EPSS Colloquium - winter-2020
Cancelled
Jan. 7, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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This event has been cancelled.
Implications of post-fire erosion for persistence of pyrogenic carbon in soil
Jan. 14, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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Fire, erosion, and soil carbon (C) dynamics overlap in space and time. Increased rates of erosion typically follow wildfires, and fire-altered or pyrogenic C (PyC, also referred to as black carbon) is redistributed vertically within soil profiles and laterally to lower landform positions along hillslopes, changing its C sequestration trajectory. However, we currently lack sufficient understanding on how and why the interaction of fire and erosional distribution of soil materials control persistence of bulk soil organic matter (SOM) and PyC in dynamic landscapes. In this talk, we present results from wildfires that occurred in the Sierra Nevada Mountains (USA) to demonstrate how the composition (based on stable isotope composition of 13C and 15N, and NMR analysis of OM composition) and magnitude of pyrogenic carbon redistributed by soil erosion varies considerably depending on fire severity and geomorphology of the landscape. Our findings also show that PyC is preferentially transported by erosion in high severity burn slopes, compared to areas affected by low and medium severity fires. Findings of this study are critical for better integration of biogeochemical and geomorphological approaches to derive improved representation of mechanisms that regulate SOM persistence in dynamic landscapes that routinely experience more than one perturbation.
Global Seismic Imaging in the era of super-computers: on the morphology of compositionally distinct "piles" at the base of the mantle
Jan. 21, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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The existence of two large low shear velocity provinces (LLSVPs) at the base of the earth's mantle with a strong equatorial "degree 2" signature has been known since the development of the first global seismic tomographic models of the earth's lower mantle. In the last few decades, their existence has been confirmed in many studies, and they are generally associated with the global upwelling flow in the lower mantle, but their precise nature and role in the earth's evolution and present-day dynamics is still debated. Several indirect clues point to their stability for the last 250-300 Ma, and possibly much further back in geological time. It has been suggested that they may be of a different composition than surrounding regions and some authors have argued that they may be denser than the ambient mantle to a significant height above the CMB. One of the key questions in furthering our understanding of the LLSVPs is how high they extend as coherent and compositionally distinct structures above the core-mantle boundary (CMB). Because of their very strong long wavelength signature, which persists to at least 1500 km above the core-mantle boundary, a common assumption is that they are compact structures across that depth range. I will discuss these ideas in the light of recent full waveform tomography images that shows the presence of ~20 broad low velocity plumes, rooted in the LLSVPs, that rise quasi-vertically from the CMB to around 1000 km depth, and then meander through the upper part of the mantle towards major hotspots.
Tidal Tomography: What an often-neglected phenomenon known as Earth tides can tell us about buoyancy in the deepest part of the mantle
Jan. 28, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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Earth’s mantle is a key component of the Earth system: its circulation drives plate tectonics, the long-term recycling of Earth’s volatiles, and as such, holds fundamental implications for the Earth’s surface environment. In order to understand this evolution, a key parameter of the mantle must be known, namely its buoyancy. In this talk, I will discuss how Earth’s body tide can provide fresh and independent constraints on deep mantle buoyancy through a newly developed technique called Tidal Tomography. This comes at a time when other interesting and exciting data sets sensitive to deep mantle buoyancy, e.g., Stoneley modes, have been brought to bear, and we will explore our conclusions in the context of other recent finds.
At the speed of volcanic eruptions
Feb. 4, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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What happens under volcanoes in the months leading up to eruption? How does the magmatic system prepare for an eruption? And why are some eruptions more explosive than others? Crystal clocks are providing some answers to these questions. The timescales of chemical diffusion operate over minutes to years prior to eruption. This talk will examine new constraints on volcanic run-up, forecasting and eruption dynamics.
Earth's Continental Crust through time
Feb. 11, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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The nature of Earth's crust that lies above sea level (continental crust) is explored using geochemical data for fine-grained terrigenous sediments (loess, shales, and glacial diamictites). The data point to a fundamental change in the composition of the crust exposed to weathering and erosion during the Late Archean and may have implications for the dominant tectonic processes operating at these times.
