Comparison of two approaches to quantify anthropogenic CO2in the ocean: Results from the northern Indian Ocean (original) (raw)
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Global Biogeochemical Cycles, 2001
This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along the World Ocean Circulation Experiment cruise I1 [Goyet et al., 1999; Sabine et al., 1999]. These two studies employed two different approaches to separate the anthropogenic CO2 signal from the large natural background variability. Sabine et al. [1999] used the AC* approach first described by Gruber et al. [1996], whereas Goyet et al. [1999] used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account t•br variations due to physical processes including mixing and the CO2 solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 concentrations estimated using the AC* approach average 12 + 4/•mol kg-• higher in this depth range than concentrations estimated using the MIX approach. Below •0800 m, the MIX approach estimates slightly higher anthropogenic CO2 concentrations and a deeper vertical penetration. Despite this compensatory effect, water column inventories estimated in the 0-3000 m depth range by the AC* approach are generally •020% higher than those estimated by the MIX approach, with this difference being statistically significant beyond the 0.001 level. We examine possible causes fbr these differences and identify a number of critical additional measurements that will make it possible to discriminate better between the two approaches.
Comparison of two approaches to quantify anthropogenic CO2 in the ocean' Results
2001
This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along the World Ocean Circulation Experiment cruise I1 (Goyet et al., 1999; Sabine et al., 1999). These two studies employed two different approaches to separate the anthropogenic CO2 signal from the large natural background variability. Sabine et al. (1999) used the AC* approach first described by Gruber et al. (1996), whereas Goyet et al. (1999) used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account tbr variations due to physical processes including mixing and the CO2 solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 conce...
Journal of Marine Research, 1999
As part of a cooperative effort of the Joint Global Ocean Flux Study (JGOFS) and of the World Ocean Circulation Experiment (WOCE) program, we have measured total CO 2 (TCO 2) and total alkalinity (TA) along three sections in the northern Indian Ocean. One section through the Gulf of Aden to the Arabian Sea is parallel to the coast of Yemen. One section is across the Arabian Sea along the nominal 9N latitude and the other section is across the Bay of Bengal along the nominal 10N latitude. The measurements were performed on board R/V Knorr in September-October 1995. The primary purpose of this work is to understand the penetration of anthropogenic CO 2 along these ocean sections. Here, we present a novel approach to the calculation of anthropogenic CO 2 in the ocean based upon the fundamentals of water-sources mixing. Consequently, we rst describe the observations and mixing of water-sources before we describe the quanti cation of anthropogenic CO 2 concentrationsin these waters. The data show large spatial variations in surface seawater of both total CO 2 (up to 50 µmol kg 2 1) and total alkalinity (up to 40 µmol kg 2 1). The variations are mainly associated with physical processes characterized by water masses of different temperature and salinity. For example, at depths we observed low TCO 2 concentration at longitude 54E 6 2E associated with the low-salinity water mass owing northward. The contrasts between the sections across the Arabian Sea and the Bay of Bengal emphasize the large property differences between the two ocean basins. Multiparametric analyses on the data clearly show the relative contributions of different water-sources in each of the ocean sections. The mixing coefficients calculated from the multiparametric analyses are further used to quantify anthropogenic CO 2 concentrations in each water-source. The results indicate that the surface water-sources contain 47.8, 42.1 and 50.4 µmol kg 2 1 in the Gulf of Aden, the Arabian Sea and the Bay of Bengal, respectively. In the surface waters there is slightly more anthropogenic CO 2 across the Bay of Bengal than across the Arabian Sea. In contrast, anthropogenic CO 2 has penetrated signi cantly deeper in the Gulf of Aden than in the Arabian Sea and in the Bay of Bengal.
