Human-induced desalinization of Manzala lagoon, Nile delta, Egypt: Evidence from isotopic analysis of benthic invertebrates

This study combines isotopic (Sr-87/Sr-86, delta O-18 and delta C-13) With paleontological data to derive a paleosalinity proxy in order to determine the impact of the Aswan High Dam on Manzala lagoon in Egypt's Nile delta. A nalyses were made on 17 invertebrate taxa (molluscs, crustaceans, foraminif era, serpulid worms). These were collected in 17 surficial samples and 18 s amples from two cores collected at the two salinity extremes of the lagoon. Of the three isotopic systems, Sr isotopes (Sr-87/Sr-86) best record salin ity changes within the lagoon. The main control on C isotopes within shell material is the mixing of fresh and marine waters entering the lagoon; thus it is also a useful paleosalinity indicator. The oxygen isotopic compositi on of shells increased with decreasing salinity: this is consistent with th e observation that fresh waters feeding the lagoon are enriched in O-18 wit h respect to Mediterranean seawater. However, the gradient in delta O-18 be tween these end-members is not sufficient to allow us to use O isotopes as a recorder of paleosalinity. Sr isotopes fi om a core on the landward margi n of the lagoon documents a decreased salinity shift from approximately 4-1 3 ppt to 1-2 ppt since about 1950. The major reduction of salinity in Manza la is due to a dramatic increase in freshwater discharge from drains into t he lagoon and effects of Aswan High Dam closure in 1964. This salinity chan ge has been the most significant in the past 100 years. This method, combin ing paleontological and isotopic analyses of sediment cores, provides docum entation of environmental deterioration and thus holds promise for the stud y of anthropogenically-induced salinity changes in other deltaic systems fr om around the world.