Tufa dams in Tigray (northern Ethiopia) as Late Pleistocene-Holocene climate proxies

The geomorphological–stratigraphic study of the backfill deposits of two tufa dams, Mai Makden and Tsabati Mariam, supported by 14C dates, has provided a detailed description of the environmental changes occurred during the Holocene. In particular, the evolution of the investigated tufa dams points out the occurrence of some century-scale stages of tufa deposition (10.9–9.3; 8.4–7.4; 6.6–5.5 kyrs BP) interrupted by intervals characterized by lower or absent deposition and dam incision (9.3–8.4; 7.4–6.6 kyrs BP). Since 5.5–2.5 kyrs BP, the deposition rates of tufa progressively declined until stream erosion incised the dams down to the underlying bedrock. The sequence of tufa events in Tigray seems to be parallel to the record of lake-level fluctuations in the Ethiopian Rift with the high stands corresponding to the main deposition stages of tufa and the low stands corresponding to the non-deposition/erosion intervals. Quite interesting, in this context, is the progressive lowering of the lake levels after ca. 5.5 kyrs cal BP. The tufa record of Tigray also shows some correlation with the cooling–warming stages of the Mediterranean Sea and, more in general, with the main peaks of the Holocene global temperatures. The above relationships emphasize the primary role of rainfall and air temperature as factors controlling the deposition of tufa. This does not exclude human impact as an additional factor for the deposition of tufa.