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Manizza, M, Keeling RF, Nevison CD.  2012.  On the processes controlling the seasonal cycles of the air-sea fluxes of O2 and N2O: A modelling study. Tellus Series B-Chemical and Physical Meteorology. 64   10.3402/tellusb.v64i0.18429   AbstractWebsite

The seasonal dynamics of the air-sea gas flux of oxygen (O-2) are controlled by multiple processes occurring simultaneously. Previous studies showed how to separate the thermal component from the total O-2 flux to quantify the residual oxygen flux due to biological processes. However, this biological signal includes the effect of both net euphotic zone production (NEZP) and subsurface water ventilation. To help understand and separate these two components, we use a large-scale ocean general circulation model (OGCM), globally configured, and coupled to a biogeochemical model. The combined model implements not only the oceanic cycle of O-2 but also the cycles of nitrous oxide (N2O), argon (Ar) and nitrogen (N-2). For this study, we apply a technique to distinguish the fluxes of O-2 driven separately by thermal forcing, NEZP, and address the role of ocean ventilation by carrying separate O-2 components in the model driven by solubility, NEZP and ventilation. Model results show that the ventilation component can be neglected in summer compared to the production and thermal components polewards but not equatorward of 30 degrees in each hemisphere. This also implies that neglecting the role of ventilation in the subtropical areas would lead to overestimation of the component of O-2 flux due to NEZP by 20-30%. Model results also show that the ventilation components of air-sea O-2 and N2O fluxes are strongly anti-correlated in a ratio that reflects the subsurface tracer/tracer relationships (similar to 0.1 mmol N2O/mol O-2) as derived from observations. The results support the use of simple scaling relationships linking together the thermally driven fluxes of Ar, N-2 and O-2. Furthermore, our study also shows that for latitudes polewards of 30 degrees of both hemispheres, the Garcia and Keeling (2001) climatology, when compared to our model results, has a phasing error with the fluxes being too early by similar to 2-3 weeks.

Keeling, RF, Manning AC, Dubey MK.  2011.  The atmospheric signature of carbon capture and storage. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 369:2113-2132.   10.1098/rsta.2011.0016   AbstractWebsite

Compared with other industrial processes, carbon capture and storage (CCS) will have an unusual impact on atmospheric composition by reducing the CO(2) released from fossil-fuel combustion plants, but not reducing the associated O(2) loss. CO(2) that leaks into the air from below-ground CCS sites will also be unusual in lacking the O(2) deficit normally associated with typical land CO(2) sources, such as from combustion or ecosystem exchanges. CCS may also produce distinct isotopic changes in atmospheric CO(2). Using simple models and calculations, we estimate the impact of CCS or leakage on regional atmospheric composition. We also estimate the possible impact on global atmospheric composition, assuming that the technology is widely adopted. Because of its unique signature, CCS may be especially amenable to monitoring, both regionally and globally, using atmospheric observing systems. Measurements of the O(2)/N(2) ratio and the CO(2) concentration in the proximity of a CCS site may allow detection of point leaks of the order of 1000 ton CO(2) yr(-1) from a CCS reservoir up to 1km from the source. Measurements of O(2)/N(2) and CO(2) in background air from a global network may allow quantification of global and hemispheric capture rates from CCS to the order of +/- 0.4 PgCyr(-1).

Rodenbeck, C, Le Quere C, Heimann M, Keeling RF.  2008.  Interannual variability in oceanic biogeochemical processes inferred by inversion of atmospheric O2/N2 and CO2 data. Tellus Series B-Chemical and Physical Meteorology. 60:685-705.   10.1111/j.1600-0889.2008.00375.x   AbstractWebsite

Atmospheric measurements of O(2)/N(2) and CO(2) at up to nine sites have been used to infer the interannual variations in oceanic O(2) exchange with an inverse method. The method distinguishes the regional contributions of three latitudinal bands, partly the individual contributions of the North Pacific and the North Atlantic also. The interannual variations of the inferred O(2) fluxes in the tropical band correlate significantly with the El Nino/Southern Oscillation. Tropical O(2) variations appear to be dominated by the ventilation of the O(2) minimum zone from variations in Pacific equatorial upwelling. The interannual variations of the northern and southern extratropical bands are of similar amplitude, though the attribution to mechanisms is less clear. The interannual variations estimated by the inverse method are larger than those estimated by the current generation of global ocean biogeochemistry models, especially in the North Atlantic, suggesting that the representation of biological processes plays a role. The comparison further suggests that O(2) variability is a more stringent test to validate models than CO(2) variability, because the processes driving O(2) variability combine in the same direction and amplify the underlying climatic signal.

