– NOVEMBER 28, 2019
Cutting-edge tech comes to new Rice lab on heels of Science study
Undersea telecommunications cables that connect the continents may help measure earthquakes and detect other seismic events, according to a newly published paper based on research conducted by a Rice University professor and his colleagues.
But proof that the technique works might not have happened without some arduous ditch-digging.
Scientists led by University of California, Berkeley, graduate student Nate Lindsey and Rice geophysicist Jonathan Ajo-Franklin confirmed that telecommunication cables may be useful to collect seismic readings from the seafloor and at great distances.
Their high-profile paper in this week’s Science serves as a nice homecoming for Ajo-Franklin, a 1998 Rice alumnus (Brown College) who rejoined the university this summer as a professor of Earth, environmental and planetary sciences. (A separate article in the issue further describes the research.)
Ajo-Franklin comes to Rice from his position as a staff scientist at Lawrence Berkeley National Laboratory (LBNL), where he and Lindsey, a National Science Foundation graduate research fellow, led an experiment that turned 20 kilometers of fiber-optic cable into the equivalent of 10,000 seismic stations along the ocean floor. The fiber was made available by the Monterey Bay Aquarium Research Institute (MBARI) and co-author Craig Dawe.
During a four-day experiment in Monterey Bay, they recorded a 3.5 magnitude quake and seismic scattering from underwater fault zones using a technique called distributed acoustic sensing (DAS).
DAS employs a photonic device that sends short pulses of laser light down the cable and detects backscattering created by strain in the cable caused by stretching. With interferometry, used to decode interference between the returning signals, they measured the strain signals every 2 meters (6 feet), effectively turning a 20-kilometer cable into 10,000 individual motion sensors.
Researchers had previously confirmed DAS with land-based “dark” fibers, optical fibers buried underground but unused or leased for short-term use, unlike “lit” internet fibers. The latest advance could significantly advance scientists’ reach.
The technique was inspired by a study that led Ajo-Franklin and Lindsey to Fairbanks, Alaska, five years ago. “It involved laying fibers and using ambient noise to detect zones of permafrost thaw in the Arctic,” Ajo-Franklin said. “It was a backbreaking effort to get the fibers in the ground.
“I remember we had a long conversation; like, ‘We know there are fibers in the ground already for telecom,’” he said. “We thought, ‘What if we use those instead of digging these kilometer-long trenches?’ That was the genesis of the idea.”
Ajo-Franklin said the new study is “on the frontier of seismology, the first time anyone has used offshore fiber-optic cables for looking at these types of oceanographic signals or for imaging fault structures. One of the blank spots in the seismographic network worldwide is in the oceans.”
The ultimate goal, he said, is to use the dense fiber-optic networks around the world — probably more than 10 million kilometers in all, on both land and under the sea — as sensitive measures of Earth’s movement, allowing earthquake monitoring in regions that don’t have expensive ground stations like those that dot much of earthquake-prone California and the Pacific Coast.
“The existing seismic network tends to have high-precision instruments, but is relatively sparse, whereas this gives you access to a much denser array,” Ajo-Franklin said. “These systems are sensitive to changes of nanometers to hundreds of picometers for every meter of length. That is a one-part-in-a-billion change.”
“The beauty of fiber-optic seismology is that you can use existing telecommunications cables without having to put out 10,000 seismometers,” Lindsey said. “You just walk out to the site and connect the instrument to the end of the fiber.”
During the underwater test, according to a University of California, Berkeley, press release, the technique allowed the researchers to measure a broad range of frequencies of seismic waves from a magnitude 3.4 earthquake that occurred 45 kilometers inland near Gilroy, California, and map multiple known and previously unmapped submarine fault zones, part of the San Gregorio Fault system. They also were able to detect steady-state ocean waves — so-called ocean microseisms — as well as storm waves, all of which matched buoy and land seismic measurements.
Ajo-Franklin plans to next test fiber-optic monitoring of seismic events in a geothermal area south of Southern California’s Salton Sea. This work, partnering with LBNL where Ajo-Franklin remains a faculty scientist, will also evaluate temperature profiling using the same network.
