Sea-level responses to rapid sediment erosion and deposition in Taiwan

Abstract

Numerous studies have shown that sediment deposition can perturb sea level by several meters over millennial timescales by modifying the gravity field, crustal elevation, and sediment thickness. Relatively few studies have focused on the complementary role of erosion on sea-level change despite its effects on the same quantities, partly because many rapidly eroding mountains are too far from shorelines to strongly perturb sea level at the coast. Taiwan, a mountainous island eroding rapidly within tens of km of the coast, offers an opportunity to investigate the joint influences of rapid onshore erosion and rapid offshore deposition on sea-level change. Here we develop a sediment loading history for Taiwan since the previous interglacial (∼120 ka) by compiling published erosion and deposition rate measurements and by applying a geometric marine sediment deposition and compaction model for sites without deposition rate measurements. We use the resulting sediment redistribution history to drive sea-level responses in a gravitationally self-consistent sea-level model. Our simulations show that the effects of rapid onshore erosion outweigh the effects of rapid offshore deposition along Taiwan's east coast. Along the east coast of Taiwan, sediment redistribution induces rapid sea-level fall, a response that differs in sign from the coastal sea-level rise induced by rapid sediment redistribution in many other river systems around the world. The spatial extent of the modeled sea-level fall is sensitive to the Earth model, particularly the effective elastic thickness of the lithosphere, a sensitivity that we describe in further detail in the Discussion. These results suggest that sediment redistribution could have generated sea-level changes of >10m on the east coast of Taiwan since 10 ka and >100m since 120 ka. This can account for some of the discrepancy between observed and modeled paleo-sea-level marker elevations, which reduces estimates of tectonically driven rock uplift rates inferred from the elevation differences between paleo-sea-level markers and modeled sea level. This highlights the importance of accounting for erosional unloading in interpretations of paleo-sea-level reconstructions and associated estimates of tectonically driven uplift rates.