February 5, 2021
3:00pm in Zoom
Cailey Condit from University of Washington
Slow earthquakes in subduction zones: constraints from the geologic, petrologic, and constituent realms

Abstract: Subduction zones host destructive megathrust earthquakes, one of the deadliest natural hazards on earth. Deformation within these zones is largely localized along the subduction plate interface, a lithologically heterogenous and chemically active fault and shear zone. At the transition between the shallow locked and deeper creeping interface, we have recently recognized a new set of slip behaviors: episodic tremor and slip (ETS) often called slow earthquakes. ETS occurs in predominately warm subduction zones below the seismogenic zone in a fluid rich environment at near lithostatic pore fluid pressures. Occurring episodically over weeks to months, these slow earthquakes may ultimately trigger megathrust events and play a key role in the slip budget of some subduction zones. However, we currently do not have a mechanistic understanding of how they occur.

In this talk I will show observations from the exhumed rock record, results from petrologic modeling, and analysis of constitutive relations to offer new constraints on the mechanisms of slow earthquakes. I will show that viscous deformation cannot accommodate slow slip strain rates along the plate interface, and that frictional deformation made possible by elevated pore fluid pressures is important. I will also demonstrate that in situ metamorphic dehydration from the subducting slab is a ready source of fluids for these high pore fluid pressures, indicating that metamorphism is an important process in the production of deep slow earthquakes.

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February 12, 2021
3:00pm in Zoom
Maya Gomes from Johns Hopkins University
How do sulfur isotopes record information about Earth’s earliest ecosystems?

Abstract: The sedimentary pyrite sulfur isotope (d34S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite d34S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide d34S geochemistry. Pyrite d34S values often capture d34S signatures of porewater sulfide at the location of pyrite formation. However, microbial mats are dynamic environments in which biogeochemical cycling shifts vertically on diurnal cycles. Therefore, there is a need to study how the location of pyrite formation impacts pyrite d34S patterns in these dynamic systems. I will present diurnal porewater sulfide d34S trends and d34S values of pyrite and iron monosulfides from Middle Island Sinkhole, Lake Huron. The sediment water-interface of this sinkhole hosts a low-oxygen cyanobacterial mat ecosystem, which serves as a useful location to explore preservation of sedimentary pyrite d34S signatures in early Earth environments. I will show that, despite large (up to ~25‰) variations in d34S values over cm-depth scales linked to changes in net sulfate consumption in sediment pore waters, d34S values of pyrite are similar to porewater sulfide d34S values near the mat surface. Oxidative sulfur cycling and other microbial activity promotes pyrite formation in and immediately adjacent to the microbial mat and that iron geochemistry limits further pyrite formation with depth in the sediment. These results imply that primary d34S signatures of pyrite deposited in organic-rich, iron-poor microbial mat environments capture information about microbial sulfur cycling and environmental conditions at the mat surface and are only minimally affected by deeper sedimentary processes during early diagenesis.

February 19, 2021
3:00pm in Zoom
Carl Tape from University of Alaska, Fairbanks
Earthquakes and Earth Structure in the EarthScope Era (2014-present)

Abstract: The subsurface structure of Alaska contains clues to its formation and evolution over geologic time. Today, tectonic forces of subduction and collision are manifest in earthquake activity across the entire state. The NSF-sponsored EarthScope Seismic Array was fully deployed in 2017, enabling new high-quality geophysical data collection in large, remote regions of the state. I will discuss recent and ongoing studies of earthquakes and seismic imaging in Alaska, with a focus on crustal faulting. I will also convey how 3D seismic wavefield simulations can be used to understand the complexities of the recorded wavefield, as well as to guide improvements to tomographic models of crustal and upper mantle structure.

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March 12, 2021
3:00pm in Zoom
Forrest Horton from WHOI
A carbonatite volcano provides a fortuitous window into the deep carbon cycle

Abstract: The origin of carbonatites—unusual igneous rocks composed mostly of carbonate minerals—is a classical geology problem that remains largely unresolved. Do carbonatitic magmas form (i) during low-degree partial melting of the mantle, (ii) by melting of metasedimentary material, (iii) via liquid immiscibility with silicate magmas, or (iv) by some combination of these processes? A solitary carbonatite volcano in Afghanistan provides new insights. The location and 40Ar/39Ar eruption age of the volcano provide unambiguous tectonic context. The isotopic compositions (thallium, strontium, and neodymium) of the Afghanistan carbonatites indicate that they contain recycled components and constrain the timescales of recycling. These observations emphasize the critical role of carbonatitic magmas in the deep carbon cycle.

