Recibidü en marzo de 1999. aceptado en ~e~i r e m b r e de i999 ABSTRACTSeawater was sarnpled from different depths in the North Atlantic Ocean (Canary Islands region) and distributed among three different labs for the determination of titration alkalinity. Analysis was performed by potentiornetric rnethods. involving titration in a closed cell, titration in an open cell and a two end-point acid addition method. The precisíon, which is the sarnple reproducibility taken from the mean sranaarci aeviarion Íor repiicate measurements, was ~erween 0.45 anci 0.90 prnoi icg-' for ine individual labs. Accuracy, here taken as the deviation for the values of a lab from the rnean of al1 three, was mostly below 1 pmol -kg-' and never exceeded O. 1% of the sarnple value. Mean standard deviation for al1 labs and al1 sarnples was 0.87 prnol . kg-', once the individual methods were calibrated using Ciencias Marinas, Vol. 26, No. 1,2000 certified reference material (CRM). Without CRM calibration, the mean standard deviation would increase to 2.8 p n o l . kg-'. The conclusion is that current high precision rnethods for alkalinity measurements calibrated with CRMs are able to reach similar accuracy as the measurement of total dissolved inorganic carbon by coulornetry and therefore allow for the precise determination of the oceanic carbon dioxide system by using the two measured parameters.Key words: alkalinity, potentiometry, seawater, certified reference material RESUMEN Se determinó la alcalinidad de muestras de agua de mar recogidas a diferentes profundidades en el Océano Atlántico Norte y distribuidas entre tres laboratorios, utilizando tres variantes potenciométricas: valoración en cubeta cerrada, valoración en cubeta abierta y un método de adición a doble punto final. La precisión, la reproducibilidad de la muestra obtenida a partir de la desviación estándar media para medidas replicadas, fue entre 0.45 y 0.90 p o l . kg-' para cada laboratorio. La exactitud, tomada como la desviación de los valores de un laboratorio respecto de la media de los tres, fue mayoritariamente menor que 1 pmol . kg-' y nunca excedió 0.1% del valor de la muestra. La desviación media estándar para todos los laboratorios y todas las muestras fue de 0.87 pmol . kg-', una vez que los métodos individuales se calibraron usando material certificado de referencia (CRM). Sin la calibración con CRM, la desviación estándar media se incrementaria a 2.8 pmol . kg-'. Se concluye que la precisión real de los métodos para la medida de la alcalinidad calibrada con CRM es capaz de alcanzar exactitudes similares a las medidas de carbono inorgánico total por coulometría y por lo tanto permite la determinación precisa del sistema de carbono oceánico usando la medida de las dos variables.Palabras clave: alcalinidad, potenciometría, agua de mar, material certificado de referencia.

[1] In the eastern North Atlantic, carbon dioxide fugacity (fCO 2 ) in the upper mixed layer and discrete pH and total alkalinity measurements in the upper 2000 m were studied during three cruises (winter, spring, and summer 2001) within the framework of the Programme Océan Multidisciplinaire Méso Echelle (POMME) project. This extensive region is located between 39°and 45°N and 16°and 21°W. The mesoscale variability of fCO 2 on the sea surface and in the atmosphere during each season was determined to understand the mechanisms of evolution that control the spatial and temporal variability of fCO 2 together with an estimation of the fluxes of CO 2 between the atmosphere and the ocean. If we consider the observation to be 22 days per cruise, the region was in-taking 0.30 Tg C during the winter cruise and 0.36 Tg C during the spring cruise, whereas it was out-gassing 0.07 Tg C during the summer cruise. These values are clear indications that the area is acting as a sink of CO 2 on an annual scale, with an estimated flux value of À1.1 mol m À2 yr À1 , which is over twice as much as the mean global flux of À0.5 mol m À2 yr À1 (Takahashi et al., 2002). The changes with time observed in the fCO 2 values over the surface layer between the winter and the spring cruises have been described considering thermodynamics, gas exchange, water transport, and biological activity in the area. The estimation of the subduction of inorganic carbon yielded a value of 0.25 Pg C yr À1 , which is approximately 10% of the global net oceanic CO 2 sink flux.Citation: González Dávila, M., J. M. Santana-Casiano, L. Merlivat, L. Barbero-Muñoz, and E. V. Dafner (2005), Fluxes of CO 2 between the atmosphere and the ocean during the POMME project in the northeast Atlantic Ocean during 2001,

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