A few years ago, we were surfing at Cardiff-by-the-Sea over the magical reefs that lend the area incredible surfing conditions and shapely wave formation. Something out of the ordinary caught my eye – paleosols sandwiched in the beach cliff! At the south end of San Elijo State Beach, near the rivermouth – there was a perfectly preserved sequence of three deeply weathered paleosols (ancient, buried soils). Even more exciting was the realization that Cardiff Reef itself (the shore platform extending out to sea) was part of the paleosol sequence! Who could have imagined – surfing atop an ancient soil!

After a short review of the geological history of the region, it was apparent that we were dealing with REALLY OLD soils – between 50 and 55 million years old! Even more exciting was that these soils had the telltale signs of intense, tropical weathering: the profiles were dominated by kaolinite and were very thick, approximately 3 meters deep. So, our surfing/geology side project began. We returned in January of 2021 to describe and sample the ancient soils at three locations where they are naturally exposed in the beach cliffs: San Elijo, Black’s Beach, and Torrey Pines. The objective of this work was to constrain the paleoclimate conditions (mean annual precipitation, mean annual temperature) during soil formation. Critically, this period in geological history (known as the Paleocene-Eocene “thermal maximum” was the last time that atmospheric CO2 was as high as is projected for the year 2100 (between ~1000 and 2000 ppmv CO2, we are currently at ~420 ppmv). Therefore, studying soils of the past can help to constrain how climate and ecosystems will respond to modern global warming, and provide unprecedented insight into the climate evolution of present day southern California during the Paleocene-Eocene thermal maximum.

Abstract: Modern deserts of southern California are known for their minimally weathered soils and low annual precipitation. However, newly described early Eocene (50-55 million-year-old) paleosols (ancient, buried soils) in beach cliffs of northern San Diego County, California reveal severe weathering under a greenhouse climate millions of years ago in present-day California. Here we provide a reconstruction of climate and weathering intensity on land during the early to mid-Eocene in southern California using the geochemistry and morphology of deeply weathered nonmarine sedimentary rocks. Early Eocene (~55 Ma) kaolinitic Oxisol paleosols developed atop a parent material of alluvial quartzite that was subject to intense subaerial alteration (CIA >98), characteristic of severe weathering under subhumid tropical conditions.  Less altered Middle Eocene (50 Ma), high shrink-swell (Vertisol) paleosols developed in siliciclastic sediments are also highly weathered (CIA >75), though to a lower degree relative to the older Oxisols, suggesting a decline in weathering intensity over ~5 Ma. Subtidal to supratidal mangrove paleosols also Eocene (~50 Ma) in age contain pyritized root traces and coalified wood which altogether allow for a reconstruction of landscape ecology and climate. We discuss paleoclimate transitions across landscapes and implications for early Eocene climate and weathering in present-day southern California.

The Mars Science Laboratory onboard Curiosity rover has been traversing up the slopes of Mt. Sharp since landing on Mars in 2012. In late 2021, the rover entered a new region of Mt. Sharp known as the “Sulfate-bearing unit” because of the widespread orbital detections of sulfate minerals from orbit (Figure 1).

The sulfate-bearing unit has spectral signatures of hydrated sulfates and is estimated to be ~700 meters thick in the northwest portion of Mt. Sharp (Rapin et al., 2021). Large-scale (5-10 meter) trough cross beds in the lowermost unit are consistent with an eolian sequence. Stratigraphically above the basal unit, a possible deflationary surface (the “Marker bed”) is topped by what appears to be a fluvial depositional system, which is consistent with an alternating wet-dry climate regime for this unit, rather than a monotonic shift to arid conditions. Since alternating wet-dry conditions in terrestrial environments can lead to regional-scale subaerial weathering of sediments, it is possible that individual weathering profiles could have formed in the sulfate-bearing unit during periods of subaerial exposure.

The identification of what appears to be the first paleosol at Gale Crater (Rapin et al., 2022) (Figure 1) is consistent with the alternating wet-dry-hypothesis for the origin of the sulfate-bearing unit. Polygonal mudcracks and sulfate nodules are common features of sulfate-rich lake margin soils and are also common in more developed smectitic (montmorillonite/nontronite-rich), shrink-swell soils, classified as Vertisols in US taxonomy. These observations now present a unique opportunity for comparisons with terrestrial weathering profiles containing similar features such as mudcracks and nodular sulfate minerals which together can provide a reference frame for evaluating a subaerial weathering hypothesis for the origin of altered sediments at this location.

We have submitted a proposal the the NASA SSW program to fund a terrestrial analog study to evaluate a pedogenic origin for features observed in the sulfate-bearing unit by examining sulfate-rich paleosols with rover-like instruments (Figure 2). Wish us luck!

Rapin, W., Dromart, G., Rubin, D., Deit, L. Le, Mangold, N., Edgar, L. A., et al. (2021). Alternating wet and dry depositional environments recorded in the stratigraphy of Mount Sharp at Gale crater , Mars. Geology, 49(7), 842–846.

Rapin, W., Sheppard, R., Dromart, G., Schieber, J., Kah, L., Rubin, D., et al. (2022). The Curiosity rover is exploring a key sulfate-bearing orbital facies. Lunar and Planetary Science Conference, 2473.

