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Ancient Fossils and Modern Climate Change: The Work of Jennifer McElwain

Science & Tech  (tags: scientist, climate change, fossils, paleontology )

- 3960 days ago -
Dr. McElwain is a paleontologist who specializes in plant fossils. Her work relates to both mass extinctions and modern climate change issues.


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Monday May 12, 2008, 10:17 pm
Dr. Jennifer McElwain's laboratories at University College Dublin in Ireland are stocked with powerful microscopes for studying the delicate structure of leaves, acids for eating away the rock around fossils, and growth chambers for running experiments on modern plants. Over the past five years, she and her team have made two expeditions to Greenland where they collected over two and half tons of fossils all the while roughing it in tents, cooking on a gas stove, and washing in a stream. The point of all this work? Counting microscopic pores on the surface of tiny fossilized leaves many no bigger than a pencil eraser.

That might seem like a trivial task but in fact, such details are important clues about the major climate changes that have shaped life over the course of Earth's 4.6 billion year history. Jennifer's research helps unravel global environmental cycles that allow us to understand how human activities affect the planet's climate today and in turn, how those activities will impact the diversity of life on Earth tomorrow.

The keys to Jennifer's research are microscopic pores on the surfaces of leaves called stomata (singular: stoma) which plants use to "breathe." Plants need carbon dioxide, just as we need oxygen, and stomata allow the plant to take in carbon dioxide to perform photosynthesis. In the process of photosynthesis, the plant will chemically convert that gas into sugar, which the plant can use to fuel cellular processes, grow, and reproduce.

Stomata, which means "mouths" in Greek, do indeed resemble tiny mouths surrounded by swollen lips. The "lips" are actually individual cells (called guard cells) that can swell up even further to close off the stomata. But why would a plant want to close off its stomata, effectively cutting it off from essential carbon dioxide? Well, plants also need water, and any time that a stoma is open, the plant loses water (along with oxygen, one of the waste products of photosynthesis). By closing the stoma when the plant has enough carbon dioxide, the plant can preserve its water and prevent itself from drying out.

Jennifer studies stomata that are preserved on the surfaces of fossil leaves. But what do stomata have to do with climate change? As an undergraduate in Ireland, Jennifer discovered that the number of stomata per square inch of leaf surface can reveal different aspects of the atmosphere in which that plant lived. Since then, she has continued in this vein of research. As Jennifer puts it, "Plants are wonderfully in tune with their environments, so there are many proxies or signals that we can obtain from fossil plants. We can work out the temperature they lived in, the atmospheric environment, and the carbon dioxide concentration."

It works like this. Stomata control a tradeoff for the plant: they allow carbon dioxide in, but they also let precious water escape. A plant that could get enough carbon dioxide with fewer stomata would have an advantage since it would be better able to conserve its water. Levels of carbon dioxide in Earth's atmosphere change over time so at times when the atmosphere is carbon-dioxide-rich, plants can get away with having fewer stomata since each individual stoma will be able to bring in more carbon dioxide. During those high-carbon-dioxide times, plants with fewer stomata will have an advantage and will be common. On the other hand, when carbon dioxide levels are low, plants need many stomata in order to scrape together enough carbon dioxide to survive. During low-carbon-dioxide times, plants with more stomata will have an advantage and will be common.

Jennifer uses stomata as indicators of carbon dioxide levels at different points in Earth's history. Experiments help her figure out the exact relationship between stomata and carbon dioxide. Using growth chambers, she can simulate the temperature, light level, and atmospheric conditions common at different times in the deep past and at different places on Earth. So even when it's subzero in Chicago, her seedlings might feel as though they are growing in sunny California or in the humid swamps of the Jurassic. Studying how modern plants respond to these environments helps Jennifer understand how the characteristics of long extinct plants were affected by their environments.

Since carbon dioxide levels directly affect global temperatures, Jennifer's work also provides an important picture of how Earth's climate has changed over time. Carbon dioxide is a well known greenhouse gas meaning that it acts like the glass in a greenhouse, trapping radiation from the sun and heating things up: the higher the level of carbon dioxide in Earth's atmosphere, the higher the global temperature. The greenhouse effect is powerful: without any greenhouse gasses in the atmosphere, global temperatures would be about 20 C lower than today! And because we continue to release greenhouse gasses into the atmosphere as we drive our cars, manufacture products, and produce electricity, we can expect global temperatures to rise in the future.

