Tropical North Atlantic Ocean Research Articles

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO2 uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO3) and silicate (Si(OH)4) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35mmolm−2d−1, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1millionkm2. We estimate that 0.15TmolCy−1 is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on Corg export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.

Potential role of Atlantic Warm Pool-induced freshwater forcing in the Atlantic Meridional Overturning Circulation: ocean–sea ice model simulations

Recent studies have indicated that the multidecadal variations of the Atlantic Warm Pool (AWP) can induce a significant freshwater change in the tropical North Atlantic Ocean. In this paper, the potential effect of the AWP-induced freshwater flux on the Atlantic Meridional Overturning Circulation (AMOC) is studied by performing a series of ocean–sea ice model experiments. Our model experiments demonstrate that ocean response to the anomalous AWP-induced freshwater flux is primarily dominated by the basin-scale gyre circulation adjustments with a time scale of about two decades. The positive (negative) freshwater anomaly leads to an anticyclonic (cyclonic) circulation overlapping the subtropical gyre. This strengthens (weakens) the Gulf Stream and the recirculation in the interior ocean, thus increases warm (cold) water advection to the north and decreases cold (warm) water advection to the south, producing an upper ocean temperature dipole in the midlatitude. As the freshwater (salty water) is advected to the North Atlantic deep convection region, the AMOC and its associated northward heat transport gradually decreases (increases), which in turn lead to an inter-hemispheric SST seesaw. In the equilibrium state, a comma-shaped SST anomaly pattern develops in the extratropical region, with the largest amplitude over the subpolar region and an extension along the east side of the basin and into the subtropical North Atlantic. Based on our model experiments, we argue that the multidecadal AWP-induced freshwater flux can affect the AMOC, which plays a negative feedback role that acts to recover the AMOC after it is weakened or strengthened. The sensitivity of AMOC response to the AWP-induced freshwater forcing amplitude is also examined and discussed.

Cause of severe droughts in Southwest China during 1951–2010

The cause of severe droughts over the Southwest China (SWC) during the local dry season is investigated based on the station rainfall data and the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data during 1951–2010. The droughts are in general consistent with local anomalous descent in the middle troposphere. The diagnosis of the vertical motion (omega) equation indicates that the local descent is primarily maintained by the anomalous cold temperature advection processes. Both the advection of anomalous temperature by mean wind and the advection of mean temperature by anomalous wind contribute to maintaining the anomalous descent over the SWC region. A composite analysis shows that the circulation anomaly over SWC is induced by remote forcing from the tropical Pacific and North Atlantic Oceans. During La Nina years, enhanced heating over the Maritime Continent induces anomalous downward motion over SWC through the connection of local Hadley circulation. Adiabatic warming associated with the downward motion helps to set up and maintain the local anomalous anticyclone. Another possible route is through the North Atlantic-Asia teleconnection, in which downstream Rossby wave energy propagation plays a crucial role. A negative-phase North Atlantic Oscillation may trigger a large-scale wave train pattern that induces an anomalous anticyclone over the subtropical Asia and promotes the dry condition over SWC.

Hurricane Initiation: An Hypothesis

A hurricane initiation mechanism, believed to be new, is proposed for the eastern tropical North Atlantic Ocean. It starts with an outbreak of warm dry air from the Sahara Desert moving out over a fairly large region of ocean just west of the big bulge of Africa. Critical to the hypothesis is the experimental fact that heat diffuses significantly slower in air than water vapor does. In summer and early fall the desert air of the outbreak is warmer than the ocean surface it first encounters. Thus this air layer is cooled from below, which is initially stabilizing. However, water vapor diffuses up into the dry air faster than the air’s heat diffuses down to the sea surface, all over the generating region simultaneously. Consequently, a horizontally large layer of air somewhat above the sea surface becomes buoyant (less dense) and rises up as a unit, and the pressure of this layer decreases by the perfect gas law. Then the water vapor in the ascending air condenses around dust particles brought in from the desert, releasing heat and producing an additional upward acceleration of the already ascending air. Atmospheric pressure lowers further in accordance with Bernoulli’s law: where the (vertical) speed is greatest, the pressure is least. Measurements are suggested to validate the hypothesis if they do not already exist.

