Penetration Of Anthropogenic CO2 Research Articles

Atmospheric deposition of inorganic nutrients to the Western North Pacific Ocean

We evaluated the potential impacts of atmospheric deposition on marine productivity and inorganic carbon chemistry in the northwestern Pacific Ocean (8–39°N, 125–157°E). The nutrient concentration in atmospheric total suspended particles decreased exponentially with increasing distance from the closest land-mass (Asia), clearly revealing anthropogenic and terrestrial contributions. The predicted mean depositional fluxes of inorganic nitrogen were approximately 34 and 15 μmol m−2 d−1 to the west and east of 140°E, respectively, which were at least two orders of magnitude greater than the inorganic phosphorus flux. On average, atmospheric particulate deposition would support 3–4% of the net primary production along the surveyed tracks, which is equivalent to ~2% of the dissolved carbon increment caused by the penetration of anthropogenic CO2. Our observations generally fell within the ranges observed over the past 18 years, despite an increasing trend of atmospheric pollution in the source regions during the same period, which implies high temporal and spatial variabilities of atmospheric nutrient concentration in the study area. Continued atmospheric anthropogenic nitrogen deposition may alter the relative abundances of nitrogen and phosphorus.

Deep oceans may acidify faster than anticipated due to global warming

Oceans worldwide are undergoing acidification due to the penetration of anthropogenic CO2 from the atmosphere 1–4 . The rate of acidification generally diminishes with increasing depth. Yet, slowing down of the thermohaline circulation due to global warming could reduce the pH in the deep oceans, as more organic material would decompose with a longer residence time. To elucidate this process, a time-series study at a climatically sensitive region with sufficient duration and resolution is needed. Here we show that deep waters in the Sea of Japan are undergoing reduced ventilation, reducing the pH of seawater. As a result, the acidification rate near the bottom of the Sea of Japan is 27% higher than the rate at the surface, which is the same as that predicted assuming an air–sea CO2 equilibrium. This reduced ventilation may be due to global warming and, as an oceanic microcosm with its own deep- and bottom-water formations, the Sea of Japan provides an insight into how future warming might alter the deep-ocean acidification. Changes in deep-water ventilation could potentially cause acidification from organic matter breakdown. The Sea of Japan has an acidification rate 27% higher at depth than at the surface, showing how reduced ventilation from warming could impact the deep ocean.

Assessing seasonal changes in carbonate parameters across small spatial gradients in the Northeastern Chukchi Sea

Observations of the marine carbonate system were made in 2010 in the northeastern Chukchi Sea to constrain the seasonal progression of carbonate mineral saturation states (Ω) throughout the water column and determine the air–sea flux of carbon dioxide (CO2). As sea ice retreats from the Chukchi Shelf, primary production consumes dissolved inorganic carbon (DIC) in the euphotic zone causing pH and carbonate mineral saturation states to increase. Throughout the summer and early autumn months of 2010, saturation states for calcite and aragonite ranged from 2.5 to 4.0 and 1.5 to 2.5, respectively, well about the saturation horizon of 1.0. Much of the organic matter produced during the bloom was vertically exported from the relatively small study area leading to an uptake of CO2 from the atmosphere of at least 340,000kg-C. The exported organic matter settled near the bottom and was remineralized back into DIC, causing concentrations to increase sharply, particularly in autumn months, driving down pH to as low as 7.75 and suppressing the concentrations of important carbonate minerals to the point that aragonite became undersaturated. The data showed a definitive seasonal progression of this process with aragonite becoming partially undersaturated along the bottom in September, and broadly undersaturated in October. While carbonate saturation states would naturally be suppressed by the high rates of export production and the accumulation of DIC near the bottom, the penetration of anthropogenic CO2 into the water column (ocean acidification) has caused these observed undersaturations, which will likely expand as CO2 levels in the atmosphere continue to rise in the coming decades.

