Natural gamma-ray emissions are potentially powerful indicators of terrigenous sediment composition. At the Ceara Rise, such data can provide insight to the evolution of terrigenous fluxes from the South American continent during the late Neogene. Although log-derived natural gamma emissions data have relatively lower noise levels than core-derived data, they are limited by a vertical resolution on the order of 0.5-1 m. Conversely, natural gamma-ray emissions data collected from cored sediments during Leg 154, although noisier, have a depth resolution on the order of 10 cm. Thus, the core data provide both a strong complement to downhole measurements of natural gamma as well as higher vertical (depth) resolution. Combining the two measurements provides for reconstruction of variations in terrigenous sedimentation over the past 35 m.y. encompassing both orbital (~0.01-~0.1 m.y.) and tectonic (>0.1 m.y.) time scales. In this paper we present a method to obtain robust discretely sampled, core-derived natural gamma-ray data from multiple holes. A “stacked” data set derived from multiple holes has less noise (lower mean square error) at all natural gamma-ray emissions channels, and thus is superior to core-derived natural gamma data taken from a single APC/XCB hole. A comparison of the stacked core-derived natural gamma data to the log-derived natural gamma data for a 12-m.y.-long Neogene sequence from Site 926 documents the general attenuation of log-derived measurements relative to stacked core-derived measurements for most orbital-scale and tectonic-scale oscillations. This result suggests that high-quality core-derived measurements of natural gamma-ray emissions can, under the correct circumstances, provide superior resolution and reliability to log-derived measurements over a large range of time scales.
1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
2Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A. tking@gsosun1.gso.uri.edu
3Present address: Corning School of Ocean Studies, Maine Maritime Academy, Castine, ME 04420, U.S.A.
|
|