HR: 09:45h
AN: OS11A-08 INVITED     [PDF]
TI: Platinum Group Element Fractionations in Sediments in the Gulf of Papua, New Guinea
AU: * Martin, C E
EM: candace.martin@stonebow.otago.ac.nz
AF: Dept. of Geology, University of Otago, P.O. Box 56, Dunedin, 9001 New Zealand
AU: Peucker-Ehrenbrink, B
EM: behrenbrink@whoi.edu
AF: WHOI, MS#25, Woods Hole, MA 02543-1541 United States
AU: Ravizza, G
EM: ravizza@hawaii.edu
AF: Dept. of Geology & Geophysics, SOEST, 712 POST, University of Hawaii, Manoa, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Brunskill, G
EM: gbrunski@mail.aims.gov.au
AF: Australian Institute of Marine Science, PMB 3, Townsville, QLD 4810 Australia
AB: The elemental concentration pattern for the platinum group elements (PGEs), and their periodic table neighbors Au and Re in modern seawater, is highly fractionated relative to their proportions in the upper continental crust, mantle or meteorites. This may be due to a combination of factors, including chemical weathering and estuarine fractionations. As yet little is known about the geochemical behaviors of the PGEs in estuaries. Because of the importance of the wet tropics in contributing solutes and sediments to the oceans, the Gulf of Papua is a good location to investigate PGE fractionations. The Fly River is one of the largest rivers draining into the Gulf of Papua and is and among the world's top rivers in terms of discharge and sediment yield. By analyzing both the source (river sediment) and sinks, we can begin to understand both the on-land and estuarine fractionations that take place in this system. We have measured the concentrations of Pt, Pd, Ir and Os in surface sediments from the Gulf of Papua, New Guinea and compare these to the concentrations and concentration ratios of Os/Ir and Pt/Pd in Fly River sediment. The Os/Ir of sediment from the upper Fly River is $\sim$4. This value is elevated in comparison to the crust, mantle and meteorites, all of which have Os/Ir of about 1. By contrast, the Os/Ir ratio of seawater is approximately 110. Sediments in the Fly River Delta and Gulf of Papua have Os/Ir ratios that are all lower than the upper Fly River, and may indicate that Ir is preferentially scavenged in the oxic sediments that are typical of the Gulf of Papua. Although a correlation between organic carbon and Os burial was previously indicated, the Ir concentrations do not appear to be correlated with the organic carbon. In the central Gulf, farther from the influence of the Fly river and nearer to the Purari and Kikori Rivers, both of which may have a larger ultramafic component in their catchments, the Os/Ir ratio in the sediments approaches the mantle value of 1, indicating a predominance of a detrital source in those sediments. The other sediments in the Gulf of Papua are interpreted to have been affected by variable amounts of oxic scavenging. Preferential removal of Ir in the estuarine environment can help to explain the extremely elevated Os/Ir of seawater. Although the variations are more subtle for Pt/Pd, there is evidence of preferential removal of Pd relative to Pt in the estuarine and marginal marine sediments. The relatively high Pt/Pd in seawater ($\sim$3) compared to the crust ($\sim$ 1.8) can be explained by oxic scavenging of Pd in sediments such as those in the Gulf of Papua.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting