Research

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Thermal histories of lunar rocks
Late Quaternary climate reconstruction using cosmogenic noble gases
Geochemical tracers of glacial erosion processes
Tectonic and geomorphic evolution of the southern Tibetan plateau
Noble gas diffusion kinetics and mechanisms

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Thermal histories of lunar rocks

The flux of impactors to the inner Solar System through time is poorly understood. Crater counting suggests that the impactor flux began declining at ~4 Ga and became constant c. 2.8 Ga. However, astronomical observations and models suggest that pulses of higher impactor flux affected the inner Solar System at different points in time. The Late Heavy Bombardment (LHB) is a postulated pulse of high impact flux at ~3.9 Ga. Establishing whether the LHB actually occurred is critical to our understanding not only of planetary-scale processes throughout our Solar System but also of when and how life on Earth developed.

The most widely cited evidence for the LHB comes from application of the 40Ar/39Ar chronometer in studies to Apollo mission lunar rocks. Frequent apparent 40Ar/39Ar plateau ages of 3.9 Ga are observed in Apollo samples that crystallized before 3.9 Ga and interpreted to indicate an unusually high impactor flux, causing heating and resetting of the 40Ar/39Ar system at that time. This approach is problematic for several reasons. First, most Apollo sample age spectra show clear evidence of partial resetting of the 40Ar/39Ar system. While ignored in most studies identifying plateau ages, partial resetting due to multiple impact heating events likely biases the interpretation of plateau ages to falsely identify a peak in impact events. Second, in most previous lunar argon measurements the sample temperature was not well controlled and/or monitored, preventing sample-specific diffusion kinetics from being obtained. Sample-specific diffusion kinetics would allow for the entire 40Ar/39Ar age spectra to be modeled and for the timing and duration of impact event(s) to be quantitatively constrained. Third, the Apollo samples cover ~4 % of the lunar surface area. Thus even if the interpretation of 3.9 Ga apparent plateau ages from Apollo samples were robust, it is unclear whether such a signal is present at a larger spatial scale and raises the question of whether an impactor flux pulse is required to explain these data.

With my postdoctoral mentor Darren Mark, I am collecting modern, high fidelity 40Ar/39Ar datasets from lunar meteorites originating from the far side of the Moon. By focusing on lunar far side meteorites, we will greatly expand the spatial extent of our knowledge about the lunar impact history. Our experimental design for step-heating analysis will allow sample-specific diffusion kinetics to be recovered for each meteorite sample, which in turn will enable us to model the observed 40Ar/39Ar age spectra to place quantitative constraints on the timing and duration of impact event(s). We are pairing 40Ar/39Ar observations with diffusion measurements on experimentally shocked analogue materials in order to assess how mineral structural changes induced by lunar impact events may have modified the kinetics of argon diffusion, which will be important to understand when trying to reconstruct the thermal histories of lunar samples. This approach will enable us to address the following questions: (1) Is there evidence for impact events at 3.9 Ga on the far side of the Moon? (2) Do we find evidence for younger pulses of impact heating on the lunar far side?, and (3) Did the kinetics of argon diffusion in lunar samples change as a function of time due to structural changes induced by impact events?

In addition to my current work on lunar meteorites, I have also investigated the diffusion kinetics and systematics of cosmogenic neon in Apollo sample 76535, the oldest lunar sample to lack any evidence of impact-related shock which has been used to argue for the presence of an early core dynamo on the Moon. My neon measurements in anorthite grains from 76535 complimented argon measurements in the same sample, and provided strong evidence that 76535 has not experienced any heating other than that due to solar insolation since being deposited at the lunar surface 142 million years ago. You can read about this work in our recent JGR Planets paper.

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Late Quaternary climate reconstruction with cosmogenic noble gases

Temperature in continental settings is one of the most important yet difficult to ascertain properties of Earth’s past climate. Many proxies from non-marine settings depend on numerous parameters in the environment and rarely constrain absolute temperature changes through time. However, quantitative constraints on past temperatures in continental environments are critical if we hope to use records of Earth’s past climate to predict the impacts of future global warming.

