Planktonic foraminifera counts, oxygen isotopes and SST reconstrucion for sediment coreGS07-150-10/3MC-B

The North Brazil Current in the western equatorial Atlantic is the main route for poleward heat transfer and therefore is a key component of the climate system. To understand the parameters that influence this region, we present a paleoceanographic reconstruction of the last 40 kyr based on study of sediment core MC 10/3. This reconstruction is based on: i) the oxygen isotope composition of the planktonic foraminifer Globigerinoides ruber (white); ii) census counts of the planktonic foraminifera assemblage to estimate sea surface temperature (SST) via the Modern Analogue Technique (SSTMAT) and to deduce characteristics of the water column; and iii) values of δ18Oivc-sw (a proxy for sea surface salinity (SSS)). The oxygen isotope composition of G. ruber showed a strong shift of greater than 1‰ after 21 cal kyr BP. Such a change can be attributed mainly to a salinity reduction of two units, as well as a slight SST increase (~ 1 °C), between 21 and 17.5 kyr BP. This change may be related to a southward displacement of the Intertropical Convergence Zone (ITCZ) and its belt of convective activity, which would have increased precipitation over the region and diminished surface salinity. The structure of the planktonic foraminiferal assemblage also indicates such conditions. The productive/thermocline-dwelling Globigerinita glutinata showed higher abundance prior to 21 cal kyr BP. Other productive/deep-dwellers, i.e., Neogloboquadrina dutertrei, Globorotalia truncatulinoides (left- and right-coiling) and Globorotalia inflata, were slightly more abundant prior to 21 cal kyr BP and became less frequent with the development of low-salinity surface water that created more stratified and oligotrophic conditions in the water column. However, the warm/oligotrophic surface-dwellers (G. ruber, Globigerinoides sacculifer and Globigerinella siphonifera) became more abundant after 21 cal kyr BP. The species Globorotalia menardii was nearly absent during the Last Glacial Maximum (LGM), and its growth in abundance, mainly after 17.5 cal kyr BP, may indicate the resumption of the Agulhas leakage, which would have reached the study area via the South Equatorial Current (SEC). The return of high salinity values synchronous with the G. menardii increase also denotes the influence of Agulhas leakage into the western equatorial Atlantic

Micro-optodes in sea ice: A new approach to investigate oxygen dynamics during sea ice formation

Oxygen micro-optodes were used to measure oxygen dynamics directly within the microstructure of sea ice by freezing the sensors into the ice during its formation. The experiment was conducted in a 4 m3 mesocosm filled with artificial seawater and inoculated with a unialgal culture of the common Antarctic ice diatom Fragilariopsis cylindrus (Bacillariophyceae) to a final chlorophyll a (chl a) concentration of 11 µg l-1. Ice growth was initiated 7 d after inoculation by reducing the air temperature to -10 ± 2°C and terminated 17 d later. The final ice thickness was 27 cm. One optode was frozen into grease ice and 2 others into the skeletal layer of the growing ice sheet. Increasing oxygen concentrations during ice crystal formation at the water surface and the ice-water interface revealed a strong inclusion of oxygen, which was either physically trapped and/or the result of photosynthesising diatoms. The major portion of oxygen was present as gas bubbles due to super-saturation as a result of increasing salinity and oxygen production by diatoms. An increase in salinity due to a concurrent decrease in ice temperatures during subsequent sea ice development reduced the maximum concentration of dissolved oxygen within brine. Thus, dissolved oxygen concentrations decreased over time, whereas gaseous oxygen was released to the atmosphere and seawater. The sensors are a significant advance on more conventional microelectrodes, because the recordings can be temperature and salinity compensated in order to obtain precise measurements of oxygen dynamics with regard to total (dissolved and gaseous) and dissolved oxygen in sea ice. Optodes do not consume oxygen during measurement over a long period under extreme conditions, which is another advantage for long-term deployment in the field