EPSS Colloquium - fall-2019

Oct. 1, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Curtis Williams - UC Davis

The source of volatiles in Earth’s interior place first order constraints on planet formation models, including accretion timescales, thermal evolution, composition, and bulk planetary redox states. The ratio of the two primordial neon isotopes, 20Ne/22Ne, provides a powerful tool to assess the source contributing volatiles to Earth’s mantle as the 20Ne/22Ne ratio is significantly different for the three potential sources: nebular gas, solar wind irradiated material, and CI chondrites. However, accurately determining the 20Ne/22Ne ratio of Earth’s interior is challenging because of pervasive atmospheric contamination and the low abundance of neon in mantle-derived basalts. In this presentation, I will show new high-precision neon isotopic measurements that provide robust evidence for a reservoir of nebular gas preserved in Earth’s deep mantle today. This observation requires the proto-Earth to have grown large enough to have gravitationally captured and dissolved nebular gases into a magma ocean prior to dissipation of nebular gas in the protoplanetary disk. This is consistent with the inference of planet formation at ~1 AU in a gas-rich, nebular environment using the Atacama Large Millimeter Array. Therefore, the gravitational capture of nebular gases could be a common feature associated with the embryo stage of terrestrial planet formation. Finally, I observe distinct 20Ne/22Ne ratios between mantle plumes derived from the Earth’s lower mantle and mid-ocean-ridge basalts that samples the upper mantle, requiring limited mass exchange between these reservoirs over Earth’s history. Addition of a chondritic component to the shallower upper mantle during the later stages of Earth’s accretion and recycling of seawater-derived neon during plate tectonic processes can explain the distinct 20Ne/22Ne ratio.

Oct. 8, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Ralph Lorenz - JHU

Saturn's giant moon Titan has been revealed to be remarkably Earth-like, with a landscape of vast dunefields, river channels and lakes under a smoggy sky punctuated by methane downpours. Titan serves as a frigid laboratory in which the same processes that shape our own planet can be seen in action under exotic conditions. Titan has a rich inventory of complex organic molecules that may provide clues how the building blocks of life are assembled. I will review findings from the epic Cassini-Huygens mission, at Saturn 2004-2017. I will also discuss prospects for future exploration: NASA recently announced the selection of the JHU Applied Physics Lab's Dragonfly concept for the next New Frontiers mission. Dragonfly will launch in 2026, to arrive in 2034: it is a rotorcraft lander, able to repeatedly take off and fly tens of kilometers in Titan's dense atmosphere and low gravity to sample the surface composition in a wide range of geological settings. Initially landing in the dunefields nearby, it will traverse to the 80km Selk impact crater, making geomorphological, meteorological and even seismological investigations over more than 2 years.

Oct. 15, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Emily Cooperdock - USC

Serpentinized ultramafic rocks are found in most subduction-related metamorphic complexes as massive bodies, exotic blocks, or fine-grained matrix within shear zones. When present, serpentinites play a critical role in fluid-transport, chemical cycling, and rheologic boundaries within subduction zone complexes. However, their propensity for fluid alteration and deformation can lead to a complex chemical and physical history that is often difficult to interpret. Here we study the multi-stage history of the serpentinites associated with the subduction complex on Syros, Greece. We use major, trace element, and stable isotope geochemistry to investigate the melt and fluid-history of serpentinites, and magnetite (U-Th)/He thermochronometry and trace elements to constrain their low-temperature cooling history. On the island of Syros, Greece, serpentinites are associated with remarkably preserved blueschist-and-eclogite-facies rocks that experienced HP-LT subduction in the Eocene. Whole rock trace and major element geochemistry, and stable isotope (δD and δ18O) analyses on the serpentinites indicate that the precursor mantle rocks likely derived from a mid-ocean ridge or hyper-extended margin, and experienced low-temperature serpentinization by seawater, with minor overprinting by oxidized, sedimentary fluids during subduction. Magnetite (U-Th)/He thermochronometry of internal fragments from large grains within a chlorite schist and a serpentinite record Mid-Miocene exhumation-related cooling ages, whereas smaller grains from the serpentinite record mineral formation associated with Pliocene normal faulting. Magnetite trace elements record evolving fluid-chemistry during magnetite growth associated with blackwall alteration and more recent faulting. These results reveal evidence for multiple episodes of fluid-rock alteration from pre- to post-subduction, which has implications for the cooling history and local geochemical exchanges of this HP-LT terrane. Particularly, magnetite trace element and (U-Th)/He thermochronometry is a relatively new technique that can provide key timing constraints on fluid-rock interactions.

