Over the edge: The role of cross-shelf sediment transport by submarine canyons and bottom currents in signal propagation research

Whether sedimentary signals propagate from the terrestrial source to the ocean sink depends on the efficiency of sediment transport across the shelf. Continental shelves are low-relief areas that vary widely in spatial extent along different continental margins and with varying sea level and can store large amounts of sediment. During high sea level, the efficiency of cross-shelf transport is increased if 1) the shelf is dissected by submarine canyons that convey terrestrial sediments derived from river outlets or near-coast longshore currents or if 2) shelf-bottom currents relocate sediment into submarine canyons and over the shelf edge.

1) Since the Last Glacial Maximum, sea level has risen by ~120 m. As a consequence, most coastlines have migrated landward, inundating large shelf areas, some of which are now dissected by submarine canyons. However, with only 4% of the world’s canyons (n=183) reaching today's coastline, these canyons remain the exception. We identified the main controls on whether a submarine canyon head remains connected to terrestrial sediment input during sea-level rise. Shore-connected canyons preferentially occur along continental margins with narrow and steep shelves, such as the Mediterranean margin and the Pacific coast of Central and South America. Moreover, our analysis supports the occurrence of such canyons offshore river basins that are characterized by resistant bedrock and high water discharge. Such rivers deliver coarse-grained sediment to submarine canyons, which erode the canyon head and floor and such systems are most likely to efficiently propagate environmental signals to the deep sea.

2) Offshore the narrow shelf of north-central Chile (29-33°S) turbidite activity ceased with increasing Holocene aridity. In contrast, offshore the humid south-central Chile coast (36-40°S), shelf-bottom currents transport sediment into canyons but also across the across a wide shelf (40-60 km) and onto the continental slope. Here, sediment archives on the continental slope record frequent turbidite deposition during highstand conditions, although most of the depocenters are not connected via canyons to terrestrial sediment sources. High sediment supply, combined with a wide shelf on which shelf-bottom currents move sediment towards the shelf edge, controls Holocene turbidite sedimentation. Moreover, shelf currents can move sediment along large shore-parallel distances deviating sediment from their intuitive transport pathway. Sediment from the Nile river is transported along the Levant shelf. Offshore northern Israel, the shelf narrows and its slope is incised by submarine canyons. Consequently, sediment is re-routed down the continental slope and builds upslope-migrating sediment waves on the continental rise. These strata hence integrate climatic signals from the Nile outlet, sea-level modulations of shelf currents and authigenic sediment-wave development 500 km away but not simply down slope of the original sediment source.

Our analyses offer new insights into the formation and maintenance of submarine canyons and shelf currents that are required to efficiently transport sediments, pollutants, and organic carbon from rivers to the deep ocean floor. The position of canyon heads with regard to the sediment source and the extent and transport capacity of shelf currents have to be accounted for in source-to-sink signal propagation research.