Lacustrine carbonate towers of Lake Abhe, Djibouti: Interplay of hydrologic and microbial processes Laura M. DeMott a, ⁎, Christopher A. Scholz a , Mohamed Osman Awaleh b a Department of Earth and Environmental Sciences, Syracuse University, 204 Heroy Geology Lab, Syracuse, NY 13244, United States of America b Institut des Sciences de la Terre, Centre d'Etudes et de Recherches de Djibouti (CERD), Route de l'aéroport, B. 486, Djibouti, Djibouti abstract article info Article history: Received 27 March 2021 Received in revised form 19 August 2021 Accepted 20 August 2021 Available online 26 August 2021 Editor: Dr. Catherine Chagué Lake Abhe, located at the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rift trends, is a hypersaline and alkaline closed lake that is known for its exposures of massive carbonate chimneys on the flat- lying sediments of the lake's eastern margin. This study describes the morphological, textural, and petrographic characteristics of these chimneys at the basin- to microscopic-scale in order to constrain the carbonate deposi- tional system in the context of basin hydroclimate history. Chimneys occur in three main fields and are assessed according to 1) large-scale (>5 m) morphological variations, 2) meso-scale (cm to <5 m) textures, and 3) micro- scale (<1 cm) fabrics. The dominant chimney fabric is a porous crystalline framework of trigonal prismatic and dendritic calcite crystals that locally contain spherulites, sickle-cell calcite fabrics, and entombed microbial cocci. In addition to the crystalline chimneys, other carbonate sediments include stromatolitic crusts composed of microdigitate laminated columns dominated by crystalline fan microfabrics, as well as carbonate-rich mud- stones and relatively rare, localized, carbonate-rich diatomites. Chimneys are interpreted to form primarily as products of mixing between hydrothermal sublacustrine springs and lake waters during lake highstand intervals, while stromatolitic crusts are interpreted to form during lowstand lake levels. Our interpretation of mixing pro- cesses is supported by the δ 18 O composition of chimney calcite, which is representative of lake water and area hot spring endmembers. The observation of stromatolitic crusts with more positive values of δ 13 C and δ 18 O than crys- talline chimneys indicates that crusts formed during periods of high evaporation and low lake level. Crusts also contain Mg-silicate minerals, which are not present in crystalline chimneys, and further support the interpreta- tion of lowstand depositional conditions. Although the age of chimney formation is not well constrained, evi- dence from seismic reflection data suggests a pattern of chimney formation during lake level rise and highstand times, followed by lake level fall, subaerial exposure and weathering that occurred at several times throughout the Late Pleistocene and Holocene. © 2021 Elsevier B.V. All rights reserved. Keywords: Lake Abhe Lacustrine carbonate Stromatolite Hydrothermal 1. Introduction Groundwater-influenced carbonate sedimentation in extensional basin lakes has led to the formation of a number of spectacular land- scapes, including the towers and mounds of porous, freshwater lime- stones in the western United States, in Pyramid Lake (Benson, 1994), Winnemucca Dry Lake (DeMott and Scholz, 2020), Searles Lake (Guo and Chafetz, 2012), and Mono Lake (e.g., Council and Bennett, 1993; Brasier et al., 2018). Submerged lacustrine carbonate towers and mounds are also observed on the lake floors of modern rift lakes, includ- ing Lake Van (Turkey) (López-García et al., 2005; Cukur et al., 2015) and Lake Tanganyika (Democratic Republic of Congo, Burundi, Tanzania, and Zambia) (Tiercelin et al., 1989; Casanova and Hillaire-Marcel, 1992; Stoffers and Botz, 1994). Another such rift-associated carbonate tower system is present at Lake Abhe, Republic of Djibouti. Although chimneys are relatively rare, carbonate deposits, and stro- matolites in particular, are present in numerous lakes of the East African Rift, although they may be more abundant in lakes of the eastern branch of the rift due to increased hydrothermal activity compared to the western branch (Casanova, 1994). A variety of carbonate sediments, including stromatolites, grainstones, mudstones, and tufas, have been described from Lake Turkana (Abell et al., 1982; Hargrave et al., 2014). A similar assemblage of carbonate facies, including oolites, coquinas, and microbialites, has been described from Lake Tanganyika (Cohen and Thouin, 1987). Stromatolites have been described from a number of lakes, including Lake Bogoria (Kenya) (Casanova, 1994; McCall, 2010), Lake Magadi (Kenya) (Casanova, 1994), and Lake Hayk (Ethiopia) (Ghinassi et al., 2012), among others. Many of these Sedimentary Geology 424 (2021) 105983 ⁎ Corresponding author at: U.S. Geological Survey, New York Water Science Center, 425 Jordan Road, Troy, NY 12180-8349, United States of America. E-mail addresses: lmdemott@syr.edu (L.M. DeMott), cascholz@syr.edu (C.A. Scholz). https://doi.org/10.1016/j.sedgeo.2021.105983 0037-0738/© 2021 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo