Environmental changes and the rise and fall of civilizations in the northern Horn of Africa: An approach combining deltaD analyses of land-plant derived fatty acids with multiple proxies in soil

Abstract

The domains of the ancient polities D'MT and Aksum in the Horn of Africa's highlands are a superior natural system for evaluating roles of environmental change on the rise and fall of civilizations. To compare environmental changes of the times of the two polities, we analyzed stable hydrogen isotopic ratios (dD) of land-plant derived fatty acids (n-C26-30) and other proxies from soil sequences spanning the Holocene from the region. Three results suggest that trends in dD values unambiguously reflect changes in rainfall. First, increases in dD coincide with dry periods inferred from studies of eastern African lakes. Second, changes in dD values were parallel among sections during overlapping time intervals. Third, consideration of vegetation history did not alter directions of trends in dD values over time. By unambiguously recording precipitation, the dD values also enhanced interpretations of proxies that are affected by both climate and land clearing. Both D'MT (ca 2750-2350 cal y BP) and the Aksumite (ca 2100-1250 cal y BP) rose during wetter intervals of the drier part of the Holocene (after ca 6000 cal y BP). Analyses of charred matter indicated that fire had been a common agent of land clearing in all sites. The influence of climate on fire varied, however. Prior to the emergence of D'MT, dD values were correlated with C4:C3 plant ratios estimated from d13C values. There are no C4 trees and precipitation may have been the main influence on canopy openness. After ca 4300 cal y BP, there was no significant relationship between dD and C4:C3 plant ratios suggesting that factors such as fire influenced canopy openness regardless of climate. Furthermore, the impact of land clearance differed between sites and between D'MT and the Aksumite's times. In one site, the interval from 3550 cal y BP to the decline of D'MT had several anomalies that suggested dramatic increases in thermal severity of fire and human impact. Among these were a large contribution of charred matter to a high% total organic carbon that low hydrogen and oxygen indices suggest was severely altered by other factors than humification. These results support hypotheses about the rise of civilizations being favored by specific climatic conditions but suggest that patterns of land clearing differed during the declines of D'MT and the Aksumite.