Future Texas hurricanes: Fast like Ike or slow like Harvey?

– JULY 6, 2020

Climate change will make fast-moving storms more likely in late 21st-century Texas

Climate change will intensify winds that steer hurricanes north over Texas in the final 25 years of this century, increasing the odds for fast-moving storms like 2008’s Ike compared with slow-movers like 2017’s Harvey, according to new research.

Hurricane Harvey as seen from the International Space Station on Aug. 28, 2017

Hurricane Harvey as seen from the International Space Station on Aug. 28, 2017. (Photo courtesy of Randy Bresnik/NASA)

The study published online July 3 in Nature Communications examined regional atmospheric wind patterns that are likely to exist over Texas from 2075-2100 as Earth’s climate changes due to increased greenhouse emissions.

The research began in Houston as Harvey deluged the city with 30-40 inches of rain over five days. Rice University researchers riding out the storm began collaborating with colleagues from Columbia University’s Lamont-Doherty Earth Observatory (LDEO) and Harvard University to explore whether climate change would increase the likelihood of slow-moving rainmakers like Harvey.

“We find that the probability of having strong northward steering winds will increase with climate change, meaning hurricanes over Texas will be more likely to move like Ike than Harvey,” said study lead author Pedram Hassanzadeh of Rice.

Pedram Hassanzadeh

Pedram Hassanzadeh

Harvey caused an estimated $125 billion in damage, matching 2005’s Katrina as the costliest hurricane in U.S. history. Ike was marked by coastal flooding and high winds that caused $38 billion damage across several states. It was the second-costliest U.S. hurricane at the time and has since moved to sixth. Ike struck Galveston around 2 a.m. Sept. 13, 2008, crossed Texas in less than one day and caused record power outages from Arkansas to Ohio on Sept. 14.

Hassanzadeh, a fluid dynamicist, atmospheric modeler and assistant professor of both mechanical engineering and Earth, environmental and planetary sciences, said the findings don’t suggest that slow-moving storms like Harvey won’t happen in late 21st century. Rather, they suggest that storms during the period will be more likely to be fast-moving than slow-moving. The study found the chances that a Texas hurricane will be fast-moving as opposed to slow-moving will rise by about 50% in the last quarter of the 21st century compared with the final quarter of the 20th century.

Suzana Camargo

Suzana Camargo

“These results are very interesting, given that a previous study that considered the Atlantic basin as a whole noticed a trend for slower-moving storms in the past 30 years,” said study co-author Suzana Camargo, LDEO’s Marie Tharp Lamont Research Professor. “By contrast, our study focused on changes at the end of the 21st century and shows that we need to consider much smaller regional scales, as their trends might differ from the average across much larger regions.”

Hassanzadeh said the researchers used more than a dozen different computer models to produce several hundred simulations and found that “all of them agreed on an increase in northward steering winds over Texas.”

Steering winds are strong currents in the lower 10 kilometers of the atmosphere that move hurricanes.

Map depicting total rainfall from 2017's Hurricane Harvey

Map depicting total rainfall from 2017’s Hurricane Harvey. (Image courtesy of NOAA)

“It doesn’t happen a lot, in studying the climate system, that you get such a robust regional signal in wind patterns,” he said.

Harvey was the first hurricane Hassanzadeh experienced. He’d moved to Houston the previous year and was stunned by the slow-motion destruction that played out as bayous, creeks and rivers in and around the city topped their banks.

“I was sitting at home watching, just looking at the rain when (study co-author) Laurence (Yeung) emailed a bunch of us, asking ‘What’s going on? Why is this thing not moving?’” Hassanzadeh recalled. “That got things going. People started replying. That’s the good thing about being surrounded by smart people. Laurence got us started, and things took off.”

Laurence Yeung

Laurence Yeung

Ebrahim Nabizadeh

Ebrahim Nabizadeh

Yeung, an atmospheric chemist, Hassanzadeh and two other Rice professors on the original email, atmospheric scientist Dan Cohan and flooding expert Phil Bedient, won one of the first grants from Rice’s Houston Engagement and Recovery Effort (HERE), a research fund Rice established in response to Harvey.

