Showing papers in "Global Biogeochemical Cycles in 1994"

Redfield ratios of remineralization determined by nutrient data analysis

Abstract: 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 [open quotes]preformed[close quotes] 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/C[sub org]/[sup O][sub 2] ratios 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 C[sub org]/C[sub inorg] regeneration ratio decreases from approximately 7 [+-] 13 at 400 m to 3 [+-] 1 at 1000 m and to 1 [+-] 05 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in themore » Indian and Pacific oceans 74 refs, 8 figs, 3 tabs« less

Climatic, edaphic, and biotic controls over storage and turnover of carbon in soils

Abstract: Soil carbon, a major component of the global carbon inventory, has significant potential for change with changing climate and human land use. We applied the Century ecosystem model to a series of forest and grassland sites distributed globally to examine large-scale controls over soil carbon. Key site-specific parameters influencing soil carbon dynamics are soil texture and foliar lignin content; accordingly, we perturbed these variables at each site to establish a range of carbon concentrations and turnover times. We examined the simulated soil carbon stores, turnover times, and C:N ratios for correlations with patterns of independent variables. Results showed that soil carbon is related linearly to soil texture, increasing as clay content increases, that soil carbon stores and turnover time are related to mean annual temperature by negative exponential functions, and that heterotrophic respiration originates from recent detritus (∼50%), microbial turnover (∼30%), and soil organic matter (∼20%) with modest variations between forest and grassland ecosystems. The effect of changing temperature on soil organic carbon (SOC) estimated by Century is dSOC/dT= 183e−0.034T. Global extrapolation of this relationship leads to an estimated sensitivity of soil C storage to a temperature of −11.1 Pg° C−1, excluding extreme arid and organic soils. In Century, net primary production (NPP) and soil carbon are closely coupled through the N cycle, so that as temperatures increase, accelerated N release first results in fertilization responses, increasing C inputs. The Century-predicted effect of temperature on carbon storage is modified by as much as 100% by the N cycle feedback. Century-estimated soil C sensitivity (−11.1 Pg° C−1) is similar to losses predicted with a simple data-based calculation (−14.1 Pg° C−1). Inclusion of the N cycle is important for even first-order predictions of terrestrial carbon balance. If the NPP-SOC feedback is disrupted by land use or other disturbances, then SOC sensitivity can greatly exceed that estimated in our simulations. Century results further suggest that if climate change results in drying of organic soils (peats), soil carbon loss rates can be high.

Field and laboratory studies of methane oxidation in an anoxic marine sediment: Evidence for a methanogen-sulfate reducer consortium

Abstract: Field and laboratory studies of anoxic sediments from Cape Lookout Bight, North Carolina, suggest that anaerobic methane oxidation is mediated by a consortium of methanogenic and sulfate-reducing bacteria. A seasonal survey of methane oxidation and CO2 reduction rates indicates that methane production was confined to sulfate-depleted sediments at all times of year, while methane oxidation occurred in two modes. In the summer, methane oxidation was confined to sulfate-depleted sediments and occurred at rates lower than those of CO2 reduction. In the winter, net methane oxidation occurred in an interval at the base of the sulfate-containing zone. Sediment incubation experiments suggest both methanogens and sulfate reducers were responsible for the observed methane oxidation. In one incubation experiment both modes of oxidation were partially inhibited by 2-bromoethanesulfonic acid (a specific inhibitor of methanogens). This evidence, along with the apparent confinement of methane oxidation to sulfate-depleted sediments in the summer, indicates that methanogenic bacteria are involved in methane oxidation. In a second incubation experiment, net methane oxidation was induced by adding sulfate to homogenized methanogenic sediments, suggesting that sulfate reducers also play a role in the process. We hypothesize that methanogens oxidize methane and produce hydrogen via a reversal of CO2 reduction. The hydrogen is efficiently removed and maintained at low concentrations by sulfate reducers. Pore water H2 concentrations in the sediment incubation experiments (while net methane oxidation was occurring) were low enough that methanogenic bacteria could derive sufficient energy for growth from the oxidation of methane. The methanogen-sulfate reducer consortium is consistent not only with the results of this study, but may also be a feasible mechanism for previously documented anaerobic methane oxidation in both freshwater and marine environments.

Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization

Abstract: 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 application of this important paleoceanographic tool.