Anthropogenic carbon changes in the Irminger Basin (1981–2006): Coupling δ13CDIC and DIC observations

The North Atlantic subpolar gyre is considered to be one of the strongest marine anthropogenic CO2 sinks, a consequence of extensive deep convection occurring during winter. Observations collected in this region since 1981 have shown large changes in Dissolved Inorganic Carbon (DIC) concentrations in intermediate and deep waters, which have been attributed to both anthropogenic CO2 penetration and natural variability in the ocean carbon cycle (Wanninkhof et al., 2010). In this context, we describe new δ13CDIC observations obtained in the Irminger Basin during two OVIDE cruises (2002 and 2006) which we compare to historical data (TTO-NAS 1981) in order to estimate the oceanic 13C Suess Effect over the more than twenty years that separates these surveys. The data reveal a significant decrease in δ13CDIC, of between −0.3‰ and −0.4‰ from 1981 to 2006. The anthropogenic change, extracted by using the extended Multi Linear Regression (eMLR) approach, explains 75% of this signal for oldest water mass and 90% for youngest. The reminding signal is due to the natural processes, such as remineralization and vertical mixing. The eMLR method was also applied to DIC measurements which i) reveal strong relationships between the increase of anthropogenic CO2 and the oceanic 13C Suess Effect over the whole water column during the 25-year period and ii) support the hypothesis of change in the Cant storage rate in the Irminger Basin between 1981 and 2006.

Decadal increases in anthropogenic CO2 along 20°S in the South Indian Ocean

We used high‐quality data for dissolved inorganic carbon and related water properties in the Indian Ocean along 20°S (World Ocean Circulation Experiment Hydrographic Program line I3) and 24°S (I4) obtained 8 years apart (1995–2003/2004) to estimate decadal‐scale increases of anthropogenic CO2 in the interior of the South Indian Ocean. Significant increases were detected to about 1800 m depth in the longitude range 35–45°E. In the upper thermocline subtropical subsurface water and Indian Central Water, anthropogenic CO2 increased an average of 7.9 ± 1.1 and 7.7 ± 0.5 μmol kg−1, respectively, whereas in the lower thermocline Antarctic Intermediate Water, the increase was 3.8 ± 0.7 μmol kg−1. A significant increase was also detected in Circumpolar Deep Water (2.5 ± 1.0 μmol kg−1). The estimated uptake rate of anthropogenic CO2 along the I3/I4 line over this time interval was 1.0 ± 0.1 mol m−2 a−1. Seasonal variations, which are influential in this ocean because of the Indian monsoon, did not affect detection of the anthropogenic CO2 signals. Comparisons with previous studies showed that increases of anthropogenic CO2 became larger in the most recent decade and that the CO2 uptake rate was similar to that in the South Pacific (1.0 ± 0.4 mol m−2 a−1) but higher than those in the South Atlantic (0.6 ± 0.1 mol m−2 a−1) and North Pacific (0.5 ± 0.1 mol m−2 a−1) Oceans. Deep penetration of anthropogenic CO2 is possibly associated with the higher uptake rate.

Life on the margin: implications of ocean acidification on Mg-calcite, high latitude and cold-water marine calcifiers

Future anthropogenic emissions of CO2 and the resulting ocean acidification may have severe consequences for marine calcifying organisms and ecosystems. Marine calcifiers depositing calcitic hard parts that contain significant concentrations of magnesium, i.e. Mg-calcite, and calcify- ing organisms living in high latitude and/or cold-water environments are at immediate risk to ocean acidification and decreasing seawater carbonate saturation because they are currently immersed in seawater that is just slightly supersaturated with respect to the carbonate phases they secrete. Under the present rate of CO2 emissions, model calculations show that high latitude ocean waters could reach undersaturation with respect to aragonite in just a few decades. Thus, before this happens these waters will be undersaturated with respect to Mg-calcite minerals of higher solubility than that of aragonite. Similarly, tropical surface seawater could become undersaturated with respect to Mg- calcite minerals containing ≥12 mole percent (mol%) MgCO3 during this century. As a result of these changes in surface seawater chemistry and further penetration of anthropogenic CO2 into the ocean interior, we suggest that (1) the magnesium content of calcitic hard parts will decrease in many ocean environments, (2) the relative proportion of calcifiers depositing stable carbonate minerals, such as calcite and low Mg-calcite, will increase and (3) the average magnesium content of carbonate sedi- ments will decrease. Furthermore, the highest latitude and deepest depth at which cold-water corals and other calcifiers currently exist will move towards lower latitudes and shallower depth, respec- tively. These changes suggest that anthropogenic emissions of CO2 may be currently pushing the oceans towards an episode characteristic of a 'calcite sea.'

