## 7 The Ground Thermal Gradient Model
With surface temperatures much warmer than those of the aquifer, it is possible to have undersaturated spring waters that may exert chemical erosion on previous tufa deposits. The geomorphological-stratigraphic analysis of tufa dams from Eastern Africa (Northern Ethiopia) and the Mediterranean basin (Central Italy) highlights the control exerted by climate changes at the global scale [69, 70, 71]. Despite the differences in latitude and climate, the aggradation of tufa dams started in both cases at the Pleistocene-Holocene transition (before 9510 \(\pm\) 100 \({}^{14}\)C yr BP--11,080-10,590 yr cal BP in Northern Ethiopia and before 9310 \(\pm\) 100 \({}^{14}\)C yr BP--10,211-10,184 yr cal BP in Central Italy) and turned to decline in the Middle Holocene (around 5610 \(\pm\) 70 \({}^{14}\)C yr BP--6450-6305 yr cal BP in Northern Ethiopia and around 6190 \(\pm\) 70 \({}^{14}\)C yr BP--7240-6990 yr cal BP in Central Italy) followed by short-lived alternating stages of incision/ deposition (since ca. 4710 \(\pm\) 70 \({}^{14}\)C yr BP--5580-5320 yr cal BP in Northern Ethiopia and 4610 \(\pm\) 100 \({}^{14}\)C yr BP--5600-5050 yr cal BP in Central Italy) until the end of tufa deposition (after 2380 \(\pm\) 50 \({}^{14}\)C yr BP--2710-2340 yr cal BP in Northern Ethiopia and 2826 \(\pm\) 60 \({}^{14}\)C yr BP--3060-2840 yr cal BP in Central Italy) with the subsequent incision of the dams down to the underlying bedrock [(20)] (Figure 8). Comparing the ages of tufa deposits with the Holocene climate changes [(69; 70; 71)] confirms the ground thermal gradient model reliability: It is interesting to notice that both in Eastern Africa and south-central Europe the deposition occurred with warming stages, while a general absence of tufa deposits and the erosion of previous ones characterizes cooling stages, even with temperatures not low enough to prevent tufa deposition [(20; 33; 37)]. Good support for the ground thermal gradient model comes from the temporal distribution of tufa deposits in the northern hemisphere during the Late Pleistocene (MIS 2-3-4), a time interval characterized by the abrupt occurrence of several very cold periods known as Heinrich (H) events [(72; 73)] and changes toward much warmer periods named Dansgaard-Oeschger (D-O) events [(74; 70)]. Comparing the U/Th dates of tufa deposits with the distribution of the relevant warm/cold temperatures peaks (Table 1) shows that, apart from a few exceptions, the overwhelming majority formed with rising temperatures to warm peaks, even in very low thermal conditions [(11)]. Even if with exceptions [(48; 75)], the aridification trend recorded in southern Europe and Eastern Africa since the middle Holocene [(76; 77; 78; 79; 80; 81)] also contributed to the progressive decline of tufa deposition, the short-lived phases of tufa erosion/aggradation recorded in both areas since the Middle Holocene [(20; 34)] may have been the combined effect of the high-frequency cold/warm and dry/wet climatic fluctuations which affected southern Europe [(70; 80; 81)] and East Africa [(78; 79)], recorded by a pronounced erosion phase of the tufa dam at Triponzo, Italy [(38)] and by a clear gap in the backfill sequence at Mai Makden, Ethiopia [(37)]. Calacreous tufa deposition is currently highly reduced [(82; 83)] or completely absent, likely due to the progressive climate cooling and, most likely, the Little Ice Age cold spell [(70; 84)]. A renewed increase of tufa deposition rates could result from ongoing global warming [(85)]. Figure 8: The final incision down to the bedrock of the Holocene tufa dam of Triponzo (Central Italy).