Spatial and temporal distribution of tropical biomass burning

Abstract: A database for the spatial and temporal distribution of the amount of biomass burned in tropical America, Africa, and Asia during the late 1970s is presented with a resolution of 5{degrees} latitude x 5{degrees} longitude. The sources of burning in each grid cell have been quantified. Savanna fires, shifting cultivation, deforestation, fuel wood use, and burning of agricultural residues contribute about 50, 24, 10, 11, and 5%, respectively, of total biomass burned in the tropics. Savanna fires dominate in tropical Africa, and forest fires dominant in tropical Asia. A similar amount of biomass is burned from forest and savanna fires in tropical America. The distribution of biomass burned monthly during the dry season has been derived for each grid cell using the seasonal cycles of surface ozone concentrations. Land use changes during the last decade could have a profound impact on the amount of biomass burned and the amount of trace gases and aerosol particles emitted. 32 refs., 3 figs., 3 tabs.

Variations of marine plankton δ13C with latitude, temperature, and dissolved CO2 in the world ocean

Abstract: Variations of the 13C 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 pCO2 through geological time from the δ13C of sedimentary organic matter. These assumptions are that (1) concentrations of CO2 in 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]aq may 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 ϵp and [CO2]aq in the modern ocean. The large residual variance for regressions of ϵp against [CO2]aq suggests that factors other than [CO2]aq strongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO2]aq cannot be easily calibrated using δ13CPOC data from the modern ocean.

Modeling carbon biogeochemistry in agricultural soils

Abstract: An existing model of C and N dynamics in soils was supplemented with a plant growth submodel and cropping practice routines (fertilization, irrigation, tillage, crop rotation, and manure amendments) to study the biogeochemistry of soil carbon in arable lands. The new model was validated against field results for short-term (1–9 years) decomposition experiments, the seasonal pattern of soil CO2 respiration, and long-term (100 years) soil carbon storage dynamics. A series of sensitivity runs investigated the impact of varying agricultural practices on soil organic carbon (SOC) sequestration. The tests were simulated for corn (maize) plots over a range of soil and climate conditions typical of the United States. The largest carbon sequestration occurred with manure additions; the results were very sensitive to soil texture (more clay led to greater sequestration). Increased N fertilization generally enhanced carbon sequestration, but the results were sensitive to soil texture, initial soil carbon content, and annual precipitation. Reduced tillage also generally (but not always) increased SOC content, though the results were very sensitive to soil texture, initial SOC content, and annual precipitation. A series of long-term simulations investigated the SOC equilibrium for various agricultural practices, soil and climate conditions, and crop rotations. Equilibrium SOC content increased with decreasing temperatures, increasing clay content, enhanced N fertilization, manure amendments, and crops with higher residue yield. Time to equilibrium appears to be one hundred to several hundred years. In all cases, equilibration time was longer for increasing SOC content than for decreasing SOC content. Efforts to enhance carbon sequestration in agricultural soils would do well to focus on those specific areas and agricultural practices with the greatest potential for increasing soil carbon content.

Soil‐atmosphere exchange of nitrous oxide, nitric oxide, and methane under secondary succession of pasture to forest in the Atlantic lowlands of Costa Rica

Abstract: We investigated changes in soil-atmosphere flux of CH4, N2O, and NO resulting from the succession of pasture to forest in the Atlantic lowlands of Costa Rica. We studied a dozen sites intensively for over one year in order to measure rates and to understand controlling mechanisms for gas exchange. CH4 flux was controlled primarily by soil moisture content. Soil consumption of atmospheric CH4 was greatest when soils were relatively dry. Forest soils consumed CH4 while pasture soils which had poor drainage generally produced CH4. The seasonal pattern of N2O emissions from forest soils was related exponentially to soil water-filled pore space. Annual average N2O emissions correlated with soil exchangeable NO3− concentrations. Soil-atmosphere NO flux was greatest when soils were relatively dry. We found the largest NO emissions from abandoned pasture sites. Combining these data with those from another study in the Atlantic lowlands of Costa Rica that focused on deforestation, we present a 50-year chronosequence of trace gas emissions that extends from natural conditions, through disturbance and natural recovery. The soil-atmosphere fluxes of CH4 and N2O and of NO may be restored to predisturbance rates during secondary succession. The changes in trace gas emissions following deforestation, through pasture use and secondary succession, may be explained conceptually through reference to two major controlling factors, nitrogen availability and soil-atmosphere diffusive exchange of gases as it is influenced by soil moisture content and soil compaction.