Depth distributions of alkalinity, TCO2 and at SEATS time-series site in the northern South China Sea

Abstract In this study, measurements of titration alkalinity (TA), total dissolved carbon dioxide (TCO2), and δ13C of TCO2 ( δ 13 C TCO 2 ) throughout the water column at the SouthEast Asian time-series study (SEATS) site were investigated in order to understand better the fundamental processes controlling their vertical distributions in the South China Sea (SCS). The linear correlations between TA and salinity in the shallow waters, as identified by the mixing line between the surface water and salinity maximum water suggested the predominant control of physical mixing on the variability of TA. In contrast, TCO2 and δ 13 C TCO 2 showed the non-conservative behavior in the respective TCO2 and δ 13 C TCO 2 vs. salinity plot due to the effect of biological production. A stoichiometric model further showed that the depth profile of NTA (=TA×salinity/35) largely reflects the increase of preformed NTA in the shallow waters, whereas carbonate dissolution was responsible for the continuous increase of NTA in the deep waters. A one-dimensional diffusion–advection model further revealed that the carbonate dissolution could account for 28% of NTCO2 (=TCO2×salinity/35) increase in deep waters, and the remaining 72% of NTCO2 was from organic decomposition. Calculation of excess TA further showed that it emerged well above the aragonite and calcite saturation depths at 600 and 2500 m, respectively, indicating that some biologically, chemically, and physically-mediated processes must be involved to provide excess TA into the shallow waters. The decrease in δ 13 C TCO 2 with depth primarily resulted from organic decomposition. The influence of anthropogenic CO2 throughout the water column was assessed with the carbon chemistry and the isotope-based approach in this study. Both methods obtained nearly the same results in which the signal of anthropogenic CO2 decreased exponentially with depth, and its penetration depth were found to be at ∼1000 m. The inventory of anthropogenic CO2 in the water column was estimated to be ∼16.6 mol C m−2, which was less than that reported in the northwest Pacific at the same latitude, presumably due to the enhanced upwelling in the SCS. Such an anthropogenic CO2 penetration had led to decreases of the saturation levels of aragonite and calcite by 17% and 14%, respectively, in the surface water, and an upward migration of aragonite saturation depth by ∼100 m since industrial revolution.

Numerical Simulations of Uptake of Anthropogenic Co2 in the North Pacific

AbstractA basin‐wide ocean general circulation model with the open southern boundary condition is employed to study the uptake and distribution of anthropogenic CO2in the North Pacific, which is compared with the simulated results from the rigid southern boundary model. Simulated results show that the both western North Pacific and eastern equatorial Pacific are two important sinks of anthropogenic CO2. Larger isopycnal diffusivity produces larger exchange fluxes of anthropogenic CO2in the western North Pacific but smaller fluxes in the equatorial region. Compared with the results from the rigid boundary model, the air‐sea exchange flux of anthropogenic CO2 is larger in these two regions in the open boundary model. During the period 1800~1997, the North Pacific has taken up 23.75 GtC of anthropogenic CO2 in RUN1. The subpolar region is an important export region of the absorbed anthropogenic CO2, where 38%~54% of the absorbed anthropogenic CO2 is transported to other regions, whereas the region of 20°N~30°N is an important storing region, which amounts to 24% of the total storage in the North Pacific. The open boundary has a significant impact on the uptake and storage of anthropogenic CO2 in the area south of 10°N. Compared with the data‐based estimate, although the model underestimates the penetration of anthropogenic CO2 in the western North Pacific and overestimates the penetration in the eastern North Pacific, the model generally well estimates the storage of anthropogenic CO2 in the North Pacific.

Tracer Monitored Titrations: Measurement of Total Alkalinity

We introduce a new titration methodology, tracer monitored titration (TMT), in which analyses are free of volumetric and gravimetric measurements and insensitive to pump precision and reproducibility. Spectrophotometric monitoring of titrant dilution, rather than volume increment, lays the burden of analytical performance solely on the spectrophotometer. In the method described here, the titrant is a standardized mixture of acid-base indicator and strong acid. Dilution of a pulse of titrant in a titration vessel is tracked using the total indicator concentration measured spectrophotometrically. The concentrations of reacted and unreacted indicator species, derived from Beer's law, are used to calculate the relative proportions of titrant and sample in addition to the equilibrium position (pH) of the titration mixture. Because the method does not require volumetric or gravimetric additions of titrant, simple low-precision pumps can be used. Here, we demonstrate application of TMT for analysis of total alkalinity (A(T)). High-precision, high-accuracy seawater A(T) measurements are crucial for understanding, for example, the marine CaCO3 budget and saturation state, anthropogenic CO2 penetration into the oceans, calcareous phytoplankton blooms, and coral reef dynamics. We present data from 286 titrations on three types of total alkalinity standards: Na2CO3 in 0.7 mol kg x soln(-1) NaCl, NaOH in 0.7 mol kg x soln(-1) NaCl, and a seawater Certified Reference Material (CRM). Based on Na2CO3 standards, the accuracy and precision are +/-0.2 and +/-0.1% (4 and 2 micromol kg x soln(-1) for A(T) approximately 2100-2500 micromol kg x soln(-1), n = 242), using low-precision solenoid pumps to introduce sample and titrant. Similar accuracy and precision were found for analyses run 42 days after the initial experiments. Excellent performance is achieved by optimizing the spectrophotometric detection system and relying upon basic chemical thermodynamics for calculating the equivalence point. Although applied to acid-base titrations in this paper, the approach should be generally applicable to other types of titrations.

