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Universitaet Bremen (2014)

A new hydrogen-isotope approach to understand North African monsoon changes in the Holocene

Haese, Barbara

Titre : A new hydrogen-isotope approach to understand North African monsoon changes in the Holocene

Auteur : Haese, Barbara

Université de soutenance : Universitaet Bremen

Grade : Doctor of Philosophy (PhD) 2014

Résumé
The aim of this thesis is to improve the understanding of the hydrological evolution of the North-West African monsoon system during the Holocene period. In particular the focus is placed on the drastic regime shift from humid and vegetated conditions of the mid-Holocene to the arid present-day conditions in North-West Africa. The timing of this regime shift is investigated in order to determine if the transition from the African Humid Period (approx. from 16,000 – 6,000 years ago) to dry present-day conditions occurred gradually or rapidly. In order to achieve this objective, variations in the isotopic composition of precipitation in relation to rainfall amount are scrutinized. To reach this goal the coupled atmosphere-land surface model ECHAM5-JSBACH is enhanced by the inclusion of a stable water isotope diagnostic module which traces beside the “normal water” H16 2 O the heavier water isotopes H18 2 O and HDO. The ECHAM5-JSBACH-wiso model is able to simulate the isotopic composition of precipitation (δ18OP and δDP) comparably well as the stand-alone ECHAM5-wiso model. In order to analyze the sensitivity of fractionation processes over land, a set of simulations with various implementations of these processes over the land surface are compared. The simulations distinguish between no fractionation, fractionation included in the evaporation flux (from bare soil), and fractionation included in both evaporation and transpiration (from water transport through plants) fluxes. While the isotopic composition of the soil water may change for δ18O by up to +8, the simulated δ18O in precipitation shows only slight differences in the order of ±1. For evaluation of the simulated isotope composition over the 20th century in North Africa, a nudged ECHAM5-JSBACH-wiso experiment is performed over the period 1958 to 2002. It is shown that the model simulates the climatology as well as interannual variability of precipitation and its isotopic composition in good agreement with observations. Furthermore, it is illustrated that the amount of Sahelian precipitation and δDP are correlated, with a Pearson correlation coefficient r = 0.71. Based on these model results the observed isotope variations are quantitatively calibrated with respect to changes of the precipitation amount. According to the model results, changes of -5 in δDP can be related to an increase of approximately 100 mm in rainfall amount under present-day conditions. In order to further investigate the evolution of the North-West African hydrological cycle during the Holocene, the precipitation and the vegetation cover simulated by the fully coupled Earth System model COSMOS are analyzed. Both variables indicate i a gradual transition from the African Humid Period into the dry present-day conditions. Based on this transient experiment, time-slice simulations, performed with the ECHAM5-JSBACH-wiso model, are carried out. These time-slice simulations reveal an amplification as well as a southwards shift of the North African rain belt from midHolocene to present day. Due to the negative relation between the amount of precipitation and its isotopic composition, the simulated δD in precipitation is about -18 more depleted in the mid-Holocene experiment, compared to the present-day one, in the West Sahel region. The findings of the model studies are supported by novel proxy data derived from δD measurements on leaf waxes in the marine sediment core GeoB7920 from the North-West African coast. In summary, all results of these studies indicate that the drastic regime shift of vegetation and rainfall amount in North-West Africa during mid-Holocene was gradual.

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Page publiée le 6 mars 2015, mise à jour le 8 janvier 2019