FY 1987

Carbonate chemistry of the North Pacific Ocean

Chen, C.T.A., M.R. Rodman, C.L. Wei, E.J. Olson, R.A. Feely, and J.F. Gendron

DOE Technical Report TR034, DOE/NBB-0079, 176 pp (1986)

Data collected by us along two longitudinal cruises serve as the main data sources, and supplementary data sets in the literature are selected for delineating the distribution of physical and chemical properties in a wide area of the North Pacific Ocean. Oxygen, pH, alkalinity, total CO, and nutrients are interrelated parameters. Along our two longitudinal sections they all show a core structure underlying the salinity minimum layer. From these oxidation-related parameters we conclude that the subsurface water of the eastern North Pacific Ocean is older than that of the western North Pacific Ocean. Alkalinity data can be used as a water mass tracer. Different water masses reveal their own mixing trends which can be identified when we examine the correlation of normalized alkalinity with temperature. The vertical distribution of the normalized alkalinity shows a maximum core at a depth of about 2500 m in the North Pacific Ocean. Calcium carbonate dissolution and circulation in the deep and bottom layers contribute to the formation of the normalized alkalinity maximum core. Our calcium and alkalinity data indicate a CaCO dissolution rate of 0.060 and 0.053 µmol/kg/yr, respectively, referenced to the Weddell Sea Deep Water for waters deeper than 2000 m. The organic carbon decomposition rate is 0.107 µmol/kg/yr and the oxygen consumption rate is 0.13 µmol/kg/yr, also for waters below 2000 m. Our analysis of carbonate data shows that about 25% of the increase in total inorganic CO in deep water, in its journey from the surface of the Southern Ocean to the depth of the North Pacific, results from inorganic CaCO dissolution. No significant difference in the inorganic carbon/organic carbon ratio exists between our two sections. We found, however, that the eastern section has a higher total TCO input than that of the western section. The degree of saturation with respect to calcite and aragonite was calculated from all available data sets. Four selected cross-sections, three longitudinal and one latitudinal, and two three-dimensional graphs show that a large volume of the North Pacific is undersaturated with respect to CaCO. The saturation horizon generally shows a shoaling from west to east and from south to north in the North Pacific Ocean. It was found that the lysocline falls at a depth much deeper (about 2500 m deeper) than the saturation horizon of calcite and several hundred meters shallower than the calcium carbonate compensation depth. Our results appear to support the kinetic point of view on the CaCO dissolution mechanisms. Our calculations on the excess CO show that its penetration depth is strongly related to circulation. The shallowest penetration depth is less than 300 m found in the eastern equatorial region where upwelling prevails an the deepest penetration depth is deeper than 2000 m off Japan where an interaction of Oyashio and Kuroshio currents is found. These results agree with conclusions drawn based on freons, tritium and carbon-14 data. Overall the North Pacific contains 14.7 ± 4 × 10 g excess carbon.