Asmeret Asefaw Berhe
Fire, erosion, and soil carbon (C) dynamics overlap in space and time. Increased rates of erosion typically follow wildfires, and fire-altered or pyrogenic C (PyC, also referred to as black carbon) is redistributed vertically within soil profiles and laterally to lower landform positions along hillslopes, changing its C sequestration trajectory. However, we currently lack sufficient understanding on how and why the interaction of fire and erosional distribution of soil materials control persistence of bulk soil organic matter (SOM) and PyC in dynamic landscapes. In this talk, we present results from wildfires that occurred in the Sierra Nevada Mountains (USA) to demonstrate how the composition (based on stable isotope composition of 13C and 15N, and NMR analysis of OM composition) and magnitude of pyrogenic carbon redistributed by soil erosion varies considerably depending on fire severity and geomorphology of the landscape. Our findings also show that PyC is preferentially transported by erosion in high severity burn slopes, compared to areas affected by low and medium severity fires. Findings of this study are critical for better integration of biogeochemical and geomorphological approaches to derive improved representation of mechanisms that regulate SOM persistence in dynamic landscapes that routinely experience more than one perturbation.
UC Berkeley and Collège de France
The existence of two large low shear velocity provinces (LLSVPs) at the base of the earth's mantle with a strong equatorial "degree 2" signature has been known since the development of the first global seismic tomographic models of the earth's lower mantle. In the last few decades, their existence has been confirmed in many studies, and they are generally associated with the global upwelling flow in the lower mantle, but their precise nature and role in the earth's evolution and present-day dynamics is still debated. Several indirect clues point to their stability for the last 250-300 Ma, and possibly much further back in geological time. It has been suggested that they may be of a different composition than surrounding regions and some authors have argued that they may be denser than the ambient mantle to a significant height above the CMB. One of the key questions in furthering our understanding of the LLSVPs is how high they extend as coherent and compositionally distinct structures above the core-mantle boundary (CMB). Because of their very strong long wavelength signature, which persists to at least 1500 km above the core-mantle boundary, a common assumption is that they are compact structures across that depth range. I will discuss these ideas in the light of recent full waveform tomography images that shows the presence of ~20 broad low velocity plumes, rooted in the LLSVPs, that rise quasi-vertically from the CMB to around 1000 km depth, and then meander through the upper part of the mantle towards major hotspots.
Earth’s mantle is a key component of the Earth system: its circulation drives plate tectonics, the long-term recycling of Earth’s volatiles, and as such, holds fundamental implications for the Earth’s surface environment. In order to understand this evolution, a key parameter of the mantle must be known, namely its buoyancy. In this talk, I will discuss how Earth’s body tide can provide fresh and independent constraints on deep mantle buoyancy through a newly developed technique called Tidal Tomography. This comes at a time when other interesting and exciting data sets sensitive to deep mantle buoyancy, e.g., Stoneley modes, have been brought to bear, and we will explore our conclusions in the context of other recent finds.
What happens under volcanoes in the months leading up to eruption? How does the magmatic system prepare for an eruption? And why are some eruptions more explosive than others? Crystal clocks are providing some answers to these questions. The timescales of chemical diffusion operate over minutes to years prior to eruption. This talk will examine new constraints on volcanic run-up, forecasting and eruption dynamics.
UC Santa Barbara
The nature of Earth's crust that lies above sea level (continental crust) is explored using geochemical data for fine-grained terrigenous sediments (loess, shales, and glacial diamictites). The data point to a fundamental change in the composition of the crust exposed to weathering and erosion during the Late Archean and may have implications for the dominant tectonic processes operating at these times.
UC Santa Cruz
Despite centuries of research, we still do not understand some of the most fundamental aspects of earthquakes including how they start, how they propagate, and why they stop. Grain-scale processes that control fault strength can be studied in lab experiments, but we are limited in making ties to field observation by our inability to pinpoint earthquake structures in natural fault zones. Here, I discuss using coseismic temperature proxies to map earthquake slip in the field, elucidate grain-scale strength at those temperatures, and estimate how energy is partitioned during earthquakes. During earthquakes, faults heat up due to their frictional resistance. Sometimes, the temperature rise during earthquakes makes the rocks hot enough to melt. However, solidified frictional melt (pseudotachylyte) is uncommon in the rock record, and other paleoseismic temperature proxies have only recently been established. The dearth of pseudotachylyte led researchers to hypothesize that faults get very weak during earthquakes, and hence do not produce much heat. If faults dramatically weaken during slip, there are important implications for how earthquakes propagate, and hence why some earthquakes grow to be very large. I use a new sub-solidus temperature proxy, biomarker thermal maturity, to identify temperature rise on faults in a variety of tectonic settings. With results from this new temperature proxy, we revisit some outstanding questions in fault mechanics such as: Where does earthquake slip occurs in a fault zone? Can creeping faults host earthquakes? Does lithology control rupture propagation? How is energy partitioned during earthquakes? and, How strong are faults during earthquakes?
The oxidation state of a rocky planet’s mantle has a strong influence on the composition of secondary atmospheres formed through outgassing. Understanding how mantle oxidation state depends on planetary bulk composition and how it changes with time due to planetary differentiation, atmospheric outgassing, and interactions with a planet’s volatile envelope is crucial to accurately predicting atmospheric compositions of rocky exoplanets. Distinguishing biosignature gases from geosignatures (false positive biosignatures produced by natural geological processes) will depend on understanding the geological processes operating on planets of different size and composition compared to the Earth. In this talk, I discuss models that explore how bulk composition, atmospheric escape and planetary differentiation (metal-silicate separation) influence mantle oxidation state and the resulting outgassed atmosphere. These and future models can help select the best exoplanetary targets for detailed characterization by future telescopes like the James Webb Space Telescope (JWST) by identifying planets with lower chances for producing false positive biosignatures.