Winter mesoscale variations of carbonate system parameters and estimates of CO₂ fluxes in the Gulf of Cadiz, northeast Atlantic Ocean (February 1998)

Abstract

[1] Further observations with small spatial and temporal resolutions conducted during different seasons are required in order fully to understand the role that shelves play in the global carbon cycle. The components of the carbonate system (total alkalinity, pH in the total scale, total dissolved inorganic carbon, and CO 2 fugacity), and dissolved oxygen, nutrients, and phytopigments were studied along the mesoscale section over the continental shelf and slope in the Gulf of Cadiz in February 1998. All the chemical properties clearly distinguish four different water masses: Gulf of Cadiz Water, North Atlantic Surface Water, North Atlantic Central Water, and Mediterranean Water. From the thermohaline properties and applied chemical conservative tracers for each water mass, a mixing model has been established which explains more than 96% of the variability in the distribution of chemical properties. The relative variation of nutrients and carbon concentrations resulting from the regeneration of organic matter was estimated. The contribution of Mediterranean water to the waters at the traverse of Cadiz varies from 15% to 40% according to this model. The difference of f CO 2 between seawater and atmosphere (Áf CO 2 = À35 matm) shows that the surface seawater in the Gulf of Cadiz is a sink for atmospheric CO 2 during winter with an average calculated net CO 2 flux across the airsea interface of about À19.5 ± 3.5 mmol m À2 d À1 . We estimate that at the traverse of Cadiz the shallow core of Mediterranean outflow carries out 1.2 . 10 4 to 2.4 . 10 4 mol inorganic carbon s À1 . This estimate is 1 order of magnitude lower than that calculated for the Mediterranean outflow in the Strait of Gibraltar.

Anthropogenic and natural CO<sub>2</sub> exchange through the Strait of Gibraltar

“…The A T concentrations measured and associated to the occurrence of NASW have been observed previously in the Strait of Gibraltar as well as in the nearby Gulf of Cadiz Aït-Ameur and Goyet, 2006). Also, available C T values for the North East Atlantic Central Water (Ríos et al, 1995;González-Dávila et al, 2003) are consistent with those reported here in the inflow of the Atlantic water passing through the Strait.…”

“…3f, Table 2), the discrepancies existing between the carbon exchanges in the literature can be attributable to the values of the water transport associated to each water layer. High values of carbon exchange result when water transports given by Béthoux (1979) are used for fluxes computation, as already underlined by other authors (Gómez, 2003;Aït-Ameur and Goyet, 2006). This circumstance may be due to the fact that the early work by Béthoux (1979) did not consider properly the interface definition for calculations.…”

Factors controlling pCO<sub>2</sub> variability in the eastern Gulf of Cádiz (SW Iberian Península)

“…The Gulf of Cádiz acts as a source of CO2 to the atmosphere during the months of summer (ST2, ST6) and autumn (ST3, ST7), and as a sink in spring (ST1, ST5) and winter (ST4, ST8). Previous studies conducted in the Gulf of Cádiz are consistent with the behaviour found in this study (González-Dávila et al, 2003;Aït-Ameur and Goyet, 2006;Ribas-Ribas et al, 2011).…”

“…The Gulf of Cádiz, during the period of this sampling, acted as a sink of CO2, with a mean rate of -0.18 ± 1.32 mmol m -2 d -1 , that would give rise to an annual flux of -0.07 mol C m -2 yr -1 . The findings of previous studies carried out in the Gulf of Cádiz coincide with the behaviour observed in this study (Santana-Casiano et al, 2002;González-Dávila et al, 2003;Huertas et al, 2006;de la Paz et al, 2009;Ribas-Ribas et al, 2011), with the exception of the study by Aït-Ameur and Goyet (2006) in which it was estimated that the Gulf of Cádiz acts as a source of CO2 to the atmosphere, although that study only corresponds to the summer season.…”

“…Guadalquivir, Tinto, Odiel and Guadiana. Various studies have been conducted in this area to evaluate the variability of the sea surface pCO2, although they cover smaller areas and shorter duration of time than this work (González-Dávila et al, 2003;Aït-Ameur and Goyet, 2006;Huertas et al, 2006;Ribas-Ribas et al, 2011) or only a specific area like the Strait of Gibraltar (Dafner et al, 2001;Santana-Casiano et al, 2002;de la Paz et al, 2009). All of these studies, however, have determined that this zone behaves as a global sink of CO2, with seasonal variations mainly induced by the combination of the fluctuations of biomass concentration and temperature.…”