Cosmogenic nuclides are produced in the uppermost few meters of the Earth’s crust by cosmic-ray particle interactions with atomic nuclei, making them attractive targets for studying Earth surface processes. Some of the first cosmogenic nuclide measurements revealed that the cosmogenic noble gases 3He and 21Ne are diffusively lost at Earth surface temperatures in common silicate minerals like quartz and feldspars. Viewed as a fatal limitation for geologic applications since then, the open-system behavior of cosmogenic noble gases can, in fact, be exploited to quantitatively reconstruct temperatures at Earth’s surface.

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Observations (black) and predicted (color) cosmogenic 3He abundances in a Holocene-age moraine boulder from the Quelccaya Ice Cap area, Peru. Our observations are consistent with little temperature change over the Holocene. From Tremblay et al. (2014b).

My Ph.D. research focused on evaluating the viability of paleothermometry using cosmogenic noble gases. To do this, we combined mathematical modeling and calibrated geologic tests with experimental research quantifying the diffusion kinetics of 3He and 21Ne in quartz and feldspars. Through this integrative experimental, modeling, and observational approach, we showed that both the 3He–quartz and 21Ne–feldspar systems can be used to reconstruct Earth surface temperatures during the Quaternary, and that the 21Ne–quartz and 21Ne–feldspar systems can also be utilized at higher temperatures and longer exposure durations on other planetary surfaces.

Having demonstrated the technique’s viability, my current and future research now involves applications of cosmogenic noble gas paleothermometry to geologic problems, with a particular focus on reconstructing Late Quaternary temperatures in key localities. With a suite of samples from rock avalanche deposits in Yosemite Valley, California, I am reconstructing temperatures through the Holocene. This reconstruction will help inform other Holocene proxy datasets from the Sierra Nevada, which are largely qualitative in nature. I am also reconstructing temperatures during the last deglaciation in the Maritime Alps, Italy, where there is also a dearth in quantitative paleoproxy records. This work is part of a longer-term research endeavor to use existing glacier chronologies and sample sets to reconstruct late Quaternary temperatures across the European Alps.

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Geochemical tracers of glacial erosion processes

Alpine glaciers play a prominent role in shaping the Earth’s surface environment through erosion. Glacial erosion modifies topography in mountainous landscapes, delivers sediment to basins, and enhances silicate weathering. These latter two effects are thought to generate positive feedbacks in the climate cycle by removing CO2 from the atmosphere, which reduces global temperatures and can thus promote more extensive glaciation. Many have argued that these feedbacks operated over the last 2–3 million years as erosion rates around the world increased and northern hemisphere and high altitude regions became extensively glaciated.

Despite the importance of glacial erosion in shaping Earth’s surface, our understanding of glacial erosion mechanisms is incomplete. Glaciers erode primarily by two processes: abrasion of the bed by rock particles dragged in basal ice, and removal of bedrock blocks by stress gradient-induced failure, known as quarrying. Due to the difficulty of observing these processes beneath glaciers directly, our understanding of abrasion and quarrying is largely based in theoretical considerations. Abrasion rates are thought to be controlled by ice sliding velocity and rock debris concentration in basal ice; the latter is dependent on up-glacier debris generation by quarrying and supraglacial erosion. Quarrying rates ought to depend on numerous parameters, including bed roughness, sliding velocity, the amplitude and frequency of water pressure fluctuations, and rock fracture distribution.

Many of the parameters influencing abrasion and quarrying rates vary systematically in space within glaciated catchments. If we could identify the origin of sediment generated by abrasion and quarrying, we could examine the relative influence of these parameters and develop a stronger mechanistic understanding for what drives these processes. To this end, I am examining the origin of sediment generated by glacial quarrying in eastern Sierra Nevada, California, using low-temperature thermochronometry.

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Cartoon demonstrating how the distribution of thermochronometric ages from a moraine (red) in a formerly-glaciated catchment with a certain bedrock age-elevation relationship (yellow) might look if glacial quarrying were focused below the former glacier’s equilibrium line altitude (ELA). 

