Geocheminar - fall-2021

Sept. 23, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Kara Brugman - Carnegie EPS

Seminar Description coming soon.

Sept. 30, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

Seminar Description coming soon.

Oct. 7, 2021
3:30 p.m. - 4:30 p.m.
Zoom

Presented By:

• Xuefeng (Nick) Peng - U. South Carolina

Salt marsh sediments are known hot spots for nitrogen cycling, including the production and consumption of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent. Coastal eutrophication, particularly elevated nitrogen loading from the application of fertilizers, is accelerating nitrogen cycling processes in salt marsh sediments. We examine the impact of long-term fertilization on nitrogen cycling processes with a focus on N2O dynamics in a New England salt marsh. By combining 15N-tracer experiments with numerical modeling, we found that both nitrification and denitrification contribute to net N2O production in fertilized sediments. Long-term fertilization increased the relative importance of nitrification to N2O production, likely a result of increased oxygen penetration from nutrient-induced increases in marsh elevation. Our current work investigates the contribution and molecular underpinning of fungal N2O production in salt marsh sediments.

Oct. 14, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Tushar Mittal - MIT

Despite significant advancements in understanding crustal melt transport, determining the shallow magmatic architecture at any given volcanic system remains a significant challenge with geophysical and geochemical methods alone. In particular, it is very difficult to assess the state of magma reservoirs (mush vs molten magma reservoirs) due to spatial resolution limitations. In this talk, I will present a complementary approach using a set of magma dynamics models - mechanical models for magma reservoirs, conduits, and porous mushes - to constrain the magmatic architecture of volcanic systems using surface observations such as ground deformation, effusion rate history, and volcanic gas fluxes. To illustrate the utility of this approach, I will summarize the results of a couple of case studies - eruptions of some of the largest volcanic events in Earth history (continental flood basalts) and the largest historical submarine eruption (Mayotte 2018-2021 eruption). The magma dynamics framework allows us to robustly constrain the presence of a single vs multiple magma reservoirs and the presence of deep crustal mushes feeding the surface eruptions. I will discuss some implications of these results with regards to erupted magma geochemistry and how combining geochemistry with magma dynamics models can provide important constraints on magmatic architecture.

Oct. 21, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Elizabeth Niespolo - Caltech/Princeton

Many Pleistocene archaeological and paleontological sites beyond the c. 50-thousand-year 14C limit remain poorly constrained in age or undated entirely. Yet, they host key evidence about terrestrial ecosystems, the biological and cultural evolution of H. sapiens, and human geographic range expansion out of Africa. Many such deposits host giant avian eggshells, as large flightless birds producing giant eggs have resided on five continents in the Pleistocene and eggs served as a food source for foraging humans. Eggshells are furthermore made of calcite and are resistant to diagenesis in deep time, making them potential candidates for uranium-series (230Th/U) geochronology; however, eggshells do not have primary U in them, rendering conventional 230Th/U dating ineffective. Laser ablation measurements that compare modern and ancient avian eggshells indicate that while modern eggshells have negligible U, ancient eggshells host significant concentrations of U and Th that vary with the eggshells’ petrographic structures. I’ll share a novel approach to dating eggshells, first tested with ostrich eggshells, called 230Th/U “burial dating”, which explicitly accounts for U in ostrich eggshell acquired from soil pore water. U and Th concentration profiles from laser ablation data optimize subsampling approaches to correct for post-depositional U uptake, and 232Th/U profiles allow screening to avoid “dirty” samples that may produce imprecise, inaccurate 230Th/U ages. Careful subsampling and the use of a simple model of U uptake provide reliability criteria inherent to the U-Th data to determine accurate 230Th/U burial ages. Resultant 230Th/U burial ages of ostrich eggshells from African archaeological sites preserve stratigraphic order and agree with independent dates. In other avian eggshells, laser ablation data suggest primary petrographic structures control secondary uptake of U, indicating that 230Th/U burial dating may apply to well-preserved eggshells of other avian taxa.

