Research interests

My research focuses on geology, soils, plants, and climate change. For my PhD work, I am studying ancient, buried soils (paleosols). Specifically, I use paleosols to examine past climates on Earth and the evolution of life.

Paleosols on Mars are also high-priority locations for biosignature investigation and Mars Sample Return, so my work also examines Mars-like paleosols from Earth to determine the biosignature preservation potential of martian paleosols.

I recently began a position as a postdoctoral scientist on the Mars 2020 Perseverance Rover surface operations team. Current projects include the alteration history and biosignature preservation potential of rocks in in the delta front at Jezero Crater.

1. What is the alteration history of rocks at the delta front in Jezero Crater, Mars?

A study focused on sedimentary rocks at “Hogwallow Flats”, Jezero Crater, Mars

Ancient (>3.5 billion-year-old) delta deposits at Jezero Crater, Mars (NASA/JPL/JHUA/ASU)
Hogwallow Flats: Light-toned, platy rocks overlie darker-toned, mottled rocks suggests alteration after deposition (NASA/JPL/ASU)

The rocks within Jezero crater are thought to have been deposited by an ancient delta system billions of years ago when Mars had liquid water and habitable surface conditions. The informally-named “Shenandoah Formation” spans more than 50 meters of vertical stratigraphy at the delta front region within Jezero Crater, Mars. The Hogwallow Flats (HWF) member of the Shenandoah Formation is a ~5 vertical meter stack of light-toned, platy and fractured sedimentary rocks that alternate in section with shaly, dark-toned, extensively mottled and deformed strata.

The sedimentary rocks at Hogwallow Flats were likely deposited underwater during the formation of the delta. Altogether these rocks appear to be significantly different in texture, color and morphology compared to other rocks in the Shenandoah Formation, possibly suggesting these rocks have experienced diagenesis. Strata at HWF appears to be heterolithic siltstones and fine sandstones with diagenetic features such as striking color variations (mottling), concretions and nodular features, diverse coatings, lumpy textures, a general absence of bedforms, filled fractures, and sulfate-enriched bedrock. These features suggest that diagenesis has pervasively affected rocks at HWF. Similarly, strata observed at Yori Pass (YP) at a similar elevation to HWF shows comparable stratigraphy and diagenetic features, suggesting HWF may be a laterally- continuous unit with widespread distribution in the delta front.

This work uses Mastcam-Z multispectral images from the Perseverance Mars Rover in conjunction with geochemical data from the SuperCam instrument to constrain the depositional and alteration history of HWF. A second objective of this work is to relate diagenesis to the biosignature preservation potential of rocks at Hogwallow Flats.

Figure 1. Altered sedimentary rocks at Hogwallow Flats as seen by Mastcam-Z (enhanced color). A-C, Light-toned, platy, fractured rocks that alternate in-section with dark-toned, mottled, shaly rocks; D-E, nodular features in bedrock; F, putative vein fill with positive relief; G, extensively mottled, lumpy, dark-toned clast with putative dark coatings; H, Fractures and wind-abraded nodules in light-toned bedrock. Arrows
in A, C and G show extensive color variations of strata. All images are from Mastcam-Z sol 461 zcam08482 L0 quicklook mosaic

2. Are there signs of life preserved in Earth’s oldest (3-billion year old) soils? Do they preserve organo-sulfur compounds like ancient rocks on Mars?

Artist depiction of the Martian surface during hypothesized warm and wet climates (~4.1-3.7 Ga, right side) compared with the dry, hyperarid and frigid surface of Mars during more recent geologic history (> 3.0 Ga, left side). Credit: Simon Fraser University

Paleosols (weathering profiles) from the Archean (2.6 billion years ago, Ga) contain the oldest undisputed evidence of life on the surface of the Earth which appear to be carbonanceous microfossils that may represent cyanobacterial mats living on the soil surface. However, recent discoveries of much older (3 – 3.7 Ga) sedimentary rocks which appear to be weathering profiles also contain tantalizing evidence of life, including putative carbonaceous microfossils and isotopically light carbon and sulfur.

The enduring question remains: On Earth, when did life first emerge on land?

In this work we examine two of Earth’s oldest putative soils: a 3.0 Ga coastal-plain paleosol from the Farrel Quartzite, Pilbara desert, Western Australia, and a 3.7 Ga coastal plain paleosol from the Isua Supercrustal Belt, Greenland. Both show morphological and geochemical evidence of surface (subaerial) acid sulfate weathering, which is a soil-forming process commonly observed on Earth today. Like ancient environments on Mars, these putative ancient soils from Earth were rich in sulfur and sulfate minerals which may have aided the preservation of organic carbon over billions of years (Eigenbrode et al. 2018). Because the Curiosity Mars rover discovered kerogen-like organic matter and organo-sulfur compounds in 3.5 Ga mudstones at the surface of Mars, we analyze these paleosols with instruments configured to operate like the Sample Analysis at Mars (SAM) instrument onboard Curiosity Mars rover.