Distribution and inventory of anthropogenic CO2 in the Southern Ocean: Comparison of three data‐based methods

The Southern Ocean is thought to play an important role in the context of global warming and anthropogenic emissions of CO2 due to its high sensitivity to both climate change and changes in the carbon cycle. Assessing the penetration of anthropogenic CO2 (Cant) into the Southern Ocean is therefore highly relevant to reduce the uncertainties attached to both the present knowledge of anthropogenic carbon inventories and predictions made by current ocean carbon models. This study compares different data‐based approaches for estimating the distribution of Cant in the ocean: a recently developed method based on the composite Tracer Combining Oxygen, Inorganic Carbon, and Total Alkalinity (TrOCA) and the “historical” back‐calculation methods (the so‐called ΔC* and preformed dissolved inorganic carbon methods). Note that the back‐calculation technique requires special care when used in the Southern Ocean, where surface oxygen can significantly deviate from equilibrium with the atmosphere. All three methods were applied to data collected at the Indian‐Atlantic boundary (WOCE line I6), where significant transient tracer concentrations were observed in deep and bottom waters. North of 50°S, distribution and inventories of Cant are coherent with previous data‐based and model estimates, but we found larger storage of Cant south of 50°S as compared to the midlatitude region. In that, our results disagree with most previous estimates and suggest that the global inventory of anthropogenic CO2 in the Southern Ocean could be much larger than what is currently believed.

Recent developments in chemical oceanography of the East (Japan) Sea with an emphasis on CREAMS findings: A review

The understanding on the chemistry of the East (Japan) Sea, a typical mid-latitude marginal sea, has been dramatically improved through the CREAMS expeditions, an international cooperative study, carried out during 1990s. The CREAMS studies confirmed that the East Sea has undergone dramatic changes during the last 50–60 years. One of the most prominent characteristics of these changes is a rapid decrease of dissolved oxygen in deep waters. There even has been a claim that the East Sea might become an anoxic sea by next 200 years. While the causes for these changes are still under investigation, it has been shown that these changes are mainly due to the modification in the mode of deep water ventilation system in the East Sea: a slow down and complete stop of bottom water formation accompanied by an enhancement of upper water formation instead. A simple moving-boundary box model (MBBM) was developed in order to quantify the processes involved in such changes for the last 50–60 years. The model predicts that the East Sea may remain as a well-oxygenated sea despites recent rapid oxygen decreases in deep waters in association with structural changes such as a shrinking of oxygen-depleted deeper waters and an expansion of oxygen-rich upper in the East Sea in next few decades. The sedimentary record, however, shows that the East Sea has undergone oscillation between well-oxygenated environment and anoxic environment during last glacial period in association with the eustatic sea-level change. Several flooding processes such as intrusion of cold Oyashio Current and less saline, nutrient-rich seawaters from East China Sea and Yellow Sea has been proposed. Being a semi-closed basin, the carbon cycle of the East Sea has been a subject of CREAMS investigation. The East Sea serves as a strong sink of atmospheric CO2; penetration of anthropogenic CO2 all the way to the bottom is clear with its very rapid conveyor-belt system.

