3He/4He and 4He/20Ne ratios in pore water and gas in Pacific deep-sea sediments

We have measured the 3He/3He and 3He/20 Ne ratios of thirty-nine pore water and gas samples in deep-sea sediments collected at twelve sites on the Pacific Ocean bottom during the cruises of Deep Sea Drilling Project Legs 87, 89, 90 and 92. The 3He/4He and 4He/20Ne ratios vary from 0.000000215 to 0.00000165 and from 0.29 to 20, respectively. He in the sample is composed of four components: (1) atmospheric He dissolved in seawater; (2) atmospheric He with mantle-derived He in Pacific bottom water; (3) in situ radiogenic He in the sediment; and (4) crustal He in the basement rock. Assuming that the 20Ne contents are constant with the value of seawater, the depth variations in the 4He/20Ne ratios at five Sites, 583D, 594, 597A, 598A and 504B, may provide useful information on 4He flux at the ocean bottom. The estimated 4He fluxes vary from 2000 to 40000 atoms cm**-2 s**-1 and are one to three orders of magnitude less than those calculated from the excess He in deep ocean water. An overall similarity between the geographical distribution of the 3He/4He ratios and heat flow data is found in the study area, between the East Pacific Rise across the Pacific Ocean and the Japanese Islands. The tendency is well explained by a conventional sea-floor spreading model.

Calcium carbonate, organic carbon, d13C and C/N composition of ODP Sites 186-1150 and 186-1151

Past changes in sea-surface productivity in the Oyashio Current are evaluated on the basis of abundances of biological constituents in sediments from Leg 186 sites. Organic carbon contents at Sites 1150 and 1151 are moderate (0.5 to 1.5 wt%) and have an algal origin as indicated by low C/N ratios (<10) and by carbon isotopic compositions ranging from -23.4 to -21.3. A decreasing trend in organic carbon contents, carbon isotope ratios, and C/N ratios occurs with depth at both sites, probably as a consequence of diagenetic degradation of organic matter. Mass accumulation rates (MARs) determined for organic carbon and carbonates at Sites 1150 and 1151 show an abrupt increase between ~5 and 7 Ma. Similar results have been reported for sites in the Indian Ocean and the Pacific Ocean for the same time interval. As it has been previously suggested, the observed increase in MAR for both carbonate and organic carbon at Leg 186 sites probably resulted from augmented nutrient supply either from continental sources or from a more vigorous ocean circulation.

Seal census raw data during POLARSTERN expedition ANT-XV/3 (PS48)

The density, species composition, and possible change in the status of pack ice seals within the Weddell Sea were investigated during the 1997/1998 summer cruise of the RV "Polarstern" (ANT-XV/3, PS48). Comparisons were made with previous surveys in the Weddell Sea where it was assumed that all seals were counted in a narrow strip on either side oft he ship or aircraft. A total of 15 aerial censuses were flown during the period 23 January - 7 March 1998 in the area bounded by 07°08' and 45°33' West longitude. The censused area in the eastern Weddell Sea was largely devoid of pack ice while a well circumscribed pack ice field remained in the western Weddell Sea. A total of 3,636 (95.4 %) crabeater seals, 21 (0.5 %) Ross seals, 45 (1.2 %) leopard seals and 111 (2.9 %) Weddell seals were observed on the pack ice during a total of 1,356.57 linear nautical miles (244.2 nm) of transect line censused. At a mean density of 21.16 1/nm**2 over an area of 244.2 nm, it is the highest densities on record for crabeater seals, density of up to 411.7 1/nm**2 being found in small areas. The overall high densities of seals (30.18 1/nm**2) recorded for the eastern Weddell Sea (27.46 1/nm**2, 0.27 1/nm**2, and 0.66 1/nm**2 for crabeater, leopard and Weddell seals respectively) is a consequence of the drastically reduced ice cover and the inverse relationship that exists between cover and seal densities. Ross seal densities (0.08 1/nm**2) were the lowest on record fort the area. It is suggested that seals largely remain within the confines of the pack ice despite seasonal and annual changes in its distribution. Indications are that in 1998 the El Niño has manifested itself in the Weddell Sea, markedly influencing the density and distribution of pack ice seals.