“Without that, we couldn’t have done this work,” Hassanzadeh said. The HERE grant allowed Rice co-author Ebrahim Nabizadeh, a graduate student in mechanical engineering, to work for several months, analyzing the first of hundreds of computer simulations based on large-scale climate models.

The day Harvey made landfall, Hassanzadeh also had reached out to Columbia’s Chia-Ying Lee, an expert in both tropical storms and climate downscaling, procedures that use known information at large scales to make projections at local scales. Lee and Camargo used information from the large-scale simulations to make a regional model that simulated storms’ tracks over Texas in a warming climate.

Chia-Ying Lee

Chia-Ying Lee

“One challenge of studying the impact of climate change on hurricanes at a regional level is the lack of data,” said Lee, a Lamont Assistant Research Professor at LDEO. “At Columbia University, we have developed a downscaling model that uses physics-based statistics to connect large-scale atmospheric conditions to the formation, movement and intensity of hurricanes. The model’s physical basis allowed us to account for the impact of climate change, and its statistical features allowed us to simulate a sufficient number of Texas storms.”

Hassanzadeh said, “Once we found that robust signal, where all the models agreed, we thought, ‘There should be a robust mechanism that’s causing this.’”

He reached out to tropical climate dynamicist Ding Ma of Harvard to get another perspective.

“We were able to show that changes in two important processes were joining forces and resulting in the strong signal from the models,” said Ma, a postdoctoral researcher in Earth and planetary sciences.

Ding Ma

Ding Ma

One of the processes was the Atlantic subtropical high, or Bermuda high, a semipermanent area of high pressure that forms over the Atlantic Ocean during the summer, and the other was the North American monsoon, an uptick in rainfall and thunderstorms over the southwestern U.S. and northwestern Mexico that typically occurs between July and September. Hassanzadeh said recent studies have shown that each of these are projected to change as Earth’s climate warms.

“The subtropical high is a clockwise circulation to the east that is projected to intensify and shift westward, producing more northward winds over Texas,” he said. “The North American monsoon, to the west, produces a clockwise circulation high in the troposphere. That circulation is expected to weaken, resulting in increased, high-level northward winds over Texas.”

Hassanzadeh said the increased northward winds from both east and west “gives you a strong reinforcing effect over the whole troposphere, up to about 10 kilometers, over Texas. This has important implications for the movement of future Texas hurricanes.”

Models showed that the effect extended into western Louisiana, but the picture became murkier as the researchers looked further east, he said.

Map depicting total rainfall from 2008's Hurricane Ike

Map depicting total rainfall from 2008’s Hurricane Ike. (Image by Hal Pierce/SSAI/NASA

“You don’t have the robust signal like you do over Texas,” Hassanzadeh said. “If you look at Florida, for instance, there’s a lot of variation in the models. This shows how important it is to conduct studies that focus on climate impacts in specific regions. If we had looked at all of North America, for example, and tried to average over the whole region, we would have missed this localized mechanism over Texas.”

Bedient is the Herman Brown Professor of Engineering and department chair of civil and environmental engineering and director of Rice’s Severe Storm Prediction, Education and Evacuation from Disasters Center. Cohan is an associate professor of civil and environmental engineering. Yeung is the Maurice Ewing Career Development Assistant Professor in Earth Systems Science in the Department of Earth, Environmental and Planetary Sciences.

The research was supported by the National Science Foundation, NASA, the Gulf Research Program of the National Academies of Sciences, Engineering and Medicine’s Early-Career Research Fellowship Program, Rice’s Houston Engagement and Recovery Effort Fund, Columbia’s Center for Climate and Life Fellows Program, the National Oceanic and Atmospheric Administration and the New York State Energy Research and Development Authority. Computational resources were provided by the National Science Foundation’s Extreme Science and Engineering Discovery Environment, the National Center for Atmospheric Research’s Computational and Information Systems Lab and Rice’s Center for Research Computing.

