Quantifying the flux of CaCO ₃ and organic carbon from the surface ocean using in situ measurements of O ₂, N ₂, pCO ₂, and pH

Ocean acidification from anthropogenic CO ₂ has focused our attention on the importance of understanding the rates and mechanisms of CaCO ₃ formation so that changes can be monitored and feedbacks predicted. We present a method for determining the rate of CaCO ₃ production using in situ measureme nts of fCO ₂ and pH in surface waters of the eastern subarctic Pacific Ocean. These quantities were determined on a surface mooring every 3 h for a period of about 9 months in 2007 at Ocean Station Papa (50N, 145W). We use the data in a simple surface ocean, mass balance model of dissolved inorganic carbon (DIC) and alkalinity (Alk) to constrain the CaCO ₃: organic carbon (OC) production ratio to be approximately 0.5. A CaCO ₃ production rate of 8 mmol CaCO ₃ m ^-2 d ^-1 in the summer of 2007 (1.2 mol m ^-2 yr ^-1) is derived by combining the CaCO ₃: OC ratio with the a net organic carbon production rate (2.5 mol C m ^-2 yr ^-1) determined from in situ measurements of oxygen and nitrogen gas concentrations measured on the same mooring (Emerson and Stump, 2010). Carbonate chemistry data from a meridional hydrographic section in this area in 2008 indicate that isopycnal surfaces that outcrop in the winter in the subarctic Pacific and deepen southward into the subtropics are a much stronger source for alkalinity than vertical mixing. This pathway has a high enough Alk:DIC ratio to support the CaCO ₃:OC production rate implied by the fCO ₂ and pH data.