H. J. W. de Baar

Dissolved iron at subnanomolar levels in the Southern Ocean as determined by ship-board analysis

The biogeochemistry of iron was investigated in remote waters of the Antarctic ocean in the austral summer of 1995/1996. A sensitive flow injection analyser, based on in-line preconcentration and luminol chemiluminescence detection (FIA-CL), was used for underway surface measurements and vertical profiling during a fine scale survey in the Polar Front (PF) in the Atlantic sector of the Southern Ocean. Results indicated a dynamic environment with dissolved iron concentrations ranging from 0.05 to 0.3 nM. Distributions are consistent with chlorophyll a and fCO2 suggesting a coupling with these parameters. The highest iron concentrations (0.3 nM) were found where chlorophyll a abundance was highest and CO2 undersaturation the most pronounced. The vertical distribution (upper 500 m) revealed a nutrient type distribution with low subsurface values (0.1 nM) gradually increasing to0.3 nM at depth. # 1998 Elsevier Science B.V. All rights reserved.

Deep dissolved iron profiles in the eastern North Atlantic in relation to water masses

Concentrations of dissolved iron (DFe, 0.2μm) were determined at two stations in the Biscay Abyssal Plain (North East Atlantic) in March 2002. DFe concentrations in the surface layer (0.23–0.34 nM) were typical of winter conditions in this area. At 1000 m, DFe concentrations increased to 0.62–0.86 nM. This feature is consistent with the production of DFe by remineralization of the biogenic material. However, at this depth, Mediterranean Outflow Water (MOW) could be an additional source of DFe. Below 2500 m, DFe concentrations were constant (0.75 ± 0.04 nM). An interesting feature of the profiles was the intermediate maximum of DFe (1.19–1.12 nM) around 2000 m, associated with the Labrador Sea Water (LSW). We suggest that the iron enrichment of LSW occurred when this water mass reached the continental margin, likely in the vicinity of the Goban plateau. Vertical distributions were highly dependent on water masses encountered.

Responses of seabirds, in particular prions (Pachyptila sp.), to small-scale processes in the Antarctic Polar Front

Small-scale distribution patterns of seabirds in the Antarctic Polar Front (APF) were investigated in relation to other biological, physical, and chemical features during the ANT-XIII/2 research cruise of R.V. Polarstern from December 1995 to January 1996. The APF is characterized by steep gradients in sea-surface temperature and salinity. Within the APF, gradient zones were closely associated with elevated levels of primary production, chlorophyll-a (chl-a) concentrations, and zooplankton densities. Even broad-billed prions (‘Pachyptila vittata-group’), which dominated the seabird community by 83% in carbon requirements, showed small-scale distributional patterns that were positively related to primary production, chl-a, and total zooplankton densities. The findings demonstrate a close, direct link between fine-scale physical processes in the APF and biological activity through several food web levels up to that of zooplankton-eating seabirds. Broad-billed prions appeared to forage on very small copepods (Oithona spp.) in close association with the front. Fish- and squid-eating predators showed poor correlations with small-scale spatial structures of the APF. However, in a wider band around the APF, most top predators did occur in elevated densities, showing gradual spatio-temporal diffusion of the impact of the APF on higher trophic levels.

Sea-trials of three different methods for measuring non-volatile dissolved organic carbon in seawater during the JGOFS North Atlantic pilot study

Abstract In 1989 the shipboard comparison of three different methods for measuring concentrations of dissolved organic carbon (DOC) resulted in reasonable agreement, with differences likely as a result of a combination of calibration offsets and general instability of the instruments at sea. Typical concentrations were about 80–140 μmol in the upper 1000 m of the water column, with a tendency towards higher values (100–150 μmol) in the surface waters. These elevated surface water levels are below the 200–300 μmol values recently reported for the Atlantic and the Pacific Oceans. In 1990 the small dataset obtained with a further modified High Temperature Combustion instrument showed similar trends. Within the analytical error there is no significant offset between two HTCO methods and one wet oxidation technique.

