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Wells, KC, Millet DB, Bousserez N, Henze DK, Griffis TJ, Chaliyakunnel S, Dlugokencky EJ, Saikawa E, Xiang G, Prinn RG, O'Doherty S, Young D, Weiss RF, Dutton GS, Elkins JW, Krummel PB, Langenfelds R, Steele LP.  2018.  Top-down constraints on global N2O emissions at optimal resolution: application of a new dimension reduction technique. Atmospheric Chemistry and Physics. 18:735-756.   10.5194/acp-18-735-2018   AbstractWebsite

We present top-down constraints on global monthly N2O emissions for 2011 from a multi-inversion approach and an ensemble of surface observations. The inversions employ the GEOS-Chem adjoint and an array of aggregation strategies to test how well current observations can constrain the spatial distribution of global N2O emissions. The strategies include (1) a standard 4D-Var inversion at native model resolution (4 degrees x 5 degrees), (2) an inversion for six continental and three ocean regions, and (3) a fast 4D-Var inversion based on a novel dimension reduction technique employing randomized singular value decomposition (SVD). The optimized global flux ranges from 15.9 TgNyr(-1) (SVD-based inversion) to 17.5-17.7 TgNyr(-1) (continental-scale, standard 4D-Var inversions), with the former better capturing the extratropical N2O background measured during the HIAPER Pole-to-Pole Observations (HIPPO) airborne campaigns. We find that the tropics provide a greater contribution to the global N2O flux than is predicted by the prior bottom-up inventories, likely due to underestimated agricultural and oceanic emissions. We infer an overestimate of natural soil emissions in the extratropics and find that predicted emissions are seasonally biased in northern midlatitudes. Here, optimized fluxes exhibit a springtime peak consistent with the timing of spring fertilizer and manure application, soil thawing, and elevated soil moisture. Finally, the inversions reveal a major emission underestimate in the US Corn Belt in the bottom-up inventory used here. We extensively test the impact of initial conditions on the analysis and recommend formally optimizing the initial N2O distribution to avoid biasing the inferred fluxes. We find that the SVD-based approach provides a powerful framework for deriving emission information from N2O observations: by defining the optimal resolution of the solution based on the information content of the inversion, it provides spatial information that is lost when aggregating to political or geographic regions, while also providing more temporal information than a standard 4D-Var inversion.

Nisbet, E, Weiss R.  2010.  Top-down versus bottom-up. Science. 328:1241-1243.   10.1126/science.1189936   AbstractWebsite

Greenhouse gas emissions are currently quantified from statistical data without testing the results against the actual increases of these gases in the atmosphere. This is like dieting without weighing oneself. Data are produced by greenhouse gas emitters of all sizes, from factory or farm to nation, and are quoted to high precision—yet misreporting occurs, whether by simple error, ignorance, or intention. But now scientists on both sides of the Atlantic are arguing that regulation of greenhouse gas emissions can have integrity only if verified by direct atmospheric measurements.

Kroopnic.P, Weiss RF, Craig H.  1972.  Total CO2,13C, and dissolved oxygen -18O at GEOSECS II in the North Atlantic. Earth and Planetary Science Letters. 16:103-110.   10.1016/0012-821x(72)90242-7   AbstractWebsite

This paper presents profiles of ΣCO2, δ13C in ΣCO2, dissolved O2, and δ18O in dissolved O2, measured at Geosecs II in the North Atlantic. The O2 minimum at 1000 m is accompanied by a minimum in δ13C and a sharp maximum in δ18O; ΣCO2 increases downward through this layer with a slope change. All four parameters are remarkably uniform in the deep and bottom water below the O2 minimum, almost to the precision of measurement. Relative to data previously reported from this area of the Atlantic, our ΣCO2 values are 3% lower than those of Li et al. [10], and our δ13C values are up to 2‰ greater than those of Deuser and Hunt [12]. Also, our δ18O enrichments in dissolved O2 are very much less than Pacific values reported by Dole and coworkers [15]. All of these differences are attributed principally to bacterial O2 consumption during sample storage by previous workers, due to lack of, or inadequate, poisoning.In contrast to the North Atlantic, there is a very large gradient of dissolved O2 in the vertical profile of North Pacific Deep Water; however, if themean deep-water O2 concentration is compared with the uniform value in North Atlantic Deep Water, the O2 and ΣCO2 differences in North Atlantic and North Pacific Deep Water are essentially equimolar at 160 μm/kg. If 77% of deep-water O2 consumption is used for oxidation of organic carbon (the R-K-R “model plankton” value), the increase in ΣCO2 in Pacific deep water is about 25% due to dissolution of carbonate, and 75% due to oxidation of organic matter, in the vertical particulate flux. These proportions are in agreement with those estimated from alkalinity-ΣCO2 variations [10]. Our δ13C measurements in the Atlantic are quite consistent with the ΣCO2-O2-alkalinity variations between the Atlantic and Pacific deep water; thus the disagreement previously noted [14] is attributed to storage effects on δ13C measurements by previous workers, as noted above.