Journal of Marine Research, 1999
As part of a cooperative effort of the Joint Global Ocean Flux Study (JGOFS) and of the World Ocean Circulation Experiment (WOCE) program, we have measured total CO 2 (TCO 2) and total alkalinity (TA) along three sections in the northern Indian Ocean. One section through the Gulf of Aden to the Arabian Sea is parallel to the coast of Yemen. One section is across the Arabian Sea along the nominal 9N latitude and the other section is across the Bay of Bengal along the nominal 10N latitude. The measurements were performed on board R/V Knorr in September-October 1995. The primary purpose of this work is to understand the penetration of anthropogenic CO 2 along these ocean sections. Here, we present a novel approach to the calculation of anthropogenic CO 2 in the ocean based upon the fundamentals of water-sources mixing. Consequently, we rst describe the observations and mixing of water-sources before we describe the quanti cation of anthropogenic CO 2 concentrationsin these waters. The data show large spatial variations in surface seawater of both total CO 2 (up to 50 µmol kg 2 1) and total alkalinity (up to 40 µmol kg 2 1). The variations are mainly associated with physical processes characterized by water masses of different temperature and salinity. For example, at depths we observed low TCO 2 concentration at longitude 54E 6 2E associated with the low-salinity water mass owing northward. The contrasts between the sections across the Arabian Sea and the Bay of Bengal emphasize the large property differences between the two ocean basins. Multiparametric analyses on the data clearly show the relative contributions of different water-sources in each of the ocean sections. The mixing coefficients calculated from the multiparametric analyses are further used to quantify anthropogenic CO 2 concentrations in each water-source. The results indicate that the surface water-sources contain 47.8, 42.1 and 50.4 µmol kg 2 1 in the Gulf of Aden, the Arabian Sea and the Bay of Bengal, respectively. In the surface waters there is slightly more anthropogenic CO 2 across the Bay of Bengal than across the Arabian Sea. In contrast, anthropogenic CO 2 has penetrated signi cantly deeper in the Gulf of Aden than in the Arabian Sea and in the Bay of Bengal.
Journal of Marine Research, 1999
As part of a cooperative effort of the Joint Global Ocean Flux Study (JGOFS) and of the World Ocean Circulation Experiment (WOCE) program, we have measured total CO 2 (TCO 2 ) and total alkalinity (TA) along three sections in the northern Indian Ocean. One section through the Gulf of Aden to the Arabian Sea is parallel to the coast of Yemen. One section is across the Arabian Sea along the nominal 9N latitude and the other section is across the Bay of Bengal along the nominal 10N latitude. The measurements were performed on board R/V Knorr in September-October 1995. The primary purpose of this work is to understand the penetration of anthropogenic CO 2 along these ocean sections. Here, we present a novel approach to the calculation of anthropogenic CO 2 in the ocean based upon the fundamentals of water-sources mixing. Consequently, we rst describe the observations and mixing of water-sources before we describe the quanti cation of anthropogenic CO 2 concentrationsin these waters. The data show large spatial variations in surface seawater of both total CO 2 (up to 50 µmol kg 2 1 ) and total alkalinity (up to 40 µmol kg 2 1 ). The variations are mainly associated with physical processes characterized by water masses of different temperature and salinity. For example, at depths we observed low TCO 2 concentration at longitude 54E 6 2E associated with the low-salinity water mass owing northward. The contrasts between the sections across the Arabian Sea and the Bay of Bengal emphasize the large property differences between the two ocean basins. Multiparametric analyses on the data clearly show the relative contributions of different water-sources in each of the ocean sections. The mixing coefficients calculated from the multiparametric analyses are further used to quantify anthropogenic CO 2 concentrations in each water-source. The results indicate that the surface water-sources contain 47.8, 42.1 and 50.4 µmol kg 2 1 in the Gulf of Aden, the Arabian Sea and the Bay of Bengal, respectively. In the surface waters there is slightly more anthropogenic CO 2 across the Bay of Bengal than across the Arabian Sea. In contrast, anthropogenic CO 2 has penetrated signi cantly deeper in the Gulf of Aden than in the Arabian Sea and in the Bay of Bengal.
