The hidden langauge of water and matter : The water trackers part I

Isotopes and the birth of geochemistry and biogeochemistry

Scientific progress often follows nonlinear paths. Discoveries in one area, initially seeming entirely unrelated to another, can profoundly impact the latter. For instance, when scientists studied atoms and their isotopes, they found that understanding isotopes illuminated entirely new fields. Insights into isotopes have helped explain the movement of water and matter across Earth and revealed events from millions of years ago, significantly influencing geoscience.

We learn early that water is H₂O—a single molecule with two hydrogen atoms and one oxygen atom. However, not all water is the same. The hydrogen (H) and oxygen (O) in water can vary, forming isotopes. This variability transforms water from a uniform substance into a diverse collection of isotopic forms. Its a polyculture, not a monoculture. These isotopic differences are crucial for tracking water’s movement through oceans, air, soil, lakes, rivers, aquifers, and living organisms.

The most common form of hydrogen has a nucleus consisting of a single proton. However, hydrogen can also exist as deuterium, where the nucleus contains both a proton and a neutron. These are isotopes of hydrogen, with deuterium being the heavier form. Similarly, oxygen’s most common isotope, oxygen-16, has a nucleus with 8 protons and 8 neutrons. Another isotope, oxygen-18, includes 8 protons and 10 neutrons. This gives water four main forms, depending on whether hydrogen or oxygen isotopes vary. Rarer forms, like water with oxygen-17, also exist. Thus, water is a polyculture of isotopes.

These isotopes form the hidden language of water. When we learn to read this language, we learn a way to track water. The hydrological cycle is a complex system—breathing, moving, flowing, splitting up, and phase transitioning. An understanding of the grammar of isotopes gives us information about how tree water turns into rain, how glacier melt ends up in someone’s well water, how vegetation’s roots bring up groundwater in the dry season, how ocean currents dive deep, how storms form, and how soil moisture evaporates. These isotopic iotas trace the Earth’s processes, hiding inside cloud droplets, plant xylem, wetlands, gurgling brooks, and bubbling springs. Scientists have used isotopes to establish evidence for the significance of small water cycle and precipitation-recycling phenomena. Their trail can tell us where our society’s water supply is coming from, what factors influence it, and provide helpful information to those working to restore the water cycle. These isotopic signatures, surprisingly, also tell us what ancient ocean water temperatures were and how much it rained millions of years ago. They are flecks containing the fingerprints of history. They tell us how the water cycle shifted in the past and what we need to do to ensure a healthy future for our water systems.

Isotopes affect more than just water. The isotopic composition of carbon, oxygen, sulfur, and methane also shifts as they circulate through Earth’s systems. Phase changes and interactions alter isotopic ratios, such as when carbon transitions between living and non-living forms. Geochemistry explores Earth’s chemical processes across land, water, and air. Biogeochemistry extends this by linking these processes with biological systems, forming a feedback loop that connects life, climate, and geomorphology.

Harold Urey, geochemistry, deuterium, heavy water and the paleo-thermometer

Harold Urey is considered by many to be the father of geochemistry. He opened a scientific portal to a landscape so vast that there was room for his students to, in turn, open doors to entirely new sub-disciplines. He and his extraordinary students were key players in the growth of biogeochemistry and many of its subfields, such as water isotope hydrology. Urey encouraged his students to “choose significant problems. Even if your ideas are wrong…something good will turn up. And that will lead you to something else.”

Born in a small Indiana town, Urey grew up in poverty, educated in one-room schoolhouses. He worked as a teacher and laborer to afford university. Later, as a professor, he was known for his deep focus, impatience, pride, and childlike curiosity. He explored diverse scientific questions, from dinosaur extinction to ancient oceans and lunar geochemistry. He once remarked, “I’d love to go to the moon; I think I’d go even if I knew I could never get back.”

