SUMMARY AND CONCLUSIONS

Detailed analysis of siliciclastic grain size in association with major element geochemistry and clay mineralogy were carried out on the carbonate-free fractions of sediments from Site 1144 on the northern margin of the SCS.

The smectite/(illite + chlorite) ratio and CIA exhibit minor changes with global climate changes, indicating that Site 1144 records no major changes of chemical weathering in the sediment source areas.

Two grain size populations with the highest variability through time were identified at 2.5-5 µm (clay) and 20-40 µm (silt). Glacial periods are characterized by higher proportions of silt size than interglacial stages. An increase in the grain size (silt proportion) can be observed between 1000 and 600 ka.

Glacial stages are also characterized by higher contents of quartz, higher sediment accumulation rates, lower reflectance (L*, lightness), and higher SiO2/Al2O3 and SiO2/K2O ratios than interglacial periods. Glacial grain size increases seem to be mainly related to an increase in quartz content in the siliciclastic fraction.

Two hypotheses can explain such changes in sediment supply, grain size, and composition:

  1. Drier conditions and intensified winter monsoon transport of eolian loess to the northern part of the SCS during glacial periods (in this case, glacial periods would be associated with a decrease in the summer monsoon rainfall and/or an increase in the winter monsoon transport), and/or
  2. An effect of sea level changes in the detrital material transport from continent or shelf to the deep ocean.

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