The last deglacial sea-level & climate changes.Coral reef records in Checreef: South Pacific: Tahiti (French Polynesia) IODP expedition

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences


The timing and course of the last deglaciation (23,000-6,000 calendar years BP - cal. yr BP) are essential components for understanding the dynamics of large ice sheets and their effects on Earth's isostasy as well as the complex relationship between freshwater fluxes to the ocean, thermohaline circulation and, hence, global climate during the Late Pleistocene and the Holocene. This proposal is related to the IODP proposal #519 concerning the coral reef records of Tahiti and the Australian Great Barrier Reef to establish the course of sea-level rise, climate variability and reef response during the last deglaciation. It includes : 1) the study of more than 600 m of reef cores with an exceptional recovery that were retrieved from 37 holes ranging from 40 to 120 m water depth around Tahiti during the IODP Expedition #310 « Tahiti Sea Level ». Distinctive levels of relict reefs covering most, if not all, the last deglaciation were drilled and therefore confirmed the significance of these features as unique archives of abrupt global sea-level rise and climate change. 2) Complimentary investigations of the Tahiti reef slopes, and 3) a site survey cruise on the Great Barrier Reef to generate high resolution bathymetric and seismic data that will be used to select suitable targets for an IODP drilling expedition corresponding to the part 2 of the IODP drilling proposal #519. The general scientific objectives of this proposal are threefold : a. To establish the course of post-glacial sea-level rise during the Last Deglaciation . b. To define SSTs and SSSs variations during the Last Deglaciation when solar insolation, sea level, and atmospheric CO2 levels were different from today. c. To analyze the impact of sea-level and environmental changes on reef development during the Last Deglaciation, with a special emphasis on the comprehensive reconstruction of environmental changes.


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Description Background: This project was funded to help ensure major UK involvement in leading successful IODP drilling of the Australian Great Barrier Reef. The original grant proposal formed part of a larger ESF proposal (CHECREEF) submitted through the EuroMARC Programme. The CHECREEF project involved research related to two IODP reef drilling targets: Tahiti (drilled as Expedition 310) and the Australian Great Barrier Reef (recently drilled as Expedition 325). In both cases the key objectives of the IODP drilling were to elucidate the rates, mechanisms and drivers of sea level and tropical climate change during the last deglaciation and during previous glacial periods. Our NERC-funded contribution was to help lead in the Site Survey Cruise for the Australian Great Barrier Reef component of this work, and more specifically to help lead in the identification of sites and targets for subsequent drilling.

Site Survey Cruise: The NERC grant made a significant contribution to funding the Site Survey Cruise on the Great Barrier Reef using CSIRO ship 'Southern Surveyor' on cruise SS07/2007, 26th September to 16th October 2007. As a consequence of the funding, Tudhope (Edinburgh) and Thomas (Oxford) were participants in that cruise and were thus able to make a substantive contribution to the collection of the seabed samples and data (seismic, swath imaging and AUV photographic data) required to identify drilling targets and to ensure that all IODP and Australian safety and environmental considerations were satisfied prior to approval of drilling.

U-series dating: A key objective of the NERC-funded work was to provide preliminary age estimates of dredged material to guide subsequent IODP drilling. U-Th dating was performed at the University of Oxford. Six corals from the drowned reefs on the shelf edge were measured: 2 from the northerly Ribbon Reef site in 46-48 m of water and 4 from the Hydrographers Passage to the south in 88-92 m of water. These produced ages ranging from 23±21 years to 19,190±80 years. This indicated that, although the reef-like features observed by multibeam bathymetry were experiencing modern coral growth, there was material of deglacial age on the surface of some of these features. This was the first confirmation that these features comprised deglacial reefs, and therefore demonstrated the potential for IODP drilling. A further three corals from a much deeper water site (Gloria Knolls) were also dated. These samples required a stepwise cleaning procedure and multiple measurements to correct of initial 230Th contamination and hence provide reliable ages. These samples returned ages from 1070±30 years to 302,000±19,000 years, providing insight into the age of formation of the knolls as well as the longevity of the deep marine ecosystem they are sustaining.

IODP Expedition 325: The research reported here made a significant contribution to ensuring the successful IODP drilling of the Great Barrier Reef as Expedition 325 (GBREC) in early 2009. In particular, the selection of drill sites on the basis of seabed and sub-seabed surveys and the U-Th dating of dredged samples proved to be an outstanding success as the drill cores successfully sampled all the key time intervals that had been targeted. This included reefs that grew during the last glacial maximum (the first time such reefs have been sampled), through the first half of the deglaciation, including periods of extremely rapid sea level rise, and during previous glacial periods. This has provided a unique set of samples with which to address the key scientific objectives concerning the rates, mechanisms and drivers of sea level and tropical climate change. Alex Thomas (Oxford) and Alexander Tudhope (Edinburgh) are Expedition scientists in this work, and (with Henderson in Oxford) have been funded through subsequent NERC IODP grants to ensure continued UK leadership in the science.
Exploitation Route This work led to the successful scientific drilling of the Great Barrier Reef of Australia (IODP Expedition 325) to reveal new insights into the rates and timings of past sea level and climate change in the region.
Sectors Environment