Thursday, April 21, 2011

One upon another

kw: science, geology, interpretation

I tend to have a messy desk. Things I don't deal with immediately get other stuff piled on top, until there are several layers. Then when I remember something I have to do something about, I need to think, "When was that?" Once I know that, I know how deep I have to dig to find it. I usually know approximately where to look. I also know that the older stuff is on the bottom.

Geology, the profession I was educated for, works in a reverse fashion. Most places, things are also in order, with the younger stuff on top or shallower than older stuff. Because most dirt, soil, and rock materials are laid down in order, and in layers, one of the first pieces of geological jargon a new geology student has to learn is "stratigraphy." The prefix "strat-" means "layer".

A short definition of stratigraphy is "The study and interpretation of layering in geological and archaeological deposits." Archaeology is just geology applied to layers that may contain fossils or artifacts or other human or hominid remains. More recent stuff, in other words. When archaeologists dig into cave deposits to recover bones and artifacts, they are very careful to record which layer in which each item was found; they keep good stratigraphic records. Similarly, when palaeontologists retrieve fossils or other traces of ancient living things, or other materials of interest such as mineral deposits, they must keep good stratigraphic records.

What is of interest to me today is the variety of sub-disciplines that make up the large field of stratigraphic interpretation. Four words are used to describe four different means of interpreting the layering found in Earth's crust, to derive the relative older-younger relationships:
  1. Lithostratigraphy is the study of the sequence of rock types. It is well known that many geological processes take place over large areas. The filling in of an ocean basin may be a generally uniform process that covers hundreds of miles. Sand dune fields that accompany desert formation may also cover many miles. So if in one large outcrop you find the sequence (from bottom to top) rocky sand, shaly mud, fine sand, dune sand; then several miles away there is an outcrop with the same sequence and similar thicknesses of each layer, it is likely that the two outcrops are related. Both cover a similar time span, and a few events within that time span can be correlated between the two outcrops.
  2. Biostratigraphy is the study of sequences of fossils, often within layers that are lithologically uniform, and of course from layer to layer. Many kinds of animals and some plants were widespread, even having worldwide distribution during the period in which they flourished. Whenever you find one of these "index fossils" as they are called, you can state that the layer in which it was found has a certain geological relationship, wherever on Earth it may be.
  3. Stable Isotope Stratigraphy is not usually worldwide in scope, but serves to correlate things like a certain concentration of an isotope such as O18 throughout a region that experienced a similar climatic history, such as a large lake or a semi-enclosed ocean basin. Most frequently, it is the pattern in the rise and fall of the isotope's concentration that is mapped and used to correlate from place to place.
  4. Magnetostratigraphy is the most recent stratigraphic tool, being only about fifty years old. It is based on the learning in the early 1960s that Earth's magnetic field has reversed itself periodically. By taking a series of magnetic measurements across a layered outcrop, the positions of reversals can be mapped and correlated from place to place. This was a worldwide phenomenon.
These four provide relative timing but not absolute timing. A rough measure of absolute timing can be determined by observing the rate at which certain kinds of sediments are laid down, particularly those that form yearly layers that can be distinguished in the rock. Varved shales are an example, and have been used to determine absolute times, in certain specific locations, for deposits as old as 400,000 years (but imagine counting all those thin layers!).

The absolute time scale needed to tie all of the stratigraphic data together was provided during the Twentieth Century by radiostratigraphy, which uses long-lived radioactive isotopes to age-date a number of common rock types. There are a number of different time ranges that are related to various radioisotopes. For example, the most common isotope of Uranium, U-238, is one of the longest, with a half life just over 4.5 billion (4500 million) years. Because of its feeble activity, it is not useful for "short" spans of time of less than a few million years. At the other end of the scale, the isotope C-14 has a half life of 5,730 years. While it is hard to determine very short time spans of a decade or less, it is well suited to time ranges between 100 and 75,000 years. Other isotopes can be used to cover intermediate spans of time.

One very useful synergy between methods is to gather well-dated specimens of index fossils. That way, for most uses, if you know that a certain fossil animal only lived between 105 and 108 million years ago, finding this fossil immediately places the rock you found it in within this time range. Finding multiple index fossils and gathering a suite of them across an outcrop gives you a series of good measures of the chronostratigraphy of the deposit, from which you can interpolate the points in between as needed.

The aim of stratigraphic study is to pin down the chronostratigraphy, by whatever method(s). Once a geologist knows that, a lot of other pieces of information can be correlated to develop a proper interpretation of the events that occurred to put those rocks in that place. The events are the goal; the rocks are the evidence.

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