Wednesday, February 07, 2007

JSW #4: Learning about sediment dispersal from the age of single grains

This is post 4 of 5 for Just Science week (Feb 5th-9th).

Earlier posts this week:

Post #1: Sediment thickness in world's oceans
Post #2: Submarine Geomorphology
Post #3: Experimental Sedimentary Systems


If I could talk to the sand grains in an ancient sedimentary deposit (I guess I already, more accurately, if they could hear me) I would ask: "Where did you come from? Where were you born and where did you grow up? Did you travel a lot before coming here?" If the sand grain could provide answers, we could then start to reconstruct the source areas of ancient sedimentary environments with great accuracy. Mountain belts get eroded...that material ends up as sediment...if you wanna learn something about ancient mountain belts, look at the sediments that they produced.

Zircons can, in some cases, answer some of these questions of provenance (albeit often with a lot of inference and interpretation...but, hey that's what we do).

For example, we can extract these little suckers from a sandstone (which is laborious and tedious process in and of itself....i'll go into that another time) and then determine their age using radiometric dating. I am not even close to an expert on geochronometry ... for this I refer you to another geo-blog, Apparent Dip, which has posts about this subject from time to time.

So, what do you do with the age of a zircon that is in a sedimentary deposit? Typically you date a bunch of grains (50-100) and end up with a distribution of ages. Some are old, some are young. Here's an analogy: let's say you have a few dollars of change in your pocket. Although they are all in your pocket now (the deposit) they probably have different dates on them and are of different morphologies (if they are all the same, that's a pretty special situation).

We end up plotting the ages in a histogram and analyze the different groupings of ages. If we are lucky, we already know something about the potential source area and see if we can make some comparisions (if it hasn't been completely eroded and/or consumed in some way).

Me and some colleagues presented some preliminary data last year at a geology meeting held in Mendoza, Argentina, called "Backbone of the Americas". The figure below is an example of one of these histograms. These plots are preliminary and do not include the requisite error bars, number of grains considered statistically significant, etc. but are posted here as an example of the application (stay tuned for actual interpretations and results in the future).

On the x-axis is age (in millions of years) and relative probability on the y-axis. These four different plots are from different stratigrpahic units within a preserved sedimentary basin (oldest deposits on bottom). A thorough comparison of significant peaks (i.e., lots of grains of similar age) from plot to plot can help us identify trends. One pattern in this particular example is the introduction of younger zircons (the peaks around 80 Ma) in the three upper plots as compared to the bottom one. In this case, the depositional age is very close to the youngest zircons here because the basin was fed by a volcanic arc. So, the younger depositional units were receiving new, baby (or toddler) zircons as sediment.

Anyway....this is just the tip of the iceberg. I'll post again about this and provide links from my fellow geo-bloggers out there who do more of this kind of work are can probably explain it better.

If you want to know the ghory details of this preliminary can read the abstract (PDF).



Lab Lemming said...

Chevron doesn't mind you posting this stuff? Your oil companies are way mellower than ours.

Brian said...

1) i'm not an employee of Chevron

2) there's no way to make $$$ from this they could care less