The pathways and properties of the Amazon River Plume in the tropical North Atlantic Ocean

The Amazon River Plume spreads across the tropical North Atlantic creating a barrier to vertical mixing. Here using a 1/6° HYCOM model and data from three research cruises in May–June 2010, September–October 2011, and July 2012, we investigate the pathways and properties of the plume. Four plume pathways for export of freshwater from the western tropical North Atlantic are identified. These consist of direct and indirect pathways to the northwest, and eastward pathways toward the subtropical gyre and toward Africa in the North Equatorial Counter Current. Because of the seasonality and cooccurrence of these pathways, plume characteristics are highly variable. Two pathways export water to the Caribbean, however the time scales associated with those direct and indirect pathways (3 versus 6+ months) differ, leading to different salinity characteristics of the plume water. Models results show that the Amazon river and tropical precipitation have similar magnitude impact on the observed seasonal cycle of freshwater within the western tropical Atlantic and at the 8°N, 38°W PIRATA mooring. Freshwater associated with the Amazon also influences surface salinity in winter as far as 20W in the model. The mean plume salinity minimum leads maximum discharge, highlighting the importance of currents and advection rather than discharge in maintaining plume properties. Plume pathways are tied to the underlying current structure, with the North Equatorial Counter Current jet preventing direct freshwater transport into the southern hemisphere. The plume influences underlying currents as well, generating vertical current shear that leads to enhanced eddy stirring and mixing in the model simulations.

Oxygen isotopes in tree rings record variation in precipitation δ18O and amount effects in the south of Mexico.

[1] Natural archives of oxygen isotopes in precipitation may be used to study changes in the hydrological cycle in the tropics, but their interpretation is not straightforward. We studied to which degree tree rings of Mimosa acantholoba from southern Mexico record variation in isotopic composition of precipitation and which climatic processes influence oxygen isotopes in tree rings (δ18Otr). Interannual variation in δ18Otr was highly synchronized between trees and closely related to isotopic composition of rain measured at San Salvador, 710 km to the southwest. Correlations with δ13C, growth, or local climate variables (temperature, cloud cover, vapor pressure deficit (VPD)) were relatively low, indicating weak plant physiological influences. Interannual variation in δ18Otr correlated negatively with local rainfall amount and intensity. Correlations with the amount of precipitation extended along a 1000 km long stretch of the Pacific Central American coast, probably as a result of organized storm systems uniformly affecting rainfall in the region and its isotope signal; episodic heavy precipitation events, of which some are related to cyclones, deposit strongly 18O-depleted rain in the region and seem to have affected the δ18Otr signal. Large-scale controls on the isotope signature include variation in sea surface temperatures of tropical north Atlantic and Pacific Ocean. In conclusion, we show that δ18Otr of M. acantholoba can be used as a proxy for source water δ18O and that interannual variation in δ18Oprec is caused by a regional amount effect. This contrasts with δ18O signatures at continental sites where cumulative rainout processes dominate and thus provide a proxy for precipitation integrated over a much larger scale. Our results confirm that processes influencing climate-isotope relations differ between sites located, e.g., in the western Amazon versus coastal Mexico, and that tree ring isotope records can help in disentangling the processes influencing precipitation δ18O.

Both soluble and colloidal iron phases control dissolved iron variability in the tropical North Atlantic Ocean

