The emerging field of astropedology is the study of ancient soils on Earth and other planetary bodies. Examination of the complex factors that control the preservation of organic matter and other ...biosignatures in ancient soils is a high priority for current and future missions to Mars. Though previously defined by biological activity, an updated definition of soil as planetary surfaces altered in place by biological, chemical or physical processes was adopted in 2017 by the Soil Science Society of America in response to mounting evidence of pedogenic-like features on Mars. Ancient (4.1–3.7 billion year old Byr) phyllosilicate-rich surface environments on Mars show evidence of sustained subaerial weathering of sediments with liquid water at circumneutral pH, which is a soil-forming process. The accumulation of buried, fossilized soils, or paleosols, has been widely observed on Earth, and recent investigations suggest paleosol-like features may be widespread across the surface of Mars. However, the complex array of preservation and degradation factors controlling the fate of biosignatures in paleosols remains unexplored. This paper identifies the dominant factors contributing to the preservation and degradation of organic carbon in paleosols through the geological record on Earth, and offers suggestions for prioritizing locations for in situ biosignature detection and Mars Sample Return across a diverse array of potential paleosols and paleoenvironments of early Mars. 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.
Nonmarine rocks in sea cliffs of southern California store a detailed record of weathering under tropical conditions millions of years ago, where today the climate is much drier and cooler. This work ...examines early Eocene (~ 50-55 million-year-old) deeply weathered paleosols (ancient, buried soils) exposed in marine terraces of northern San Diego County, California, and uses their geochemistry and mineralogy to reconstruct climate and weathering intensity during early Eocene greenhouse climates. These Eocene warm spikes have been modeled as prequels for ongoing anthropogenic global warming driven by a spike in atmospheric CO
. Paleocene-Eocene thermal maximum (PETM, ~ 55 Ma) kaolinitic paleosols developed in volcaniclastic conglomerates are evidence of intense weathering (CIA > 98) under warm and wet conditions (mean annual temperature MAT of ~ 17 °C ± 4.4 °C and mean annual precipitation MAP of ~ 1500 ± 299 mm). Geologically younger Early Eocene climatic optimum (EECO, 50 Ma) high shrink-swell (Vertisol) paleosols developed in coarse sandstones are also intensely weathered (CIA > 80) with MAT estimates of ~ 20 °C ± 4.4 °C but have lower estimated MAP (~ 1100 ± 299 mm), suggesting a less humid climate for the EECO greenhouse spike than for the earlier PETM greenhouse spike.
Problematic fossils are described from Late Ediacaran to Early Cambrian red sandstones of the Arumbera Sandstone, Grant Bluff, and Central Mount Stuart Formations in central Australia, within a new ...systematic classification of Vendobionta. Arumberia banksi has been one of the most problematic of Ediacaran fossils, at first considered a fossil and then a sedimentary or organo-sedimentary structure. Our re-examination of the type material and collection of new material reveals misconceptions about its topology: it was a recessive fossil on the bed top, protruding down from the counterpart overlying slab. The concave-up body was 3 mm thick and chambered above a diffuse lower surface, so not a sedimentary structure. Also re-evaluated is the discoid fossil Hallidaya brueri, here including "Skinnera brooksi' as its upper surface. A new species (Noffkarkys storaaslii gen. et sp. nov.) is a multilobed frond with regular, fine, trapezoidal quilts. Three new records of Trepassia wardae, Dickinsonia costata, and Ernietta plateauensis are reported from the Arumbera and Grant Bluff Formations. Reevaluation of palaeomagnetic and biostratigraphic data suggest an hiatus of 26 million years at the Ediacaran-Cambrian boundary within the Arumbera Formation, but some of this missing time is filled by the Grant Bluff and Central Mount Stuart Formations.
Natural climate solutions have been proposed as a way to mitigate climate change by removing CO 2 and other greenhouse gases from the atmosphere and increasing carbon storage in ecosystems. The ...adoption of such practices is required at large spatial and temporal scales, which means that local implementation across different land use and conservation sectors must be coordinated at landscape and regional levels. Here, we describe the spatiotemporal domains of research in the field of climate solutions and, as a first approximation, we use the Pacific Northwest (PNW) of the United States as a model system to evaluate the potential for coordinated implementations. By combining estimates of soil organic carbon stocks and CO 2 fluxes with projected changes in climate, we show how land use may be prioritized to improve carbon drawdown and permanence across multiple sectors at local to regional scales. Our consideration of geographical context acknowledges some of the ecological and social challenges of climate change mitigation efforts for the implementation of scalable solutions.
