What does a Serbian cave tell you about the weather 2,500 years ago?

As with most types of weather, scientists suspect that the flow of jet streams is affected by climate change. Data for the past century and a half indicate that the northern jet stream became stronger during that period. But a century is not a long time in climatological terms, and it is not entirely clear whether strengthening is a natural phenomenon.

In research published in the journal Geology, Miova Li of Fujian Normal University and Slobodan Markovic of the University of Novi Sad in Serbia shed new light on this question. Climatologists routinely examine ancient air trapped in polar ice for information about the state of the climate hundreds or even thousands of years ago. The researchers suggest that something very similar could be achieved by looking at the chemical composition of rock formations in a pair of Serbian caves.

As with many caves, the floors of both Śrjanska Cave and Prikonoska Cave, both located in southeastern Serbia, are dotted with thin spiers of rock called stalagmites. These rocks are formed very slowly by water falling from the rocks above. Each drop carries dissolved minerals. If water drips on the same spot over many years, stalagmites gradually form as the minerals precipitate. More importantly, analyzing the minerals that make up stalagmites can reveal information about the water from which they were formed.

The researchers were interested in Serbia specifically because of the influence of its location on the weather. The type of rain that falls on the country depends on the strength of the Arctic jet stream. When it blows strongly, it tends to blow away the clouds that have formed over the Atlantic Ocean. When they are weak, clouds tend to drift in from the Mediterranean Sea instead.

These two bodies of water have unique chemical signatures. Mediterranean waters contain a rare isotope of oxygen known as oxygen-18, in which the element’s eight protons are linked to ten neutrons instead of the usual eight. Water from the Atlantic Ocean has less. By examining the ratios of this isotope in the stalagmite layers, the researchers hope they can discover when Serbia was exposed to more Atlantic storms or more Mediterranean storms, and thus what the jet stream was doing.

Drs Li, Markovic and their colleagues examined two stalagmites, one 380 mm specimen from Śrjanska and one 238 mm from Brekonoska. Using traces of two other elements, uranium and thorium, they were able to date both stalagmites. Those at Śrjanska grew between 434 BC and 1913, while those at Šrjeńska Cave formed between 798 BC and 404 BC. They then analyzed 581 samples of an oxygen-containing mineral called calcium carbonate.

The researchers concluded that the North Atlantic jet stream appears stronger today than at any time in the past 2,500 years. Their confidence in their method was strengthened when they were able to see in the rocks the imprints of climate events known to have occurred from other sources, such as the Roman warm period, which lasted between 300 and 200 BC, and the cold period of the Dark Ages. Which ranged from approximately 300 to 700. Interestingly, the data suggests that the jet stream was stronger during cold periods and weaker during warmer periods, the opposite of the trend seen today.

Why the modern jet stream seems to do the opposite is not yet clear. Researchers raise as one possibility the atmospheric influence of the so-called global warming hole in the North Atlantic. This is a stationary bubble of unusually cold water that extends from the Hudson Strait in northern Canada almost to the west coast of Ireland. This is thought to be caused by the melting of the Greenland ice sheets and the weakening of large oceanic conveyor belts that transport warm water from the tropics to the high northern latitudes. Perhaps more data from other stalagmites in other caves will help unravel the mystery.

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