Laurence Yeung

Laurence Yeung wins NSF CAREER Award to study biosphere’s history

EEPS Virtual Graduation Ceremony

Sylvia Dee, Caroline Masiello and Mark Torres receive grant to study environmental implications of COVID-19


Grants to Rice faculty support diagnostic, environmental, social projects

The Rice University COVID-19 Research Fund Oversight and Review Committee announced it will fund six additional projects by faculty working to mitigate the effects of the new coronavirus.

Researchers at Rice, some with the help of off-campus colleagues, plan to develop a device that rapidly identifies high-risk COVID-19 patients; a mobile phone-based test to detect the virus; a project to show how images, narratives and histories shape pandemic response; a study of how COVID-19 response policies impact air quality; a survey of Harris County residents to identify barriers to staying at home; and a study of the environmental impact of COVID-19 in Texas.

Six new projects represent the second round to be backed by the fund; the initial four projects were announced on April 20. The application window has recently closed and additional awards will be announced in the coming weeks, according to the committee led by Marcia O’Malley, the Stanley C. Moore Professor of Mechanical Engineering and a professor of electrical and computer engineering and of computer science. O’Malley is a special adviser to the provost on educational and research initiatives for collaborative health.

The EEPS-led project proposed by Sylvia Dee, Ted Loch-TemzelidesCaroline Masiello and Mark Torres will take advantage of a “crucial but short-lived research window” to evaluate the short-term impacts of rapid environmental mitigation during the coronavirus crisis and how environmental pollution and economic activity affect each other. The crisis, they suggest, provides a glimpse of how Earth’s environment and its climate system might respond to aggressive, fast-paced carbon-mitigation. It also provides an opportunity to assess which sectors of the economy — energy production, the restaurant industry or grocery supply chains — contribute maximally to environmental pollution, given explicit knowledge of closure and shelter-in-place policy timelines.

To aggressively monitor and capture environmental change from several months before the pandemic through the return to business as usual, undergraduate researchers will gather and synthesize data to build a mapping software tool for Texas. Users will be able to zoom in on their home counties and see how COVID-19 policies affected local environmental pollution conditions in real time, in both mapped and graphical visualizations.

Dee and Torres are assistant professors of Earth, environmental and planetary sciences. Loch-Temzelides is the George and Cynthia Mitchell Chair in Sustainable Development and a professor of economics. Masiello is a professor of Earth, environmental and planetary sciences.




Three Earth, Environmental and Planetary Sciences 2020 Ph.D. graduates are awarded prestigious National Science Foundation (NSF) Postdoctoral Fellowships – a record for the department. Brandee Carlson, Tian Dong and Andrew Moodie, all from the same laboratory group, receive the highly competitive grant after submitting research proposals to the Division of Earth Sciences at NSF.  The scope of the evaluation considers the scientific merits of the proposal, and the potential for transformative research as well as professional development by training recipients for research and leadership positions.  The grants provide two years of salary and research support at an institution of the fellows choosing.

These postdoctoral fellowships are only offered to early-career scientists, so student supervisors are relied upon to discuss fellowship opportunities with their students during their graduate careers.  Assistant professor Jeff Nittrouer, primary advisor for Carlson, Dong, and Moodie, strongly encouraged them to apply to the NSF program and is thrilled with the results.

“I could not be more proud of them,” says Nittrouer. “Collectively, they’ve shown how a laboratory raises the bar and thrives, demonstrating that scientific success comes from collaborations with fellow students and colleagues, both here at Rice and globally.”


EEPS 2020 Ph.D. graduates (L to R) Tian Dong, Brandee Carlson and Andrew Moodie all receive NSF Earth Sciences Postdoctoral Fellowships




“In the past six years, Brandee, Andrew, Tian, and Chen Wu [PhD, 2020] have cultivated a special culture: inclusiveness and sharing of ideas and resources, typifying the mantra that the sum of the parts is greater than the whole.” -Dr. Jeff Nittrouer






In terms of their upcoming research ventures, they’ll rely on recent experiences, in particular, working in far-flung localities and remote environments.