Carbon distribution, fluxes, and budgets in the subtropical North Atlantic Ocean (24.5°N)

Hydrographic CO2 system data obtained from World Ocean Circulation Experiment (WOCE) transatlantic zonal section A5 across 24.5°N and Florida Straits are described. By combining CO2 measurements with hydrographic velocity calculations, the zonal and vertical variability of meridional fluxes of total alkalinity (TA), total dissolved inorganic carbon (TIC), and anthropogenic CO2 (δC) are estimated. The resulting CO2 fluxes are examined in four geostrophic mid‐ocean zones as well as in the Gulf Stream flow through Florida Straits and in the surface ageostrophic Ekman flow. This method allows an estimate of the net budgets of these chemical species in the system considering together the Arctic and Atlantic Oceans north of 24.5°N. Taking into account the net flux contribution through Bering Strait, total meridional transports of chemical properties across 24.5°N latitude are also estimated. The slightly divergent net TA budget (−460 ± 200 kmol s−1) suggests that the North Atlantic is a small alkalinity source. The divergent TIC budget (−2430 ± 200 kmol s−1 or −0.92 ± 0.08 GtC yr−1) suggests that the North Atlantic is a source of TIC and therefore a net sink for atmospheric CO2. This value is twice as large as a previous estimation made from a poorly sampled section. Surprisingly, the North Atlantic Ocean appears to act as a net sink of anthropogenic CO2 (+630 ± 200 kmol s−1 or +0.24 ± 0.08 GtC yr−1) and therefore a weak source of anthropogenic CO2 to the atmosphere. Its main contributor is the intense northward flux in the Florida Current (+1280 ± 100 kmol s−1). The calculations imply a divergent inorganic carbon budget of −3060 ± 200 kmol s−1 or −1.16 ± 0.08 GtC yr−1 in preindustrial times (TIC278, when the molar fraction of CO2 in the atmosphere was 278.2 ppm). This means that the North Atlantic would have had a 25% stronger divergence of TIC prior to the beginning of anthropogenic CO2 penetration.

Can CFCs be used to determine anthropogenic CO2?

From model simulations of the uptake of CFC‐11, CFC‐12 and anthropogenic CO2, we investigate the method of using CFC‐derived water ages to determine the anthropogenic CO2 accumulation in the ocean. The CFC age method is best suited to water younger than 30 years where CFC water ages are most reliable. For water younger than 30 years, the CFC‐12 and CFC‐11 age method can estimate the global 1980–1990 accumulation of anthropogenic CO2 to within 10% and 22%, respectively. With the exception of the Southern Ocean south of 50°S and the equatorial upwelling areas, these CFC‐derived estimates of the water column inventory of anthropogenic CO2 are within 10% of the simulated values. Our results suggest that the CFC age method provides another way of determining the accumulation of anthropogenic CO2 in the ocean. We applied the CFC age method to observed CFC data to estimate the accumulation of anthropogenic CO2 in the North Pacific (20°N–65°N) between 1980 and 1990. For water younger than 30 years the CFC‐derived estimate of accumulation is 1.1 Gt C. In comparison, the estimate from the simulated CFC distribution is 1.3 Gt C. Our model simulation overestimated the accumulation north of 40°N and underestimated the penetration of anthropogenic CO2 into the subtropical gyre. From the observed CFC data we estimate the anthropogenic CO2 uptake by the North Pacific to be 1.1 Gt C/yr for the 1980s.

Distribution of anthropogenic CO2 in the Pacific Ocean

This work presents an estimate of anthropogenic CO2 in the Pacific Ocean based on measurements from the WOCE/JGOFS/OACES global CO2 survey. These estimates used a modified version of the ΔC* technique. Modifications include a revised preformed alkalinity term, a correction for denitrification, and an evaluation of the disequilibrium terms using an optimum multiparameter analysis. The total anthropogenic CO2 inventory over an area from 120°E to 70°W and 70°S to 65°N (excluding the South China Sea, the Yellow Sea, the Japan/East Sea, and the Sea of Okhotsk) was 44.5 ± 5 Pg C in 1994. Approximately 28 Pg C was located in the Southern Hemisphere and 16.5 Pg C was located north of the equator. The deepest penetration of anthropogenic CO2 is found at about 50°S. The shallowest penetration is found just north of the equator. Very shallow anthropogenic CO2 penetration is also generally observed in the high‐latitude Southern Ocean. One exception to this is found in the far southwestern Pacific where there is evidence of anthropogenic CO2 in the northward moving bottom waters. In the North Pacific a strong zonal gradient is observed in the anthropogenic CO2 penetration depth with the deepest penetration in the western Pacific. The Pacific has the largest total inventory in all of the southern latitudes despite the fact that it generally has the lowest average inventory when normalized to a unit area. The lack of deep and bottom water formation in the North Pacific means that the North Pacific inventories are smaller than the North Atlantic.