Global Biogeochemical Cycles, 2005
1] The DC* method of is widely used to estimate the distribution of anthropogenic carbon in the ocean; however, as yet, no thorough assessment of its accuracy has been made. Here we provide a critical re-assessment of the method and determine its accuracy by applying it to synthetic data from a global ocean biogeochemistry model, for which we know the ''true'' anthropogenic CO 2 distribution. Our results indicate that the DC* method tends to overestimate anthropogenic carbon in relatively young waters but underestimate it in older waters. Main sources of these biases are (1) the time evolution of the air-sea CO 2 disequilibrium, which is not properly accounted for in the DC* method, (2) a pCFC ventilation age bias that arises from mixing, and (3) errors in identifying the different end-member water types. We largely support the findings of Hall et al. , who have also identified the first two bias sources. An extrapolation of the errors that we quantified on a number of representative isopycnals to the global ocean suggests a positive bias of about 7% in the DC*-derived global anthropogenic CO 2 inventory. The magnitude of this bias is within the previously estimated 20% uncertainty of the method, but regional biases can be larger. Finally, we propose two improvements to the DC* method in order to account for the evolution of airsea CO 2 disequilibrium and the ventilation age mixing bias. Citation: Matsumoto, K., and N. Gruber (2005), How accurate is the estimation of anthropogenic carbon in the ocean? An evaluation of the DC* method, Global Biogeochem. Cycles, 19, GB3014,
Biogeosciences, 2009
The subtropical Indian Ocean along 32 • S was for the first time simultaneously sampled in 2002 for inorganic carbon and transient tracers. The vertical distribution and inventory of anthropogenic carbon (C ANT) from five different methods: four database methods (C*, TrOCA, TTD and IPSL) and a simulation from the OCCAM model are compared and discussed along with the observed CFC-12 and CCl 4 distributions. In the surface layer, where carbonbased methods are uncertain, TTD and OCCAM yield the same result (7±0.2 molC m −2), helping to specify the surface C ANT inventory. Below the mixed-layer, the comparison suggests that C ANT penetrates deeper and more uniformly into the Antarctic Intermediate Water layer limit than estimated from the much utilized C* method. Significant CFC-12 and CCl 4 values are detected in bottom waters, associated with Antarctic Bottom Water. In this layer, except for C* and OCCAM, the other methods detect significant C ANT values. Consequently, the lowest inventory is calculated using the C* method (24±2 molC m −2) or OCCAM (24.4±2.8 molC m −2) while TrOCA, TTD, and IPSL lead to higher inventories (28.1±2.2, 28.9±2.3 and 30.8±2.5 molC m −2 respectively). Overall and despite the uncertainties each method is evaluated using its relationship with tracers and the knowledge about water masses in the subtropical Indian Ocean. Along 32 • S our best estimate for the mean C ANT specific inventory is 28±2 molC m −2. Comparison exercises for data-based C ANT methods along with
Introduction to Special Section: Ocean Measurements and Models of Carbon Sources and Sinks
Global Biogeochemical Cycles, 2001
This issue of Global Biogeochemical Cycles contains a remarkable set of papers, which critically evaluate a variety of model-and observation-based approaches addressing the oceanic distribution, storage, and transport of CO2. Three of the papers are concerned with observation-based estimates of excess (or anthropogenic) CO2 [Coatanoan et al., this issue; Sabine and Feely, this issue; Chen, this issuel. They focus on the approaches, assumptions, and uncertainties involved in detecting the excess CO2 signal above the ocean's large and variable natural dissolved inorganic carbon (DIC) background. A further paper [Orr e! al., this issue] deals with modeling of the uptake of excess CO2, including a comparison of model results with observation-based estimates. A companion article published in the previous issue of this journal by Sarmiento e! al. [2000] addresses the preindustrial or "natural" carbon cycle and.particularly the role of the ocean in transporting carbon between the Northern and Southern Hemispheres. 