by Hussein Sayani (@hsayani)
Scientists will often name instruments and equipment in the
lab. I like to think it’s out of pride and love, kind of like how people name
their cars, and not because we talk about them all the time and just want to
blend in with normal people waiting in line for coffee. For example, our Delta V plus mass spectrometer and Kiel carbonate device are called Matilda and Damien
(named after certain movie characters). My friends know this, so I’ll
occasionally get calls that essentially go: “I haven’t heard from you in a
while. How’s Damien doing?” See, it makes us sound normal! Anyway …
The NENIMF houses two ion microprobes, the Cameca IMS 3F and
the Cameca IMS 1280. For this project, we’ve been using the IMS 1280, a very
large beastie affectionately known as The Princess.
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| The obligatory glamour shots |
A quick rundown of how this ion microprobe works:
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| Schematic of the IMS 1280, borrowed from the NENIMF site. |
A duoplasmatron (a type of ion source) is used to generate a
primary beam of oxygen ions. This primary
beam then digs into the sample, ejecting ions from its surface (this is called
sputtering). Various lenses and
deflectors are used to channel the sputtered ions towards a large magnet which
separates the ions by tiny differences in mass (every element has a different
mass). After being separated, our
charged particles are sent to the ion counters which tell us how much of each
ion is present in the sample. Pretty
nifty, right? For my samples, the measurement
process takes about 15 minutes per spot. We measure about 60-65 spots a day and the sampling is only partially automated. Hooray for 17 hour work da … wait, what?
The Princess and her bling:
Preparing samples for The Princess is a little different. With the other analytic techniques we
employ, we usually mill some coral powder and then dissolve it in acid. The
Princess on the other hand, prefers smooth, flat, and shiny gold discs. SIMS
samples are prepared by cutting of a small section from the slab. The coral skeleton
is very porous, so the side of the coral section that we want to measure is
embedded in epoxy to create an even surface. After some polishing to completely
flatten and smooth this side, a small slice is taken and attached to a glass slide
with more epoxy. Then come the hours and hours of polishing the slide to get it
perfectly smooth and flat. Finally, the slide is coated in gold (ooh shiny!).
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| The thin sections (slightly larger than a quarter) are locked into a steel mount and loaded into the SIMS (red arrow). |
Since the slice of coral on the slide is very thin (hence the name thin section), we can use a microscope shine a light through it and see the various different crystals that make up the skeleton. Besides being very cool, this lets us know what features within the skeleton to avoid (as different features have different chemical compositions).
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| The left side is what we see on the SIMS. The right side is what the slide actually looks like. You need a map of both to know what to avoid (e.g the black line running down our column - see the white arrow). |
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| I'm sorry, Earth! |
Loading the sample into the SIMS and waiting for the
instrument to reach high vacuum can take a couple hours, so it’s best to know beforehand
where you want to make your measurements. After spending some time familiarizing
myself with each slide (and series of slides as we have 2-3 per coral), I use the
microscope to build a map to help me navigate each slide. Back in January I did
this using paper, scissors, and tape (and you thought science was complicated).
Out of kindness to trees and the planet, this time I made the maps on my laptop
instead.
Now that we’re all familiar with the SIMS technique, let’s
take a look at what this beastie can do. We’re interested in figuring out past
ocean temperature, and so we’ll be measuring the ratio of strontium to calcium
(Sr/Ca) at each spot on the coral as this is a commonly used proxy for such reconstructions. Remember, our
goal is to determine how useful, reliable, and reproducible SIMS measurements
are. So the first thing we want to check
is whether or not our SIMS measurements look like temperature. We measured
Sr/Ca at almost weekly resolution (one measurements every 6-8 days). As you can see on the graph (light red line), the
data looks very noisy and it’s hard to see any long-term trends. What we want to see is an annual cycle (a warm
summer period and a cool winter period), and to get that we’ll have to do some
averaging. Since we’ll be comparing
these Sr/Ca measurements to monthly ocean temperatures, a monthly running
average is appropriate. Applying this to
the data set, we get a Sr/Ca record that happens to look a lot like temperature
(thick red and black lines)! This is great because it tells us that we’re doing
something right.
We still need to test how reliable this technique is – i.e.
can we measure similar samples and get the same numbers? To test this, we
measured samples from different corals that grew at the same time. The idea is to get the same Sr/Ca record for
each of them. Surprisingly, the records
from two of these overlapping corals were an almost 75% match. That is much
better than what we were expecting.
There is much more data left to analyze, but it looks very
promising. We’re hoping that the 800 measurements we’ve made will provide us
with a roadmap to the most efficient and effective way to use this technique to
test the integrity of our fossil corals.
