Sauveur Belviso

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Dimethyl sulfide production during natural phytoplanktonic blooms

Abstract Dimethyl sulfide (DMS), produced by biological activity in seawater, is the principal gaseous form of sulfur released to the atmosphere by the ocean and plays an important part in the biogeochemical sulfur cycle. The production of DMS in seawater tanks has been quantified during phytoplankton bloom simulations for the growth and senescence phases of biomass. This gas production during the senescence phase is 7–26 times higher than during the growth phase. Thus, DMS production by the senescence process could be one of the major mechanisms for the generation of DMS in seawater.

Seasonal variation of atmospheric dimethylsulfide at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric concentrations of dimethylsulfide (DMS) were carried out for two years in a marine site at remote area: the Amsterdam Island (37°50′S–77°31′E) located in the southern Indian Ocean. DMS concentrations were also measured in seawater. A seasonal variation is observed for both DMS in the atmosphere and in the sea-surface. The monthly averages of DMS concentrations in the surface coastal seawater and in the atmosphere ranged, respectively, from 0.3 to 2.0 nmol l-1 and from 1.4 to 11.3 nmol m-3 (34 to 274 pptv), with the highest values in summer. The monthly variation of sea-to-air flux of DMS from the southern Indian Ocean ranges from 0.7 to 4.4 μmol m-2 d-1. A factor of 2.3 is observed between summer and winter with mean DMS fluxes of 3.0 and 1.3 μmol m-2 d-1, respectively.

Potential impact of climate change on marine dimethyl sulfide emissions

Dimethyl sulfide (DMS) is a biogenic compound produced in sea-surface water and outgased to the atmosphere. Once in the atmosphere, DMS is a significant source of cloud condensation nuclei in the unpolluted marine atmosphere. It has been postulated that climate may be partly modulated by variations in DMS production through a DMS-cloud condensation nuclei-albedo feedback. We present here a modelled estimation of the response of DMS sea-water concentrations and DMS fluxes to climate change, following previous work on marine DMS modeling (Aumont et al., 2002) and on the global warming impact on marine biology (Bopp et al., 2001). An atmosphere—ocean general circulation model (GCM) was coupled to a marine biogeochemical scheme and used without flux correction to simulate climate response to increased greenhouse gases (a 1% increase per year in atmospheric CO2 until it has doubled). The predicted global distribution of DMS at 1 × CO2 compares reasonably well with observations; however, in the high latitudes, very elevated concentrations of DMS due to spring and summer blooms of Phaeocystis can not be reproduced. At 2 × CO2, the model estimates a small increase of global DMS flux to the atmosphere (+2%) but with large spatial heterogeneities (from −15% to +30% for the zonal mean). Mechanisms affecting DMS fluxes are changes in (1) marine biological productivity, (2) relative abundance of phytoplankton species and (3) wind intensity. The mean DMS flux perturbation we simulate represents a small negative feedback on global warming; however, the large regional changes may significantly impact regional temperature and precipitation patterns.

Comparison of global climatological maps of sea surface dimethyl sulfide

We have examined differences in regional and seasonal variability among sevenglobal climatologies of sea-surface dimethyl sulfide (DMS) concentrations. We foundlarge differences between recent climatologies and that typically used by mostatmospheric sulfur models. The relative uncertainty (1s/mean) in the latitudinaldistribution of the annual mean DMS concentration increases from about 50% in tropicaland temperate regions to nearly 100% in the high latitudes. We also compared theseclimatologies to new measurements in the North Atlantic Ocean taken during the 2001Programme Oce´an Multidisciplinaire Me´so Echelle (POMME) expeditions.

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric sulfur dioxide (SO2) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50′ S–77°30′ E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (222Rn) as continental tracer. Average monthly SO2 concentrations range from less than 0.2 to 3.9 nmol m-3 (annual average = 0.7 nmol m-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO2 concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO2 concentrations within ±30%. Comparing between computed and measured SO2 concentrations allowed us to estimate a yield of SO2 from DMS oxidation of about 70%.

Size distribution of dimethylsulfoniopropionate (DMSP) in areas of the tropical northeastern Atlantic Ocean and the Mediterranean Sea

Abstract Size fractionation of dimethylsulfoniopropionate (DMSP) was carried out in 2 areas of the Mediterranean Sea and 2 areas of the tropical northeastern Atlantic Ocean. In the Mediterranean Sea, particles in the size range 10–200 μm collected 15 m deep accounted for 16.0 ± 4.6% and 39% of total particulate DMSP off Villefranche/Mer and off Banyuls/Mer, respectively. In the Atlantic Ocean, the study of the vertical size distribution of particulate DMSP revealed that DMSP containing particles in the size range 10–200 μm tend to accumulate at the pycnocline and accounted for 26.0 ± 7.2% and 36.3 ± 11.6% of total particulate DMSP at sites 18°30′N, 21°W and 21°N, 31°W, respectively. Although particulate DMSP in mixed layer waters of the tropical northeastern Atlantic Ocean was carried mainly by particles in the size range 0.7–10 μm, no significant correlation was found between concentration of dissolved compounds (dissolved DMSP + DMS in samples filtered through GF/F filters) and this DMSP fraction. Dissolved DMSP + DMS levels were significantly correlated ( r 2 = 0.33, n = 24, P = 0.002) only with levels of particulate DMSP in the size range 10–200 μm. Since particulate DMSP in the size range 10–200 μm appeared not to be associated with microphytoplanktonic populations (diatoms and dinoflagellates), it is suggested that some heterotrophic organisms (microzooplankton) and/or detrital microscopic material (aggregates, fecal pellets) could play a key role in controlling the concentrations of dissolved DMSP and DMS in these waters. Depth profiles of DMSP levels in the size range 0.7–10 μm revealed that DMSP covaried with diadinoxanthin and zeaxanthin, two nonphotosynthetic carotenoids with photoprotective properties. This result provides the first in-situ indication of light-dependent DMSP accumulation in nannophytoplankton (most likely prymnesiophytes) and prochlorophytes.