Urey entered many fields. He worked by first becoming conversant in a field, then trying to formulate a theory to encompass the many facts that had yet to be correlated. He was prolific. His post-doctoral student, Sam Epstein, said, “Harold Urey had a thousand ideas. He thought of doing a thousand important things. Maybe 998 were shooting from the hip; he hadn’t really thought it out. But two of the ideas would be very unusual, and that’s all you need, just two—even one good idea. With people like that, you don’t concentrate on whether they were wrong or that they changed their mind. The important issue is to recognize their good ideas and take them seriously” These ideas all tied back into his larger goal of understanding biogeochemical cycles across space and time. Great scientists come in the form of hedgehogs or foxes. Hedgehogs like to unify everything into one theory, like Pascal, Einstein, and Grothendieck. Foxes solve many types of problems without tying them to one universal idea, like Goethe, Feynman, and Terence Tao. Urey was a hedgehog who might have appeared from afar to be a fox.

As a young professor, Urey observed stellar light patterns suggesting a heavier hydrogen isotope. He hypothesized that letting liquid hydrogen evaporate would leave behind heavier hydrogen, measurable with a mass spectrometer. This method worked, and in 1931, Urey discovered deuterium, earning a Nobel Prize. His achievement—a testament to determination and ingenuity—made headlines and cemented his reputation.

The discovery showed that water could take different forms, such as "heavy water" containing deuterium. Urey speculated in a 1935 Scientific American article that heavy water could advance our understanding of living processes.

Urey next devised a paleo-thermometer to measure ancient ocean temperatures. He hypothesized that carbonaceous shells, created by organisms like foraminifera, recorded water isotope ratios influenced by ocean temperatures. Colder water contains fewer heavy isotopes due to reduced evaporation, and these isotopes are integrated into shells. Over time, these shells become part of rock layers, allowing scientists to date the layers and infer ocean temperatures from isotopic data.

Sam Epstein and the oxygen isotopes of water

Although Urey frequently talked about his paleo-thermometer idea, it was initially met with little interest. Eventually, he found a Canadian post-doc named Sam Epstein, who recalled, “Harold Urey had this crazy problem that nobody believed could be done... But this problem was considered to be a wild [goose] chase; and if I’d had any sense, I would have gone back.”

Epstein joined Urey’s team at the University of Chicago, where he found a surprisingly informal scientific culture. In Canada, professors were addressed formally as “Sir.” At Chicago, even Enrico Fermi, a decorated physicist and Nobel laureate, faced candid critiques during his lectures.

Epstein, dove into the project with resolve. He built a mass spectrometer to analyze shells and studied how isotopic ratios correlated with salinity. He found that saline water contained more heavy isotopes due to processes like inland rain stripping heavy isotopes from atmospheric moisture.

Epstein recalled, “We had to know something about the isotopic composition of ocean water, because the isotopic composition of the ocean is related to the isotopic composition of the shells…. we showed that the isotopic composition of ocean water varied because as the water evaporates, the lighter isotopes are preferentially removed. And of course, the isotopic distribution of ocean water is connected with.. rain and precipitation. It turned out that the isotopic composition of this water varies in the world in a systematic way. The heavier-isotope water falls initially in the warm areas, but as the air masses lose this heavy water, it becomes lighter and lighter and lighter, resulting in rains in the northern latitudes consisting of isotopically lighter waters. And the consequence of that turns out to be enormous.” They had found a way to find the temperature of ancient oceans.

Reflecting years later, Epstein remarked, “I don’t think any of us ever thought that this would develop into the field that it eventually became. Later on, it became quite clear that you can measure isotopic effects for anything. You can reexamine all of chemistry.”

The Chicago Group

At the University of Chicago in the late 1940s and 1950s, a collective began to form, their synergy driving significant scientific progress. After Sam Epstein joined Urey’s group, others followed: Stanley Miller (who, with Urey, developed a famous theory about the origin of life, suggesting that lightning striking ancient wetlands could form amino acids), Irving Friedman, and Harmon Craig. Urey was a great mentor who helped his students flourish. Adding to the mix was Harry Libby, a professor across the hallway from Urey. Together, this collective gained fame as the so-called Chicago group.

