Modeling orbital induced variations in circum-Mediterranean climate

Abstract

The climate of the Earth varies both irregular and (quasi)periodic over a broad range of time-scales. The variations with periods of some ten-thousands of years are caused by variations in the shape of the orbit of the Earth and the orientation of the rotation axis of the Earth. These variations strongly affect the strength and the spatial and seasonal pattern of the insolation received by the Earth. This leads to climatic oscillations which are often recorded in sedimentary archives. One example is the deposition of sapropels (organic-rich black layers) which are ubiquitously present in the deeper parts of the Mediterranean Sea throughout at least the last 13 million years. The proxy data related to sapropels clearly point to a precession (date of perihelion) dominated oscillation in the climate system. However, also an obliquity (tilt of the rotation axis of the Earth) signal is found in the sapropel record, despite the weak obliquity signal in the insolation at low latitudes. The paleoclimatic origin of sapropels is not fully understood. The most used hypothesis is that sapropels are deposited when the discharge of the river Nile (which is determined by the strength of the African summer monsoon) is high. However, the role of discharge from European rivers and the role of precipitation over the Mediterranean Sea is not clear. Another uncertainty is the timing of the deposition of sapropels, i.e., the youngest sapropel lags precession by about 3,000 years and it is not clear whether this is the case for all sapropels. In this research the orbital signals in the African summer monsoon and in the European climate are studied with climate models. The results show that a strong precession signal is present in the discharge of the river Nile which is in agreement with the hypothesis. Furthermore, an obliquity signal exists in the discharge which is caused by remote influences from high latitudes. In addition to the orbital signals in the discharge of the river Nile, the river discharge from the northern borderlands and the precipitation over and around the Mediterranean Sea also show a precession as well as an obliquity signal. The precession signal in the precipitation is in agreement with a pollen record from Greece which is also described in this thesis. The model results show no clear indications for a lagged response of the circum-Mediterranean climate system to precession. However, a clear (vegetation induced) lag was found in the circulation in the Atlantic Ocean. It is not fully excluded that a similar mechanism can also cause a lag in the deposition of sapropels in the Mediterranean Sea. In order to determine which influence (from the south or from the north) is most important for the circulation in the Mediterranean Sea a model for the Mediterranean Sea is used. It turns out that the influence of increased Nile discharge is strictly located to the Eastern coast while increased discharge from European rivers and increased precipitation cause lower salinity over the entire Mediterranean Sea, especially over locations which are important for the deep circulation (i.e., where deep water is formed). The main conclusion drawn from this research is that model results indicate that the influence of the river Nile on the circulation in the Mediterranean Sea is smaller than thought before and that the deposition of sapropels could also be caused by changes in the hydrological cycle over the northern borderlands and over the Mediterranean Sea.