Inorganic carbon and biological oceanography above a shallow oxygen minimum in the entrance to the Gulf of California in the Mexican Pacific

Abstract

The eastern Pacific intermediate oxygen minimum layer (OML) is particularly well‐developed and shoals close to the Mexican coast. We obtained hydrographic profiles including oxygen concentration [O], measured dissolved inorganic carbon (DIC), continuous surface pCO2 in water and air and took biological data south of the Gulf of California in March 2005. The core of the OML, with close to zero [O] was centered around 475 m with a thickness of the core varying between 80 m and 700 m and the upper limit of the core ranging from 500 m to 80 m. At the surface [O] was close to air‐saturation and showed no relation with the depth of the OML contrary to DIC and pCO2 concentrations. Below 50‐m depth the changes in DIC and Apparent Oxygen Utilization (AOU) yielding a molar ratio of δDIC = 0.79 x δAOU. When the OML shoaled, surface temperature, chlorophyll concentration [CHL], and depth‐integrated zooplankton increased, resulting in positive correlations among plankton biomass, pCO2, and DIC in surface waters. When [CHL] is recalculated as particulate organic carbon (POC), a linear relationship with the sum of DIC and POC is observed as expected for concomitant transport of DIC and inorganic nutrients supporting POC formation. Neglecting ventilation of carbon into the atmosphere and the production of dissolved organic carbon, the results suggested that ~16% of the carbon transported up from the OML was present in surface waters in the form of POC and the rest in inorganic form.

Air-sea CO2fluxes above the stratified oxygen minimum zone in the coastal region off Mexico

“…In this region, we expected to find a strong carbon source to the atmosphere (>5 mmol C m −2 d −1 ), however, the region behaved only as a weak source (maximum FCO 2 of 3.3 mmol C m −2 d −1 ). In addition previous studies have reported a negative relationship between the oxycline depth and the Δ p CO 2 value at the surface, indicating influence from the OMZ [ Maske et al ., ]. In the present study, however, we observed a large range of values on the surface (Figure ), which resulted from the combination of diverse surface processes.…”

“…In the NETP off Mexico, the oxycline depth and upper limit for the OMZ are associated with StSsW [Cepeda- Morales et al, 2013], where changes in this water mass depth may be driven by interannual, seasonal, and local phenomena that affect vertical mixing [Maske et al, 2010;Cepeda-Morales et al, 2013]. The high productivity of the region leads to significant organic matter remineralization at the oxycline and within the OMZ core, therefore producing DIC [Paulmier et al, 2006], increasing pCO 2 , and helping to maintain the OMZ [Paulmier et al, 2006[Paulmier et al, , 2011.…”

“…Oxycline depth for every profile was defined as described by Maske et al . [], using the depth of maximum oxygen difference over 4 m depth calculated for every meter: Δ[O]/m = ([O] Z−2 – [O] Z+−2 ])/4, where Z is depth in meters until the maximum gradient was determined.…”

“…Percent oxygen saturation was calculated using the Ocean Data View software [Schlitzer, R., Ocean Data View, http://odv.awi.de, 2013], which uses the method proposed by Weiss [1970]. Oxycline depth for every profile was defined as described by Maske et al [2010], using the depth of maximum oxygen difference over 4 m depth calculated for every meter:…”

“…The most extensive OMZ is found in the North Eastern Tropical Pacific (hereafter NETP). Here the concentration of oxygen is less than 45 μmol kg −1 at depths shallower than 100 m [ Fernández‐Álamo and Färber‐Lorda , ; Cepeda‐Morales et al ., ], and the DIC concentration is greater than 2200 μmol kg −1 within 50 m of the surface [ Maske et al ., ]. The CO 2 flux in the OMZ of the NETP, however, has not yet been estimated and its contribution to the global carbon balance has not been defined.…”