Holocene evolution of Lake Faiyum (Egypt): Insights from elemental and molecular proxies

Abstract

Lake Faiyum provides a comprehensive Holocene sediment record, offering more insight into Nile flood deposition than fluvial sediments in the Nile Valley. Geochemical analysis of lake sediments reveals changes in climate, productivity, organic matter source and composition, salinity, and redox conditions. We integrated lipid biomarker and elemental data from core F1–08 taken at the southern margin of the lake. Our findings indicate that the Early Holocene (ca. >10–8.2 cal. ka BP) was characterized by a humid climate with high lake-levels. This is supported by low Ti/Al and Zr/Al ratios, low terrestrial/aquatic n-alkane ratios (TARs), and a significant contribution of freshwater algal homologues (low TOC/TN ratio). Salinity was low, indicated by low Sr/Ba and S/TOC ratios. Nutrient-rich Nile discharge fostered productivity and oxygen depletion, promoting organic carbon preservation. During the period from ca. 8.2 to 6.2 cal. ka BP (within the Middle Holocene), the lake experienced a slight increase in salinity, which caused a change from freshwater to brackish water conditions accompanied by a slight lake-level drop and delta-like wet land, indicated by increased Sr/Ba and S/TOC ratios. Brackish conditions in the lake coincide with the appearance and sharp increase of various botryococcenes, caused by elevated algal productivity including Botryococcus braunii and typified by low TOC/TN ratios. The Middle to Late Holocene (ca. 6.2 cal. ka BP to present) witnessed gradual aridification and declining lake-levels, organic productivity, and preservation. Evidence includes high Ti/Al and Zr/Al ratios, low TOC contents, and reduced contribution of freshwater algal homologues (higher TOC/TN) and a sharp decrease of botryococcenes. Salinity gradually increased, reflected by higher Sr/Ba and S/TOC ratios. An increase in Mo enrichment since ca. 4.2 cal. ka BP, without a corresponding rise in TOC contents, may be due to human-induced hydrological changes or the transport of these elements to the lake through wind and limited rainfall. Elevated Ti levels at that time may be linked to increased aridity in the Ethiopian Highlands, potentially impacting Nile floods ca. 4.2 cal. ka BP and contributing to the decline of the Old Kingdom in Egypt.