Methane in the Baltic and North Seas and a Reassessment of the Marine Emissions of Methane

Abstract: During three measurement campaigns on the Baltic and North Seas, atmospheric and dissolved methane was determined with an automated gas chromatographic system. Area-weighted mean saturation values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, February and July 1992) and 126 ± 8% (south central North Sea, September 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept of regional gas transfer coefficients. Our estimates computed with two different air-sea exchange models lie in the range of 11-18 Tg CH4 yr-1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three-layer model to numerically simulate the time dependent variation of the oceanic flux to the atmosphere. The model calculations indicate that even with increasing tropospheric methane concentration, the ocean will remain a source of atmospheric methane.

The Dole Effect and its variations during the last 130,000 years as measured in the Vostok Ice Core

Abstract: We review the current understanding of the Dole effect (the observed difference between the {delta}{sup 18}O of atmospheric O{sub 2} and that of seawater) and its causes, extended the record of variations in the Dole effect back to 130 kyr before present using data on the {delta}{sup 18}O of O{sub 2} obtained from studying the Vostok ice core, and discuss the significance of temporal variations. The Dole effect reflects oxygen isotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation, and evapotranspiration). Our best prediction of the present-day Dole effect, +20.8 {per_thousand}, is considerably lower than the observed value, +23.5 {per_thousand}, and we discuss possible causes of this discrepancy. During the past 130 kyr, the Dole effect has been 0.05 {per_thousand} lower than the present value, on average. The standard deviation of the Dole effect from the mean has been only {+-} {per_thousand}, and the Dole effect is nearly unchanged between glacial maxima and interglacial periods. The small variability in the Dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant. Most periodic variability in the Dole effect is in the precession band, suggesting that changes in this global biogeochemical termmore » reflects variations in low-latitude land hydrology and productivity or possibly variability in low-latitude oceanic productivity. 65 refs., 4 figs., 3 tabs.« less

Potential distribution of methane hydrates in the world's oceans

Abstract: Estimates of the magnitudes and spatial distribution of potential oceanic methane hydrate reservoirs have been made from pressure-temperature phase relations and a plausible range of thermal gradients, sediment porosities, and pore fillings taken from published sources, based on two major theories of gas hydrate formation (1) in situ bacterial production and (2) pore fluid expulsion models The implications of these two models on eventual atmospheric methane release, due to global warming, are briefly examined The calculated range of methane volumes in oceanic gas hydrates is 264 to 1391 x 10{sup 15} m{sup 3}, with the most likely value on the lower end of this range The results for the bacterial model show a preferential distribution of hydrates at mid- to high latitudes, with an equatorial enhancement in the case of the fluid migration model The latter model also generates a deeper and thicker hydrate stability zone at most latitudes than does the former Preliminary results suggest that the hydrate distribution predicted by the fluid migration model may be more consistent with observations However, this preliminary finding is based on a very limited sample size, and there are high uncertainties in the assumptions The volume of methane hydrate within the uppermostmore » 1 m of the hydrate stability zone and within 1{degrees}-2{degrees}C of the equilibrium curve, assuming in situ bacterial generation, is 093-632 x 10{sup 12} m{sup 3}, or 00035-0012% of the maximal estimated hydrate reservoir Nevertheless this volume, if released uniformly over the next 100 years, is comparable to current CH{sub 4} release rates for several important CH{sub 4} sources Corresponding CH{sub 4} volumes calculated using the fluid migration model are nearly 2 orders of magnitude lower 52 refs, 2 figs, 5 tabs« less

Carbon isotope fractionation by marine phytoplankton in culture: The effects of CO2 concentration, pH, temperature, and species

Abstract: Closed cultures of marine phytoplankton were established under variable conditions of CO2 concentration, temperature, growth rate (by light limitation), and pH (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 with Skeletonema costatum and Emiliania huxleyi, increasing CO2 concentrations led to increased carbon isotope discrimination (resulting in organic matter progressively depleted in δ13C, i.e., a greater, more negative ϵp). ϵp values for E. huxleyi were 8–10‰ less than for S. costatum under identical conditions. For the S. costatum cultures, there was nearly a 20 ‰ range in [CO2aq]-dependent ϵp. The effect was nonlinear with a leveling off at high [CO2aq]. Over a pH range of 7.5–8.3 but at a constant [CO2aq] there was a variation in carbon isotope fractionation by S. costatum of about 9 ‰ with a minimum at pH 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 for S. costatum over 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 CO2 concentrations. Hindcasts of ancient atmospheric pCO2 from measurements of δ13C of organic carbon in marine sediments will require careful calibration because of the variety of possible factors that influence δ13Corg.