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March 26, 2021
3:00pm in Zoom
Rajdeep Dasgupta from Rice University
The Acquisition of Life-essential Volatile Elements (mostly Nitrogen) on Earth and Other Inner Solar System Rocky Bodies

Abstract: Long-term chemical habitability of rocky planetary surfaces rely heavily on the supply of life-essential, volatile elements from the interior. Therefore, the origin and abundance of carbon (C), sulfur (S), nitrogen (N), and hydrogen (H) in the combined atmosphere-crust-mantle reservoir are key. Most studies to date approached the origin of major volatiles on Earth and rocky Solar System planets using isotopic fingerprinting of putative, undifferentiated building blocks and their comparison with terrestrial rocks. However, a critical step of planetary accretion, i.e., core-mantle-atmosphere differentiation for planetesimals, planetary embryos, and growing planets are often overlooked. In this talk, I will discuss some new observations both from laboratory experiments and meteoritics to constrain the origin of terrestrial major volatiles in general and nitrogen (and to some extent carbon) in particular. A byproduct will be constraints on the conditions and styles of rocky planet formation and growth that likely led to Earth-like rocky planets with their observed inventory of major volatiles.

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April 9, 2021
3:00pm in Zoom
Michael Manga from Berkeley
Submarine volcanic eruptions

Abstract: I will summarize some open questions about underwater volcanic eruptions and what we have learned from a couple that occurred in the past decade. I will address the following questions: how are submarine eruptions different from those on land? What is the fate of erupted material?

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April 16, 2021
3:00pm in Zoom
Chandra Turpen from UMD
The Access Network: Organizing with Students around Equity

Abstract: The Access Network is an organization that supports vibrant interactions among students and faculty who advocate for equity work in the physical sciences across nine university-based sites. While each site differs, they share a commitment to five core principles: 1) fostering supportive learning communities, 2) engaging students in authentic science, 3) developing students’ professional skills, 4) empowering students to take ownership of their education, and 5) increasing diversity and equity in the physical sciences. The talk will start by sharing our network’s goals and activities. The second half of this talk will use McGee and Bentley’s framework of “equity ethic” (EE) to understand how Access student leaders adopt and refine a commitment to equity and social justice work within the physical sciences. In McGee and Bentley’s study of STEM students of color, they define EE as students’ sense of altruism and collectivism within and outside of their communities. Through interviews with student leaders, we model components of students’ EEs and how their EEs are influenced by their participation in Access. Student accounts illustrate that they are invested in improving equity within their disciplinary communities and see progress toward these goals as an important measure of success. Our findings highlight how students are already infusing an EE into their professional physics activities. This research suggests that student leaders benefit from having opportunities to articulate and refine critiques of disciplinary culture, and connect their EE to their professional practices.

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April 23, 2021
3:00pm in Zoom
James Kirkpatrick from McGill University
How fault surface geometry controls earthquakes

Abstract: Earthquakes happen when faults are pushed to failure by plate motions. The geometry of fault surfaces is thought to control both fault strength and the rupture process. However, because earthquakes rupture faults at depth beneath the Earth’s surface, active faults are difficult to observe and relating earthquake behaviour to a specific fault attribute remains challenging. Here, I will present field observations of exposures of ancient, exhumed faults, which demonstrate that at meters-scale, all faults exhibit a fractal-like geometry. This fundamental geometry is a function of the strength of the rocks they cut, and is fractal because rock strength varies with scale. However, fault surfaces are also complex. They are always smoother in the direction of slip because they wear down during slip. To investigate fault geometry at the scale of earthquake ruptures, I will compare the field observations to 3-D seismic reflection data from the Costa Rica subduction zone. The megathrust fault shows characteristics similar to those observed in the field, but the geometry is spatially variable with a characteristic length scale comparable to the spacing of normal faults in the lower plate (a few kilometers). Geomorphological and seismological characteristics of the subduction zone suggest that the geometric variability causes strength heterogeneity that controls earthquake rupture characteristics.

April 30, 2021
3:00pm in Zoom
Aradhna Tripati from UCLA
Frontiers of carbonate clumped isotope geochemistry, within an inclusive science framework

Abstract: The emergence of new proxies enables us to resolve fundamental questions about Earth’s evolution. A promising tool for the study of environmental change through time is the carbonate clumped isotope thermometer. In principle, this technique can provide a thermodynamically based estimate of carbonate mineral formation temperature and a relatively assumption-free calculation of water 18O/16O ratios. Over the past fourteen years, I have studied the systematics of carbonate clumped isotopes in foraminifera and coccoliths and other geological archives including lacustrine and soil carbonates. In this talk, I will discuss contributions I have made to analytical advances to develop its usability for paleoceanographic and paleoclimatic reconstructions, highlight applications to reconstruct ocean temperatures and terrestrial hydroclimates in the past, including proxy-model comparisons to understand underlying dynamical mechanisms, and new developments relating to the physical geochemistry of clumped isotopes in carbonates where we have used both theory and experiments to quantify kinetic isotope effects, and have developed the novel capability to make measurements of multiple clumped isotope species in carbonates. I will describe the social context for this scientific research, and how this has led to efforts within my research group, in the Center for Diverse Leadership in Science, and with partners, to support belonging, equity, justice and innovation in the geosciences and within higher education.

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

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