Louis Derry on Understanding Earth's Biogeochemical Cycles
Scientist Interview: July 2010
According to a recent analysis of Essential Science IndicatorsSM from Thomson Reuters, the work of Dr. Louis Derry entered the top 1% in the field of Geosciences with the highest total citations of any new entrant. In the Web of Science®, his record shows 25 papers cited 437 times from January 1, 2000 to May 29, 2010.
Derry is Professor of Geological Sciences at Cornell University in Ithaca, New York.
Would you tell us a bit about your educational background and research experiences?
I did undergraduate work at Colorado College, and then worked in the mineral and petroleum industries for a few years. I eventually went on to graduate studies, and I received a Ph.D. from Harvard University in Geological Sciences/Geochemistry.
I worked for two to three years at the Centre de Recherches Pétrographiques et Géochimiques (CNRS) in Nancy, France. I eventually came back to the US and joined Cornell University, first as a research fellow and then as a faculty member
My research experiences have varied quite a bit, from the trace element chemistry of raw petroleum at Chevron, to Precambrian sedimentary geochemistry as a graduate student, to the study of weathering and erosion processes in the Himalayan orogen, to modern biogeochemical cycling in atmosphere-plant-soil-water-rock systems.
I've been lucky enough to be able to work on problems over very different spatial and temporal scales. The challenges and problems are different, but the fundamentally cool thing about science is that many seemingly different phenomena share important basic processes that can be understood in similar ways.
What would you say is the main focus of your research?
Figure 1:
Snow sampling in Colorado.
View this and two other larger images with
descriptions in the tabs below.
That's a bit hard to answer. But if I had to pick a theme, it's trying to understand the functioning of Earth's biogeochemical cycles and how they change in response to various "forcing factors," whether these be evolutionary events, tectonic events, climate change, land use change, and so on.
Since the Earth is a highly coupled system, changes in biogeochemical cycles feed back and impact climate, evolution, and so on, so developing a simple mathematical framework for understanding these complex interactions is also something that interests me. The challenge is to connect the observations to the computations in a useful way—it's too easy to let models of these systems get way ahead of the constraints.
So, we need to think creatively, but also remember that just because we calculated something doesn't mean it actually works that way. At this stage in our understanding, models may be most useful in identifying where critical measurements could really advance our understanding, by helping to frame well-posed questions that can be answered with observations
Several of your highly cited papers deal with strontium, particularly in the Himalayas. Would you talk a little about this aspect of your work? Why is strontium so important?
The interesting thing about answering your question is that strontium isn't important, at least not by itself! Strontium is a trace element that is not essential for most biological or geological process (it does help make fireworks red), but it has similar biogeochemical behavior to magnesium and calcium, as it sits immediately below those elements in the periodic table. Mg and Ca really are important for many reasons.
But the isotopic ratio of strontium varies in systematic and useful ways across different parts of the Earth, because of the slow decay of 87-rubidium to 87-strontium. Over geological time (tens to hundreds of million years), the ratio of 87-strontium to 86-strontium varies as a result.
So, if we take two rocks with different geological histories, they may have quite different 87Sr/86Sr ratios, and we can measure these differences quite precisely. When the strontium in those rocks is leached out into a stream, or picked up by a plant, or ends up in the oceans, we can trace its source as a result.
Since strontium behaves a lot like calcium and magnesium, we can then say something about the behavior of these much more interesting elements. They are vitally important biologically and geologically, and are very closely coupled to the global carbon cycle.
So, in a slightly roundabout way, all this work on strontium gets us critical data that we can use to trace the local scale or global scale cycles of calcium, magnesium, and carbon. Until quite recently, we didn't really have the means to trace calcium and magnesium in an analogous fashion, and even with some new developments in that area, strontium remains very valuable as a tracer.
Another of your highly cited papers has to do with silica cycling (Derry LA, et al., "Biological control of terrestrial silica cycling and export fluxes to watersheds," Nature 433[7027]:728-31, February 2005). Please tell us about this paper and related work.