Oct. 22, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Kate Maher - Stanford

Earth’s biosphere is thought to exert a substantial influence on regolith evolution and chemical weathering rates. However, ecosystems are also highly efficient at retaining and recycling nutrients. Thus, although the ecological demand for rock-derived nutrients (e.g., P, Ca, K) may exceed the rates of regolith supply, ecosystems use a variety of retention and recycling strategies to armor themselves against nutrient limitations. To evaluate the balance between nutrient recycling and new nutrient input, we combined a plant model with a weathering model that accounts for erosion, water flow, regolith thickness, mineral solubilization rates, secondary minerals, and nutrient storage in organic and mineral phases. Although the model predicts strong correlations between weathering and plant growth, the relationship is due to the underlying dependence of both on nutrient rejuvenation from erosion. Collectively, model results suggest that plant productivity is not an unmitigated driver of weathering rates but is instead a commensal partner in regolith evolution. The model results also place constraints on how the biosphere may have influenced the relationship between silicate weathering rates and atmospheric carbon dioxide through Earth history.

Oct. 29, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Christopher Russell - UCLA EPSS

The Sun is powered by a nuclear furnace deep inside. This energy source is very constant but the outer layers of the Sun are very dynamic and have a 22 year magnetic cycle that can have important effects on our technological systems. We look at the behavior of the Sun and its periodicity; how the solar wind and Earth are affected by the solar cycle and solar activity and how we need to study the Sun to fully understand it.

Nov. 5, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Heather Knutson - Caltech

Nearly a decade has passed since the discovery that planets with sizes intermediate between that of the Earth and Neptune (“super-Earths” or “mini-Neptunes”, depending on their densities) dominate the observed population of close-in exoplanets. These planets have no solar system analogue, yet 30% of Sun-like stars appear to have at least one (and often more) interior to Mercury’s orbit. Did these planets form in situ, or did they migrate inward from a more distant formation location? Either way, the implications for our understanding of planet formation are bound to be significant. In my talk I will describe current efforts to address this question by characterizing the bulk densities and compositions of these planets and searching for outer gas giant companions.

Nov. 12, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Clara Blattler - University of Chicago

The chemical composition of seawater is a critical boundary condition for reconstructing biogeochemical cycles, interpreting geochemical proxy measurements, and understanding the co-evolution of life and Earth’s surface environments. However, much about the evolution of seawater chemistry remains unknown, and very few quantitative constraints exist on Precambrian ocean composition. I will present a new method using the geochemical behavior of marine evaporite deposits to constrain seawater chemistry, in particular the relative concentrations of sulfate, calcium, and carbonate alkalinity. Using observations of evaporite deposits from the Neogene through the Archean, I will explore the implications of this work for understanding the Great Oxidation Event and Earth’s habitability.

Nov. 19, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Greg Beroza - Stanford

Earthquakes don’t happen on a schedule, so seismologists record continuous ground motion data, typically at 100 samples per second, 365 days a year. Increasingly capable and affordable sensors have led to a dramatic increase in the volume of continuous seismic data being recorded. Seismologists have to sift through those massive data sets to find not only the large, infrequent earthquakes, but also the much more numerous, but still important, small earthquakes. This need to extract as much information as we can from large data sets motivates a new generation of more efficient, robust, and scalable earthquake monitoring approaches based on machine learning techniques. I will present details of deep learning tools we have developed for earthquake signal detection, denoising, discrimination, and earthquake characterization. These algorithms employ different training strategies from unsupervised to supervised approaches using real or semi-synthetic data, but they are all implementations of deep neural networks. We use different network architectures based on the character of the problem to be solved. These include convolutional networks, convolutional-recurrent networks, and autoencoders. I will present examples that demonstrate the power of these approaches for developing dramatically improved earthquake catalogs and for providing new insight into earthquake processes.

Nov. 26, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• June Wicks - JHU

Interpretations of terrestrial exoplanet observations requires better knowledge of matter at extreme conditions, as planetary formation and subsequent evolution is fundamentally controlled by the properties of minerals at high pressure. In this talk I will describe how we use laser-driven dynamic compression to push experimental access to unprecedented pressures and temperatures. Using laser and x-ray laser facilities around the world, we mineral physicists have an ever-expanding set of drivers and diagnostics to interrogate the properties of matter. I’ll describe some of these experiments, interrogating the phase diagram and equation of state of our favorite terrestrial building blocks, and discuss how the high pressure frontier of science will contribute to the next generation of exoplanet questions.

Dec. 3, 2019
3:30 p.m. - 4:30 p.m.
3853 Slichter

Presented By:

• Anton Artemyev - UCLA

Seminar Description coming soon.