Mapping mantle heterogeneities through redox state
Feb. 18, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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To further our understanding of the communication between the deep Earth and its surface and atmosphere, we explore how redox state affects mantle mineralogy, density, seismic velocities and melting and in turn what this could mean for convection and thermochemical evolution of the planet. Mantle ferric iron Fe3+content and its distribution are largely unconstrained and are influenced by the disproportionation of Fe2+ through the formation of Earth’s most abundant mineral bridgmanite, and the presence of other minor elements such as aluminum and water. Here we show that redox-induced density contrast (~1-2%) affects mantle convection and may potentially cause the oxidation of the upper mantle. Our geodynamic simulations suggest that such a density contrast causes a rapid ascent and accumulation of oxidized material in the upper mantle, with descent of the denser, reduced material to the core–mantle boundary. The resulting heterogeneous redox conditions in Earth’s interior may have contributed to the large low-shear velocity provinces in the lower mantle and the rise of oxygen in Earth’s atmosphere. Using a combination of laser-heated diamond anvil cell experiments, Monte Carlo simulations, geodynamic modeling, and seismic forward modeling calculations, our work shows the importance of ferric iron content and its effects on overall rock mineralogy in the lower mantle. Therefore, geophysical anomalies in the mantle may reflect not only differences in bulk composition or temperature, but also mantle oxidation state.
Taking the Temperature of Earthquakes
Feb. 25, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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Despite centuries of research, we still do not understand some of the most fundamental aspects of earthquakes including how they start, how they propagate, and why they stop. Grain-scale processes that control fault strength can be studied in lab experiments, but we are limited in making ties to field observation by our inability to pinpoint earthquake structures in natural fault zones. Here, I discuss using coseismic temperature proxies to map earthquake slip in the field, elucidate grain-scale strength at those temperatures, and estimate how energy is partitioned during earthquakes. During earthquakes, faults heat up due to their frictional resistance. Sometimes, the temperature rise during earthquakes makes the rocks hot enough to melt. However, solidified frictional melt (pseudotachylyte) is uncommon in the rock record, and other paleoseismic temperature proxies have only recently been established. The dearth of pseudotachylyte led researchers to hypothesize that faults get very weak during earthquakes, and hence do not produce much heat. If faults dramatically weaken during slip, there are important implications for how earthquakes propagate, and hence why some earthquakes grow to be very large. I use a new sub-solidus temperature proxy, biomarker thermal maturity, to identify temperature rise on faults in a variety of tectonic settings. With results from this new temperature proxy, we revisit some outstanding questions in fault mechanics such as: Where does earthquake slip occurs in a fault zone? Can creeping faults host earthquakes? Does lithology control rupture propagation? How is energy partitioned during earthquakes? and, How strong are faults during earthquakes?
Atmosphere-Mantle feedbacks: importance of mantle oxidation state
March 3, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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The oxidation state of a rocky planet’s mantle has a strong influence on the composition of secondary atmospheres formed through outgassing. Understanding how mantle oxidation state depends on planetary bulk composition and how it changes with time due to planetary differentiation, atmospheric outgassing, and interactions with a planet’s volatile envelope is crucial to accurately predicting atmospheric compositions of rocky exoplanets. Distinguishing biosignature gases from geosignatures (false positive biosignatures produced by natural geological processes) will depend on understanding the geological processes operating on planets of different size and composition compared to the Earth. In this talk, I discuss models that explore how bulk composition, atmospheric escape and planetary differentiation (metal-silicate separation) influence mantle oxidation state and the resulting outgassed atmosphere. These and future models can help select the best exoplanetary targets for detailed characterization by future telescopes like the James Webb Space Telescope (JWST) by identifying planets with lower chances for producing false positive biosignatures.
The role of dust on Earth’s climate: insights from the ice core record and modern ecosystems
March 10, 2020
3:30 p.m. - 4:30 p.m.
Slichter 3853
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It may seem insignificant, too tiny to have an impact on Earth’s surface, but it can travel thousands of miles, sustain life, and record variations in climate. Fine grained mineral particles—otherwise known as dust—can tell us about how ecosystems respond to aridification, land-use intensification, or rapid climate transitions both in the present and thousands to millions of years ago. In this talk, I will discuss how variations in the dust cycle alter soil nutrients and what geochemical variations of dust preserved in the ice core record can tell us about predominant wind directions, glacial sediment supply, and atmospheric transport.