Nathalie Lefèvre

Modeling the geochemical cycle of iron in the oceans and its impact on atmospheric CO2 concentrations

Iron occurs at very low concentrations in seawater and seems to be a limiting factor for primary production in the equatorial Pacific and the Southern Ocean. The global distribution of iron is still not well understood because of a lack of data and the complex chemistry of iron. We develop a 10-box model to study the oceanic distribution of iron and its effect on atmospheric CO2 concentration. Subject to our assumptions, we find that a lack of interocean fractionation of deep sea iron concentrations, as suggested by Johnson et al. [1997a], is not readily explained by a balance of eolian deposition, scavenging, and regeneration. Incorporation of organic complexation in the model, as suggested by Johnson et al., to reduce the scavenging rate of iron when concentrations fall below some ligand-stabilized concentration, is one solution to this difficulty. Alternatively, the deep-sea concentration may be more variable than the current, rather sparse data coverage suggests. In the model, deep-sea iron concentrations are responsive to the atmospheric source, even if we adopt stabilization of concentrations by a ligand as modeled by Johnson et al. [l997a]. In the Southern Ocean, where the model suggests iron supply has an important limiting effect on the biota, more than 99% of the iron supply to the surface in the present day comes from upwelling and not from the local atmospheric flux. In the context of glacial-interglacial changes to atmospheric CO2 the model suggests that increasing atmospheric iron to the entire global ocean by a factor of 2, leads to decreases in atmospheric CO2 of 10–30 ppm, depending on assumptions. However, in our model, CO2 concentrations are almost unaffected by changes in Southern Ocean atmospheric fluxes alone, unless these are unrealistically large (> 100 times present day). The effect on atmospheric carbon dioxide is slightly stronger if accompanied by increased stratification of the Southern Ocean. The model suggests that eolian “iron fertilization” of the ocean could have importantly influenced glacial atmospheric CO2 concentrations but that other processes must also be at work to account for the full magnitude of the glacial-interglacial change.

Variability of pCO2 in the tropical Atlantic in 1995

Atmospheric and oceanic partial pressures of CO2 (pCO2) have been recorded automatically along two Atlantic meridional transects in 1995. The tropical Atlantic ocean (20°S–20°N) is generally a source of CO2 for the atmosphere, but in the region of the North Equatorial Countercurrent an undersaturation of CO2 has been observed. Undersaturations previously reported in the literature are explained by the decrease of salinity due to the high precipitations associated with the Intertropical Convergence Zone. In June 1995, strong CO2 undersaturations (ΔpCO2 = −70μatm) were observed near 8°N, which suggests, in addition of the salinity effect, an uptake of CO2 due to biological activity. This undersaturation, although weaker than in spring, also appeared at other periods of the year 1995.

A new optical sensor for PCO2 measurements in seawater

Abstract A knowledge of the distribution of the partial pressure of CO2 at the surface of the ocean is needed to quantify the absorption of atmospheric CO2 by gas exchange. Owing to the large space and time variability of this quantity at the surface of the ocean, an experimental approach aiming to make time-series measurements from unattended platforms must be envisaged. In this context we have developed a PCO2 sensor which meets the requirements for being installed on a buoy for approximately 1 year. The principle of the sensor is based on a colorimetric method. The selected dye is thymol blue. Its spectra and variation of its pK vs. temperature have been determined. A dedicated optical spectrophotometer has been built making absorbance measurements at three wavelengths. The sensor includes a sensitive volume enclosed with two silicone membranes and filled with thymol blue in an ionic medium. It is related to the spectrophotometer with silica fiber optics. Two calibration experiments were made over a fugacity of CO2 (fCO2) range extending from 340 μatm to 590 μatm. They lasted 3 and 4 days, respectively and showed that in the range 340–600 μatm a variation of ± 1 μatm is seen by the sensor and that the standard error of the fit of the two calibration curves is, respectively, ± 3 μatm and ±5 μatm.

PCO2, chemical properties, and estimated new production in the equatorial Pacific in January–March 1991

