What Fossils Can Tell Us About Climate Change

Do microscopic fossils hold the key to understanding climate change? Scientists studying tiny marine shellfish called ostracodes have found that they harbour in their shells is a...
14 April 2008


I'll be honest. There are two ways to do things: the easy way and the hard way. Why do I bother to start out an essay this way? Well, I am a student at East Carolina University in downtown Greenville, North Carolina, seeking my Master's Degree in Geology. Yep, that means I chose the (semi) hard way. Before I get into the meat of this article, as it were, I want to say that, like all starting researchers, I suspect the only thing I will have mastered by the time I leave is migraines and the art of brewing tea.

So what do I do that will, hopefully, pole-vault me into a PhD and many years of research beyond? I would like to think I am helping myself in my efforts to acquire land that will be good for future tea production, where upon I can make millions of dollars, rule the tea industry and enjoy the best teas this world has to offer.

However, that aside, I guess I am better referred to as a palaeo-climatologist (sometimes paleo-ecologist) by proxy. My true field is micro-palaeontology or the study of fossils too small to be seen (in detail) without the aid of some sort of ocular device (microscope). I was first introduced to microfossils in a palaeontology class at Mary Washington University.

The project was a detailed study of the climate patterns of the Holocene (the last 10,000 years) using ostracodes (a type of microfossil) in lacustrine (freshwater) sediments collected from two sites in a loch on the west coast of Ireland. Ostracodes are water-dwelling crustaceans, which in some ways resemble shrimps.

Each species of ostracode has a defined set of conditions in which it prefers to live, including temperature and salinity (salt content) and the amount of organic material suspended in the water. Therefore, by looking at the species present, you can determine what the water conditions were like at the time the animals lived.

To do this, cores are collected from the sediments that settle on a lake-bottom. The deepest sediments are the oldest, whilst those closer to the surface have been more recently laid down. By sifting through them at specified intervals, and charting which ostracode species are present, it's possible to work out how the water conditions have changed over time. For example, one species of ostracode, Cyclocypris, likes lots of organic material. Candona, another species, are omnipresent no matter what the conditions, whilst a third species, Limnocytherina, prefer clear water and will only tolerate low levels of organic matter.

Plotting the ratio of Limnocytherina to Cyclocypris in the cores from the Irish loch showed larger populations of Limnocytherina towards the bottoms of the cores, and higher populations of Cyclocypris at the tops. This tells us that the lake must have started off with very little organic material in the water, favouring Limnocytherina, but over time the organic content increased, making it more ideal for Cyclocypris. This process of filling a lake with sediments and organic material is called eutrophication and says something about the climate of the lake.

Ostracodes also build themselves a calcium carbonate-based shell which, on average, they moult eight or more times throughout their lives. Cyclocypris ovum, one of the ostracode species useful in deciphering previous climate conditions.After shedding each shell they grow a new one, and in the process take a geologic snapshot of the conditions of the surrounding water, which is what provides the minerals for the new shell to form. The chemical elements that make up these minerals, such as carbon or oxygen, are also found in more than one form in nature and in different relative abundances. These different forms are known as isotopes, and certain biological systems prefer to use one isotope over another.

Photosynthesis, for example, which is the process by which plants use the energy in sunlight to produce food, preferentially uses the lighter carbon-12 (C12) isotope over the slightly heavier carbon-13 (C13). This means that, in a lake, the plant material is enriched for (contains more of) the lighter C12, whilst the left-over C13 remains in the water. Now, when a moulting ostracode takes carbon from the water to form a new shell it will incorporate a higher amount of C13 because the water has more of it. So scientists can use this isotopic signature to tell what the climate was doing when the shell formed.

Oxygen can also be used in a similar way, although in this case physics and chemistry, rather than biology, are the dominant forces. When water evaporates from the ocean at the Earth's equator, oxygen-18 (O18), the heavier oxygen isotope, finds it harder to evaporate because it weighs more, so the water vapour formed contains relatively more of the lighter oxygen-16 (O16).

Consequently, the ocean becomes enriched in O18 and airborne water vapour enriched for O16. When the vapour condenses and falls as rain the first atoms to fall out of the sky, because they are heavier, are any O18s. Therefore, more of the O18 comes back to earth. Repeated cycles of evaporation, rain, evaporation and rain work like a whisky still, distilling the water vapour so that it becomes almost exclusively O16. The level of rainfall will therefore affect the ratio of 016 to O18; very dry conditions lead to an increase in O18 because excess evaporation takes away the O16, and an ice-age also depletes O16 because the ice holds onto the light isotope (which fell as snow), leaving behind more O18 to fall as rain elsewhere. When an ostracode creates a shell it records these differences between the levels of the oxygen isotopes, just like it does the carbon isotopes, creating an ancient rainfall record.

So, if I sift through my sediments at specified intervals, I can take the shells out from each interval and use a mass spectrometer to tell me the ratios of carbon and oxygen isotopes present in each shell. From these results it's then possible to reconstruct a picture of the climate as it must have been at that time. For instance, higher C13 isotope ratios indicate a warmer climate because for this to occur plant life must have scavenged the carbon-12. And if the lake is high in O18 it's likely that there were ice caps around at the time (to hold the O16) or that there are patterns of intense evaporation (arid climate) that is drawing off the lighter O16 in water.

Thankfully someone else has done the really hard work by coming up with an equation that converts oxygen isotope values into temperature in °C for oceanic environments. The equation doesn't work so well for lacustrine (freshwater) environments, but one out of two isn't that bad!

Overall, microfossils have an ever growing list of uses and possibilities to help determine climate and I, as a student, must iron out what is good, what is bad and what is just plain ugly...


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