Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre

[EN]Labile Fe(II) distributions were investigated in the Sub-Tropical South Atlantic and the Southern Ocean during the BONUS-GoodHope cruise from 34 to 57_ S (February? March 2008). Concentrations ranged from below the detection limit (0.009 nM) to values as high 5 as 0.125 nM. In the surface mixed layer, labile Fe(II) concentrations were always higher than the detection limit, with values higher than 0.060nM south of 47_ S, representing between 39% and 63% of dissolved Fe (DFe). Biological production was evidenced. At intermediate depth, local maxima were observed, with the highest values in the Sub-Tropical domain at around 200 m, and represented more than 70% of DFe. Remineralization processes were likely responsible for those sub-surface maxima. Below 1500 m, concentrations were close to or below the detection limit, except at two stations (at the vicinity of the Agulhas ridge and in the north of the Weddell Sea Gyre) where values remained as high as _0.030?0.050 nM. Hydrothermal or sediment inputs may provide Fe(II) to these deep waters. Fe(II) half life times (t1/2) at 4 _C were measured in the upper and deep waters and ranged from 2.9 to 11.3min, and from 10.0 to 72.3 min, respectively. Measured values compared quite well in the upper waters with theoretical values from two published models, but not in the deep waters. This may be due to the lack of knowledge for some parameters in the models and/or to organic complexation of Fe(II) that impact its oxidation rates. This study helped to considerably increase the Fe(II) data set in the Ocean and to better understand the Fe redox cycle.

Biological and physical controls in the Southern Ocean on past millennial-scale atmospheric CO2 changes

Millennial-scale climate changes during the last glacial period and deglaciation were accompanied by rapid changes in atmospheric CO2 that remain unexplained. While the role of the Southern Ocean as a ’control valve’ on ocean–atmosphere CO2 exchange has been emphasized, the exact nature of this role, in particular the relative contributions of physical (for example, ocean dynamics and air–sea gas exchange) versus biological processes (for example, export productivity), remains poorly constrained. Here we combine reconstructions of bottom-water [O2], export production and 14C ventilation ages in the sub-Antarctic Atlantic, and show that atmospheric CO2 pulses during the last glacial- and deglacial periods were consistently accompanied by decreases in the biological export of carbon and increases in deep-ocean ventilation via southern-sourced water masses. These findings demonstrate how the Southern Ocean’s ’organic carbon pump’ has exerted a tight control on atmospheric CO2, and thus global climate, specifically via a synergy of both physical and biological processes.

(Table 2) Uranium concentrations and isotopic ratios in surface waters of the Atlantic Ocean in April 1977

Uranium concentrations and isotope ratios were measured in thirteen surface-water samples collected across the entire Atlantic Ocean. The mean isotope ratio was 1.15+/-0.01, and the mean concentration 3.1+/-0.2 µg/l.

Geochemistry of planktonic foraminifera from DSDP sites in the Atlantic and Pacific Ocean

The lithium content of planktonic foraminiferal calcite has been determined to evaluate temporal variability of seawater Li concentrations over the past 116 m.y. Mean foraminiferal calcite lithium/calcium in each time interval is no more than 16% greater nor 25% less than the mean Li/Ca of all samples. Li/Ca minima are observed for samples from 50-60 m.y. and 80-90 m.y., with Li/Ca about 25% lower than in adjacent time intervals. At no time during the past 40 m.y does mean Li/Ca appear to be higher than that at present. Subject to the limitations imposed by sample coverage and diagenesis, a similar conclusion holds for the past 116 m.y. Coupled with an oceanic mass balance model for Li, these data suggest that: (1) oceanic Li concentrations and, therefore, high-temperature hydrothermal circulation fluxes during the past 40 m.y. (and perhaps the past 100 m.y.) have not been more than perhaps 30-40% greater than at present for intervals any longer than a million years at most, and (2) these fluxes were not a factor of two higher 100 m.y. ago. By inference, variations in oceanic crustal generation rates over these time periods are similarly limited. Decreases in hydrothermal circulation fluxes and crustal generation rates or fluctuations up to 20% in these rates of a few million years duration are not necessarily ruled out by the Li/Ca data. The lack of variability in Li/Ca over time is not unequivocal evidence that hydrothermal fluxes have not varied because the rates of removal processes may be linked to changes in input fluxes.