2O production in rivers and estuaries was modeled as a function of nitrification and denitrification rates, which were related to external nitrogen (N) inputs. N loading rates were estimated as a function of environmental parameters in the watersheds using two existing models that we adapted for global databases. Model estimated export of dissolved inorganic nitrogen (DIN) by world rivers to estuaries in 1990 is 20.8 Tg N yr−1; approximately 75% is estimated to be anthropogenic. DIN export to the Atlantic and Indian Oceans is similar (5.4 Tg N yr−1 and 4.6 Tg N yr−1, respectively); inputs to the Pacific are approximately 50% greater. China and southeast Asia account for over 50% of DIN export by world rivers. Globally, anthropogenic DIN export is predominately attributed to fertilizer N, followed by sewage and atmospheric deposition. About 8% of the total N inputs to the terrestrial environment can be accounted for as DIN export by rivers. Worldwide N2O emissions from rivers (55%), estuaries (11%), and continental shelves (33%) are calculated to be 1.9 Tg N yr−1. For rivers and estuaries, approximately 90% of N2O emissions are in the northern hemisphere in line with the regional distribution of DIN export by rivers. China and India account for about 50% of N2O emissions from rivers and estuaries. About 1% of the N input from fertilizers, atmospheric deposition, and sewage to watersheds is lost as N2O in rivers and estuaries. Globally, rivers and estuaries could account for approximately 20% of the current global anthropogenic N2O emissions and are similar in magnitude to a number of previously identified sources including direct emissions of N2O from soils induced by anthropogenic N inputs.

Characterizing patterns of global land use: An analysis of global croplands data

Abstract: Human activities have shaped significantly the state of terrestrial ecosystems throughout the world. One of the most direct manifestations of human activity within the biosphere has been the conversion of natural ecosystems to croplands. In this study, we present an analysis of the geographic distribution and spatial extent of permanent croplands. This analysis represents the area in permanent croplands during the early 1990s for each grid cell on a global 5 min (∼10 km) resolution latitude-longitude grid. To create this data set, we have combined a satellite-derived land cover data set with a variety of national and subnational agricultural inventory data. A simple calibration algorithm was used so that the spatial land cover data were generally consistent with nonspatial agricultural inventory data. The spatial distribution of croplands represented in this analysis presents a quantitative depiction of global agricultural geography. The regions of the world known to have intense cultivation (e.g., the North American corn belt, the European wheat-corn belt, the Ganges floodplain, and eastern China) are clearly portrayed in this analysis. It also captures the less intensely cultivated regions of the world, usually surrounding the regions mentioned above, and regions characterized by subsistence agriculture (e.g., Sahelian Africa). Data generated from this kind of analysis can be used within global climate models and global ecosystem models to assess the importance of permanent croplands on environmental processes. In particular, these data, combined with models, could help evaluate the role of changing land cover on regional climate and carbon cycling. Future efforts will need to concentrate on other land use systems, including pastures and regions of shifting cultivation. Furthermore, land use and land cover data must be extended to include an historical dimension so as to evaluate the changing state of the biosphere over time. This article contains supplementary material.

Phosphorus accumulation in marine sediments and the oceanic phosphorus cycle

Abstract: Ideas about key factors in the oceanic mass balance of dissolved, reactive phosphate have changed substantially. I present an integrated overview of these here, with an emphasis on evaluating the burial sinks for P and defining areas needing further research. The major source of reactive P to the ocean is river input. Reactive P is delivered to the oceanic sediment-water interface primarily in particulate organic matter. P scavenged by hydrothermal iron-rich oxyhydroxide particles, with uptake in proportion to deep water phosphate concentrations, represents a substantially smaller flux to the sediment-water interface. Diagenetic transformations are important influences on the form of reactive P burial in marine sediments. P burial occurs with organic carbon burial and as P associated with iron-rich oxyhydroxide particles and coatings. Formation of authigenic P-rich phases, presumably apatite, at the expense of organic P and oxide-associated P, is significant in open ocean marine sediments. The authigenic P sink may represent a substantially larger portion of the sedimentary burial than indicated by previous estimates focused on P burial in organic-rich continental margin sediments.