Accumulation and uptake of anthropogenic CO2 in the Southern Ocean, south of Australia between 1968 and 1996

Increases in the anthropogenic CO2 inventory for Southern Ocean waters to the south of Australia were estimated by comparing measurements made 28 years apart in 1968 and 1996. For this period, the deepest penetration of anthropogenic CO2 is consistent with the depth of 28 year old water from CFC‐11 age estimates. Significant accumulation of anthropogenic CO2 (13±10 μmol kg−1) was found in Antarctic Bottom Water (AABW) from 500–4500 m south of 58°S suggesting AABW to be an important water mass for the storage of anthropogenic CO2 in the deep ocean. Deep penetration of anthropogenic CO2 (<1900 m) was found in the sub‐Antarctic zone (SAZ) between 48°S and 51°S while little accumulation (<400 m) was observed south of 53°S. A simple one dimensional box model was used to investigate the anthropogenic CO2 uptake potential in the SAZ based on observed CFC‐11 concentrations and the calculated anthropogenic CO2 inventory. Formation and export of sub‐Antarctic Mode Water (SAMW) was estimated by reproducing the observed present day SAMW CFC‐11 concentration using time histories of atmospheric CFC‐11 concentrations. The anthropogenic CO2 uptake for the SAZ (45°–50°S) ranged from 0.73 to 0.86 μmol kg yr−1. The calculated range of SAMW formation and export rates only increased the anthropogenic CO2 uptake potential for the SAZ by as much as 18% (<0.13 μmol kg−1 yr−1). Extrapolating our estimate to the circumpolar SAZ gives an anthropogenic CO2 uptake of 0.07–0.08 GTC yr−1. Even though SAMW formation does not considerably enhance anthropogenic CO2 uptake in the SAZ, it provides an important mechanism for transporting anthropogenic CO2 into the ocean interior. SAMW therefore may be important for anthropogenic CO2 uptake on a larger regional scale.

The distribution of δ in total dissolved inorganic carbon in the central North Pacific Ocean along 175°E and implications for anthropogenic CO2 penetration

We measured the quantity of d 13 C in total dissolved inorganic carbon in seawater collected between 488N and 158S along . 1758E in the central North Pacific during the Northwest Pacific Carbon Cycle Study program NOPACCS in 1993 and 1994, focusing on the distribution of d 13 C on isopycnal surfaces of 25.6, 26.0, 26.4, 26.8 and 27.2 s in the main u pycnocline of the North Pacific. The distribution of d 13 C was controlled largely by biological cycling. Using apparent oxygen utilization as a measure of biological effects on d 13 C variation, we calculated preformed values of d 13 C at the time when the water left the surface. The preformed d 13 C showed a trend of increasing southward in the subtropical region on all the surfaces. Variations in preformed d 13 C on the surfaces between 25.6 and 26.4 s in the subtropical region most likely u

Comparison of methods to determine the anthropogenic CO2 invasion into the Atlantic Ocean

A comparison of different methods for estimating the anthropogenic CO2 burden in the Atlantic Ocean is performed using referenced, high quality total dissolved inorganic carbon (DIC) data. The dataset is from two cruises through the center of the basin between 62°N and 43°S in 1991 and 1993. The specific anthropogenic input is determined utilizing empirical procedures as described in Gruber et al. (1996) and Chen and Millero (1979) to correct for remineralization and to estimate preanthropogenic endmembers. These estimates are compared with output of the Princeton ocean biogeochemical model and the NCAR ocean model. The results show that the specific inventories of anthropogenic carbon agree to within 20% but with different storage and uptake patterns. The empirical estimates differ because of assumptions about mixing and winter outcrop endmembers. The same remineralization quotients (Redfield ratios) were used for each method. Varying these constants within the range of literature values causes changes in specific inventories of similar magnitude as the differences observed with different methodologies. Comparison of anthropogenic CO2 uptake and chlorofluorocarbon ages suggests that the anthropogenic CO2 penetration in the North Atlantic is too shallow following the procedure according to Gruber et al. (1996), and too deep using those of Chen and Millero (1979). The results support these previous observations in that the uptake of CO2 in the North Atlantic is disproportionate to its surface area. This is caused by a combination of deep water formation and deep winter mixed layers.

