Reverse Weathering Reactions within Recent Nearshore Marine Sediments, Kaneohe Bay, Oahu

Abstract

The purpose of this study is to present the results of mineralogical and petrochemical analyses of the solid phase components and the inorganic chemistry of the interstitial waters of the Recent anoxic sediments of Kaneohe Bay, Oahu. Nineteen shallow 1-4 meter gravity cores of the lagoonal sediments of Kaneohe Bay were analyzed for pore water chemistry and seven were subjected to detailed mineralogical and petrochemical analyses.

The pore waters of the sediment column show depletions in dissolved SO =4, Ca++, Mg++ and Sr++ accompanied by increases in titration alkalinity, NH4 + , PO 4 -3 and Si02 with respect to the overlying seawater with increasing subbottom depth. Na+, Cl-, K+ and Fetot exhibit minor departures from overlying bay waters assuming that depletions of Na+ and Cl- are the result of an influx of meteoric ground water from beneath the bay's floor. The bay may be divided into two parts on the basis of the rates of pore water diagenesis: in the southern part of the bay, S0 =4 is completely depleted within 80cm subbottom depth, whereas in the northern part, complete S0 =4 reduction does not occur at depths to 350cm. The southern sediments are contaminated by raw, high C/N sewage, resulting in an increased metabolic reduction rate of S0 =4 by anerobic bacteria over that observed in the unpolluted northern bay. Calculation of S0 =4 consumed versus alkalinity plus NH=4 produced indicates a relationship in which roughly one-half of the "produced alkalinity" has been consumed in the formation of authigenic minerals, primarily nontronite and aragonite.

Quantitative mineralogical and petrochemical analyses of the solid phase components reveal the loss of amorphous iron-oxyhydroxides, biogenic opaline silica, and amorphous aluminosilicate with increasing subbottom depth. Pyrite formation occurs immediately below the sedimentwater interface. Scanning Electron Microscope observations show a hierarchy of morphologies with depth: single l-micron crystals to 30- micron diameter framboids. Pyrite formation accounts for the lack of detectable S= within the pore waters and is dependent on the availability of pore water iron derived from the dissolution of amorphous iron-oxyhydroxides. The amount of pyrite present below 40cm subbottom depth exceeds the amount which could be formed by the complete reduction of buried pore water S0=4 suggesting the importance of bioturbation in the mixing of pore and overlying seawaters.

Authigenic nontronite and mixed-layer smectite-illite are being formed as the result of the reaction of amorphous aluminosilicate with pore water Si02 from opal dissolution and pore water Fe and/or other cations. In those cores where sufficent dissolved iron exists in the pore water, nontronite forms, whereas when dissolved iron is not present as evidenced by the presence of dissolved S= in the pore water, a mixed-layer smectite-illite is formed. The amount of smectite formed is limited by the amount of opal which dissolves. For Kaneohe Bay sediments an average of 0.12 weight percent authigenic smectite is added annually to the sediment column. Minor amounts of authigenic plagioclase, phillipsite, clinoptilolite, analcime, sepiolite, siderite and apatite are also being formed within the sediments.

The relationship between reduced pore water Fe and smectite formation suggests that reverse weathering reactions resulting in either authigenic nontronite or mixed-layer smectite-illite may occur in all anoxic marine sediments rich in terrigeneously-derived, poorly-crystalline "kaolinite" and containing enriched pore water Si02. Assuming that 10 percent of the total flux of the world river sediments delivered to the ocean is deposited in Kaneohe Bay-type environments and that rates of reaction are similar to those observed in Kaneohe Bay, then approximately 6 percent of the CO2 consumed by rock weathering may be returned annually to the atmosphere by these reactions.