Holocene aggradation/degradation phases of tufa dams in northern Ethiopia and central Italy: a palaeoclimatic comparison between East Africa and Mediterranean Europe

摘要

The C-14 dating of buried soils and peat layers from tufa dammed swampy-lacustrine sequences allows us to outline the aggradation/degradation sequence of Holocene tufa dams in central Italy and northern Ethiopia. In northern Ethiopia, the aggradation of tufa dams started before 9,510 +/- 100 C-14 yr BP (11,080-10,590 yr cal BP), turned to decline around 5,610 +/- 70 C-14 yr BP (6,450-6,305 yr cal BP) with short-lived alternating stages of incision/deposition since ca. 4,710 +/- 70 C-14 yr BP (5,580-5,320 yr cal BP). After ca. 2,380 +/- 50 C-14 yr BP (2,710-2,340 yr cal BP), fluvial incision completely cut the tufa dams. In central Italy, the aggradation of tufa dams started before 9,310 +/- 100 C-14 yr BP (10,211-10,184 yr cal BP) and declined after 6,190 +/- 70 C-14 yr BP (7,240-6,990 yr cal BP). From ca. 4,610 +/- 100 C-14 yr BP (5,600-5,050 yr cal BP), alternating stages of dam erosion/aggradation occurred, eventually followed by a phase of deep incision down to the present valley floors. The occurrence of similar trends of tufa dam aggradation/erosion in Mediterranean Europe and East Africa, as compared with the patterns of Holocene climate in both areas seems to indicate that cold/warm and dry/wet fluctuations have controlled the aggradation/erosion phases of tufa dams including their ultimate incision by streams. On the other hand, the comparable evolution of tufa dams in areas quite different in geographical position points out the significant morphogenetic role of climate changes at the global/supra-regional level.