EPSS Colloquium - fall-2015

Sept. 24, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Blarie Schoene - Princeton

How continental crust is created, preserved and recycled, and whether or not these processes have changed through Earth history are important for a) understanding the geochemical and petrological stratification of the crust and b) quantifying long term geochemical and isotopic cycling in the Earth’s crust and mantle. Developing models for crustal evolution requires robust geochronology on both the short and long timescales, targeting relatively rapid geologic phenomena (e.g. magma production and differentiation) as well as long term secular change. This contribution highlights recent efforts to better apply high-precision U-Pb geochronology to continental magmatic systems and to develop techniques comparing magmatic systems through Earth history.

Models describing the transfer of mass and heat through the crust during orogenesis demand age constraints with increasing precision and accuracy. While modern ID-TIMS U-Pb geochronology can resolve the timescales of zircon crystallization in single pulses of magma, much work is needed to relate dates to processes such as magma production, transport, differentiation, and emplacement. Our recent work focuses on integrating zircon crysallization ages and geochemistry to both understand the growth history of single zircons on <50 ka timescales and to build a framework for longer timescale geochemical evolution of two Alpine magmatic systems. To compare differences in magmatic differentiation during crustal magmatism from the Archean to present, we develop statistical methodologies for analyzing large geochemical databases (Earthchem, etc.). Substantial differences in both crustal inputs (basalts) and indicators of differentiation to high-Si compositions suggest either secular changes in magmatic/metamorphic processes during crustal genesis and modification, or preservation bias. These results motivate further detailed investigation of Archean terranes, although robust comparison between any number of orogenic belts, Archean or modern, require geochronology with precision that is relevant to tectonomagmatic processes. Sub-million year precision is now achievable in Archean rocks by ID-TIMS U-Pb geochronology, but necessitates careful integration of field, geochemical, and geochronological data with numerical modelling studies.

Oct. 1, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Lingsen Meng - UCLA

The 2015 Mw 7.8 Nepal-Gorkha earthquake with casualties of over 9,000 people is the most devastating disaster to strike Nepal since the 1934 Nepal–Bihar earthquake. Its rupture process is imaged by teleseismic back-projections (BP) of seismograms recorded by three large regional networks in Australia, North America and Europe. The source images of all three arrays reveal a unilateral eastward rupture. But the propagation directions and speeds differ significantly between different arrays. To understand the spatial uncertainties of the BP analysis, we image four moderate-size aftershocks recorded by all three arrays exactly as done for the mainshock. We find that the apparent source locations inferred from BPs are systematically biased from the catalog location, which can be explained by the uncertainty of the seismic velocity derived from the 1D reference model (e.g. PREM). We introduced a slowness error term in travel time that successfully mitigates the source location discrepancies of different arrays. The calibrated BPs are mutually consistent and reveal a unilateral rupture propagating eastward at a speed of 2.7 km/s over ~150km along the down-dip edge of the locked Himalaya thrust zone, predominantly located in a relatively narrow and deep swath. In that sense, the 2015 Nepal earthquake is an incomplete rupture that failed to rupture the entire Himalayan décollement to the surface, which can be regarded as an intermediate event during the interseismic period of larger and complete Himalyan ruptures. The high-frequency ground shaking in the near-field region rarely exceeds the European Macroseismic Intensity (EMS) 8, lower than anticipated by the Ground motion prediction Equations (GMPE) given the magnitude and proximity to the rupture. Such modest level of ground motions can be explained in part by the deep and thus further away high-frequency source radiations from the Kathmandu basin.

Oct. 8, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Seth John - USC

Zinc and cadmium are important micronutrients in the ocean, where biological productivity draws down Zn and Cd concentrations in surface waters and fractionates Zn and Cd stable isotopes. New global datasets of seawater dissolved stable isotope ratios (?66Zn and ?114Cd) provide unique insight into biogeochemical cycling of these elements. Cd is governed by simple nutrient dynamics, with preferential uptake of lighter Cd isotopes in the surface ocean and remineralization in the deep. A combination of observations, experiments, and modeling suggest that Zn cycling is more complicated, with an ‘inverse’ isotope profile caused by preferential adsorption of heavier Zn isotopes onto sinking particles. With a more complete understanding of Zn and Cd isotope cycling in the modern ocean, we can begin to use them as tracers in the geological record. For example, ?66Zn and ?114Cd in a post-snowball Earth cap carbonate hint at weak surface-ocean productivity during deglaciation.

