In its simplest form, an isotope is a chemical element, such as hydrogen, or iron or aluminium. Many (but not all) elements have several isotopes, that is forms of an element with different masses.
Lithium is what is known as an alkali metal. This means it inhabits the far left of the periodic table, with chemical properties similar to sodium and potassium. It is the lightest metal in existence. In its pure metal form, it reacts explosively with water (see video in second column).
It is currently used extensively in lithium ion batteries that power most electronic devices, and in treating bipolar disorder in humans. We use it to tell us about climate cycles and processes in the present and the past.
Lithium Isotopes and our Project
Tracing weathering in the past is tricky, because you have to determine the absence of rock, i.e. that which has been dissolved. We mainly use the isotope ratio of lithium (7Li/6Li), which starts out in silicate rocks on the continents. As they are dissolved, Li is washed into rivers. At the same time, the isotope ratio is subtly changed by the weathering process. The lithium is then washed into the oceans, where the Li isotope ratio of seawater is changed by the Li isotope ratio of the rivers. Ultimately, carbonates such as limestone form from the ocean water, and provide an archive for changing ocean chemistry. Because the ocean chemistry is changed by continental weathering, ocean carbonates provide an archive for changing weathering.
We use lithium because it is biologically neutral (so isn’t affected by plants or microbes), and because it is only present in silicate rocks on the continents. This means that weathering of carbonates (which does not affect long term CO2) does not affect Li, and we therefore have a tracer that traces exactly what we want to know about.
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Quantifying the link between weathering and past CO2 levels
Testing the link between weathering and CO2 – evidence from extreme climate events
North China craton: A unique window into Earth’s middle age