The size partitioning of dissolved iron (dFe, <0.4μm) into soluble (sFe, <0.02μm) and colloidal (0.02μm<cFe<0.4μm) phases was investigated at seven stations in the tropical North Atlantic Ocean, and the results are compared to the dFe size fractionation study of Bergquist et al. (2007) in the same region. Downwind of the North African dust plumes, cFe comprised 80±7% of the surface dFe pool at six stations, supporting the hypothesis that atmospherically-derived Fe is maintained in the colloidal size fraction. At the deep chlorophyll maximum, colloidal Fe had minimum concentrations or was completely absent, suggesting that cFe was either preferentially taken up by microbes and/or scavenged/aggregated at these depths. At remineralization depths, sFe was the dominant fraction both in the subtropical gyre-like stations (76% sFe; [sFe]=0.42±0.03nmol/kg) and in the oxygen minimum zone (56% sFe; [sFe]=0.65±0.03nmol/kg). Only at remineralization depths of stations with intermediate oxygen concentrations (100–110μmol/kg) did colloidal Fe dominate (contributing 58% of dFe) , indicating that cFe may be serving as a conduit of dFe loss during mixing of high-Fe OMZ and low-Fe gyre waters. North Atlantic Deep Water (NADW) had a typical sFe concentration of 0.34±0.05nmol/kg. In the deepest samples composed of a NADW/Antarctic Bottom Water mixture where the bottom water may have attained a ∼0.1nmol/kg hydrothermal Fe input during transit past the Mid-Atlantic Ridge, sFe did not increase coincidentally with dFe, indicating that any potential hydrothermal Fe contribution was colloidal. In general, the results of this study counter the previous hypothesis of Bergquist et al. (2007) that the colloidal Fe fraction predominately controls dFe variability, instead suggesting that both soluble and colloidal Fe are variable, and both contribute to the observed dFe variability throughout the North Atlantic. The nearly 50–50% dFe partitioning into soluble and colloidal phases below the DCM suggest one of two partitioning mechanisms persists: (1) soluble and colloidal Fe exchange rates reach a “steady state,” over which regional, uniquely-partitioned Fe sources can be overlain, or (2) the partitioning of Fe-binding ligands between the two size fractions is variable in the open ocean and directly controls dFe partitioning.

Pathways of Superoxide (O2–) Decay in the Eastern Tropical North Atlantic

Superoxide (O2(-): IUPAC name dioxide (•1-)) is an important transient reactive oxygen species (ROS) in the ocean formed as an intermediate in the redox transformation of oxygen (O2) into hydrogen peroxide (H2O2) and vice versa. This highly reactive and very short-lived radical anion can be produced both via photochemical and biological processes in the ocean. In this paper we examine the decomposition rate of O2(-) throughout the water column, using new data collected in the Eastern Tropical North Atlantic (ETNA) Ocean. For this approach we applied a semi factorial experimental design to identify and quantify the pathways of the major identified sinks in the ocean. In this work we occupied six stations, two on the West African continental shelf and four open ocean stations, including the CVOO time series site adjacent to Cape Verde. Our results indicate that, in the surface ocean impacted by Saharan aerosols and coastal sediment resuspension, the main decay pathways for superoxide are via reactions with Mn(II) and organic matter.

Impact of the marine atmospheric boundary layer conditions on VSLS abundances in the eastern tropical and subtropical North Atlantic Ocean

Abstract. During the DRIVE~(Diurnal and Regional Variability of Halogen Emissions) ship campaign we investigated the variability of the halogenated very short-lived substances (VSLS) bromoform (CHBr3), dibromomethane (CH2Br2) and methyl iodide (CH3I) in the marine atmospheric boundary layer in the eastern tropical and subtropical North Atlantic Ocean during May/June 2010. The highest VSLS mixing ratios were found near the Mauritanian coast and close to Lisbon (Portugal). With backward trajectories we identified predominantly air masses from the open North Atlantic with some coastal influence in the Mauritanian upwelling area, due to the prevailing NW winds. The maximum VSLS mixing ratios above the Mauritanian upwelling were 8.92 ppt for bromoform, 3.14 ppt for dibromomethane and 3.29 ppt for methyl iodide, with an observed maximum range of the daily mean up to 50% for bromoform, 26% for dibromomethane and 56% for methyl iodide. The influence of various meteorological parameters – such as wind, surface air pressure, surface air and surface water temperature, humidity and marine atmospheric boundary layer (MABL) height – on VSLS concentrations and fluxes was investigated. The strongest relationship was found between the MABL height and bromoform, dibromomethane and methyl iodide abundances. Lowest MABL heights above the Mauritanian upwelling area coincide with highest VSLS mixing ratios and vice versa above the open ocean. Significant high anti-correlations confirm this relationship for the whole cruise. We conclude that especially above oceanic upwelling systems, in addition to sea–air fluxes, MABL height variations can influence atmospheric VSLS mixing ratios, occasionally leading to elevated atmospheric abundances. This may add to the postulated missing VSLS sources in the Mauritanian upwelling region (Quack et al., 2007).

