Northern Weddell Gyre Research Articles

Late summer particulate organic carbon export and twilight zone remineralisation in the Atlantic sector of the Southern Ocean

Abstract. As part of the GEOTRACES Bonus-GoodHope (BGH) expedition (January–March 2008) in the Atlantic sector of the Southern Ocean, particulate organic carbon (POC) export was examined from the surface to the mesopelagic twilight zone using water column distributions of total 234Th and biogenic particulate Ba (Baxs). Surface POC export production was estimated from steady state and non steady state modelling of 234Th fluxes, which were converted into POC fluxes, using the POC/234Th ratio of large, potentially sinking particles (> 53 μm) collected via in situ pumps. Deficits in 234Th activities were observed at all stations from the surface to the bottom of the mixed layer, yielding 234Th export fluxes from the upper 100 m of 496 ± 214 dpm m−2 d−1 to 1195 ± 158 dpm m−2 d−1 for the steady state model and of 149 ±517 dpm m−2 d−1 to 1217 ± 231 dpm m−2 d−1 for the non steady state model. Using the POC/234Thp ratio of sinking particles (ratios varied from 1.7 ± 0.2 μmol dpm−1 to 4.8 ± 1.9 μmol dpm−1) POC export production at 100 m was calculated to range between 0.9 ± 0.4 and 5.1 ± 2.1 mmol C m−2 d−1,assuming steady state and between 0.3 ± 0.9 m−2 d−1 and 4.9 ± 3.3 mmol C m−2 d−1, assuming non steady state. From the comparison of both approaches, it appears that during late summer export decreased by 56 to 16% for the area between the sub-Antarctic zone and the southern Antarctic Circumpolar Current Front (SACCF), whereas it remained rather constant over time in the HNLC area south of the SACCF. POC export represented only 6 to 54% of new production, indicating that export efficiency was, in general, low, except in the vicinity of the SACCF, where export represented 56% of new production. Attenuation of the POC sinking flux in the upper mesopelagic waters (100–600 m depth interval) was evidenced both, from excess 234Th activities and from particulate biogenic Ba (Baxs) accumulation. Excess 234Th activities, reflected by 234Th/238U ratios as large as 1.21 ± 0.05, are attributed to remineralisation/disaggregation of 234Th-bearing particles. The accumulation of excess 234Th in the 100–600 m depth interval ranged from 458 ± 633 dpm m−2 d−1 to 3068 ± 897 dpm m−2 d−1, assuming steady state. Using the POC/234Thp ratio of sinking particles (> 53 μm), this 234Th accumulation flux was converted into a POC remineralisation flux which ranged between 0.9 ± 1.2 mmol C m−2 d−1 and 9.2 ± 2.9 mmol C m−2 d−1. Mesopelagic particulate biogenic Ba has been reported to reflect bacterial degradation of organic matter and to be related to oxygen consumption and bacterial carbon respiration. We observed that the highest Baxs contents (reaching up to > 1000 pM), in general, occurred between 200 and 400 m. Depth-weighted average mesopelagic Baxs (meso-Baxs) values were converted into respired C fluxes, which ranged between 0.23 and 6.4 mmol C m−2 d−1, in good agreement with 234Th-based remineralisation fluxes. A major outcome from this study is the observed significant positive correlation between POC remineralisation as estimated from meso-Baxs contents and from 234Th excess (R2 = 0.73; excluding 2 outliers). Remineralisation of POC in the twilight zone was particularly efficient relative to POC export resulting in negligible bathypelagic (> 600 m) POC export fluxes in the sub-Antarctic zone, the Polar Front zone and the northern Weddell Gyre, while the subtropical zone as well as the vicinity of the SACCF had significant deep POC fluxes.