These analyses include SAM-Evolved Gas Analysis (SAM-EGA, laboratory analog instrument at NASA Johnson Space Center) and SAM-Gas Chromatography Mass Spectroscopy (SAM-GCMS, lab analog instrument at the Johnson Biosignature Lab, Georgetown University). We also examine carbon isotope ratios of organic carbon isolated from the two paleosols.

What did we find? Organic carbon up to 1.6 wt. %, possible preservation of organo-sulfur compounds, and carbon isotopic evidence consistent with microbial life. SAM-GCMS and nuclear magnetic resonance (NMR) spectroscopy to identify organic molecules is currently being performed to determine the possible types of organic carbon compounds present in samples.

3. Evolved gas analysis of Mars-analog paleosols

NASA Curiosity Mars Rover has been integral for understanding the geology and past habitability of ancient near-surface environments of Mars (Photo; NASA/JPL/ University of Arizona)

The Sample Analysis at Mars (SAM) onboard NASA Curiosity Mars Rover performs evolved gas analysis (EGA) on rock and sediment samples at Gale Crater, Mars. Evolved gas analysis involves heating a sample to ~850 C and analyzing volatile gases released from the thermal decomposition of the sample. Volatile gases released during SAM – EGA provide important constraints on mineralogy and organic content of Martian rocks and sediments. However, there has been no effort to characterize mineralogy or the organic content of ancient soils (paleosols) on Earth that are similar to potential Martian paleosols. Paleosols on Mars were recently named a high priority location for biosignature detection and Mars Sample Return, so we seek to determine if the organic carbon preserved within terrestrial paleosols can be detected with Mars rover-like instruments. One approach is to analyze mineralogy and organic content of Mars-analog paleosols with instruments configured to operate similarly to those onboard Curiosity Mars Rover, such as a SAM-EGA analog instrument here on Earth.

We performed evolved gas analysis at NASA Johnson Space Center to examine the mineralogy and organic carbon content of paleosols from the Painted Hills in Eastern Oregon. We use a Setaram Labsys EVO coupled to a Pfeiffer Thermostar mass spectrometer configured to Curiosity Rover-like analytical conditions (Pressure maintained at 30 mbar, Helium used as a carrier gas, ramped combustion to ~900 degrees C).

Setaram Labsys EVO = SAM-EGA analog instrument

The objectives of this study were to characterize the mineralogy and organic carbon content of Mars-analog paleosols. Early Oligocene (33 Ma) paleosols rich in dioctahedral phyllosilicates and amorphous colloids were collected from the Painted Hills in eastern Oregon. Three paleosol types were analyzed with a thermal and evolved gas analyzer configured to operate similarly to the Sample Analysis at Mars Evolved Gas Analysis instrument (SAM-EGA) onboard the Mars Science Laboratory (MSL) Curiosity rover. We also used X-ray diffraction and visible/near infrared spectroscopy to constrain the mineralogy of these smectite-rich samples (up to ~90 wt. % Al and Fe clay minerals). The results: lots of crayon drawings (below).

We even got to visit the Lunar Lab at NASA JSC where the moon rocks live!

4. Back to Earth: Spatial and temporal domains of natural climate solutions

Natural climate solutions (NCS) have been proposed to mitigate climate change by removing CO2 from the atmosphere and increasing organic carbon permanence in terrestrial ecosystems. Adoption is required at global scales, but implementation of NCS has been limited by the lack of a systematic framework to prioritize ecosystem restoration or conservation at local and regional scales. Carbon sequestration policies implemented uniformly fail to consider regional ecological feedback systems and tradeoffs among finite natural resources and can also have detrimental effects on food production. By combining estimates of soil organic carbon (C) stocks, projected precipitation changes, land use, and landscape-level analysis of carbon and water flux within Oregon and Washington (below), we show that NCS efforts should be prioritized in natural areas with low soil C and projected precipitation increases. On the other hand, we argue that conservation and not NCS may be more appropriate for regions with high soil C stocks and decreases in projected precipitation.

Workflow for prioritization of natural climate solutions (NCS) areas of implementation or conservation across regional and sub-regional spatial scales. Step 1 uses baseline data of a socio-ecological context to identify target areas for both conservation and implementation practices.  Step 2 combines those target areas with future projections of precipitation using both a high emission and low emission scenario to rank areas by priority. 