Because of the powerful impact that carbon dioxide levels have on the greenhouse effect and global climate, Jennifer's fossil stomata provide a broad picture of past global temperatures. Stomata can be used to directly estimate past carbon dioxide levels, and those carbon dioxide levels can then be used to make an indirect estimate of temperature. Typically (although there are exceptions to the rule), fossils with many stomata (low carbon dioxide) came from times of low global temperature, and fossils with few stomata (high carbon dioxide) came from times of high global temperatures.

Jennifer has been using fossilized leaf stomata to investigate some of the mysteries of the past including what caused different extinction events. Though the word sounds disastrous, extinction is a normal process in the history of life. Lineages go extinct for different reasons perhaps a random event (like a hurricane) hit them particularly hard or perhaps they were out-competed by another species. Regardless, extinction is an important aspect of evolution. Extinctions can free resources and niche-space of which another lineage might be able to take advantage, leading to a new evolutionary trajectory. However, at particular points during the history of life, extinctions of massive proportions have occurred, killing off a large percentage of species alive at the time and altering the course of evolution. These events are called mass extinctions. Mass extinctions are easy to identify in the fossil record but figuring out what caused them is much more difficult.

Jennifer is particularly interested in the cause of the end-Triassic mass extinction some 200 million years ago. This extinction decimated species in marine reefs and killed around 50% of North American vertebrates, including the phytosaur. But what caused this extinction? Many hypotheses had been proposed - meteorites, global cooling, global warming, sea level rise, and sea level fall - but no conclusive evidence had yet supported one hypothesis in particular. Jennifer decided to study the climate at the time of this event to see what clues it might offer.

Jennifer collected fossil leaves deposited before, during, and after the mass extinction event. She discovered a major drop in the number of stomata on leaves coinciding with the mass extinction and that meant major increases in carbon dioxide and global temperatures right at the time of the mass extinction. Her calculations suggested that global temperatures would have risen 5 C - such a global warming translates into a regional warming of up to 16 C!

Jennifer's data on fossil stomata supported the hypothesis that global warming played a role in the end-Triassic mass extinction. To further investigate this hypothesis, she completed two more studies:

First, she used mathematical models of plant physiology to simulate how a plant would respond to the warmer climate at the end-Triassic. These models suggested that end-Triassic plants especially plants with large leaves would have overheated at those temperatures.

Second, she catalogued which terrestrial plant lineages went extinct during this extinction event and found that large-leafed lineages were particularly likely to go extinct exactly what one would expect to find if global warming had contributed to the mass extinction! As Jennifer puts it, "This is a really nice result. It shows that our predictions from modern ecological understandings of plants were borne out in the fossil record."

Together, these various lines of evidence (changes in fossil stomata, mathematical models of plant physiology, and studies of diversity before and after the extinction event) strongly support the hypothesis that global warming played a role in the end-Triassic mass extinction. Such intense global warming would likely have had a cascade of consequences for end-Triassic ecosystems. Some lineages might have gone extinct as a direct result of this warming for example, large-leafed plant species that overheated. Those extinctions could have had a ripple effect on other species that depended on them, potentially even causing extinctions of carnivores like the phytosaur. And the increased carbon dioxide levels along with higher temperatures could have had similar effects on ocean life. Although no one is yet certain of the exact sequence of events, it is clear that global warming of this magnitude has the potential to cause major changes in ecosystems.

Life's history has been occasionally interrupted by larger and smaller extinction events. Some (like the end-Triassic extinction already described) wiped out a huge portion of all organisms living at the time; other events were more localized. In the mid-Jurassic (about 180 million years ago), ocean-dwellers experienced one of these localized extinction events that killed off 33-53% of marine species living at the time.
Unraveling the cause of this extinction has been a challenge, but we do have a few important leads. From the geologic record, we know that these extinctions occurred around the same time that a mysterious layer of black rock was deposited. This black shale, sometimes several meters thick, appears in sediments around the world. Scientists think that this shale was formed when the world's oceans became depleted of oxygen. During this time, dead marine organisms formed a carbon-rich layer on the ocean floor that was eventually transformed into black shale. Because this oceanic oxygen depletion event (and many other events like it) seems to be correlated with times of unusually intense volcanic activity, many scientists have hypothesized that atmospheric or climatic changes may have played a role in triggering these depletion events, though they have not yet worked out all the details. But how exactly could volcanoes and volcanic events have changed the atmospheric composition in the mid-Jurassic in the first place? Jennifer set out to investigate.

