Sediment analyses of unprecedented accuracy reveal phases of stability during periods of strong climate change
The lake level of the Dead Sea is currently dropping by more than one metre every year – mainly because of the heavy water consumption in the catchment area. However, very strong lake level drops due to climate changes are also known from earlier times. At the end of the last ice age, for example, the water level dropped by almost 250 metres within a few millennia. A study published today in the journal Scientific Reports now provides new insights into the exact course of this process. Daniela Müller and Achim Brauer from the German Research Centre for Geosciences (GFZ) in Potsdam, together with colleagues from the Hebrew University of Jerusalem, studied 15,000-year-old sediments from the Dead Sea and the surrounding area using newly developed methods. With unprecedented accuracy, they show that the long period of drought was interrupted by wet periods lasting ten to a hundred years. This also offers new insights into the settlement history of this region, which is significant for human development, and enables better assessments of current and future developments driven by climate change.
The water cycle at the Dead Sea – then and now
In highly sensitive regions such as the Eastern Mediterranean, where water availability is an important factor for socio-economic and political development, it is crucial to understand how the water cycle is changing in response to climate change. Geologists can achieve this by assessing strong hydroclimatic changes that occurred several millennia back in time. For example, during the transition from the last ice age to the Holocene, the water level of Lake Lisan dropped by about 240 metres in the period 24-11 thousand years ago, which eventually led to its transition into today’s Dead Sea.
Sediments as witnesses of time
The sediments at the edge of lake Lisan near the archaeological site of Masada and from the bottom of what is now the Dead Sea are unique witnesses to this development. In their new study, researchers led by Achim Brauer, head of Section 4.3 “Climate Dynamics and Landscape Evolution” at the German Research Centre for Geosciences Potsdam, and doctoral student Daniela Müller together with colleagues from the Geological Survey Israel and the Hebrew University of Jerusalem, analysed these sediments with unprecedented precision. The investigations took place within the framework of the PALEX project ‘Paleohydrology and Extreme Floods from the Dead Sea ICDP Core’, which is funded by the German Research Foundation (DFG).
New high-resolution methods for sediment analysis
For this study, new high-resolution analytical methods were developed at the GFZ to gain precise information from the stratification of the sediments and their geochemical composition, even about seasonal deposition processes and thus about the type, duration and course of climatic phases.
In particular, the combination of light microscopic methods with so-called 2D element mapping using X-ray fluorescence scanners is new. This enables the precise identification and localisation of elements in the sediments. Important and challenging for this is the preparation of the sediments for this analysis: The moisture must be removed from them by freeze-drying – not easy given the high salt content of the Dead Sea and its affinity for water. Then the sediments are impregnated in synthetic resin and thin sections are made from them. In all this, the microstructure must not be altered.
Pause in climate change: humid phases interrupted long dry periods
The researchers found out that the dramatic long-term drop in the lake level due to increasing dryness was interrupted several times by wetter phases when climate change took breaks. “In this study, we were able for the first time to precisely determine the duration of these phases with several decades and in one case up to centuries by counting annual layers in the sediment,” says Daniela Müller, lead author of the study. The exact reason for these pauses in the climate change of this region still remain elusive. Possible links to North Atlantic climate are suspected.
“What was particularly surprising was that during these wetter phases, in some cases over several decades, there we even did not find any traces of extreme floods, which are typical for this region even today and during wetter times in the past,” Müller explains.
Consequences for archaeological considerations and future climate scenarios
These results are of further interest for archaeological considerations because they coincide with the time when the Natufian culture settled in this region. Climatically stable phases could have favoured the cultural developments.
“The study shows that strong climatic changes in the past have been very dynamic and included phases of relative stability. We learn from this that climate change is not linear, but that phases of strong changes alternate with calm phases,” says Achim Brauer.