Local variation in climate and land use during the time of the major kingdoms of the Tigray Plateau in Ethiopia and Eritrea

The Tigray Plateau of Ethiopia and Eritrea is vulnerable to environmental change, yet environmental influences on the rise and fall of the civilizations that once existed there are almost unexplored. We sampled sections of gully walls for palaeoenvironmental proxies from two sites: 1) Adi Kolen on the southern outskirts of the Plateau's most developed former empire, the Aksumite, and 2) Adigrat near polities dating to at least ca. 3000 cal yr BP. A multi-proxy approach for examining local variation in palaeoenvironments was evaluated that included stable isotopic and elemental analyses (δ13CSOM, δ15N, %TOC, and %TN) of soil, and charcoal identification. An increase in δ15N values from older soils in Adi Kolen (4400 cal yr BP) and Adigrat (2900 cal yr BP) until 1200 cal yr BP is not explained by changes in δ15N that occur with time in an unchanging environment. It may instead indicate an overall decrease in rainfall from the earlier times until 1200 cal yr BP. In one Adigrat section, the decreases in organic δ13C and increases in C/N molar ratios from older to younger soil could have resulted from changes that occur over time, per se. In the remaining sections, however, δ13CSOM trends more likely reflect changes in the biomass of C4 relative to C3 plants (% C4 biomass). Changes in% C4 biomass may reflect climate and/or land use. Deciphering which may be aided by analyses of the other proxies. Identified charcoal suggests that both sites supported some juniper forest types until very recently but that forests may have been a more important and dynamic component of Adigrat's vegetation history than Adi Kolen's. If environment affected the trajectories of the kingdoms of the Tigray Plateau, these results suggest that the exact nature of the changes in climate differed among kingdoms. The kingdoms prior to 1200 cal yr BP may have been exposed to increasing aridity punctuated with relatively wetter intervals. Thereafter, general changes in climate are not apparent. Land clearing dynamics are likely to have had a more consistent effect on the trajectories of kingdoms than climate changes.