September 3, 2021
3:00pm in PLS 1140
Sarah Stamps from Virginia Tech
Clues About the Break-up of the African Continent

Abstract: The African continent is slowly fragmenting along the East African Rift System (EARS) across east Africa. What drives this fragmentation process and how exactly is the Earth's lithosphere moving along the EARS? In this seminar, I present results that shows how the tectonic plates of the EARS are slowly moving and deforming, which causes hazardous seismic and volcanic activity.  Using modern-day, high precision positioning measurements from Global Navigation Satellite Systems (GNSS) and advanced numerical methods, we investigate the physics driving surface motions to understand the driving forces causing the break-up of Africa. We find that deformation is characterized by east-west extension and the major forces driving extension are derived from topography gradients.

September 10, 2021
3:00pm in PLS 1140
Jonathan Tucker from Smithsonian
Linking deep volatile cycles from subduction to eruption

Abstract: The plate tectonic cycling of volatiles like water, halogens, and noble gases between Earth’s interior and exterior has been responsible for maintaining a habitable planetary surface environment for billions of years and is a key control on mantle convection and Earth’s thermal evolution. However, the history and mechanism of volatile exchange is poorly constrained. I will discuss novel observational evidence that deep Earth cycles of water, chlorine, argon, and xenon are linked and preserve seawater provenance throughout mantle circulation and their eventual eruption at mid-ocean ridges. The results imply that subduction of volatiles derived from unfractionated seawater is the major control on the distribution of diverse volatile species in the mantle. I will show that subducted hydrous minerals formed in equilibrium with seawater are incapable of preserving seawater noble gas signatures, requiring hydrous minerals to be formed in closed systems. Furthermore, I will use temporal constraints on xenon subduction to estimate that the water flux into the mantle during the Archean was much lower than modern rate, consistent with slower plate velocities or a change in the nature of convective recycling in the Archean.

September 17, 2021
3:00pm in PLS 1140
Freya George from Johns Hopkins University
From grain-scale records to global volatile cycling: rhythmic garnet zoning and mass transfer during subduction

Abstract: The chemical and isotopic zoning exhibited by many metamorphic minerals serves as a time-resolved record of processes occurring during both mineral growth and periods of diffusive relaxation. Using garnet from high pressure–low temperature metamorphic rocks derived from a global suite of subduction zones, oscillatory zoning—rhythmic cycles of peaks and troughs in element concentration—is shown to be near ubiquitous. Therefore, this zoning likely reflects some fundamental dynamic or petrogenetic process operating during garnet crystallization in these settings of significant mass transfer. Here, oxygen isotopes are used to explore the notion that elemental oscillations are buffered by infiltrating grain boundary fluids. Our results imply that fine-scale elemental and isotopic zoning in garnet is decoupled, and may be driven by distinct processes of dispersal and advection. Data and modeling results presented here also suggest that while local fluid-filled porosities generally behave as closed systems during prograde metamorphism, grain boundaries may be transiently flushed and buffered by large volumes of external fluids without a need for evident fluid channelization structures. Together, these norms and exceptions point to the significant spatiotemporal heterogeneity of these settings, and therefore the continuing opportunities the geological record presents to assess the nature of global volatile cycling.

September 24, 2021
3:00pm in PLS 1140
Delta Merner from Union of Concerned Scientists
Science, Climate Litigation, and the Law

Abstract: The science is clear: emissions of heat-trapping gases, especially from the burning of fossil fuels, are driving global climate change. Scientists across a range of disciplines are helping to inform societal decisions about how to limit the effects of climate change, protect people from climate impacts, and hold accountable those responsible for climate-related damages. As climate change impacts accelerate and powerful interests continue to stand in the way of science-based climate policy, communities on the frontlines are increasingly turning to the courts. Litigation may be an important tool to hold governments and businesses accountable for actions and inactions that have contributed to climate change’s advance. Climate litigation, informed by science, may help those affected take action against responsible parties to limit climate harms.

This talk will discuss the role of science it climate litigation, with a specific focus on advances in attribution science. We will also look at climate litigation trends and current cases in Maryland. Finally, well discuss how different research, including your own, can help inform litigation. The speaker will happily discuss career paths for PhDs outside of the academy.

October 8, 2021
3:00pm in PLS 1140
Shreeram Inamdar from University of Delaware
Ghosts of landuse past: How have milldams and their legacies influenced riparian ecosystems and what are the implications for watershed management?

Abstract: Hundreds to thousands of milldams were constructed every few miles on creeks and rivers by early settlers in the mid-Atlantic USA. Widespread agricultural erosion coupled with the dams resulted in large amounts of sediment deposition in mid-Atlantic valley bottoms. This legacy sediment deposition has significantly altered the structure and functions of riparian ecosystems.  While most of the milldams are gone, many still remain and continue to affect riparian ecosystem processes. In this talk, I will highlight some of the key questions that we are investigating through ongoing NSF and USDA grants, including: (a) How do existing milldams affect hydrologic conditions, redox environments, and nutrient cycling, upstream and downstream of milldams? (b) Do wet conditions upstream of milldams serve as hotspots for nitrogen removal? (c) How do riparian soils, groundwater, and stream water quality evolve after dam removal? And (d) What are the implications of these structures and their legacies for contemporary and future water quality and watershed management? Our early observations from these projects confirm some of our hypotheses but also reveal surprising and unexpected results. While wet conditions upstream of dams resulted in low nitrate-N in riparian soils (likely due to denitrification removal) we also found very high ammonium-N concentrations in near-stream soils and groundwater. Stream water-riparian groundwater interactions were very heterogenous and influenced by topographic and soil conditions unique to milldams. Milldam removals did not alter riparian groundwater and stream water nitrogen substantially, but did register a significant, rapid change in soil 15N isotopes. Given that riparian zones are a key component of the water quality portfolio, understanding these changes and legacy effects is critical for better management, policies, and decision making with regard to milldams and riparian ecosystems.

