Carbonate sediment budget of Kailua Bay, Oahu, Hawaii
Multispectral and 3D imagery throughout courtesy Ebitari Isoun
Jodi N. Harney
Kailua Bay on windward (northeast) Oahu is the field study area for my dissertation research in coastal geology. I primarily study carbonate sediments, their origin, and their relationship to reef development and coastal evolution. My current project is constructing a "sediment budget" in which I quantify the production, storage, and loss of carbonate sediment in Kailua Bay during the last 5000 years. This page will introduce you to the concept of sediment budgets, the parameters we employ, and the significance of the project. A link at the bottom of this page will lead you to details on the study area, and successive links will take you through general aspects of carbonate sediment production and storage in this coastal region.
Parameters of a sediment budget Sediment budgets are quantitative estimates of particulate sources, sinks, and losses within a geographically well-defined natural system. Among shallow-marine carbonate reefs, a comprehensive sediment budget (e.g. Hubbard et al. 1990, Scoffin et al. 1980, Land 1979, Stearn et al. 1977) must address the following processes (illustrated in a figure below):
Parameters of a sediment budget
Sediment budgets are quantitative estimates of particulate sources, sinks, and losses within a geographically well-defined natural system. Among shallow-marine carbonate reefs, a comprehensive sediment budget (e.g. Hubbard et al. 1990, Scoffin et al. 1980, Land 1979, Stearn et al. 1977) must address the following processes (illustrated in a figure below):
Temporal and spatial boundary conditions must be specified for the region and period over which the model will operate:
(1) Time scale
(2) Geographic region of interest
(3) Benthic community structure
We ask the following questions:
We pursue these questions for Kailua Bay using:
The significance of a high-resolution sediment budget lies in its capacity for improving knowledge of reef productivity, benthic diversity, response to stresses, and geologic development through time. This can provide researchers with a better understanding of how reefs function as complex systems of living and geologic components and how they respond to changes in boundary conditions. This is useful for improving various problems such as: understanding trends in reef biodiversity; coastal erosion and sediment processes; the architecture of carbonate strata; and other needs that are best addressed with observational data from the reef environment.
Kailua Bay is located on the northeast (windward) coast of Oahu in the Hawaiian Archipelago. The embayed shoreline lies between volcanic features that define the geographic limits of the system (Lanikai and the Mokulua Islands to the east [left side of the image]; Mokapu Peninsula to the northwest [right side of the image]). The seaward extent of the model region is approximately contiguous with the 30 m depth contour and with the seaward base of the deep reef margin.
The bay experiences an annual range of wind, wave, and water quality conditions. Trade winds blow onshore at 10-25 kts 90% of the summer, generating waves 1-3 m in height with periods of 6-9 s. During the winter, trade winds blow at variable speeds 50-80% of the season, and storms in the North Pacific deliver large swells (4 m height, 10-20 s periods) that may refract into the bay. Observational data sets describing conditions are available from various sources (principally NOS buoys and weather stations) extending through recent decades. Learn more about the study area by visiting my Kailua page.
Click to see a larger version.
A georeferenced, orthographically-rectified, digital mosaic of airborne multi-spectral data (above) illustrates the distribution of benthic substrates in Kailua. Dark areas are consolidated substrate, living coral, and algae; light areas are typically sandy regions. The sandy shoreline grades seaward to a shallow (<5 m depth), fossil limestone surface with a veneer of sand (hardgrounds). Various benthic environments of this region are composed of sand, rubble, limestone outcrops, meadows of the calcareous alga Halimeda and fleshy algae such as Sargassum, and mounds of Porites lobata like the one below.
Photo courtesy of Eric Grossman.
The shoreface is dominated by a broad (4 sq km), shallow fringing reef 5-20 m in depth. The inner reef platform lies in 5-8 m water depth and is composed largely of fossil reef outcrops with 25-50% living coral cover. The outer reef platform extends seaward to 20 m depth as spur and groove topography with 50-100% living coral cover. Along the reef margin 3 km offshore, the fore-reef slopes steeply seaward (in some places as talus and elsewhere as living coral cover) to abut a sand field at -25 m (Hampton et al. 1998). The scleractinian coral genera Porites, Montipora, and Pocillopora characterize the reef platform, and their distribution, abundance, and morphology varies with depth and hydrodynamic energy. A photo of the reef slope is shown below.
Photo courtesy of Eric Grossman.
Coralline (red) algae are also prolific inhabitants of the benthic community at all depths. Branching Porolithon gardineri is most abundant in shallow waters, while encrusting Porolithon onkodes and other species are found in all depths and habitats.
Age and composition of sediments
Beach and submarine sediments of Kailua Bay are primarily composed of carbonate skeletal fragments produced by two processes: mechanical, chemical, and biological destruction of reef framework limestone (coralline algae and coral) into rubble, sand, and silt; and direct sedimentation upon the death of organisms such as Halimeda, molluscs, and foraminifera. Although coral is abundant atop the reef platform, it is a minor constituent (<15%) of most sediment assemblages. Shoreface sands are instead dominated by coralline algae (up to 60%) and Halimeda (up to 35%). Other constituents include molluscs (15-20%), benthic foraminifera (<10%), and echinoderms (<5%).
Despite an apparently healthy ecosystem engaging in active carbonate production, sediments in Kailua are dominantly fossil-aged. Of 20 accelerator-radiocarbon ages, only one dates post-1950; twelve ages are 500-1000 calendar years before present (cal yr BP); five are 2000-5000 cal yr BP. Sand composition and age across the shoreface broadly reflect a spatial relationship to carbonate production. Corals and coralline algae, principle builders of the reef framework, are younger and more abundant in sands along the channel axis and in offshore areas, while Halimeda, molluscs, and foraminifera are younger and more dominant in sediments landward of the main region of framework-building. The figures below illustrate our age data (Harney et al. in press)
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