Author manuscript, published in "Geochimica et Coscomochimica Acta 111 (2013) 140-161" DOI: 10.1016/j.gca.2012.10.040

1 Environmental changes and the rise and fall of civilizations in the northern Horn of Africa: an approach combining δD analyses of land-plant derived fatty acids with multiple proxies in soil. Abstract 1 The domains of the ancient polities D'MT and Aksum in the Horn of Africa's highlands are a 2 superior natural system for evaluating roles of environmental change on the rise and fall of 3 civilizations. To compare environmental changes of the times of the two polities, we 4 analyzed stable hydrogen isotopic ratios (δD) of land-plant derived fatty acids (n-C 26–30) and 5 other proxies from soil sequences spanning the Holocene from the region. Three results 6 suggest that trends in δD values unambiguously reflect changes in rainfall. First, increases in 7 δD coincide with dry periods inferred from studies of eastern African lakes. Second, changes 8 in δD values were parallel among sections during overlapping time intervals. Third, 9 consideration of vegetation history did not alter directions of trends in δD values over time. 10 By unambiguously recording precipitation, the δD values also enhanced interpretations of 11 proxies that are affected by both climate and land clearing. 12 Both D'MT (ca 2750–2350 cal y BP) and the Aksumite (ca 2100–1250 cal y BP) rose 13 during wetter intervals of the drier part of the Holocene (after ca 6000 cal y BP). Analyses of 14 charred matter indicated that fire had been a common agent of land clearing in all sites. The 15 influence of climate on fire varied, however. Prior to the emergence of D'MT, δD values were 16 correlated with C 4 :C 3 plant ratios estimated from δ 13 C values. There are no C 4 trees and 17 precipitation may have been the main influence on canopy openness. After ca 4300 cal y 18 BP, there was no significant relationship between δD and C 4 :C 3 plant ratios suggesting that 19 factors such as fire influenced canopy openness regardless of climate. Furthermore, the 20 impact of land clearance differed between sites and between D'MT and the Aksumite's times. 21 In one site, the interval from 3550 cal y BP to the decline of D'MT had several anomalies that 22 suggested dramatic increases in thermal severity of fire and human impact. Among these 23 were a large contribution of charred matter to a high % total organic carbon that low 24 hydrogen and …