Park, S, Li S, Mühle J, O'Doherty S, Weiss RF, Fang X, Reimann S, Prinn RG.  2018.  Toward resolving the budget discrepancy of ozone-depleting carbon tetrachloride (CCl4): an analysis of top-down emissions from China. Atmos. Chem. Phys.. 18:11729-11738.: Copernicus Publications   10.5194/acp-18-11729-2018   Abstract

Carbon tetrachloride (CCl4) is a first-generation ozone-depleting substance, and its emissive use and production were globally banned by the Montreal Protocol with a 2010 phase-out; however, production and consumption for non-dispersive use as a chemical feedstock and as a process agent are still allowed. This study uses the high frequency and magnitude of CCl4 pollution events from an 8-year real-time atmospheric measurement record obtained at Gosan station (a regional background monitoring site in East Asia) to present evidence of significant unreported emissions of CCl4. Top-down emissions of CCl4 amounting to 23.6±7.1Gg yr−1 from 2011 to 2015 are estimated for China, in contrast to the most recently reported, post-2010, Chinese bottom-up emissions of 4.3–5.2Ggyr−1. The missing emissions ( ∼ 19Ggyr−1) for China contribute to approximately 54% of global CCl4 emissions. It is also shown that 89 % ± 6% of CCl4 enhancements observed at Gosan are related to CCl4 emissions from the production of CH3Cl, CH2Cl2, CHCl3 and C2Cl4 and its usage as a feedstock and process agent in chemical manufacturing industries. Specific sources and processes are identified using statistical methods, and it is considered highly unlikely that CCl4 is emitted by dispersive uses such as old landfills, contaminated soils and solvent usage. It is thus crucial to implement technical improvements and better regulation strategies to reduce evaporative losses of CCl4 occurring at the factory and/or process levels.

Muhle, J, Lueker TJ, Su Y, Miller BR, Prather KA, Weiss RF.  2007.  Trace gas and particulate emissions from the 2003 southern California wildfires. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd007350   AbstractWebsite

In October 2003, thirteen major wildfires in southern California burned more than 300,000 hectares of mainly chaparral biome. High-precision in situ trace gas and particle measurements of the wildfire plumes in La Jolla, California, showed a high degree of correlation among carbon dioxide (CO(2)), methane (CH(4)), nonmethane hydrocarbons, and methyl halide mixing ratios, as well as with particle number concentrations (10-300 nm and 500-2500 nm aerodynamic diameter). Aerosol time-of-flight mass spectrometry of individual aerosol particles (50-2500 nm range) showed that 70-85% had typical biomass burning signatures (levoglucosan coupled with potassium). Only 5-18% of particles in the 50 - 300 nm range had vehicle signatures. Molar trace gas enhancement ratios (ERs) versus ethane and CO(2) were calculated and showed a narrow age distribution, consistent with the short distance from the wildfires. ERs for N(2)O and CH(3)CCl(3) versus CO(2) were determined, but correlations were poor. No significant CH(2)Cl(2) or CHCl(3) emissions were detected. CO2 emissions from the nearby Cedar fire were estimated both with a simple Lagrangian atmospheric transport model and a burned area approach and extrapolated to 11 Tg CO(2) for the total burned area in southern California. Total CO(2), CH(4), C(2)-hydrocarbons, benzene, toluene, methyl chloride, methyl iodide, and PM(2.5) emissions were similar to 0.2-3.5% of yearly global extratropical forest fire emissions and more than 28% of CH(4), C(6)H(6), and PM(2.5) 2003 San Diego and South Coast Air Basins anthropogenic emissions. Particle distributions and single particle chemistry are discussed. PM(2.5) considerably exceeded the EPA short-term exposure limit.