Once again, Jennifer turned to fossil leaves deposited before, during, and after the extinction and oxygen depletion events. The stomata on the leaves confirmed that those events coincided with high levels of carbon dioxide in the atmosphere (i.e., few stomata), but they also turned up something totally unexpected. Just before the extinction event, there was a sudden spike in the density of stomata on fossil plants suggesting a low point in carbon dioxide levels just before the extinctions! But during the extinction event, carbon dioxide levels were quite high. So carbon dioxide levels must have increased remarkably rapidly from a low shortly before the mass extinction to a high during the extinctions just 50,000 years later.

What could have caused such a rapid increase in carbon dioxide levels? Jennifer thought that some major tectonic disturbance must have been responsible but what could it have been? One key clue was the geology of Antarctica and South Africa. In both these regions, geologists have found huge beds of once-molten rock alongside and sandwiched into deposits of scorched coal. Dating techniques suggest that these rock beds were formed around the same time that carbon dioxide levels skyrocketed in the mid-Jurassic. Furthermore, this all coincided with the period in which Gondwana - an ancient super continent - broke up into the modern land masses that we know today.

Jennifer has pieced these clues together into a plausible hypothesis to explain the rapid increase in carbon dioxide levels. It is likely that as Gondwana fragmented, floods of extremely hot (over 1000 C!), molten rock oozed out from the Earth's interior. With this volcanic outpouring would have come a release of greenhouse gasses but even worse, some of this molten rock may have worked its way into underground coal deposits from Carboniferous and Permian times and set them on fire. This would have led to massive underground coal fires releasing enormous amounts of methane (another important greenhouse gas) and carbon dioxide in a short period of time. Of course, future studies will provide more data relevant to this hypothesis, but at least initially, it seems to fit well with what we know about geologic activity at that time.

Jennifer points out that those coal fires (and the extinctions they may have triggered) bear a remarkable similarity to humans' reliance on fossil fuels today. "It's very analogous to what we're doing today. Humans are burning coals today that were formed in Carboniferous and Permian times. So what we are doing is totally equivalent to what these dolorite sills [the molten rock that oozed into coal deposits] were doing in the Jurassic they're rapidly burning carbon that was stored for millions and millions of years before, and they're putting the whole carbon cycle out of balance."

Jennifer's work helps us piece together the big picture of how Earth's climate has changed over time and how those changes have affected life's diversity. She relies on diverse sources of data (from mathematical models to experiments to basic geology) combined with her detailed studies of fossil leaves - details as seemingly trivial as the number of tiny pores on the surface of an ancient leaf - to reconstruct the climates in which those plants once lived.

This work highlights the fundamental similarities between catastrophic events in Earth's history and the sorts of environmental changes that humans are bringing about today, with our steady production of carbon dioxide and other greenhouse gasses through the burning of fossil fuels. Thus, an understanding of Earth's history may help us predict our impact on global environmental cycles tomorrow. As Jennifer explains, "The importance of the work that the carbon cycle typically takes about one million years to come back into balance (or equilibrium) after a perturbation, so even if we stopped burning fossil fuels today, we still would feel the effects of what we've done today for the next 200 years and it could take hundreds of thousands to even a million years for it all to come back into equilibrium. So people think, 'Oh, we don't need to worry because...the climate's not going to warm in the next ten or 50 years,' but if we think of our children or our children's children, there are going to be major changes because of what we're doing today. And that's just how the carbon cycle works there's a lag time."
Jennifer is particularly interested in how such environmental changes are likely to affect global biodiversity: "The big question we're addressing and want to address in the future is how do changes in greenhouse gasses - increases in greenhouse gasses and global warming - influence the biodiversity and ecology of natural ecosystems, because this is particularly relevant to today." Next she intends to revisit fossils from the end-Triassic mass extinction to find out if different sorts of organisms are affected differently by climate change. As she explains, "What I'm trying to do by studying the Triassic/Jurassic boundary is to work out if there are any particular ecological traits - rarity or dominance or reproductive specialization - that increase a species' vulnerability to extinction, that make it more likely to go extinct as global climate changes. I'm hoping that if I can study enough of these events in the fossil record, I might be able to make broad predictions on which species today will be most vulnerable as global warming continues over the next 50 to 100 years. That's the big burning question at the moment." and she just got a new grant from the European Union to answer it, so stay tuned!
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