The structure and significance of early Holocene laminated lake sediments in the Faiyum Depression (Egypt) with special reference to diatoms

Lake Qarun lies in the Egyptian Faiyum Depression and is the modern remnant of a much larger Holocene lake. Variations in water level occurred during the Holocene as a result of palaeoclimate changes and, since ca. 4000 bp, there have been hydrological interventions. Past lake levels have been inferred from the locations of archaeological sites, former beaches and exposed lake sediments, but there is no continuous Holocene palaeolimnogical record for the lake. To explore the potential of this record, three sediment cores (10.4-21.4m long) were collected from terrestrial locations on the southern margin of the lake in 2008. The basal sections of all three cores consisted of thinly laminated diatom marl sediments overlying coarse sand. The nature of these laminations was investigated in thin sections from two cores (QARU9 and 10). Quantitative very high-resolution diatom analysis revealed seasonal succession patterns of Aulacoseira and Stephanodiscus species within diatom-rich laminae. Elemental microprobe analysis of one thin section (QARU10) confirmed the presence of alternating Ca- and Si-rich laminae. The results indicate annually deposited sediment sequences with seasonality signals provided by microlayers of diatoms, calcite and clastic material. According to diatom counting, mean varve thicknesses equated to 6.0 (QARU9) and 9.4 (QARU10) annual diatom laminae accumulated cm?1. Radiocarbon dates from a third core (adjacent to QARU9) indicated that the QARU9 thin section material was bracketed between 8693 and 9935 cal bp. Diatom microstratigraphy was used to infer seasonality in varve forming processes. Gross sediment stratigraphy indicated that varve forming conditions had persisted for >1500 years. The presence of early Holocene varved sediments in palaeo-Lake Qarun provides an exceptional palaeoenvironmental archive for northeast Africa.