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The reef core of the Ediacaran Salient Platform is spectacularly exposed atop the Interbedded Succession of the Upper Miette Group on the east face of Mount Machray as viewed from Salient Mountain, British Columbia.
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Microbialitic metadolomite in the Paleoproterozoic (~2.05 Ga) Nash Fork formation, “valley of the stromatolites”, Medicine Bow Mountains, Wyoming.
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The Ignacio Quartzite (of contested age, though likely Cambrian) unconformably overlies the Mesoproterozoic (1.433 Ga) Eolus Granite in the Needle Mountains, Colorado.
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The East Rackla River valley viewed from the north, looking towards Mount Mervyn. The Dawson Thrust runs parallel to the river valley.
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The K-Pg boundary claystone (lower pencil) just below the IrZ tonstein (upper pencil) within the IrZ coal at Herpejunk, Hell Creek, Montana
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B.S. Thesis. Advisor: Suzanne Mahlburg Kay
My first significant computational work - originally a side project to work on as a grad student while waiting for Blair's clean lab to be built. We were surprised by the strength and consistency of the geochemical trends we observed on Gyr timescales. The simplest signals directly reflect secular mantle cooling, but others hint at a change in crustal evolution that appears to coincide temporally with oxidation of the surface Earth in the Great Oxygenation Event.
Here we used the weighted bootstrap resampling approach from our 2012 paper to address the longstanding question of volcanic-plutonic parity. The results indicate that felsic plutons are not (on average) significantly more cumulate than felsic volcanics, favor fractional crystallization as the predominant mechanism of geochemical differentiation, and suggest the influence of magmatic water content on magma stalling and intrusion. This paper was my first foray into high-performance computing, running ~1.3 million pMELTS simulations to invert for P-T paths and water contents that produce differentiation trends similar to the those observed in the natural dataset.
Ph.D. Dissertation. Advisor: Blair Schoene
Here we investigated the variation in the amount of zircon expected to saturate from magmas of different ages throughout Earth history by running full MELTS simulations along with trace element partitioning calculations (on a local Princeton University cluster) for each of the ~70,000 samples in the Keller & Schoene (2012) dataset. We found that older magmas crystallize substantially less zircon per unit mass due to the geochemical consequences of secular mantle cooling – suggesting that a larger volume of felsic magmatism is required to explain the Archean (and perhaps the Hadean) zircon record than previously considered. We note also that anorthosite flotation crust is largely zircon free and thus invisible to zircon-based crustal growth models, with implications explored in the supplement.
In this longer-format paper we examine in detail the record of basaltic rocks preserved in the continental crust, building on the weighted bootstrap resampling procedure of Keller and Schoene, 2012. A range of rapid trace element variations in the basaltic record can be explained as a result of mantle melting systematics (changing partition coefficients as a result of Grt and Cpx-out). Both trace element signatures of slab fluid input and major element signatures of calc-alkaline vs tholeiitic differentiation are remarkably stable and consistently arc-like, strongly suggesting the occurence of subduction and plate tectonics throughout the preserved rock record (back to at least 3.85 Ga). While some non-plate tectonic models may produce some flux melting, a virtually constant proportion of flux to decompression melting in the preserved continental record is a tall order for any non-plate model for Archean tectonics.
“Weighted Means Considered Harmful” – try the alternative here!
A model framework for the interpretation of mineral age spectra in stratigraphic context
It has long been observed that the sequence of sedimentary rocks deposited in the past half-billion years often sharply overlies older igneous or metamorphic basement at an erosional surface known as the Great Unconformity. We provide evidence that this unconformity may record rapid erosion during Neoproterozoic “snowball Earth” glaciations. We show that the extent of Phanerozoic sedimentation in shallow continental seas can be accurately reproduced by modeling the accommodation space produced by the proposed glacial erosion, underlining the importance of glaciation as a means for lowering erosional base level. These results provide constraints on the sedimentary and geochemical environment in which the first multicellular animals evolved and diversified in the “Cambrian explosion” following the unconformity.
See also Perspective by Seth Burgess
The oldest known minerals on Earth are Hadean (> 4.0 Ga) zircons from the Jack Hills, Australia. We present the first application to such Hadean zircons of stepwise chemical abrasion isotope dilution thermal ionization mass spectrometry with trace element analysis (stepwise CA-ID-TIMS-TEA). We examine the evolution in U-Pb age and trace element chemistry of zircon domains accessed by successive chemical abrasion steps in the context of the geologic history of the Jack Hills zircons.