Silicon is, after oxygen, the second most abundant element in the Earth, but like calcium, we didn't have good tracers for its behavior. In a manner similar to the argument for strontium, we used germanium as a tracer for silicon. The ratio of germanium to silicon turns out to be a very useful tracer for silicon, or its hydrated form, "silica."
Using germanium (by itself of no particular interest except to semiconductor manufacturers) as a tracer we found out a number of interesting and unexpected things. One of them was that the amount of silica cycling through plants was very large, and that most of the silica dissolved in river waters had been through a plant once or perhaps many times before it washed downstream.
"...the fundamentally cool thing about science is that many seemingly different phenomena share important basic processes that can be understood in similar ways."
Most everyone thought that the cycle of silica was simple—that it weathered out of rocks, went into streams, and was carried off to the oceans. But we found that this important element, one of the most studied of all by geologists, had a major environmental pathway that had been missed.
We hadn't really been looking for it, and so were a little slow realizing what we were seeing, but it turns out that good work by soil scientists had more or less predicted this kind of behavior. It's just that they didn't have a way of tracing the sources and movement of silica until we helped move germanium from being the answer to a trivia question to a useful biogeochemical tracer.
Are there any projects you have forthcoming that you are free to discuss?
One that we are working on is quantifying the carbon dioxide fluxes that come out of mountain ranges. For years many people, including us, have tried to quantify how atmospheric carbon dioxide is consumed by weathering in mountain ranges, and how much weathering reactions are enhanced by mountain-building events and subsequent erosion of the steep and high topography.
But mountain-building events also squeeze and heat rocks, and in doing so can drive off carbon dioxide in reactions that occur at 10-20 kilometers depth and 300–500°C. So a proper assessment of the total carbon budget of a mountain-building event, like the collision between India and Asia that continues to create the Himalayas, must include both the source and the sink.
Recently we were able to estimate the source of carbon dioxide in the central Himalayas from these deep reactions for the first time, and found it was a lot larger than anyone expected. The carbon dioxide output seems to be closely related to the heat output of the range as well, a very interesting and useful result.
So now we're trying to get good estimates of the output of heat and carbon dioxide in other parts of the Himalayan range so that we can better evaluate and quantify how the global carbon cycle couples with mountain-building events.
In what directions do you see your field (or key aspects thereof) going in the next decade?
The shortage of crystal balls being what it is, I'm sure whatever I say will be amusing in 10 years. However, a few things seem likely to be important. A new generation of mass spectrometers has made a whole suite of new tracer measurements possible, using subtle variations in the isotopic ratios of many metals.
This has opened up a really wide range of questions at all scales in biogeochemistry, but the challenge will be to develop a good quantitative understanding of what these fancy new data are really telling us. That will require some thoughtful integration of mathematical/computational models with new data.
Coupling good new measurements with more sophisticated system modeling efforts I think will be where the really exciting advances in the field over the next decade are likely to be. There—I said it, so we can all have a good laugh in 10 years.
Louis A. Derry
Department of Earth & Atmospheric Sciences
Cornell University
Ithaca, NY, USA
LOUIS DERRY'S MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:
Derry LA, et al., "Biological control of terrestrial silica cycling and export fluxes to watersheds," Nature 433(7027): 728-31, 17 February 2005 with 72 cites. Source: Essential Science Indicators from Thomson Reuters .
KEYWORDS: BIOGEOCHEMICAL CYCLES, FORCING FACTORS, EVOLUTIONARY EVENTS, TECTONIC EVENTS, CLIMATE CHANGE, HIGHLY COUPLED SYSTEMS, MODELS, STRONTIUM, TRACE ELEMENTS, HIMALAYAS, MAGNESIUM, CALCIUM, CARBON CYCLE, SILICON, GERMANIUM, SILICA, CARBON DIOXIDE FLUXES, MOUNTAIN RANGES.
Image 1:
Image 1:
Snow sampling in Colorado.
Image 2:
Image 2:
Sampling a river in Zambales, Philippines.
Image 3:
Image 3:
Crystal Geyser, UT, a natural CO2 source.
Image 4:
Image 4:
At a limestone gorge in Ardeche, France