Research Task 7 Electrochemical methods for autonomous chemical monitoring in the ocean in Oxygen Minimum Zones

PhD position. Supervisors: Dr Véronique Garçon, Prof. Pierre Gros, Dr Pierre Temple-Boyer and Dr Lars Damgaard Host: CNRS with secondments to UPS and LAAS, and Unisense.

The Oxygen Minimum Zones (OMZs), known as suboxic layers (O_²<20 µM), have contracted and expanded during cold and warm periods at paleoceanographical scales.⁽¹⁾ In the OMZs, the main biogeochemical anomalies in the carbon and nitrogen cycles are generally focused in the oxycline.⁽²⁾ The objective is to determine the control mechanisms of the OMZ bacterial activity its intensity and variability, acting simultaneously on the oxygen, carbon and nitrogen cycle at the oxycline. This requires chemical sensors to be developed for autonomous mid- to long-term deployment. The ESR working on this Research Task will include use and further refine the STOX oxygen sensor developed by Unisense. This sensor has already been applied in OMZ regions of the Arabian Sea, off Namibia, and off the South American west coast. It can reproducibly determine O_² at concentrations << 100 nM, and some sensors can even be calibrated down to 1-2 nM oxygen. The sensor is unique by a built-in zero calibration ability allowing in situ detection of extremely low concentrations. With these refinements made the voltametric method developed for silicate measurements^(3,4,5) will be adapted to determine phosphate concentrations over the concentration range found in the open and coastal oceans. Silicon and polymer-based microtechnologies developed at LAAS will be used to integrate with electrochemical methods of phosphate detection in liquid phase. Thus, a phosphate ion microsensor will be produced in collaboration with Unisense. Finally, appropriate surface treatments against biofouling will be implemented and the phosphate sensor will undergo in situ trials, investigating for the first time in situ variations in oxygen and phosphate across marine oxyclines.

⁽¹⁾ Cannariato, K.G. & Kennett J.P., 1999. Geol. 27, 975-978. ⁽²⁾ Paulmier, A., et al., 2006. GRL, 33, L20601, doi:10.1029/2006GL026801. ⁽³⁾ Lacombe, M., 2007, PhD thesis UPS. ⁽⁴⁾ Lacombe, M., et al., 2007. Mar. Chem., doi:10.1016/j.marchem.2007.05.002 ⁽⁵⁾ Lacombe, M., et al., 2008. Talanta, 77 (2), 744-750.