CO2 MEASUREMENTS ON THE PIRATA NETWORK Nathalie Lefèvre, IRD LOCEAN Antoine Guillot, DT INSU Brest Laurence Beaumont, Théo Danguy, DT INSU Nathalie.lefevre@locean-ipsl.upmc.fr CARBOOCEAN, annual meeting, Gran Canaria, 4-8 December 2006

Scientific objectives European project CARBOOCEAN (theme 1, WP4) Estimate the flux of CO2 at the air-sea interface by developing an observational network for CO2 Network for the tropical Atlantic:  2 merchant ships (France-Brazil, red line, France-French Guiana, green line) equipped with an automated CO2 system to measure atmospheric and oceanic CO2 underway by infrared detection.  2 CO2 sensors for PIRATA moorings at 6oS, 10oW and 8oN, 38oW (black dots) using a colorimetric method (CARIOCA buoy technology). The CO2 network is part of the European project CARBOOCEAN. One of the objectives of this project is to better estimate the CO2 flux at the air-sea interface and its variability. The approach used is the development of an observational network for the Atlantic ocean and to a lesser extent for the Southern ocean. At IRD LOCEAN we have proposed to equip 2 merchant ships (France-Brazil and France- French Guiana) and 2 moorings to assess the CO2 flux and its variability in the tropical Atlantic. This work is funded under CARBOOCEAN and the network is being setup. EGEE 3 cruise (in blue) See poster

Choice of the PIRATA moorings for installing CO2 sensors CO2 sensors on PIRATA moorings: 6oS, 10oW: CO2 time-series in the SEC 8oN, 38oW: CO2 time-series in the NECC SeaWiFs, Septembre 2003 Impact of the Amazon outflow on the CO2 distribution during the retroflection of the North Brazil Current (NBC) Advection far East in the North Equatorial Counter-Current (NECC) 40W 10W Surface circulation is shown: 2 main current systems, the NECC and the SEC. The plan is to have one CO2 sensor in each current, eastern and western parts of the tropical Atlantic. The CO2 variability is affected by SST, S, biological activity. We know nothing about the river impact on the CO2 distribution (freshwater input & biological activity). Amazon waters propagate very far east when the NECC is at its maximum (autumn). In the east the impact of the upwelling supplying CO2-rich waters is monitored at PIRATA 6S, 10W. 10S

Installation of the first CO2 sensor on the PIRATA mooring at 6oS, 10oW during the EGEE 3 cruise (June 2006) Argos antenna We measure the partial pressure (or more correctly the fugacity) of CO2 (CO2 sensor) and the dissolved oxygen concentration (optode Aanderaa 3835 that can operate down to 300m). CO2 sensor and oxygen optode Box with electronics CO2 sensors built at DT INSU (Antoine Guillot, Théo Danguy, Laurence Beaumont)

Comparison IR-CARIOCA

Data of the CO2 sensor since the 7th of June 2006 Hourly measurements of CO2, SST and O2 sent by ARGOS The upwelling starts at the end of June so the temperature decreases and the surface waters, initially undersaturated in CO2 (below the atmospheric value of 367 micro-atm, becomes supersaturated. SST decrease due to the upwelling (June-September) CO2 increase fCO2 = -19.54 * SST + 895.84 r2 = -0.88

Correlation between fCO2 and SST and SSS Detrended and normalized hourly fCO2, sea surface temperature and sea surface salinity (interpolated from PIRATA daily records) as a function of time (in days). Data from 8 June to 7 October 2006. Colder and fresher seawater associated with higher CO2 concentrations

Oxygen measurements using the Aanderaa optode 3835 Pipe connection (20 cm) Too large amplitude for O2 diurnal cycle (~20 mmol/ kg) after 3 months

Diurnal cycle consistent with biological activity Dissolved inorganic carbon (DIC) calculated using TA=f(S) and fCO2 Decrease of DIC during the day (photosynthesis) Increase of DIC during the night (convection & respiration) 14-21 Sep. 2006

Diurnal cycle detected on both DIC and fCO2 DIC and fCO2 variability after the upwelling season Example for 17-23 November 2006 Diurnal cycle detected on both DIC and fCO2

Conclusions Very encouraging results: CO2 data since June Source of CO2 from 1.19 (June) to 6.82 m.mol.m-2.d-1 (September) fCO2 SST Satisfactory results at the PIRATA mooring, no biofouling detected on the CO2 sensor, good CO2 data in agreement with the physical properties (water mass variability and impact of the upwelling). The sensor needs to be replaced each year during the PIRATA cruise. The existing one is sent for calibration, renewal of the dye and a spare one is installed on the mooring with new batteries. The year after, we swap the sensors. We hope to extend the CO2 network by equipping other PIRATA moorings...

Plan for 2007 CO2 sensor to be installed at 8oN, 38oW during the PIRATA Brazil cruise in 2007 Replace the sensor (lifetime of 1 year) during EGEE 5 (June 2007) to continue the time-series (the existing one is sent for calibration, and for renewal of the dye at the DT INSU while a spare one is installed).