Isotopes?

Isotopes?

A term thrown in so many different fields of science but never really successfully explained outside the realm of the super nerdy. They’re pretty simple, really- essentially just flavors of the same atom with different numbers of neutrons. More neutrons, heavier isotope. Too many neutrons, and your isotope becomes unstable, radioactively decaying over time to a different version of that element or perhaps another element entirely. With that brief introduction I’d like to explain one stable isotope system that is particularly interesting to me because it allows scientists to take a piece of the earth and reconstruct ancient environments.

C12 and C13 are stable isotopes of carbon- they both occur naturally in the environment and do not undergo any natural physical transformations over time. However, because of a small difference in their molecular weights due to the “extra” neutron in the C13 isotope, these two isotopes are processed quite differently in the environment.

Being slightly heavier means that C13 is a bit more difficult for biological systems to process. Most biological processes are adapted to using the lighter isotope, which is far more abundant.  When air diffuses into plant leaves via stomates, the tiny pores that suck up carbon dioxide for photosynthesis, CO2 is tightly bound by an enzyme before it can diffuse out again. In most cases this enzyme is Rubisco (see post: enzymes in the environment) . It turns out that Rubisco preferentially binds to C12, causing C13-enriched air to be released back into the atmosphere. Since most plants take up CO2 via Rubisco (this is known as the C3 photosynthetic pathway), most plant tissue on Earth is depleted in C13 relative to the atmosphere.

However, when a plant keeps its stomata open to take up CO2, a problem emerges- transpiration. Water loss occurs primarily through these same pores that plants must keep open if they want to feed themselves. In hot, arid environments, this puts your normal C3 plants in a sticky situation. They must open their stomata to eat, but risk losing dangerous amounts of water when they do so.

Millions of years ago, a group of plants evolved a rather elegant solution to this problem, known as the C4 photosynthetic pathway. They co-opted an enzyme already present in mitochondria for cellular respiration and gave it Rubisco’s job. This enzyme, known as PEP-carboxylase (I’ll call it PEPC here for simplicity), has a much higher affinity for CO2 than Rubisco- in fact, it binds CO2 so tightly that leaf stomata only need to be open for a fraction of the time they would otherwise. The high affinity of PEPC for CO2 also means that it doesn’t “distinguish” C12 from C13- it grabs whatever CO2 molecule is closest and binds tightly.

What does it matter that two classes of plants fractionate C13 differently? Scientists now have the tools to analyze the molecular composition of plant tissue and can determine a plant’s specific C13/C12 ratio.  C3 and C4 plants have distinct C13/C12 ratios and are easy to distinguish once isotopic analysis has been performed. For living plants, this would not be a terribly illuminating exercise- there are other anatomical and taxonomical ways to distinguish C3 and C4 plants that would be much more straightforward and less expensive.

But what about dead plants? Soil organic matter is composed principally of decomposed plant material, but even the most knowledgeable soil scientists aren’t able to look at a soil and say exactly what plants produced it. If we could, however, the soil would tell us numerous things. Accumulating over hundreds to tens of thousands of years, the soil profile from bedrock to the surface essentially represents a continuum of accumulated material that represents different floral and faunal assemblages, climate regimes and major environmental disturbances.

However, a complex series of transformation processes take place as plant material is decomposed and moved down the soil profile, some of which lead to C13 accumulation while others lead to C13 depletion. Carbon compounds are sorbed to surfaces, eaten by microbes, recycled, taken up by plants, leached, oxidized, and protected, to name a few. Given the inherent complexity of these systems, how can scientists can’t always sample down a soil pit and accurately describe species assemblages at different times using carbon isotopes alone.

A more fruitful path has involved obtaining environmental  samples that have undergone relatively little decomposition, such as cores of sediment from the bottom of a lake, or a core of peat from an inundated field. The plant material within samples that have been buried or otherwise protected from decomposition will be relatively similar, at least at the molecular level, to the original plant tissue, and can thus provide meaningful information about a past environment. For example, a sudden switch from C3 to C4 dominated plant material could indicate a transition from a cooler, wetter climate to a warmer, drier one. Stable carbon isotopes have proved incredibly valuable in tracing the spread of human agriculture, which can often lead to rather dramatic changes in the isotopic signature of a sample.

Still don’t think isotopes are interesting? If you have a friend whose fidelity to vegetarianism is in question, sending a sample of their hair to an isotope lab should resolve the situation. Chances are, if you’re a vegetarian your C13 levels will be relatively high, indicating a more plant-rich diet (a disproportionate number of the world’s major crops are C4 plants).

Fossil forams provide surprising insight into ice age oceans

In the North Atlantic, ocean water circulation patterns have far-reaching effects on global climate. Convective mixing is a dominant process due to thermal stratification of the water column. At low latitudes, warm, low-density surface waters float over a mass of much colder, high-density subsurface water. As warm surface water travels north, the temperature difference between surface and subsurface is diminished. Nutrient-depleted surface water cools and sinks, forcing deep water to rise. As deep water rises to the ocean surface, it brings a fresh pulse of nutrients that causes enhanced ocean productivity near the poles.

The formation of North Atlantic deepwater, or NADW, and the continual circulation of warm, subtropical water, play an important role in moderating Arctic climates. In colder intervals of Earth’s history such as the Last Glacial Maximum (LGM) 20,000 years ago, diminished thermal stratification reduces open ocean convection. Less surface water is transported poleward, and the water that is does not have the same warming effect on the local atmosphere and land surfaces.

