Award: OCE-1233706

The major goals of the project were to collect marine carbon cycle and carbon dioxide measurements in the eastern Pacific Ocean during the 2013 East Pacific GEOTRACES cruise. Our collaborative effort was focused on collecting samples for inorganic and organic carbon, from the surface ocean to the deep ocean, between Peru and Tahiti. The oceanic region experiences low oxygen conditions oxygen deficient zone, ODV) from water depths of about 100 m to 1,000 m, and it is an important area for fisheries associated with upwelling along the coast. Due to the influence of pH on ocean chemistry, this region provides a large range of marine seawater carbonate chemistry that influences the ocean cycling of trace elements and isotopes. On the cruise we sampled for dissolved inorganic carbon (DIC) and total alkalinity (TA), and dissolved organic carbon (DOC), with subsequent laboratory analyses of these samples. Highly precise and accurate measurements of DIC, TA and DOC were made to meet the high-bar criteria of quality that has been set for the CLIVAR-Repeat hydrography program and U.S. time series efforts (i.e., BATS and HOT). Across the Peru-Tahiti section, very large horizontal gradients in surface/mixed layer dissolved inorganic carbon (DIC) and total alkalinity (TA) from the nutrient-rich, low-oxygen coastal upwelling region adjacent to Peru to the oligotrophic central Pacific. Near the coast of Peru, upwelling of CO2 rich waters from the oxygen deficient zone (ODV) impinged at the surface with very high partial pressures of CO2 (pCO2; >800-1,200 µatm), and low pH (7.55 to 7.8). These waters were also undersaturated with respect to aragonite, a common calcium carbonate (CaCO3) mineral. These chemical conditions indicate that the shelf calcareous sediments are 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 dissolved inorganic carbon (DIC) and pCO2 have increased by as much as 3% and 20%, respectively, while pH and saturation state for aragonite have decreased by as much as 0.063 and 0.54, respectively. In intermediate waters (~200-500 m), dissolved oxygen has decreased (loss of up to 15%) and nitrate increased (gain of up to 30% ) over the past twenty years and this likely reflects the westward expansion of the oxygen deficient zone across the central Eastern South Pacific Ocean. Over the same period, DIC and pCO2 increased by as much as 5% and 20%, respectively, while pH and saturation state for argonite decreased by ~20% respectively. Such rapid change in pH and CO2–carbonate chemistry over the past twenty years has significant implications for changing the thermodynamics and solubility of intermediate water TEIs, but also for modifying the marine ecosystem of the upper waters in the eastern South Pacific. Last Modified: 05/16/2017 Submitted by: Nicholas R Bates