Department of Earth and Environmental Sciences Tulane University How did rivers behave when sea level was ~120 m lower than present, 20,000 years ago? (39240-AC8) Torbjörn E. Törnqvist Department of Earth and Environmental Sciences Tulane University

Conventional wisdom… During sea-level highstands (like the present day), most sediment transported from the continents is trapped in the shallow coastal oceans, known as the continental shelf In contrast, during sea-level lowstands, like the Last Glacial Maximum (LGM) 20,000 years ago, it is believed that the world’s rivers discharged their water and sediment at the shelf edge, thus flushing sediment directly into the deep ocean, as illustrated by the next cartoon

Posamentier et al. (1988)

Conventional wisdom… During sea-level highstands (like the present day), most sediment transported from the continents is trapped in the shallow coastal oceans, known as the continental shelf In contrast, during sea-level lowstands, like the Last Glacial Maximum (LGM) 20,000 years ago, it is believed that the world’s rivers discharge their water and sediment at the shelf edge, thus flushing sediment directly into the deep ocean, as illustrated by the next cartoon But does this conventional wisdom always hold? The following cartoon shows what happens if sea level remains on the continental shelf during lowstand (scenario B)

Testing the conventional model… Using the geophysical model of collaborator Glenn Milne (University of Durham, UK) we calculate the position of the shoreline during the last sea-level lowstand to test how common scenarios A and B are Specifically, we analyze the position of the lowstand shoreline in relation to the shelf edge The next slides show this relationship in our two study areas (the northern Gulf of Mexico and the Bay of Biscay)

lowstand shoreline in black; modern shoreline in white

shelf edge in purple

Testing the conventional model… Using the geophysical model of collaborator Glenn Milne (University of Durham, UK) we calculate the position of the shoreline during the last sea-level lowstand to test how common scenarios A and B are Specifically, we analyze the position of the lowstand shoreline in relation to the shelf edge The next slides show this relationship in our two study areas (the northern Gulf of Mexico and the Bay of Biscay) Topographic/bathymetric profiles illustrate the striking contrast between the two areas: The Gulf of Mexico adheres to the conventional model The Bay of Biscay shows a lowstand shoreline that remains well updip of the shelf edge

circles show calculated shelf-edge positions defined by curvature; location of topographic/bathymetric profiles shown in preceding map

Conclusions and challenges… The lowstand (LGM) shoreline does not always coincide with the shelf edge Sea-level lowstand does not always cause the formation of cross-shelf valleys and shelf-margin deltas Sediment transported from the continents does not necessarily reach the deep ocean during sea-level lowstand Our understanding of how the shelf edge forms is still surprisingly limited