Dissolved iron in the tropical North Atlantic Ocean

Seawater from 27 stations was sampled in a zig-zag transect across the tropical North Atlantic from Barbados to the Cape Verde Islands, 7–20°N and 21–58°W. Under the Northwest African dust plume, surface dissolved Fe (dFe) was elevated, averaging 0.55±0.29nmol/kg (1SD), but showed a wide range from 0.12 to 1.31nmol/kg with no clear longitudinal or latitudinal gradients. A subsurface dFe maximum in excess of 1nmol/kg was evident coincident with the oxygen minimum zone. Our goal was to determine whether this maximum occurred as a result of remineralization of high Fe:C organic material formed in the dust-laden (Fe-rich) surface ocean or as a result of lateral mixing of a coastal Fe signal from the African margin. We found that dFe is directly related to apparent oxygen utilization (AOU) with the same slope across all depths sampled, which implies a remineralization source throughout the water column. A continental margin dFe supply was discounted because a modeled margin Fe source should cause curvature in the Fe–AOU relationship that was not observed. We also did not see a significant dissolved manganese (dMn) plume emanating from the margin. Regenerated Fe:C ratios calculated from the dFe:AOU relationships ranged from 9.6 to 12.4μmol Fe/mol C, which is much higher than ratios found in HNLC and dust-poor regions. Deep waters measured at one station in the eastern tropical North Atlantic reflected typical NADW concentrations until 4000m, where the dFe increased to 0.82nmol/kg in four discrete samples down to 5000m. We propose that this increase in dFe may reflect dFe inputs to deep waters as they traverse the Mid-Atlantic Ridge and enter the eastern basin.

Diversity and distribution of microbial eukaryotes in the deep tropical and subtropical North Atlantic Ocean

Employing a combination of 4',6-diamidino-2-phenylindole and fluorescein isothiocyanate (DAPI-FITC) staining and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH), we distinguished a variety of taxonomic and morphological types of eukaryotic microbes in the central and deep water masses of the tropical and subtropical North Atlantic Ocean. Samples were taken along a transect across the tropical Atlantic, along the equatorial upwelling and into the West-African upwelling region. Samples were collected as deep as 7000m in the Romanche Fracture Zone within the Mid-Atlantic Ridge. Approximately 50–70% of FISH-identified eukaryotes in deep water masses belong to one of seven groups: kinetoplastids, labyrinthulomycetes, fungi, diplonemids, group II alveolates, MAST 4 (stramenopiles), and an unidentified organism with a peculiar nuclear morphology. A smaller percentage of total eukaryotes was identified in the Central Water, especially in the oxygen minimum zone, than in deep water masses. CARD-FISH probes designed to identify broad taxonomic groups revealed kinetoplastids and fungi were more abundant than noted in previous studies employing 18S rRNA gene clone libraries. Group II alveolates, in contrast, were much less prevalent than previously reported. On a second survey, eukaryotic microbes were enumerated in the deep-sea basins below the North Atlantic subtropical gyre including the Vema Fracture Zone, which is another prominent trench in the Mid-Atlantic Ridge. The abundance of eukaryotes and chlorophyll concentrations were significantly different between the two cruises, which covered very different hydrographic regimes with associated high and low levels of primary production, respectively.