Ediacaran to Cambrian red sandstones of the Northern Territory, including Arumbera Sandstone, and Grant Bluff and Central Mount Stuart formations, have been reexamined and sampled in order to ...reconstruct paleoenvironments from sedimentary facies and paleosols. Sedimentary facies include green-gray calcareous sandstone (Member III of Arumbera Sandstone only), red siltstone with calcareous nodules (also only Member III), and also non-calcareous red-green siltstone, interbedded red siltstone and sandstone, and trough bedded sandstone. The green-gray facies has diverse and distinctive Cambrian marine trace fossils, not found in the red beds which have instead an assemblage of problematic body fossils including Arumberia, Noffkarkys, Hallidaya, Ernietta, Trepassia, and Dickinsonia. The red facies are interpreted as fluvial paleochannels (trough cross bedded), fluvial levees (interbedded siltstone and sandstone), poorly drained floodplain (red-green siltstone), and well drained floodplain or low terrace (calcareous nodular). The carbonate nodules have micritic, replacive fabrics and a significant positive correlation of δ13C and δ18O characteristic of pedogenic carbonate of Calcid soils. Other red beds have pseudomorphous sand crystals after gypsum, like those of Gypsid soils. Individual red beds show negative strain and mass transfer for cationic bases typical of soils, as well as gradational alteration down from fossiliferous tops characteristic of soils. Their varied degree of pedogenic differentiation is identified as Psamment, Aquent and Ochrept paleosols. Phosphorus depletion in the paleosols is evidence that organisms lived in the paleosols, and were not introduced by transient marine incursions. The problematic quilted fossils Arumberia, Noffkarkys and Hallidaya formed a distinctive inland terrestrial polsterland during the Ediacaran and early Cambrian.
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•Ediacaran-Cambrian Arumbera, Grant Bluff and Central Mount Stuart Formations have many paleosols.•Carbonate nodules have positive correlation of δ13C and δ18O of Calcid soils.•Pseudomorphous sand crystals after gypsum are like those of Gypsid soils.•Phosphorus depletion in the paleosols is evidence that organisms lived in the paleosols.•Arumberia, Noffkarkys and Hallidaya formed a distinctive inland terrestrial polsterland.
Ancient, buried soils, or paleosols, may have been preserved in the geological record on Mars, and are considered high‐priority targets for biosignature investigation. Studies of paleosols on Earth ...that are similar in composition to putative martian paleosols can provide a reference frame for constraining their organic preservation potential on Mars. However, terrestrial paleosols typically preserve only trace amounts of organic carbon, and determining what carbon is original is complicated by diagenesis and additions of modern carbon. The objectives of this study were (a) to determine whether organic carbon in Mars‐analog paleosols can be detected with thermal and evolved gas analysis, and (b) constrain the age of organic carbon using radiocarbon (14C) dating. Oligocene (33 Ma) paleosols from Oregon were examined with an instrument similar to the Sample Analysis at Mars Evolved Gas Analysis instrument onboard the Mars Science Laboratory Curiosity rover. Trace amounts of organic carbon and fragments of organic molecules were observed in all samples. Total organic carbon (TOC) ranged from 0.002 to 0.032 ± 0.006 wt. %. The near‐surface horizons of paleosols had significantly higher TOC relative to subsurface layers. Radiocarbon dating of four samples revealed an organic carbon age of ∼6,200–14,500 years before present and a fraction modern ranging from 0.16 to 0.46. Modeled abundances of modern carbon in bulk samples ranged from 0.41%–3.1% ± 0.11%, which were consistent with additions of small amounts of modern organic carbon. This work demonstrates that martian paleosols are a potential high priority location for in‐situ biosignature investigation.