Oct. 15, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Lorraine Lisiecki - UCSB

Although detailed age models exist for some marine sediment core records of the last glacial cycle (0-150 ka), age models for many other cores rely on the stratigraphic correlation of benthic d18O, which records changes in ice volume and deep ocean temperature. The large amount of data available for the last glacial cycle offers the opportunity to significantly improve upon benthic d18O compilations, such as the "LR04" global stack [Lisiecki and Raymo, 2005]. Not only are the age constraints for the LR04 stack now outdated, but a single global alignment target neglects regional differences of several kiloyears in the timing of benthic d18O change during glacial terminations. Here we present regional stacks that characterize mean benthic d18O change for eight ocean regions and a volume-weighted global stack of data from 263 cores. Age models for these stacks are based on radiocarbon data from 0-40 ka, correlation to a layer-counted Greenland ice from 40-56 ka, and correlation to radiometrically dated speleothems from 56-150 ka. These regional stacks offer better stratigraphic alignment targets than the LR04 global stack and, furthermore, suggest that the LR04 stack is biased 1-2.5 kyr too old throughout the Pleistocene. Finally, we compare global and regional benthic d18O responses with records of sea level and climate change over the last glacial cycle.

Oct. 22, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Tim Lyons - UCR

New geochemical proxy data are suggesting a far more dynamic history for initial oxygenation of Earth’s biosphere. Most notably, novel tracers are exposing a middle chapter in Earth history with surprisingly low oxygen contents in the atmosphere, the deep ocean, and the shallow ocean. These challenging conditions were the backdrop for the initial emergence of complex life and set the stage for the rise of animals. This talk will explore this critical interval in Earth’s evolution, the conditions before and after, the transitions, and the advances and debates about these first biological radiations.

Oct. 29, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Dave McComas (Distinguished Alumni) -

The Sun produces a million mile per hour wind of hot ionized gas that flows out all directions in space all of the time. This solar wind interacts with the planets and other objects in the solar system and, at Earth, produces both beautiful aurora and dangerous space weather that can kill orbiting satellites. Further out, the solar wind inflates a bubble in the local interstellar medium that helps protect the entire solar system from dangerous galactic cosmic radiation. Over the past decade our knowledge of the outer reaches of this bubble – our heliosphere – have grown immeasurably with both direct sampling by the two Voyager spacecraft in these distant reaches and the first remote imaging of the global interaction by the Interstellar Boundary Explorer – IBEX. This talk tells their story. More information here

Nov. 5, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Matt Siegler - SSI

We present evidence that the hydrogen deposits near the lunar poles are primarily off-polar and antipodal to each other. Using models of thermal stability of water ice, we find this is suggestive of the lunar pole having migrated from its current spin axis by >5.5 deg. The direction and magnitude of this inferred "True Polar Wander" are shown to correspond with a theoretical thermal anomaly below the Procellarum KREEP Terrain. In this sense, we believe the polar volatiles serve as a record of lunar interior evolution.

Nov. 12, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Issac Smith - SWRI

A wealth of information about the climate cycle and ice budget of Mars is recorded within the layers of Martian polar caps. Using primarily subsurface observations with the Shallow Radar (SHARAD) instrument on board Mars Reconnaissance Orbiter (MRO), along with optical observations of the surface, we study the stratigraphic record of the ice caps to detect alternating periods of accumulation and erosion.

Analysis of subsurface stratigraphy on both poles, especially near to geomorphic features such as the ubiquitous spiral troughs, tells us of climatic changes that occur on different time scales. The youngest water ice deposits on the planet belong to the northern cap and provide a record of ice age advance and subsequent retreat. The southern cap is in general much older than the northern cap, but very young, massive CO2 deposits reside near to the surface and reveal periodic atmospheric collapse on a shortened time scale. Both of these observations agree well with climate models, which have predicted both the ice age cycle and periodic atmospheric collapse.

In this talk, I will discuss each of these observations and broadly discuss their significance. Included will be many pretty pictures of the poles and descriptions of the formation of major landforms that have proven enigmatic until the arrival of MRO.

Nov. 19, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Christopher Russell - UCLA

What are those bright spots? How did that big lonely mountain form all by itself? Are those all impact craters? Why are there no large craters? Where is the ice that you said was there? Is there haze or isn't there? Why is Ceres shooting energetic electrons at Dawn? How do you know it is salt if you don't know what type of salt it might be?

Nov. 26, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• thanksgiving -

Seminar Description coming soon.

Dec. 3, 2015
4 p.m. - 5 p.m.
Geology 3656

Presented By:

• Rob Lillis - Berkeley

Results from MAVEN's mission to Mars

The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has been orbiting Mars since 21 September 2014 and collecting data in science mode since 16 November 2014. The science objectives of the MAVEN mission are to characterize the upper atmosphere and ionospheric structure and composition, the interactions of the sun and the solar wind with the planet, and the processes driving loss of gas from the atmosphere to space. Our goal is to understand the chain of processes leading to escape today, learn how to extrapolate back in time, and determine the integrated escape of atmosphere over Martian history. Measurements are being collected from all of the science instruments in our normal mapping orbit and through multiple “deep dip” campaigns. Results are providing a first-time detailed look at the upper-atmospheric system surrounding Mars, and are elucidating the key processes and history of the atmosphere. Preliminary results will be integrated into a coherent view of the processes controlling the upper-atmosphere system and the escape to space. In this seminar, an overview will be given of the set of science results from the first ~6 months of data from the mission.