Thompson, RL, Patra PK, Ishijima K, Saikawa E, Corazza M, Karstens U, Wilson C, Bergamaschi P, Dlugokencky E, Sweeney C, Prinn RG, Weiss RF, O'Doherty S, Fraser PJ, Steele LP, Krummel PB, Saunois M, Chipperfield M, Bousquet P.  2014.  TransCom N2O model inter-comparison - Part 1: Assessing the influence of transport and surface fluxes on tropospheric N2O variability. Atmospheric Chemistry and Physics. 14:4349-4368.   10.5194/acp-14-4349-2014   AbstractWebsite

We present a comparison of chemistry-transport models (TransCom-N2O) to examine the importance of atmospheric transport and surface fluxes on the variability of N2O mixing ratios in the troposphere. Six different models and two model variants participated in the inter-comparison and simulations were made for the period 2006 to 2009. In addition to N2O, simulations of CFC-12 and SF6 were made by a subset of four of the models to provide information on the models' proficiency in stratosphere-troposphere exchange (STE) and meridional transport, respectively. The same prior emissions were used by all models to restrict differences among models to transport and chemistry alone. Four different N2O flux scenarios totalling between 14 and 17 TgN yr(-1) (for 2005) globally were also compared. The modelled N2O mixing ratios were assessed against observations from in situ stations, discrete air sampling networks and aircraft. All models adequately captured the large-scale patterns of N2O and the vertical gradient from the troposphere to the stratosphere and most models also adequately captured the N2O tropospheric growth rate. However, all models underestimated the inter-hemispheric N2O gradient by at least 0.33 parts per billion (ppb), equivalent to 1.5 TgN, which, even after accounting for an overestimate of emissions in the Southern Ocean of circa 1.0 TgN, points to a likely underestimate of the Northern Hemisphere source by up to 0.5 TgN and/or an overestimate of STE in the Northern Hemisphere. Comparison with aircraft data reveal that the models over-estimate the amplitude of the N2O seasonal cycle at Hawaii (21 degrees N, 158 degrees W) below circa 6000 m, suggesting an overestimate of the importance of stratosphere to troposphere transport in the lower troposphere at this latitude. In the Northern Hemisphere, most of the models that provided CFC-12 simulations captured the phase of the CFC-12, seasonal cycle, indicating a reasonable representation of the timing of STE. However, for N2O all models simulated a too early minimum by 2 to 3 months owing to errors in the seasonal cycle in the prior soil emissions, which was not adequately represented by the terrestrial biosphere model. In the Southern Hemisphere, most models failed to capture the N2O and CFC-12 seasonality at Cape Grim, Tasmania, and all failed at the South Pole, whereas for SF6, all models could capture the seasonality at all sites, suggesting that there are large errors in modelled vertical transport in high southern latitudes.

Thompson, RL, Ishijima K, Saikawa E, Corazza M, Karstens U, Patra PK, Bergamaschi P, Chevallier F, Dlugokencky E, Prinn RG, Weiss RF, O'Doherty S, Fraser PJ, Steele LP, Krummel PB, Vermeulen A, Tohjima Y, Jordan A, Haszpra L, Steinbacher M, Van der Laan S, Aalto T, Meinhardt F, Popa ME, Moncrieff J, Bousquet P.  2014.  TransCom N2O model inter-comparison - Part 2: Atmospheric inversion estimates of N2O emissions. Atmospheric Chemistry and Physics. 14:6177-6194.   10.5194/acp-14-6177-2014   AbstractWebsite

