ALBERT

Description: The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

Description: The partial pressure of CO2 (pCO2) was measured during the 1995 South-West Monsoon in the Arabian Sea. The Arabian Sea was characterized throughout by a moderate supersaturation of 12–30 µatm. The stable atmospheric pCO2 level was around 345 µatm. An extreme supersaturation was found in areas of coastal upwelling off the Omani coast with pCO2 peak values in surface waters of 750 µatm. Such two-fold saturation (218%) is rarely found elsewhere in open ocean environments. We also encountered cold upwelled water 300 nm off the Omani coast in the region of Ekman pumping, which was also characterized by a strongly elevated seawater pCO2 of up to 525 µatm. Due to the strong monsoonal wind forcing the Arabian Sea as a whole and the areas of upwelling in particular represent a significant source of atmospheric CO2 with flux densities from around 2 mmol m−2 d−1 in the open ocean to 119 mmol m−2 d−1 in coastal upwelling. Local air masses passing the area of coastal upwelling showed increasing CO2 concentrations, which are consistent with such strong emissions.

Description: We show the distribution of nutrients, oxygen, total dissolved inorganic carbon (CT) and total alkalinity (AT) along three sections close to the Canary Islands, between 18°W and the African coast during Meteor 37/2 cruise (January 1997). From the thermohaline properties of Eastern North Atlantic Central Water (ENACW), Mediterranean Water (MW), Antarctic Intermediate Water (AAIW) and North Atlantic Deep Water (NADW), a mixing model has been established based on the water mass description. It can explain most of the variabilities found in the distribution of the chemical variables, including the carbon system, and it is validated through the use of conservative chemical variables like ‘NO.’ From nutrients, oxygen, AT and CT, the chemical characterisation of the water masses was performed by calculating the concentration of these variables in the previously defined thermohaline end-members. The relative variation of nutrient concentrations, resulting from the regeneration of organic matter, was estimated. Close to the African shelf-break, a poleward undercurrent conveying as much as a 11% of AAIW was observed only in the southern section (28.5°N). From the chemical and thermohaline properties of the end-members, a comparison with data from other oceanic regions was made in respect to conservative chemical variables (‘NO’). In addition, a north–south gradient in the ventilation pattern of water masses is observed from the residuals of the model.

Description: Two newly designed underway systems for the measurement of CO2 partial pressure (pCO2) in seawater and the atmosphere are described. Results of an intercomparison experiment carried out in the North Sea are presented. A remarkable agreement between the two simultaneously measured (pCO2) data sets was observed even though the spatial variability in surface pCO2 was high. The average difference of all l -min averages of the seawater pCO2 was as low as 0.15 μatm with a standard deviation of 1.2 μatm indicating that no systematic difference is present. A closer examination of the profiles shows that differences tend to be highest during maxima of the pCO2 gradient (up to 14 μatm/min). The time constants of both systems were estimated from laboratory experiments to 45 s, respectively, 75 s thus quantitatively underlining their capability of a fast response to pCO2 changes

Description: The effect of dissolution from particulates into the supernatant solution in sediment trap sample cups has been measured for fatty acids. A mooring array with time series sediment traps was deployed in the northeast Atlantic Ocean (59°N, 21°W) for 14 months. Selected representative samples from the trap at 2200 m (poisoned with NaN3) were analyzed for total and free fatty acids in both the solution and particulate phase by means of gas chromatography‐mass spectrometry with an ion trap detector. The flux contribution of the dissolved total fatty acids (∑ DTFA) was found to be between 15 and 75% of the total flux (∑ TTFA, sum of the fluxes of total fatty acids in both particles and supernatants). Dissolved free fatty acids (∑ DFFA) represented 25–88% of the total flux of free fatty acids (∑ TFFA). Absolute concentrations of total and free fatty acids in both compartments are discussed in terms of the processes controlling the distribution between the two phases, for example, readsorption. Sample handling, poisoning, bacterial activity, and swimmers may also affect fatty acid distribution. Flux data (sum of particulate and dissolved fluxes) are presented for individual fatty acids. Also, the degree of dissolution of individual fatty acids is shown for one sample (dissolved fraction ranging between 16 and 98% of total flux).

Description: Temporal trends in oceanic dissolved inorganic carbon (DIC) and δ13C-DIC were reconstructed along five isopycnals in the upper 1000 m of the North Atlantic Ocean using a back-calculation approach. The mean anthropogenic DIC increase was 1.21 ± 0.07 μmol kg−1 yr−1 and the mean 13C decrease was −0.026 ± 0.002‰ yr−1, both in good agreement with the results from previous studies. The observed δ13C-DIC perturbation ratio is −0.024 ± 0.003‰ (μmol kg−1)−1. Our results indicate that the North Atlantic is able to maintain equilibrium with the anthropogenic perturbation for DIC and follows it with decadal time lag for δ13C. A CFC-calibrated one-dimensional isopycnal advection-diffusion model is used to evaluate temporal DIC and δ13C trends and perturbation ratios of the reconstructions. We investigate the time history of the air-sea CO2 and 13C disequilibria in the North Atlantic and discuss the importance of physical and biological processes in maintaining them. We find evidence that the North Atlantic Ocean is characterized by enhanced uptake of anthropogenic CO2. Also, we use the model to examine how the time rate of change of δ13C depends on changes in the temporal evolution of δ13C in the atmosphere. The model evolution explains the curious result that the time rate of change of surface water δ13C in the North Atlantic Ocean can exceed that observed concurrently in the atmosphere. Finally we introduce a powerful way of estimating the global air-sea pCO2 disequilibrium based on the oceanic δ13C-DIC perturbation ratio.

