Excess 210Pb as an indicator of flood-stage sediments in prograding, Wax Lake Delta, USA

Abstract

Throughout the world, many deltas are eroding as a consequence of sea level rise and diminished sediment delivery from impounded river systems. Contrary to this global trend, Wax Lake Delta (WLD), Louisiana, USA recently formed (∼1952) and is prograding as sediments delivered by a Mississippi River diversion accumulate at the coastal margin. The scientific community has debated about the degree to which deltas build through large-scale flood events, versus more regular seasonal flooding. Here, we use WLD as a natural laboratory to investigate the relative contributions to deltaic land growth of sediments delivered by large-scale flood events compared to sediments delivered during lesser river flows. We collected six sediment cores from a representative island in WLD and measured the stratigraphic distribution of excess 210Pb (i.e. 210PbTotal – 226Ra). Our data showed alternating layers of sediments with and without excess 210Pb. We attributed the deposition of sediments without excess 210Pb to large-scale flood events and the deposition of sediments with excess 210Pb to lesser river flows. With this approach, we found that 53% of the island-top sediments in WLD were deposited by large-scale flood events and 47% of the sediments were deposited during lesser river flows. Our results show that excess 210Pb has the potential to be a useful indicator of sedimentation mode on active delta surfaces and our results quantify the relative contribution of large-scale flood events to land growth and maintenance on a prograding delta.