This study examines N2O emission estimates from five different atmospheric inversion frameworks based on chemistry transport models (CTMs). The five frameworks differ in the choice of CTM, meteorological data, prior uncertainties and inversion method but use the same prior emissions and observation data set. The posterior modelled atmospheric N2O mole fractions are compared to observations to assess the performance of the inversions and to help diagnose problems in the modelled transport. Additionally, the mean emissions for 2006 to 2008 are compared in terms of the spatial distribution and seasonality. Overall, there is a good agreement among the inversions for the mean global total emission, which ranges from 16.1 to 18.7 TgN yr(-1) and is consistent with previous estimates. Ocean emissions represent between 31 and 38% of the global total compared to widely varying previous estimates of 24 to 38%. Emissions from the northern mid- to high latitudes are likely to be more important, with a consistent shift in emissions from the tropics and subtropics to the mid- to high latitudes in the Northern Hemisphere; the emission ratio for 0-30A degrees N to 30-90A degrees N ranges from 1.5 to 1.9 compared with 2.9 to 3.0 in previous estimates. The largest discrepancies across inversions are seen for the regions of South and East Asia and for tropical and South America owing to the poor observational constraint for these areas and to considerable differences in the modelled transport, especially inter-hemispheric exchange rates and tropical convective mixing. Estimates of the seasonal cycle in N2O emissions are also sensitive to errors in modelled stratosphere-to-troposphere transport in the tropics and southern extratropics. Overall, the results show a convergence in the global and regional emissions compared to previous independent studies.

Weiss, RF.  1991.  Transient tracers in the ocean, tropical Atlantic study: chlorofluorocarbon measurements. Scripps Institution of Oceanography Reference Series. :159., San Diego; La Jolla: University of California ; Scripps Institution of Oceanography Abstract
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Waugh, DW, Vollmer MK, Weiss RF, Haine TWN, Hall TM.  2002.  Transit time distributions in Lake Issyk-Kul. Geophysical Research Letters. 29   10.1029/2002gl016201   AbstractWebsite

[1] Measurements of sulfur hexafluoride (SF6)and chlorofluorocarbons (CFCs) are used to constrain the timescales for deep-water renewal in Lake Issyk-Kul. As these tracers have different tropospheric histories their combination provides more transport information than one tracer alone. In particular, from these measurements the mean, Gamma, and standard deviation, sigma, of the distributions of transit times since water made last contact with the surface can be tightly constrained. Gamma is older than the age determined from SF6 and younger than the ages from the CFCs, and increases from around 4 yrs at 200 m to around 10.5 yrs at the deepest location (655 m). sigma also increases with depth and equals around 0.7 to 0.8 Gamma, which corresponds to large ranges of transit times, and implies mixing processes play a major role in the transport. The approach used can also be applied to similar tracer measurements in the oceans and groundwaters to constrain transport in these geophysical systems.

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Saunois, M, Bousquet P, Poulter B, Peregon A, Ciais P, Canadell JG, Dlugokencky EJ, Etiope G, Bastviken D, Houweling S, Janssens-Maenhout G, Tubiello FN, Castaldi S, Jackson RB, Alexe M, Arora VK, Beerling DJ, Bergamaschi P, Blake DR, Brailsford G, Bruhwiler L, Crevoisier C, Crill P, Covey K, Frankenberg C, Gedney N, Hoglund-Isaksson L, Ishizawa M, Ito A, Joos F, Kim HS, Kleinen T, Krummel P, Lamarque JF, Langenfelds R, Locatelli R, Machida T, Maksyutov S, Melton JR, Morino I, Naik V, O'Doherty S, Parmentier FJ, Patra PK, Peng CH, Peng SS, Peters GP, Pison I, Prinn R, Ramonet M, Riley WJ, Saito M, Santini M, Schroeder R, Simpson IJ, Spahni R, Takizawa A, Thornton BF, Tian HQ, Tohjima Y, Viovy N, Voulgarakis A, Weiss R, Wilton DJ, Wiltshire A, Worthy D, Wunch D, Xu XY, Yoshida Y, Zhang BW, Zhang Z, Zhu QA.  2017.  Variability and quasi-decadal changes in the methane budget over the period 2000-2012. Atmospheric Chemistry and Physics. 17:11135-11161.   10.5194/acp-17-11135-2017   AbstractWebsite