The annual fCO2 cycle and the air’sea CO2 flux in the sub-Antarctic Ocean

The sub-Antarctic zone (SAZ) lies between the subtropical convergence (STC) and the sub-Antarctic front (SAF), and is considered one of the strongest oceanic sinks of atmospheric CO2. The strong sink results from high winds and seasonally low sea surface fugacities of CO2 (fCO2), relative to atmospheric fCO2. The region of the SAZ, and immediately south, is also subject to mode and intermediate water formation, yielding a penetration of anthropogenic CO2 below the mixed layer. A detailed analysis of continuous measurements made during the same season and year, February — March 1993, shows a coherent pattern of fCO2 distributions at the eastern (WOCE/SR3 at about 145°E) and western edges (WOCE/I6 at 30°E) of the Indian sector of the Southern Ocean. A strong CO2 sink develops in the Austral summer (ΔfCO2 μ - 50 μatm) in both the eastern (110°-150°E) and western regions (20°-90°E). The strong CO2 sink in summer is due to the formation of a shallow seasonal mixed-layer (about 100 m). The CO2 drawdown in the surface water is consistent with biologically mediated drawdown of carbon over summer. In austral winter, surface fCO2 is close to equilibrium with the atmosphere (ΔfCO2 ± 5 μatm), and the net CO2 exchange is small compared to summer. The near-equilibrium values in winter are associated with the formation of deep winter mixed-layers (up to 700 m). For years 1992–95, the annual CO2 uptake for the Indian Ocean sector of the sub Antarctic Zone (40°-50°S, 20°-150°E) is estimated to be about 0.4 GtC yr-1. Extrapolating this estimate to the entire sub-Antarctic zone suggests the uptake in the circumpolar SAZ is approaching 1 GtC yr-1.

The δ13C anomaly in the northeastern Atlantic

The δ13C of dissolved inorganic carbon was measured on samples collected at 49°N in the northeast Atlantic in January 1994. Deeper than 2000 m, δ13C exhibits the same negative correlation versus dissolved phosphate that is observed elsewhere in the deep Atlantic. Upward from 2000 m to about 600 m, δ13C shifts to values more negative than expected from the correlation with nutrients at depth, which is likely due to penetration of anthropogenic CO2. From these data, the profile of the anthropogenic δ13C decrease is calculated by using either dissolved phosphate or apparent oxygen utilization as a proxy for the preanthropogenic δ13C distribution. The shape of the anthropogenic anomaly profile derived from phosphate is similar to that of the increase in dissolved inorganic carbon derived by others in the same area. The reconstruction from oxygen utilization results in a lower estimate of the anthropogenic δ13C decrease in the upper water column, and the vertical anomaly profile is less similar to that of the dissolved inorganic carbon increase. A 13C budget for the atmosphere, ocean, and terrestrial biosphere indicates that within the range of probable ocean CO2 uptake the ratio of δ13C to inorganic carbon change should be mostly influenced by the 13C inventory change of the biosphere. However, the uncertainty in the ratio we derive prevents a strong contraint on the size of the exchangeable biosphere.

On the relations between the oceanic uptake of CO2 and its carbon isotopes

The recent proposals to estimate the oceanic uptake of CO2 by monitoring the oceanic change in 13C/12C isotope ratio [Quay et al., 1992] or the air‐sea 13C/12C isotopic disequilibrium [Tans et al., 1993] is reviewed. Because the history of atmospheric CO2 and 13CO2 since preindustrial times is almost the same and increasing in an almost exponential fashion, the oceanic penetration depth of both tracers must be the same. This dynamic constraint permits the establishment of yet a third method to estimate the global ocean uptake of CO2 from 13C measurements. Using available observations in conjunction with canonical values for global carbon cycle parameters, the three methods yield inconsistent oceanic CO2 uptake rates for the time period 1970–1990, ranging from 0.6 to 3.1 GtC yr−1. However, uncertainties in the available carbon cycle data must be taken into account. Using a nonlinear estimation procedure, a consistent scenario with an oceanic CO2 uptake rate of 2.1±0.9 GtC yr−1 can be established. The method also permits an investigation of the sensitivities of the different approaches. An analysis of the results of two three‐dimensional simulations with the Hamburg model of the oceanic carbon cycle shows that the 13C isotope indeed tracks the oceanic penetration of anthropogenic CO2. Because of its different time history, bomb produced radiocarbon, as measured at the time of the Geochemical Ocean Sections Study (GEOSECS), correlates not as well to excess carbon.