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Abstract:
Sediment supply and sea level interact to control sediment flux to deep-water submarine fans. Although some fans continue to be active during times of rising sea level, the source of sediment is not always clear and may be dominated by reworking in high energy coastal areas rather than reflecting erosional signals directly from the source drainage basin. We present new age and geochemical provenance data from cores covering the last ~20 ka that show continuous deep-water sedimentation through the Indus submarine canyon since at least ~11 ka, despite the cessation of sedimentation on the upper fan around that time. Large turbidity flows mantled terraces >200 m above the thalweg throughout the Holocene and their deposits show trends in grain size and geochemistry that we interpret to record direct supply from the river mouth and little reworking of older deposits eroded by longshore currents, storm waves, slumping, or sliding, at least at 4.7–9.0 ka. We use Nd–Sr isotope compositions to show that sediments within the canyon and in the shelf clinoform to the east of the canyon are similar to the Holocene river mouth, suggesting direct supply from the Indus River to the eastern clinoform and into the canyon. The sediment storage time on the shelf before redeposition would be no more than ~8 k.y., and likely much less during the Early-Mid Holocene (4.7–9.0 ka). Sr–Nd isotopes also indicate that significant reworking of sediment older than 8 ka during sea level rise is excluded. Thus, coherent erosional pulses (signals) in the river, likely caused by climatic disturbances, are communicated to the canyon at least since ~9.0 ka, suggesting that sediment supply, modulated by climate in the Indus basin, dominates over sea level in controlling canyon sedimentation in high sediment supply settings, although the vast majority of the sediment supply is stored on the shelf and in the delta. Our study for the first time allows the differentiation between reworking within the canyon and direct supply from the river in a major submarine canyon.
