SUMMARY AND CONCLUDING REMARKS

Carbonate contents of late Pliocene sediments from Site 1143 display a cyclicity, with lower contents during glacial and higher contents during interglacial stages. Whereas dilution by terrigenous detrital matter input seems to be an important factor for low carbonate contents during glacials, enhanced productivity, indicated by Ba enrichments, seems to have led to a higher carbonate flux during interglacials. The changes in productivity are probably associated with oceanographic changes (e.g., stronger inflow of nutrient-rich water masses from the Sunda shelf during interglacials).

The terrigenous detrital matter composition of Site 1143 sediments fluctuates owing to multiple processes, including changes in provenance and weathering intensity caused by monsoonal climate variability and sea level fluctuations, resulting in a varying shelf area and the Mekong River Fan structure.

A clear effect of the onset of major Northern Hemisphere glaciation cycles at 2.73 Ma (Tiedemann et al., 1994) on sediments from Site 1143 can not be observed, although gradual or long-term changes in sediment composition are present during the investigated interval. For example, there is a shift toward lower TiO2 (cfb) values at ~2.9 Ma and an increase in Al2O3 (cfb) variations at ~2.55 Ma. These changes may be explained by an increasing contribution from the Mekong River, delivering material with low Ti/Al contents and a subsequent gradual increase in weathering, causing higher Al2O3 (cfb) (probably reflecting kaolinite) contents. Both could be the result of an enhanced summer monsoon.

The manganese and phosphate geochemistry of Site 1143 sediments seems to be strongly dominated by biological productivity and the availability of carbonate as an adsorbing agent for phosphate-rich iron oxides and/or substrates for the formation of manganese carbonates or CFA. During strong interglacials and corresponding times of enhanced productivity, Mn may be remobilized from continental margin sediments deposited within an oxygen-minimum zone and subsequently exported into the pelagic region, where it is oxidized and settles to form Mn-enriched layers in the sediment.

Pyrite concretions were detected as enrichments of iron, sulfide, and chalcophilic trace elements in the sediments. These layers are believed to be of minor significance and represent a rather small and locally restricted phenomenon.

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