Dating sediment layers
If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.
Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks.
By: Rob Brown There are many proxies paleoecologists use to determine past environments and communities (insects, pollen, diatoms, packrat middens, tree rings, etc.), many of which have been discussed on this blog previously.
These proxies can be used to answer questions ranging from seasonal to millennial time scales.
Varved sediments offer a unique situation where the temporal resolution necessary to determine annual to decadal changes relevant to a human lifetime can be achieved.
Simply put, a varve is an annual layer of sediment that forms in distinct layers (Figure 1).
With the exception of tree rings, which were previously discussed on this post by Erin our reconstructions are often limited by errors in dating methods.
However in some lakes, sediments are deposited in visible annual layers called varves.
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QUESTION: Can we date sedimentary rocks using radiometric dating techniques?
ANSWER: Sedimentary rocks cannot be dated directly using radiometric dating, which is based on the idea that when rocks are in liquid form, their radiometric clock resets.
A single year’s deposit includes a light (summer) layer and a dark (winter) layer.
Varves don’t form in all lakes, in fact they are found in very few.
Dating the particles which make up the rock wouldn’t give you the age of the rock itself.