Mercury species concentrations near Casey station, Antarctica and along the SR3 CASO-GEOTRACES transect

We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolution south-to-north section (27 stations from 65.50°S to 44.00°S) with up to 15 depths (0-4440 m) between Antarctica and Tasmania (Australia) along the 140°E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a study of mercury speciation in the 'snow-sea ice-seawater' continuum was conducted at a coastal site, near the Australian Casey station (66.40°S; 101.14°E). In the open ocean waters, total Hg (Hg(T)) concentrations varied from 0.63 to 2.76 pmol/L with 'transient-type' vertical profiles and a latitudinal distribution suggesting an atmospheric mercury source south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightly higher mean Hg(T) concentrations (1.35 ± 0.39 pmol/L) were measured in Antarctic Bottom Water (AABW) compared to Antarctic Intermediate water (AAIW) (1.15 ± 0.22 pmol/L). Labile Hg (Hg(R)) concentrations varied from 0.01 to 2.28 pmol/L, with a distribution showing that the Hg(T) enrichment south of the SPF consisted mainly of Hg(R) (67 ± 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 ± 23%). Methylated mercury species (MeHg(T)) concentrations ranged from 0.02 to 0.86 pmol/L. All vertical MeHg(T) profiles exhibited roughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depth up to an intermediate depth maximum. As for Hg(T), low mean MeHg(T) concentrations were associated with AAIW, and higher ones with AABW. The maximum of MeHg(T) concentration at each station was systematically observed within the oxygen minimum zone, with a statistically significant MeHg(T) vs Apparent Oxygen Utilization (AOU) relationship (p <0.001). The proportion of Hg(T) as methylated species was lower than 5% in the surface waters, around 50% in deep waters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station Hg(T) and Hg(R) concentrations found in the 'snow-sea ice-seawater' continuum were one order of magnitude higher than those measured in open ocean waters. The distribution of Hg(T) there suggests an atmospheric Hg deposition with snow and a fractionation process during sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurring with the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHg(T) concentrations in the sea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol/L. The MeHg(T) vertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basal ice. Whereas Hg(T) concentrations in the water samples collected between the Antarctic continent and Tasmania are comparable to recent measurements made in the other parts of the World Ocean (e.g., Soerensen et al., 2010; doi:10.1021/es903839n), the Hg species distribution suggests distinct features in the Southern Ocean Hg cycle: (i) a net atmospheric Hg deposition on surface water near the ice edge, (ii) the Hg enrichment in brine during sea ice formation, and (iii) a net methylation of Hg south of the SPF.

Downhole variations in grain sizes at DSDP/ODP Hole 504B

The maximum grain sizes of plagioclase and magnetite in the groundmass of the sheeted dike complex drilled at Hole 504B have been measured. Downhole variations through a 440-m-long section show a crude zig-zag pattern consisting of a gradual decrease or increase followed by an abrupt jump. The gradual decrease or increase in grain size extends over many lithologic units, and hence, does not reflect variations in grain size within a single dike. Such a zig-zag pattern is well explained by grain-size variations through multiple dikes. By using the observed inclination of sheeted dikes of 81° ± 2.5°, thickness of the multiple dikes varies from 0.7 to 8.5 m and averages to 4 ± 1 m. The average thickness of individual dikes forming multiple dikes is 0.8 m. We expect such multiple dikes to be formed during rifting events beneath mid-oceanic spreading ridges. If the average expansion at rifting episodes is twice as wide as the average width of the multiple dike units, the full spreading rate of 7.2 cm/yr of Cocos Ridge gives 112 ± 33 yr for a time interval of the rifting. A simple one-dimensional conductive cooling model is applied to solidification of multiple dikes. Numerical simulations show that the grain-size variations observed through the drill hole are more consistent with a model where a new injection of a dike occurs periodically with a constant time interval rather than one where the next dike intrudes just after the solidification of the previous one. Grain-size variations within simple dikes from Iritono, Japan, and those for Makaopuhi lava lake, Hawaii, show that square root of crystallization time is linearly correlated with the logarithm of plagioclase size. By using an empirically derived relationship between these two variables, the variations of plagioclase size through Hole 504B are directly compared with the calculated times for crystallization. Each rifting episode at the Costa Rica Rift lasts for several years, and periodic injection of a new dike occurs into the center of a previously solidified multiple dike at time intervals varying from 1 to 12 months.

