Under my microscope, I can see tiny calcium carbonate shells that could fit on the head of a pin. These remnants of prehistoric life forms look like tiny snails or clams. The microfossils were trapped beneath the Mediterranean Sea for millennia, until a research ship collecting sediment cores from the seafloor retrieved them and sent them to me at Universität Hamburg’s Institute for Geology. The fossils offer us insights into the climate during the Earth’s recent geological history.
For my latest research project, I looked for traces of the chemical element barium in the calcium carbonate shells. Why? Because the level of barium tells me how much freshwater flowed from the Nile into the Mediterranean back then, and how much rainfall there was in the Nile catchment area. There are only very small amounts of barium in seawater; the vast majority of the barium only came with the flow of freshwater from the Nile. The more precipitation there is, and the more water flows into the Nile’s catchment area, the more barium is leached from the riverbed, transported to the Mediterranean and ultimately accumulates in microscopic life forms’ calcium carbonate shells.
This new method allows researchers to draw direct conclusions on precipitation levels and can also be used to support other findings – such as those from analyzing vegetation data. The study shows: in sediment cores from the eastern Mediterranean – the area where Cyprus now lies – the layers starting from roughly 12,000 years ago, the beginning of the current interglacial, contain much higher barium levels. This trend peaked ca. 9,000 years ago, remaining constant for roughly a millennia before levels began dropping again. This development matches that of sunlight – though with a slight chronological lag. Given the higher amounts of sunlight, the tropical rain belt shifted to the north, the West African summer monsoon intensified and more freshwater flowed into the sea.
The consequences of this inflow were drastic. Since freshwater is lighter than saltwater, it rested atop the seawater like a lid. As a result, the vertical water circulation was interrupted and the deeper water layers were no longer oxygenated. Starting at a depth of 1,800 meters, there was no more oxygen and no more life – for 4,000 years. The eastern Mediterranean was essentially like a giant version of a hypoxic lake, which we sometimes see in particularly hot summers. Making matters worse, this condition was reinforced by mild winters: the surface water never grew cold and heavy enough to sink lower and “kick-start” the water circulation.
Throughout our history, seawater circulation has only been interrupted roughly every 23,000 years. Accordingly, many researchers are currently investigating this unusual phenomenon. My findings can be used to help verify the results of computational climate models, and to help interpret today’s climate changes. For example, if the current warming results in a shift of the rain belt in the Sahel zone, it will mean more freshwater in the Nile and my study will become especially relevant for those people living in the Mediterranean.
Valerie Menke is a geologist whose work currently focuses on using microscopic life forms to reconstruct climate changes that took place in the Mediterranean during the Earth’s recent geological history.
Author: Valerie Menke