Bedrock low-temperature thermochron-ometric ages vary systematically with elevation in the eastern Sierra Nevada. Sediment generated in this location therefore carries a geo-chemical “fingerprint” of its source elevation in its thermochronometric age. In formerly-glaciated Sierra Nevada catchments with uniform bedrock lithology, we have sampled sediment from Last Glacial Maximum (LGM) moraines for apatite (U-Th)/He thermochronometry. Given that abra-sion generates fine-grained sediment, we are focusing on coarser grain sediment in these moraines, which can be generated by two processes: quarrying, and supraglacial erosion mechanisms such as rockfall that deliver coarse material to the glacier surface. The apatite (U-Th)/He age distribution in the sediment, paired with the relationship between apatite (U-Th)/He age and elevation, will allow me to identify spatial patterns in the origin of sediment produced by either quarrying or rockfall. This geochemical approach has recently been applied to understand the spatial dependence of different erosion processes in a fluvial catchment in the Sierra Nevada, but has not been widely utilized to understand glacial erosion processes.

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Tectonic and geomorphic evolution of the southern Tibetan plateau

Through the NSF Continental Dynamics project “Lhasa Block: Top to Bottom,” a group of interdisciplinary researchers and I are investigating the evolution of the southern Tibetan Plateau margin. This project aims to provide a robust mass balance for the crust in southern Tibet since the onset of India–Asia collision and in doing so potentially resolve the mass imbalance in this collisional system that exists in our current state of knowledge. One critical piece of this endeavor is determining how much material has been removed from the system due to erosion. To do this, I am using a suite of thermochronometers (specifically apatite 4He/3He, apatite (U-Th)/He, and zircon (U-Th)/He) to samples we collected across the southern plateau margin in conjunction with state-of-the-art numerical modeling to map out spatial and temporal patterns in erosion rates. Our initial work (Tremblay et al. 2015, PNAS) revealed that erosion rates in Southern Tibet were as rapid as rates in the Himalaya between 20 and 10 million years ago and then abruptly shut down. Although seminal papers using thermochronometry focused on southern Tibet, this work was the first to demonstrate both when and how rapid this shutdown in erosion was. We hypothesized that this dramatic switch was due to rapid rock uplift associated with duplexing in the Himalaya and consequent large-scale drainage reorganization across southern Tibet.

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Compilation of low-temperature thermochronometry data from the southern Tibetan plateau.

To further test this hypothesis of Himalaya–Tibet evolution, we are actively building the thermochronometric dataset from southern Tibet, which will include data from 120 samples collected over a 105 km2 area. We are using [1] formal inverse methods to invert the spatially distributed thermochronometric dataset (including our data and all existing thermochronometry data in the literature) for erosion rates and [2] landscape evolution models to explore what patterns in rock uplift and drainages could account for the observed patterns in erosion.

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Noble gas diffusion kinetics and mechanisms

Measurements of noble gases are used to explore a broad range of questions about Earth and planetary geology, from near surface processes on Earth to the origins of our solar system. Because the noble gases are chemically inert, their abundances in geologic materials are ultimately controlled by the kinetics of noble gas diffusion. Understanding noble gas diffusion kinetics in different geologic materials is therefore of paramount importance to their application in addressing Earth science questions.

During my Ph.D., I conducted dozens of experiments to constrain helium and neon diffusion kinetics in quartz and feldspars. These experiments in many cases reveal complex noble gas diffusion behavior, which we can demonstrate is not a temperature-dependent phenomenon. Instead, complex diffusion behavior appears to be associated with some sample-specific material property. While this type of behavior has been observed in feldspars for decades and can be modeled mathematically, we lack a

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Example of complex argon (blue) and neon (green) diffusion behavior in labradorite, a plagioclase feldspar. From Tremblay et al. (2017).

physical understanding of what causes it. Understanding what controls complex diffusion behavior and developing a physical, mathematical framework with which to account for it is a key aspect of my ongoing research.

My current hypothesis is that aberrations from the normal crystal lattice distributed throughout a mineral grain, such as defects or radiation damage, cause complex diffusion behavior. I am gearing up conduct detailed microstructural and textural analyses of samples exhibiting complex diffusion behavior using transmission electron microscopy and synchrotron techniques to explore this possibility. I will pair this work with systematic diffusion experiments on synthetic materials with well-characterized structural defects and radiation damage densities.

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