Following the recent Global Carbon Project (GCP) synthesis of the decadal methane (CH4) budget over 2000-2012 (Saunois et al., 2016), we analyse here the same dataset with a focus on quasi-decadal and inter-annual variability in CH4 emissions. The GCP dataset integrates results from top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models (including process-based models for estimating land surface emissions and atmospheric chemistry), inventories of anthropogenic emissions, and data-driven approaches. The annual global methane emissions from top-down studies, which by construction match the observed methane growth rate within their uncertainties, all show an increase in total methane emissions over the period 2000-2012, but this increase is not linear over the 13 years. Despite differences between individual studies, the mean emission anomaly of the top-down ensemble shows no significant trend in total methane emissions over the period 2000-2006, during the plateau of atmospheric methane mole fractions, and also over the period 2008-2012, during the renewed atmospheric methane increase. However, the top-down ensemble mean produces an emission shift between 2006 and 2008, leading to 22 [16-32] Tg CH4 yr(-1) higher methane emissions over the period 2008-2012 compared to 2002-2006. This emission increase mostly originated from the tropics, with a smaller contribution from mid-latitudes and no significant change from boreal regions. The regional contributions remain uncertain in top-down studies. Tropical South America and South and East Asia seem to contribute the most to the emission increase in the tropics. However, these two regions have only limited atmospheric measurements and remain therefore poorly constrained. The sectorial partitioning of this emission increase between the periods 2002-2006 and 2008-2012 differs from one atmospheric inversion study to another. However, all top-down studies suggest smaller changes in fossil fuel emissions (from oil, gas, and coal industries) compared to the mean of the bottom-up inventories included in this study. This difference is partly driven by a smaller emission change in China from the top-down studies compared to the estimate in the Emission Database for Global Atmospheric Research (EDGARv4.2) inventory, which should be revised to smaller values in a near future. We apply isotopic signatures to the emission changes estimated for individual studies based on five emission sectors and find that for six individual top-down studies (out of eight) the average isotopic signature of the emission changes is not consistent with the observed change in atmospheric (CH4)-C-13. However, the partitioning in emission change derived from the ensemble mean is consistent with this isotopic constraint. At the global scale, the top-down ensemble mean suggests that the dominant contribution to the resumed atmospheric CH4 growth after 2006 comes from microbial sources (more from agriculture and waste sectors than from natural wetlands), with an uncertain but smaller contribution from fossil CH4 emissions. In addition, a decrease in biomass burning emissions (in agreement with the biomass burning emission databases) makes the balance of sources consistent with atmospheric (CH4)-C-13 observations. In most of the top-down studies included here, OH concentrations are considered constant over the years (seasonal variations but without any inter-annual variability). As a result, the methane loss (in particular through OH oxidation) varies mainly through the change in methane concentrations and not its oxidants. For these reasons, changes in the methane loss could not be properly investigated in this study, although it may play a significant role in the recent atmospheric methane changes as briefly discussed at the end of the paper.

Ganesan, AL, Chatterjee A, Prinn RG, Harth CM, Salameh PK, Manning AJ, Hall BD, Mühle J, Meredith LK, Weiss RF, O'Doherty S, Young D.  2013.  The variability of methane, nitrous oxide and sulfur hexafluoride in Northeast India. Atmos. Chem. Phys.. 13:10633-10644.: Copernicus Publications   10.5194/acp-13-10633-2013   AbstractWebsite

High-frequency atmospheric measurements of methane (CH4), nitrous oxide (N2O) and sulfur hexafluoride (SF6) from Darjeeling, India are presented from December 2011 (CH4)/March 2012 (N2O and SF6) through February 2013. These measurements were made on a gas chromatograph equipped with a flame ionization detector and electron capture detector, and were calibrated on the Tohoku University, the Scripps Institution of Oceanography (SIO)-98 and SIO-2005 scales for CH4, N2O and SF6, respectively. The observations show large variability and frequent pollution events in CH4 and N2O mole fractions, suggesting significant sources in the regions sampled by Darjeeling throughout the year. By contrast, SF6 mole fractions show little variability and only occasional pollution episodes, likely due to weak sources in the region. Simulations using the Numerical Atmospheric dispersion Modelling Environment (NAME) particle dispersion model suggest that many of the enhancements in the three gases result from the transport of pollutants from the densely populated Indo-Gangetic Plains of India to Darjeeling. The meteorology of the region varies considerably throughout the year from Himalayan flows in the winter to the strong south Asian summer monsoon. The model is consistent in simulating a diurnal cycle in CH4 and N2O mole fractions that is present during the winter but absent in the summer and suggests that the signals measured at Darjeeling are dominated by large-scale (~100 km) flows rather than local (<10 km) flows.

