15N enrichment in the Sahara:: In search of a global relationship

We have analysed human and animal collagen samples from three geographically and temporally distinct cemeteries at the Dakhleh Oasis, Egypt. All sites display strikingly high average values of δ15N: Kellis 1 (Late Ptolemaic-Early Roman period) 18.0‰; Kellis 2 (Romano-Christian period) 18.0‰, and 'ein Tirghi (Roman period) 17.0‰. Rainfall at Dakhleh is essentially zero. The δ15N values for humans and animals lie on the respective quasi-linear relationship between rainfall and δ15N found by Heaton et al. (1986). Data from Dakhleh and other sites suggest that a single linear trend describes the rainfall-δ15N relationship in a wide range of sites. This correlation is believed to be due to a combination of two effects: excretion of excess 15N-depleted urea in order to increase osmolality of urine (Ambrose and DeNiro, 1986a, b) and 15N-enrichment in arid-region plants, as a result of 15N-enrichment in soils. Higher δ15N values in human consumers were acquired through consumption of animal-derived protein. High δ15N in desert soils may be caused by volatilization of isotopically light ammonia formed during bacterial activity, an effect which increases near to the soil surface.