New insights on the role of organic speciation in the biogeochemical cycle of dissolved cobalt in the southeastern Atlantic and the Southern Ocean

Abstract. The organic speciation of dissolved cobalt (DCo) was investigated in the subtropical region of the southeastern Atlantic, and in the Southern Ocean in the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre, between 34°25´ S and 57°33´ S along the Greenwich Meridian during the austral summer of 2008. The organic speciation of dissolved cobalt was determined by competing ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using nioxime as a competing ligand. The concentrations of the organic ligands (L) ranged between 26 and 73 pM, and the conditional stability constants (log K'CoL) of the organic complexes of Co between 17.9 and 20.1. Most dissolved cobalt was organically complexed in the water-column (60 to >99.9%). There were clear vertical and meridional patterns in the distribution of L and the organic speciation of DCo along the section. These patterns suggest a biological source of the organic ligands in the surface waters of the subtropical domain and northern subantarctic region, potentially driven by the cyanobacteria, and a removal of the organic Co by direct or indirect biological uptake. The highest L:DCo ratio (5.81 ± 1.07 pM pM−1) observed in these surface waters reflected the combined effects of ligand production and DCo consumption. As a result of these combined effects, the calculated concentrations of inorganic Co ([Co']) were very low in the subtropical and subantarctic surface waters, generally between 10−19 and 10−17 M. In intermediate and deep waters, the South African margins can be a source of organic ligands, as it was suggested to be for DCo (Bown et al., 2011), although a significant portion of DCo (up to 15%) can be stabilized and transported as inorganic species in those DCo-enriched water-masses. Contrastingly, the distribution of L does not suggest an intense biological production of L around the Antarctic Polar Front where a diatom bloom had recently occurred. Here [Co'] can be several orders of magnitude higher than those reported in the subtropical domain, suggesting that cobalt limitation was unlikely in the ACC domain. The almost invariant L:DCo ratio of ~1 recorded in these surface waters also reflected the conservative behaviours of both L and DCo. In deeper waters higher ligand concentrations were observed in waters previously identified as DCo sources (Bown et al., 2011). At those depths the eastward increase of DCo from the Drake Passage to the Greenwich Meridian could be associated with a large scale transport and remineralisation of DCo as organic complexes; here, the fraction stabilized as inorganic Co was also significant (up to 25%) in the low oxygenated Upper Circumpolar Deep Waters. Organic speciation may thus be a central factor in the biogeochemical cycle of DCo in those areas, playing a major role in the bioavailability and the geochemistry of Co.

Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

Abstract. Silicon isotopic signatures (δ30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW δ30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the δ30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 ± 1.3 and 1.1 ± 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

Comparison of two time-variant forced eddy-permitting global ocean circulation models with hydrography of the Scotia Sea

A comparison between hydrographic observations and output from two realistically forced z-level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12–0.40) and too warm in POCM_4C (by >2 °C) suggesting problems with the air-sea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.

Comparison of two time-variant forced eddy-permitting global ocean circulation models with hydrography of the Scotia Sea

A comparison between hydrographic observations and output from two realistically forced z -level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12–0.40) and too warm in POCM_4C (by >2 °C) suggesting problems with the air-sea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.

Ba distribution in surface Southern Ocean sediments and export production estimates

We present excess Ba (Baxs) data (i.e., total Ba corrected for lithogenic Ba) for surface sediments from a north‐south transect between the Polar Front Zone and the northern Weddell Gyre in the Atlantic sector and between the Polar Front Zone and the Antarctic continent in the Indian sector. Focus is on two different processes that affect excess Ba accumulation in the sediments: sediment redistribution and excess Ba dissolution. The effect of these processes needs to be corrected for in order to convert accumulation rate into vertical rain rate, the flux component that can be linked to export production. In the Southern Ocean a major process affecting Ba accumulation rate is sediment focusing, which is corrected for using excess 230Th. This correction, however, may not always be straightforward because of boundary scavenging effects. A further major process affecting excess Ba accumulation is barite dissolution during exposure at the sediment‐water column interface. Export production estimates derived from excess 230Th and barite dissolution corrected Baxs accumulation rates (i.e., excess Ba vertical rain rates) are of the same magnitude but generally larger than export production estimates based on water column proxies (234Th‐deficit in the upper water column; particulate excess Ba enrichment in the mesopelagic water column). We believe export production values based on excess Ba vertical rain rate might be overestimated due to inaccurate assessment of the Baxs preservation rate. Barite dissolution has, in general, been taken into account by relating it to exposure time before burial depending on the rate of sediment accumulation. However, the observed decrease of excess Ba content with increasing water column depth (or increasing hydrostatic pressure) illustrates the dependence of barite preservation on degree of saturation in the deep water column in accordance with available thermodynamic data. Therefore correction for barite dissolution would not be appropriate by considering only exposure time of the barite to some uniformly undersaturated deep water but requires also that regional differences in degree of undersatuation be taken into account.