Climatological analysis of December rainfall in the Panama Canal Watershed

ABSTRACTMonthly accumulated precipitation over the tropical Panama Canal Watershed has the largest interannual variability in December, with the wettest month on record being December 2010. December accumulated precipitation over the watershed is found to vary with sea surface temperature (SST) anomalies in both the tropical North Atlantic and equatorial Pacific oceans, but a considerably stronger relationship is found with the difference between these SST anomalies. The configuration of SST anomalies in these two ocean basins during December has a strong effect on the flux of low‐level moisture over the Caribbean Sea, southern Central America, and the eastern Pacific Ocean through the modification of the Caribbean Low‐Level Jet (CLLJ) and the Chorro del Occidente Colombiano (CHOCO) jet. Wet Decembers in the watershed are associated with cool (warm) SST anomalies in the tropical Pacific (North Atlantic), a weakening (strengthening) of the CLLJ (CHOCO jet), and increased moisture convergence over and around Panama, with the opposite conditions associated with dry Decembers. The SST anomalies in these two ocean basins affect the distribution of daily precipitation during December differently, with SST anomalies in the Pacific (Atlantic) predominately associated with changes in the frequency of heavier (lighter) precipitation. The analysis shows that December 2010 followed the pattern of associations of wet Decembers but with the highest intensity of precipitation and CHOCO jet strength.

Stable isotope ratio measurements of Cu and Zn in mineral dust (bulk and size fractions) from the Taklimakan Desert and the Sahel and in aerosols from the eastern tropical North Atlantic Ocean

Accurate characterization of the stable isotope composition of Cu and Zn in major global mineral dust sources and in aerosols is central to the application of these isotope systems to the studies of global geochemical processes and cycles. We test here for the first time Cu and Zn isotope ratios within a well-defined source-receptor setting on the continent–ocean interface and determine the isotope composition of (i) bulk surface soil dust samples from the Sahel region, (ii) individual size fractions of surface dust samples from the Taklimakan Desert, and (iii) aerosols collected in the equatorial eastern North Atlantic region. This is achieved reducing the blank contribution during the ion exchange step using small resin and acid volumes and using a second ion exchange passage to purify the Cu fraction. We find no significant correlation between size fractions and isotope ratios in the two samples analyzed from the Taklimakan Desert. Mass balance calculations suggest that isotope ratios of bulk samples are within the analytical precision of the <4μm size fraction and can be used to characterize atmospheric long range transport of Cu and Zn in mineral dust from the Taklimakan Desert. The <1µm size fractions of two aerosol samples collected over the equatorial eastern North Atlantic region have Cu and Zn isotope ratios that are different to Sahel surface soil dust suggesting important non-crustal sources, in line with calculated enrichment factors, and possibly of anthropogenic origin. Using previously reported δ66Zn values for anthropogenic emission from Europe, preliminary calculations suggest that up to 55% of Zn arriving at the sampling points in the equatorial eastern North Atlantic region could be of anthropogenic origin.

The distribution of thiamin and pyridoxine in the western tropical North Atlantic Amazon River plume

B-vitamins are recognized as essential organic growth factors for many organisms, although little is known about their abundance and distribution in marine ecosystems. Despite their metabolic functions regulating important enzymatic reactions, the methodology to directly measure different B-vitamins in aquatic environments has only recently been developed. Here, we present the first direct measurements of two B-vitamins, thiamin (B1), and pyridoxine (B6), in the Amazon River plume-influenced western tropical North Atlantic (WTNA) Ocean, an area known to have high productivity, carbon (C) and dinitrogen (N2) fixation, and C sequestration. The vitamins B1 and B6 ranged in concentrations from undetectable to 230 and 40 pM, respectively. Significantly higher concentrations were measured in the surface plume water at some stations and variation with salinity was observed, suggesting a possible riverine influence on those B-vitamins. The influences of vitamins B1 and B6 on biogeochemical processes such as C and N2 fixation were investigated using a linear regression model that indicated the availability of those organic factors could affect these rates in the WTNA. In fact, significant increases in C fixation and N2 fixation were observed with increasing vitamin B1 concentrations at some low and mesohaline stations (stations 9.1 and 1; p value <0.017 and <0.03, respectively). N2 fixation was also found to have a significant positive correlation with B1 concentrations at station 1 (p value 0.029), as well as vitamin B6 at station 9.1 (p value <0.017). This work suggests that there can be a dynamic interplay between essential biogeochemical rates (C and N2 fixation) and B-vitamins, drawing attention to potential roles of B-vitamins in ecosystem dynamics, community structure, and global biogeochemistry.

