Seawater Carbonate Chemistry Distributions Across the Eastern South Pacific Ocean Sampled as Part of the GEOTRACES Project and Changes in Marine Carbonate Chemistry Over the Past 20 Years

The US GEOTRACES Eastern Pacific Zonal Transect in 2013 that sampled in the South Pacific Ocean has provided an opportunity to investigate the biogeochemical cycling of trace elements and isotopes (TEIs) and seawater carbon dioxide (CO2)-carbonate chemistry. Across the Peru to Tahiti section, the entire water column was sampled for total alkalinity (TA) and dissolved inorganic carbon (DIC), in addition to core hydrographic and chemical measurements conducted as part of the GEOTRACES cruise. From the nutrient-rich, low-oxygen coastal upwelling region adjacent to Peru to the oligotrophic central Pacific, very large horizontal gradients in marine carbonate chemistry were observed. Near the coast of Peru, upwelling of CO2-rich waters from the oxygen-deficient zone (ODZ) impinged at the surface with very high partial pressures of CO2 (pCO(2); >800-1,200 mu atm), and low pH (7.55-7.8). These waters were also undersaturated with respect to aragonite, a common calcium carbonate (CaCO3) mineral. These chemical conditions are not conducive to pelagic and shelf calcification, with shelf calcareous sediments vulnerable to CaCO3 dissolution, and to the future impacts of ocean acidification. A comparison to earlier data collected from 1991 to 1994 suggests that surface seawater DIC and pCO(2) have increased by as much as 3 and 20%, respectively, while pH and saturation state for aragonite (Omega(aragonite)) have decreased by as much as 0.063 and 0.54, respectively. In intermediate waters (similar to 200-500 m), dissolved oxygen has decreased (loss of up to -43 mu moles kg(-1)) and nitrate increased (gain of up to 5 mu moles kg(-1)) over the past 20 years and this likely reflects the westward expansion of the ODZ across the central Eastern South Pacific Ocean. Over the same period, DIC and pCO(2) increased by as much as +45 mu moles kg(-1) and +145 mu atm, respectively, while pH and Omega(aragonite) decreased by -0.091 and -0.45, respectively. Such rapid change in pH and CO2-carbonate chemistry over the past 20 years reflects substantial changes in the ODZ and water-column remineralization of organic matter with no direct influence from uptake of anthropogenic CO2. Estimates of anthropogenic carbon (i.e., C-ANT) determined using the TrOCA method showed no significant changes between 1993 and 2014 in these water masses. These findings have implications for changing the thermodynamics and solubility of intermediate water TEIs, but also for the marine ecosystem of the upper waters, especially for the vertically migrating community present in the eastern South Pacific Ocean.