Murphy, PP, Harrison DE, Feely RA, Takahashi T, Weiss RF, Gammon RH.  1998.  Variability of Δ pCO2 in the subarctic North Pacific. A comparison of results from four expeditions. Tellus Series B-Chemical and Physical Meteorology. 50:185-204.   10.1034/j.1600-0889.1998.t01-1-00006.x   AbstractWebsite

Time-space variability of surface seawater pCO(2) is examined over the region (150 degrees W-180 degrees, 46 degrees N-50 degrees N) of the subarctic North Pacific where large meridional gradients of temperature and nutrient concentrations exist. The data were collected during four trans-Pacific expeditions in three different years (1985-1987), but within the same 30-day period of the year (August-September). Systematic measurement differences between the four data sets are estimated as <10 mu atm. The inter-expedition comparison suggests that surface seawater pCO(2) in the study area is quite variable, with mean differences of up to 25 mu atm and local differences up to 60 mu atm. Spatial and interannual variability of surface seawater pCO(2) were found to contribute significant uncertainty to estimates of the mean Delta pCO(2) for the study area. Fluxes were calculated using Delta pCO(2) values from the four expeditions combined with gas exchange coefficients calculated from four different wind fields giving a range of -0.94 to +4.1 mmol CO2 m(-2) d(-1). The range of fluxes from the study area is scaled to a larger area of the North Pacific to address how this variability can translate into uncertainties in basin-wide carbon air-sea exchange fluxes.

Vollmer, MK, Weiss RF, Bootsma HA.  2002.  Ventilation of Lake Malawi/Nyasa. The East African great lakes limnology, palaeolimnology, and biodiversity. ( Odada EO, Olago DO, Eds.).:209-233., Dordrecht; Boston, MA: Kluwer Academic Publishers Abstract
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Reverdin, G, Weiss RF, Jenkins WJ.  1993.  Ventilation of the Atlantic Ocean equatorial thermocline. Journal of Geophysical Research-Oceans. 98:16289-16310.   10.1029/93jc00976   AbstractWebsite

The ventilation of the lower equatorial Atlantic thermocline (100 m to 400 m) is investigated from the distributions of salinity, oxygen, the chlorofluoromethanes F-11 and F-12, tritium, and the excess helium 3; 1983 data suggest a correlation on isopycnal surfaces between oxygen, chlorofluoromethanes and to a lesser extent, with the apparent ages derived from F-11/F-12 and helium 3/tritium. The apparent ages increase with depth from 10 years on sigma(THETA)=26.5 to over 15 years near 400 m. The tracers are largely undersaturated with respect to the surface concentrations corresponding to the apparent ages. We investigate the ventilation processes responsible for these distributions with simple circulation models, where water from the subtropical gyres reaches isopycnally the equatorial thermocline. The few model parameters are determined by fitting the simulations to the chlorofluoromethane distributions in 1983. The sensitivity of these parameters to expected errors on the past surface concentrations and to vertical mixing is comparable and can result in an error of more than 20% in the model parameters. In 1983, model [H-3] and [He-3]* are too large, in particular when no diapycnal mixing is modeled, but the differences could have arisen from errors on past surface concentrations. During the 1980s, die model overestimates the increase in chlorofluoromethanes, in particular F-12. The fitted model inflow from the southern subtropical gyre between 110 m and 400 m is 14 x 10(6) m3 s-1, in the range of expected transports by the western boundary current near Brazil. The model oxygen utilization rate is on the order of 12 mmol kg-1 yr-1 at 100 m and 5 mmol kg-1 yr-1 at 400 m (below sigma(theta)=27.1).

