Far-field effects of the Nile damming on the silica cycle in the Southeastern Mediterranean Sea

Silica plays a key role in the growth of silicifying primary producers (e.g., diatoms) and hence the ocean carbon pump. The Mediterranean Sea's eastern Levantine Basin (ELB) is a low silica (and low N and P) ultra-oligotrophic basin. Before 1965, Nile autumn floods were a major source of dissolved silica (DSi) and other nutrients to primary producers of the ELB continental shelf, also known as the Nilotic cell. The construction of the Aswan High Dam (AHD) in the mid-1960s, blocked these floods, drastically diminishing the autumn-diatom blooms offshore the Nile delta. However, the far-reaching and long-lasting effects of the Nile damming on the Si cycle in the ELB remain unclear. Here, we studied the changes in DSi in the surface water offshore Israel and the distribution of biogenic silica in deep-sea short sediment cores, collected hundreds of kilometers from the Nile outlet, at depths range of 1100-1900 m, offshore the ELB Israeli coast. We show post dam reduction and termination in flood related seasonality of DSi and a concurrent decrease (of up to 79 %) in biogenic silica (BSi) accumulation rates in surficial sediments relative to underlying sediments. These changes reflect the effects of Si (dissolved and particulate) retention by the AHD on diatoms production, export and burial in the ELB. This far-field effect was demonstrated in deep-sea areas subjected to intense lateral transport of resuspended sediments from the shelf via intermediate nepheloid layers and to coastal water intrusions, along the path of the pre-dam, flood plumes. Our core records show that the AHD worsened nutrient-diminished, exceptionally unfavorable conditions for diatoms that persisted in the deep ELB at least during the last four millennia.