Late Holocene sea level, Gilbert and Ellice Islands, west central Pacific Ocean

Abstract

Abstract Four coastal deposits are recognised: two intertidal and/or supratidal—calm:weather and stormy-weather beach breccia and conglomerate; and two subtidal—back-reef breccia and biohermal reef rock. High-level outcrops of the subtidal facies provide evidence for six second-order transgressions of Late Holocene age. Most of these have been radiocarbon dated from Tridacna shells. Where Tridacna samples were available instead, all coral dates were rejected because of secondary aragonite contamination. The ages of the second-order transgressions correlate well with transgressions recorded from New Zealand and fall on a curve representing the first-order Flandrian Transgression and subsequent regression which in these islands reached a maximum of about +2.4 m 2760 radiocarbon years b.p. This maximum is about 1250 years younger than the +2.1 m maximum recorded for the northern part of New Zealand, and thus fits a predicted difference in timing of the Flandrian Transgression maxima caused by oceanic salinity changes. The net fall in sea level of 2.4 m from the Flandrian Transgression maximum has been of major importance in the development of atoll islets, which, mainly, if not wholly, as a result of this fall, have been built from sand taken from the shallow margins of the lagoonal floor. It may be no coincidence that the earliest Micronesian and Polynesian settlements date from about the period of the transgression maximum.