Geochemistry, Mineralogy, and Stable Isotopic Results from Ala Wai Estuarine Sediments: Records of Hypereutrophication and Abiotic Whitings

The geochemistry, mineralogy, and stable isotopic composition of sediments cored from the Ala Wai Canal, described for the first time here, provide a record of past changes in salinity, oxygenation, and eutrophication in a shallow, subtropical artificial estuary. Sediments of the canal are rich in organic carbon (ca. 1-8%) and calcium carbonate (ca. 6-68%). The carbonate fraction contains a mixture of aragonite (ca. 2-25%), magnesian calcite (ca. 1-38%), and calcite (0-11 %). The majority of this carbonate seems to be a direct result of biogenically induced inorganic precipitation from the water column. This interpretation is supported by historical measurements of hypereutrophication in the water column, the appreciable lack of biogenic carbonate in the sediments, the presence of carbonate throughout the canal, the fine grain size and mixed marine mineralogy of the carbonate, the significant positive correlation between CaC03 and uranium scavenged from the water column, the lack of detectable carbonate in associated fluvial sediments, the similarity between the isotopic composition of the carbonates and that of the total dissolved carbon in the present water column, and the positive covariance between accumulation rates of CaC03 and organic carbon in portions of the back basin core. Supersaturation with respect to these phases appears favored by high primary productivity and accompanying CO2 drawdown in warm surface waters. The process of precipitation is analogous to marine whitings and inorganic CaC03 precipitation in lakes, but to our knowledge this is the first reported occurrence documented from an estuarine system. Temporal variations in paleoproductivity, bottom water oxygenation, and changes in the water balance of the canal are assessed on the basis of CaC03 and organic carbon flux rates and by downcore variations in the isotopic composition of organic carbon, CaC03 , and benthic foraminifera. We demonstrate that the canal was, and continues to be, highly productive and that the back, landlocked basin of the canal has undergone episodes of progressive eutrophication at least twice since 1935. The first phase of eutrophication is marked by an upsection increase in the stable carbon isotopic gradient between surface and deep waters. During this time the back basin became hydrologically closed and its waters became fresher, as I University of Hawai'i School of Ocean and Earth Science and Technology contribution no. 3591 and UNIHI-SEAGRANT contribution no. JC-94-41. This study was funded by a grant to Craig R. Glenn from the donors of The Petroleum Research Fund, administered by the American Chemical Society, and from funds provided to Gary M. McMurtry and Eric H. De Carlo from the University of Hawai'i Sea Grant Program, Project R/ME-IPD. The National Science Foundation provided funds for the University of Hawai'i Young Scholars Program in 1991 and 1992, during which cores for this study were taken, as well as provided funds for the UH Research Experience for Undergraduates Program, which supported Jennifer Benaman's stipend. Manuscript accepted 29 July 1994. 2 Department of Geology and Geophysics, University of Hawai'i, Honolulu, Hawai'i 96822. 3 Department of Oceanography, University of Hawai'i, Honolulu, Hawai'i 96822. 4 Department of Civil Engineering, Florida Institute of Technology, Melbourne, Florida 32901. 5 Current address: Department of Civil Engineering, University of Texas, Austin, Texas 78712.