Influence of orbital forcing and solar activity on water isotopes in precipitation during the mid- and late Holocene
In this study we investigate the impact of mid- and late Holocene orbital forcing and solar activity on variations of the oxygen isotopic composition in precipitation. The investigation is motivated by a recently published speleothem δ 18O record from the well-monitored Bunker Cave in Germany. The record reveals some high variability on multi-centennial to millennial scales that does not linearly correspond to orbital forcing. Our model study is based on a set of novel climate simulations performed with the atmosphere general circulation model ECHAM5-wiso enhanced by explicit water isotope diagnostics. From the performed model experiments, we derive the following major results: (1) the response of both orbital and solar forcing lead to changes in surface temperatures and δ 18O in precipitation with similar magnitudes during the mid- and late Holocene. (2) Past δ 18O anomalies correspond to changing temperatures in the orbital driven simulations. This does not hold true if an additional solar forcing is added. (3) Two orbital driven mid-Holocene experiments, simulating the mean climate state approximately 5000 and 6000 yr ago, yield very similar results. However, if an identical additional solar activity-induced forcing is added, the simulated changes of surface temperatures as well as δ 18O between both periods differ. We conclude from our simulation results that non-linear effects and feedbacks of the orbital and solar activity forcing substantially alter the δ 18O in precipitation pattern and its relation to temperature change.