Trace metals in the oceans: evolution, biology and global change

All living organisms require several essential trace metal elements. During biological evolution of prokaryotes and later on also eukaryotes several metals became incorporated as essential factors in many biochemical functions more or less in accordance with the abundance of these metals on the planet. As a result the biological importance of first row transition metals can be ranked roughly in the order Fe, Zn, Cu, Mn, Co, Ni. The second row metals Ag and Cd or third row metals like Hg and Pb appear to have no biological function, except possibly for Cd. Iron (Fe) being the fourth most abundant element in the crust of the planet has also played a role to temper the evolution of biogenic oxygen (O2) in the atmosphere and oceans. Yet eventually O2 has taken over the biosphere where now both atmosphere and ocean are strongly oxidizing. Inside every living cell the primordial reducing conditions have remained however. Therefore enzyme systems based on metal couples Fe-Mn and Cu-Zn are required to protect the cell interior from damage by reactive oxygen species. The key role of metals in these and many other enzymes as well as in protein folding is one of the major vectors in biological diversity at both the molecular and the species level. Abundance and biological role of metals in the oceans are being discovered only since 1976 and many questions remain. Until now many metals appear to be tightly linked with the large scale biological cycling. Recently the significance of very fine colloids as well as dissolved organic metal-complexes has been shown. Plankton ecosystems now appear to be governed by co-limitation of several essential metals, where the biological fractionation of their stable isotopes is expected to give rise to significant shifts in isotopic ratio for any given metal element. Co-limitation of plankton growth is consistent also with observed interactions between metals. Firstly substitution of for example Co or Cd for Zn is known for some but not all phytoplankton taxa. Secondly the cellular uptake of one metal, e.g. Cd, may respond to a complex matrix of other metals like Zn, Mn and possibly Fe. By mining and industrial use of metals, land use change and irrigation, mankind has greatly modified the abundances and mutual ratio’s of metals in the biosphere. Biota can to some extent resist the ensuing external stress through cellular homeostasis. However at highly elevated levels or excessive metal ratio’s several biological species cannot longer exist and major shifts of ecosystems and their diversity do occur. Likely such changes have already taken place for centuries and as such have largely gone unnoticed. The past decade was the onset of the iron age in oceanography. Nowadays Fe is known to be a severely limiting element in some 40 % of the world oceans. This limitation is sometimes relieved by aeolian supply of continental dust, but most Fe supply is actually from below emanating of reducing sediments. With adequate Fe there is a systematic response by the class of very large oceanic diatoms, their massive blooms then giving rise to uptake of CO2 and export of both opal (SiO2) and organic matter into the deep-sea. Hence the supply of Fe to ocean waters is one of the major controls of plankton ecosystems and ocean element cycling (C, Si, N, P) over time scales ranging from weeks to the 100,000 year periodicity of glaciations. Understanding the cycling and biological function of metals in the oceans is a prerequisite for understanding the role of the oceans in global change of past, present and future.

Biological versus physical processes as drivers of large oscillations of the air-sea CO2 flux in the Antarctic marginal ice zone during summer

The fugacity of CO2 andabund ance of chlorophyll a (Chla) were determined in two long transects from the Polar Front to the Antarctic Continent in austral summer, December 1995–January 1996. Large undersaturations of CO2 in the surface water were observedcoinciding with high Chl a content. In the major hydrographic regions the mean air–sea fluxes were foundto range from � 3 to +7 mmol m � 2 d � 1 making these regions act as a sink as well as a source for CO2. In the total 40-d period, the summation of the several strong source and sink regions revealed an overall modest net source of 0.3 mmol m � 2 d � 1 , this basedon the Wanninkhof (J. Geophys. Res. 97 (1992) 7373) quadratic relationship at in situ windspeed. A simple budget approach was used to quantify the role of phytoplankton blooms in the inorganic carbonate system of the Antarctic seas in a time frame spanning several weeks. The major controlling physical factors such as air–sea flux, Ekman pumping and upwelling are included. Net community production varies between � 9 and +7 mmol m � 2 d � 1 , because of the large oscillations in the dominance of autotrophic (CO2 fixation) versus heterotrophic (CO2 respiration) activity. Here the mixedlayer d epth is the major controlling factor. When integratedover time the gross influx andefflux of CO 2 from air to sea is large, but the net residual air/sea exchange is a modest efflux from sea to atmosphere.