Plain Language Summary
Ancient, buried soils, or paleosols, may have been preserved in the geological record on Mars. On Earth, paleosols that are billions of years old contain signs of ancient life (biosignatures) and therefore paleosols on Mars are considered high‐priority locations for biosignature investigation. One way to determine the biosignature preservation potential of possible martian paleosols is to examine organic carbon preservation in paleosols from Earth that resemble martian paleosols. The objectives of this work were to determine if carbon in Mars‐analog paleosols can be detected by an instrument configured to operate like the Sample Analysis at Mars Evolved Gas Analyzer onboard Curiosity rover, and to use radiocarbon dating to determine if samples contain modern carbon. Trace amounts of organic carbon and fragments of organic molecules were observed. Like modern soils, the near‐surface horizons of all paleosols had significantly higher amounts of carbon relative to subsurface layers. Radiocarbon dating revealed an organic carbon age of ∼6,200–14,500 years before present. Application of an isotopic mixing model suggested these dates could be explained by slight additions of modern carbon to ancient samples. These results demonstrate that putative martian paleosols are a potential high priority location for in‐situ biosignature investigation.
Key Points
Trace amounts of organic carbon (OC) in Mars‐analog paleosols were detected with thermal and evolved gas analysis
The near‐surface horizons of 33‐million‐year‐old paleosols had significantly higher OC content relative to deeper layers
Radiocarbon dates of approximately 6–14 thousand years were consistent with small amounts of modern carbon in paleosol samples
Excavation of a green breccia lens in the otherwise shaley Hudspeth Formation of central Oregon recovered a surprising variety of fossils of different kinds: plant impressions, mollusc shells, and ...vertebrate bones and teeth. Some ammonites, such as Mortoniceras inflatum, which date the deposit as late Albian (103 Ma), were preserved with shell intact below, but dissolved above back to sutures. Intact ammonites were outnumbered by angular ammonite fragments in the size range 1–2 cm. Marine clams, snails and ammonites were found in the same bed as a variety of fossil plants, including horsetails, ferns, cycads, and conifers. Also recovered were bones of fish, ichthyosaurs, dinosaurs, and pterosaurs. Orientation of large fossil logs and branches reveal derivation from the east, where a shoreline of shingle beaches and alluvial fans created the interfingering Gable Creek Conglomerate. The matrix-supported green breccia is massive and ungraded, and interpreted as a mass flow deposit. Enrichment in phosphorus compared with shale above and below is evidence that its fine-grained component includes guano. This guano and unusual fragmentation of ammonites are interpreted as evidence that the Oregon pterosaur, Bennettazhia oregonensis, was a mollusc-eater and formed large colonies on nearby cliffs, like modern gull rookeries.
Ancient (4.1–3.7-billion-year-old) layered sedimentary rocks on Mars are rich in clay minerals which formed from aqueous alteration of the Martian surface. Many of these sedimentary rocks appear to ...be composed of vertical sequences of rocks containing Fe/Mg clay minerals overlain by rocks with Al clay minerals that altogether resemble paleosols (ancient, buried soils) from Earth. The types and properties of minerals in paleosols can be used to constrain the environmental conditions during formation to better understand weathering and diagenesis on Mars. This work examines the mineralogy and diagenetic alteration of volcaniclastic paleosols from the Eocene-Oligocene (43–28 Ma) Clarno and John Day Formations in eastern Oregon as a Mars-analog site. Here, paleosols rich in Al phyllosilicates and amorphous colloids overlie paleosols with Fe/Mg smectites that altogether span a sequence of ~ 500 individual profiles across hundreds of meters of vertical stratigraphy. Samples collected from three of these paleosol profiles were analyzed with visible/near-infrared (VNIR) spectroscopy, X-ray diffraction (XRD), and evolved gas analysis (EGA) configured to operate like the SAM-EGA instrument onboard Curiosity Mars Rover. Strongly crystalline Al/Fe dioctahedral phyllosilicates (montmorillonite and nontronite) were the major phases identified in all samples with all methods. Minor phases included the zeolite mineral clinoptilolite, as well as andesine, cristobalite, opal-CT and gypsum. Evolved H2O was detected in all samples and was consistent with adsorbed water and the dehydroxylation of a dioctahedral phyllosilicate, and differences in H2O evolutions between montmorillonite and nontronite were readily observable. Detections of hematite and zeolites suggested paleosols were affected by burial reddening and zeolitization, but absence of illite and chlorite suggest that potash metasomatism and other, more severe diagenetic alterations had not occurred. The high clay mineral content of the observed paleosols (up to 95 wt%) may have minimized diagenetic alteration over geological time scales. Martian paleosols rich in Al and Fe smectites may have also resisted severe diagenetic alteration, which is favorable for future in-situ examination. Results from this work can help differentiate paleosols and weathering profiles from other types of sedimentary rocks in the geological record of Mars.