“The environment is extremely important. You’re surrounded by a bunch of bright people; you learn by osmosis. It’s very important to be in a place where lots and lots of radiation is coming at you. It’s like sunshine—you absorb it, and you become a good scientist,” Sam Epstein reflected on the Chicago scene.

Irving Friedman and the hydrogen isotopes of water

Irving Friedman grew up in New York, and then did a PhD in figuring out the origin of granite at the University of Chicago. As a post-doc, Urey set him the problem to explain the hydrogen water isotopes ratios of different waters of the world, which paralleled the work Epstein was doing with the heavy oxygen isotopes of water. It would help with the development of the paleo-thermometer. More than any other pairs of isotopes, the heavy and lighter hydrogen get separated by geological processes, a consequence of the isotopes having the biggest percentage difference in weight. They thus provide a distinct signature.

The ratio of the heavy hydrogen isotope of water to the normal hydrogen isotope of water can be measured in thousandths. This ratio can be written in a form that is adjusted around a value of 0 for a standardized water, and given the name delta-hydrogen. So a negative delta value means the water has more of the lighter isotope than does the standardized water. Here are some values he got for delta : for rivers in California, Sacramento River - 1.4 , San Joaquin River -2.3. For east coast rivers that receive Atlantic Ocean air : Connecticut River -2.1; Susquehanna River -0.2. For the Colorado River at Yuma, Arizona -6.1, Missouri River at Kansas City -7.1, Columbia River at Trail, British Columbia -10.1, Apalachicola River of Florida +4.2.

These numbers contain information about the hydrological cycle. Learning to read this language of the isotopes like a hydrologist is akin to a learning to read the elements of the landscape like a nature tracker, or learning how to read the clouds like a meteorologist.

There are different conditions that contribute to each number being the value it is. When moisture travels inland, each time it rains, more of the heavier hydrogen isotope will drop out. So moisture far inland will have lower numbers. Colder weather also leads to lower numbers. If there is a lot of evaporation, then heavier isotopes will be left behind, and there will be higher numbers. So one has to tease out the story from the numbers using various contextual clues.

In the case of the California, we can surmise that the rivers have relatively higher deuterium values because the ocean moisture did not have to travel too far, before it dropped down into the watershed. As the wind ascends into the mountains of Colorado, the heavier deuterium will have dropped out more. High up in the Colorado river the delta value is thus very negative. As the Colorado river flows into Arizona, there will be bodies of waters that has experienced quite a bit of evaporation contributing to it. So the values will be low but not too low. Because the values in Trail, British Columbia are so low, it suggests the ocean air probably blew quite a long distance inland before it got there. So there are probably winds blowing north across North America that go to British Columbia. Florida’s river has quite high numbers. Because it is not far inland its probably not the distance travlled by the ocean moisture that leads to the high numbers. Rather, this suggests the watershed there experiences a lot of evaporation, leaving behind high deuterium numbers.

Friedman compared the data he got with the data Epstein got, by putting the data on a graph where one axis delineates the ratio of hydrogen isotopes of water, and the other the axis delineates the ratio of the oxygen isotopes of water. Intriguingly all the data points end up along a straight line.

Why are the points on a straight line?

An analogy is in order. Lets say you have water with two types of alcohol in it, ethanol and methanol. The boiling points of both are lower than water. The air will have a certain ratio of methanol and ethanol vapor. As you boil the alcohol, the air will increase in both ethanol and methanol. Each corresponding increase in temperature will lead to a corresponding increase in ethanol and methanol, but the amount it increases isn’t random, it follows the same ratio. That ratio forms the slope of the ratio line for the alchohol.

The situation with the methanol and the ethanol can be seen as analagous to the situation with the hydrogen isotopes and the oxygen isotopes of water. For every increase of the oxygen isotope ratio, the hydrogen isotope ratio would increase by 8 times as much. The slope of the line is thus 8. The colder parts of the ocean will be further right on the line than the warmer parts of the ocean, as the colder parts of the ocean will have more heavy isotopes left behind after evaporation.