CARAIB - A global model of terrestrial biological productivity

Abstract: CARAIB, a mechanistic model of carbon assimilation in the biosphere estimates the net primary productivity (NPP) of the continental vegetation on a grid of 1° × 1° in latitude and longitude The model considers the annual and diurnal cycles It is based on the coupling of the three following submodels; a leaf assimilation model including estimates of stomatal conductance and leaf respiration, a canopy model describing principally the radiative transfer through the foliage, and a wood respiration model Present-day climate and vegetation characteristics allow the discrimination between ecotypes In particular, specific information on vegetation distribution and properties is successfully used at four levels; the leaf physiological level, the plant level, the ecosystem level, and the global level The productivity determined by the CARAIB model is compared with local measurements and empirical estimates showing a good agreement with a global value of 65 Gt C yr−1 The sensitivity of the model to the diurnal cycle and to the abundance of C4 species is also tested The productivity slightly decreases (10%) when the diurnal cycle of the temperature is neglected By contrast, neglecting the diurnal cycle of solar irradiance produces unrealistically high values of NPP Even if the importance of this increase would presumably be reduced by the coupling of CARAIB with a nutrient cycle model, this test emphasizes the key role of the diurnal cycle in a mechanistic model of the NPP Uncertainties on the abundance and spatial distribution of C4 plants may cause errors in the NPP estimates, however, as demonstrated by two sensitivity tests, these errors are certainly lower than 10% at the global scale as shown by two tests

Influence of water table on carbon dioxide, carbon monoxide, and methane fluxes from Taiga Bog microcosms

Abstract: Hydrological changes, particularly alterations in water table level, may largely overshadow the more direct effects of global temperature increase upon carbon cycling in arctic and subarctic wetlands. Frozen cores (n=40) of intact soils and vegetation were collected from a bog near Fairbanks, Alaska, and fluxes of CO{sub 2}, CH{sub 4}, and Co in response to water table variation were studied under controlled conditions in the Duke University phytotron. Core microcosms thawed to a 20-cm depth over 30 days under a 20 hour photoperiod with a day/night temperature regime of 20/10{degrees}C. After 30 days the water table in 20 microcosms was decreased from the soil surface to -15 cm and maintained at the soil surface in 20 control cores. Outward fluxes of CO{sub 2} (9-16 g m{sup -2}d{sup -1}) and CO (3-4 mg m{sup -2}d{sup -1}) were greatest during early thaw and decreased to near zero for both gases before the water table treatment started. Lower water table tripled CO{sub 2} flux to the atmosphere when compared with control cores. Carbon monoxide was emitted at low rates from high water table cores and consumed by low water table cores. Methane fluxes were low (<1 mg m{sup -2}d{sup -1}) in all coresmore » during thaw. High water table cores increased CH{sub 4} flux to 8-9 mg m{sup -2}d{sup -1} over 70 days and remained high relative to the low water table cores (<0.74 mg m{sup -2}d{sup -1}). Although drying of wetland taiga soils may decrease CH{sub 4} emissions to the atmosphere, the associated increase in CO{sub 2} due to aerobic respiration will likely increase the global warming potential of gas emissions from these soils. 43 refs., 4 figs.« less

Climate controls on temporal variability of methane flux from a poor fen in southeastern New Hampshire: Measurement and modeling

