Determination and behaviour of platinum group and associated trace-elements in ocean sediments
The occurrence and chemistries of the platinum group elements (PGE's; Ru, Rh, Pd, Os, Ir, Pt) and gold (Au) are assessed. Current understanding of their geochemical cycling is discussed with relevance to oxidation-reduction processes in ocean sediments, and their general mobility around redox fronts. Samples of representative ocean sediment were selected, and a range of instrumentation used to constrain their inorganic geochemistry. A combination of ICP-AES and ICP-MS techniques enable the use of a single digestion procedure for the determination of a wide range of major- and trace-elements. A recently developed hydride generator AFS system was selected for the semi-automated determination of arsenic (As), selenium (Se) and antimony (Sb), and a Coulometric method was chosen to determine organic carbon (Corg) and calcium carbonate (CaC03). Available dissolution techniques are discussed, and a microwave dissolution / cation-exchange procedure refined to enable the determination of low (parts per billion; ng/g) concentrations of PGE's in sediments. A diagenetic case study was undertaken to confirm PGE enrichments above background concentrations in an active redox-controlled sedimentary environment. Samples of a qeochemically well-constrained Mediterranean core (MC07s; ...,7 ka) were digested by microwave dissolution, and analysed by ICP-MS. Clear enrichments were found all PGE's from the redox front. An Early Pliocene (3.5 Ma) turbidite (DOP Section 952A-14H-04), buried at 128 mbsf (metres below sea floor), was selected from.the Madeira Abyssal Plain (MAP) to study the longer-term stability of such PGE enrichments. The colouration suggested the unit was organic-rich and contained a fossil redox front. Pore-water data demonstrates that this unit is now experiencing a second phase of sulphate-reducing conditions. A total geochemical analysis of the core, including PGE determinations, confirmed the presence of a relic redox front, and allowed an assessment of the behaviour of platinum group and trace-elements within this system. Comparisons of PGE data from this unit with more recent « 7 ka) enrichments of the Mediterranean core suggest that post-depositional mobility of the PGE's can occur at redox boundaries within fine-grained ocean sediments of any age. Great care is needed when using PGE spikes in ancient sediments to argue for a purely extra-terrestrial source, as has recently been the case with KIT boundary mass-extinction theorists.