“We’re going to do seismic imaging over an active geothermal zone there to see if we can identify the faults that provide fluids to deep reservoirs,” he said. “It’s a dark fiber project where we’re planning to utilize fiber owned by a telecom company.”
Some of that work will come to his new lab, Ajo-Franklin said. “We just purchased a DAS box for Rice and will have a facility to test cables and try different installations and special environments,” he said. “And we’ll use this technology in field deployments around the world in places that are seismically interesting or have structural imaging targets.”
Though southeast Texas lacks the seismic activity that characterizes regions around fault zones, Ajo-Franklin plans to take full advantage of benefits he sees in the local environment.
“If you’re interested in ambient noise imaging, this is actually a good place to be,” he said. “We work a lot on what’s called ambient noise seismology, using all the random noises propagating through the Earth. If you do a bit of signal processing, you get something that looks like an active-source seismic section.”
He said DAS could be used to measure the effects of hydraulic fracturing for oil production, as well as wave action in the Gulf of Mexico and local hydrogeologic processes.
The research was funded by the U.S. Department of Energy, the National Science Foundation, and the David and Lucile Packard Foundation.
– NOVEMBER 26, 2019
Richard Gordon is honored by scientific society
AAAS fellows are elected by their peers, and Gordon and Miranda are among 443 new fellows announced this week by the 120,000-member association. Fellows are selected for scientifically or socially distinguished efforts to advance science or its applications.
Gordon, the W.M. Keck Professor of Earth, Environmental and Planetary Sciences, was selected “for distinguished contributions to the fields of tectonic geophysics and geodesy through forefront research on diffuse oceanic plate boundaries and true polar wander.”
Gordon joined Rice in 1995 and studies the movement and deformation of tectonic plates, the patterns those movements leave in the paleogeographic record and the geophysical consequences of tectonic motions and deformation, including earthquakes, the building of mountain ranges and the evolution of ocean basins. He has discovered and investigated a number of diffuse boundaries that separate distinct tectonic plates in the oceans, and his investigations of “true” polar wander, instances when Earth shifted relative to its spin axis, have helped explain puzzling features of the paleomagnetic record. Gordon’s many honors include the American Geophysical Union’s Macelwane Medal in 1989 and the Geological Society of America’s Day Medal in 2002.
The 2019 AAAS Fellows will be acknowledged in the Nov. 29 issue of Science and honored at a Feb. 15 ceremony at the 2020 AAAS annual meeting in Seattle.
Sulfur isotopic compositions of deep arc cumulates
Cin-Ty A. Lee, Monica Erdman, Wenbo Yang, Lynn Ingram, Emily J. Chin, Donald J. DePaolo
Earth and Planetary Science Letters, Volume 500, 15 October 2018, Pages 76-85
Heavy sulfur isotopic compositions of arc lavas suggest a seawater component in the sulfur budget of arc lavas, but whether the seawater signature derives from the subducting slab or from magma interactions with lithologies in the upper plate is unclear. To see through the effects of degassing or crustal processing, a study was conducted on the S isotopic composition of deep arc cumulates from 45–90 km beneath the Sierra Nevada batholith in California, a Cretaceous continental arc. These cumulates represent the crystal line of descent from magmatic differentiation of hydrous arc basalts. The deepest (up to 60–90 km) and most primitive cumulates are low in Fe and have high molar Mg/(Mg + Fe), whereas the shallow and more evolved cumulates are high in Fe and have low Mg/(Mg + Fe). Bulk rock S correlates with Fe and negatively with Mg/(Mg + Fe). The most primitive cumulates are isotopically similar to the Earth’s mantle whereas the more evolved cumulates are heavier by 6‰ in the direction of seawater sulfate. The mantle-like S isotopic signatures of the primitive cumulates indicate that the contribution of slab-derived sulfate to arc lavas may not be as large as widely thought. Heavy S isotopic signatures are seen only in the evolved arc cumulates, which suggests that the seawater signature of arc lavas may not all derive directly from the slab, but perhaps during magma interaction with pre-arc crust. In continental arcs, pre-arc crust is dominated by accreted marine metasediments and metabasalts, and in island arcs, by seawater altered oceanic crust in the upper plate. The limited contribution of slab sulfate to the mantle source of Sierran arc magmas, if generalizable, suggests that sulfate in the subducting slab is efficiently released well before the arc magmatic front. Such a scenario would be consistent with the higher solubility of sulfate in aqueous fluids compared to that of sulfide. In summary, it is suggested here that the upper plate, in the form of seawater altered crust and sediments, may be as or more important for the sulfur budget in arc magmas than the subducting slab. Early loss of sulfate from the slab during subduction suggests that the dominant S species transported to the deep mantle is in the reduced form – sulfide.
