Integration of core, logging and drilling data in modern reefal carbonates to improve core location and recovery estimates (IODP Expedition 310)

Abstract:

Palaeoclimate and sea-level studies commonly rely on interpretations based on depth. The location and quantity of core recovered during a drilling expedition has direct implications for subsequent applications of core measurements. A key objective of Integrated Ocean Drilling Program (IODP) Expedition 310 is to establish the course of postglacial sea-level rise for Tahiti in the South Pacific, for which assigning the correct depth to recovered core is fundamental. By convention, core is placed at the top of the core barrel run from which it was collected. Unless core recovery is 100%, this results in depth errors that are inversely proportional to the recovery. The difficulties of locating core are compounded for the Tahiti boreholes, where primary cavities in the carbonate reef successions are frequently present. Only through the integration of core with continuous geophysical measurements of the borehole can accurate depth positioning be achieved. High-resolution optical and acoustic images of the borehole wall allow effective visual correlation of the core and logging datasets. The final integrated depths form the underlying framework for all subsequent scientific analyses of recovered core employing evaluations that rely on depth. Furthermore, the integration process allows true core recoveries to be accurately estimated.

The depth of core below modern-day sea level provides the underlying framework on which many palaeoclimate and sealevel studies base their interpretations. Integrated Ocean Drilling Program (IODP) Expedition 310 aims to constrain sea-level changes since the last deglaciation at Tahiti in French Polynesia. For this, a misplacement of a core section can lead directly to an equivalent error in past sea-level estimates. However, the depth of recovered core is important for all the principal objectives of the Tahiti Sea Level Expedition. These are as follows (Expedition 310 Scientists 2006).

(1) To establish the course of postglacial sea-level rise (i.e. the exact shape of the deglaciation curve) for the period 20-10 ka. Coral reefs, such as that surrounding Tahiti, are excellent sealevel indicators. The accurate dating of these allows the timing of deglaciation events to be determined, thus improving understanding of the mechanisms that drive glacial-interglacial cycles. This furthers understanding of the dynamics of large ice sheets (Lindstrom & MacAyeal 1993) and their effects on Earth's isostasy (Lambeck 1993; Peltier 1994) and global climate (e.g. Stocker & Wright 1991). Previously constructed sea-level curves (e.g. Fairbanks 1989; Bard et al. 1990a, b, 1996; Chappell & Polach 1991; Edwards et al. 1993) do not encompass the entire deglaciation or are based in regions where large and discontinuous tectonic movements may bias the record. Tahiti is ideally located in a region of relative tectonic stability at considerable distance from former Pliocene ice sheets.

(2) To define sea surface temperature (SST) variations for the region over the period 20-10 ka. Coral reefs allow past SSTs to be recovered from stable isotope and trace element ratios preserved in coral skeletons (Guilderson et al. 1994; McCulloch et al. 1996; Beck et al. 1997; McCulloch et al. 1999). Analyses from the Tahiti reefs will further knowledge of regional variation of SSTs and climatic variability, as well as global variation and relative timing of postglacial climate change in both hemispheres.

(3) To analyse the impact of sea-level changes on reef growth and geometry. This will allow assessments of the impact of glacial meltwater phases and the morphological and sedimentological evolution of the foreslopes (highstand v. lowstand processes).

Reconstruction of sea-level curves relies on absolute dating of in situ corals by radiometric methods in conjunction with palaeobathymetric information that indicates absolute sea levels. The depth of core from which this information is obtained forms the underlying framework in which to accurately record sea-level changes. Detailed assessment of past reef morphologies is also directly reliant upon core depth positioning. Sea surface temperature studies use the results of sea-level studies and the underlying depth framework to correlate between different analyses and interpret results in a wider context. To maintain a good control on drilling depths and ensure high recovery in reef rocks short core runs, generally of 1.5 m or less, were used in Tahiti. However, precise depth positioning can be achieved only in intervals where recovered core can be integrated with continuous records of the nature of the borehole. In intervals of low or disturbed core recovery, drilling records and downhole geophysical logs provide the only way to characterize the borehole section. Furthermore, in coral reefs regions of high porosity and large void spaces are common and, without careful integration of discrete core pieces with continuous records of the borehole walls, significant depth errors can result. Magnitudes are roughly related to the size of the cavities, with depth inaccuracies of up to 2 m for the material recovered in the Tahiti boreholes. Unless corrected for, these errors will propagate through subsequent analyses, such as sea-level studies, that depend on the assumption that core depths are accurate.