Hydrogen isotope composition of pore waters and interlayer water in sediments of ODP Leg 129 holes

Hydrogen isotope compositions have been measured on pore waters from sediments of Leg 129 sites in the Pigafetta and East Mariana basins (central western Pacific). Total water (pore + sorbed waters) contents and their dD have been analyzed for three samples that contain smectite but no zeolite so that sorbed water can be attributed to interlayer water. The H budget for pore and total waters implies that interlayer water is 20 per mil to 30 per mil depleted in D compared to pore water. Because the interlayer/total water molar ratio (0.25 to 0.5) in smectitic sediments is very high, interlayer water represents an important reservoir of D-depleted water in sediments. dD depth profiles for pore water at Sites 800 and 801 show breaks related to chert and radiolarite layers and are relatively vertical below. Above these chert units, pore waters are similar to modern seawater but below, they are between -10 per mil and -5.5 per mil. These values could represent little modified pre-Miocene seawater values, which were D-depleted because of the absence of polar caps, and were preserved from diffusive exchange with modern seawater by the relatively impermeable overlying chert layers. At Site 802, dD values of the pore waters show a decrease in the Miocene tuffs from 0 per mil values at the top to -8 per mil at 250 mbsf. Below, dD values are relatively uniform at about -8ë. Miocene tuffs are undergoing low water/rock alteration. A positive covariation of dD and Cl content of pore water in the tuffs suggests that the increase of dD values could result from secondary smectite formation. Low diffusive exchange coupled with D enrichment due to alteration of preglacial waters could explain the observed profile.

Geochemistry of ODP Leg 115 basalts

Melting-phase relations at high pressures and Sr-Nd isotopic compositions are reported for basalts collected from the western Indian Ocean during Ocean Drilling Program Leg 115. Based on the concentrations of high-field-strength elements, we have subdivided the basalts into eight groups. A tholeiitic primary magma estimated using an olivine maximum fractionation model is representative of depleted lavas. This melt is in equilibrium with lherzolite minerals at 1.3 GPa and 1330°C under dry conditions. Also, an alkaline primary magma, representative of enriched lavas, is not saturated with orthopyroxene under dry conditions, but it is saturated with lherzolite minerals under CO2-saturated conditions at 1.7 GPa and 1350°C. These results imply that the tholeiitic magmas were segregated from mantle diapirs at shallower levels than the alkaline magmas. The highest 143Nd/144Nd value is obtained for the most depleted tholeiitic basalts, and the lowest value corresponds to the enriched alkaline basalt. The Sr isotopes of the basalts range from 0.70378 to 0.70449 and are inversely correlated with the Nd isotopic values. The present experimental and geochemical data suggest that depleted mantle material is underlain by the enriched material in the upper mantle beneath the region.

Concentration of organic compounds in aerosols and surface waters of the East Atlantic and Antarctic

The data on content and composition of lipids and aliphatic hydrocarbons (HC) in aerosols and surface waters obtained during the spring-summer periods of 2001 and 2003 along the vessel route from the North Sea to the Antarctic and backwards are presented. It was shown that the distribution of organic compounds is caused by influence of zonal supply of eolian matter from land, anthropogenic, and marine autochtonous sources. Concentrations of organic compounds in the aerosols varied from 0.22 to 13.04 ng/m**3 for lipids and from 0.04 to 7.03 ng/m**3 for aliphatic HC; in surface waters, it from 9 to 84 and from 1 to 53 µg/l, respectively. There is correlation between fluxes of lithogenic fraction of the aerosols, HC, and lipids. Growth of productivity in the aquatic area increases levels of the HC in the surface waters but to a lower degree than HC supply with oil contamination.