Abstract: Three scales of temporal variability were present in methane (CH 4) flux data collected during a 2.5 year (mid-1990n1992) study at a small, poor fen in southeastern New Hampshire. (1) There was a strong seasonality to the fluxes (high in summer); monthly average fluxes range from 21.4 mg CH 4nm m2nd m1n(February 1992) to 639.0 mg CH 4nm m2d m1n(July 1991). Annual fluxes were 68.8 g CH 4nm m2n(1991) and 69.8 g CH 4nm m2n(1992). (2) There was interannual variability; distribution of flux intensity was very different from 1991 to 1992, particularly the timing and rapidity of the onset of higher fluxes in the spring. (3) There was a high degree of variability in CH 4nflux during the warm season; four successive weekly flux rates in July 1991 were 957, 1044, 170, and 491 mg CH 4nm m2nd m1. Fluxes were correlated with peat temperature (r 2=0.44) but only weakly with depth to water table (r 2n= 0.14 for warm season data). Warm season fluxes appeared to be suppressed by rainstorms. Along with methane flux data we present an analysis of this temporal variability in flux, using a peatland soil climate model developed for this site. The model was driven by daily air temperature, precipitation, and net radiation; it calculated daily soil temperature and moisture profiles, water table location, and ice layer thickness. Temperature profiles were generally in good agreement with field data. Depth to water table simulations were good in 1992, fair in 1990, and poor in the summer of 1991. Using model-simulated peat climate and correlations to methane flux developed from the field data, simulated methane fluxes exhibited the same three modes of temporal variability that were present in the field flux data, though the model underestimated peak fluxes in 1990 and 1991. We conclude that temporal variability in flux is significantly influenced by climate/weather variability at all three scales and that rainfall appears to suppress methane flux for at least several days at this site.

Nitrogenous fertilizers: Global distribution of consumption and associated emissions of nitrous oxide and ammonia

Abstract: The global distribution of nitrogen input via application of chemical nitrogenous fertilizers to agricultural ecosystems is presented. The suite of 1° (latitude/longitude) resolution data bases includes primary data on fertilizer consumption, as well as supporting data sets defining the distribution and intensity of agriculture associated with fertilizer use. The data were developed from a variety of sources and reflect conditions for the mid-1980s. East Asia, where fertilizer use is increasing at ∼10%/year, accounted for ∼37% of the total, while North America and western Europe, where fertilizer use is leveling off, accounted for 17% and 14% of global use, respectively. Former centrally planned economies of Europe consumed one fifth of the 1984 total, but rapid increases in the 1980s are slowing, and consumption trends are variable. The most widely used chemical nitrogenous fertilizer is urea which accounted for 40% of the world's total in the mid-1980s. While almost every country consumes urea, ∼75% of the large East Asian fertilizer use is supplied by this one fertilizer. Ammonium nitrate, used primarily in the former centrally planned economies of Europe, in West Asia, and in Africa, accounted for about one quarter of global consumption. These data were used to estimate distributions of the annual emission of nitrous oxide (N2O) and of ammonia (NH3) associated with the use of fertilizers. Applying published ranges of emission coefficients for fertilizer types in the data base yields a median emission of 0.1 Tg N2O-N, with lower and upper values of 0.03 and 2.0 Tg N2O-N in 1984. This equals <1% to ∼3% of the total nitrogen applied via commercial fertilizers and represents <1% to 15% of the annual emission of N2O from terrestrial sources. Assuming that the ∼4% annual increase in consumption of nitrogenous fertilizers during the 1980s corresponds to a ∼4% rise in the release of N2O-N, yearly increases in emissions from fertilizer use are <0.01 to 0.08 Tg N2O-N equal to <1% to 3% of the current growth of atmospheric nitrous oxide. However, since no measurements of fertilizer-derived nitrous oxide emissions are available for agricultural environments in the tropics/subtropics, where ∼40% of fertilizer N is consumed and where consumption is increasing rapidly, relative contributions of climatic regions to current and future emissions remain uncertain. Ammonia emission coefficients for simple groups of fertilizer types were applied to derive the global distribution of ammonia volatilization associated with nitrogenous fertilizer consumption. The 1984 total of ∼5–7 Tg NH3-N, about 10–15% of the annual ammonia source, is concentrated overwhelmingly in subtropical Asia owing to the dominant use of urea with high rates of volatilization. However, the paucity of measurements in representative ecological and management environments suggests that the magnitude and distribution of current and future ammonia emission from fertilizers is still poorly known.

A three dimensional time‐dependent approach to calibrating sediment trap fluxes

Abstract: We conducted an experiment to test explicitly the accuracy of upper ocean sediment trap fluxes using the particle-reactive radionuclide 234Th (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 total 234Th were collected every 8 m between the surface and 96 m and immediately combined for analysis to obtain the vertically integrated activity of 234Th. 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 predicted 234Th flux from the watercolumn profiles was not significantly different from zero (−30 ± 140 disintegrations per minute/m2/d). The measured trap 234Th 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 the true particle flux.