Methane fluxes on boreal peatlands of different fertility and the effect of long-term experimental lowering of the water table on flux rates

Abstract: Methane (CH4) fluxes were measured at 17 peatland sites with different nutritional and hydrological characteristics in the southern and middle boreal zones in Finland by a static chamber technique. Many of the natural peatlands also had counterparts drained for forestry 30–50 years ago. The mean emissions from May to September were 8.0 g CH4 m−2 for natural ombrogenous bogs and 19.0 g CH4 m−2 for natural minerogenous fens thus being higher than the 2 g CH4 m−2 yr−1 estimated for the Canadian peatlands. Change in the mean water table level had greater effect on CH4 fluxes on natural fens than on natural bogs. The mean CH4 emissions on drained bogs and fens were 3.9 g CH4 m−2 and 0.3 g CH4m−2, respectively. Some drained fens even had CH4 uptake from the atmosphere. The change in the mean water table had the lowest effect on CH4 fluxes on drained peatlands. The CH4 fluxes on peatlands (natural fens and bogs and drained peatlands) are associated with peat aeration, nutrient level, vegetation cover, peat compaction, peat temperature, and finally with microbial processes responsible for the net release of CH4. We could explain 67% of the variation in mean CH4 fluxes on Finnish peatlands by measuring the mean water table, peat bulk density, and peat pH. The present results can be used to predict the possible changes in CH4 emissions on peatlands if the climate is drying in north. For example, lowering of the present water table by 10 cm would induce a 70% reduction in the CH4 emissions from fens and a 45% reduction from bogs.

Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance

Abstract: Terrestrial ecosystems are thought to be a major sink for carbon at the present time. The endeavor to find this terrestrial sink and to determine the mechanisms responsible has dominated terrestrial research on the global carbon cycle for years. Some of the mechanisms advanced to explain the “missing sink” are also negative feedbacks to a global warming. Here we distinguish between mechanisms likely to act as feedbacks to a global warming and other mechanisms consistent with a terrestrial sink that are not feedbacks to a global warming. One of the postulated negative feedback mechanisms that also helps explain the current “missing sink” is based on the theory that carbon should accumulate in vegetation as a result of a warming-enhanced mineralization of nitrogen in soil organic matter. The theory assumes that mineralized N is neither retained in the soil (through reimmobilization by microbial biomass) nor lost from the ecosystem, but rather becomes available for plant growth. None of these assumptions is supported yet by field data. In contrast, trends across existing climatic gradients suggest that warmer temperatures will lead to a decrease in the C:N ratio of soils (i.e., the mineralized N remains in soil). Data pertaining to temporal variability in the global carbon balance are conflicting with respect to the question of whether increasing temperatures cause a release or storage of terrestrial carbon. The answer seems to depend in part on time scale. Most likely, multiple mechanisms, including some that release carbon and others that accumulate it, account for the present net accumulation of carbon on land. However, a positive feedback between temperature and the release of CO2 to the atmosphere by terrestrial respiration seems likely to grow in importance and could change significantly the role that terrestrial ecosystems play in the global carbon balance.

Net community production of dissolved organic carbon

Abstract: Each ycar large amounts of carbon, with a residence time of months, accumulate in the surface layer of the ocean as semilabile dissolved organic carbon (DOC). This material is transported long distances, contributing to the interhemispheric transfer and deep ocean export of carbon. The fraction of net community production resulting in the accumulation of semilabile DOC is estimated here by mass balance during periods of net phytoplankton production in three diverse environments: the Ross Sea polynya, the Equatorial Pacific Ocean, and the Sargasso Sea. In the eutrophic systems of the Ross Sea and the Equatorial Pacific, net DOC production generally fell between 10 and 20% of net community production. For the 1995 spring bloom in the Sargasso Sea. net DOC production was 59-70% of the net community production. Net DOC production was maximal during the period of deep convective overturn of the water column, indicating linkage between the processes. Following the Sargasso Sea spring bloom and into the summer period, net DOC production was nil over the upper 250 m so that net DOC production was reduced to ∼8% of net community production on a seasonal timescale. Consideration of the various types of production regimes in the ocean indicates that the global net production of semilabile DOC is ∼17% of global new production. Regions of the world's oceans with the greatest contributions to global net community production, such as equatorial and coastal upwelling areas, contribute most to the global production of semilabile DOC.

Mid-Holocene land-surface conditions in northern Africa and the Arabian Peninsula: A data set for the analysis of biogeophysical feedbacks in the climate system

Abstract: Large changes in the extent of northern subtropical arid regions during the Holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales. Models that omit biogeophysical feedback, however, are unable to account for the full magnitude of African monsoon amplification and extension during the early to middle Holocene (˜9500–5000 years B.P.). A data set describing land-surface conditions 6000 years B.P. on a 1° × 1° grid across northern Africa and the Arabian Peninsula has been prepared from published maps and other sources of palaeoenvironmental data, with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the Palaeoclimate Modelling Intercomparison Project. The data set includes information on the percentage of each grid cell occupied by specific vegetation types (steppe, savanna, xerophytic woods/scrub, tropical deciduous forest, and tropical montane evergreen forest), open water (lakes), and wetlands, plus information on the flow direction of major drainage channels for use in large-scale palaeohydrological modeling. The data set is available in digital form by anonymous ftp.

