I utilize paleomagnetism and fundamental rock magnetism as tools to investigate problems in the planetary sciences. By studying the remanent magnetism recorded within rocks from differentiated planetary bodies, I can learn about core processes that facilitate the generation of dynamo magnetic fields within the Earth, Moon, and planetesimals. Determining the longevities and paleointensities of dynamo fields that initially magnetized rocks also provides insight into the long-term thermal evolution (i.e., effects of secular cooling) of planetary bodies. I also use paleomagnetism to understand impact cratering events, which are the most ubiquitous modifiers of planetary surfaces across the solar system. Impact events produce heat, shock, and sometimes hydrothermal systems that are all capable of resetting magnetization within impactites and target rocks via thermal, shock, and chemical processes. Therefore, I am able to use a combination of paleomagnetic and rock magnetic characterization to investigate shock pressures, temperatures, structural changes, and post-impact chemical alteration experienced by cratered planetary surfaces.
Probing the hydrothermal system of the Chicxulub impact crater.
2020; 6 (22): eaaz3053
The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth's crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 * 105 km3 of Earth's crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 106 years.
View details for DOI 10.1126/sciadv.aaz3053
View details for PubMedID 32523986
- Explosive interaction of impact melt and seawater following the Chicxulub impact event GEOLOGY 2020; 48 (2): 108–12
Probing space to understand Earth
Nature Reviews Earth & Environment
2020; 1: 170-181
View details for DOI 10.1038/s43017-020-0029-y
The first day of the Cenozoic
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2019; 116 (39): 19342–51
Highly expanded Cretaceous-Paleogene (K-Pg) boundary section from the Chicxulub peak ring, recovered by International Ocean Discovery Program (IODP)-International Continental Scientific Drilling Program (ICDP) Expedition 364, provides an unprecedented window into the immediate aftermath of the impact. Site M0077 includes ∼130 m of impact melt rock and suevite deposited the first day of the Cenozoic covered by <1 m of micrite-rich carbonate deposited over subsequent weeks to years. We present an interpreted series of events based on analyses of these drill cores. Within minutes of the impact, centrally uplifted basement rock collapsed outward to form a peak ring capped in melt rock. Within tens of minutes, the peak ring was covered in ∼40 m of brecciated impact melt rock and coarse-grained suevite, including clasts possibly generated by melt-water interactions during ocean resurge. Within an hour, resurge crested the peak ring, depositing a 10-m-thick layer of suevite with increased particle roundness and sorting. Within hours, the full resurge deposit formed through settling and seiches, resulting in an 80-m-thick fining-upward, sorted suevite in the flooded crater. Within a day, the reflected rim-wave tsunami reached the crater, depositing a cross-bedded sand-to-fine gravel layer enriched in polycyclic aromatic hydrocarbons overlain by charcoal fragments. Generation of a deep crater open to the ocean allowed rapid flooding and sediment accumulation rates among the highest known in the geologic record. The high-resolution section provides insight into the impact environmental effects, including charcoal as evidence for impact-induced wildfires and a paucity of sulfur-rich evaporites from the target supporting rapid global cooling and darkness as extinction mechanisms.
View details for DOI 10.1073/pnas.1909479116
View details for Web of Science ID 000487532900027
View details for PubMedID 31501350
View details for PubMedCentralID PMC6765282
- Drilling-induced and logging-related features illustrated from IODP-ICDP Expedition 364 downhole logs and borehole imaging tools SCIENTIFIC DRILLING 2018; 24: 1–13
- Lunar Swirl Morphology Constrains the Geometry, Magnetization, and Origins of Lunar Magnetic Anomalies JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 2018; 123 (8): 2223–41
Rapid recovery of life at ground zero of the end-Cretaceous mass extinction
2018; 558 (7709): 288-+
The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth1,2. It was caused by the impact of an asteroid3,4 on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago 5 , forming the Chicxulub impact crater6,7. After the mass extinction, the recovery of the global marine ecosystem-measured as primary productivity-was geographically heterogeneous 8 ; export production in the Gulf of Mexico and North Atlantic-western Tethys was slower than in most other regions8-11, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning 12 , on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors-trophic interactions 13 , species incumbency and competitive exclusion by opportunists 14 -and 'chance'8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.