Our consideration of geography for prioritizing NCS across landscapes acknowledges ecological and socioeconomic challenges in favor of an integrated view for local implementation as a path for scaling up climate change responses. We introduce a compelling vision for policy on climate solutions, which includes the co-benefits of conservation for socioeconomic equity and sustainable development. This prioritization protocol can be adapted at the local level to guide policy for targeting the highest priority locations for NCS. 

Carbon-water tradeoffs in a hypothetical Pacific Northwest transect. Inset figure shows relationship between evapotranspiration (ET), gross primary productivity (GPP), and leaf area index (LAI). Boxplots show the carbon-water tradeoff (GPP and ET) across ecosystems (data from Baldocci and Penuelas, 2018).  Modelled soil organic carbon (SOC) data from were compiled by randomly selecting a point in each ecosystem (except wetlands) across a transect from Cape Perpetua, OR to Burns, OR. Mean derived SOC values are listed next to each data point and quantiles of the distribution were computed with Quantile Regression Forests. The mean was computed using the default random forests algorithm. The 0.05 and 0.95 quantiles (gray shaded area) present the lower and upper boundaries of a 90% prediction interval and are used as a measure of prediction uncertainty. Representative wetland SOC is after Hinson et al. (2017) using National Wetlands Inventory data and the U.S. Soil Survey Geographic Database to model the area-weighted wetland SOC to 100 cm depth across all U.S. west coast tidal wetlands 

5. Neoproterozoic (~700 Ma) Snowball Earth in the Grand Canyon

About 700 million years ago, the surface of the Earth may have partially or almost entirely froze over. What was the cause?

About 700 million years ago during one of Earth’s “icehouse” climates, the planet appears to have almost entirely frozen over. The reasons leading to this “snowball Earth” are poorly understood. However, there is evidence of a rapid diversification of life including the evolution of most eukaryotic lineages just prior to the first glaciation event. One hypothesis is that increased chemical weathering on land led to a global drawdown in atmospheric CO2, because the weathering of silicate minerals consumes CO2 from the atmosphere. We went to the Grand Canyon to investigate marine and continental deposits shortly before the onset of glaciation to test the hypothesis that an increase in weathering and productivity on land and at sea may have driven the onset of Neoproterozoic snowball Earth. We hiked down the North Rim’s Nankoweap trail to describe and sample continental deposits near Nankoweap Creek near the Colorado River. We discovered Tonian (~776–729 Ma) sedimentary deposits which showed geochemical and morphological evidence consistent with subaerial weathering, which is a soil-forming process. One explanation for the well-studied carbon isotope excursions in these sediments is exposure to subaerial weathering during sea level regression.

6. Stuck in the clay! Organic matter preservation in clay-rich rocks of Earth and Mars

Our Mars analog site: Eocene / Oligocene clay-rich paleosols (buried Alfisols and Inceptisols) at the Painted Hills Unit of the John Day Fossil Beds National Monument, Oregon (Photo: William Horton)

Rocks with high amounts of clay have been detected in thousands of locations across the surface of Mars. In these rocks, organic matter has been found. The abundant clay-rich rocks on Mars preserve information about past habitable climates on Mars, and may aid in preservation of organic matter over billions of years. However, it is not well understood how clay mineralogy or other variables like depth of burial influence the long-term preservation of organics in terrestrial rocks. The question emerges:

What factors control organic matter preservation in clay-rich rocks for millions or even billions of years?

We first use a series of clay-rich paleosols (fossil soils) on Earth to show that soils with iron/magnesium smectite clays that formed under reducing conditions can preserve the greatest amount of organic matter.

Next, a compilation of previously published data and original research spanning a diverse suite of paleosols from the Pleistocene (1 Myr) to the Archean (3.7 Byr) show that redox state is the predominant control for the organic matter content of paleosols. Most notably, the chemically reduced surface horizons (layers) of Archean (2.3 Byr) paleosols have organic matter concentrations ranging from 0.014–0.25%. However, clay mineralogy, amorphous phase abundance, diagenetic alteration and sulfur content are all significant factors that influence the preservation of organic carbon. The surface layers of paleosols that formed under chemically reducing conditions with high amounts of iron/magnesium smectites and amorphous colloids should be considered high priority locations for biosignature investigation within subaerial paleoenvironments on Mars.

7. A record of drought stress preserved in fossil wood and soils

 Do carbon and oxygen isotopes in fossil cellulose and soil carbonate reflect past atmospheric water deficit?

