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

• Published in: Frontiers in Marine Science
• Main Author: Bates, Nicholas Robert
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: Pacific; Ocean acidification
• Online Access: https://eprints.soton.ac.uk/426295/; https://eprints.soton.ac.uk/426295/1/fmars_05_00398.pdf

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 (CO 2 )-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 CO 2 -rich waters from the oxygen-deficient zone (ODZ) impinged at the surface with very high partial pressures of CO 2 (pCO 2 >800-1,200 μatm), 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 CaCO 3 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 (Ωaragonite) 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 -43 μmoles kg -1 ) and nitrate increased (gain of up to 5 μmoles 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 μmoles kg -1 and +145 μatm, respectively, while pH and Ωaragonite decreased by -0.091 and -0.45, respectively. Such rapid change in pH and CO 2 -carbonate chemistry over the past 20 years reflects substantial changes in ...