Ra-226 in marine barite: new constraints on supported Ra-226

Following the pioneering work of Paytan et al. [A. Paytan et al., Geochim. Cosmochim. Acta 60 (1996) 4313-4319], new investigations have been undertak en on marine barite separated from central and western equatorial Pacific s ediments. These investigations are designed to achieve a better quantificat ion of supported Ra-226 activities in the separated samples, enabling the u se of excess Ra-226 (Ra-226(ex)) in marine barite as a tool for dating Holo cene sediments. Instrumental neutron activation analysis allowed first to m easure significant U-238 activities in the separated samples, attributable to highly insoluble minerals that, like barite, are resistant to the chemic al treatment used in this study (TiO2 minerals, zircon, monazite). Within t hese U-rich minerals, Ra-226 activities are in secular equilibrium with U-2 38. Second, the study of the Ra-226 Signal in old barite samples that do no t contain any residual Ra-226(ex) activities provided accurate constraints on the amount of Ra-226 produced by unsupported Th-230(ex), remaining Ra-22 6 activities (corrected for the U-238 contribution) appeared to decrease wi th depth with a slope comparable to that defined by the bulk Th-230(ex) act ivities. The consistency of this deep trend allowed its extrapolation upwar ds to the sediment top in order to obtain an estimate of the Ra-226 activit ies produced by unsupported Th-230 in th, Holocene barite samples. The tota l correction for supported Ra-226 in the Holocene barite samples used in th is study therefore consists of the sum of Ra-226 activities in secular equi librium with U-238 within minerals and Ra-226 activities produced by unsupp orted Th-230 thus estimated. The resulting correction is low for samples se parated from central equatorial Pacific sediments compared with both the to tal Ra-226 activities measured in barite and results of previous studies. T he magnitude of the correction is nevertheless higher for samples from west ern equatorial Pacific sediments, which display a lower barite purity. The corrected Ra-226- in barite profiles are in agreement with the general tren d observed for long half-life radionuclides with a similar mixed layer in t he sediment top, thus adding confidence in the use of this tool. Below the mixed layer, the exponential decay of Ra-226(ex) activities in barite allow ed the determination of sedimentation rates that agree with the estimates f or the two investigated sites. Ultimately, one of the main uses for the Ra- 226-in barite dating method will be its application to Holocene sediments f rom the Southern Ocean where dating is often problematic. (C) 2001 Elsevier Science B.V. All rights reserved.