Measurements of the partial pressure of CO2 (PCO2) at the sea surface, dichlorodifluoromethane (F12), salinity, temperature, oxygen, nutrients, wind, and current velocities were made during a cruise (January–March 1991) in the equatorial Pacific from Panama to Noumea via Tahiti. In the western Pacific (140°W to 165°E) the westward South Equatorial Current is well established. Distributions of tracers show extrema near the equator in the eastern Pacific (from 95°W to 140°W), indicating that the upwelling is especially active in this area. The zonal distribution of chemical tracers is not regular because of intrusions of warmer water from the north associated with equatorial long waves. The temporal changes in PCO2 result from thermodynamic changes, biological activity, and gas exchange with the atmosphere. In order to compare the magnitude of these processes, we assess the variations of PCO2 (dPCO2) between two stations as the sum of thermodynamic changes driven by temperature and salinity changes, air-sea exchange computed from observed wind and difference of PCO2 between the sea and the atmosphere, and the biological activity estimated from the nitrate decrease and C:N ratio (106:16). The resulting assessed change in PCO2 is in agreement with the observed change for 42 pairs of stations. Each of these pairs of stations is thus considered as representing a simple water mass advected by the measured currents between the two stations so that daily fluxes can be estimated. The contribution of CO2 outgassing to dPCO2 is low, between −0.2 to −0.0 μatm d−1. The thermodynamical dPCO2 averages 0.7±0.2 μatm d−1 in the mixed layer. The biological dPCO2 (-1.5±0.5 μatm d−1) is the highest in absolute value implying an average value of new production along the equator of 72±25 mmolC m−2 d−1 (0.9±0.3 gC m−2 d−1) for the equatorial Pacific (130°W-165°E). This value is very high and the overestimation could result from the simplistic description of the advection and mixing of water. An attempt to account for these processes by constraining the net heat flux to 100 W m−2 [Weare et al., 1981] reduces the estimate of new production to 58 mmolC m−2 d−1 (0.7 gC m−2 d−1 ). A mean upwelling velocity of 0.5±0.1 m d−1 east of 140°W is calculated, based on F12 undersaturations.

Assessing the seasonality of the oceanic sink for CO2 in the northern hemisphere

Seasonal CO2 fluxes are estimated from quarterly maps of ΔpCO2 (difference between the oceanic and atmospheric partial pressure of CO2) and associated error maps. ΔpCO2 maps were interpolated from pCO2 measurements in the North Atlantic and the North Pacific Oceans using an objective mapping technique. Negative values correspond to an uptake of CO2 by the ocean. The CO2 flux for the North Atlantic Ocean, between 10°N and 80°N, ranges from −0.69 GtC/yr, for the first quarter (January-March), to −0.19 GtC/yr for the third quarter (July-September) using the gas exchange coefficient of Tans et al. [1990], satellite wind speeds, and a correction for the skin effect. On annual average, the North Atlantic ocean (north of 10°N) is a sink of CO2 ranging from −0.23 ± 0.08 GtC/yr (gas exchange coefficient of Liss and Merlivat [1986] with Esbensen and Kushnir [1981] wind field) to −0.48 ±0.17 GtC/yr (gas exchange coefficient of Tans et al. with satellite wind field). The CO2 flux for the North Pacific, between 15°N and 65°N, ranges from −0.66 GtC/yr from April to June to zero from July to September. For the Atlantic, the errors are generally small, that is, less than 0.19 GtC/yr, but for the Pacific considerably larger uncertainties are generated due to the less extensive data coverage. The northern hemisphere ocean (north of 10°N) is a net sink of CO2 to the atmosphere which is stronger in spring (April-June), due to the biological activity, with an estimate of −1.23 ± 0.40 GtC/yr averaged over this period. The annual mean northern hemisphere ocean flux is −0.86 ± 0.61 GtC/yr.

Air-sea CO2 fluxes in the equatorial pacific in January-March 1991

The partial pressure of CO[sub 2] in surface seawater and in air were continuously measured during the cruise ALIZE II (January 1991-March 1991) in the equatorial Pacific from Panama to Noumea via Tahiti. It provides a large set of data for estimating the air-sea CO[sub 2] flux in the equatorial Pacific. An average flux is calculated between 2.5 N and 2.5S. This estimation ranges from 5 to 8.5 mmol m[sup [minus]2]d[sup [minus]1] according to the CO[sub 2] exchange coefficient used. 4 refs., 18 figs., 3 tabs.

Origin of CO2 undersaturation in the western tropical Atlantic

Underway fCO 2 has been measured from two merchant ships sailing from France to French Guyana and France to Brazil, and during two zonal cruises from Africa to French Guyana. In the western Tropical Atlantic, the strongest undersaturation is associated with the Amazon discharge near 55.W. In the 5.S.10.N, 65.35.W region, the carbon system is strongly correlated to salinity and robust empirical relationships could be determined. This region is a sink of CO2 in May.June during the high-flow period of the Amazon river. The eastward propagation of Amazon waters is observed when the retroflection of the North Brazil Current takes place. In August 2008, freshwater is observed as far as 40.W when the North Equatorial Counter Current is quite strong. The Amazon plume, defined as salinities less than 34.9, is a sink of CO2 of 0.96 mmol m.2 d.1. Further east, near 27.W, CO2 undersaturation is recorded thoughout the year between 5.N and 8.N. This is caused by the high precipitation associated with the presence of the intertropical convergence zone (ITCZ). Removing the temperature effect leads to low (high) fCO2 associated with low (high) salinities in boreal summer (winter), which is consistent with the seasonal migration of the ITCZ.

