Modelling Stable Isotopes in the Eemian and Holocene Hydrological Cycles

Abstract

The climate of interglacials within the Quaternary is of particular interest for palaeoclimatology due to its similarity to present-day conditions. This thesis evolves around a set of simulations with the isotope-enabled version of the atmospheric general circulation model ECHAM4. Stable isotopes of oxygen and hydrogen, respectively, are transported in the modelled hydrological cycle, altered by fractionation during phase changes. The simulations represent time slices of the Eemian (124 ka BP) and a series of six mid- to late Holocene time slices (6 to 1 ka BP), respectively. Boundary conditions include sea surface temperatures from a coupled atmosphere-ocean general circulation model (ECHO-G) and insolation anomalies according to changes in orbital parameters. The study focuses on the analysis of the isotopic composition of precipitation. Anomalies of delta-18-O are interpreted in terms of anomalies of precipitation amount and air temperature, respectively. The simulation results are compared to measured isotope values from various climate archives. The physical processes that lead to the simulated isotopic composition of precipitation are shown for case studies and from a global perspective. A prominent feature of both Eemian and Holocene conditions is the insolation anomaly during boreal summer months. Tropical and subtropical African climate is affected significantly by increased zonal flow that increases rainfall amount and decreases air temperature. The spatial distribution of delta-18-O in the simulations indicates that changes in zonal moisture transport rather than a meridional shift in the ITCZ alter the hydrological cycle and trigger the observed northward extension of the rainfall area. Knowledge about the annual distribution of rainfall is crucial for accurate analysis of isotopic signatures from climate archives. This thesis shows orbitally induced changes of the seasonality and associated delta-18-O values. The extent and type of the anomalies varies greatly with location. Some tropical areas may experience a shift from a double to a single cycle, while delta-18-O from extra-tropical regions are more affected by temperature changes without significant changes in the timing of the main precipitation season.