Dynamics of fossil fuel CO2 neutralization by marine CaCO3

Abstract: A detailed model of the ocean circulation and carbon cycle was coupled to a mechanistic model of CaCO3 diagenesis in deep sea sediments to simulate the millennium-scale response of the oceans to future fossil fuel CO2 emissions to the atmosphere and deep sea. Simulations of deep sea injection of CO2 show that CaCO3 dissolution is sensitive to passage of high-CO2 waters through the Atlantic Ocean, but CaCO3 dissolution has a negligible impact on atmospheric pCO2 or the atmospheric stabilization CO2 emission in the coming centuries. The ultimate fate of the fossil fuel CO2 will be to react with CaCO3 on the seafloor and on land. An initial CaCO3 dissolution spike reverses the net sedimentation rate in the ocean until it is attenuated by an enhanced vertical gradient of alkalinity after about 1000 years. The magnitude of the initial spike is sensitive to assumptions about the kinetics for CaCO3 dissolution, but subsequent behavior appears to be less model dependent. Neutralization by seafloor CaCO3 occurs on a timescale of 5–6 kyr, and is limited to at most 60–70% of the fossil fuel release, even if the fossil fuel release is smaller than the seafloor erodible inventory of CaCO3. Additional neutralization by terrestrial CaCO3 restores a balance between CaCO3 weathering and seafloor accumulation on a timescale of 8.5 kyr, while the deficit of seafloor CaCO3 (the lysocline) is replenished with an e-folding timescale of approximately 18 kyr. The final equilibrium with CaCO3 leaves 7–8% of the fossil fuel CO2 remaining in the atmosphere, to be neutralized by the silicate rock cycle on a time frame of hundreds of thousands of years.

Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex

Abstract: We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July-August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) < poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) < intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) < rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May-June) and fall (September-October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub-dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze-up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3.

Relationship between ecosystem productivity and photosynthetically active radiation for northern peatlands

Abstract: We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe, NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = alpha PPFD P(sub max)/(alpha PPFD + P(sub max) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = beta PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = -2.0 micro mol m(exp -2) s(exp -1) for bogs and -2.7 micro mol m(exp -2) s(exp -1)) for fens) and lower NEE at moderate and high light levels (P(sub max)= 5.2 micro mol m(exp -2) s(exp -1) for bogs and 10.8 micro mol m(exp -2) s(exp -1) for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = -2.4 micro mol m(exp -2) s(exp -1)) and NEE rates (alpha = 0.020 and P(sub max)= 9.2 micro mol m(exp -2) s(exp -1)) than the upland ecosystems (closed canopy forest, grassland, and cropland). Despite this low productivity, northern peatland soil carbon pools are generally 5-50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.

Anthropogenic CO2 in the Atlantic Ocean

Abstract: The anthropogenic CO2 in the Atlantic Ocean is separated from the large natural variability of dissolved inorganic carbon using the method developed by Gruber et al [1996] Surface concentrations of anthropogenic CO2 are found to be highest in the tropical to subtropical regions and to decrease toward the high latitudes They are very close to what is expected from thermodynamic considerations assuming that the surface ocean followed the atmospheric CO2 perturbation Highest specific inventories (inventory per square meter) of anthropogenic CO2 occur in the subtropical convergence zones Large differences exist between the North and South Atlantic high latitudes: In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior; north of 50°N it has even reached the bottom By contrast, waters south of 50°S contain relatively little anthropogenic CO2, and hence specific inventories are very low An anthropogenic CO2 inventory of about 22 ± 5 Gt C is estimated for the Atlantic north of the equator for 1982, and 18 ± 4 Gt C is estimated for the Atlantic south of the equator for 1989 The Princeton ocean biogeochemistry model predicts anthropogenic CO2 inventories of 200 Gt C (North Atlantic, 1982) and 177 Gt C (South Atlantic, 1989) for the same regions in good agreement with the observed inventories Important differences exist on a more regional scale, associated with known deficiencies of the model

Importance of suspended participates in riverine delivery of bioavailable nitrogen to coastal zones

Abstract: Total nitrogen (TN) loadings in riverine sediments and their coastal depocenters were compared for 11 river systems worldwide to assess the potential impact of riverine particulates on coastal nitrogen budgets. Strong relationships between sediment specific surface area and TN allow these impacts to be estimated without the intense sampling normally required to achieve such budgets. About half of the systems showed higher nitrogen loadings in the riverine sediments than those from the coastal depocenter. In spite of uncertainties, these comparisons indicate that large, turbid rivers, such as the Amazon, Huanghe, and the Mississippi, deliver sediments that in turn release significant or major fractions of the total riverine nitrogen delivery. Riverine particulates must therefore be considered an essential factor in watershed nutrient loading to coastal ecosystems and may affect delivered nutrient ratios as well as total nutrient loading. The relative importance of particulate versus dissolved delivery has decreased over recent decades in the Mississippi as a result of damming and fertilizer use in the watershed.