CaCO3 content, terrigenous components, foraminifera abundance and age of sediment core MD95-2006, Barra Fan, UK

Lithology, lithic petrology, planktonic foraminiferal abundances, and clastic grain sizes have been determined in a 30 m-long core recovered from the Barra Fan off northwest Scotland. The record extends back to around 45 kyr B.P., with sedimentation rates ranging between 50 and 200 cm/kyr. The abundance of ice-rafted debris indicates 16 glacimarine events, including temporal equivalents to Heinrich events 1-4. Enhanced concentrations of basaltic material derived from the British Tertiary Province suggest that the glacimarine sediments record variations in a glacial source on the Hebrides shelf margin. Glacimarine zones are separated by silty intervals with high planktonic foraminifera concentrations that reflect an interstadial circulation regime in the Rockall Trough. The results suggest that the last British Ice Sheet fluctuated with a periodicity of 2000-3000 years, in common with the Dansgaard-Oeschger climate cycle.

Chemistry of basalts of ODP Hole 111-504B

The effect of oxygen fugacity (fO2) on the partition relationship of Mg and Fe between Plagioclase and sillicate liquid was investigated at 1 atm for basaltic samples recovered during ODP Leg 111 from Hole 504B. Samples 111-504B-143R-2 (Piece 8) and 111-504B-169R-1 (Piece 1) have Plagioclase as the liquidus phase. The distribution coefficient of Mg between Plagioclase and melt is constant at about 0.04 against the variation of fO2, whereas that of Fe (total Fe) varies from 0.3 at f(O2) = 0.2 atm to 0.03 at f(o2) = 10**-11.5 at 1200°C. The distribution coefficient of Mg is slightly higher than that calculated from the phenocryst and bulk-rock compositions, suggesting a kinetic disequilibrium effect on the distribution of Mg in Plagioclase. Because Mg, Fe, and Fe3+ have similar diffusion coefficients in silicate melt, the disequilibrium effect is greatly reduced for the exchange reaction of Mg and total Fe between Plagioclase and liquid. The exchange partition coefficient is highly dependent on fo2, with log fo2 ranging from -0.7 to - 11.5 at approximately 1200°C. Using this relationship, the f(O2) of crystallization of the magmas is estimated to be near the one defined by the fayalite-quartz-magnetite assemblage.

Main components and isotopic composition of gases from mineral waters of the Taman Peninsula and Caucasus

Helium isotope composition as an indicator of the mantle-derived component was studied in gases from mineral springs, stratal waters, and mud volcanoes developed west of the Teberda River valley (10 objects) and two springs in the central segment of the Greater Caucasus orogen between the active El'brus and Kazbek volcanoes. In the western segment of the orogen ratios of 3He/4He = R_corr vary from 46x10**-8 to 114x10**-8 (from 0.33 to 0.81 R_atm, where R_atm = 1.4x10**-6 is the atmospheric ratio). They are substantially lower relative to ratios in the vicinity of El'brus and Kazbek and close to those in samples from the central segment (from 70x10**-8 to 134x10**-8 (from 0.50 to 0.96 R_atm), as well as to ratios previously recorded in the Caucasian Mineral Waters (CMW) area. Moreover, concentration of 3He in them is notably higher than its crustal radiogenic level characteristic of mud volcanoes in the Taman Peninsula, where 3He/4He varies from 1.4x10**-8 to 2.8x10**-8 (from 0.01 to 0.02 R_atm). Nitrogen-methane gas from northern piedmonts of the western Caucasus also contains nonatmogenic components including radiogenic 40Ar (40Ar/36Ar = 900), excessive nitrogen (~87% of total N2 concentration in sample) and mantle He. These data specify distribution of mantle derivates along the orogen strike and age of intrusive magmatic activity in its different segments.

Hydrocarbon gar in sediments of the Costa Rica ubduction zone

At the active continental margin off Costa Rica substantial amounts of hydrocarbon gases are encountered in sediments. The molecular composition (C1-C3) of free hydrocarbon gas as well as the isotopic composition (d13C of methane and ethane and D of methane) was analysed on core samples (ranging between 50 and 380 m depth) collected at sites 1040-1043 which was drilled during ODP Leg 170. In addition, the molecular composition of the C1-C3 hydrocarbons and the d13C composition of C1 and C2 hydrocarbons was determined on adsorbed gas from selected depth intervals at Site 1041 (50-380 mbsf). The molecular composition, and stable carbon and hydrogen isotope signature of low molecular weight hydrocarbons from core sediments and gas pockets indicate that most of the gas was generated by microbial CO2-reduction. Beside d13C values of about -80 per mil for methane (which is typical for microbially- generated methane) extremely light d13C values of -55 per mil were measured for ethane. The carbon isotope composition of methane and ethane, as well as the C1/(C2+C3) ratio display distinct trends with increasing depth. Gas mixing calculations indicate that the percentage of thermally-generated ethane increases from 10% at about 75 mbsf to almost 80% at 380 mbsf. The fraction of thermogenic methane in this depth interval is calculated to range from 0.03 to 1.8% of the total methane. The small contribution of thermogenic methane would increase the d13C value by <1 per mil. Therefore, the increase of d13C of methane (by about 12 per mil) with depth cannot be explained by gas mixing alone. Instead, the observed d13C trend is caused by successive isotope depletion of the methane precursor within the sedimentary organic matter due to progressing microbial gas generation.