Of all Harold Urey’s students, Friedman is the one who most focused on water for the rest of his career after Chicago. At the US Geological Survey, he initiated a department to study isotopes. He applied stable isotopes to study oceans, rivers, lakes, glaciers, hot springs, and the atmosphere. Some there call Friedman the father of the field of water isotope hydrology.

Harmon Craig and the Local Meteoric Water Lines

In the late 1940s Harmon Craig joined Urey’s group at the University of Chicago as a graduate student. He decided to research geochemistry and isotope ratios because he said it “allows you to study everything on earth”. In order to do it “you have to know something about everything, from nuclear cross-sections to photosynthesis.”

Craig’s style was blunt, bold, and aggressive, which constrasted with Epstein, who was friendly, socially intelligent, and grew a wide network of friends. Epstein said of Harmon “He’d come and say some stupid thing on purpose, and I’d say, “That’s not right.” And by the time I got through with him, I was completely drained. But nevertheless, just talking with a guy like that makes you think.”

Urey was working on projects to date the ancient oceans with the sea creatures, and to determine what killed the dinosaurs. Across the hall Harry Libby was working on radiocarbon dating with trees and other organisms. Craig worked with both Urey and Libby. Craig’s project involved understanding how the ratio of carbon isotopes changed in various biogeochemical processes. Sea animals, dinosaurs, trees, and organisms in general, during photosynthesis and metabolism would incorporate more of the carbon 12 isotope rather than the carbon 13, into their bodies. Craig’s work would turn out to be important for the field of paleontology - the carbon 12 to 13 ratio in the fossil record, and combined with Libby’s more famous carbon 14 dating method, which he was developing at the same time, has helped scientists figure out ancient life on earth.

Craig would go onto a swashbuckling lifestyle in science. He travelled the world collecting water samples, diving into oceans, drilling ice cores in Greenland, and once got burnt by a geyser in Yellowstone. He got into quite a few scrapes, being forced onto gunboats in Zaire, and getting robbed in Tanzania by thieves brandishing spears. He fought funding agencies who wanted him to explain his proposed science projects in a more formal way. His work was data driven, organized and mathematical, yet he also worked more by the seat of his pants, looking for exciting things, that led to new research vistas. He said “So many times it’s happened in my own work that serendipity and just plain good luck intervened − you start up some problem with a completely wrong idea and it takes you to something even more interesting than if the original idea had been correct. Nature is bountiful with her rewards if you are willing to take a chance.”

Harmon Craig collected and analyzed 400 samples of rain, lakes, and rivers from around the world. Following in the footsteps of Friedman, he plotted these data points on a graph, with the deuterium water isotope ratio on one axis and the oxygen water isotope ratio on the other. He established a global mean standard to which these isotope ratios could be compared, calling this reference line the Global Meteoric Water Line (GMWL). Meteoric water refers to water derived from precipitation.

[Global Meteoric Water Line from Craig 1961]

Craig also identified phenomena that Friedman had not noted—behaviors that diverged from the GMWL. These deviations reflected local uniqueness and provided valuable insights into how the water cycle operates at specific geographic locations and how it connects with other regions.

One of the new behaviors Craig observed involved points below the GMWL. These occurred in closed basins, where water from lakes and rivers could not drain back into the ocean. Additionally, he identified anomalous behaviors at the upper end of the line, where points veered off to the right. The slope of this line was 5, compared to the GMWL slope of 8. This meant that for every unit increase in the hydrogen isotope ratio, the oxygen isotope ratio increased only fivefold instead of eightfold. But why was this the case?

To understand, imagine a jar of water with a closed lid. Both liquid and vapor coexist in dynamic equilibrium, with liquid molecules evaporating into the air at the same rate as vapor molecules condensing back into the liquid. If the water cools, the isotope ratios shift: for every oxygen isotope that condenses out, eight hydrogen isotopes do as well. This jar condensation scenario mirrors how rain condenses in the atmosphere. When rain forms, the air is saturated, and the process operates at equilibrium. Consequently, rainwater worldwide exhibits a slope of 8 when plotted on the isotope ratio graph. These rains then replenish lakes and rivers.

