35. Land-Ocean Sediment Transfer in Palaeotimes, and Implications for Present-Day Natural Fluvial Fluxes

Detrital fluxes to the ocean during the period from the late Jurassic to the Pliocene ranged between 2.7–5.2 Gt year^–1, but increase up to 9.6–15.5 Gt –1 year in the Pliocene-Holocene. Of the Holocene flux, 11–12 Gt year^–1 was carried by rivers. This estimate is suggested as the natural component of present-day fluvial sediment flux to the ocean. The contemporary flux is 1.1- to 2-fold higher, due to human disturbance of the land. Small basins make a major contribution to this increase, while many large rivers demonstrate stable fluxes. This is due to sediment trapping within the fluvial system, with sediment residence time increasing with increasing basin size. Over long time spans sediment delivery ratios (SDR) are less dependent on basin size. At a global land-ocean scale, the SDR is estimated to be in the range 5–20% for timescales ranging from decades to centuries but its value increases to 77% for the last 150 million years.