Saturday, April 6, 2013

Quick and Dirty Dating

So many untouched cores of coral!
by Pamela Grothe

Week 1: Scripps

I started my two-week jaunt to California last week at Chris Charles’ lab at Scripps Institution of Oceanography, Kim’s alma mater.  After several expeditions in the field, Kim has collected quite a collection of fossil coral from the Line Islands (and in particular from Palmyra), all of which have not been dated or geochemically analyzed. Since Scripps is just down the road from UC Irvine, we figured it would be a great opportunity to inventory the collection that exists there and take samples from them to date in addition to my samples from Christmas Island. Unfortunately, half the collection was off-site in storage bays located in rattlesnake habitat, so I was only able to inventory half the existing collection. That just means I will have to return another day to inventory the rest! In all, I found 48 cores that have not been analyzed yet that I measured, photographed, categorized, and sampled.

Week 2: UC Irvine

Doing what I do best, exploring the area on a 10 mile trail
run in Crystal Cove State Park on Easter Sunday.
I haven't seen hills like this since I left Boulder!
After enjoying my time in beautiful La Jolla, I bummed a ride up to Irvine on Friday afternoon from the guy who works at the Cal Tech Marine Lab, where I’m staying. It just so happened that he was in the area picking up sea urchins and saved me the headache of public transportation to head up the coast! With Kim’s connections at Cal Tech, I was able to stay at their Marine Lab right on the water in Corona Del Mar! The accommodations are far from luxurious but you can’t beat cheap oceanfront lodging.

I spent Saturday in the lab washing and drying the samples I grabbed at Scripps, and then took Easter Sunday to myself to explore the area and get caught up on some homework since I’m missing two weeks of classes.

And so the dating process begins! Carbon-14 is a radioactive element that decays with a known half-life of 5,730 years. Corals take up carbon from the water while it’s living, including stable 12C and radioactive 14C. Once it dies, 14C begins to decay while the stable 12C remains constant, thus changing the 14C to 12C ratio. By comparing the measured 14C/12C ratio with the 14C/12C ratio of the atmosphere an age can be calculated.  This is the radiocarbon age, which is not the true age because the 14C/12C ratio of the atmosphere has not been constant through the past. Luckily the experts have worked this out and we can just apply a calibration curve to our data (in my case a curve that also takes into account the reservoir age of the water in which the corals grew) and we get a true calendar age!

The AMS. Much bigger than our mas spec!
Carbon-14 atoms are rare and so in order to measure them you need an accelerator mass spectrometer, such as the one here at UC Irvine (pictured on right). These instruments are equipped with an ion source that bombards the sample with cesium ions, which are then focused into the accelerator by magnets. In the accelerator, they collide with Argon gas that is circulating throughout in order to break up molecular ions. The filtered ions then enter the detector and are measured. Only a special few get to operate this, so for this part, my only participation was watching the experts load my sample wheel from outside the high voltage cage.

Cartoon schematic illustrating the different
components of the AMS.
But there’s a lot of prep work to get the samples ready for the AMS. Luckily for me, I’m performing the “quick and dirty” method, also known as the fast screening method, so my prep time is reduced dramatically. Normally, the samples that go into the wheel are graphitized, a process that takes the original sample and converts it into graphite (leaching with HCl to clean the sample, dissolving in phosphoric acid to convert sample to a CO2 gas, and then graphitizing the CO2 gas to graphite). However, the quick and dirty removes all those steps and just uses the powdered sample mixed with pure iron!

Of course, that being said, the process of physically handling 85 samples still took three days. The samples I collected at Scripps needed to be crushed. Iron has to be weighed out to between 5.0 and 6.5 milligrams (about half-way through I became good enough to just eye ball this saving more time). Then 0.3 milligrams of the powdered sample needs to be weighed out and mixed with iron (this you can’t eye ball because the beam current works best with 0.3 milligrams of sample so there’s not much wiggle room here). The last step is pouring this tiny amount of sample into a pin-sized hole without spilling and then pressing it to 400 psi so that it is no longer loose powder. It took a bit of muscle memory before getting the tilt and pour method down without spilling my sample everywhere! But the biggest time consuming part of all of this is cleaning every tool in-between samples, as you don’t want to contaminate your next sample. It’s crazy how mush waste I generated in just prepping my two wheels as nothing is reused in this lab because of possible contamination.

Now that the samples are prepped and ready, it’s time to let the AMS do its job and wait for the final results.

Straight forward prep work for rapid screening dating method - no wet chemistry involved here!