Computer Science
Scientific paper
Apr 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001e%26psl.187..147v&link_type=abstract
Earth and Planetary Science Letters, Volume 187, Issue 1-2, p. 147-161.
Computer Science
7
Scientific paper
Following the pioneering work of Paytan et al. [A. Paytan et al., Geochim. Cosmochim. Acta 60 (1996) 4313-4319], new investigations have been undertaken on marine barite separated from central and western equatorial Pacific sediments. These investigations are designed to achieve a better quantification of supported 226Ra activities in the separated samples, enabling the use of excess 226Ra (226Raex) in marine barite as a tool for dating Holocene sediments. Instrumental neutron activation analysis allowed first to measure significant 238U activities in the separated samples, attributable to highly insoluble minerals that, like barite, are resistant to the chemical treatment used in this study (TiO2 minerals, zircon, monazite). Within these U-rich minerals, 226Ra activities are in secular equilibrium with 238U. Second, the study of the 226Ra signal in old barite samples that do not contain any residual 226Raex activities provided accurate constraints on the amount of 226Ra produced by unsupported 230Th: the remaining 226Ra activities (corrected for the 238U contribution) appeared to decrease with depth with a slope comparable to that defined by the bulk 230Thex activities. The consistency of this deep trend allowed its extrapolation upwards to the sediment top in order to obtain an estimate of the 226Ra activities produced by unsupported 230Th in the Holocene barite samples. The total correction for supported 226Ra in the Holocene barite samples used in this study therefore consists of the sum of 226Ra activities in secular equilibrium with 238U within minerals and 226Ra activities produced by unsupported 230Th thus estimated. The resulting correction is low for samples separated from central equatorial Pacific sediments compared with both the total 226Ra activities measured in barite and results of previous studies. The magnitude of the correction is nevertheless higher for samples from western equatorial Pacific sediments, which display a lower barite purity. The corrected 226Ra-in barite profiles are in agreement with the general trend observed for long half-life radionuclides with a similar mixed layer in the sediment top, thus adding confidence in the use of this tool. Below the mixed layer, the exponential decay of 226Raex activities in barite allowed the determination of sedimentation rates that agree with the estimates for the two investigated sites. Ultimately, one of the main uses for the 226Ra-in barite dating method will be its application to Holocene sediments from the Southern Ocean where dating is often problematic.
Reyss Jean Louis
van Beek P.
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