Trace elements and U-Pb ages in petrified wood as indicators of paleo-hydrologic events
Hehe Jiang, Cin-Ty Lee, William G.Parker
Chemical Geology, Volume 493, 20 August 2018, Pages 266-280
Subsurface fluid systems are important for chemical weathering, ore formation and thermal evolution of the crust. Changes in the dynamics and distribution of subsurface fluid flow systems are controlled by changes in global and regional terrestrial climate, tectonics, and elevation. This paper concerns the dating of changes in ancient subsurface hydrologic systems. However, direct dating of water-rock interaction is challenging because of the lack of appropriate materials to date and the more open and complex nature of subsurface flow regimes. Here, we explore the prospects of using U-Pb dating of petrified (silicified) wood as a means of quantifying continental paleo-hydrology. Oxidizing fluids, often of meteoric origin, tend to leach and mobilize U from the country rock, but when such waters contact organic-rich material, U can become reduced and immobilized, resulting in U-rich silicified wood. We present in situ laser ablation ICPMS analyses of U-Pb isotopes and trace elements in petrified wood from the Upper Triassic Chinle Formation (225–208 Myr) in the Petrified Forest National Park in Arizona (USA), allowing us to establish a generalized workflow for making meaningful paleo-hydrologic interpretations of the U-Pb systematics of silicified wood. Wood characterized by brownish colors and preservation of cellular structure have low Fe contents and positive Ce anomalies, indicating silicification in reducing environments and isolation in relatively reduced conditions after silicification, resulting in closed system behavior of U and Pb. Wood characterized by vivid colors (orange, red, etc.) and little to no preservation of cellular structure are much higher in Fe and exhibit negative Ce anomalies, indicating influence by more oxidized fluids. The brownish samples yield U-Pb ages clustered between 250 and 200 Ma with a peak coinciding with the time of deposition (~220 Ma), which indicates that fossilization largely took place almost immediately after deposition and that U-Pb in quartz faithfully retains the time of such fossilization. In contrast, the orangish-reddish-whitish samples yield younger U-Pb ages, defining distinct errorchron ages, which reflect subsequent generations of quartz crystallization. Scatter associated with errorchrons are likely due to local (mm- to cm-scale) U or Pb mobility or variable initial Pb composition. Distinct younger age peaks appear to correlate with the timing of regional unconformities associated with tectonic or epeirogenic uplift. We suggest that uplift and exhumation may initiate the onset of oxidizing fluid systems, resulting in leaching and transport of U from the surroundings, followed by subsequent generations of quartz precipitation. In summary, U-Pb dating of petrified wood or silicified organic material, has high potential for dating paleo-hydrologic events. However, due to complexities in terrestrial hydrologic systems, interpretations of U-Pb systematics must be informed by accompanying geochemical and textural observations.