Evaluation of terrestrial carbon cycle models through simulations of the seasonal cycle of atmospheric CO2: First results of a model intercomparison study

Abstract: Results of an intercomparison among terrestrial biogeochemical models (TBMs) are reported, in which one diagnostic and five prognostic models have been run with the same long-term climate forcing. Monthly fields of net ecosystem production (NEP), which is the difference between net primary production (NPP) and heterotrophic respiration RH, at 0.5° resolution have been generated for the terrestrial biosphere. The monthly estimates of NEP in conjunction with seasonal CO2 flux fields generated by the seasonal Hamburg Model of the Oceanic Carbon Cycle (HAMOCC3) and fossil fuel source fields were subsequently coupled to the three-dimensional atmospheric tracer transport model TM2 forced by observed winds. The resulting simulated seasonal signal of the atmospheric CO2 concentration extracted at the grid cells corresponding to the locations of 27 background monitoring stations of the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory network is compared with measurements from these sites. The Simple Diagnostic Biosphere Model (SDBM1), which is tuned to the atmospheric CO2 concentration at five monitoring stations in the northern hemisphere, successfully reproduced the seasonal signal of CO2 at the other monitoring stations. The SDBM1 simulations confirm that the north-south gradient in the amplitude of the atmospheric CO2 signal results from the greater northern hemisphere land area and the more pronounced seasonality of radiation and temperature in higher latitudes. In southern latitudes, ocean-atmosphere gas exchange plays an important role in determining the seasonal signal of CO2. Most of the five prognostic models (i.e., models driven by climatic inputs) included in the intercomparison predict in the northern hemisphere a reasonably accurate seasonal cycle in terms of amplitude and, to some extent, also with respect to phase. In the tropics, however, the prognostic models generally tend to overpredict the net seasonal exchanges and stronger seasonal cycles than indicated by the diagnostic model and by observations. The differences from the observed seasonal signal of CO2 may be caused by shortcomings in the phenology algorithms of the prognostic models or by not properly considering the effects of land use and vegetation fires on CO2 fluxes between the atmosphere and terrestrial biosphere.

Effects of pasture management on N2O and NO emissions from soils in the humid tropics of Costa Rica

Abstract: Emissions of nitrous oxide (N2O) and nitric oxide (NO) from agricultural soils in the tropics are important in the global budgets of these trace gases. We made monthly measurements of N2O and NO emissions from pastures with three different management systems on volcanic soils in northwestern Costa Rica: traditional (no N input from fertilizer or legumes), pastures with a grass-legume combination, and pastures fertilized with 300 kg N ha−11 yr−1. Average annual N2O emissions were 2.7 ng N cm−2 h−1 from the traditional pastures, 4.9 ng N cm−2 h−1 from the grass-legume pastures, and 25.8 ng N cm−2 h−1 from the fertilized pastures. Average annual NO emissions were 0.9, 1.3, and 5.3 ng N cm−2 h−1 from traditional, grass-legume and fertilized pastures, respectively. In a separate experiment the effects of ammonium, nitrate, and urea-based fertilizer mixtures on nitrogen oxide fluxes were compared. We measured nitrogen oxide fluxes following four different fertilization events. Nitrogen oxide fluxes were among the highest ever measured. The difference in soil water content between the fertilization events had a far greater effect on N2O and NO emissions than the effect of fertilizer composition. We conclude that the concept of “emission factors” for calculating N2O and NO emissions from different types of N fertilizer is flawed because environmental factors are more important than the type of N fertilizer. To estimate fertilizer-induced N2O emission in tropical agriculture, stratification according to soil moisture regime is more useful than stratification according to fertilizer composition.

Testing global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration

Abstract: We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO ≈ O2 + CO2), which is conservative with respect to terrestrial photosynthesis and respiration. We then compare observations of APO to the simulations of an atmospheric transport model which uses ocean-model air-sea fluxes and fossil fuel combustion estimates as lower boundary conditions. We present observations of the annual-average concentrations of CO2, O2, and APO at 10 stations in a north-south transect. The observations of APO show a significant interhemispheric gradient decreasing towards the north. We use air-sea CO2, O2, and N2 fluxes from the Princeton ocean biogeochemistry model, the Hamburg model of the ocean carbon cycle, and the Lawrence Livermore ocean biogeochemistry model to drive the TM2 atmospheric transport model. The latitudinal variations in annual-average APO predicted by the combined models are distinctly different from the observations. All three models significantly underestimate the interhemispheric difference in APO, suggesting that they underestimate the net southward transport of the sum of O2 and CO2 in the oceans. Uncertainties in the model-observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the physical and biological components of the ocean models. Potential deficiencies in the physical components of the ocean models, which have previously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the APO observations. These deficiencies include the inadequate parameterization of subgrid-scale isopycnal eddy mixing, a lack of subgrid-scale vertical convection, too much Antarctic sea-ice formation, and an overestimation of vertical diffusivities in the main thermocline.