Impact of physical processes on the seasonal distribution of the fugacity of CO2 in the western tropical Atlantic

The fugacity of CO2 (fCO2) has been measured underway during three quasi-synoptic cruises in the western tropical Atlantic in March/April 2009 and July/August 2010 in the region 6°S–15°N, 52°W–24°W. The distribution of fCO2 is related to the main features of the ocean circulation. Temperature exerts a dominant control on the distribution of fCO2 in March/April whereas salinity plays an important role in July/August due to the more developed North Equatorial Countercurrent (NECC) carrying Amazon water and to the high precipitation associated with the presence of the Intertropical Convergence Zone (ITCZ). The main surface currents are characterized by different fCO2. Overall, the NECC carries less saline waters with lower fCO2 compared to the South Equatorial Current (SEC). The North Equatorial Current (NEC) is usually characterized by CO2 undersaturation in winter and supersaturation in summer. Using empirical fCO2-SST-SSS relationships, two seasonal maps of fCO2 are constructed for March 2009 and July 2010. The region is a sink of CO2 of 0.40 mmol m−2d−1 in March, explained by the winter cooling in the northern hemisphere, whereas it is a source of CO2 of 1.32 mmol m−2d−1 in July. The equatorial region is a source of CO2 throughout the year due to the upwelling supplying CO2-rich waters to the surface. However, the evolution of fCO2 over time, determined from all the available cruises in a small area, 1°S–1°N, 32°W–28°W, suggests that the source of CO2 has decreased in February-March from 1983 to 2011 or has remained constant in October-November from 1991 to 2010.

Seasonal and interannual variability of sea‐air CO2 fluxes in the tropical Atlantic affected by the Amazon River plume

CO2 fugacities obtained from a merchant ship sailing from France to French Guyana were used to explore the seasonal and interannual variability of the sea-air CO2 exchange in the western tropical North Atlantic (TNA; 5–14°N, 41–52°W). Two distinct oceanic water masses were identified in the area associated to the main surface currents, i.e., the North Brazil Current (NBC) and the North Equatorial Current (NEC). The NBC was characterized by permanent CO2 oversaturation throughout the studied period, contrasting with the seasonal pattern identified in the NEC. The NBC retroflection was the main contributor to the North Equatorial Counter Current (NECC), thus spreading into the central TNA, the Amazon River plume, and the CO2-rich waters probably originated from the equatorial upwelling. Strong CO2 undersaturation was associated to the Amazon River plume. Total inorganic carbon drawdown due to biological activity was estimated to be 154 µmol kg−1 within the river plume. As a consequence, the studied area acted as a net sink of atmospheric CO2 (from −72.2 ± 10.2 mmol m−2 month−1 in February to 14.3 ± 4.5 mmol m−2 month−1 in May). This contrasted with the net CO2 efflux estimated by the main global sea-air CO2 flux climatologies. Interannual sea surface temperature changes in the TNA caused by large-scale climatic events could determine the direction and intensity of the sea-air CO2 fluxes in the NEC. Positive temperature anomalies observed in the TNA led to an almost permanent CO2 outgassing in the NEC in 2010.

Sea water fugacity of CO2 at the PIRATA mooring at 6°S,10°W

In order to better understand the variability of surface CO2 in the Tropical Atlantic, a CARIOCA sensor has been installed on a PIRATA mooring at 6.S, 10.W in June 2006. The fugacity of CO2 (fCO2) is recorded hourly from 7 June 2006 to 30 October 2009 with two important data gaps. From July to September, an upwelling develops and a decrease in sea surface temperature (SST) is observed, associated with an fCO2 increase. However, the highest fCO2 is observed in October, after the upwelling season, due to the warming of surface waters. The region is a net source of CO2 to the atmosphere of 2.10 ) 0.69 molm.2 yr-1 in 2007. The monthly flux is maximum (3.21 ) 0.8molm-2 yr-1) in November (averaged over 2006 and 2008). High frequency variability is observed throughout the time series but is particularly pronounced after the upwelling season. Biological and thermodynamic processes explain the diurnal variability. Dissolved inorganic carbon (DIC) is calculated from (alkalinity) TA and fCO2 using an empirical TA. salinity relationship determined for the eastern equatorial Atlantic. Net community production (NCP) is calculated from DIC daily changes and ranges from 9 to 41 mmol m-2 d-1, which is consistent with previous measurements in this region.