Magadiite, a rare hydrous sodium-silicate mineral [naSi 7 o 13 (oH) 3 ·4(H 2 O)], was discovered about 50 years ago in sediments around Lake Magadi, a hypersaline alkaline lake fed by hot springs in the semiarid southern Kenya Rift Valley. today this harsh lacustrine environment excludes most organisms except microbial extremophiles, a few invertebrates (mostly insects), highly adapted fish (Alcolapia sp.), and birds including flamingos. Burrows discovered in outcrops of the High Magadi Beds (~25-9 ka) that predate the modern saline (trona) pan show that beetles and other invertebrates inhabit this extreme environment when conditions become more favourable. Burrows (cm-scale) preserved in magadiite in the High Magadi Beds are filled with mud, silt and sand from overlying sediments. Their stratigraphic context reveals upward-shallowing cycles from mud to interlaminated mud-magadiite to magadiite in dm-scale units. The burrows were formed when the lake floor became fresher and oxygenated, after a period when magadiite precipitated in shallow saline waters. the burrows, probably produced by beetles, show that trace fossils can provide evidence for short-term (possibly years to decades) changes in the contemporary environment that might not otherwise be recognised or preserved physically or chemically in the sediment record.Saline lakes precipitating evaporites are among the most extreme environments on Earth and include the most important stressors for metazoan life 1-3 . Hypersalinity is commonly accompanied by high pH (hyperalkalinity), anoxia and high turbidity, all of which are major stressors in underfilled evaporitic lakes 4-7 . Although metazoan diversity is typically low in these harsh settings, microbial communities often flourish 8,9 . Soda lakes frequently have very high microbial biomass (mainly photoautotrophs) and productivity 10,11 . Such lakes, with their carbonate-rich waters, are most common in regions with volcanic bedrock or hydrothermal recharge 12,13 . Lake Magadi, a hypersaline (>300 g/kg TDS: Total Dissolved Solids) alkaline (pH: 10-11) soda lake, 0-2 m deep, lies in faulted volcanic terrain in the axial depression of the southern Kenya Rift 14-16 just south of the equator (1°53′S). Modern lake sediments, cores and Quaternary deposits exposed around its margins have provided details of the sedimentary facies, including thick deposits of trona [Na 3 (CO 3 )(HCO 3 )·2H 2 O] that underlie the modern lake floor 14,16,17 . The Pleistocene sediment record includes abundant bedded, nodular and intrusive chert (including dykes) of diverse origins and the rare sodium-silicate mineral, magadiite [NaSi 7 O 13 (OH) 3 ·4(H 2 O)] 18-23 . Lake Magadi, the most saline of the major lakes in the East African Rift, is one of the most chemically stressful lacustrine settings on the planet 24 . In this paper, we describe and interpret unusual trace fossils that are preserved in terminal Pleistocene magadiite. This soft silicate mineral, a precursor of quartzose chert, is an improbable host for invertebrate ichnof...
