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!

Hello from Southern California!

by Pamela Grothe

Not all trips in the Cobb lab are to exotic places, but we still travel to cool places nationally too, visiting other labs and collaborating with others (as Stacy and Hussein have already blogged about). I should probably introduce myself first. Hi, I’m Pamela Grothe, Kim’s newest PhD student, blogging from Southern California. I started at Georgia Tech last August and am just beginning to wrap up my second semester. A bit about myself, I’m a native Virginian, however, I’m a Coloradan at heart. I moved to Atlanta from Boulder after spending the last four years there, working as a research assistant in a NOAA lab and completing my master’s degree at CU. Everyone tells me I’m crazy for trading Boulder for Atlanta, but hey, it’s ONLY five years, right! Other than science, I love to run and race. I also love to bike and swim, hike and camp, scuba dive and kayak, and whatever else that involves the outdoors – I think that is par for course for anybody working in the earth sciences.

My Research

A ridge of fossil coral rubble  on Christmas Island

I fall on the coral side of the Cobb Lab (despite the photo of me in a caving helmet). For my research, I will be working on a new method of using hundreds of short (5-20yr-long) fossil coral segments (a.k.a. rubble) to reconstruct the El Nino-Southern Oscillation (ENSO) and tropical Pacific climate variability over the last 7,000 years in the central tropical Pacific. To date, paleo-ENSO variability has relied on exceedingly rare, decades-long fossil coral sequences, 16 of which are presented in Kim’s new Science paper spanning the last 7,000 years. However, smaller fossil coral rubble pieces are plentiful on Christmas Island (see photo at right, and which the gang graciously collected samples of last May). The plan is to use these numerous but shorter rubble pieces to produce a more statistically robust paleoclimate reconstruction – it’s all about the numbers! That’s the gist of the project, which I am only just beginning and super excited about!

Southern California

With any type of paleo-reconstruction, in order to put your record in context, you have to know how old your sample is! And that’s why I’m in California. I’m here to date the fossil coral that the group collected all across Christmas Island last May. My new approach of using hundreds of fossil coral rubble presents a unique challenge to me though. As you recall from Stacy’s dating lesson on U/Th dating, it is a quite intensive and time-consuming technique and thus very inefficient for quickly dating hundreds of sample at a time. So I’ve been in touch with collaborators Dr. John Southon and Dr. Guaciara do Santos at University of California Irvine’s Keck Accelerator Mass Spectrometry Lab, working with them on a method to quickly date lots of sample using high throughput radiocarbon dating (up to 60 per day)!

So in a nutshell, that’s who I am, what I’m working on and why I’m in California. Next up will be more details on my actual trip, which also includes a side trip to Chris Charles' lab Scripps Institution of Oceanography in La Jolla to sample fossil corals that Kim collected back in 1998!