$^{226}$Ra in marine barite: new constraints on supported $^{226}$Ra - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Earth and Planetary Science Letters Année : 2001

$^{226}$Ra in marine barite: new constraints on supported $^{226}$Ra

Résumé

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 $^{226}$Ra activities in the separated samples, enabling the use of excess $^{226}$Ra ($^{226}$Raex) 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, $^{226}$Ra activities are in secular equilibrium with $^{238}$U. Second, the study of the $^{226}$Ra signal in old barite samples that do not contain any residual $^{226}$Raex activities provided accurate constraints on the amount of $^{226}$Ra produced by unsupported $^{230}$Th: the remaining $^{226}$Ra activities (corrected for the $^{238}$U contribution) appeared to decrease with depth with a slope comparable to that defined by the bulk $^{230}$Thex activities. The consistency of this deep trend allowed its extrapolation upwards to the sediment top in order to obtain an estimate of the $^{226}$Ra activities produced by unsupported $^{230}$Th 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 $^{226}$Ra activities in secular equilibrium with $^{238}$U within minerals and $^{226}$Ra activities produced by unsupported $^{230}$Th thus estimated. The resulting correction is low for samples separated from central equatorial Pacific sediments compared with both the total $^{226}$Ra 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 $^{226}$Ra-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 $^{226}$Raex 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 $^{226}$Ra-in barite dating method will be its application to Holocene sediments from the Southern Ocean where dating is often problematic.

Dates et versions

hal-03126078 , version 1 (28-03-2024)

Identifiants

Citer

Pieter van Beek, Jean-Louis Reyss. $^{226}$Ra in marine barite: new constraints on supported $^{226}$Ra. Earth and Planetary Science Letters, 2001, 187 (1-2), pp.147-161. ⟨10.1016/S0012-821X(01)00288-6⟩. ⟨hal-03126078⟩
23 Consultations
0 Téléchargements

Altmetric

Partager

Gmail Facebook X LinkedIn More