The redox “filter” beneath magmatic orogens and the formation of continental crust
Ming Tang, Monica Erdman, Graham Eldridge and Cin-Ty A. Lee
Science Advances 2018: Vol. 4, no. 5, eaar4444, DOI: 10.1126/sciadv.aar4444
The two most important magmatic differentiation series on Earth are the Fe-enriching tholeiitic series, which dominates the oceanic crust and island arcs, and the Fe-depleting calc-alkaline series, which dominates the continental crust and continental arcs. It is well known that calc-alkaline magmas are more oxidized when they erupt and are preferentially found in regions of thick crust, but why these quantities should be related remains unexplained. We use the redox-sensitive behavior of europium (Eu) in deep-seated, plagioclase-free arc cumulates to directly constrain the redox evolution of arc magmas at depth. Primitive arc cumulates have negative Eu anomalies, which, in the absence of plagioclase, can only be explained by Eu being partly reduced. We show that primitive arc magmas begin with low oxygen fugacities, similar to that of mid-ocean ridge basalts, but increase in oxygen fugacity by over two orders of magnitude during magmatic differentiation. This intracrustal oxidation is attended by Fe depletion coupled with fractionation of Fe-rich garnet. We conclude that garnet fractionation, owing to its preference for ferrous over ferric iron, results in simultaneous oxidation and Fe depletion of the magma. Favored at high pressure and water content, garnet fractionation explains the correlation between crustal thickness, oxygen fugacity, and the calc-alkaline character of arc magmas.
Charles Duncan Award for Outstanding Academic Achievement
Rajdeep Dasgupta, professor of Earth, environmental and planetary sciences, received the Duncan Award, which is presented by Rice deans upon the recommendation of senior faculty. It honors tenure-track or tenured faculty members who have less than 10 years of experience.
Cin-Ty Lee, Hehe Jiang, Elli Ronay, Daniel Minisini, Jackson Stiles, Matt Neal
Scientific Reportsvolume 8, Article number: 4197 (2018)
On greater than million year timescales, carbon in the ocean-atmosphere-biosphere system is controlled by geologic inputs of CO2 through volcanic and metamorphic degassing. High atmospheric CO2 and warm climates in the Cretaceous have been attributed to enhanced volcanic emissions of CO2 through more rapid spreading at mid-ocean ridges and, in particular, to a global flare-up in continental arc volcanism. Here, we show that global flare-ups in continental arc magmatism also enhance the global flux of nutrients into the ocean through production of windblown ash. We show that up to 75% of Si, Fe and P is leached from windblown ash during and shortly after deposition, with soluble Si, Fe and P inputs from ash alone in the Cretaceous being higher than the combined input of dust and rivers today. Ash-derived nutrient inputs may have increased the efficiency of biological productivity and organic carbon preservation in the Cretaceous, possibly explaining why the carbon isotopic signature of Cretaceous seawater was high. Variations in volcanic activity, particularly continental arcs, have the potential of profoundly altering carbon cycling at the Earth’s surface by increasing inputs of CO2 and ash-borne nutrients, which together enhance biological productivity and burial of organic carbon, generating an abundance of hydrocarbon source rocks.
An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle
Julia Ribeiro and Cin-Ty Lee
The depth of slab dehydration is thought to be controlled by the thermal state of the downgoing slab: cold slabs are thought to mostly dehydrate beneath the arc front while warmer slabs should mostly dehydrate beneath the fore-arc. Cold subduction zone lavas are thus predicted to have interacted with greater extent of water-rich fluids released from the downgoing slab, and should thus display higher water content and be elevated in slab-fluid proxies (i.e., high Ba/Th, H2O/Ce, Rb/Th, etc.) compared to hot subduction zone lavas. Arc lavas, however, display similar slab-fluid signatures regardless of the thermal state of the slab, suggesting more complexity to volatile cycling in subduction zones. Here, we explore whether the serpentinized fore-arc mantle may be an important fluid reservoir in subduction zones and whether it can contribute to arc magma generation by being dragged down with the slab. Using simple mass balance and fluid dynamics calculations, we show that the dragged-down fore-arc mantle could provide enough water (∼7–78% of the total water injected at the trenches) to account for the water outfluxes released beneath the volcanic arc. Hence, we propose that the water captured by arc magmas may not all derive directly from the slab, but a significant component may be indirectly slab-derived via dehydration of dragged-down fore-arc serpentinites. Fore-arc serpentinite dehydration, if universal, could be a process that explains the similar geochemical fingerprint (i.e., in slab fluid proxies) of arc magmas.