Platinum and palladium abundances in marine sediments and their geochemical behavior in marine environments.

Abstract

A 284 marine sediments of terrigenous, hemipelagic and pelagic origin were collected from the different environments, i.e., in the seven sea areas around the Japanese islands (terrigenous sediments), on the Mariana Ridge (hemipelagic sediments), and in the Central Pacific (pelagic sediments). These samples were analyzed together with 31 lake sediments and 41 marine shales for Pt and Pd by atomic absorption spectrometry using a graphite furnace atomizer after solvent extraction separation. The average abundances of Pt and Pd are markedly higher in the pelagic sediments than the terrigenous sediments, with the hemipelagic sediments showing intermediate abundance. Most of the samples analyzed in this study are richer in Pt than Pd, whereas siliceous ooze collected from the central Pacific high productivity zone was generally richer in Pd than Pt. Although the Pt and Pd abundances and water depths of the sampling locations are not correlated, there are clear negative correlations between the Pt and Pd abundances and the sedimentation rates in the studied regions. It is considered that most of the Pt and Pd in marine sediments is originally derived from weathering of crustal materials, and both the elements have been precipitated and accumulated as the elemental and oxide forms by the involvements in biological transport processes. The great enrichments of Pt and Pd in pelagic sediments may result from the very low sedimentation rates and strongly oxidizing environments. The different vertical distributions of Pt or Pd and organic C concentrations in the central Pacific cores suggests that the formation of organometallic complexes of Pt and Pd, and migration of both elements during the early diagenesis are not so significant except in very rare cases. The crustal abundances for Pt and Pd are tentatively estimated as 2.7 ppb and 1.9 ppb, respectively.