Winter fluxes of CO2 and CH4 from subalpine soils in Rocky Mountain National Park, Colorado

Abstract: Fluxes of CO2 and CH 4 through a seasonal snowpack were measured in and adjacent to a subalpine wetland in Rocky Mountain National Park, Colorado. Gas diffusion through the snow was controlled by gas production or consumption in the soil and by physical snowpack properties. The snowpack insulated soils from cold midwinter air temperatures allowing microbial activity to continue through the winter. All soil types studied were net sources of CO2 to the atmosphere through the winter, whereas saturated soils in the wetland center were net emitters of CH 4 and soils adjacent to the wetland were net CH 4 consumers. Most sites showed similar temporal patterns in winter gas fluxes; the lowest fluxes occurred in early winter, and maximum fluxes occurred at the onset of snowmelt. Temporal changes in fluxes probably were related to changes in soil-moisture conditions and hydrology because soil temperatures were relatively constant under the snowpack. Average winter CO2 fluxes were 42.3, 31.2, and 14.6 mmol m -2 d -• over dry, moist, and saturated soils, respectively, which accounted for 8 to 23% of the gross annual CO2 emissions from these soils. Average winter CH 4 fluxes were -0.016, 0.274, and 2.87 mmol m -2 d -1 over dry, moist, and saturated soils, respectively. Microbial activity under snow cover accounted for 12% of the annual CH 4 consumption in dry soils and 58 and 12% of the annual CH 4 emitted from moist and saturated soils, respectively. The observed ranges in CO2 and CH 4 flux through snow indicated that winter fluxes are an important part of the annual carbon budget in seasonally snow-covered terrains.

Soil carbon stocks and their rates of accumulation and loss in a boreal forest landscape

Abstract: Boreal forests and wetlands are thought to be significant carbon sinks, and they could become net C sources as the Earth warms. Most of the C of boreal forest ecosystems is stored in the moss layer and in the soil. The objective of this study was to estimate soil C stocks (including moss layers) and rates of accumulation and loss for a 733 km2 area of the BOReal Ecosystem-Atmosphere Study site in northern Manitoba, using data from smaller-scale intensive field studies. A simple process-based model developed from measurements of soil C inventories and radiocarbon was used to relate soil C storage and dynamics to soil drainage and forest stand age. Soil C stocks covary with soil drainage class, with the largest C stocks occurring in poorly drained sites. Estimated rates of soil C accumulation or loss are sensitive to the estimated decomposition constants for the large pool of deep soil C, and improved understanding of deep soil C decomposition is needed. While the upper moss layers regrow and accumulate C after fires, the deep C dynamics vary across the landscape, from a small net sink to a significant source. Estimated net soil C accumulation, averaged for the entire 733 km2 area, was 20 g C m−2 yr−1 (28 g C m−2 yr−1 accumulation in surface mosses offset by 8 g C m−2 yr−1 lost from deep C pools) in a year with no fire. Most of the C accumulated in poorly and very poorly drained soils (peatlands and wetlands). Burning of the moss layer in only 1% of uplands would offset the C stored in the remaining 99% of the area. Significant interannual variability in C storage is expected because of the irregular occurrence of fire in space and time. The effects of climate change and management on fire frequency and on decomposition of immense deep soil C stocks are key to understanding future C budgets in boreal forests.

Carbon storage during biodegradation of municipal solid waste components in laboratory‐scale landfills

Abstract: The objective of this research was to measure the amount of carbon associated with the major biodegradable components of municipal solid waste (MSW) that remains in long-term storage after anaerobic decomposition in landfills. Tests were conducted in quadruplicate in 2-L reactors operated to obtain maximum decomposition. Measured carbon storage factors (CSFs) for grass, leaves, branches, food waste, coated paper, old newsprint, old corrugated containers, office paper, and MSW were 0.32, 0.54, 0.38, 0.08, 0.34, 0.42, 0.26, 0.05, and 0.22 kg C sequestered dry kg−1, respectively. These values were then used to estimate an overall CSF for MSW that varied from 0.274 to 0.302 kg C sequestered wet kg−1 for waste mixtures that exclude and include recycling, respectively. On the basis of an overall CSF for MSW and data on global MSW generation, global carbon sequestration from MSW burial is estimated to be at least 119 million metric tons per year.

