The “Amount Effect” is a phenomenon observed in the stable isotope composition of rain water related to how much rain has fallen.
Water is composed of the elements of hydrogen and oxygen. Hydrogen has two stable isotopes, 1H (hydrogen, most abundant) and 2H (deuterium). Oxygen has several stable isotopes, the most abundant being 16O (common) and 18O (rarer). As a body of water (like an ocean or a lake) evaporates, water composed of the lighter isotopes of hydrogen and/or oxygen evaporates first. The result is that clouds (water vapor in the air) is isotopically ‘lighter’ than the original body of water, and the lake or ocean becomes isotopically ‘heavy’. How much heavier the lake becomes depends upon the air temperature. And it’s the lake water that is consumed by animals and goes into forming minerals that we can analyze.
This is the basic premise for much of what we do using isotopes of oxygen from rocks, bones, or teeth to understand what past weather patterns were like. We can make estimates of temperature based upon the relative amounts of heavy and light isotopes of oxygen and hydrogen.
But it gets more complex.
You see, what I describe above works perfectly well in areas of cooler temperatures, relatively low humidity and not a lot of rain, but in places with high humidity, lots of precipitation, and warm, warm temperatures (like the tropics), the evaporation cycle which makes the clouds lighter than the lakes is disrupted. In those places, when it rains, the rainwater is isotopically ‘lighter’ than might have been predicted otherwise, making lakes also lighter than we would expect.
This is the Amount Effect.
It is then up to us, the scientists, to distinguish between isotopic variations due to temperature changes and those due to changes in the amount of precipitation.