Enrichment of silicate and CO2 and circulation of the bottom water in the Weddell Sea

Deep and bottom water from the Enderby Basin, which is strongly enriched in silicate, enters the Weddell Sea off Kapp Norvegia parallel to the coast. However, the bottom water in this region originates from the northern Weddell Sea, indicating a southward return flow of bottom water west of the prime meridian. The eastern Weddell Sea margin was identified as the place where a significant silicate enrichment (at least 15 μmol kg-1) and a weak CO2 enrichment of the bottom water occurs, related to a regional recirculation cell. The deep and bottom water continue their course through the Weddell Sea along the base of the continental slope, where further to the west they are underridden by a thin layer of new, silicate-poor bottom water. A silicate maximum and weak TCO2 maximum are formed at the interface between deep and bottom water at approximately 4000 m. This silicate maximum occurs in the central Weddell Sea as well. This indicates an exchange of the deep water between the boundaries and the interior of the Weddell basin; the northwestern Weddell Sea was identified as an important site for this. Bottom layer enrichment of CO2 in the central Weddell Sea (3 μmol kg-1) is comparable to that in the eastern Weddell Sea, but silicate enrichment in the former is much less than in the latter. The extent of bottom layer enrichment suggests that about 2% of the primary production reaches the seafloor, supporting the view that the biological pump mechanism in this area is effectively transporting downward a significant amount of CO2.

Radium isotopes as a tracer of sediment-water column exchange in the North Sea

Sediment-water column exchange plays an important role in coastal biogeochemistry. We utilize short-lived radium isotopes (224Ra and 223Ra) to understand and quantify the dominant processes governing sediment-water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats and North Sea sediments as the dominant source. Pore waters show 100-fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment-water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation-based Ra flux estimates and the majority of previously reported Ra flux estimates for other regions. Ra-based estimates of benthic alkalinity fluxes compare well to observed values, and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.

Fe-Binding Dissolved Organic Ligands in the Oxic and Suboxic Waters of the Black Sea

In the oxygen-rich layer of the Black Sea, above the permanent halocline, the Fe and nitrate concentrations are low where fluorescence is relatively high , indicating uptake by phytoplankton. In this study we used ligand exchange adsorptive cathodic stripping voltammetry (CLE-aCSV), using 2-(2-Thiazolylazo)-p-cresol (TAC) as measuring ligand, to investigate the role of Fe-binding dissolved organic ligands in keeping Fe in the dissolved phase and potentially biologically available. The conditional stability constant, logK´, was between 21 and 22 in most samples, which is on average lower than in ocean water. The Fe-binding dissolved organic ligand concentrations varied between 0.35 and 4.81 nEq of M Fe, which was higher than the dissolved concentration of Fe (DFe) as found in most samples. At two stations ligands were saturated in the surface. At one station ligands were saturated near the oxycline, where ligand concentrations seemed to increase, indicating that they play a role in keeping Fe in the dissolved phase across the redox gradient. At the fluorescence maximum (between 40 and 50 m), the dissolved organic ligand binding capacity (alphaFeL=K´*[L´]) of Fe was at its highest while the concentration DFe was at its lowest. Here, we find a statistically significant, positive relationship between fluorescence and the logarithm of alphaFeL, along with fluorescence and the ratio of the total ligand concentration over DFe. These relationships are best explained by phytoplankton utilizing Fe from Fe-binding organic ligands, resulting in an increase in free Fe-binding ligands.

The contribution of ocean-leaving DMS to the global atmospheric burdens of DMS, MSA, SO2, and NSS SO4=: DMS, MSA, SO2, NSS SO4=BURDENS OF OCEANIC DMS ORIGIN

[1] The contribution of ocean-derived DMS to the atmospheric burdens of a variety of sulphur compounds (DMS, MSA, SO2, and nss SO4=) is quantified from season to season. Such quantification, especially for nss SO4= (the climate-relevant product of DMS oxidation), is essential for the quantification of the radiative forcing of climate that may be attributable to marine phytoplankton under possible future climate conditions. Three-dimensional chemical transport modeling up to the stratosphere is used as a tool in realizing this aim. Global data sets on oceanic and terrestrial sulphur sources are used as input. We find that the contribution of ocean-leaving DMS to the global annually averaged column burdens of the modeled compounds is considerable: 11.9 mumol m(-2) (98% of total global burden) for DMS; 0.95 mumol m(-2) (94% of total global burden) for MSA; 2.8 mumol m(-2) (32% of total global burden) for SO2 and 2.5 mumol m(-2) (18% of total global burden) for nss SO4=. The mean annual contribution of DMS to the climate-relevant nss SO4= column burden is greatest in the relatively pristine Southern Hemisphere, where it is estimated at 43%. This contribution is only 9% in the Northern Hemisphere, where anthropogenic sulphur sources are overwhelming. The marine algal-derived input of the other modeled sulphur compounds ( DMS, MSA, and SO2) is also greatest in the Southern Hemisphere where a lower oxidative capacity of the atmosphere, a larger sea-to-air transfer of DMS and a larger emission surface area lead to an elevation of the atmospheric DMS burden.