Diazotrophic bacteria respond to Saharan dust additions

Three bioassay experiments were performed to study the effects of nutrient and Saharan dust additions on natural diazotrophic communities in the tropical North Atlantic Ocean. Samples for nucleic acid analysis were collected at the beginning and end of 48 h incubations. TaqMan probes specific to 7 diazotrophic phylotypes, viz. filamentous cyanobacteria (Trichodesmium spp.), unicellular cyanobacterial (UCYN) Groups A, B, and C, Gamma A and P Proteobacteria, and Cluster III, were used to quantify nifH DNA abundances. N-2 fixation rates were measured in the same experiments using the N-15(2) gas bubble injection method. N-2 fixation was co-limited by P and Fe. Total nifH abundances increased relative to the control with additions of either Fe or P or both in combination. Additions of dissolved N, alone or in combination with phosphate, induced increases in UCYN-A and Gamma A nifH compared with the control. Saharan dust additions significantly stimulated fixation rates. Abundances of all cyanobacterial and Gamma A nifH phylotypes at least doubled after Saharan dust additions where surface water dissolved Fe concentrations were <2 nmol l(-1). Laboratory experiments with cultures of T. erythraeum demonstrated that dust addition promoted colony formation and the persistence of T. erythraeum biomass relative to cultures to which no Fe was added. Our results with both field and laboratory experiments indicate that Saharan dust positively affects diazotrophic phylotype abundances and changes T. erythraeum colony morphology.

Fluxes and distribution of dissolved iron in the eastern (sub‐) tropical North Atlantic Ocean

Aeolian dust transport from the Saharan/Sahel desert regions is considered the dominant external input of iron (Fe) to the surface waters of the eastern (sub‐) tropical North Atlantic Ocean. To test this hypothesis, we investigated the sources of dissolved Fe (DFe) and quantified DFe fluxes to the surface ocean in this region. In winter 2008, surface water DFe concentrations varied between &lt;0.1 nM and 0.37 nM, with an average of 0.13 ± 0.07 nM DFe (n = 194). A strong correlation between mixed layer averaged concentrations of dissolved aluminum (DAl), a proxy for dust input, and DFe indicated dust as a source of DFe to the surface ocean. The importance of Aeolian nutrient input was further confirmed by an increase of 0.1 nM DFe and 0.05 μM phosphate during a repeat transect before and after a dust event. An exponential decrease of DFe with increasing distance from the African continent, suggested that continental shelf waters were a source of DFe to the northern part of our study area. Relatively high Fe:C ratios of up to 3 × 10−5 (C derived from apparent oxygen utilization (AOU)) indicated an external source of Fe to these African continental shelf waters. Below the wind mixed layer along 12°N, enhanced DFe concentrations (&gt;1.5 nM) correlated positively with apparent oxygen utilization (AOU) and showed the importance of organic matter remineralization as an DFe source. As a consequence, vertical diffusive mixing formed an important Fe flux to the surface ocean in this region, even surpassing that of a major dust event.

