Stalagmites
© Fredy Thuerig/Shutterstock
Stalagmites are upward-growing mounds of mineral deposits that have built up due to water dripping onto the floor of a cave, and they reveal quite the history of a land.
Around 4,200 years ago Mesopotamia's first empire, the Akkadian fell, coinciding with major transformations in Egypt and the Indus Valley, the two other great civilizations of the time. A study of stalagmites in Iran suggests a widespread climatic event may have been responsible for all three.

Civilizations rise and fall for many reasons, and the causes of the Akkadian Empire's demise remain controversial. The coincidence of timing with far away events has led some historians to propose a climatic cause. The nature, and even existence, of this event has been unclear, however, coming as it did in the middle of the Holocene era of largely stable temperatures, with no known upsurge in volcanic activity or change in solar output.

However, when a team led by The University of Oxford's Dr Stacy Carolin studied a stalagmite from Gol-e-Zard Cave in Iran's Alborz Mountains formed between 5,200 and 3,700 years ago they saw something certainly happened around the relevant time. The team report in the Proceedings of the National Academy of Sciences there were sharp spikes in the amount of magnesium relative to calcium 4,510 and 4,260 years ago, coinciding with slower growth and changes in the stone's oxygen isotopes. These changes lasted 110 and 290 years, respectively before the stalagmite composition returned to previous levels.

The Akkadian Empire
© Zunkir via Wikimedia Commons CC-by-SA 3.0
The Akkadian Empire was the first great empire of Mesopotamia, preceding the better known Babylonians and Sumerians. The explanation for its fall has come from mountains on the shores of the distant Caspian Sea.
The industry and mining of ancient civilizations sometimes left its mark on the planet, but we know of no mechanism by which the Akkadians could have had an impact on such distant caves. Therefore it seems likely that whatever was causing the chemical change brought down the Akkadians, rather than their fall altering the chemistry of distant caves.

The change in the stalagmite's composition appears to be the result of increased dust falling in the mountains, which in turn seems to be a consequence of drier conditions to the west. Today, dry years in the deserts of Syria and Iraq are associated with increased dust deposition in Tehran, just 50 kilometers (30 miles) from Gol-e-Zard. The slow growth of the stalagmite could be a sign of locally drier conditions as well.

Sediments from the Red Sea and the Gulf of Oman, among other paleoclimatic proxies, have previously been used to suggest western Asia experienced at least one major dry period around this time, but the dating for these was too imprecise to tie them confidently to the Akkadian collapse. The stalagmites, on the other hand, have an error of just 31 years.

There is a major debate among historians as to how much climate change has contributed to civilization collapse - one that has become hotter as it becomes more relevant for us. We don't know why Mesopotamia dried out during this period, but it seems it took down one civilization, and severely affected two others.