Brandee Carlson heads to the University of Colorado, Boulder, to collaborate with Prof. Irina Overeem in the Institute of Arctic and Alpine Research. Dr. Carlson is exploring delta front processes of Arctic rivers, focusing research in Greenland, where under warming climate conditions, river sediment supply is increasing due to rapidly retreating glaciers and thawing permafrost. Dr. Carlson plans to investigate how failures on multiple Arctic deltas vary by water and sediment discharge.  Her work includes several field campaigns combined with CU’s extensive remote sensing capabilities. The project dovetails with her previous work on the Yellow River delta but will ultimately expand her expertise to include sediment transport at a variety of delta fronts and climate conditions.

Tian Dong will study how physical processes shape river morphology, working with Dr. Timothy Goudge at the University of Texas at Austin. Tian will develop new metrics to distinguish between meandering and braided river patterns, from sediment deposits, drill cores, remote sensing, and the rock record. The goals are to identify the prevalence of these river types for the past eon of earth’s history and improve groundwater reservoir models.  Ultimately, the metrics may be translatable to the paleoclimate record of other terrestrial planets, including Mars.

Andrew Moodie will collaborate between Stanford University and the University of Texas, working with Drs. Jef Caers and Paola Passalacqua, respectively. Dr. Moodie’s project seeks to improve an understanding of subsurface delta sediment distribution and ground fluid movement, using machine learning algorithms.  The aim is to distinguish how multiple natural and anthropogenic factors, including as sea level change and infrastructure development, influence delta systems.

According to Andrew, “Our understanding of subsurface fluid transport in river deltas is limited.  Improving our ability to manage water resources and mitigate pollutant transport lowers risk to societal health.  And predicting ground-fluid transport relies on models to constrain subsurface composition, however due to the complexity of river-delta environments, accurate assessments are difficult. Using machine learning to better constrain environmental heterogeneity will benefit society and the cultures that live on deltas globally”.

Although the group splits at the end of the academic term, all agree that their experiences at Rice have collectively enhanced their future as scientists and mentors.

“Lessons learned abroad were brought back here” [to Rice] says Brandee. “Countless hours of discussion, sharing ideas on white boards and helping write each other’s codes all enhanced our scientific successes.”

Nittrouer concludes, “Rice’s motto is ‘Unconventional Wisdom’.  When I view the accomplishments of these students, I can’t help but applaud their unconventional generosity, humility, and determination.  These students selflessly helped one another, and thanks to the support here at Rice, there was no lack for opportunity”.

Melodie French wins National Science Foundation CAREER Award

– JANUARY 31, 2020

Fed grant backs Rice earthquake research

HOUSTON – (Jan. 31, 2020) – Rice University geologist Melodie French is crushing it in her quest to understand the physics responsible for earthquakes.

Rice University geologist Melodie French has earned a National Science Foundation CAREER Award to support her investigation of the tectonic roots of earthquakes and tsunamis. (Credit: Jeff Fitlow/Rice University)

Rice University geologist Melodie French has earned a National Science Foundation CAREER Award to support her investigation of the tectonic roots of earthquakes and tsunamis. Photo by Jeff Fitlow

The assistant professor of Earth, environmental and planetary science has earned a prestigious CAREER Award, a five-year National Science Foundation (NSF) grant for $600,000 to support her investigation of the tectonic roots of earthquakes and tsunamis.

CAREER awards support the research and educational development of young scholars likely to become leaders in their fields. The grants, among the most competitive awarded by the NSF, go to fewer than 400 scholars each year across all disciplines.

For French, the award gives her Rice lab the opportunity to study rocks exhumed from subduction zones at plate boundaries that are often the source of megathrust earthquakes and tsunamis. Her lab squeezes rock samples to characterize the strength of the rocks deep underground where the plates meet.

“Fundamentally, we hope to learn how the material properties of the rocks themselves control where earthquakes happen, how big one might become, what causes an earthquake to sometimes arrest after only a small amount of slip or what allows some to grow quite large,” French said.

“A lot of geophysics involves putting out instruments to see signals that propagate to the Earth’s surface,” she said. “But we try to understand the properties of the rocks that allow these different phenomena to happen.”

That generally involves putting rocks under extreme stress. “We squish rocks at different temperatures and pressures and at different rates while measuring force and strain in as many dimensions as we can,” French said. “That gives us a full picture of how the rocks deform under different conditions.”

