LAKE ABHE TOWER CARBONATE FABRICS AS A POTENTIAL PROXY FOR HYDROCLIMATE VARIATION IN THE AFAR RIFT

Lake Abhe (Afar Rift) is located at the convergence of the Red Sea, Gulf of Aden, and Main Ethiopian Rift trends (East African Rift). The lake elevation is ~240 m above sea level and it is the only extant lake located in a tectonic triple junction. Abhe is currently a closed basin lake, fed by the Awash River, and has not been subject to marine incursions that have affected the Danakil and Assal regions. While the lake is currently hyperalkaline, saline, and rapidly desiccating, throughout the Quaternary lake levels have fluctuated dramatically. At present, the lake covers an area of ~320 km², with a maximum depth of ~37 m; at past highstand intervals lake area reached ~6000 km² and was ~170 m deeper. During highstands, thermal groundwaters emerging along faults led to deposition of large (up to 60 m high) carbonate tower structures. These towers are often aligned linearly along fault trends and are commonly associated with hot springs and fumaroles; some actively vent steam. These types of hydrothermal-lacustrine carbonate deposits may serve as useful proxy indicators for hydroclimatic variations in lacustrine basins; similar carbonate towers in lakes of the Great Basin have been used to examine such temporal changes. The Abhe towers have similarities in gross morphology to these deposits but exhibit textural differences. The Abhe towers have two distinct textural fabrics: 1) Highly porous crystalline calcite and 2) dense, nodular, laminated calcite. These distinct textures likely represent two main phases of carbonate accretion resulting from variations in lake level and water chemistry. Both phases exhibit a mixture of biogenic and abiogenic microfabrics, consistent with a sublacustrine-hydrothermal mixing origin, and indicating potential microbial influence on deposition. The porous crystalline calcite is associated with the chimney, or “central pipe”, of the tower structures, and likely indicates times of high groundwater flux into the lake basin and rising or high lake levels. The dense laminated calcite typically forms as exterior coatings on towers and shorelines and may accumulate during intervals of lower groundwater flux and falling lake levels. Trace element geochemistry from EDS and XRF may provide insight into the nature of these fluxes and the relationship to regional hydroclimate.