While most large river-deltas in the world are facing the risk of subsidence and erosion in the Anthropocene, it is suspected that the Changjiang submerged delta (CSD) could be subjected to the impacts of the world's largest dam, the Three Gorges Dam (TGD). Here we firstly indicate that the CSD went through high accumulation (1958–1978); slight accumulation (1978–1997), slight erosion (1997–2002); and high accumulation (2002–2009), despite the 70% reduction of the sediment load from upstream since the operation of the TGD in 2003. Meanwhile, at the depocenter of the submerged delta, the accumulation maintained a high rate of 10 cm/yr during 1958–2009. This suggests on a longer term, the distal sediment source from the upstream had little effect on the CSD. Within this time frame the changes in the partition of sediment load among the branching channels of the Changjiang Estuary could likely control the shifting of the depocenter of the CSD on a decadal time scale. Episodic extreme floods and storm surges also increased the magnitude of deposition and erosion of the CSD on short-term scales. A re-evaluation of the impacts of TGD on the CSD is urgently needed.

Geological sequestration of atmospheric carbon dioxide (CO2) can be achieved by the erosion of organic carbon (OC) from the terrestrial biosphere and its burial in long-lived marine sediments. Rivers on mountain islands of Oceania in the western Pacific have very high rates of OC export to the ocean, yet its preservation offshore remains poorly constrained. Here we use the OC content (Corg, %), radiocarbon (Δ 14Corg) and stable isotope (δ13Corg) composition of sediments offshore Taiwan to assess the fate of terrestrial OC, using surface, sub-surface and Holocene sediments. We account for rock-derived OC to assess the preservation of OC eroded from the terrestrial biosphere and the associated CO2 sink during flood discharges (hyperpycnal river plumes) and when river inputs are dispersed more widely (hypopycnal). The Corg, Δ14Corg and δ 13Corg of marine sediment traps and cores indicate that during flood discharges, terrestrial OC can be transferred efficiently down submarine canyons to the deep ocean and accumulates offshore with little evidence for terrestrial OC loss. In marine sediments fed by dispersive river inputs, the Corg, Δ14Corg and δ 13Corg are consistent with mixing of terrestrial OC with marine OC and suggest that efficient preservation of terrestrial OC (>70%) is also associated with hypopycnal delivery. Sub-surface and Holocene sediments indicate that this preservation is long-lived on millennial timescales. Re-burial of rock-derived OC is pervasive. Our findings from Taiwan suggest that erosion and offshore burial of OC from the terrestrial biosphere may sequester >8 TgC yr−1 across Oceania, a significant geological CO2 sink which requires better constraint. We postulate that mountain islands of Oceania provide a strong link between tectonic uplift and the carbon cycle, one moderated by the climatic variability which controls terrestrial OC delivery to the ocean.