δ18O in mollusk shells from Pliocene Lake Hadar and modern Ethiopian lakes: implications for history of the Ethiopian monsoon

Two of the five lacustrine intervals in the largely fluvial Hadar Formation, Afar, Ethiopia, occur in the Sidi Hakoma Member deposited 3.4–3.2 Ma. In a perspective of the δ18O of 11 modern Ethiopian lakes and their shells, the δ18O of the Hadar fossil shells provide a snapshot of the nature of ancient Lake Hadar and Ethiopia’s climate in the Pliocene. Ethiopia’s modern lakes both in the Rift and on the Western Plateau are fed by drainage of Plateau rain with its well established barely negative δ18OSMOW of −1.3‰. Except for the man-made Lake Koka reservoir, all other Ethiopian lakes are isotopically quite positive ranging from +5.4 to +16.0‰, indicating how significant evaporation is in their water budget. Shells from lakes with extant mollusk populations are mostly in isotopic equilibrium with the δ18O and temperature of their lake water. The upper transgressive interval in the Sidi Hakoma Member is the largest one in the Formation beginning at its base with the ‘Gastropod Beds’ beach deposits. Mollusks from shell beds other than the ‘Gastropod Beds’ show more positive and more variable δ18O between shells, with internal variations within shells as much as 7‰. At these times the site must have been underlain by a shallow partially isolated embayment of Lake Hadar which underwent rapid expansions and then contractions by evaporation, within the few year lifetimes of the individual mollusks. The results from the ‘Gastropod Beds’ are of most significance for interpreting the overall paleoclimate at Hadar. Their uniformly negative δ18OPDB shell values that average −6.7‰ represent a much less evaporated stage of Lake Hadar when its δ18OSMOW was 8‰ lower than the spectrum of modern lakes in Ethiopia, and indeed even 3‰ or more lower than average modern Plateau rain. To explain such negative values we hypothesize that the Atlantic-derived air mass component to the Ethiopian monsoon was persistently strengthened during Pliocene summers, which intensified the amount and the negative isotopic character of rainfall onto both the Afar and the Ethiopian Plateaus that drained to Lake Hadar. A similar phenomenon characterized the brief periodic pluvial episodes of the Quaternary, including the latest in the early Holocene, known as the African Humid Period. In contrast to the hot semi-desert steppe conditions of today’s western Afar, the diverse abundant terrestrial fossil fauna at Hadar, including the early hominid Australopithecus afarensis, is explained by the wetter, and probably cooler, summers that persisted throughout the Late Pliocene.