•Paleosols from the Oligocene (33 Ma) John Day Formation in Oregon were assessed with a multi-instrument suite relevant to Mars exploration.•Strongly crystalline Al/Fe dioctahedral phyllosilicates were the major phases identified in all samples (up to ~95 wt%).•Detections of fine-grained hematite and zeolites suggested paleosols were affected by burial reddening and zeolitization.•Absence of illite and chlorite suggest that illitization and other, more severe diagenetic alterations had not occurred.•The high clay mineral content of the observed paleosols (up to 95 wt%) may have minimized diagenetic alteration over geological time scales.•Results from this work can help distinguish paleosols from other types of sedimentary rocks in the geological record of Mars.
Paleosols are characterized with new chemical and petrographic data from the Tonian (776–729 Ma) Chuar Group in the eastern Grand Canyon, Arizona. Paleosols have been discovered at 66 stratigraphic ...levels in a 1622 m sequence largely of marine shales and thin intertidal stromatolitic dolostones. A purple paleosol association includes gypsum “desert roses” in profiles like Gypsids of hyperarid climates. A red paleosol association includes calcareous nodules profiles like Calcids of semiarid climates. Degree of weathering and phosphorus depletion of the paleosols is evidence of microbial-earth communities gaining in productivity during the Tonian. The gypsic to calcic soil transition in the modern Atacama Desert of Chile corresponds with a transition from simple actinobacterial-cyanobacterial microbial earths to complex algal-fungal communities of markedly greater weathering effectiveness and carbon sequestration. A similar change in biological weathering may have induced Snowball Earth 12 Myrs after deposition of the Chuar Group. Paleosols are found only at levels of the Chuar Group where δ13C isotopic composition of both organic matter and of carbonate are below marine and mantle values. Although such inflections in isotopic time series have been considered chemostratigraphic markers of global oceanic composition, in this case we interpret them to reflect a change in sea level from shallow marine to terrestrial. A notable inflection at about 752 Ma in the Chuar Group corresponds with the recently recognized Konnarock Glaciation of Virginia, and may have been a time of global sea-level fall. Correlation of Precambrian marine sequences by wiggle matching isotopic time series may still be effective, but because of sea-level change rather than for the chemical oceanographic reasons originally envisaged.
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•Paleosols are common in Tonian (776–729 Ma) Chuar Group in the Grand Canyon, Arizona.•Purple paleosols include gypsum “desert roses” in profiles like Gypsids of hyperarid climates.•A red paleosols include calcareous nodules profiles like Calcids of semiarid climates.•Paleosols reveal increased biological weathering on land as a possible cause of Snowball Earth.•Isotopic chemostratigraphy of the Chuar Group reflects sea level change not ocean chemistry.
Behavior of Columbian mammoths (Mammuthus columbi) is revealed by a newly discovered trackway at the Pleistocene locality of Fossil Lake, Oregon. 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 all heading generally west. The tracks are in the Marble Bluff biotite tuff (43.2 cal ka), which forms a surface horizon to the Pogani silty clay loam paleosol (Natrargid), with a cracked surface and a columnar-structured, subsurface (Bn) horizon, like soils under desert soda pans with alkali shrubland. Directly below is the Yada silty clay paleosol (Xeroll) with crumb textured surface (A) horizon like grassland soils. The Pasiwa loam (Psamment) is a thin brown siltstone, with sparse roots and burrows of lake-margin early successional vegetation. The Pui sandy loam (Aquent) is well-bedded tuff and sand (A) with subhorizontal calcareous rhizomes and adventitious roots, like those of lake-margin tule reeds (Schoenoplectus acutus). Columbian mammoths may have moved like modern elephants with infants in matriarchal groups through landscapes of sagebrush and grassland, and this trackway includes a limping female attended by concerned juveniles. Grassland paleosols common in the Fossil Lake Formation, are now rare in the same region, perhaps related to extinction of proboscidean and equine grazers.
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•43.2 cal ka paleosol has 117 tracks of Columbian mammoth in central Oregon.•Main trackway 20 m long was limping, and overriding tracks of concerned juveniles.•Trackway is on alkali shrubland paleosol (Natrargid) with bare earth between bushes.•Other Mollisol paleosols with mammoth bones record mammoth steppe in Oregon.
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