Geology and surfing: An Eocene (50-55 Ma) tropical greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA

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!

A sequence of Eocene (~50 Ma) deeply weathered paleosols (green and red layers) in the beach cliff at the south end of San Elijo State Beach, Cardiff, CA.
Cardiff Reef, San Elijo State Beach, Cardiff, CA (Getty images)

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.

The lowermost paleosol profile (mottled green-red due to burial gleization) composes the shore platform and goes out to sea to make the incredible surfing spot known to local surfers as “Cardiff Reef”. Massive and extensive sand-filled polygonal mudcracks are diagnostic of a Vertisol (high shrink-swell soil).

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.

Erosional remnant of a kaolinitic Oxisol paleosol (55 Ma) in the Mt. Soledad Formation, Black’s Beach, La Jolla, CA (directly below the “Indian Trail” at UCSD).

Did the Curiosity rover just encounter an ancient soil profile on Mars?

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.

Figure 1. Putative paleosol within the basal sulfate-bearing unit (LSu) at Gale crater (a-b) inferred from decimeter-sized concretions (c), dark-toned nodular beds (d) and sulfate-enriched polygonal mudcracks (e-f), adapted from Rapin et al., 2022

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!

Figure 2. Desert roses (A-D) and crystals (E-F) in hand specimens of terrestrial paleosols, and G-H, comparisons with concretions at Gale crater: A, clay pseudomorph of gypsum desert rose from By horizon of Thamberalg pedotype in Ediacaran (599 Ma) Ranford Formation, Donkey Creek, Western Australia; B, silica pseudomorphs of gypsum desert roses in By horizon of Muru pedotype from Ediacaran (547 Ma) Ediacara Member in Brachina Gorge, South Australia; C, barite desert roses from Oligocene (20 Ma) Rockenberg Formation near Rockenberg, Germany; D, barite desert roses from By horizon of pedotype from Early Permian (270 Ma) Garber Sandstone near Cimarron City, Oklahoma; E, silica pseudomorphs of mirabilite in A horizon of Viku pedotype from Cryogenian (640 Ma) Reynella Siltstone Member Hallett Cove, South Australia; F, ripidolite pseudomorphs of kieserite in By horizon of Isi pedotype from Archean (3700 Ma) Isua Greenstone near Isukasia, Greenland; G, Mastcam image (Sol 1277) showing concretions from the Murray Formation; H, Mastcam image (Sol 3396) showing concretions and evaporite pseudomorphs from the clay-sulfate transition. Terrestrial specimens in Condon Collection of Museum of Natural and Cultural History, University of Oregon are (a), R4185 (b), F6345 (d), field photographs (c, e-f).

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.

Introducing the Mars Organic Molecule Analyzer (MOMA)

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!

(Rover) eyes on the prize

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.

Mastcam-Z. Photo: Jim Bell
Photo: NASA/JPL-Caltech

What a shock! That plant is locked in a rock.

~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.

Fascinating fossils

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

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Termites are incredible organisms. Many species are farmers: they gather plant material, bring it back to their fungi-colonized mound (usually Ascomycota), let the fungi digest the plant, then eat the fungi.

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Nautiloids from the Ordovician Stairway Sandstone, Maloney Creek, Northern Territory

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Uluru (Ayers Rock), a Precambrian (550 Ma) monolith

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Unknown fossil

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Ordovician (~460 Ma) clams, Mount Watt, Northern Territory. The furthest I have ever been from home. -25.33063687950072, 133.89224836154077

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Trilobite trails (Cruziana), subsurface burrows of one of the earliest animals, Ordovician Mount Watt, Northern Territory

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Ancient seafloor with Cruziana

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Stromatolites (ID?) from the Johnny’s Creek Member of the  ~770 Ma Bitter Springs Formation, Northern Territory

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No idea, ID?

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“Dalmation” rock, a limestone, Johnny’s Creek Member of the Neoproterozoic Bitter Springs Formation: Interesting sedimentary texture, possibly altered by methanogens.

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Glacier rock: Conglomerate, diamictite and
sandstone of the 720 to 660 Ma “snowball earth” Areyonga Formation. It appears to be stromatolitic.

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Early Cambrian (~520 Ma) Bunyerichnus sp. Ross River, Northern Territory

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Nuclear control rod

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 Inzeria intia stromatolite from the 1 Ga Bitter Springs Formation.Summer 2018 - Australia 179

Searching for signs of life in ancient Mars soils

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.


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…

clay catalysis RNA

Ten reasons why soil matters to you

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!



The wonderful world of science at Swanton Pacific Ranch

About the Ranch

Swanton Pacific Ranch is located in Santa Cruz County at the northern reaches of California’s Central Coast and the Monterey Bay. The 3,200 acre property is a landscape composed of a majestic redwood forest, lush riverine ecosystems and expansive coastal grassland overlooking the bay and the Pacific Ocean.

Ranch Overview

Recognized for its high biodiversity and abundant resources, this area provides a valuable opportunity to study the methods of resource conservation applied through sustainable management techniques. The ranch was donated to the California Polytechnic State University in 1993 by the late Al Smith. A Cal Poly graduate and founder of Orchard Supply Hardware, Al had specific goals, “…that Swanton Pacific Ranch be maintained as a working ranch and used exclusively for agriculture, recreational, educational purposes”.

This educational and research facility is owned by the Cal Poly Corporation and managed by the College of Agriculture, Food and Environmental Sciences. Faculty, graduate students and undergraduates actively pursue research opportunities, utilizing the forest, range, and watershed resources within the ranch. The ranch hosts a variety of functions some of which include the production of certified natural beef, “U-pick” certified organic apples, hosting of professional meetings and workshops, and courses offered by the Department of Natural Resource Management.


Natural Diversity as a Living Laboratory

Situated at the northwest end of Santa Cruz County and occupying circa 30 square miles of sharply contrasted terrain, the Scott Creek Watershed concentrates within its geomorphological boundaries, at least 10-12% of California’s flora, both native and introduced.

The Scott Creek Watershed and its environs, is more than an aggregation of
600+ native species (subspecies, varieties and forms), representing 282+ genera and 90+ families: it is that rare occurrence, a living window into California’s evolutionary past, still relatively undeveloped by human activity and spared the habitat degradation that has befallen much of the coastal ecology elsewhere in our state.