 Soils, plants and the atmosphere are intricately connected because plants reside in soils and exchange gases with the atmosphere. It is uncertain how modern climate change will alter the global water balance, which is important for predicting future drought stress across the global soil-plant-atmosphere system. A key question driving this work is: With modern climate change, where on Earth will get wetter, and where will get drier? It is known that drought stress leaves an imprint on the isotope ratios of carbon and oxygen within modern tree-ring cellulose and soil carbonate, but this relationship has never been investigated in fossil trees or paleosols to see if it can be extended to the fossil record, thus allowing for estimates of drought stress across biomes during past climate change events. First, I compiled previously published isotope data from modern trees growing around the world in different climates. I found that the isotope ratios of carbon and oxygen in the wood of modern trees and in soil carbonate were significantly correlated to the vapor pressure deficit (a metric of drought stress) in the atmosphere when each formed. I then used these data as a “training set” to extend predictions to the fossil record by developing a function for measuring drought stress that can be applied to fossil wood and paleosols, discussed in detail below.

Eocene 52 ma fossil metasequoia from lac de gras, canada
Eocene (52 million years old) fossil Metasequoia, “Dawn redwood” Lac de Gras, Canada Courtesy of Prince of Wales Northern Heritage Centre
These and other suitably preserved samples may hold a record of vapour pressure deficit in the ratios of carbon and oxygen isotopes.

Predicting past atmospheric moisture levels: During past greenhouse climates, where on Earth did it get wetter? Where did it get drier?

The drying power of air, or vapour pressure deficit (VPD), is an important measurement of potential plant stress and productivity. A geological record of VPD is needed for paleoclimate studies attempting to forecast future climate from past greenhouse spikes, but at present there are few quantitative atmospheric moisture proxies that can be applied to fossil material.  Here we show that VPD leaves a permanent record in the slope of the significant positive correlation between stable isotopes of carbon and oxygen (13C and 18O) in cellulose and pedogenic carbonate, which can be used to reconstruct VPD.

The effect of increasing vapor pressure deficit (VPD, the drying power of the air) on stable isotope ratios in modern plant cellulose and soil carbonate (From Broz et al. 2020 in review)

Our new compilation of data across four continents shows that the slope (S) of the orthogonal least squares regression of 13C and 18O isotope ratios is related to VPD within and across biomes. We discuss physiological and biophysical mechanisms that contribute to the significant positive correlation observed between VPD and (S), which we argue can be used as a proxy that could help reconstruct past and predict future climatic conditions. As one application, we used this proxy to estimate VPD of 0.44 kPa ±0.23 kPa for cellulose of Eocene (45 Ma) Metasequoia from Axel Heiberg Island, and 0.80 kPa ±0.52 kPa for Oligocene (26 Ma) pedogenic carbonate from Oregon, both of which are consistent with other existing records. 

8. Paleontology: Life and times of Pleistocene (43 thousand years ago) Columbian Mammoths in eastern Oregon

We report the discovery of a Columbian mammoth (Mammathus columbi) trackway and the buried, fossil soils on which they resided. Trackways are rare, and can reveal the behavior, habits, and community structure of extinct organisms like this one.

gregshineJuly and mammoth excavation 435July and mammoth excavation 41926283326798_01f6310d64_o
Our 8 by 20 m excavation of the mammoth trackway found 117 tracks, including one 20-m-long adult trail, partial trackways of 3 additional adults, a yearling and a baby.

9. Environmental toxicology – Arsenic and lead transport in soils and water

-Arsenic and Selenium in central California soils and wines
Sampling soils at a central California vineyard for determination of arsenic and selenium

10. Water remediation with organic materials: Can worm castings (vermicompost) remove excess nitrogen from agricultural wastewaters?

-Nitrate removal from solution with vermicomposts: kinetics, mechanisms and point of zero-charge determination
July 2016 101

Laboratory batch study with dairy-manure vermicompost

11. Vermicompost (worm castings) as a replacement for synthetic fertilizer in specialty crop production

In specialty crops like strawberry (Fragaria x ananassa), can vermicompost application at some “goldilocks” level allow for similar growth and fruit yield compared to conventional synthetic fertilizer application?
-Nitrogen dynamics of strawberry growth in vermicompost-amended media
Greenhouse strawberry growth trials with dairy-manure vermicompost, Cal Poly State University, San Luis Obispo, CA

12. Plant ecology and evolution

Do geographic barriers disrupt gene flow between two subspecies of native mint plants? How effective are geographic boundaries at maintaining distinct subspecies?
-Testing subspecies limits in Monardella villosa
Monardella villosa ssp. franciscana (courtesy of CalPhotos)

13. Astropedology: Soils in Space

Soils on Earth and Mars: What are the abiotic pathways of pedogenesis on Earth and elsewhere?
Possible thin acid sulfate paleosol at the ~3.5 billon year old Sheepbed member of Yellowknife formation, Gale Crater, Mars (Courtesy of NASA/JPL)

14. The role of mineral surfaces in the origin of life

Where did life on earth begin?
-Alkaline hydrothermal vents and phyllosilicate clays as locations for the origin of life
Spontaneous polymerization of amino acids in clay interlayer space (from Erastova et al., 2017)

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