Cryptogamic ground covers as analogues for early terrestrial biospheres: Initiation and evolution of biologically mediated proto‐soils
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Abstract Modern cryptogamic ground covers (CGCs), comprising assemblages of bryophytes (hornworts, liverworts, mosses), fungi, bacteria, lichens and algae, are thought to resemble early divergent terrestrial communities. However, limited in situ plant and other fossils in the rock record, and a lack of CGC‐like soils reported in the pre‐Silurian sedimentological record, have hindered understanding of the structure, composition and interactions within the earliest CGCs. A key question is how the earliest CGC‐like organisms drove weathering on primordial terrestrial surfaces (regolith), leading to the early stages of soil development as proto‐soils, and subsequently contributing to large‐scale biogeochemical shifts in the Earth System. Here, we employed a novel qualitative, quantitative and multi‐dimensional imaging approach through X‐ray micro‐computed tomography, scanning electron, and optical microscopy to investigate whether different combinations of modern CGC organisms from primordial‐like settings in Iceland develop organism‐specific soil forming features at the macro‐ and micro‐scales. Additionally, we analysed CGCs growing on hard rocky substrates to investigate the initiation of weathering processes non‐destructively in 3D. We show that thalloid CGC organisms (liverworts, hornworts) develop thin organic layers at the surface (<1 cm) with limited subsurface structural development, whereas leafy mosses and communities of mixed organisms form profiles that are thicker (up to ~ 7 cm), structurally more complex, and more organic‐rich. We term these thin layers and profiles proto‐soils. Component analyses from X‐ray micro‐computed tomography data show that thickness and structure of these proto‐soils are determined by the type of colonising organism(s), suggesting that the evolution of more complex soils through the Palaeozoic may have been driven by a shift in body plan of CGC‐like organisms from flattened and appressed to upright and leafy. Our results provide a framework for identifying CGC‐like proto‐soils in the rock record and a new proxy for understanding organism–soil interactions in ancient terrestrial biospheres and their contribution to the early stages of soil formation.Keywords:
Biogeochemical Cycle
Regolith
Regolith
Soil production function
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[1] Large areas distributed on the Earth's surface are covered by regolith, an unconsolidated heterogeneous material overlying bedrock. In high-latitude areas, most of the land surface has been reworked and eroded by both glacial and fluvial processes, leaving only remnants of formerly extensive regolith covers. In an effort to further the understanding of weathering patterns and processes in old regolith covers, a comprehensive study of localities spread across Norway was carried out. On the basis of the distribution of minerals and elements within regolith, as well as its internal structure and geomorphologic setting, we ascertained that it was formed in situ and originated in pre-Quaternary times. There are similarities between the study sites with respect to regolith thickness, zonation, and composition. The Chemical Index of Alteration (CIA) and the Weathering Index of Parker (WIP) suggests that the degree of chemical weathering in the regolith is advanced compared to the parental bedrock with a maximum change of over 80%, which indicates a substantial increase in the proportion of secondary versus primary minerals. Mineral analysis identified kaolinite and gibbsite, which are considered indicative of advanced weathering and therefore support this observation. On the basis of statistical relationships between different grain size fractions (<125 μm), we observed a consistent pattern, which revealed that physical weathering becomes progressively less important in the production of grains smaller than 32 μm. On the basis of this finding, we infer that chemical weathering progressively dominates the production of fine silt, very fine silt, and clay, whereas physical weathering primarily controls the production of grain size fractions larger than 32 μm. This particular pattern is suggested to be an intrinsic feature in the formation of weathered high-latitude regolith.
Regolith
Bedrock
Silt
Soil production function
Parent material
Saprolite
Gibbsite
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Abstract The porosity of the upper layers of regolith is key to the interaction of an airless planetary body with precipitating radiation, but it remains difficult to characterize. One of the effects that is governed by regolith properties is Energetic Neutral Atom (ENA) emission in the form of reflected and neutralized solar wind protons. We simulate this process for the surface of the Moon by implementing a regolith grain stacking in the ion‐solid‐interaction software SDTrimSP‐3D, finding that proton reflection significantly depends on the regolith porosity. Via comparison with ENA measurements by Chandrayaan‐1, we derive a globally averaged porosity of the uppermost regolith layers of . These results indicate a highly porous, fairy‐castle‐like nature of the upper lunar regolith, as well as its importance for the interaction with impacting ions. Our simulations further outline the possibility of future regolith studies with ENA measurements, for example, by the BepiColombo mission to Mercury.
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Lunar soil
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Saxicolous species of lichens are able to induce and accelerate weathering of their rocksubstrate, and effects of lichens on substrate can be attributed to both physical and chemical causes.This paper is focused on biotic weathering actions of epilithic and endolithic species on the differentrock types (sandstones and volcanogenic rocks) in Antarctica. The patterns, mechanisms, processes andneoformations of rock-weathering resulting from lichen colonization are expounded in detail.Furthermore, it is pointed out that, for a better understanding of the impacts of lichens onenvironments, the studies on the rate of biotic weathering and the comprehensive involvement of thelichen effects on weathering of natural rocks remain to be carried out in Antarctica.
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Regolith
Soil production function
Subaerial
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Abstract The production, distribution, and evolution of lunar regolith are critical in deciphering the lunar bombardment history and comprehending the transport of materials across the lunar surface, which are still not well understood. In this study, we conducted a comprehensive investigation of factors influencing the production and distribution of lunar regolith by individual simple craters. Combining our results for the impact‐generated regolith volume with a lunar production function, we developed an analytical model to describe the regolith growth process. We found that the strength of bedrock significantly affects the crater size and hence the volume of regolith produced especially for subdecameter impactors. The regolith volume produced by an individual impact crater is quantitatively characterized as a function of crater diameter and preimpact regolith thickness. This regolith production is primarily determined by how much bedrock is shattered, followed by the impact‐induced volume change of target material and lastly by the regolith volume created by secondary cratering processes. When a single crater forms, preimpact regolith thickness greatly affects the regolith distribution pattern; a larger fraction of the regolith will be distributed outside the crater rim for a deeper preimpact regolith layer. Our regolith evolution model can serve as a good first‐order estimation of the regolith growth process that provides better constraints on the regolith buffering trend than previous studies. This model also suggests that, when ignoring the contribution from large, distant impacts, the regolith growth process is dominated by impact craters at scales from a meter to a few hectometers.
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Bedrock
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