GEOCHEMISTRY OF A PISTON CORE FROM ONTONG-JAVA PLATEAU (WESTERN EQUATORIAL PACIFIC) - EVIDENCE FOR SEDIMENT REDISTRIBUTION AND CHANGES IN PALEOPRODUCTIVITY

We discuss geochemical proxies, reflecting processes of primary produc tivity, CaCO3 dissolution, and sediment redistribution in a piston cor e (RNDB 74P) from the Ontong Java Plateau. Due to the shallow water de pth, biogenic carbonate is well preserved and a very good delta(18)O s tratigraphy is available down to isotopic stage 11. Th-230(ex) gives e vidence that the sediment accumulation pattern is driven mainly by pro cesses of sediment focusing or winnowing. Due to the constant producti on of Th-230 in the water column, the bulk sediment accumulation rates could be corrected for the particle rain deriving from the water colu mn above. The Th-230(ex)0/CaCO3 ratio reflects the well-known Pacific CaCO3 preservation pattern with ice growth dissolution spikes and degl acial preservation spikes. The record of the grain size fraction >63 m u m supports these results. The downcore concentrations and accumulati on rates of barium (Ba) are on a higher level during interglacials and show several peaks. Normalization of Ba with Th-230(ex)0 delivers a m ore uniform level of the Ba accumulation rates throughout the core. Th is pattern suggests a constantly higher biological productivity (nearl y tenfold) in this area throughout the past 200 kyr compared with an o pen ocean environment. Barium peaks observed at the climatic transitio ns 2/1 and 6/5 and in stage 5 are in contrast to a predicted reduction of interglacial productivity at this location. A possible explanation might be the onset of the modern circulation pattern. The transition from Ba-enriched deep water to lower contents in the Atlantic might ha ve resulted in an enhanced deposition of Ba in the Pacific.