Keeling, RF, Blaine T, Paplawsky B, Katz L, Atwood C, Brockwell T.  2004.  Measurement of changes in atmospheric Ar/N2 ratio using a rapid-switching, single-capillary mass spectrometer system. Tellus Series B-Chemical and Physical Meteorology. 56:322-338.   10.1111/j.1600-0889.2004.00117.x   AbstractWebsite

The atmospheric Ar/N-2 ratio is expected to undergo very slight variations due to exchanges of Ar and N-2 across the air-sea interface, driven by ocean solubility changes. Observations of these variations may provide useful constraints on large-scale fluxes of heat across the air-sea interface. A mass spectrometer system is described that incorporates a magnet with a wide exit face, allowing a large mass spread, and incorporates an inlet with rapid (5 s) switching of sources gases through a single capillary, thus achieving high precision in the comparison of sample and reference gases. The system allows simultaneous measurement of Ar/N-2, O-2/N, and CO2/N-2 ratios. The system achieves a short-term precision in Ar/N-2 of 10 per meg for a 10 s integration, which can be averaged to achieve an internal precision of a few per meg in the comparison of reference gases. Results for Ar/N-2 are reported from flasks samples collected from nine stations in a north-to-south global network over about a 1 yr period. The imprecision on an individual flask, as estimated from replicate agreement, is 11 per meg. This imprecision is dominated by real variability between samples at the time of analysis. Seasonal cycles are marginally resolved at the extra-tropical stations with amplitudes of 5 to 15 per meg. Annual-mean values are constant between stations to within 5 per meg. The results are compared with a numerical simulation of the cycles and gradients in Ar/N-2 based on the TM2 tracer transport model in combination with air-sea Ar and N-2 fluxes derived from climatological air-sea heat fluxes. The possibility is suggested that Ar/N-2 ratios may be detectably enriched near the ground by gravimetric or thermal fractionation under conditions of strong surface inversions.

Lueker, TJ, Walker SJ, Vollmer MK, Keeling RF, Nevison CD, Weiss RF, Garcia HE.  2003.  Coastal upwelling air-sea fluxes revealed in atmospheric observations of O2/N2, CO2 and N2O. Geophysical Research Letters. 30   10.1029/2002gl016615   AbstractWebsite

[1] We capture water column ventilation resulting from coastal upwelling in continuous records of O-2/N-2, CO2, and N2O at Trinidad, California. Our records reveal the gas exchange response time of the ocean to the upwelling and ensuing biological production. Satellite and buoy wind data allow extrapolation of our records to assess coastal upwelling air-sea fluxes of O-2 and N2O. We improve on previous regional estimates of N2O flux in coastal and continental shelf region of the western U. S. We characterize the source of N2O as being predominately from nitrification based on the O-2/N2O emissions ratio observed in our atmospheric records.

Battle, M, Bender M, Hendricks MB, Ho DT, Mika R, McKinley G, Fan SM, Blaine T, Keeling RF.  2003.  Measurements and models of the atmospheric Ar/N2 ratio. Geophysical Research Letters. 30   10.1029/2003gl017411   AbstractWebsite

[1] The Ar/N-2 ratio of air measured at 6 globally distributed sites shows annual cycles with amplitudes of 12 to 37 parts in 10(6). Summertime maxima reflect the atmospheric Ar enrichment driven by seasonal warming and degassing of the oceans. Paired models of air-sea heat fluxes and atmospheric tracer transport predict seasonal cycles in the Ar/N-2 ratio that agree with observations, within uncertainties.

Keeling, RF, Garcia HE.  2002.  The change in oceanic O2 inventory associated with recent global warming. Proceedings of the National Academy of Sciences of the United States of America. 99:7848-7853.   10.1073/pnas.122154899   AbstractWebsite

Oceans general circulation models predict that global warming may cause a decrease in the oceanic O-2 inventory and an associated O-2 outgassing. An independent argument is presented here in support of this prediction based on observational evidence of the ocean's biogeochemical response to natural warming. On time scales from seasonal to centennial, natural O-2 flux/heat flux ratios are shown to occur in a range of 2 to 10 nmol of O-2 per joule of warming, with larger ratios typically occurring at higher latitudes and overlongertime scales. The ratios are several times larger than would be expected solely from the effect of heating on the O-2 solubility, indicating that most of the O-2 exchange is biologically mediated through links between heating and stratification. The change in oceanic O-2 inventory through the 1990s is estimated to be 0.3 +/- 0.4 X 10(14) mol of O-2 per year based on scaling the observed anomalous long-term ocean warming by natural O-2 flux/heating ratios and allowing for uncertainty due to decadal variability. Implications are discussed for carbon budgets based on observed changes in atmospheric O-2/N-2 ratio and based on observed changes in ocean dissolved inorganic carbon.