Direct measurements reveal insignificant storage of anthropogenic CO2 in the Abyssal Weddell Sea

In the northern Weddell Gyre at the prime meridian, Total TCO2 changes in the Weddell Sea Bottom Water (WSBW) have been investigated. Following a suggestion by [Poisson and Chen, 1987], the TCO2 difference at potential temperatures of 0.2°C and −0.8°C was determined using data from 1996 and 1998. No significant difference was found to similar differences for the years 1973 and 1981 reported by Poisson and Chen. Thus, over a period of 25 years an at most minor amount of anthropogenic CO2 has penetrated into the WSBW at this location. This suggests that this abyssal subpolar region is relatively unimportant for the storage of anthropogenic CO2. The same core of WSBW exhibited a marked increase of chlorofluorocarbon (CFC). For the Southern Ocean, therefore, CFCs are apparently of limited value as analogues of anthropogenic CO2, in contrast to some other ocean provinces.

Spring distribution of larger (>64 μm) protozoans in the Atlantic sector of the Southern Ocean

Distribution of larger protozoans (armoured dinoflagellates, tintinnids, heliozoans, radiolarians and foraminiferans >64 μm) is presented for three major water masses of the Southern Ocean: the Polar Front region (PFr), the southern Antarctic Circumpolar Current (southern ACC) and the northern Weddell Gyre. Sampling took place during the SO-JGOFS cruise ANT X/6 of R/V Polarstern (October–November 1992) along a meridional transect at 6°W between 48°00′S and 59°30′S. Multinet samples (64 μm mesh size) were taken at six stations from the surface down to 500 m depth at five different depth intervals. In the upper 100 m of the water column abundances of larger protozoans varied between 94 and 10,930 ind. m –3, with highest abundances in the PFr, where phytoplankton blooms occurred, and lowest values in the Antarctic Circumpolar Current–Weddell Gyre Boundary (AWB). Foraminiferans and polycystine and smaller (<300 μm) phaeodarian radiolarians dominated larger protozoan assemblages in the PFr. In open water of the southern ACC, tintinnids, armoured dinoflagellates, foraminiferans and smaller (<300 μm) phaeodarian radiolarians were equally important. The heliozoans Sticholonche spp. and nassellarian radiolarians dominated assemblages in the Weddell Gyre and AWB. Larger protozoan biomasses ranged between 2 and 674 μg C m −3 and were always dominated by larger (>300 μm) phaeodarians. Highest biomasses were found in the AWB between 200 and 500 m depth. Standing stocks of larger protozoans constituted a negligible fraction of zooplankton biomass in the upper 200 m of the water column. In deeper layers of the ice-covered Weddell Gyre and AWB their biomasses, dominated by larger (>300 μm) phaeodarians, was significant contributing up to 45% to total larger protozoan and metazoan biomass. Analysis of correlation between distribution patterns and environmental conditions at the stations sampled indicate that spring distribution patterns of heterotrophic armoured dinoflagellates, polycystine radiolarians and foraminiferans follow productivity in the water column. Of the protozoan groups studied the smaller (<300 μm) phaeodarian radiolarians also showed a significant correlation with productivity during spring, however, results from previous studies do not suggest a consistent pattern. Spring distribution patterns of other larger protozoans were not related to differences in productivity in the water column, and effects such as ice-cover, grazing or silica limitation might be determining. Dead radiolarian skeletons constituted on average 27, 8 and 11% of the population of nassellarians, spumellarians and smaller (<300 μm) phaeodarians, respectively. The contribution of dead radiolarian skeletons to total radiolarian stocks varied with depth and water mass. Differences between live and skeleton assemblages composition were observed. These differences should be taken into consideration when interpreting the geological record.

