Wednesday, October 10, 2012

Every coral has a story

by Hussein Sayani (@hsayani)

I like to think of paleoclimatologists as detectives trying to solve a mystery, which in our case is how the tropical Pacific climate has changed over the last thousand years.

Our research sites - the Line Islands in the central tropical Pacific

An example of a core taken from a Porites coral growing near 
Palmyra Island. X-ray images (right) are usually used to help 
us figure out where to make our measurements.
Satellites have only been measuring climate for the last couple decades.  So we need to somehow figure out what happened hundreds of years ago. That’s where corals come in. Reef-building corals (these are the large stony types) continuously form a hard calcium-carbonate skeleton. The chemical composition of each “layer” of skeleton the coral makes will depend on things such as temperature, how much it rained, ocean circulation, etc.  By measuring how the chemistry of these layers is different in the past, we can figure out how climate was different in the past.  And so, ladies and gentlemen, we have our “eyewitnesses”.

Paleoclimatologists typically extract climate information from corals by measuring the changes in elemental composition (usually calcium, strontium, and magnesium) and/or changes in the type of oxygen atoms present.

As climate detectives, we always have to question if the coral is telling us the truth.  In most cases we can trust our coral’s testimony, but occasionally our witnesses’ memory can become a little fuzzy.  This can especially be a problem when our witnesses are very old (i.e fossil corals). We of course thoroughly screen all our fossil corals and remove the bad ones, but we have to face that fact that there is no such thing as a perfectly preserved fossil coral.

The surface of pristine coral's skeleton is usually very smooth (left panel). Sometimes additional 
crystals will form on the skeleton's surface as the coral sits in seawater (right panel) or when a
fossil coral sitting on a beach gets rained on.  These features are invisible to the naked eye, but 
can be seen using a scanning electron microscope. 

The sharp needles covering the coral in the image above are one flavor of alteration (or diagenesis) that a coral skeleton can undergo. This diagenetic stuff, has a very different chemical composition, and can easily contaminate our samples making our climate reconstructions very wrong. The upside here is that in many cases, the diagenetic stuff is just lining the outside of the coral skeleton and the inside is usually not altered. So what we need is a way to pick what we’re measuring - something we can’t do with our usual analytical techniques.

Enter the SIMS! 

The ion microprobe lets us make very, very small measurements on coral.  Using this technique, we can measure spots on the coral approximately 10-20 microns wide.  This is less than half the thickness of human hair, and over 50 times smaller than the spots we usually measure on coral. Are you excited?  You should be!  This means we finally have a way to measure only the good parts of bad coral and still get reliable climate information. 

SIMS hasn’t been widely used in coral work so far, thus we’re in slightly unchartered territory here.  So the first order of business is to convince ourselves that SIMS coral measurements are reliable and reproducible (i.e. that we repeatedly get the truth from a coral that isn’t lying).  This is what I'm currently investigating at WHOI.

Up Next:  The 411 on SIMS.