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Investigations into a model-proxy discrepancy between precipitation and hydrogen stable isotopes in mid-Holocene northern Africa

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Alexander Thompson1, Clay Tabor2, Chris Poulsen1 

1Department of Earth and Environmental Sciences, University of Michigan
2Center for Integrative Geosciences, University of Connecticut 

Depleted mid-Holocene (MH, 6,000 years BP) northern African leaf wax isotopic values have been directly interpreted via the amount effect to indicate up to 10 times higher mean annual rainfall relative to the pre-industrial era (PI). Ten leaf wax δD records from northern Africa show an average depletion of 13.4‰ in the MH relative to the PI; however, investigation of this isotopic difference has yet to be performed with isotope-enabled climate models. Here, we use iCESM, a water isotope-enabled earth system model, to simulate climate and water isotopic compositions for the MH and PI. In contrast to the amount effect, we find that the change in isotopic composition of rainfall (δDP) does not uniformly follow MH northern African increases in rainfall rate. δDP is relatively depleted by 24‰ during the MH in the east (10.4–16.1°N, 25– 35°E) due to the continental effect associated with an increase in low-level monsoonal flow. In the west (25.6–31.3°N, 10°W–10°E) and in contrast with leaf wax records, δDP is relatively enriched by 6.5‰ during the MH due to a strengthening of the Saharan Heat Low. However, simulated MH soil water δD (δDS) exhibits a strong depletion (by 17‰ in the west and 30‰ in the east) relative to PI values, and thus provides better agreement with northern African leaf wax δD records than δDP (RMSE of 11.34‰ for δDS compared to 17.01‰ for δDP). MH δDS depletion occurs due to expanded vegetation extent in northern Africa and the resulting changes to the evapotranspiration (ET) flux. In the PI desert, the ET flux is dominated by evaporation, which relative to the atmosphere, enriches soil water with heavy isotopes. Alternatively, the transpiration to evapotranspiration ratio (T:ET) increases in the vegetated MH (by 1300% in the west and 60% in the east), and, since transpiration is non-fractionating, the soil water does not undergo the same enrichment. As a result, the simulated MH soil water pool is less enriched, or more depleted, than the PI. Overall, our results suggest that leaf waxes incorporate an integrated isotopic signal of both soil water and precipitation. Based on these findings, we suggest that northern Africa leaf wax δD records may be recording a more nuanced signal of surface hydrology, where local vegetational processes can overprint the amount effect signal from mean annual rainfall.