The lab conducts experiments on both exposed surface rocks that were once deep within subduction zones and rock acquired by drilling for core samples.

Rice University geologist Melodie French and graduate student Ben Belzer work with a rock sample. French has been granted a National Science Foundation CAREER Award to study the tectonic roots of earthquakes and tsunamis. (Credit: Jeff Fitlow/Rice University)

Rice University geologist Melodie French and graduate student Ben Belzer work with a rock sample. French has been granted a National Science Foundation CAREER Award to study the tectonic roots of earthquakes and tsunamis. Photo by Jeff Fitlow

“I’m working with (Rice Professor) Juli Morgan on a subduction zone off of New Zealand where they drilled through part of the fault zone and brought rock up from about 500 meters deep,” French said. “But many big earthquakes happen much deeper than we could ever drill. So we need to go into the field to find ancient subduction rocks that have somehow managed to come to the surface.”

French is not sure if it will ever be possible to accurately predict earthquakes. “But one thing we can do is create better hazard maps to help us understand what regions should be prepared for quakes,” she said.

French is a native of Maine who earned her bachelor’s degree at Oberlin College, a master’s at the University of Wisconsin-Madison and a Ph.D. at Texas A&M University.

The award, co-funded by the NSF’s Geophysics, Tectonics and Marine Geology and Geophysics programs, will also provide inquiry-based educational opportunities in scientific instrument design and use to K-12 students as well as undergraduate and graduate-level students.


Read the award abstract at https://www.nsf.gov/awardsearch/showAward?AWD_ID=1945264&HistoricalAwards=false.

Follow Rice News and Media Relations via Twitter @RiceUNews.

Related materials:

Rheology and Deformation (French Lab): https://mefrench.com and the latest issue of Outcroppings- https://earthscience.rice.edu/wp-content/uploads/2020/02/French-lab-spread.pdf

An interview with Melodie French: http://earthscience.rice.edu/wp-content/uploads/2016/09/2016-Outcroppings-MelodieFrench.pdf

Earth, Environmental and Planetary Sciences: https://earthscience.rice.edu

Wiess School of Natural Sciences: https://naturalsciences.rice.edu

Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation’s top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,962 undergraduates and 3,027 graduate students, Rice’s undergraduate student-to-faculty ratio is just under 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for lots of race/class interaction and No. 4 for quality of life by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger’s Personal Finance.

Bally Alumni Gabor Tari wins AAPG Ambassador Service Award

From AAPG Explorer (December 2019): 

Gabor Tari is the recipient of the Vlastimila “Vlasta” Dvořáková International Ambassador Service Award. This award is given to those who have promoted growth and awareness of the AAPG organization internationally, outside the United States, and created opportunities for the Association to reach a wider audience of geoscientists worldwide. Tari is being honored for exception role in being the main driver of catalysts for many international events in the Europe Region.

AAPG awards, approved by the Executive Committee, are presented annually to recognize individuals for service to the profession, the science, the Association and the public.

Biographies and citations of all award winners will be included in a future AAPG BULLETIN. (Image courtesy of AAPG)


Gabor Tari received his Ph.D. from Rice in 1994.  He lives in Austria and is the Group Chief Scientist for Geology at OMV Exploration and Production Company.


Climate prompts change at Rice

Martha Lou Broussard Wins Prestigious AAPG Award

Martha Lou Broussard

Martha Lou Broussard is this year’s recipient of the AAPG Halbouty Outstanding Leadership Award, a very high honor. Martha Lou has played a key role in the success of AAPG and our own EEPS.  She was the first woman to graduate from our department. She played key roles in AAPG and established the AAPG student expo, which is now the primary way the energy industry recruits students. Today, she continues to help our department stay in touch with alumni and help our students in professional development and networking. Under the Broussard fellowship, she continues to support and foster the careers of women in earth sciences.

We are lucky to have Martha Lou!
-Cin-Ty Lee
From AAPG Explorer: 

AAPG Honorary Member Martha Lou Broussard is this year’s Michel T. Halbouty Outstanding Leadership Award recipient.

