Ancient Rainfall

On clues to the pattern of future rainfall in the Middle East — that might be buried in the past.

Scientists at the Geological Survey in Israel hope to determine the last 4,000 years of rainfall in the Eastern Mediterranean region — by dissecting cave formations. When rainwater soaks into the Earth, it percolates downward, picking up minerals. Sometimes this water drips into caves — where it adds layers to subterranean wonderlands filled with stone icicles, rippled stone curtains, columns and sheets.

The Israeli scientists have carefully sliced into cave features to figure out the amount of rain that fell in the past. What they’ve found matches archaeological records of ancient floods and droughts. And although recent conditions in the Middle East have been pretty dry, they’ve been both drier and wetter at various times in the past. In fact, comparing the rainfall with the region’s archaeology suggests the ebb and flow of civilizations in the region might be synchronized with the cycle of rain.

These scientists are able to do this work because rain carries a chemical signature that’s linked to the amount of rain that falls.

More Notes

In order to understand the nifty trick these researchers pulled off to determine the rainfall, its important to know a little bit about the element oxygen and isotope ratios.

The chemical signature that Miryam Bar-Matthews and her colleagues are measuring is the so-called “oxygen isotope ratio.”

Ratios compare one number to another. For example, if I have a basket of 5 apples and 6 oranges, the ratio of apples to oranges would be 5 to 6.

Atoms of oxygen come in several flavors. These are called “isotopes”. The most common form weighs 16 atomic mass units (or a.m.u’s). But a smaller number of oxygen atoms have a weight of 18 atomic mass units. An isotope ratio would compare O18 to O16.

Water is made up of one oxygen atom (any flavor) and two hydrogen atoms. If you grabbed a bucket of water, most of the water molecules would contain an oxygen with an a.m.u. of 16 — in shorthand, you write O16. The amount of water molecules with O18 — the heavier oxygen — depends on where the water came from (a lake or stream, an ocean or river), and the room temperature.

Why the room temperature? These different isotopes of oxygen make water with O16 behave differently than water with O18. For example, if it’s really hot in the room, water with O16 will evaporate faster. Because it is lighter, it is slightly easier for O16 turn into steam and escape the bucket. How much faster it evaporates depends on the temperature.

The temperature would also affect how water with O18 and water with O16 will also interact with the cave formations.

From conducting an experiment over 11 years, the researchers observed that if the amount of rainfall goes up, the O18 ratio goes down. This is the “rule” that describes the connection between oxygen 18 and how much rain falls.

So the researchers measured the O18 ratio in the cave formations. But in order for that number to make any sense they had to figure out two other things:

  1. Where did the rain come from?
  2. What was the land temperature when the rain was flowing down the cave formations?

The answer to the first question is not too difficult for this region. It turns out that all the rain carried to the caves originally evaporated from the Eastern Mediterranean Sea and turned into rain clouds. There aren’t any other sources of water! To determine what the ratio of O18 to O16 has been in the Eastern Mediterranean through time, the researchers looked at the fossilized bodies of tiny one-celled sea creatures — plankton — from the Eastern Mediterranean.

The second question involves some approximations. It turns out that the plankton records can also give an estimate of the surface temperature of the Eastern Mediterranean. Because the temperature of the sea is often related to the temperature on land, the researchers used the sea surface temperature to approximate the land temperature.

Now, they had all the pieces of information they needed. The researchers could use the temperature and plankton records to correct the cave formation O18 ratio and determine how much rain fell over the last 250,000 years.

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