2. Storage of Excess CO• The term "excess CO2" refers to carbon inventory or concentration differences within an environmental reservoir (e.g., the ocean, atmosphere, fossil fuel reserves or the terrestrial biosphere) relative to inventories or concentrations that existed during the preindustrial era. Analyses of high-resolution ice cores [Smith et al., 1999; Indermuehle et al., 1999] reveal that atmospheric levels of CO2 have varied by no more than-20 btatm (1 btatm-0.101325 Pa) through most of the Holocene. Around the year 1750 atmospheric levels started to rise from the late-Holocene level of-280 uatm, initially owing to excess CO2 released by land use changes and later owing to fossil fuel combustion. It is generally assumed that prior to 1750 the global carbon cycle was in a steady state that has now been significantly perturbed as a direct result of human activity. Hence the preindustrial era against which excess CO2 levels are assessed ended around 1750. In estimating or modeling oceanic levels of excess CO2, it is almost invariably assumed that changes in the ocean's dissolved carbon content since 1750 have been caused exclusively by an increased net air-to-sea flux driven by the anthropogenic increase of the pCO2 of the atmosphere. The uptake of excess CO2 by the oceans during the 1980s was-2 Pg C yr-• [Siegenthaler and
Use of SF 6 to estimate anthropogenic CO 2 in the upper ocean
Journal of Geophysical Research, 2008
The highest concentrations of anthropogenic carbon (C ant) are found in the upper layers of the world ocean. However, this is where seasonal variability of inorganic carbon and related parameters due to thermal and biological effects complicates use of back-calculation approaches for C ant. Tracer based approaches to C ant estimation are unaffected by biological variability and have found wide application. However, slowdown , even reversal, of the atmospheric growth of chlorofluorocarbons (CFCs) restricts use of these tracers for C ant estimation for waters ventilated since the mid 1990s. Here we apply SF 6 , a tracer that continues to increase in the atmosphere, as a basis for the C ant estimation, using samples collected in the midlatitude North Atlantic in 2004. C ant estimates derived from water mass transit time distributions (TTDs) calculated with SF 6 are compared to those based on CFC-12. For recently ventilated waters (pCFC-12 > 450ppt),theuncertaintyofSF6basedestimatesofCantis450 ppt), the uncertainty of SF 6 based estimates of C ant is 450ppt),theuncertaintyofSF6basedestimatesofCantis6 mmol kg À1 less than that of CFC-12 based estimates. CFC-12 based estimates remain more reliable for older (deeper) water masses, as a result of the longer input history and more readily detectable concentrations of CFC-12. Historical data suggest that the near-surface saturation of CFC-12 has increased over time, in inverse proportion to its atmospheric growth rate. Use of a time-dependent saturation of CFC-12 in TTD calculations appears to provide more reliable estimation of C ant .
Earth and Planetary Science …, 2006
Measurements of 231 Pa, 230 Th and 232 Th concentrations have been made on five water-column profiles along the western margin of the Madagascar and Mascarene Basins in the southern Indian Ocean. These measurements help to fill a significant gap in the global coverage of water-column 232 Th, 230 Th and 231 Pa data. 232 Th concentrations vary, but generally increase with depth, suggesting higher particle loading in deeper waters, and the presence of a significant dissolved fraction of 232 Th. 230 Th concentrations increase with depth, and profiles are similar to the average of existing data from other regions. 231 Pa concentrations, on the other hand, show significant depth structure, apparently reflecting the various water masses sampled at this location. The modified remnants of North Atlantic Deep Water are found at a depth of 6 2000 m and exhibit elevated 231 Pa concentrations exported from the South Atlantic. Antarctic Intermediate and Bottom Waters have lower 231 Pa, probably due to scavenging onto opal particles during transit from the Southern Ocean. The differences between water masses raises a question: which water mass is important in controlling the 231 Pa/ 230 Th ratio in underlying sediments? A simple one-dimensional model is used to demonstrate that the 230 Th and 231 Pa exported to sea-floor sediments last equilibrates with waters close to the seafloor (within 6 1000 m), rather than averaging the whole water column. These findings suggest that 231 Pa xs / 230 Th xs in sediments provides information primarily about deep-water masses. In this region, sedimentary records will therefore provide information about the past flow of Antarctic Bottom Water into the Indian Ocean. Interpretation of data from other regions, such as the North Atlantic where this proxy has most successfully been applied, requires careful consideration of regional oceanography and knowledge of the composition of the water masses being investigated. D