Production of oceanic nitrous oxide by ammonia-oxidizing archaea

Abstract. The recent finding that microbial ammonia oxidation in the ocean is performed by archaea to a greater extent than by bacteria has drastically changed the view on oceanic nitrification. The numerical dominance of archaeal ammonia-oxidizers (AOA) over their bacterial counterparts (AOB) in large parts of the ocean leads to the hypothesis that AOA rather than AOB could be the key organisms for the oceanic production of the strong greenhouse gas nitrous oxide (N2O) that occurs as a by-product of nitrification. Very recently, enrichment cultures of marine ammonia-oxidizing archaea have been reported to produce N2O. Here, we demonstrate that archaeal ammonia monooxygenase genes (amoA) were detectable throughout the water column of the eastern tropical North Atlantic (ETNA) and eastern tropical South Pacific (ETSP) Oceans. Particularly in the ETNA, comparable patterns of abundance and expression of archaeal amoA genes and N2O co-occurred in the oxygen minimum, whereas the abundances of bacterial amoA genes were negligible. Moreover, selective inhibition of archaea in seawater incubations from the ETNA decreased the N2O production significantly. In studies with the only cultivated marine archaeal ammonia-oxidizer Nitrosopumilus maritimus SCM1, we provide the first direct evidence for N2O production in a pure culture of AOA, excluding the involvement of other microorganisms as possibly present in enrichments. N. maritimus showed high N2O production rates under low oxygen concentrations comparable to concentrations existing in the oxycline of the ETNA, whereas the N2O production from two AOB cultures was comparably low under similar conditions. Based on our findings, we hypothesize that the production of N2O in tropical ocean areas results mainly from archaeal nitrification and will be affected by the predicted decrease in dissolved oxygen in the ocean.

The distinct nitrogen isotopic compositions of low and high molecular weight marine DON

To constrain the sources and cycling of bulk and size-fractionated marine dissolved organic nitrogen (DON), samples were collected for concentration and isotopic (δ15N) analysis from the western tropical North Atlantic Ocean as well as over the North Australian shelf. Bulk DON concentrations are typically between 4 and 6μM, with low molecular weight (LMW, i.e., <1000Da) DON accounting for 60 to 70% of total DON. The δ15N of both bulk and high molecular weight (HMW, i.e., >1000Da to <0.2μm) DON from the western tropical North Atlantic is similar to previous measurements, as well as to bulk and HMW DON collected off the North Australian Shelf, ~4‰. Here we report the first measurement of marine LMW DON δ15N, and a coherent pattern emerges where LMW DON δ15N<HMW DON δ15N. An analytical concern is that the bulk DON δ15N as calculated from our coupled HMW and LMW measurements is often lower than that measured directly for the bulk DON. Despite this discrepancy, the self-consistency of the data in other regards argues for the robustness of the basic observation of lower δ15N in LMW relative to HMW DON. One explanation for this isotopic difference is considered most likely, based on the model that DON δ15N elevation relative to PON is fundamentally due to DON production from PON without isotopic fractionation, coupled with DON destruction occurring with fractionation. In this model, HMW DON loss must occur predominantly through the breakage of N-containing bonds, for which isotope fractionation should be substantial, whereas LMW DON loss is partly due to direct assimilation by phytoplankton and other microbes, which appears to occur with only minor isotopic fractionation. However, an alternative hypothesis exists: LMW and HMW DON may have distinct sources, for example, prokaryotic and eukaryotic plankton, respectively. In either case, it appears that most LMW DON has not passed through the HMW DON pool, as this would require an unrealistically large isotope fractionation at the HMW to LMW conversion. Finally, neither the concentration nor the δ15N of the bulk or either size fraction of DON varies with in situ N2 fixation rates measured at the time of sample collection. However, the δ15N of both bulk and LMW DON from the western tropical North Atlantic shows significant differences between samples collected six months apart, which may be due to seasonally variable stimulation of primary production by the Amazon River.

Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean

Abstract. Sea-to-air and diapycnal fluxes of nitrous oxide (N2O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Sea-to-air fluxes as well as diapycnal fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N2O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N2O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-to-air and diapycnal N2O fluxes indicated that the mean sea-to-air flux is about three to four times larger than the diapycnal flux. Neither vertical and horizontal advection nor biological production were found sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. From our observations we conclude that common parameterizations for the gas transfer velocity likely overestimate the air-sea gas exchange within highly productive upwelling zones.

Erratum: Corrigendum: The Amazon basin in transition

Nature 481, 321–328 (2012) In the ‘Natural and anthropogenic climatic variation’ section of this Review, we incorrectly referred to the North Atlantic Oscillation as a contributor to the 2005 Amazonian droughts. We should instead have referred to the Atlantic Multidecadal Oscillation, which is an oceanic phenomenon related to anomalies in sea surface temperature in the tropical north Atlantic Ocean and can affect drought in the Amazon Basin.