Precise chronology of the last interglacial period: ^(234)U-^(230)Th data from fossil coral reefs in the Bahamas

Abstract

A detailed study of ^(238)U-^(234)U-^(230)Th ages was made for different coral species from two Bahamian reefs to determine the time scale of the sea-level high during the last interglacial period, using recently developed mass spectrometric techniques for the measurement of U and Th isotopes. Thirty-seven coral samples were analyzed in replicate. Typical errors at 125 ky are ±1.5 ky (2σ), and 3-ky time intervals appear to be well resolved. This high precision permits detailed chronologic study with time resolution adequate to define stages of the reef's history. These data demonstrate that the time between transgression and regression of the ocean in the last interglacial was about 12 ky, after which sea level fell rapidly at more than ∼2 m/ky. The high sea-level stand began possibly by 132 ky and certainly by 129 ky ago, when sea level reached ∼6 m above present mean low sea level. High sea level was sustained until 120 ky and then fell rapidly. This time period covers the Milankovitch insolation peak at 65°N but is not sharply defined, and the sea-level high lasted from about 4 ky before the insolation peak to 8 ky after it. We find no evidence for a double peak of sea-level rise in the last interglacial episode (that is, high stands at both ∼125 and 142 ky). There appears to be some discrepancy between this precise chronology for a coral reef on a tectonically stable platform and the chronology assigned to deep-sea sediments. The present results show that the duration of the last interglacial led to a 12-ky period of high sea-level stand which is much longer than the insolation peak. The study area consists of two sizeable bank/barrier coral reefs of Sangamon age that crop out along the coast northwest of Cockburn Town on San Salvador Island and at Devil's Point on Great Inagua Island. The initial stage of reef development is not revealed at either site, but the main parts of both reefs are well preserved. At Cockburn Town, the beginning of the reef was marked by colonization of Acropora palmata on hardground areas and possibly other corals at 132 ky ago. Patch reefs at the southeastern flank of the developing reef were growing by 129 to 126 ky ago. By 126 to 123 ky ago, the crest of the reef was formed by the frame-building coral A. palmata, and associated patch reefs flourished along its flanks. A. palmata coral heads grew upward to a mean low sea level ∼6 m above the present one. The reef began to decline as sea level dropped at about 123 to 120 ky ago. Near two measured profiles at Devil's Point, the top of a rubblestone, dominated by A. cervicornis, and patch-reef corals preserved in growth position have been beveled off and represent an ancient wave-cut surface. ^(230)Th ages on corals below and above this surface indicate that it was cut at about 125 ± 1 ky ago.

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