Sediment Trapping and Bypassing in Active Continental Margin Settings: New Insight from MARGINS Source-to-Sink Studies

Continental margin sediments contain a wealth of information on changing climatic, terrestrial and oceanographic conditions over time, but their interpretation depends on our ability to understand the myriad controls on sediment dispersal and accumulation. Traditional models consider sea level to be the primary control on the partitioning of sediments among near shore, shelf and deep sea environments. However, recent studies have highlighted the importance other major factors influencing the ultimate fate of river-derived sediments. The NZ Waipaoa focus area and nearby regions exemplify active margin settings where the fate of riverine inputs during current highstand conditions is strongly modulated by changing sediment yield of the rivers, tectonic accommodation on narrow continental shelves, and energetic oceanographic conditions. Whereas tectonic deformation of active margins such as the NZ North Island east coast creates abundant accommodation in shelf synclines and the potential to preserve a high-resolution stratigraphic record, this accommodation is rapidly depleted by high sediment yields of small mountainous rivers characteristic of these regions. For the Waipaoa focus area, shelf accommodation and sediment input by the rivers was roughly in balance throughout most of the Holocene, resulting in highly efficient shelf sediment trapping. However, huge increases in sediment yield in response to deforestation have resulted in the present situation where roughly 75% of modern sediments dispersed to the shelf by the river escapes the Waipaoa shelf. Further to the north off the Waiapu River, increased sediment yields due to deforestation have resulted in a river that routinely exceeds a sediment concentration of 40 g/l, allowing river inflow to the shelf with hyperpycnal characteristics. Strong wave and current conditions stimulate the export of massive quantities from the adjacent shelf in the bottom boundary layer.