Holocene sedimentary and aquatic biogeochemical responses reflected in Ordy Pond, Oahʻu, Hawai'i, and, contemporary modeling of submarine groundwater discharge in Kahana Bay, Oahʻu, Hawaiʻi
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2002-12
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University of Hawaii at Manoa
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This dissertation explores aquatic geochemistry in two settings on O'ahu. Ordy Pond biogeochemistry responds rapidly to environmental changes, while Kahana Bay is influenced by significant groundwater and surface water influx and mixing with coastal ocean water. Thus, Ordy Pond is a better tool for studying Oahu's paleoenvironments, and Kahana Bay demonstrates the applicability of natural chemical tracers to quantify submarine groundwater discharge (SGD). Work in Ordy Pond consisted of modern water column and particulate production time-series analyses, and sediment core study. Air temperature changes appeared to control the seasonal water chemistry. When the pond was thermally stratified in the spring/summer, epilimnion primary productivity increased dramatically as indicated by dissolved O2, dissolved inorganic carbon, S13CDIC, and particulate production rates. Once the density gradient waned, the anoxic hypolimnion mixed with the epilimnion, organic matter productivity fell, and surface waters became suboxic to anoxic. Primary productivity is believed to have been controlled by the availability of sunlight; the longer daily photoperiod during stratification resulted in greater organic matter production, while greater mixing in winter reduced the photoperiod, increased phytoplankton respiration, and reduced organic matter production. The carbonate mineral flux increased during elevated primary productivity due to increased carbonate mineral saturation states. Modern particulate production closely reflected the water chemistry, and the biogeochemical processes in the modern pond were used to interpret the Holocene environmental history of the area through analysis of pond sediments. The sediments revealed three dramatic paleoenvironmental events: 1) the most recent post-glacial sea level rise around Oʻahu ~ 9.7 kya; 2) sea level fall ~1 kya following Oahu's mid Holocene Sea level high stand; and 3) Western human contact with O'ahu and the introduction of plantation-scale agriculture. Finally, in a separate study of the submarine ground water discharge to Oahu's coastal waters, Kahana Bay was found to experience a much greater exchange with the surrounding environment than Ordy Pound. Total SGD in Kahana Bay is significant (90 x 106 L d-1), providing 16% as much terrestrial water, five times as much phosphorus, and two times as much nitrogen as surface water runoff.
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Geology and Geophysics; no. 4250
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