Assessment of a global climatology of oceanic dimethylsulfide (DMS) concentrations based on SeaWiFS imagery (1998-2001)

A method is developed to estimate sea-surface particulate dimethylsulfoniopropionate (DMSPp) and dimethylsulfide (DMS) concentrations from sea-surface concentrations of chlorophyll a (Chl a). When compared with previous studies, the 1° × 1° global climatology of oceanic DMS concentrations computed from 4 years (1998-2001) of Chl a measurements derived from SeaWiFS (satellite-based, sea-viewing wide field of view sensor) exhibits lower seasonal variability in the southern hemisphere than in the northern hemisphere. A first evaluation of the method shows that it reasonably well represents DMSPp and DMS in the North Atlantic subtropical gyre, in large blooms of mixed populations of diatoms and Phaeocystis spp., and in massive blooms of Phaeocystis spp. but fails for large, almost pure blooms of diatoms. DMSPp and DMS concentrations derived from SeaWiFS were also compared with spatially and temporally coincident in situ measurements acquired independently in the Atlantic between 39°N and 45°N and in sub- tropical and subantarctic Indian Ocean surface waters. Moderate spring and summer phytoplankton blooms there exhib- ited similar trends in DMSPp and DMS levels vs. moderate blooms of mixed populations of prymnesiophytes and dinoflagellates investigated by others. Measured DMS largely exceeded simulated DMS concentrations, whereas mea- sured and simulated DMSPp levels were in close agreement. DMS accumulation is tentatively attributed to dinoflagellate DMSP lyase activity.

Dimethylsulfide, aerosols, and condensation nuclei over the tropical northeastern Atlantic Ocean

Concentration of dimethylsuifide (DMS) in seawater, concentrations of DMS and sulfur dioxide (SO2) in the atmosphere, concentrations of methanesulfonate (MSA) and non-sea-salt sulfate (nss-SO42−) in size-segregated aerosols, and number concentration of condensation nuclei (CN) were measured during September and October 1991 in the northern tropical Atlantic Ocean in order to assess the role of DMS in CN production over this oceanic area. Radon-222 activity and aerosol ionic composition were used to distinguish air masses with oceanic, continental, and/or polluted characters. No obvious covariation appeared between DMS and its oxidation products (SO2, H2SO4, and MSA) over the whole period of the experiment. However, the division of data into subsets according to continental tracer information allowed us to show that SO2 and nss-SO4 concentrations correlated with DMS concentration in unpolluted air masses. MSA and nss-SO42− were found to be mainly concentrated in particles with diameters < of 0.6 μm. Daily mean nss-SO42− in the <0.6-μm-diameter range and CN concentration were correlated (R = 0.91, n = 17, P < 0.001), which suggests that H2SO4 is an important CN precursor. Atmospheric DMS and CN number daily mean concentrations also correlated (R = 0.82, n = 21, P < 0.001). However, the CN population was strongly influenced by continental inputs less than 500 km downwind of Africa, whereas DMS seemed to be able to affect the CN number concentration at about 1500 km from this continent.

Spatial and temporal variability of atmospheric sulfur-containing gases and particles during the Albatross campaign

To investigate the oxidation chemistry of dimethylsulfide (DMS) in the marine atmosphere, atmospheric DMS, SO2, as well as several DMS oxidation products in aerosol phase such as non-sea-salt sulfate (nss-SO4), methanesulfonate (MSA), and dimethylsulfoxide (DMSOp) have been measured during the Albatross campaign in the Atlantic Ocean from October 9 to November 2, 1996. Long-range transport, local sea-to-air flux of DMS (FDMS), marine boundary layer (MBL) height variation, and photochemistry were found to be the major factors controlling atmospheric DMS concentration which ranged from 29 to 396 parts per trillion by volume (pptv) (mean of 120±68 pptv) over the cruise. The spatial variability of MSA and DMSOp follows the latitudinal variations of FDMS. A 2-day period of intensive photochemistry associated with quite stable atmospheric conditions south of the equator allowed the observation of anticorrelated diurnal variations between DMS and its main oxidation products. A chemical box model describing sulfur chemistry in the marine atmosphere was used to reproduce these variations and investigate coherence of experimentally calculated fluxes FDMS with observed DMS atmospheric concentrations. The model results reveal that the measured OH levels are not sufficient to explain the observed DMS daytime variation. Oxidizing species other than OH, probably BrO, must be involved in the oxidation of DMS to reproduce the observed data.