Robert M. Key

Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models

We have compared simulations of anthropogenic CO2 in the four three-dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85±0.35 Pg C yr−1 for the 1980–1989 average. Regionally, the Southern Ocean dominates the present-day air-sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real oceans present uptake, based on comparison of regional data-based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the data-based estimates could be provided regionally.

Evaluation of ocean carbon cycle models with data-based metrics

New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

Anthropogenic CO2 inventory of the Indian Ocean

This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified ΔC* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6±2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1±1 Pg C for the same area. Approximately 6.7±1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ±3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61–0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.

Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity

[1] Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically. INDEX TERMS: 4275 Oceanography: General: Remote sensing and electromagnetic processes (0689); 4532 Oceanography: Physical: General circulation; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; KEYWORDS: biogeochemical cycles, particle export, vertical exchange

A global model for the early diagenesis of organic carbon and organic phosphorus in marine sediments

Sediments are the main repository in the oceanic cycles of carbon (C) and phosphorus (P). In order to relate the deposition of organic C and organic P from the water column, and ultimate burial in sediments, we present a model for the early diagenesis of organic matter in marine sediments. This general diagenetic model was developed for inclusion in global circulation models and is based on a single master variable, the sedimentation rate. The processes included are sediment advection, particle mixing by bioturbation, porewater diffusion, organic matter degradation by aerobic respiration, sulfate reduction, and methanogenesis. Deposition fluxes of organic C and organic P, and the bottomwater concentrations of O2 and SO4= are used as input for the sediment model. First-order kinetics are assumed for the degradation of organic C and organic P (one-G model). Despite the models simplicity, the model quantitatively reproduces the rates of organic matter oxidation and the burial fluxes of organic C and P. The predicted trends of oxygen penetration depth and the relative importance of oxygen and sulfate reduction are in good agreement with field observations. The model appears to account for the critical biogeochemical and transport processes that control the fate of organic matter in sediments.

How much deep water is formed in the Southern Ocean

Three tracers are used to place constraints on the production rate of ventilated deep water in the Southern Ocean. The distribution of the water mass tracer PO4* (“phosphate star”) in the deep sea suggests that the amount of ventilated deep water produced in the Southern Ocean is equal to or greater than the outflow of North Atlantic Deep Water from the Atlantic. Radiocarbon distributions yield an export flux of water from the North Atlantic which has averaged about 15 Sv over the last several hundred years. CFC inventories are used as a direct indicator of the current production rate of ventilated deep water in the Southern Ocean. Although coverage is as yet sparse, it appears that the CFC inventory is not inconsistent with the deep water production rate required by the distributions of PO4* and radiocarbon. It has been widely accepted that the major part of the deep water production in the Southern Ocean takes place in the Weddell Sea. However, our estimate of the Southern Ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the Weddell Sea, which lie in the range 1–5 Sv. Possible reasons for this difference are discussed.

An updated anthropogenic CO2 inventory in the Atlantic Ocean

[1] This paper presents a comprehensive analysis of the basin-wide inventory of anthropogenic CO2 in the Atlantic Ocean based on high-quality inorganic carbon, alkalinity, chlorofluorocarbon, and nutrient data collected during the World Ocean Circulation Experiment (WOCE) Hydrographic Program, the Joint Global Ocean Flux Study (JGOFS), and the Ocean-Atmosphere Carbon Exchange Study (OACES) surveys of the Atlantic Ocean between 1990 and 1998. Anthropogenic CO2 was separated from the large pool of dissolved inorganic carbon using an extended version of the DC* method originally developed by Gruber et al. [1996]. The extension of the method includes the use of an optimum multiparameter analysis to determine the relative contributions from various source water types to the sample on an isopycnal surface. Total inventories of anthropogenic CO2 in the Atlantic Ocean are highest in the subtropical regions at 20� –40� , whereas anthropogenic CO2 penetrates the deepest in high-latitude regions (>40� N). The deeper penetration at high northern latitudes is largely due to the formation of deep water that feeds the Deep Western Boundary Current, which transports anthropogenic CO2 into the interior. In contrast, waters south of 50� S in the Southern Ocean contain little anthropogenic CO2. Analysis of the data collected during the 1990– 1998 period yielded a total anthropogenic CO2 inventory of 28.4 ± 4.7 Pg C in the North Atlantic (equator-70� N) and of 18.5 ± 3.9 Pg C in the South Atlantic (equator-70� S). These estimated basin-wide inventories of anthropogenic CO2 are in good agreement with previous estimates obtained by Gruber [1998], after accounting for the difference in observational periods. Our calculation of the anthropogenic CO2 inventory in the Atlantic Ocean, in conjunction with the inventories calculated previously for the Indian Ocean [Sabine et al., 1999] and for the Pacific Ocean [Sabine et al., 2002], yields a global anthropogenic CO2 inventory of 112 ± 17 Pg C that has accumulated in the world oceans during the industrial era. This global oceanic uptake accounts for approximately 29% of the total CO2 emissions from the burning of fossil fuels, land-use changes, and cement production during the past 250 years. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1635 Global Change: Oceans (4203); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4806 Oceanography: Biological and Chemical: Carbon cycling; KEYWORDS: anthropogenic CO2, Atlantic Ocean, air-sea disequilibrium

Separating natural and bomb-produced radiocarbon in the ocean : The potential alkalinity method

alkalinity method provides an estimate of surface ocean prebomb D 14 C concentrations. Predictions with the technique appear to be unbiased and have uncertainties which are less than previous techniques. INDEX TERMS: 4860 Oceanography: Biological and Chemical: Radioactivity and radioisotopes; 4894 Oceanography: Biological and Chemical: Instruments and techniques;

Consistency and synthesis of Pacific Ocean CO2 survey data

Between 1991 and 1999, carbon measurements were made on twenty-five WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different laboratories and four countries analyzed at least two of the four measurable ocean carbon parameters (DIC, TAlk, fCO2, and pH) on almost all cruises. The goal of this work is to assess the quality of the Pacific carbon survey data and to make recommendations for generating a unified data set that is consistent between cruises. Several different lines of evidence were used to examine the consistency, including comparison of calibration techniques, results from certified reference material analyses, precision of at-sea replicate analyses, agreement between shipboard analyses and replicate shore based analyses, comparison of deep water values at locations where two or more cruises overlapped or crossed, consistency with other hydrographic parameters, and internal consistency with multiple carbon parameter measurements. With the adjustments proposed here, the data can be combined to generate a Pacific Ocean data set, with over 36,000 unique sample locations analyzed for at least two carbon parameters in most cases. The best data coverage was for DIC, which has an estimated overall accuracy of ∼3 μmol kg−1. TAlk, the second most common carbon parameter analyzed, had an estimated overall accuracy of ∼5 μmol kg−1. To obtain additional details on this study, including detailed crossover plots and information on the availability of the compiled, adjusted data set, visit the Global Data Analysis Project web site at: http://cdiac.esd.ornl.gov/oceans/glodap.

Woce AMS radiocarbon. I: Pacific Ocean results (p6, p16 and p17) : 14C cycling and the oceans

AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14 C since the time of the GEOSECS survey (8/73-6/74). Near-surface Δ 14 C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25-50‰, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25‰. Changes in the 250-750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75‰ since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50‰.