Coral and Sea Level

Emma-Kate Potter is a PhD candidate at the Australian National University. She tracks the history of sea level changes by studying growth cycles of ancient coral.

Emma-Kate Potter: So in the past, sea level has gone up and down, and up and down. And coral can grow at any of those times. They particularly like to grow when sea level is going up, they grow on top of each other and they compete to try and make it to the top of the water column so they can get more light.

In 2001, Potter traveled with a research team to the West Indies and New Guinea to measure enormous reef terraces made by the ancient cycles of coral growth. Samples of the coral were taken back to the lab and dated. Researchers used the age — and elevation — of the corals to determine sea level at various times in the past. Then the data were compared to the existing scientific record of climate change over the past 100 thousand years.

Emma-Kate Potter: What I’ve done is looked at what other people have written about the time period that I have been studying and found, oh well, look, this guy says that there are these oscillations in this time period and I’m seeing something very similar. And that’s really satisfying, when you feel like … there’s a coherent picture being built up that your work is contributing to.

Excerpts of Interview with Emma-Kate Potter

On periods of, say, a hundred thousand years, it melts and then ices up again. And so the sea levels go up and down as a function of that.

The Earth’s orbit changes on timescales of tens of thousand of years to hundreds of thousands of years, and that changes how much energy we get at particular latitudes. Now, they think what happens is that the amount of energy that’s received at these kind of northern latitudes — which is where the major ice sheets are located — so in North America, over Canada, in the continental North American region and over the Scandinavian region, that the amount of energy that is received from the sun at those latitudes controls how the ice sheets grow and decay over time.

In these glacial periods, the water that is evaporated from the oceans gets precipitated out as snow and forms these ice sheets and glaciers that basically is taking all this water out of the oceans, and it’s not letting it melt and go back into the oceans. So sea level goes down in those periods.

We think that over the last million years or so, it’s gone up and down by over 100 meters every hundred thousand years or so.

One of the things that I’ve been working on in particular is I go and collect coral samples which grew a long time ago. They grew at periods of sea level occillation that happened 89 to 100 thousand years ago. If we can go and collect these corals, and we can date them to find out how old they are, we can calculate what the sea level was at that time in the past.

In the past, sea level has gone up and done, and up and down. And coral can grow at any of those times. But they particusally like to grow when sea level is going up they grow on top of each other and the compete to try and make it to the top of the water column so they can get more light.

When sea level falls again, and there might be other coral growth, but what you’re left with is this big old coral terrace which is just left sitting there

And this could happen anywhere, but at these particular locations, such as Barbados and Huon Peninsula in Papua New Guinea these? features are uplifted out of the water, so now we have free access to them and we can sample them and then, of course, date them and find out when these periods of sea level rise occurred.

Other people take big cores right through some of the coral reef to get a continuous record of this reef growth during a period of sea level rise.

What basically led to this idea of oscillating sea level during the last glacial cycle was that fact that at these locations we could see these distinct periods of reef growth that occurred and we were not seeing a continuum, if you like, of coral growth.

Or sometimes it’s just up to a tape measure and then we take a piece of the coral and bring it back to our lab, and we can measure the date of it.

The field trips that I’ve been on have been pretty simple to organize in that we don’t have big drill rigs or anything complex like that, we just get out with a hammer and chisel and collect as many samples as we can find. But then the actual processing of the samples to find out the ages of them is quite a complex thing. It’s not that we can just put the old coral rock into a machine and it tells us the age. It has to be dissolved and prepared chemically, and we use a mass spectrometer to measure the amounts of isotopes in the coral to work out its age. And so, the work that I’ve been doing, I’ve analyzed over maybe 80 of these samples and it took me a good year to get everything done. And then to sit down and try to figure out, OK, what does this actually mean? There are a lot of uncertainties that come into come into calculating the sea level — is the uplift constant, how certain are we that the ages we are measuring are accurate

I’ve started off with a project in mind, or a question in mind that I wanted to answer. And then you go along, and you collect your samples, and you analyze them, and then you think, hummm, I wonder how this fits in with another record of climate at the same period. And you go and you do an awful lot of reading.

Obviously, we can’t go back to the ocean how it was 100 thousand years ago and know exactly how that ocean was circulating and what was going on because we don’t have that ocean any more. All we have left is a record, of perhaps of the conditions in that ocean or some hints about what was going on then that’s recorded in the sediment that’s in the ocean floor

Q: Can you tell me about something that’s been particularly satisfying for you?

I think it’s coming to the end of analyzing samples and sitting down with the data and just personally for me, trying to figure out how this data fits in with other records. What I’ve done is looked at what other people have written about the time period that I have been studying and found , oh well, look, this guy says that there are these oscillations in this time period and I’m seeing something very similar. And that’s really satisfying, when you feel like there’s a coherent picture being built up that your work is contributing to.

What you have in your mind?