View details for DOI 10.1038/s41586-018-0163-6
View details for Web of Science ID 000435071400053
View details for PubMedID 29849143
View details for PubMedCentralID PMC6058194
- The Case Against an Early Lunar Dynamo Powered by Core Convection GEOPHYSICAL RESEARCH LETTERS 2018; 45 (1): 98–107
A two-billion-year history for the lunar dynamo
2017; 3 (8): e1700207
Magnetic studies of lunar rocks indicate that the Moon generated a core dynamo with surface field intensities of ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga). The field subsequently declined to <~4 μT by 3.19 Ga, but it has been unclear whether the dynamo had terminated by this time or just greatly weakened in intensity. We present analyses that demonstrate that the melt glass matrix of a young regolith breccia was magnetized in a ~5 ± 2 μT dynamo field at ~1 to ~2.5 Ga. These data extend the known lifetime of the lunar dynamo by at least 1 billion years. Such a protracted history requires an extraordinarily long-lived power source like core crystallization or precession. No single dynamo mechanism proposed thus far can explain the strong fields inferred for the period before 3.56 Ga while also allowing the dynamo to persist in such a weakened state beyond ~2.5 Ga. Therefore, our results suggest that the dynamo was powered by at least two distinct mechanisms operating during early and late lunar history.
View details for DOI 10.1126/sciadv.1700207
View details for Web of Science ID 000411589900031
View details for PubMedID 28808679
View details for PubMedCentralID PMC5550224
The fate of water within Earth and super-Earths and implications for plate tectonics
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
2017; 375 (2094)
The Earth is likely to have acquired most of its water during accretion. Internal heat of planetesimals by short-lived radioisotopes would have caused some water loss, but impacts into planetesimals were insufficiently energetic to produce further drying. Water is thought to be critical for the development of plate tectonics, because it lowers viscosities in the asthenosphere, enabling subduction. The following issue persists: if water is necessary for plate tectonics, but subduction itself hydrates the upper mantle, how is the upper mantle initially hydrated? The giant impacts of late accretion created magma lakes and oceans, which degassed during solidification to produce a heavy atmosphere. However, some water would have remained in the mantle, trapped within crystallographic defects in nominally anhydrous minerals. In this paper, we present models demonstrating that processes associated with magma ocean solidification and overturn may segregate sufficient quantities of water within the upper mantle to induce partial melting and produce a damp asthenosphere, thereby facilitating plate tectonics and, in turn, the habitability of Earth-like extrasolar planets.This article is part of the themed issue 'The origin, history and role of water in the evolution of the inner Solar System'.
View details for DOI 10.1098/rsta.2015.0394
View details for Web of Science ID 000399292900007
View details for PubMedID 28416729
View details for PubMedCentralID PMC5394257
- Further evidence for early lunar magnetism from troctolite 76535 JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 2017; 122 (1): 76–93
The formation of peak rings in large impact craters
2016; 354 (6314): 878–82
Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.
View details for DOI 10.1126/science.aah6561
View details for Web of Science ID 000388531900038
View details for PubMedID 27856906
- Reply to Comment on "Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early dynamo" EARTH AND PLANETARY SCIENCE LETTERS 2016; 450: 409–12
- A matter of minutes: Breccia dike paleomagnetism provides evidence for rapid crater modification GEOLOGY 2016; 44 (9): 723–26
- Magnetism of a very young lunar glass JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 2015; 120 (10): 1720–35
- Preservation and detectability of shock-induced magnetization JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 2015; 120 (9): 1461–75
The lunar dynamo
2014; 346 (6214): 1198-+
The inductive generation of magnetic fields in fluid planetary interiors is known as the dynamo process. Although the Moon today has no global magnetic field, it has been known since the Apollo era that the lunar rocks and crust are magnetized. Until recently, it was unclear whether this magnetization was the product of a core dynamo or fields generated externally to the Moon. New laboratory and spacecraft measurements strongly indicate that much of this magnetization is the product of an ancient core dynamo. The dynamo field persisted from at least 4.25 to 3.56 billion years ago (Ga), with an intensity reaching that of the present Earth. The field then declined by at least an order of magnitude by ∼3.3 Ga. The mechanisms for sustaining such an intense and long-lived dynamo are uncertain but may include mechanical stirring by the mantle and core crystallization.
View details for DOI 10.1126/science.1246753
View details for Web of Science ID 000349770700001
View details for PubMedID 25477467
- Decline of the lunar core dynamo EARTH AND PLANETARY SCIENCE LETTERS 2014; 404: 89–97
A Long-Lived Lunar Core Dynamo
2012; 335 (6067): 453–56
Paleomagnetic measurements indicate that a core dynamo probably existed on the Moon 4.2 billion years ago. However, the subsequent history of the lunar core dynamo is unknown. Here we report paleomagnetic, petrologic, and (40)Ar/(39)Ar thermochronometry measurements on the 3.7-billion-year-old mare basalt sample 10020. This sample contains a high-coercivity magnetization acquired in a stable field of at least ~12 microteslas. These data extend the known lifetime of the lunar dynamo by 500 million years. Such a long-lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior. The inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory.
View details for DOI 10.1126/science.1215359
View details for Web of Science ID 000299466800046
View details for PubMedID 22282809