Seasonal variations in the atmospheric O2/N2 ratio in relation to the kinetics of air‐sea gas exchange

Abstract: Observations of seasonal variations in the atmospheric O2/N2 ratio are reported at nine baseline sites in the northern and southern hemispheres. Concurrent CO2 measurements are used to correct for the effects of land biotic exchanges of O2 on the O2/N2 cycles thus allowing the residual component of the cycles due to oceanic exchanges of O2 and N2 to be calculated. The residual oceanic cycles in the northern hemisphere are nearly diametrically out of phase with the cycles in the southern hemisphere. The maxima in both hemispheres occur in summer. In both hemispheres, the middle-latitude sea level stations show the cycles with largest amplitudes and earliest phasing. Somewhat smaller amplitudes are observed at the high-latitude stations, and much smaller amplitudes are observed at the tropical stations. A model for simulating the oceanic component of the atmospheric O2/N2 cycles is presented consisting of the TM2 atmospheric tracer transport model [Heimann, 1995] driven at the lower boundary by O2 fluxes derived from observed O2 saturation anomalies in surface waters and by N2 fluxes derived from the net air-sea heat flux. The model is optimized to fit the observed atmospheric O2/N2 cycles by adjusting the air-sea gas-exchange velocity, which relates O2 anomaly to O2 flux. The optimum fit corresponds to spatially and temporally averaged exchange velocities of 24±6 cm/hr for the oceans north of 31°N and 29±12 cm/hr for the oceans south of 31° S. These velocities agree to within the uncertainties with the gas-exchange velocities expected from the Wanninkhof [1992] formulation of the air-sea gas-exchange velocity combined with European Centre for Medium-Range Weather Forecasts winds [Gibson et al., 1997] but are larger than the exchange velocities expected from the Liss and Merlivat [1986] relation using the same winds. The results imply that the gas-exchange velocity for O2, like that of CO2, may be enhanced in the open ocean by processes that were not systematically accounted for in the experiments used to derive the Liss and Merlivat relation.

Atmospheric iron supply and enhanced vertical carbon flux in the NE subarctic Pacific: Is there a connection?

Abstract: Recent studies have confirmed the relationship between iron supply and phytoplankton growth rates in all three high-nitrate low-chlorophyll (HNLC) oceanic provinces However, there is little evidence, so far, of the role of iron in altering the efficiency of the biological pump via increased downward export of particulate organic carbon (POC) The NE subarctic Pacific is unique among HNLC regions in that long time series pelagic observations and deep-moored sediment trap records exist which may provide the best opportunity thus far to test aspects of the iron hypothesis Episodic elevated levels of chlorophyll a (> 20 μg L−1) were observed 6 times between 1964 and 1976 at the former site of Ocean Station Papa (OSP) In addition, between 1984 and 1990 on at least three occasions, concurrent pulses of POC and biogenic silica were recorded in deep-moored traps at OSP Possible explanations for these events, such as lateral advection of more productive waters, iron-mediated blooms, or grazing by salp swarms are discussed and tested using an existing downward POC flux model Owing to the episodic nature of such events, no available data are sufficiently comprehensive to unequivocally rule out any of these explanations Nevertheless, from the data available, the occurrence of pelagic or deep water pulses, approximately once every 3 years, are most consistent with iron-mediated diatom blooms, and of the sinking of POC and biogenic silica (from such a bloom) to depth, respectively A comparison of the timing of these iron-mediated pulses with that of the transport probabilities of atmospheric dust supply from Asia and Alaska provides an opportunity to assess the likelihood of a coupling between the atmosphere and the ocean

The calcite lysocline as a constraint on glacial/interglacial low‐latitude production changes

Abstract: We investigate the response of the calcite lysocline to changes in the export production of the low-latitude surface ocean (the combined equatorial, tropical, and subtropical regions). We employ different CaCO3 throughput schemes in a time-dependent ocean carbon cycle model to separate the CaCO3 production/lysocline balance from the other components of the model response and to estimate the effect of dissolution driven by organic matter decomposition in surface sediments. With this model, we carry out three experiments based on previous hypotheses for the cause of glacial/interglacial atmospheric CO2 variations: (1) a simple increase in low-latitude production, (2) a decrease in its CaCO3/organic carbon rain ratio, and (3) an increase in the depth of organic carbon regeneration. None of these changes, when taken alone, can lower atmospheric CO2 to glacial levels without violating observations of the glacial lysocline depth. If a low-latitude production increase were accompanied by a decrease in the CaCO3/organic carbon rain ratio, then these simultaneous changes could lower atmospheric CO2 to observed glacial levels without causing a significant change in the ocean-average depth of the lysocline. However, even with these simultaneous changes, the required increase in production is 50% or more of the modern rate of production.