Broussard holds the distinction of being the first female geology graduate of Rice University, and she was a technical assistant to M. King Hubbert prior to going on to a successful career in the oil industry with Shell Development Company and ERICO. She returned to Rice to work as the Department of Earth Science coordinator, and now volunteers as the department’s alumni coordinator.

She is a recipient of AAPG’s Distinguished Service Award and is a past vice president and chair of the AAPG House of Delegates, but is perhaps best known for helping to start the AAPG/Society of Exploration Geoscientists Student Expo program, which today forms a key role in the recruiting program of as many as 40 exploration and production companies.


Breathing? Thank volcanoes, tectonics and bacteria

– DECEMBER 2, 2019

Study points to one cause for several mysteries linked to breathable oxygen

Earth’s breathable atmosphere is key for life, and a new study suggests that the first burst of oxygen was added by a spate of volcanic eruptions brought about by tectonics.

The evolution of life as depicted in a mural at NASA Ames Research Center in Mountain View, California.

The evolution of life as depicted in a mural at NASA Ames Research Center in Mountain View, California. The rise of oxygen from a trace element to a primary atmospheric component was an important evolutionary development. (Courtesy of NASA Ames/David J. Des Marais/Thomas W. Scattergood/Linda L. Jahnke)

The study by geoscientists at Rice University offers a new theory to help explain the appearance of significant concentrations of oxygen in Earth’s atmosphere about 2.5 billion years ago, something scientists call the Great Oxidation Event (GOE). The research appears this week in Nature Geoscience.

“What makes this unique is that it’s not just trying to explain the rise of oxygen,” said study lead author James Eguchi, a NASA postdoctoral fellow at the University of California, Riverside who conducted the work for his Ph.D. dissertation at Rice. “It’s also trying to explain some closely associated surface geochemistry, a change in the composition of carbon isotopes, that is observed in the carbonate rock record a relatively short time after the oxidation event. We’re trying explain each of those with a single mechanism that involves the deep Earth interior, tectonics and enhanced degassing of carbon dioxide from volcanoes.”

Eguchi’s co-authors are Rajdeep Dasgupta, an experimental and theoretical geochemist and professor in Rice’s Department of Earth, Environmental and Planetary Sciences, and Johnny Seales, a Rice graduate student who helped with the model calculations that validated the new theory.

Scientists have long pointed to photosynthesis — a process that produces waste oxygen — as a likely source for increased oxygen during the GOE. Dasgupta said the new theory doesn’t discount the role that the first photosynthetic organisms, cyanobacteria, played in the GOE.

James Eguchi, Johnny Seales and Rajdeep Dasgupta

Geoscientists (from left) James Eguchi, Johnny Seales and Rajdeep Dasgupta published a new theory that attempts to explain the first appearance of significant concentrations of oxygen in Earth’s atmosphere about 2.5 billion years ago as well as a puzzling shift in the ratio of carbon isotopes in carbonate minerals that followed. (Photos courtesy of Rice University)

“Most people think the rise of oxygen was linked to cyanobacteria, and they are not wrong,” he said. “The emergence of photosynthetic organisms could release oxygen. But the most important question is whether the timing of that emergence lines up with the timing of the Great Oxidation Event. As it turns out, they do not.”

Cyanobacteria were alive on Earth as much as 500 million years before the GOE. While a number of theories have been offered to explain why it might have taken that long for oxygen to show up in the atmosphere, Dasgupta said he’s not aware of any that have simultaneously tried to explain a marked change in the ratio of carbon isotopes in carbonate minerals that began about 100 million years after the GOE. Geologists refer to this as the Lomagundi Event, and it lasted several hundred million years.

One in a hundred carbon atoms are the isotope carbon-13, and the other 99 are carbon-12. This 1-to-99 ratio is well documented in carbonates that formed before and after Lomagundi, but those formed during the event have about 10% more carbon-13.

Eguchi said the explosion in cyanobacteria associated with the GOE has long been viewed as playing a role in Lomagundi.