On geologic timescales, Earth’s habitable climate is maintained by a negative feedback process wherein atmospheric CO₂ is consumed by reaction with silicate rocks during erosion and weathering. However, relative to modern continental crust, many models propose an ancient crust that was thinner, denser, and significantly lower in silica for the first 1 to 2 billion years of Earth history. Like modern oceanic crust, such mafic crust would likely be poorly exposed to the atmosphere, resulting in a less climatically stable early Earth. We find that two geologic processes (mantle cooling and atmospheric oxidation) significantly compromise some previous methods for estimating ancient crustal composition. Accounting for these factors results in estimates much closer to the composition of modern continental crust.
The Great Unconformity involves a common gap of hundreds of millions to billions of years in the geologic record. The cause of this missing time has long eluded explanation, but recently two opposing hypotheses claim either a glacial or a plate tectonic origin in the Neoproterozoic. We provide thermochronologic evidence of rock cooling and multiple kilometers of exhumation in the Cryogenian Period in support of a glacial origin for erosion contributing to the composite basement nonconformity found across the North American interior. The broad synchronicity of this cooling signal at the continental scale can only be readily explained by glacial denudation.
Possibly the best-resolved Cryogenian exhumation signal yet!
Although the causes of the five largest mass extinctions remain controversial, geochronological improvements have revealed an apparent correlation between large igneous provinces (LIPs) and periods of Phanerozoic faunal turnover. This paper establishes that this relationship is unlikely to occur by chance and defines an eruptive rate threshold, above which known continental LIPs correlate with large extinctions. Continental LIPs also have an approximately linear relationship between their eruptive rate and extinction magnitude. It is difficult to attribute the causality of any one extreme event like an extinction with certainty, but there is an overall correlation between continental LIPs and extinction events that warrants consideration.
See also the Dartmouth News story by Harini Barath!
The composition of the continental crust is important for understanding Earth's evolution on global and regional scales. We build on previous work estimating Earth's composition with a new approach using Bayesian statistics, where prior information from rocks sampled at Earth's surface is combined with information from seismic properties to estimate Earth's crustal composition at depth. To connect seismic properties and compositions, we model the seismic properties of rock compositions. Our approach provides rigorous uncertainty estimates for crust composition at depth. By quantifying the sources of this uncertainty, we are able to propose a path forward for future work to further improve compositional estimates for Earth's crust at depth.
Editor’s summary: The end-Cretaceous mass extinction, which included the elimination of all nonavian dinosaurs, occurred after the impact of a meteorite and during a stretch of large-scale volcanism. Although it is known that the impact is temporally linked to the extinction, the relative roles are hard to disentangle. Cox and Keller used an inversion scheme that is agnostic about what occurred geologically but provides best guesses for a number of variables, including carbon and sulfur dioxide release. These gases cause environmental changes, and the results may argue for a two-stage extinction related to the volcanism. This approach should be useful for disentangling other complex events in Earth systems and elsewhere. —Brent Grocholski
Undergraduate Course, Dartmouth College, Department of Earth Sciences, 2019
Co-taught: Spring 2019, with Meredith Kelly
Undergraduate Course, Dartmouth College, Department of Earth Sciences, 2020
Taught: Spring 2021 Co-taught: Spring 2020, with Xiahong Feng
Undergraduate Course, Dartmouth College, Department of Earth Sciences, 2020
Taught: Fall 2020
Graduate Seminar, Dartmouth College, Department of Earth Sciences, 2021
Co-taught: Winter 2021, with Sarah Slotznick
Undergraduate Course, Dartmouth College, Department of Earth Sciences, 2023
Co-taught: Winter 2023, with Mathieu Morlighem
Undergraduate/Graduate Course, Dartmouth College, Department of Earth Sciences, 2023
Taught: Winter 2021, Winter 2022, Winter 2023
Undergraduate Course, Dartmouth College, Department of Earth Sciences, 2023
Co-taught with Ed Meyer: Fall 2021 (MT segment), Spring 2022 (AZ, CA segment), Fall 2022 (MT segment), Fall 2023 (MT segment)
Graduate Seminar, Dartmouth College, Department of Earth Sciences, 2023
Co-taught: Fall 2023, with Marisa Palucis
Undergraduate/Graduate Course, Dartmouth College, Department of Earth Sciences, 2024
Taught: Winter 2020, Spring 2022, Spring 2024