A carbon budget for the Arctic Ocean

Abstract: We present a carbon budget for the Arctic Ocean that is based on estimates of water mass transformation and transport. The budget is constrained by conservation of mass and salt. In the model, the Eurasian and Canadian basins have been divided into five and six boxes, respectively, based on the prevailing water masses. In addition, there are three boxes representing the shelf areas. Total dissolved inorganic (C T ) and organic (TOC) carbon concentrations for the different water masses and different regions are used, together with the volume flows, to calculate the carbon transports. The carbon budget calculation shows that at present the oceanic transport into the Arctic Ocean is larger than out, that is, 3.296±0.008 Gt C yr -1 of C T transported in and 3.287±0.004 Gt C yr - transported out with the corresponding values for TOC being 0.134±0.009 Gt C yr -1 and 0.122±0.006 Gt C yr -1 , respectively. However, the outflowing waters are older than the inflowing waters and had thus been exposed to an atmosphere with lower concentration of anthropogenic carbon dioxide when entering the Arctic Ocean. When recalculating the budget to the preindustrial scenario, assuming steady state, the C T transport changes to 3.243 Gt C yr - in and 3.266 Gt C yr -1 out. To balance the preindustrial transports, assuming no change in the TOC fluxes, a direct input of atmospheric carbon dioxide of 0.011±0.014 Gt C yr -1 is required. Added to this is the burial of organic matter which is calculated as 0.013±0.010 Gt C yr -1 using a recycling efficiency of 80% [Hulth, 1995] and a new production of 0.05 Gt C yr -1 [Anderson et al., 1994]. An indirect contribution of atmospheric carbon dioxide via runoff adds 0.017±0.004 Gt C yr -1 , resulting in a preindustrial total atmospheric input of 0.041±0.018 Gt C yr -1 .

Determination of the isotopic(13C/12C) discrimination by terrestrial biology from a global network of observations

Abstract: We analyze data from the National Oceanic and Atmospheric Administration / Climate Monitoring and Diagnostics Laboratory global air sampling network in order to extract the signatures of isotopic (13C/12C) discrimination by the terrestrial biota and of fossil fuel combustion for the regions surrounding the sampling sites. We utilize measurements of carbon monoxide (CO) to give an estimate of the contribution of fossil fuel combustion to the short-term variability of carbon dioxide (CO2). In general, variations of CO2 are more strongly dominated by biological exchange, so the isotopic signature of fossil fuel combustion, while consistent with inventory estimates, is not well constrained by the observations. Conversely, results for isotope discrimination by the terrestrial biosphere are not strongly dependent on assumptions about fossil fuel combustion. Our analysis appears valid primarily for stations fairly near continental source/sink regions, particularly for midlatitude regions of the northern hemisphere. For these stations we derive a mean discrimination of −16.8 per mil (‰), with site-to-site variability of 0.8‰ (1 standard deviation) and with little or no consistent latitudinal gradient.

Air‐borne dust fluxes to a deep water sediment trap in the Sargasso Sea

Abstract: The record of atmospheric dust deposition as recorded by a deep sea sediment trap in the Sargasso Sea is presented The record is shown to be consistent with the limited available data on directly measured atmospheric dust loadings The seasonality of the sediment trap dust flux is different from that of the atmospheric deposition as a result of seasonal biological cycles in the surface water On the longer term the sediment trap dust flux undergoes quite large variations in the annual average flux from 36 to 94 mg m−2 d−1 These variations are shown to reflect changes in atmospheric transport efficiency from source regions in North Africa rather than changes in the strength of the dust source in that region The changes in the dust inputs to this area of the Sargasso Sea appear not to have changed the flux of carbon reaching the deep water, and the implications of this are discussed

Transient Climate Change and Net Ecosystem Production of the Terrestrial Biosphere

Abstract: In this sensitivity study, we have applied the Terrestrial Ecosystem Model ((TEM) version 4.1) to examine the responses of terrestrial ecosystems to transient changes in atmospheric CO2 concentration and climate in the 21st century at the scales of the globe, biomes, latitudinal gradient, and economic regions. Three predictions of transient change in climate and atmospheric CO2 concentration in the 21st century from the Integrated Global System Model developed at Massachusetts Institute of Technology were used. The TEM estimates a global annual net ecosystem production (NEP) of about 0.8 Pg C yr−1 in 1990. Global annual NEP in 2100 increases by about 2.6 Pg C yr−1 for the HHL (higher CO2 emissions and temperature increases), 1.8 Pg C yr−1 for the RRR (reference CO2 emissions and temperature increases), and 0.5 Pg C yr−1 for the LLH (lower CO2 emissions and temperature increases) climate change predictions. The boreal and tropical evergreen forests account for a large portion of the increased global annual NEP. Latitudinal distribution of total annual NEP along 0.5°-resolution latitudinal bands shifts significantly from the tropics to the northern middle and high latitudes over time. The potential CO2 uptake over the period of 1990–2100 differs substantially among the 12 economic regions of the world. As we used potential mature natural vegetation in the global extrapolation of TEM, these NEP estimates represent the potential CO2 uptake or the upper bound for long-term carbon sequestration by the terrestrial biosphere. This sensitivity study shows that the temporal dynamics and spatial distribution of carbon, nitrogen, and water fluxes of terrestrial ecosystems are very sensitive to the magnitudes and paths of transient changes in atmospheric CO2 concentration and climate in the 21st century.