MOMA is an instrument designed to detect organic molecules on Mars and will fly onboard the upcoming ExoMars 2022 Rosalind Franklin rover, due to land in June 2023 at the ~4 billion year old Oxia Planum region. Here, what appear to be ancient weathering profiles have been detected from orbital remote sensing, and the rover may encounter these rocks during its primary mission. Exciting science to come!

From this year’s Astrobiology Graduate Student Conference (Remote, 14-17 September 2021)

Topic: Searching for biosignatures in Earth’s oldest soils

Before research lockdown due to COVID-19:

Assisting with the deployment of the Mastcam-Z engineered prototype, a multispectral, high resolution 3D camera which is now flying on Mars 2020 Perseverance Rover. The prototype was designed and built by Megan Barrington and Alex Hayes, Cornell University. It was an exciting and humbling opportunity to work with members of the Mastcam-Z team during the deployment of the prototype at our Mars-analog site (near the Painted Hills, Eastern Oregon).

The Perseverance rover will seek signs of ancient life and collect rock and soil samples for possible return to Earth.

The goal of this research was to collect hyperspectral images of soil clays and other hydrated phases for use during the Mars 2020 Perseverance Rover mission.

~30 million year old leaf impression fossils of plants from the Rujada Flora from a forest roadcut through the Fisher Formation near Cottage Grove, Oregon. Specimens include leaves of sumac (Rhus varians), alder (Alnus carpinoides), tanoak ( Notholithocarpus simulans), and dawn redwood (Metasequoia occidentalis)!

The Oligocene (~30 Ma) Rujada flora (42 spp.) and the nearby Willamette flora (40 spp.) are dominated by oak (Quercus consimilis) and alder (Alnus heterodonta). The occasional fossil salamander (Palaeotaricha oligocenica) and caddis fly cases of Metasequoia needles (ichnogenus Folindusia) have also been collected in the area.

These leaves fell into and were preserved in an ancient lake. Burial in lake sediments inhibited the decay of organic carbon in the leaves, which makes for exceptional preservation today. A recent roadcut exposed these ancient rocks. Thanks!

The study of paleobotany can help us understand how plants adapted to climate change millions of years ago, which can inform our predictions of how plants today may respond to modern climate change.

A few specimens acquired in the Amadeus Basin, near Alice Springs, Northern Territory, Australia, and range in age from ~380 Ma – 1 Ga.

Adrian Broz, Greg Retallack, Briony Horgan, Lucas Silva, and Matt Polizzotto

Sequence of ~33 million year old clay-rich fossil soils (paleosols) at the Painted Hills, John Day Fossil Beds National Monument, Oregon (Photo: Jamie Francis). These paleosols are similar in mineral composition (lots of hydrated clays) and  stratigraphic distribution to clay-rich areas on Mars at Mawrth Vallis, Nili Fossae, Oxia Planum, and elsewhere. Importantly, the ExoMars 2020 rover is going to Oxia Planum, which is a westward extension of the Mawrth Vallis clay layers.

Across ancient surface environments of Mars, Where are the best places to search for past signs of life? Where are you most likely to find something interesting, if it is there?

Mars offers the tantalizing prospect of being the most immediate and accessible location to test the hypothesis that life has existed elsewhere. Current and planned missions to Mars are investigating clay-rich areas (dioctahedral and trioctahedral phyllosilicate clay-rich rocks), but it is not well understood which types of clays (there are many types) are best at preserving organic matter or other biosignatures over geological time scales.

This research seeks to prioritize locations for in-situ biosignature detection (Curiosity and Mars 2020) and Mars Sample Return. Our approach is to examine clay-rich paleosols on Earth that are strikingly similar in clay mineralogy and stratigraphy to clay sequences that have been detected on Mars. Stay tuned for future updates.

                                                                             CLAY!

Sticky, slimy stuff….

Clays are cool because:

1) Their mineralogy can record the aqueous history of a given location by constraining the temperature, pH and water:rock ratio during clay formation.

2)  they have been implicated in many origin-of life theories (especially 2:1 phyllosilicates) because they facilitate spontaneous polymerization of complex macromolecules (like RNA) and provide the structural framework for concentration and preservation of these macromolecules;

and 3) They can preserve organic matter from oxidation and radiation, possibly over billions of years…

Why soil? Well..

(Source: Penn State Soil Characterization Lab)

10. Soil is forgiving….soil is a dynamic resource that can be restored and used again in the lifetime of a human.

9. Unless you live in a houseboat, your house is likely built on soil (even a houseboat is built from wood that came from a tree growing on soil).

8. Care for a beer? Not gonna happen without those plants that just happen to grow in soil.

7. Worried about global warming? Then be happy that soil sequesters about 2x the amount of carbon found in all vegetation and the atmosphere combined!

6. Cotton doesn’t just come from the mall…You get a lot of your clothes from crops that grow on soil.

5. Ever been sick? You probably have taken an antibiotic that was derived from organisms in the soil.

4. Ever drink water from a well or stream? You could likely die of contaminated water if there was not soil to filter water for drinking.

3. Enjoy eating? You would likely starve to death if you could not eat plants that grow in soil.

2. Like breathing? You probably would not be breathing if there was not soil for plants to grow in that produce the oxygen keeping you alive.

1. Imagine a world where nothing that died decomposed. Soil microorganisms are required for breaking down dead things on the surface of earth!