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Aug. 30 |
TBA |
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Sep. 6 |
Michael Coates The University of Chicago |
Fish placed: sharks in context of the new framework for early gnathostomes |
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Sep. 13 |
Jessica Creveling Oregon State University |
Spatial Variation in Late Ordovician Glacioeustatic Sea-Level Change Abstract: In this talk, I will explore the spatial and temporal variability of sea-level change induced by the glaciation and deglaciation of a modeled Late Ordovician ice sheet covering southern Gondwana. This exercise provides insight into which field sites can provide robust estimates of the magnitude of Late Ordovician glacioeustasy. |
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Sep. 20 |
Maxim Ballmer ETH Zurich |
Large-scale compositional heterogeneity in the Earth’s mantle Seismic imaging of subducted Farallon and Tethys lithosphere in the lower mantle has been taken as evidence for whole-mantle convection, and efficient mantle mixing. However, cosmochemical constraints point to a lower-mantle composition that has a lower Mg/Si compared to upper-mantle pyrolite. Moreover, geochemical signatures of magmatic rocks indicate the long-term persistence of primordial reservoirs somewhere in the mantle. In this presentation, I establish geodynamic mechanisms for sustaining large-scale (primordial) heterogeneity in the Earth’s mantle using numerical models. Mantle flow is controlled by rock density and viscosity. Variations in intrinsic rock density, such as due to small-scale heterogeneity in basalt or iron content, can promote layering or partial layering in the mantle. Layering can be sustained in the presence of persistent whole mantle convection due to the competition of mixing vs. “unmixing” of heterogeneity, e.g. in the transition zone or near the core-mantle boundary [1]. On the other hand, lateral variations in intrinsic rock viscosity, such as due to heterogeneity in Mg/Si, can strongly affect the mixing timescales of the mantle. As one end-member regime of mantle convection, intrinsically strong rocks may remain poorly mixed through the age of the Earth, and persist as large-scale domains in the mid-mantle due to focusing of deformation along weak conveyor belts [2]. That large-scale lateral heterogeneity and/or layering can persist in the presence of whole-mantle convection can explain the stagnation of some slabs, as well as the deflection of some plumes, in the mid-mantle. These findings indeed motivate new seismic studies for rigorous testing of model predictions. [1] Ballmer, M. D., N. C. Schmerr, T. Nakagawa, and J. Ritsema (2015), Science Advances, doi:10.1126/sciadv.1500815. [2] Ballmer, M. D., C. Houser, J. W. Hernlund, R. Wentzcovitch, and K. Hirose (2017), Nature Geoscience, doi:10.1038/ngeo2898. |
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Sep. 27 |
John Marshall Massachusetts Institute of Technology |
Inter-hemispheric asymmetries in climate: exploring the coupling between the Inter-tropical Convergence Zone and the ocean Using observations and a hierarchy of models we discuss the role that the ocean’s meridional overturning cells plays in modulating the position of the Inter-tropical Convergence Zone (ITCZ). We argue that the displacement of the ITCZ north of the equator is a consequence of the ocean’s deep overturning cell in the Atlantic, the AMOC, which warms the NH troposphere relative to the SH. Since the ITCZ is found in the warm hemisphere, in the annual-mean it resides in the NH. We also discuss the role of the ocean’s subtropical cells (STCs) in the Pacific which are strongly coupled to the trade winds. Heat transport by the STCs makes the position of the ITCZ over the ocean ‘sticky’, strongly damping meridional migrations. We conclude by discussing the implications of our study for understanding observed ITCZ migrations in the current and paleo climate. |
| Oct. 4 |
TBA |
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Oct. 11 |
Jim Gaherty Columbia University |
An ocean-bottom view of mantle flow and lithosphere formation beneath Earth’s largest tectonic plate What is a plate? What is its connection to the underlying convecting mantle? These questions are central to our understanding of global geodynamics and plate tectonics, but our knowledge of basic plate properties in even very simple geological regions is poor. Observations of strong positive seismic velocity gradients within the mantle lithosphere, and a very sharp and very shallow lithosphere-asthenosphere boundary (LAB), are inconsistent with modern theoretical models of oceanic plates. They imply that non-thermal factors such as bulk composition, mineral fabric, grain size, and dehydration play important roles in controlling the formation of the lithosphere, and thus the underlying