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Mini-Monsoon In The Mediterranean - Science article on the climate puzzle in the Near East

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04.04.2003

Our post-ice-age climate is not nearly as stable as is commonly believed. New evidence for this is reported by geoscientists of the DFG Research Center Ocean Margins in the upcoming issue of the journal Science. Investigating marine sediments from the northern Red Sea, they discovered that this currently very dry region was influenced by a long wet period between 9,000 and 6,500 years ago. It was characterized by a monsoon-like system during that time. The possibility that this Mediterranean monsoon could again dictate Near East precipitation patterns in the future in connection with a sustained green-house effect cannot be ruled out.

Dr. Helge Arz and Dr. Frank Lamy, with other Bremen colleagues, have studied marine deposits from the northern Red Sea that were retrieved in Spring 1999 during a cruise of the research vessel Meteor under the scientific leadership of Dr. Jürgen Pätzold. One of their investigative techniques involved studying the remains of microorganisms in fine detail. The information recorded in these tiny chalk shells reflects the environmental conditions of the epochs during which these animals lived in the Red Sea. The scientists were able to determine that, during the period between 9,000 and 7,000 years ago, the surface waters of the Red Sea had a significantly lower salt content than they have today. Shortly after the end of the last ice age this value was about 37 grams of salt per liter of sea water (per mil). That is about 3 per mil less than today. The data also indicate that this freshening was limited to the upper 100 meters of the water column.

In addition, the geoscientists investigated the sedimentation rates and clay contents of the deposits and discovered dramatic variations in these parameters. Seven thousand years ago, near the end of the time period studied, the total rate of deposition suddenly decreased by about 25% and the clay content by 50%. The sediment input by rivers and streams made up a significantly smaller proportion than before and the wind, according to modeling calculations, became a predominating factor in contributing sand and dust from the land into the sea.

A clear decline in the salinity of the Red Sea and drastic changes in sedimentation processes, say the Bremen scientists, indicate greater temperature differences between the land and sea. "On the one hand, water temperatures in the eastern Mediterranean after the end of the last ice age were lower than today," states Dr. Lamy. "On the other, the land and overlying air masses warmed quickly in the summer season. The warm air rose and was replaced by cool moist air masses flowing in from the Mediterranean. Monsoon-like weather conditions resulted, similar to what we see today in the Indian Ocean," says the researcher. Rainfall was much heavier, especially in the summer months. Investigations from plant pollen and Israeli cave deposits give additional support to the monsoon scenario.

Whether similar monsoon conditions could play a role in future climate processes in the Near East remains speculative. Regional climate models suggest that increasing greenhouse effects in the future would result in reduced summer rains in the Mediterranean region. The Science article, however, brings these models into question - are other scenarios also possible, for example: larger temperature differences between the Mediterranean and land, Mediterranean monsoon conditions and trends toward increased rainfall. "Our data only permits us to draw conclusions about the climate of the past," explains Science author Dr. Helge Arz. "But maybe our results will provide an opportunity for more intensive discussions relating to climate modeling and measurement data. This is also supported by the fact that there is an evident relationship between climatic processes in the Near East and those in the North Atlantic. Rain in the Near East is always more abundant when the atmospheric pressure fluctuations between the Azores high and Iceland deep follow a particular pattern."

Albert Gerdes | Source: alphagalileo
Further information: oceanmargins.de

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