Keeling, RF, Stephens BB, Najjar RG, Doney SC, Archer D, Heimann M.  1998.  Seasonal variations in the atmospheric O2/N2 ratio in relation to the kinetics of air-sea gas exchange. Global Biogeochemical Cycles. 12:141-163.   10.1029/97gb02339   AbstractWebsite

Observations of seasonal variations in the atmospheric O-2/N-2 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 O-2 on the O-2/N-2 cycles thus allowing the residual component of the cycles due to oceanic exchanges of O-2 and N-2 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 O-2/N-2 cycles is presented consisting of the TM2 atmospheric tracer transport model [Heimann, 1995] driven at the lower boundary by O-2 fluxes derived from observed O-2 saturation anomalies in surface waters and by N-2 fluxes derived from the net air-sea heat flux. The model is optimized to fit the observed atmospheric O-2/N-2 cycles by adjusting the air-sea gas-exchange velocity, which relates O-2 anomaly to O-2 flux. The optimum fit corresponds to spatially and temporally averaged exchange velocities of 24+/-6 cm/hr for the oceans north of 31 degrees N and 29+/-12 cm/hr for the oceans south of 31 degrees 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 O-2, 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.

Stephens, BB, Keeling RF, Heimann M, Six KD, Murnane R, Caldeira K.  1998.  Testing global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration. Global Biogeochemical Cycles. 12:213-230.   10.1029/97gb03500   AbstractWebsite

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O-2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO approximate to O-2 + 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, O-2, 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, O-2, and N-2 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 O-2 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.

Keeling, RF, Manning AC, McEvoy EM, Shertz SR.  1998.  Methods for measuring changes in atmospheric O2 concentration and their application in southern hemisphere air. Journal of Geophysical Research-Atmospheres. 103:3381-3397.   10.1029/97jd02537   AbstractWebsite

Methods are described for measuring changes in atmospheric O-2 concentration with emphasis on gas handling procedures. Cryogenically dried air samples are collected in 5 L glass flasks at ambient pressure and analyzed against reference gases derived from high-pressure aluminum tanks. Fractionation effects are minimized by avoiding pressure and flow variations throughout the gas-handling system. The overall external reproducibility is approximately +/-3.3 per meg, with systematic errors associated with collecting samples and with storing them for 1 year reduced to the level of 3 per meg or smaller. The demonstrated short-term reproducibly of air delivered from high-presure tanks is +/-1.5 per meg, with the composition changing by at most 5 per meg by surface desorption reactions as the tank is depleted to below 3500 kPa. A 9-year survey of a suite of six reference gases showed no systematic long-term trends in relative O-2 concentrations to the level of 5 per meg. Results are presented from samples collected at Cape Grim (41 degrees S), Macquarie Island (54 degrees S) and the South Pole Station (90 degrees S). From measurements spanning 1991-1995 it is found that the O-2 concentrations at the South Pole are on average 3.6+/-1.2 per meg higher than at Cape Grim. This result runs contrary to the expectation that the air at high southern latitudes should be depleted in O-2 as a result of O-2 uptake from the Southern Ocean and may require the existence of unknown O-2 sources near Antarctica or unexpected atmospheric transport patterns.

Bender, ML, Battle M, Keeling RF.  1998.  The O2 balance of the atmosphere: A tool for studying the fate of fossil-fuel CO2. Annual Review of Energy and the Environment. 23:207-223.   10.1146/   AbstractWebsite

Carbon dioxide is a radiatively active gas whose atmospheric concentration increase is likely to affect Earth's climate. CO2 is added to the atmosphere by biomass burning and the combustion of fossil fuels. Some added CO2 remains in the atmosphere. However, substantial amounts are taken up by the oceans and land biosphere, attenuating the atmospheric increase. Atmospheric O-2 measurements provide one constraint for partitioning uptake rates between the ocean and the land biosphere. Here we review studies of atmospheric O-2 concentration variations and discuss their implications for CO2 uptake by the ocean and the land biosphere. We compare estimates of anthropogenic carbon fluxes from O-2 studies with estimates from other approaches and examine the contribution of natural ocean carbon fluxes to atmospheric O-2 variations.

Keeling, RF, Piper SC, Heimann M.  1996.  Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration. Nature. 381:218-221.   10.1038/381218a0   AbstractWebsite

THE global budget for sources and sinks of anthropogenic CO2 has been found to be out of balance unless the oceanic sink is supplemented by an additional 'missing sink', plausibly associated with land biota(1,25). A similar budgeting problem has been found for the Northern Hemisphere alone(2,3), suggesting that northern land biota may be the sought-after sink, although this interpretation is not unique(2-5); to distinguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models(2,6,7), (CO2)-C-13/(CO2)-C-12 data(2,4,5), sparse oceanic observations of p(CO2) (ref. 3) or C-13/C-12 ratios of dissolved inorganic carbon, (4,5,8) or single-latitude trends in atmospheric O-2 as detected from changes in O-2/N-2 ratio.(9,10). Here we present an extensive O-2/N-2 data set which shows simultaneous trends in O-2/N-2 in both northern and southern hemispheres and allows the O-2/N-2 gradient between the two hemispheres to be quantified. The data are consistent with a budget in which, for the 1991-94 period, the global oceans and the northern land biota each removed the equivalent of approximately 30% of fossil-fuel CO2 emissions, while the tropical land biota as a whole were not a strong source or sink.