LAB. Within the asthenosphere, estimates of mantle flow from seismic anisotropy vary widely, and the key driving mechanisms – absolute plate motion, small-scale convection, density-driven flow – are debated. There is little consensus on which of these factors are dominant, in part because observations of detailed lithosphere/asthenosphere structure are limited. To address this discrepancy, we conducted the NoMelt experiment on ~70 Ma Pacific lithosphere between the Clarion and Clipperton fracture zones. The experiment consists of a 600x400 km array of broad-band ocean-bottom seismometers (OBS) and magnetotelluric (MT) instruments, and an active-source reflection/refraction experiment. Located far from the ridge system, in an area with minimal post-formation intra-plate volcanism, the experiment provides a unique high-resolution view of the dominant processes associated with plate formation and dynamic flow in the underlying mantle. |
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Oct. 18 |
October recess (no talk) | |
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Oct. 25 |
Jennifer Kay University of Colorado Boulder (http://cires.colorado.edu/research/research-groups/jennifer-kay-group) |
Do Southern Ocean radiation biases and cloud feedbacks matter for 21st century warming? Equilibrium climate sensitivity, the warming in response to instantaneously doubling atmospheric carbon dioxide concentrations, is a common metric for climate model inter-comparison. In addition to being a well-established idealized benchmark, ECS has historically been correlated with warming in more realistic transient forcing experiments. In this talk, I will show that increased equilibrium climate sensitivity does not always lead to increased transient warming. A large, long-standing, and pervasive climate model bias is excessive absorbed shortwave radiation over the mid-latitude oceans, especially the Southern Ocean. Our recent work has “fixed” this bias using an observationally motivated modification to the shallow convection detrainment. In the improved model, we find the equilibrium climate sensitivity increases by 1.5 K. Yet, when this same “fix” is implemented in a transient climate simulation, the warming in the improved model remains very similar to the control. Why? In transient climate simulations, extratropical ocean heat uptake delays the positive cloud-sea surface temperature feedbacks over the Southern Ocean responsible for increasing equilibrium climate sensitivity. As a result, large changes in extratropical cloud biases and feedbacks have a limited affect on realistic transient warming simulations of the 20th and 21st century. |
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Nov. 1 |
Benjamin Mills University of Leeds |
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Nov. 8 |
Silliman Lecturer Richard Alley Penn State |
Avoiding optimistic extrapolations: history and physics for projecting sea-level rise
Ice sheets experienced major retreats in response to modest past warming, as shown by a growing body of data including the cosmogenic isotopes in the GISP2 rock core and the pulse of Pliocene diatoms preserved in Antarctica’s Sirius Formation. Future warming-induced ice-sheet retreat could be fast as well as large, based on physical understanding calibrated to recent ice-shelf losses and lake-drainage events. “Collapse” of Thwaites Glacier in West Antarctica, if triggered, could raise global sea levels more than 3 m in a human lifetime or less. Paleoclimatic and instrumental data are not especially useful for understanding the fastest possible changes, because we have no evidence that nature ever ran the experiment we may produce. Projections are highly sensitive to fracture behavior; uncertainties almost surely can be reduced, but are likely to remain large. Materials science generally shows that all extrapolations of material strength and stability are optimistic, a physical result that is worrisome for coastal people in a warming world with large ice shelves that could break rapidly.
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Nov. 15 |
Flint Lecturer Maureen Raymo Columbia University |
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| Nov. 22 | Thanksgiving recess (no talk) | |
| Nov. 29 |
Justin Strauss Dartmouth University |
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Dec. 6 |
Roberta Rudnick UC Santa Barbara |
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Dec. 8 |
Itay Halevy Weizmann Institute of Science |
Colloquium is held at 4:00 pm in KGL 123
Please address inquiries to the colloquium committee (Email: colloquium@earth.geology.yale.edu).
Contacts
AOCD: Nicole Shibley, Chris Kruse Geochemistry: Terry Tang, James Super, Robin Canavan Geophysics: Kierstin Daviau, Neala Creasy Paleontology: Janet Burke , Juri Miyamae Tectonics: Duncan Keller
Faculty: Kanani Lee, Alan Rooney