Anthropogenic changes in waterways produce "drought-like" layers in shelf sediments

A primary component of the global sediment cycle is the delivery of sediment from rivers to the sea, an input that fluctuates in magnitude and frequency owing to changes in precipitation. Some of these fluctuations can be recognized in the sedimentary record on the continental shelf and used to reconstruct past climatic conditions. However, recent damming and waterway diversions have affected the volume, location, and arrival intervals of alluvial deposits to the sea. Yet, the reflection of these anthropogenic endeavors on the sedimentological record and how they relate to climatic shifts is not well understood. In this study, we examined the inner continental shelf sediments in the northern Gulf of Aqaba-Eilat and the Israeli coast of the Mediterranean Sea to determine how they were impacted by 20th century anthropogenic alterations of incoming rivers. In the Gulf of Aqaba-Eilat, a drought-like upper sediment layer appeared where floods are no longer reaching the sea because of river channel diversion. This Horizon contained microplastics, timing it to after the foundation of the city of Eilat. These markers are disassociated from recorded rainfall and flood events and were not replicated where floods continued to reach the sea. In the Mediterranean, the observed drought-like changes in the sediment corresponded with the damming of the Nile. Our results show that in both cases, anthropogenically reduced load of fine alluvial (mostly flood) particles and continued winnowing caused sediments to coarsen and become more sorted with higher concentrations of larger foraminifera tests. These sedimentological markings resemble those reported for prolonged droughts, but can be differentiated by discrepancies to recent climatic records. Considering the alterations of waterways worldwide, this sedimentological mismatch may constitute a new proxy of the Anthropocene and highlights the way that human activities are altering the sediment cycle.