Submerged fossil corals : archives of diagenesis, subsidence and sea level.

Abstract

Corals are well suited as absolute markers of sea-level. Given the modern elevation of the coral, the age of coral, sea level at the coral's time of death, and the paleogrowth position of a fossil corals allow one to compute a vertical tectonic rate of motion; if the tectonic rate is known then one can use the sample's location, paleogrowth position, and age to compute a paleosea level. I studied the uranium-series isotopic composition of fossil corals from three settings: New Britain, Papua New Guinea, Hawaii and the Huon Gulf, Papua New Guinea. These three settings are distinct in their tectonic setting: New Britain corals are emergent as a result of tectonic uplift and the Hawaiian and Huon corals are submergent. Corals samples from uplifted terraces in New Britain are highly altered due high rainfall rates in the area. The measured 230Th ages of corals from the Holocene terrace are between 4.3 ± 0.03 ka to 9.0 ± 0.16 ka. Using these ages, their present elevation and the corresponding paleo sea level, I computed an average uplift rate of 1.6 ±0.4 m/ka. Uranium-series isotopic compositions of submerged corals are not better preserved than their subaerially exposed counterparts, although their diagenetic signatures differ. I report 230Th ages of three fossil reefs from the northwestern coast of Hawaii: the -400 m reef at Mahukona (136.7 ± 0.9 ka to 151.8 ± 1.7 ka), the -1000 m terrace off eastern Kohala (377.2 +13.4/-12.2 ka and 392.5 +20.5/-17.9 ka) and the -1000 m reef off northwestern Kohala (286.5 ± 1.4 ka to 342.8 ± 1.4 ka). From the submerged Hawaiian samples I identify three main trends in uranium-series isotopic composition: (1.) An increase in both 230Th/238U and 234U/238U; (2.) an increase in 230Th/238U with little change in 234U/238U; and (3.) low 230Th/238U with both low and high 234U/238U. Measured 230Th ages of submergent coral samples from the Huon Gulf range from 60 ka to infinite age (>600 ka). The measured 230Th ages of coral samples are older than we expected, but broadly increase in age with depth. Corals from three terraces (-1280 m, -1650 m, and -1950 m) represent material from Stage 11, suggesting that the model of terrace development is likely more complex than the original idea of a distinct sea-level rise event per terrace. Although I am able to identify three main trends in the uranium-series isotopic compositions of the Hawaiian samples, no clear trend emerges in the Huon Gulf setting. The subsidence rate (m/ka) computed from the most reliable ages of the deepest terraces suggests that the subsidence in the Huon Gulf averages 4 m/ka