In a lake where rain inputs significantly exceed evaporation losses, the lake's isotope ratios align with a slope of 8, consistent with the GMWL. However, if evaporation dominates and the air is dry, the situation changes. This new scenario is analogous to a jar with an open lid. The system operates under non-equilibrium conditions (referred to as kinetic fractionation), where lighter hydrogen isotopes diffuse into the air more readily than lighter oxygen isotopes. The drier the air, the greater the isotopic separation, and as the humidity approaches zero, the slope approaches 5.

Each geographic location produces its own line and slope when hydrogen and oxygen isotope ratios are plotted. These are known as Local Meteoric Water Lines (LMWL). From these lines, researchers can infer information about humidity, evaporation, and other environmental variables. For example, Western Oregon and Washington have an LMWL slope of 8.2 due to their wet and humid conditions. In contrast, North and South Dakota exhibit a slope of 7.0, and Montana's even drier conditions result in a slope of 5.0.

The Local Meteoric Water Lines became a vital part of the grammar of the isotopic language of water, offering profound insights into the hydrological and climatic processes that shape our planet.

There are scientific revolutions that advance our understandings on specific topics, like relativity, quantum mechanics, neuroscience or cell biology. There are revolutions that look at emergent patterns in all of nature, like chaos theory and dynamical systems. And then there are methodological revolutions that opens up new worlds to see, like microscopes, radar, and telescopes. The isotope paradigm shift was in some sense a experimental and methodological revolution. But it also was something more. One also had to understand the chemistry and physics of what was going on, and to build theoretical models to explain the isotopic behavior. The isotopic field was a marriage of the experimenter, collecting samples, building and using mass spectrometers to measure isotopes, and also of the theorist, building models about what was happening. Every geo-interaction and phase change influences the isotope ratios. To read the isotope code one also had to figure out how the geo-mechanisms that wrote that code. If you map the changes in the code across the globe you begin to get a holistic sense of its fractal and mulitply-forked circulations. The isotope paradigm gave rise to a systems thinking growth in the understanding of the flow of matter and water around the earth.

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Reference

Craig, Harmon. "Isotopic variations in meteoric waters." Science 133, no. 3465 (1961): 1702-1703.

Craig, Harmon. "The geochemistry of the stable carbon isotopes." Geochimica et cosmochimica acta 3, no. 2-3 (1953): 53-92

Epstein, Samuel, Ralph Buchsbaum, Heinz Lowenstam, and Harold C. Urey. "Carbonate-water isotopic temperature scale." Geological Society of America Bulletin 62, no. 4 (1951): 417-426

Epstein, Samuel, and Toshiko Mayeda. "Variation of O18 content of waters from natural sources." Geochimica et cosmochimica acta 4, no. 5 (1953): 213-224

Epstein, Sam. "The role of stable isotopes in geochemistries of all kinds." Annual Review of Earth and Planetary Sciences 25, no. 1 (1997): 1-21

Epstein interview http://oralhistories.library.caltech.edu/197/1/Epstein%2C_S._OHO.pdf

Gilfillan, E.S.J. (1934) The isotopic composition of sea water. Journal of the American Chemical Society 56, 406-408

Friedman, Irving. "Deuterium content of natural waters and other substances." Geochimica et cosmochimica acta 4, no. 1-2 (1953): 89-103.

Jouzel, J., G. Delaygue, A. Landais, V. Masson-Delmotte, C. Risi, and F. Vimeux (2013), Water isotopes as tools to document oceanic sources of precipitation, Water Resour. Res., 49, 7469–7486, doi:10.1002/2013WR013508.

Urey, Harold C., Heinz A. Lowenstam, Samuel Epstein, and Charles R. McKinney. "Measurement of paleotemperatures and temperatures of the Upper Cretaceous of England, Denmark, and the southeastern United States." Geological Society of America Bulletin 62, no. 4 (1951): 399-416.