Seasonal pH and aragonite saturation horizons in the Gulf of Alaska during the North Pacific Survey, 1956–1957

Abstract. The extent of global change in carbon system parameters can only be evaluated by comparing present with past measurements. In the northern North Pacific, where aragonite saturation horizons are among the shallowest in the world, historical measurements of carbonate parameters vary from rare to nonexistent. However, during the summer of 1956 and winter of 1957, an extensive survey of the oceanography of the Northeast Pacific, under the auspices of the Canadian Committee on Oceanography, was conducted by the Fisheries Research Board of Canada. Approximately 2500 measurements of pH at depths from surface to 2000 m were taken throughout the Gulf of Alaska, in addition to measurements of nutrient and hydrographic properties. After conversion to the contemporary total pH scale, these data revealed significant seasonal and latitudinal differences in pH in the upper 200 m. Estimates of aragonite saturation indicate that undersaturated water was a common feature of the surface mixed layer north of 51° N latitude in the winter of 1957. The North Pacific Survey data were compared with the results of a summer 2007 survey of the west coast of North America where pH levels were ~0.1 pH units lower (at a reference density of 26.2σ_θ than was found in the summer of 1956.