Geochemical analysis of sediments and interstitial water in sediment cores from the North-West African continental margin and from the Central Pacific

A. Continental slope sediments off Spanish-Sahara and Senegal contain up to 4% organic carbon and up to 0.4% total nitrogen. The highest concentrations were found in sediments from water depths between 1000 and 2000 m. The regional and vertical distribution of organic matter differs significantly. Off Spanish-Sahara the organic matter content of sediment deposited during glacial times (Wuerm, Late Riss) is high whereas sediments deposited during interglacial times (Recent, Eem) are low in organic matter. Opposite distribution was found in sediments off Senegal. The sediments contain 30 to 130 ppm of fixed nitrogen. In most sediments this corresponds to 2-8 % of the total nitrogen. Only in sediments deposited during interglacial times off Spanish-Sahara up to 20 % of the total nitrogen is contained as inorganically bound nitrogen. Positive correlations of the fixed nitrogen concentrations to the amounts of clay, alumina, and potassium suggest that it is primarily fixed to illites. The amino acid nitrogen and hexosamine nitrogen account for 17 to 26 % and 1.3 to 2.4 %, respectively of the total nitrogen content of the sediments. The concentrations vary between 200 and 850 ppm amino acid nitrogen and 20 to 70 ppm hexosamine nitrogen, both parallel the fluctiations of organic matter in the sediment. Fulvic acids, humic acids, and the total organic matter of the sediments may be clearly differentiated from one another and their amino acid and hexosamine contents and their amino acid composition: a) Fulvic acids contain only half as much amino acids as humic acids b) The molar amino acid/hexosamine ratios of the fulvic acids are half those of the humic acids and the total organic matter of the sediment c) The amino acid spectra of fulvic acids are characterized by an enrichment of aspartic acid, alanine, and methionine sulfoxide and a depletion of glycine, valine, isoleucine, leucine, tyrosine, phenylalanine, lysine, and arginine compared to the spectra of the humic acids and those of the total organic matter fraction of the sediment. d) The amino acid spectra of the humic acids and those of the total organic matter fraction of the sediments are about the same with the exception that arginine is clearly enriched in the total organic matter. In general, as indicated by the amino compounds humic acids resemble closer the total organic matter composition than the low molecular fulvic acids do. This supports the general idea that during the course of diagenesis in reducing sediments organic matter stabilizes from a fulvic-like structure to humic-like structure and finally to kerogen. The decomposition rates of single aminio acids differ significantly from one another. Generally amino acids which are preferentially contained in humic acids and the total organic matter fraction show a smaller loss with time than those preferably well documented in case of the basic amino acids lysine and arginine which- although thermally unstable- are the most stable amino acids in the sediments. A favoured incorporation of these compounds into high molecular substances as well as into clay minerals may explain their relatively high "stability" in the sediment. The nitrogen loss from the sediments due to the activity of sulphate-reducing bacteria amounts to 20-40 % of the total organic nitrogen now present. At least 40 % of the organic nitrogen which is liberated by sulphate-reducing bacteria can be explained ny decomposition of amino acids alone. B. Deep-sea sediments from the Central Pacific The deep-seas sediments contain 1 to 2 orders of magnitude less organic matter than the continental slope sediments off NW Africa, i.e. 0.04 to 0.3 % organic carbon. The fixed nitrogen content of the deep-sea sediments ranges from 60 to 270 ppm or from 20 to 45 % of the total nitrogen content. While ammonia is the prevailing inorganic nitrogen compound in anoxic pore waters, nitrate predominates in the oxic environment of the deep-sea sediments. Near the sediment/water interface interstital nitrate concentrations of around 30 µg-at. N/l were recorded. These generally increase with sediment depth by 10 to 15 µg-at. NO3- N/l. This suggests the presence of free oxygen and the activity of nitrifying bacteria in the interstitial waters. The ammonia content of the interstitial water of the oxic deep-sea sediments ranges from 2 to 60 µg-at. N/l and thus is several orders of magnitude less than in anoxic sediments. In contrast to recorded nitrate gradients towards the sediments/water interface, there are no ammonia concentration gradients. However, ammonia concentrations appear to be characteristic for certain regional areas. It is suggested that this regional differentiation is caused by ion exchange reactions involving potassium and ammonium ions rather than by different decomposition rates of organic matter. C. C/N ratios All estimated C/N ratios of surface sediments vary between 3 and 9 in the deep-sea and the continental margin, respectively. Whereas the C/N ratios generally increase with depth in the sediment cores off NW Africa they decrease in the deep-sea cores. The lowest values of around 1.3 were found in the deeper sections of the deep-sea cores, the highest of around 10 in the sediments off NW Africa. The wide range of the C/N ratios as well as their opposite behaviour with increasing sediment depth in both the deep-sea and continental margin sediment cores, can be attributed mainly to the combination of the following three factors: 1. Inorganic and organic substances bound within the latticed of clay minerals tend to decrease the C/N ratios. 2. Organic matter not protected by absorption on the clay minerals tends to increase C/N ratios 3. Diagenetic alteration of organic matter by micro-organisms tends to increase C/N ratios through preferential loss of nitrogen The diagenetic changes of the microbially decomposable organic matter results in both oxic and anoxic environments in a preferential loss of nitrogen and hence in higher C/N ratios of the organic fraction. This holds true for most of the continental margin sediments off NW Africa which contain relatively high amounts of organic matter so that factors 2 and 3 predominate there. The relative low C/N ratios of the sediments deposited during interglacial times off Spanish-Sahara, which are low in organic carbon, show the increasing influence of factor 1 - the nitrogen-rich organic substances bound to clay minerals. In the deep-sea sediments from the Central Pacific this factor completely predominates so that the C/N rations of the sediments approach that of the substance absorbed to clay minerals with decreasing organic matter content. In the deeper core sections the unprotected organic matter has been completely destroyed so that the C/N ratios of the total sediments eventually fall into the same range as those of the pure clay mineral fraction.