Description: A coulometrically-based SOMMA system for the determination of total dissolved carbon dioxide (TCO2) in a continuous mode was designed and tested at sea. The new continuous technique approached the same high accuracy and reliability associated with prior discrete TCO2 measurements. During three cruises encompassing more than 19 weeks and 6000 continuous TCO2 measurements none of the three different systems tested exhibited any hardware-related failures. We found that coulometer cell lifetimes can greatly exceed prior expectations with many of the titration cells in the continuous mode remaining accurate for up to 72 h at carbon ages exceeding 50 mg C. We suggest a practical definition based on the CRM analyses for changing coulometer cells in the continuous mode. Systematic deviations of the SOMMA pipette volume from a theoretical temperature dependence were identified both from field data comparisons and pipette calibrations. Hence pipettes should be kept at constant temperature or they must be gravimetrically calibrated over the expected temperature range. Comparison of the continuous TCO2 data together with simultaneously measured additional CO2 system parameters showed that the refitted “Mehrbach” dissociation constants for carbonic acid best-represent fCO2 when calculated from TCO2 and alkalinity over a wide range of sea-surface temperatures and salinities. Some remaining systematic differences of calculated–measured fCO2 of up to 9 μatm likely reflect uncertainty in the temperature-dependence of the “Mehrbach” constants as well as possible uncertainty in the alkalinity–salinity relationship used to estimate alkalinity in the consistency checks.

Description: Data on the carbonate system of the Northwestern Indian Ocean obtained on a cruise of F.S. Meteor during SW monsoon in July/August 1995 were compared with those of George et al. [George, M.D., Kumar, M.D., Naqvi, S.W.A., Banerjee, S., Narvekar, P.V., de Sousa, S.N., Jayakumar, D.A., 1994. A study of the carbon dioxide system in the northern Indian Ocean during premonsoon. Mar. Chem. 47, 243–254] collected during intermonsoon. In general, deep water values agreed well between the two expeditions. Surface waters, however, showed a substantial increase in dissolved inorganic carbon (CT) in the coastal regions due to strong upwelling in the SW monsoon. This was also accompanied by very high CO2 partial pressures in surface waters. The north–south gradients in vertical profiles of the measured parameters in the Arabian Sea are discussed by comparing profiles from the oligotrophic equatorial region with those from the highly productive central Arabian Sea. The effect of denitrification on regenerated CT and AT is minor, with contributions of 〈9 and 〈8 μmol kg−1, respectively, to the total amount regenerated also utilizing oxygen. The dissolution of biogenic carbonates is discussed; different approaches to define the depth, where the dissolution starts (lysocline(s), carbonate critical depth (CCrD)), are compared together with the calculation of saturation depth from carbonate concentrations. It is shown, that small differences in measured CT and AT (found between our data and those measured during GEOSECS) and different calculation approaches to the CO2 system (different dissociation constants for species involved and taking into account phosphate and silicate concentrations) can produce pronounced differences in the calculated saturation depths. However, CT and AT data suggest substantial dissolution of biogenic carbonate in the water column even above the calcite lysocline, irrespective of the procedures followed to calculate this horizon.

Description: The results of 1 year of automated pCO2 measurements in 2002/2003 onboard the car carrier M/V Falstaff are presented and analyzed with regard to the driving forces that change the seawater pCO2 in the midlatitude North Atlantic Ocean. The pCO2 in surface seawater is controlled by thermodynamics, biology, air-sea gas exchange, and physical mixing. Here we estimate the effects on the annual cycle of pCO2 and relate this property to parameters like SST, nitrate, and chlorophyll. On the basis of the amplitude in seawater pCO2 for all 4° × 5° grid boxes, this region can be separated into an eastern and western basin. The annual pCO2 cycle in the eastern basin (10°W–35°W) is less variable, which can be related to the two counteracting effects of temperature and biology; air-sea gas exchange plays a minor role when using climatological MLD. In the western basin (36°W–70°W) the pCO2 amplitude is more variable and strongly follows the thermodynamic forcing, since the biological forcing (as derived from nitrate concentrations) is decreased. Biology and air-sea exchange strongly depend on the MLD and therefore also include physical mixing effects. The pCO2 data of the analyzed region between 34°N and 52°N compare well to the Takahashi et al. [2002] climatology except for regions north of 45°N during the wintertime where the bias is significant.

Description: The penetration of anthropogenic or “excess” CO2 into the North Atlantic Ocean was studied along WOCE‐WHP section A2 from 49°N/11°W to 43°N/49°W using hydrographic data obtained during the METEOR cruise 30–2 in October/November 1994. A backcalculation technique based on measurements of temperature, salinity, oxygen, alkalinity, and total dissolved inorganic carbon was applied to identify the excess CO2. Everywhere along the transect surface water contained almost its full component of anthropogenic CO2 ( ∼62 μmol kg−1). Furthermore, anthropogenic CO2 has penetrated through the entire water column in the western basin of the North Atlantic Ocean. Even in the deepest waters (5000 m) of the western basin a mean value of 10.4 μmol kg−1 excess CO2 was calculated. The maximum penetration depth of excess CO2 in the eastern basin of the North Atlantic Ocean was ∼3500 m with values falling below 5 μmol kg−1 in greater depths. These results compare well with distributions of carbontetrachloride. They are also in agreement with the current understanding of the role of the “global ocean conveyor belt” for the uptake of anthropogenic CO2 into the deep ocean.