Severinghaus, JP, Bender ML, Keeling RF, Broecker WS.  1996.  Fractionation of soil gases by diffusion of water vapor, gravitational settling, and thermal diffusion. Geochimica Et Cosmochimica Acta. 60:1005-1018.   10.1016/0016-7037(96)00011-7   AbstractWebsite

Air sampled from the moist unsaturated zone in a sand dune exhibits depletion in the heavy isotopes of N-2 and O-2. We propose that the depletion is caused by a diffusive flux of water vapor out of the dune, which sweeps out the other gases, forcing them to diffuse back into the dune. The heavy isotopes of N-2 and O-2 diffuse back more slowly, resulting in a steady-state depletion of the heavy isotopes in the dune interior. We predict the effect's magnitude with molecular diffusion theory and reproduce it in a laboratory simulation, finding good agreement between field, theory, and lab. The magnitude of the effect is governed by the ratio of the binary diffusivities against water vapor of a pair of gases, and increases similar to linearly with the difference between the water vapor mole fraction of the site and the advectively mixed reservoir with which it is in diffusive contact (in most cases the atmosphere). The steady-state effect is given by delta(i) = [i/j/i(0)/j(0) - 1] 10(3) parts per thousand congruent to [(1 - x(H2O)/1 - x(H2O0))((Dj-H2O/Di-H2O)-1) -1] 10(3) parts per thousand, where delta(i) is the fractional deviation in permil of the gas i/gas j ratio from the advectively mixed reservoir, x(H2O) and x(H2O0) are respectively the mole fractions of water vapor at the site and in the advectively mixed reservoir, and D-i-H2O is the binary diffusion coefficient of gas i with water vapor. The effect is independent of scale at steady state, but approaches steady state with the time constant of diffusion set by the length scale. Exploiting the mechanism, we make an experimental estimate of the relative diffusivities of O-2 and N-2 against water vapor, finding that O-2 diffuses 3.6 +/- 0.3% faster than N-2 despite its greater mass. We also confirm in the study dune the presence of two additional known processes: gravitational fractionation, heretofore seen only in the unconsolidated firn of polar ice sheets, and thermal diffusion, well described in laboratory studies but not seen previously in nature. We predict that soil gases in general will exhibit the three effects described here, the water vapor flux fractionation effect, gravitational fractionation, and thermal diffusion. However, our analysis neglects Knudsen diffusion and thus may be inapplicable to fine-grained soils.

Keeling, RF.  1993.  On the role of large bubbles in air-sea gas exchange and supersaturation in the ocean. Journal of Marine Research. 51:237-271.   10.1357/0022240933223800   AbstractWebsite

A parameterization of bubble-induced gas exchange is presented in which the bubble contribution to gas exchange is expressed in terms of separate transfer velocities for ingassing (K(b)in) and outgassing (K(b)out). The difference between the ingassing and outgassing velocities (K(b)in - K(b)out) is further separated into two components, the first caused by the injection of small bubbles into the water, the second caused by gas exchange across the surface of hydrostatically compressed larger bubbles. It is argued that both K(b)out and the exchange contribution to the difference K(b)in - K(b)out should be largely independent of the dissolved concentrations of the major gases N2 and O2. A simple model is presented which allows K(b)out and the exchange contribution to the difference K(b)in - K(b)out to be estimated. The model incorporates data from laboratory simulation experiments on the bubble production spectrum. The results indicate that bubbles larger than 0.05 cm in radius, which have often been assumed to play a negligible role, contribute significantly to bubble-induced gas exchange and supersaturation in the ocean. The model is used to explore the sensitivity of bubble-induced gas exchange to the overall air entrainment rate, size and depth distributions of the bubbles, and to the gas exchange rates across the surface of individual bubbles. The model suggests that bubbles may make an important contribution to overall gas exchange at windspeeds above 10 m sec-1. In this regime gas transfer velocities should depend, not just on diffusivity, but also on the solubility of the gases. It is suggested that K(b)(out) should scale roughly as alpha-0.3D0.35 where a is the solubility and D is the diffusivity. The model results, in combination with measurements on inert gas supersaturations, suggest that the global-mean supersaturation of CO2 induced by bubbles is not larger than 0.3% and most probably is around 0.08%. A major uncertainty results from a lack of information on production rates and distributions of large bubbles. Several possible experiments are proposed for improving estimates of bubble-induced gas exchange and supersaturation.