sediment, carbonate, radiocarbon, Hawaii, coral, algae, Holocene AGE AND COMPOSITION OF CARBONATE

The origin, age, and dynamics of carbonate sediments in Kailua Bay on Oahu, Hawaii, are described. The shoreface (from shoreline to 4 km offshore) consists of a broad (5 km2) fringing coral reef ecosystem bisected by a sinuous, shore-normal, sand-filled paleostream channel 200–300 m wide. The median grain diameter of surface sands is finest on the beach face (<0.3 mm) and increases offshore along the channel axis. Kailua sands are>90 % biogenic carbonate, dominated by skeletal fragments of coralline algae (e.g. Porolithon, up to 50%) followed by the calcareous green alga Halimeda (up to 32%), coral fragments (1–24%), mollusc fragments (6–21%), and benthic foraminifera (1–10%). Sand composition and age across the shoreface are correlated to carbonate production. Corals and coralline algae, principal builders of the reef framework, are younger and more abundant in sands along the channel axis and in offshore reefal areas, while Halimeda, molluscs, and foraminifera are younger and more dominant in nearshore waters shoreward of the main region of framework-building. Shoreface sediments are relatively old. Of 20 calibrated radiocarbon dates on skeletal constituents of sand, only three are younger than 500 yr BP; six are 500–1000 yr BP; six are 1000–2000 yr BP; and five are 2000–5000 yr BP. Dated fine sands are older than medium to coarse sands and hence ma