This much about the interaction between North Atlantic circulation and climate is well understood. However, the timing of changes in NADW circulation and corresponding changes in climate remains something of a mystery. Scientists essentially face a chicken and egg problem- do climate changes shut down this oceanic conveyor belt, or does the shutdown of the conveyor belt occur first, by some other means entirely, but cause subsequent feedbacks on climate?

Currently, the climate change-induced NADW breakdown theory is popular and has been used to explain a number of abrupt climate reversals. The most prominent example is the Younger Dryas (YD), a brief cold-snap that occurred some 12 millions years ago following the end of the LGM and the retreat of continental glaciers. Proponents of this theory argue that glacial melting caused huge pulses of low-density freshwater into the north Atlantic, in precisely the region where vertical stratification is weak today and convective mixing occurs.  This influx of low-density water effectively shut down NADW formation, leading to a rapid cold reversal and a brief but dramatic rebound of continental glaciers.

A recent study using carbon isotopes found in fossil foraminifera, or forams, to date ocean water columns suggests otherwise. 14C is a heavy isotope of carbon that is produced in the upper atmosphere due to cosmic ray activity, and enters the surface ocean as a dissolved gas.  It is a popular isotope for radiometric dating, as it decays to 12C over a known period of time. The quantity of 14C remaining in a sample can thus be used to determine the sample’s age. A decreased 14C/12Cratio indicates an older sample. Indeed, numerous studies suggest that 14C depleted water is associated with decreases in convective mixing.

Fossil foraminifera, a popular organism for radiometric dating studies to reconstruct past climates

But how does one find 10,000 year old water to date and study in the first place? Scientists can’t simply put a bucket into the ocean and pull up 20,000 year old water to- they need a fossil or preserved object from the time period of interest. Some planktonic organisms such as forams leave behind a calcareous exoskeleton when they die. If buried quickly, these can be preserved for thousands or millions of years. While many planktonic organisms preferentially take up 12C over 14C, skewing the natural ratio of the two isotopes in their body tissue, forams do not significantly alter the natural 14C abundance. Examining fossil forams buried in ocean sediments thus provides a window into the past, allowing an accurate date to be ascribed to the ocean that the tiny creature existed in.

What are fossil forams from the North Atlantic telling us about ice age oceans? Proponents of the glacial melt water-induced NADW shutdown theory, and fans of “The Day After Tomorrow”, will no doubt be surprised by the finding that deepwater from the YD era actually dates to 600 years prior to the cold reversal. The shutdown of the oceanic conveyor belt prior to global cooling suggests that an unknown mechanism may in fact be driving ocean circulation, and in doing so exerting a powerful control on global climate.

Thornalley et al. 20110. The Deglacial Evolution of North Atlantic Deep Convection. Science 331: 202-205.

The rise and fall of human society with climate change

Scientists are becoming increasingly skilled at reconstructing past climates through use of a variety of “climate proxies”, including ice cores, lake sediments, fossils and tree pollen assemblages. Dendochronology, the analysis of tree rings, is now providing powerful evidence for the connection between human welfare and climate. Climate variations have influenced agricultural productivity, warfare and health of preindustrial peoples. A recent study in Science reports a high-resolution reconstruction of Central European summer precipitation and temperature for the past 2500 years, providing direct evidence that periods of social stability correspond with climactic stability, while periods of social upheaval, famine and even plague correspond with dramatic climate shifts and increased climactic variation.

To reconstruct a long-term climate record, researchers examined nearly 9000 pieces of wood from living and dead trees. Over 7000 were oak samples from France and Germany. Many of these were collected from historic buildings, or rivers and bogs that preserve ancient wood. To obtain the earliest dates possible, samples were obtained from archaeological sites. Researchers separately collected 1500 stone pine and larch wood samples from high altitudes in Austria.

To make a continuous record from the present to the past, dendochronologists first examine tree rings from live wood samples to provide a baseline for dating. From there they work back to older and older samples. The width of spacing between consecutive rings corresponds to the amount of growth experienced that year. In particularly bad years, a ring may be broken, fuzzy, or barely present. The isotopic composition of wood samples can also be taken and several important climate metrics can be extracted from it. O18, for example, is a heavy isotope of oxygen. In rainy years, paleoclimatologists expect a sample to be relatively enriched in O18. Taken together, these two sources of data provide strong evidence for both temperature and moisture conditions in a particular year.

To calibrate such a climate record, human records are extremely valuable. Weather records providing temperature and moisture data over the past 200 years were collected for Central Europe. These records allow scientists to precisely determine how weather affects ring growth in that year.

The result?  A continuous, 2500-year climate record that, when compared with archaeological and historical data, showed a stark pattern. Times of social stability and prosperity corresponded with warm, wet summers that led to high agricultural yields. Warm, stable climates coincide with the rise of the Roman Empire and peak years of medieval Europe. The opposite was also true. For example, a dramatic cold snap around ~1300 A.D. occurred directly prior to the famines and plague that spread across Europe half a century later. The decline of the Holy Roman Empire from AD ~250-600 coincides with a marked increase in climactic variability.

Though ancient peoples were clearly more susceptible to the affects of a bad harvest year than modern societies are, the powerful and fundamental connection between human welfare and climate still manifests itself today. Many scientists believe that the Holocene, the geological era of relatively mild, stable climates that led to the global dominance of the human species, is now ending. The Anthropocene, an era in which human actions are the primary climate driver, has just begun. We may do well to consider the effect of climate on human society throughout a climactically stable era when making decisions that will affect our climate in the future.

 

Buntgen et al. 2011. 2500 Years of European Climate Variability and Human Susceptibility. Science. In press.