Historical Geology/Deposition rates
In previous articles we have mentioned the sedimentation rates for various sediments. Perhaps it is time to ask ...
How do we know?
[edit | edit source]There are a number of ways we can find out the present, or at least recent, rate of sedimentary deposition.
One of the simplest involves what is known as a sediment trap, which is used to measure deposition of marine sediment. One can be seen being recovered from the ocean in the photograph to the right.
Fundamentally, it is a technologically sophisticated bucket. One aspect of its sophistication is that it has a whole set of collection bottles at the base which successively rotate into the collection position at fixed intervals. This allows geologists to measure seasonal variations in the quantity and composition of sediments.
Sediment traps are particularly useful when dealing with very fine sediments with a very low rate of deposition such as siliceous ooze, calcareous ooze, and pelagic clay.
Another method, suitable for when sedimentation rates is higher, is to take a drilling sample in which some layer corresponds to a recent event. For example, the first tests of hydrogen bombs in the 1950s are marked in the sedimentary record by the sudden appearance of cesium-137, an isotope of cesium not found in nature.
Another such time marker consists of the peak in environmental lead that occurs in 1970. Before that point, lead in sediments rose with the use of petroleum; in 1970, the U.S. Congress passed the Clean Air Act, and the lead found in sediments begins to decline.
Other methods involve a rather ingenious use of naturally occurring radioactive isotopes which are constantly being deposited on the sea floor; we shall describe these methods in detail in the article on the U-Th, U-Pa, and Ra-Pb methods of dating.
When we look at sedimentary rocks, and try to figure out their rates of deposition, we may also appeal to more conventional methods of radiometric dating. If we have (for example) a layer of pelagic claystone, itself undatable directly, sandwiched between two layers of igneous rock, which is datable by radiometric methods, then by subtracting the date of the lower layer from the date of the upper layer we get a period for the deposition of the claystone in between, and so, given the thickness of the claystone, we can figure out its average rate of deposition over that period. If we wish, we can then use our knowledge of how much more compact pelagic claystone is than the parent sediment to produce a sedimentation rate expressed in millimeters of the original sedimentary material per thousand years.
There is quantitative agreement between the rates of deposition measured in sediments today and the rate of deposition inferred for the corresponding sedimentary rocks; for example the deposition rates calculated for the calcareous material in marine limestone are the same as the deposition rates measured for calcareous ooze; and similar remarks apply to other rocks and their corresponding sediments.
This agreement is confirmation, if any is still needed, that geologists are correct in their diagnosis of the mode of deposition of the parent sediments of these rocks.