Modeling the Global Carbon Cycle: Nitrogen fertilization of the terrestrial biosphere and the “missing” CO2 sink

Abstract: The discrepancy between estimates of net terrestrial CO2 emissions derived from (1) inverse modeling of the ocean/atmosphere system and (2) modeling of land use change, better known as the “missing” CO2 sink, suggests that some changing environmental factor, such as CO2, anthropogenic N emissions, or climate, has fertilized terrestrial ecosystems. To address this question, we herein describe and apply GLOCO, a global carbon cycle model. GLOCO's ocean submodel combines a box diffusion model with representations of chemical equilibria and biological processes to simulate the distributions and cycling of inorganic and organic carbon, phosphate, and alkalinity. The terrestrial submodel divides the biosphere into seven natural biomes with dynamic carbon and nitrogen cycling in both vegetation and soils. Anthropogenic influences on the functioning of the carbon and nitrogen cycles, such as fossil fuel combustion, forestry, and agricultural development, are also incorporated in the model. Our analysis confirms previous suggestions that because temperate and boreal forests are N limited, CO2 fertilization of these forests is less than predicted by short-term CO2 response factors. Modeling of temperate/boreal forest fertilization by anthropogenic N deposition suggests that CO2 is initially sequestered at a C:N ratio of ∼100, rather than the steady state value for the ecosystem of ∼30. If N deposition is to account for the 40–70% of the fertilization of the terrestrial biosphere not explainable by CO2 fertilization and temperature increases, then we estimate that 26-30 Tg N yr−1 of anthropogenic deposition in the temperate and boreal zones would be required. Recent anthropogenic NOx and NH3 deposition fluxes at northern temperate latitudes have been estimated to be 20–28 Tg N yr−1. Thus fertilization by anthropogenic N emissions likely constitutes a significant portion of the missing CO2 sink.

Varve calibrated records of carbonate and organic carbon accumulation over the last 2000 years in the Black Sea

Abstract: 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 for Emiliania 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−2 yr−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−2 yr−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, are about the same in both settings.

Effect of altitude on the carbon-isotope composition of forest and grassland soils from Papua New Guinea

Abstract: The carbon-isotope composition of both forest and grassland soils from Papua New Guinea exhibit predictable trends with increasing altitude. Soils under pure C3 vegetation (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 by Korner et al. [1988]. Grassland soils show a decrease in δ13C value above about 1000 m, from maximum values which are close to pure C4 values (−12 to −13‰ vs. PDB) to minimum values which are indistinguishable from pure C3 values 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 CO2 is 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 C3 carbon present in the sample. It is also possible that other factors, such as moisture availability, may play some role in determining the proportions of C3 and C4 grasses 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 particulate organic carbon.

Exports of carbon and nitrogen from river basins in Canada's Atlantic Provinces

Abstract: The loss of carbon and organic nitrogen from the terrestrial ecosystem via streams and rivers is dependent on a number of factors such as basin vegetation, geography, geology, climate, and hydrology. We studied the export of dissolved carbon and nitrogen from 26 rivers varying in size from 45 to 92,500 km{sup 2} located in Atlantic Canada. Twenty-four of the basins studied were free of significant anthropogenic activity and were covered with coniferous and mixed hardwood forests. Our results showed that total organic carbon loss from the region, normalized for area, was approximately 29 kg ha {sup -1} yr{sup -1}, while inorganic C was considerably lower at 4.3 kg ha{sup -1} yr{sup -1}. We developed predictive statistical models using total precipitation, basin size, and basin slope to predict the export of organic carbon and nitrogen. Our results suggest that increases in regional precipitation will most likely increase the loss of organic carbon and nitrogen from terrestrial systems. We also found that inorganic carbon and nitrogen were not influenced by precipitation. Inorganic carbon seemed more influenced by geology, and inorganic nitrogen seemed more influenced by basin slope. 32 refs., 4 figs., 3 tabs.

Uptake of inorganic carbon and nitrate by marine plankton and the Redfield Ratio

Abstract: 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−3 content is suggested for studies of the underlying mechanisms.

Methane emissions from rice fields: Effect of soil properties

Abstract: 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 −2 to 36.3 g m−2.

