Strontium isotope stratigraphy of Kita-daito-jima Atoll, North Philippine Sea: implications for Neogene sea-level change and tectonic history

Abstract

A detailed chronology of a 432 m core from Kita-daito-jima, an atoll on the Philippine Sea plate, has been obtained by strontium isotope stratigraphy. The atoll carbonate extends to late Oligocene age. The top ∼ 100 m Plio-Pleistocene section of the core has been dolomitized, and dolomite also extends to middle Miocene sediments. Sr isotopes identify two dolomitization events, at ∼ 2 and ∼ 5 Ma. Atoll growth appears to have been continuous between 18.8 and 24.3 Ma and the fit of the growth history to a standard model provides a successful confirmation of Darwin's subsidence theory. The atoll growth appears to have commenced about 42 Ma ago (6 Ma after volcanism), producing a ∼ 2 km column of atoll carbonate, in agreement with bathymetry. The subsidence model defines an apparent loss of 360 m of the sedimentary record since the early-middle Miocene boundary. The amount in excess of the ∼ 100 m associated with sea-level fall is attributed to Plio-Pleistocene uplift at rates of ∼ 20–30 m/Ma associated with the forebulge of the Philippine Sea plate prior to its subduction. Uplift rates have also been estimated using (a) the heights of dated last-interglacial coral reefs on Kita-daito-jima and nearby Minami-daito-jima (∼ 17 m/Ma and ∼ 29 m/Ma), (b) Sr isotope ages of calcites from surface samples on Minami-daito-jima (∼ 21 m/Ma) and (c) contrasts in elevation with Okino-daito-jima 100 km away (∼ 22 m/Ma). The locations of hiatuses and ages of dolomitization indicate sea-level falls of ∼ 80 m at ∼ 17–16 Ma, ∼ 30 m at ∼ 16–15 Ma, ∼ 125 m at ∼ 11 Ma, and ∼ 90 m at ∼ 5 Ma and at ∼ 2 Ma. The data have been combined with those from Enewetak Atoll to produce a curve of sea-level change for the Neogene.