Did the last sea level lowstand always lead to cross-shelf valley formation and source-to-sink sediment flux?

It is commonly believed that the efficiency of sediment flux from continents to oceans is maximized during relative sea level (RSL) lowstands by means of cross-shelf valleys that are directly connected to the continental slope and deep marine environment. While such conditions have been documented for the last RSL lowstand along several continental margins, there is increasing evidence that radically different conditions persisted during the Last Glacial Maximum (LGM) elsewhere, with lowstand shorelines that remained on the continental shelf. Here we analyze the relationship between the LGM (21 ka) shoreline and the shelf edge for the Gulf of Mexico off the United States and the Bay of Biscay off France. A geophysical model is used to compute shoreline positions corrected for isostatic movements, and the shelf edge position is quantified by means of curvature. The conditions in the two study areas differed markedly: throughout the Gulf of Mexico study area, LGM sea level dropped to a point commonly ∼40 m below the shelf edge, consistent with conventional sequence stratigraphic models, while in the Bay of Biscay the modeled LGM shoreline remained well landward of the shelf edge, in places separated by hundreds of kilometers. These observations hint at potentially significant implications for (1) the source-to-sink sediment flux from continents to oceans and its variation in time and space, (2) sequence stratigraphic models that predict deep marine sedimentation as being particularly prominent during RSL lowstands, and (3) the occurrence of paleovalleys and related features on the continental shelf. In addition, our findings raise fundamental questions about the mechanics of shelf edge formation.