Oct. 28, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Leslie Insixiengmay - UCLA EPSS

Pyrite-structured FeOOH_x, also known as p-FeOOH_x, has been found to be a phase at lower mantle pressure and temperature conditions. From X-ray diffraction studies, p-FeOOH_x falls into the Pa-3 space group, consisting of a corner-sharing FeO6 octahedra cross-linked with hydrogen bonds, which become symmetric at high pressure. The partially filled 3d electron orbitals give rise to two electronic configurations where Fe2+ can either be in a high-spin or low-spin state. Many iron-bearing oxides and silicates are known to undergo a pressure-induced transition from the high-spin to the low-spin state at conditions of the Earth’s lower mantle. Such spin transitions are important because they shrink the volume and increase the density, affecting buoyancy relationships and seismically observable elastic wave velocities. In FeOOH_x measuring the volume is important as a means for determining hydrogen content (x), which is still unknown and may be variable, and can be determined only by comparison with equations of state of known hydrogen content. However, the spin transition in p-FeOOH_x is poorly understood with only a few experimental measurements constraining the pressure at which it occurs. Here, we investigate the high-spin to low-spin transition in p-FeOOH_x with x=1 using density functional theory (DFT) as implemented in VASP. We find the high-spin to low-spin transition to be a gradual change at both high and low temperatures with a stable mixed-spin state during the transition, calling for an alteration of the thermodynamic ideal mixing model previously proposed by other studies where the transition is sharp at low temperatures. We find the addition of a favorable interaction term between high-spin and low-spin states to the enthalpy of mixing accounts for the first principles results. I will discuss implications for determination of the hydrogen content of this phase and its significance for the deep Earth.

Nov. 4, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Paul Warren - Department of Earth, Planetary, and Space Sciences, UCLA

The meteorite Northwest Africa 12969, an ultra-magnesian harzburgite, was before this study classified as ungrouped. It consists of enstatite pyroxene and forsterite (Fo99) olivine, along with minor FeNi metal (mostly rusted) and medium-Ca pyroxene. I find mineralogical and geochemical similarities with a common type of achondrite, ureilites, which are carbon-bearing peridotites, averaging roughly 3 wt% C, mostly as graphite (and similar “poorly graphitized” carbon). The most magnesian known ureilite, ALH 82106, has Fo95 (core) olivine. Many ureilites, especially high-shock ureilites, contain trace proportions of diamond. A single grain of “at least” 100 μm is by far the largest known. NWA 12969 is moderately shocked and contains nearly 2 wt% carbon. After difficulty with sawing for thin sections, reflected light microscopy revealed numerous ultra-high relief lath-shaped grains, diamonds, clustered in association with larger graphitic carbons. Optically contiguous diamond laths (not necessarily all one crystal) are up to 120 μm long. Mild siderophile element depletions (INAA data; e.g., CI-normalized Os = 0.71, Ir = 0.68, Ni = 0.175, Au = 0.17) manifest further similarity with ureilites. This pattern probably resulted from coreward separation of several wt% of S-rich metallic melt. Not all available evidence points neatly towards affinity with known ureilites. A reported oxygen-isotopic composition resembles ureilites in being well below the terrestrial line, but falls to the high-Δ17O side of the ureilite Fo vs. Δ17O trend. Our bulk-rock result for zinc (CI-normalized, 0.076) is more depleted than normal in ureilites. More work is needed, most obviously on stable isotopes.

Nov. 18, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Edwin Schauble - Department of Earth, Planetary, and Space Sciences, UCLA

Seminar Description coming soon.

Dec. 2, 2021
3:30 p.m. - 4:30 p.m.
Geology 4677

Presented By:

• Elise Wilkes - Caltech

Position-specific carbon isotope ratios within amino acids are expected to serve as detailed tracers of biogeochemical and ecological processes, but remain largely unexplored in environmental samples due to methodological limitations. This talk will describe the first Orbitrap-based measurements of natural-abundance, position-specific carbon isotope variation in an amino acid isolated from a biological matrix. I will present a method optimized for the precise characterization of isotope ratios within serine and explore an application for probing photorespiration and serine metabolism in plants. I will also discuss the potential for generalizing these approaches to other amino acids and organic structures in natural samples, paving the way for novel isotope proxies and applications.