Next Section |
Table of Contents
This proposal succeeds a preliminary proposal submitted to the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) in September 1997 to drill drowned reefs and reef terraces at Tahiti. The present submission is a revised version of Proposal 519, which was assessed in Fall 1998 by the Environment Science Steering and Evaluation Panel (ESSEP). It includes some revised sections that address a few comments raised in the panel discussion and concerning the scientific objectives and technical/operational issues. As noted by ESSEP during its fall 1998 review, the goals are well-aligned with the high-priority objectives of the Long Range Plan (1996) and cover the two subthemes Earths Changing Climate and Causes and Effects of Sea Level Change. These aims are related to other international scientific programs such as International Geosphere-Biosphere Program (IGBP), Past Global Changes (PAGES) and PAGES/Climate Variability and Predictability (CLIVAR) interface.
1. To establish the course of postglacial sea level rise at Tahiti (i.e., to define the exact shape of the deglaciation curve for the period 20.0010.00 ka.
The expected results are the following:
- To assess the validity, timing, and amplitude of the MWP-1A event;
- To assess the maximum sea level drop during the LGM,
- To prove or disprove the sawtooth pattern of sea level rise during the last deglaciation (Locker et al., 1996), and
- To test predictions based on different ice and rheological models.
Reconstruction of sea level curves relies on absolute dating of in situ corals by radiometric methods (230Th/234U by thermal ionization mass spectrometry [TIMS], 14C by accelerator mass spectrometry [AMS]), and paleobathymetric information deduced from biological communities (corals, algae, and mollusks) that live in a sufficiently narrow or specific depth range to be useful as absolute sea level indicators.
2. To define SST variations for the region over the period 20.0010.00 ka in order gain better knowledge of
- Regional variation of SSTs in the South Pacific,
- Climatic variability and the identification of specific phenomena such as ENSO, and
- Global variation and relative timing of postglacial climate change in the southern and northern hemispheres.
Methods include stable isotope ( 18O) and trace element (Sr/Ca ratios by TIMS) analyses on high-resolution (i.e., at the monthly scale) sampling of massive coral colonies. Coupled analyses of d18O and Sr/Ca on the same sample may yield estimates of both temperature and salinity (McCulloch et al., 1996). Stable isotope 18O measurements, systematically coupled with those of 18O in coral skeletons, will provide information on other parameters (e.g., solar variations or metabolism processes). Geochemical methods will be coupled with measurements and analyses of the bandwidths and microstructural variations in the coral skeletons.
3. To analyze the impact of sea level changes on reef growth and geometry and, especially
- The impact of glacial meltwater phases (identification of reef drowning events),
- The morphological and sedimentological evolution of the foreslopes (highstand versus lowstand processes),
- Modeling of reef building, and
- Environmental changes during reef development.
A numerical model simulating reef building will be used to study the effect of a sea level jump on reef geometry and to qualitatively assess the effect of sea level fluctuations on the reef shape and age-depth relationships.
The present proposal may provide the opportunity to better constrain the deglacial history (see Peltier, 1994; Fleming et al., 1998; Okuno and Nakada, 1999) by documenting, for the first time, the LGM lowstand in well-studied cores in the far field and by comparing the MWP-1A event in the Pacific and Atlantic Oceans. Furthermore, the study of very early deglacial coral material should provide the first Sr/Ca SSTs for the LGM in the Pacific, which could then supplement Barbados sample data (Guilderson et al., 1994) and the recent study of Stage 6 corals (McCulloch et al., 1999).
Next Section |
Table of Contents