Geophysical Institute

GFI/BCCR Seminar | Alexandra Touzeau: Isotopes in the Nile Valley

Alexandra Touzeau (LSCE/UiB)

Isotopes in the Nile Valley

Alexandra Touzeau, Romain Amiot, François Fourel, François Martineau, Christophe Lécuyer

The controls on isotopes in the precipitation varies worldwide. Within the tropical belt, the main control is the “amount effect”. It corresponds to a rain-out of the precipitation during intense convective rains, leading to low δ18O values in the rainfall. These low values during strong precipitation events are opposed to high (sometimes positive) δ18O values observed in surface waters, especially in small lakes and marshes, where evaporation leads to heavy isotopes enrichment. These natural variations in the isotopic composition of water can be used to trace the origin of river water, when several sources are possible. If this isotopic composition is preserved, it can also help to reconstruct past climate variations, specially variations in the amount of precipitation over time. In the present study, our first aim was to use isotopes to estimate past precipitation levels in the source regions of the Nile river, and especially in the Ethiopian mountains. We also wanted to see if the variations of the Nile level affected populations leaving along its banks and depending on the river for irrigation. First, we measured δ18Op phosphate values in the bone and enamel of Ancient Egyptians, with the objective of recovering past δ18Ow values in the Nile River, their principal source of drinking water. We used the phosphate-water fractionation equation of Daux et al. (2008) for humans. The values reconstructed for the river water range from -4 ‰ to +4 ‰. There is a large scatter for each period, maybe caused by the strong interannual variability in the Nile discharge. Nonetheless, there is a significant trend toward higher δ18O values for more recent periods. We observe a total increase of +2.9‰ between 5500 and 2500 BP. Based on present-day relationship between δ18O values in the precipitation and the precipitation amount, we estimate that it could correspond to a decrease of precipitation by 130 mm/month during the rainy season. Second, we measured δ13C values in the carbonate fraction of the enamel and bone. At first order, and especially for people with low meat diet, these δ13C values reflect the proportion of C4-plants in the diet. The C4-plants have a specific metabolism that makes them more resistant than other plants to dry environments. Thus, we were expecting to see an increase in the proportion of C4-plants in the diet because of the observed drying trend. However, the δ13C values remained constant during the period considered, implying that the Egyptians did not change their eating habits to adapt to the climatic change.

Daux, V., Lécuyer, C., Héran, M.-A., Amiot, R., Simon, L., Fourel, F., Martineau, F., Lynnerup, N., Reychler, H., Escarguel, G., 2008. Oxygen isotope fractionation between human phosphate and water revisited. Journal of Human Evolution 55, 1138-1147.