Dissolution kinetics of calcium carbonate in equatorial Pacific sediments

Abstract: 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 constant k, formulated according to the equation developed by Keir [1980]: dissolution rate = kγ(1-Ω)n. The observed dissolution fluxes are predicted by the model when k = 5 to 100%/d and n = 4.5. This range of k values has important implications regarding the type of carbonate dissolving and its location within the sediment column. At low values of k, organic carbon rain rates to the seafloor become the dominant driving force of carbonate dissolution. At higher values of k, the degree of bottom water undersaturation becomes more important. Dissolution of carbonate within equatorial Pacific sediments can be adequately described with k = 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 CaCO3 arriving 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 mass transport rates.

Solubilities of Al, Pb, Cu, and Zn in rain sampled in the marine environment over the North Atlantic Ocean and Mediterranean Sea

Abstract: Chemical processes controlling the dissolved and particulate phase distribution of crustal (Al) and noncrustal metals (Pb, Cu, and Zn) appear to differ in marine precipitation sampled over the North Atlantic Ocean and Mediterranean Sea. Dissolved Al appears to be in equilibrium with a trivalent Al salt at rainwater pH 80% of the total Pb, Cu, and Zn concentrations are delivered to the surface oceans in the dissolved form. For a corresponding pH range, Al solubility varies from 60%. Over the wider observed pH range (of 3.5 to 6.9), the solubilities of Pb, Cu, Zn, and Al are highly variable. The use of mean trace metal solubilities for the assessment of dissolved atmospheric trace metal wet deposition fluxes, and their effects on surface ocean biogeochemistry should be constrained by taking into account rainwater pH in future estimates in global models.

El Niño‐Southern Oscillation related fluctuations of the marine carbon cycle

Abstract: 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 Nino-Southern Oscillation (ENSO) induced fluctuations of the wind field. During El Nino 1982/1983, when upwelling and biological productivity in the equatorial Pacific were strongly reduced and sea surface temperatures were increased, the oceanic CO2 partial pressure in this region decreased significantly. Consequently, in 1982/1983 the CO2 flux from the tropical ocean into the atmosphere was reduced. However, in 1983 the interannual deviations from the long-term trend in atmospheric CO2 showed 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 CO2 from the land biosphere must have been about 2 GtC.

Impact of gypsum application on the methane emission from a wetland rice field.

Abstract: 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−1 gypsum was reduced by 55–70% compared to non-amended plots. Although CH4 emission from fields with a high input of fresh organic matter was strongly enhanced, the experiments showed that the relative reduction in CH4 emission upon gypsum application was independent of organic matter addition. The reduced CH4 emission 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 CH4 emissions from rice grown on high-sulfate containing soils or gypsum-amended soils.

Air-sea carbon dioxide exchange in the North Pacific subtropical Gyre: Implications for the global carbon budget

Abstract: 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−1 for 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−1 of 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 overestimated.

The sensitivity of the terrestrial biosphere to climatic change: A simulation of the Middle Holocene

Abstract: A process-based ecosystem model, DEMETER, is used to simulate the sensitivity of the terrestrial biosphere to changes in climate. In this study, DEMETER is applied to the two following climatic regimes: (1) the modern observed climate and (2) a simulated mid-Holocene climate (6000 years before present). The mid-Holocene climate is simulated using the GENESIS global climate model, where shifts in the Earth's orbital parameters result in warmer northern continents and enhanced monsoons in Asia, North Africa, and North America. DEMETER simulates large differences between modern and mid-Holocene vegetation cover: (1) mid-Holocene boreal forests extend farther poleward than present in much of Europe, Asia, and North America, and (2) mid-Holocene North African grasslands extend substantially farther north than present. The simulated patterns of mid-Holocene vegetation are consistent with many features of the paleobotanical record. Simulated mid-Holocene global net primary productivity is approximately 3% larger than present, largely due to the increase of boreal forest and tropical grasslands relative to tundra and desert. Global vegetation carbon is higher at 6 kyr B.P. compared to present by roughly the same amount (4%). Mid-Holocene global litter carbon is larger than present by 10%, while global soil carbon is approximately 1% less. Despite the regional changes in productivity and carbon storage the simulated total carbon storage potential of the terrestrial biosphere (not including changes in peat) does not change significantly between the two simulations.

The lifetime of excess atmospheric carbon dioxide

Abstract: We explore the effects of a changing terrestrial biosphere on the atmospheric residence time of CO2 using 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 CO2 decay (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 pCO2 value. 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 CO2 and 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 CO2 under 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 CO2 lies 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 CO2 increases to 72 years, and if we remove the terrestrial component completely, then it increases further to 92 years.