SEDIMENT BUFFERING AND REWORKING PROCESSES IN FORMING THE DEEP-WATER SEDIMENT RECORD OF THE SOUTH CHINA AND ARABIAN SEAS (Invited Presentation)

Sediments deposited by continental slopes and submarine fans potentially record the evolving environmental conditions that have affected the onshore drainage basins from which they were derived, yet interpreting this record is potentially complicated by processes of reworking and storage. In the Indus basin of SW Asia incision of flood plains and terraces suggest that almost the entire sediment load delivered to the delta since 10 ka could be derived from reworking of older sediment. After the sediment reaches the ocean it may be stored in shelf clinoforms, which are then eroded into the deep basin only as sealevel begins to fall again. Only a fraction of the river discharge makes it into the canyon and even that has so far not been propagated on to the upper fan. Nd isotopes show that shelf sediment archives are significantly affected by sediment transport along the coast. Cores sampling the Holocene from the Indus Shelf show significant decoupling from the record found at the river mouth, suggesting that these are not good environmental records for the Indus Basin but are often affected by wave reworking, while the deep-water fan itself has received no sediment since 11 ka. Sediment deposited during falling sealevel would further rework and mix sediment originally eroded over a wide period of time. On the South China slope sediment chemistry and mineralogy show a coupling with monsoon intensity between 13 and 10 ka with more weathering as the monsoon became stronger. However, reduced weathering intensity before the start of monsoon weakening, coupled with indicators for dry weathering conditions during the Early Holocene monsoon maximum show that the deposition of that time is driven by reworking of material exposed and weathered on the shelf during the LGM and only reworked into deep water during a period of strong run-off and rising sea-levels. Once the shelf of the Taiwan Strait was substantially flooded after 8 ka the flux of material to the deep-water was cut-off. Deep-water, slope records do at least have the advantage of not being eroded and weathered during sealevel lowstands but tend to accumulate sediments that were originally deposited on the shelf and only later reworked. Consequently, they cannot be considered as simple measures of environmental conditions in the source drainage system at the time of their deposition.