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Edmond, JM, Jacobs SS, Gordon AL, Mantyla AW, Weiss RF.  1979.  Water column anomalies in dissolved silica over opaline pelagic sediments and the origin of the deep silica maximum. Journal of Geophysical Research-Oceans and Atmospheres. 84:7809-7826.   10.1029/JC084iC12p07809   AbstractWebsite

Recent estimates based on pore water studies and mass balance considerations put the average flux of dissolved silica from the sediments into the deep water at about 3 μmol cm−2 yr−1. This flux, if mixed uniformly in a bottom layer 100 m thick, results in an anomaly increment of 0.3 μmol kg−1 yr−1. In basins of restricted circulation containing opaline sediments the residence time of the bottom waters should be long enough that the resulting anomaly be easily resolvable using existing data. Examination of the many hundreds of detailed, precise silica profiles presently available in unpublished reports shows that this is indeed the case. The largest effects (∼50 μmol/kg) are seen in the Weddell-Enderby Basin. Intermediate anomalies (∼20 μmol/kg) are prevalent in the northern Indian Ocean. Lesser features (∼10 μmol/kg) are clearly discernable in the other antarctic basins and in the north Pacific. No anomalies are observed over the equatorial Pacific and Indian bands of opaline deposits, even in the basins east of the East Pacific Rise or in the Central Basin of the Indian Ocean. The flux of dissolved silica from the sea floor is generally unaccompanied by any resolvable oxygen consumption, indicating that relatively minor amounts of organic material reach the sediments in these regions. However, in the north Indian Ocean there are pronounced effects on all the nutrients, oxygen, and alkalinity, with regeneration approximating the Redfield prediction. In the northern Indian Ocean and the extreme northeast Pacific (northeast of 45°N, 160°W) the silica profiles increase to the bottom. This is a strong indication that the deep silica maximum observed over much of these oceans may have a large advective component, the feature itself being induced by the northward flow of the underlying low-silica bottom waters. In the Indian Ocean the data coverage is sufficient to demonstrate unequivocally the dominance of this effect.

Fine, RA, Warner MJ, Weiss RF.  1988.  Water mass modification at the Agulhas retroflection: chlorofluoromethane studies. Deep-Sea Research Part a-Oceanographic Research Papers. 35:311-332.   10.1016/0198-0149(88)90013-1   AbstractWebsite

Chlorofluoromethane (CFM) and hydrographic data from the 1983 Agulhas Retroflection cruise are used to show the importance of the region in ventilating thermocline and Intermediate Waters of the southwest Indian ocean gyre. Generally South Atlantic waters are more recently ventilated by at least two years than those of the South Indian Ocean, probably because the latter are farther downstream from the source regions near the South Atlantic subantarctic sector. A two-component mixing model shows that the outflow from the Agulhas Retroflection (14-4°C) was composed of South Indian water and at least 23% South Atlantic water. However, at the density of Indian sector Subantarctic Mode Water the inflow into the Agulhas Retroflection was well preserved in the outflow, and the South Atlantic and Indian waters appear to be ventilated by different water masses. In addition, strong interleaving was found throughout the survey area (between 14 and 4°C), characterized by correlations of negative salinity anomalies with high CFM concentrations. At the density of Antarctic Intermediate Water (AAIW) there was interleaving of both low salinity water and higher salinity Red Sea Water. Using estimates of past atmospheric ratios of two CFMs, we calculate that AAIW within the retroflection was 50–75% diluted by mixing with CFM-free water since leaving the source region. Results from the two-component mixing model, which show substantial contributions of South Atlantic water in the outflow, suggest that the return flow for the 10 Sv leakage of Indian Ocean water via the Agulhas Current into the South Atlantic [Gordon (1985) Science, 227, 1030–1033; Gordonet al. (1987) Deep-Sea Research, 34, 565–600] is occurring at thermocline and intermediate depths. A combination of active mixing in this region and similarity in the ventilation processes may be the reason that the South Atlantic and Indian thermoclines are coincident in temperature and salinity space (between 15 and 7°C) as noted by Gordon.