October 22, 2021
3:00pm in Virtual
Casey Honniball from NASA GSFC
Lunar Surface Hydration: A View from Earth

Abstract: Hydration on the lunar surface was first reported in 2009 by three spacecraft and manifested as a strong absorption at 3 µm. The hydration at 3 µm is produced by hydroxyl (OH) attached to metal cations and/or molecular water (H2O). Measurements of the 3 µm band were revolutionary but the returned spacecraft data have limitations in wavelength coverage, spatial resolution, global coverage, and lunar time of day. To bridge the gap, we use two Earth based observatories to characterize the true nature of the 3 µm band and to unambiguously detect molecular water. Using the SpeX infrared cross-dispersed spectrograph at the NASA InfraRed Telescope Facility (IRTF) at Maunakea Observatory we address diurnal variability of the 3 µm band and with the NASA/DLR Stratospheric Observatory For Infrared Astronomy (SOFIA) we observe the Moon at 6 µm where a purely H2O feature is exhibited.

Observations with the IRTF reveal total water (OH + H2O) abundances ranging from 0 to ~500 ppm H2O. From this new data set with improved thermal removal, we find diurnal variations of the 3 µm band along with variations with latitude and composition. We observe a decrease in abundance with increasing lunar local time, an asymmetric trend about the equator that favors the South, and higher abundances in highland regions. Data from SOFIA of the Clavius crater and surrounding region reveal abundances of ~100 to 412 ppm H2O from the 6 µm emission band that we attribute to molecular water on the Moon. All spectra acquired at the Clavius region exhibit a 6 µm emission band. We are unaware or any other lunar material that may exhibit an isolated 6 µm band. This is the first direct, unambiguous detection of H2O on the Moon outside the permanent shadows at the lunar poles.

November 12, 2021
3:00pm in PLS 1140
Jeff Freymueller from Michigan State University
Megathrust Slip Budget and Earthquakes along the Alaska Peninsula

Abstract: The 2020-2021 sequence of earthquakes offshore of the Alaska is of interest for several reasons. The first major earthquake in the sequence ruptured part of the famous Shumagin seismic gap, the second was a strike-slip earthquake that ruptured in the downgoing Pacific plate, and the third was the largest earthquake in the US since 1965. Campaign GPS survey data prior to the earthquakes, collected mainly from 1993-2016, also provided an unusually precise interseismic velocity field, which provided a detailed model for the along-strike variations in the interseismic slip behavior of the megathrust. In the eastern part of the Alaska Peninsula, a large fraction of the shallow megathrust is locked, but near the western end of the Peninsula the megathrust was steadily creeping. This talk will present the earthquake history of this region, the megathrust slip budget as inferred from the interseismic data, and studies of the megathrust earthquakes and their slip distribution.

November 19, 2021
3:00pm in PLS 1140
Mathurin Wamba from Princeton
Regional Tomography targeting mantle plumes by waveform inversion: the case of La Réunion Island

Abstract: Mid-plate volcanoes are well known as hotspots. They represent the surface signature of mantle plumes, nevertheless their origin and their role in geodynamics are still a challenge in the Earth sciences. Even though plate tectonics and mantle plumes were discovered at the same time, the latter cannot be explained by the former. Plumes’ birth, life and death play a fundamental role on the evolution of life on Earth and on plate-tectonic reorganization. La Réunion hotspot is known as one of the largest on the Earth, that created the Deccan volcanic traps in India (almost 2 million km2 ) and the death of more than 90 % of life on the Earth including dinosaurs ∼ 65Ma ago. So far the origin of the mantle plumes and their role in geodynamics are still unclear in Earth sciences. In that respect, we use the dataset from the French-German RHUM-RUM experiment around La Réunion hotspot (2012-2013), from IRIS data center and FDSN to extensively investigate the deep structure of the plume along its complete track from its birth to its present stage, as well as from the upper mantle to the lowermost mantle. The use of spectral element method allows us to perform forward modelling for several thousand paths across the Indian ocean. The sensitive matrix (first order approximation of the Hessian matrix) is built from the coupling of normal modes along and across the branches, by using nonlinear asymptotic theory. This waveform inversion approach enables us to resolve deep anomalies in the mantle underneath Indian ocean. The upper mantle beneath the Indian Ocean is fed by lowermost mantle structure rising from the core-mantle boundary. The lower mantle thermochemical dome associated to the South-African Large Low-Shear Velocity Province is found to be composed of several conduits. Plume branches are highlighted at ∼ 900 km depth. Thermal instability and thermochemical heterogeneities in the D" layer are likely the principal reasons of the plumes birth at the core-mantle boundary, and therefore an indicator of long-life of the Réunion hotspot.

The coordinator for the Colloquium Series is Dr. Megan Newcombe. You can contact her at

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