Geochemistry, Mineralogy, and Stable Isotopic Results from Ala Wai Estuarine Sediments: Records of Hypereutrophication and Abiotic Whitings
Geochemistry, Mineralogy, and Stable Isotopic Results from Ala Wai Estuarine Sediments: Records of Hypereutrophication and Abiotic Whitings
Date
1995-10
Authors
Glenn, C.R.
Rajan, S.
McMurtry, G.M.
Benaman, J.
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University of Hawaii Press
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Abstract
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 ofCaC03 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 indicated by the compositions and covariance in carbon and oxygen isotopic
values of the carbonates. The second phase began about the time that the
canal's sediment sill was dredged and is marked by an upsection increase in the
carbon isotopic composition of authigenic carbonates and benthic foraminifers,
reflecting a progressive increase in primary productivity and water-column stratification
through time. Oxygen isotope results suggest that the second phase is
also marked by increased freshening of the back basin as the sediment sill has
built back to the canal's surface.
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Glenn CR, Rajan S, McMurtry GM, Benaman J. 1995. Geochemistry, mineralogy, and stable isotopic results from Ala Wai estuarine sediments: records of hypereutrophication and abiotic whitings. Pac Sci 49(4): 367-399.
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