Water chemistry and iron speciation in samples taken during SWEDARP 97/98 campaign with S.A. Agulhas to the Southern Ocean

The distribution and speciation of iron was determined along a transect in the eastern Atlantic sector (6°E) of the Southern Ocean during a collaborative Scandinavian/South African Antarctic cruise conducted in late austral summer (December 1997/January 1998). Elevated concentrations of dissolved iron (>0.4 nM) were found at 60°S in the vicinity of the Spring Ice Edge (SIE) in tandem with a phytoplankton bloom, chiefly dominated by Phaeocystis sp. This bloom had developed rapidly after the loss of the seasonal sea ice cover. The iron that fuelled this bloom was mostly likely derived from sea ice melt. In the Winter Ice Edge (WIE), around 55°S, dissolved iron concentrations were low (<0.2 nM) and corresponded to lower biological productivity, biomass. In the Antarctic Polar Front, at approximately 50°S, a vertical profile of dissolved iron showed low concentrations (<0.2 nM); however, a surface survey showed higher concentrations (1-3 nM), and considerable patchiness in this dynamic frontal region. The chemical speciation of iron was dominated by organic complexation throughout the study region. Organic iron-complexing ligands ([L]) ranged from 0.9 to 3.0 nM Fe equivalents, with complex stability log K'(FeL) = 21.4-23.5. Estimated concentrations of inorganic iron (Fe') ranged from 0.03 to 0.79 pM, with the highest values found in the Phaeocystis bloom in the SIE. A vertical profile of iron-complexing ligands in the WIE showed a maximum consistent with a biological source for ligand production and near surface minimum possibly consistent with loss via photodecomposition. This work further confirms the role iron that has in the Southern Ocean in limiting primary productivity.