“Cyanobacteria prefer to take carbon-12 relative to carbon-13,” he said. “So when you start producing more organic carbon, or cyanobacteria, then the reservoir from which the carbonates are being produced is depleted in carbon-12.”

Eguchi said people tried using this to explain Lomagundi, but timing was again a problem.

A figure that illustrates how inorganic carbon cycles through the mantle more quickly than organic carbon, which contains very little of the isotope carbon-13.

This figure illustrates how inorganic carbon cycles through the mantle more quickly than organic carbon, which contains very little of the isotope carbon-13. Both inorganic and organic carbon are drawn into Earth’s mantle at subduction zones (top left). Due to different chemical behaviors, inorganic carbon tends to return through eruptions at arc volcanoes above the subduction zone (center). Organic carbon follows a longer route, as it is drawn deep into the mantle (bottom) and returns through ocean island volcanos (right). The differences in recycling times, in combination with increased volcanism, can explain isotopic carbon signatures from rocks that are associated with both the Great Oxidation Event, about 2.4 billion years ago, and the Lomagundi Event that followed. (Image by J. Eguchi/University of California, Riverside)

“When you actually look at the geologic record, the increase in the carbon-13-to-carbon-12 ratio actually occurs up to 10s of millions of years after oxygen rose,” he said. “So then it becomes difficult to explain these two events through a change in the ratio of organic carbon to carbonate.”

The scenario Eguchi, Dasgupta and Seales arrived at to explain all of these factors is:

  • A dramatic increase in tectonic activity led to the formation of hundreds of volcanoes that spewed carbon dioxide into the atmosphere.
  • The climate warmed, increasing rainfall, which in turn increased “weathering,” the chemical breakdown of rocky minerals on Earth’s barren continents.
  • Weathering produced a mineral-rich runoff that poured into the oceans, supporting a boom in both cyanobacteria and carbonates.
  • The organic and inorganic carbon from these wound up on the seafloor and was eventually recycled back into Earth’s mantle at subduction zones, where oceanic plates are dragged beneath continents.
  • When sediments remelted into the mantle, inorganic carbon, hosted in carbonates, tended to be released early, re-entering the atmosphere through arc volcanoes directly above subduction zones.
  • Organic carbon, which contained very little carbon-13, was drawn deep into the mantle and emerged hundreds of millions of years later as carbon dioxide from island hotspot volcanoes like Hawaii.

“It’s kind of a big cyclic process,” Eguchi said. “We do think the amount of cyanobacteria increased around 2.4 billion years ago. So that would drive our oxygen increase. But the increase of cyanobacteria is balanced by the increase of carbonates. So that carbon-12-to-carbon-13 ratio doesn’t change until both the carbonates and organic carbon, from cyanobacteria, get subducted deep into the Earth. When they do, geochemistry comes into play, causing these two forms of carbon to reside in the mantle for different periods of time. Carbonates are much more easily released in magmas and are released back to the surface at a very short period. Lomagundi starts when the first carbon-13-enriched carbon from carbonates returns to the surface, and it ends when the carbon-12-enriched organic carbon returns much later, rebalancing the ratio.”

Eguchi said the study emphasizes the importance of the role that deep Earth processes can play in the evolution of life at the surface.

Earth's atmosphere as seen from the International Space Station July 20, 2006

Earth’s atmosphere as seen from the International Space Station July 20, 2006. (Image courtesy of NASA)

“We’re proposing that carbon dioxide emissions were very important to this proliferation of life,” he said. “It’s really trying to tie in how these deeper processes have affected surface life on our planet in the past.”

Dasgupta is also the principal investigator on a NASA-funded effort called CLEVER Planets that is exploring how life-essential elements might come together on distant exoplanets. He said better understanding how Earth became habitable is important for studying habitability and its evolution on distant worlds.

“It looks like Earth’s history is calling for tectonics to play a big role in habitability, but that doesn’t necessarily mean that tectonics is absolutely necessary for oxygen build up,” he said. “There might be other ways of building and sustaining oxygen, and exploring those is one of the things we’re trying to do in CLEVER Planets.”

The research was supported by the National Science Foundation, NASA and the Deep Carbon Observatory.