CO2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter

Quasi-continuous fugacity of CO 2 (fCO 2 ) data were collected in the eastern Weddell Gyre and southern Antarctic Circumpolar Current (ACC) of the Southern Ocean during austral autumn 1996. Full depth Total CO 2 (TCO 2 ) sections are presented for austral autumn and winter (1992) cruises. Pronounced fCO 2 gradients were observed at the Southern Ocean fronts. In the Weddell Gyre, fCO 2 regimes appeared to coincide with surface and subsurface hydrographic regimes. The southern ACC was supersaturated with respect to CO 2 , as was part of the northern Weddell Gyre. The southern Weddell Gyre was markedly undersaturated. The great potential of autumn cooling for generating undersaturation and CO 2 uptake from the atmosphere was demonstrated. In the northeastern Weddell Gyre, upwelling of CO 2 - and salt-rich deep water was shown to play a role as the horizontal fCO 2 distribution closely resembled that of the surface salinity. The total uptake of atmospheric CO 2 by the Weddell Gyre in autumn (45 days) was calculated to be 7·10 12 g C. The deep TCO 2 distribution noticeably reflected the different water masses in the region. A new deep TCO 2 maximum was detected in the ACC, which apparently characterizes the boundary between the equatorward flowing Antarctic Bottom Water (AABW) and the Circumpolar Deep Water (CDW). East of the Weddell Gyre, the AABW stratum is much thicker (>2000 m) than more to the west, on the prime meridian (<300 m).

On the sources of Weddell Gyre Antarctic Bottom Water

In March–April 1995, as part of the World Ocean Circulation Experiment section A23, we completed 49 hydrographic stations across the Weddell Gyre and southern Antarctic Circumpolar Current, from the Antarctic continental shelf (72.5°S, 16.5°W) to South Georgia (55°S, 34.5°W). Chlorofluorocarbon (CFC‐11, CFC‐12, and CFC‐113) data collected at these stations reveal that distinct sources renew the Antarctic Bottom Water (defined as waters with potential temperatures less than 0°C) of the Weddell Gyre. Weddell Sea Bottom Water (defined as waters with potential temperatures less than −0.7°C) formed in the western Weddell Sea has CFC concentrations about 5 to 6 times higher in the eastward flowing northern Weddell Gyre than in the westward flowing southern limb. Our CFC measurements suggest that distinct sources of Weddell Sea Bottom Water exist in the western Weddell Sea, in agreement with previous descriptions based on potential temperature and salinity signals. In the northern Weddell Gyre, high CFC concentrations in Weddell Sea Deep Water, potential temperatures between 0°C and −0.7°C, confirm the long‐recognized sources for this water mass in the western and southwestern Weddell Sea. In the southern Weddell Gyre at about 20°W and along the Antarctic continental slope, Weddell Sea Deep Water with potential temperatures around −0.45°C shows a deep CFC maximum about 1000 m above the seafloor. CFC concentrations in this deep southern core are about 80% of those of new Weddell Sea Deep Water in the northern Weddell Gyre near 30°W. The A23 CFC and hydrographic data are not consistent with the hypothesis that Weddell Sea Deep Waters are derived from a single source in the western Weddell Sea. Instead, these tracers suggest that an important portion of the Weddell Sea Deep Water in the southern Weddell Gyre originates outside the western Weddell Sea, probably near the Amery Basin and environs, around 75°E. These features of the circulation and renewal of the deep Weddell Gyre should be carefully considered in simulations dealing with fluxes, pathways, and formation rates of Antarctic Bottom Water.