Long‐term variability of the terrestrial and oceanic carbon sinks and the budgets of the carbon isotopes 13C and 14C

Abstract: The long-term variability in the terrestrial and oceanic uptake of anthropogenic carbon is investigated. Ice core and direct observations of atmospheric CO2 and 13C are used for the last 200 years. An inverse method called double deconvolution is applied. It is found that the biosphere turned from a carbon source of about 0.5 Gt C yr−1 into a sink of 1 Gt C yr−1 during the first half of this century. This is in qualitative agreement with earlier reconstructions based on atmospheric CO2 data and implies a terrestrial sink to compensate land use emissions during the last five decades. Oceanic and biospheric carbon uptakes are estimated to be 0.9±1.0 and l.l±1.0 Gt C yr−1 as averaged over the 1970–1990 period. Hence ocean uptake is on the low side of current estimates, but our results may be biased as δ13C observations between 1956 and 1982 are missing. Additional uncertainties in calculated carbon sinks are due to uncertainties in model parameters and in fossil emission estimates. Prior to 1950, uncertainties are primarily related to uncertainties in the ice core δ13C data; a Monte Carlo analysis yields a l-σ uncertainty in the terrestrial and oceanic uptake of ±0.36 Gt C yr−1 when ice core data are smoothed over a 50 year period. The budget of bomb-produced radiocarbon is reinvestigated. We could not find model solutions that concomitantly match the bomb budget and the observed atmospheric δ13C and prebomb Δ14C decrease. The closure of the budget would require a relatively low oceanic and biospheric 14C uptake that conflicts with the relatively high isotopic uptake rates required to simulate the observed decrease in δ13C and Δ14C. We conclude that recent estimates of bomb test productions and/or the stratospheric 14C decrease are not compatible with published 13C and 14C observations.

Soil respiration and georespiration distinguished by transport analyses of vadose CO2, 13CO2, and 14CO2

Abstract: Georespiration and soil respiration operate on organic carbon pools of vastly different sizes and mean residence times (MRT). Both processes occur in the shallow subsurface at the Dalmeny site in southern Saskatchewan, Canada. Steady and transient, heterogeneous simulations of vadose CO2,13CO2, and 14CO2 show that at least 98% of all subsurface respiration occurs in the solum where the MRT of labile soil carbon is about 10 years. Root respiration dominates the total during the growing season. Remaining CO2 generation occurs near the capillary surface at 6.5–7.5 m depth, where δ14C of respired CO2 indicates an MRT of about 22,000 years. This value is consistent with a respiration substrate dominated by Cretaceous-age kerogen in the till. The simulated oxidation/georespiration rate at this depth is also consistent with observed depletion of kerogen C from the vadose zone during the Holocene. Field relations in this setting indicate that georespiration is controlled hydrogeologically by the development of aerobic vadose zones; we speculate that this may be more generally true on a global basis. Where soil parent materials contain ancient carbon, georespiration should be considered as a possible factor complicating studies of soil carbon turnover.

Simulating carbon dynamics along the Boreal Forest Transect Case Study (BFTCS) in central Canada: 1. Model testing

Abstract: CENTURY 4.0, a simulation model of carbon and nitrogen dynamics of terrestrial ecosystems based on the relationships between climate, soil texture, plant productivity, decomposition and human management, was tested against observed data along the boreal forest transect case study (BFTCS) in central Canada. The results show that the simulated average aboveground biomass and net N mineralization were consistent with observed data. The modeled estimates for soil carbon were consistent with those from regional-scale empirical regression models. High correlation (R2 = 0.92) with data was obtained for the simulation of soil carbon dynamics of the boreal forest, but the model overestimated soil carbon (O–20 cm) by 2–8% for fine-textured soil and underestimated soil carbon by 5–18% for sandy soil. The effects of climatic variation on temporal changes in biomass and soil carbon storage over the past century were found to be very different for southern and northern sites but relatively insensitive to site-specific soil texture. The main discrepancies between observed data and CENTURY 4.0 results are associated with the effects of soil texture and an inadequate representation of fire disturbance on C dynamics of boreal forests. Further improvements, particularly in the representation of disturbance regimes and in the simulation of slow pool C dynamics, are suggested to enhance its predictive capability.