Radiocarbon profile of Hanauma Reef, Oahu, Hawaii

Abstract

Hanauma Bay on the southeast coast of Oahu is a breached compound explosion crater invaded by the sea. Ten core holes through an active fringing reef within the bay provided 63 samples for which C 14 dates have been determined. These ages indicate that (1) the reef started growing about 7,000 yr ago; (2) most of its vertical growth was during the interval from 5,800 to 3,500 radiocarbon years ago, when its average upward growth rate was 1 m/300 yr; and (3) during the past 3,000 yr, it advanced seaward at the rate of 1 m/45 yr. The data indicate that Koko Bench at −5 m was formed at least 5,800 radiocarbon years ago when the sea stood from 5 to 9 m below its present level. This result is contrary to a previously suggested date of origin of 4,100 to 4,400 yr ago. During the past 3,500 yr, sea level on the reef has risen to its present position at a decreasing rate, but at an overall rate for the interval of 1 m/∼2,900 yr. It is unlikely that sea level ever stood appreciably higher than at present during the past 3,500 yr. Volcanic ash at the base of several cores marks the latest eruption of nearby Koko Crater, which the C 14 dates place at no later than 5,800 and possibly 7,000 or more radiocarbon years ago. In most cases, the radiocarbon dates follow a sequence that is consistent with stratigraphic position, but age inversions occur that are clearly beyond the statistically defined errors in age. They are probably caused by irregularities in the growth pattern of the reef near channels and large pockets. The profile of ages in this most extensively dated reef indicates that casual and sparse sampling of a reef could lead to dubious results in interpreting reef history and chronology of sea levels.