ABSTRACT

Sediments of the bay and coastal plain of Kailua (Oahu, Hawaii) are > 90% biogenic carbonate produced by destruction of primary reef framework (coral and encrusting coralline algae) and by direct sedimentation through the biological activity of calcifying organisms (the green alga Halimeda, the branching coralline alga Porolithion gardineri, molluscs, and benthic foraminifera). Field measurements of benthic community structure, gross carbonate production, bioerosion, and direct sedimentation in 17 physiographic zones in Kailua Bay are used to calculate modern calcareous sediment production rates in the 12 km² fringing reef system.

Total gross carbonate productivity by corals and encrusting coralline algae (based on mapped percent cover and known growth rates) occurs at an average rate of 1.22 (± 0.36) kg m^-2 y^-1 over hard substrates of the reef platform, corresponding to 0.8 (± 0.2) mm y^-1 (using a bulk density of 1.48 g cm^-3, the average of coral and coralline algae). Coralline algae contributes 42% of the total gross productivity. Bioerosion of coral and coralline algae facies at and near the reef surface (estimated from slabbed reef samples) occurs at average rates of 0.10-1.15 kg m^-2 y^-1 and releases 1,911 (± 436) m³ of unconsolidated carbonate sediment annually. Mechanical erosion (coral breakage) likely contributes an additional 315 m³ y^-1.

Carbonate sediment is also produced directly by the green alga Halimeda, branching coralline algae, molluscs, and benthic foraminifera at a combined rate of 1,822 (± 200) m³ y^-1 (using densities specific to sediment origin). The total rate of production of unconsolidated carbonate sediment in Kailua Bay is the sum of these sources, amounting to 4,048 (± 635) m³ y^-1. Normalizing gross sediment production (in kg y^-1) by reef habitat area (in m²) generates average rates of productivity (in kg m^-2 y^-1) that are directly comparable to each other and to the literature; such rates can also be employed and tested in other reef settings. In Kailua Bay, the total production of calcareous sediment corresponds to an average (normalized) rate of 0.53 (± 0.19) kg m^-2 y^-1, with 0.33 (± 0.13) kg m^-2 y^-1 contributed through erosion of the coralgal framework and 0.20 (± 0.06) kg m^-2 y^-1 contributed by direct sediment production on the reef surface.

Applying these modern sediment-production rates over the 5,000 years that Kailua Bay has been completely inundated by postglacial sea-level rise, an estimated 20.2 (± 3.2) 10⁶ m³ of unconsolidated carbonate sediment has been produced in the system. The volume of sediment stored in the various reef channels and holes in Kailua Bay is 3.7 (± 0.3) 10⁶ m³, or 19% of that produced since 5,000 y BP. The volume of sand in the modern beach is 1.0 (± 0.1) 10⁶ m³, or 5% of Holocene sediment production. The volume of carbonate sediment stored in the coastal plain is estimated, using core-log data and associated radiocarbon ages, to be 10.0 (± 1.8) 10⁶ m³, or 51% of Holocene sediment production. The remaining 25% likely represents sediment loss due to the natural processes of dissolution, attrition, and transport offshore. These export terms are not well understood and emphasize the need for sediment dynamics to be incorporated into reef and sediment budgets. Although sediment production in this reef system is prodigious, the rate of "new" sediment supplied to the beachface is less than 2% of what moves on and off the beach seasonally.