Date of Award
Doctor of Philosophy in Oceanography
Dana R. Kester
This thesis consists of a study of the relationship between trace metal chemistry and the carbon dioxide system in the marine environment. The distribution and speciation of several metals with an emphasis on manganese were studied to evaluate the effect of carbon dioxide system, water composition and biological processes. Seawater samples collected in January 1984 along the continental shelf and slope off Peru between latitude 10 to 11°S and longtitude 78 to 79°W were used for this study.
An equilibrium model was used to predict the changes of inorganic speciation of trace metals in the marine environment due to changes in water properties. The ionic strength dependence of stoichiometric association constants for both major ions and trace metal complexes was based on a Pitzer-type function. Calculations were made for oceanic and estuarine waters based on thermodynamic data at 25°C and 1 atmosphere pressure. The range of speciation in near-surface ocean waters was based on the chemical characteristics of a highly productive region off Peru. The salinity of these waters was nearly constant at about 35 o/oo. The only significant chemical property that would affect the speciation of trace metals in the region off Peru was pH which ranged from 7.75. to 8.37 in the upper 30 meters. Hydroxy complexes were the major forms of iron(III). Changes in pH were important for the speciation of Pb(II) and Cu(II) for which carbonate complexes were the major species. For Mn(II) and Fe(II), free ions and their chloro complexes contributed more than 90% of the total species. Changes composition of seawater play an important role in Mn(II) and Fe(II) speciation. The chemical speciation of Mn(II) at various salinities was also examined. The amount of MnCl+ complexes increased from less than 0.1% in world-average river water to about 30% in seawater of salinity 35 o/oo.
Measurements were made of pH, alkalinity and related biologically active chemicals in the waters off Peru. These data were used to calculate the inorganic carbon species concentrations and the degree of calcite and aragonite saturation in these waters. The total carbon dioxide and oxygen data indicated an upwelling location at about 6.5 km offshore. The water was upwelled along sq of 26.1 at a depth not greater than 100 meters. The recently upwelled water had low pH of less than 7.8 which corresponded to the low oxygen and high nutrient concentrations in this region. The highest biological activity was found at about 28 km offshore. At this station supersaturation of oxygen and a low concentration of- total carbon dioxide was found. The pH of surface water was as high as 8.36. The value of pCO2 of surface water at this station was 242 μatm which is lower than the atmospheric value of 330 μatm. The calcium carbonate saturation indicated that the I water of nearshore stations was supersaturated with both calcite and aragonite even in deep water. For the slope and oceanic stations, the waters were undersaturated with respect to calcite below 400 meters and with respect to aragonite below l00 meters. A Deffeyes' plot implied that biological activity was the important process in the water of the upper 100 meters. This conclusion was supported by the apparent oxygen utilization - total carbon dioxide relationship which had a slope close to the Redfield ratio.
Manganese in this region is particularly interesting because its oxidation state may change between the oxidizing surface waters and the low oxygen subsurface waters. The concentrations of manganese(II), dissolved manganese and total manganese in seawater of this area were measured. Manganese was preconcentrated on silica-immobilized 8-hydroxyquinoline cartridges. The manganese was eluted from the cartridge with 2N HCl-0.lN HNO3 and analyzed by a Perkin Elmer Model 5000 atomic absorption spectrophotometer with a Model 500 heated graphite furnace. The total manganese concentration was 20-40 nmol/kg at the surface of a nearshore station and it decreased to 3-5 nmol/kg at about 30 km offshore. Manganese(II) was the dominant form of dissolved manganese in this region. 1 An increase in dissolved manganese was evident in the oxygen minimum zone and in the bottom water of nearshore stations as a result of reduction processes and input from the bottom sediment.
This study indicated that in productive areas, biological processes are important in regulating the chemical properties of the water. Variations in pH were found to be highest in the euphotic zone. The distribution and chemistry of trace metals were also found to be influenced by biological activity. An increase in dissolved manganese concentration was found in the oxygen minimum zone and near bottom water of nearshore stations as a result of organic matter decomposition. Changes in pH also affected the speciation of trace metals, such as Fe(III), Pb(II) and Cu(II), because their hydroxy and carbonate complexes were the predominant species in seawater.
Thumtrakul, Wilaiwan, "THE CARBON DIOXIDE SYSTEM AND ITS ROLE ON TRACE METAL CHEMISTRY IN THE MARINE ENVIRONMENT" (1984). Open Access Dissertations. Paper 1410.