Corals are nature's own archives

Jani Tanzil & Intan Nurhati

Ms Jani Tanzil and Dr Intan Nurhati, who are both in their early 30s, have been researching corals around Singapore as part of their work at the Singapore-MIT Alliance for Research and Technology (Smart).

By examining the corals, which can grow for hundreds of years, they can detect how the climate here has changed over time.

A marine ecologist by training, Ms Tanzil is currently completing her University of Amsterdam doctorate, which is focused on corals around the Thai-Malay peninsula.

Like trees, corals add layers or bands over time.

Ms Tanzil uses luminescent and density banding patterns in corals to date and reconstruct the corals' past growth rates, in a method known as sclerochronology.

Dr Nurhati is an Indonesian climate scientist with a doctorate from the Georgia Institute of Technology in the United States. A postdoctoral associate at Smart, she studies corals around Singapore to answer questions such as whether the monsoon seasons are changing, and how industrial activities have altered marine chemistry.

Coral reefs occupy less than 0.1 per cent of the world's oceans, but scientists believe that the reefs account for and support more than a quarter of the oceans' biodiversity.

Singapore's reef area covers less than 10 sq km, but it is home to over 250 hard coral species, almost one-third of the global total.

Q: Most scientists spend much of their time in the laboratory. Do you?

Dr Intan Nurhati: We actually spend about 10 per cent of our time in the water, and our work calls for scuba diving qualifications. We dive to coral reefs and take coral samples to study past environmental changes, like how the climate and ocean chemistry have changed.

Ms Jani Tanzil: We also look into how these changes have affected the corals and coral reefs.

Q: Ever had any close shaves?

Ms Tanzil: We haven't had any close shaves, but there have been a few times when we reached the shore after a diving trip just before a huge storm hit.

Q: Why study corals?

Dr Nurhati: Most climate research requires data from a long period of time, say hundreds of years, if you want to understand what nature is doing versus what people did.

Current measurement instruments may not have existed back then. Some sites may also be too remote for sensors. To study (the climate phenomenon) El Nino, for instance, we need long temperature records from the Central Pacific ocean, but who on earth would have put sensors there 100 years ago?

So we need to be creative and look at nature's own archives. Just like trees have rings that are annual markings, some corals also have growth rings, which can be seen with X-rays and under ultraviolet light.

This means we can study corals to learn more about past events.

Corals can grow for 10, 20, even hundreds of years, and some can grow relatively fast, so you can even get monthly data, like what happened in September 1984.

Q: What are some of the things that you can learn from corals?

Dr Nurhati: The coral bands vary in their thickness and composition due to changes in factors such as water temperature, clarity and nutrient availability.

You can study many things from the coral skeleton's chemistry. It can tell us about changes in temperature, which are important if you want to know the rate of ocean warming and global warming.

We also study how ocean chemistry has changed by measuring heavy metals such as lead in our corals, and rainfall changes by measuring the chemistry proxy for salinity in corals.

If you have more rain, the ocean becomes more diluted, so the salinity falls.

Q: How are the rings or bands formed?

Ms Tanzil: For corals around Singapore, the luminescent bands are related to salinity changes.

Trees shed leaves, which decompose and create humic acids. When you have many trees near rivers, the higher river flow during the wet season washes the humic acids into the reefs and corals absorb them. In dry seasons, there is less river flow and therefore less humic acids flowing into the reefs.

The exact cause of the bandings and rings in corals is still being investigated, but my research has found that bright bands usually correspond to the south-west monsoon and the dull bands correspond to the north-east monsoon.

We also verify this by staining the coral in the reef with a harmless calcium dye. You stain it multiple times, and then collect a specimen later. The stain bands, which show up as pink lines in the coral skeleton, will allow you to infer the growth rate and pattern. I have stained corals over a two-year period just to make sure the natural bands are somewhat annual.

Q: How do you get the coral specimens?

Dr Nurhati: We core the corals using a long barrel with diamond-dusted teeth at one end, and we use pressurised air to power the drill, which rotates the coring barrel into the coral. This gives us long columns of coral.

Q: Doesn't this harm the corals?

Dr Nurhati: We don't kill or collect the whole coral, but sample only a small portion. If you look at a coral, the living part is only the outer layer, which is about 0.5cm thick, covering the coral like a blanket. Beneath this is dead skeleton.

Our samples are just 5cm in diameter, so we take only a small, thin slice of living material. Everything else is skeleton.

Ms Tanzil: To the living coral, it's like a fish taking a bite out of it. We fill the hole with epoxy, which is like plasticine but hardens like cement. With time, the living tissue will grow over the epoxy and cover it up. We also monitor the holes we patch up, and we do need a permit from the National Parks Board to core corals in Singapore waters.

Q: What do you do with the coral specimens?

Dr Nurhati: We "slab" it into two to three slices. Most of the time, we use the middle slice for analysis, and the rest we keep for our records and in case someone wants to collaborate and study other things with it.

Ms Tanzil: Then I take the slices and date the various bands. Different corals grow differently, so it's important to understand what causes the banding.

Dr Nurhati: Once the dates are assigned, I put a tracking mark in each section and drill a small portion into powder. We collect this very fine powder, weigh it, and then dilute it in acid so we have a solution, which we put into machines for different analyses.

Q: What's the experience of studying corals like to you?

Dr Nurhati: It's like having a history book of the environment, and you even get chapters.

- Feng Zengkun

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