Paleosols preserve archives of vegetation history, environmental changes, and sedimentary systems. The changes in vegetation history and environmental conditions in karst areas of the Yunnan-Guizhou ...plateau of southwestern China since the late Pleistocene were analyzed using δsup.13C, sup.14C and the Rb/Sr ratio. Our δsup.13C results reveal the coexistence of C3 and C4 plants (−10.91 to −30.45‰) in soil organic matter, something that is consistent with the present field vegetation assessment. The large vertical difference in the δsup.13C value (>4‰) is related to the cultivation of agricultural crops and to changes in environmental conditions. Furthermore, the variation in δsup.13C values was influenced by the summer monsoon climate of the Indian Ocean and east Asia. We found that total organic carbon (TOC) concentration decreased exponentially with soil depth, indicating high carbon loss. In addition, sup.14C apparent age of the soil profiles ranged from modern times to 20,605 (±63) years before the present (yrs. BP) and is positively correlated with soil depth. The combined results indicate that, since the late Pleistocene era, environmental conditions in the study area have changed intermittently on a scale of millennia from hot-humid to cold-dry conditions. We speculate that there may have been extreme climate events in the study area 14,750 years BP, which are marked by a drastic change in δsup.13C and in the Rb/Sr ratio.
This work aims to introduce and test various semiquantitative field methods and environmental magnetic measurements to help prepare future planetary missions on Mars. For analog observations, ...paleosols of loess successions in various stages of soil development were investigated and were used as models to infer environmental change during environmental change on early Mars. Methods commonly used in terrestrial soils and sediment environments, such as soil development indices and low field and frequency-dependent magnetic susceptibility, are introduced and evaluated as potential proxies to constrain paleoenvironmental conditions and climate change on Mars billions of years ago. These methods include the following: 1) low field magnetic susceptibility, which may aid in the identification of weathered horizons (e.g., palaeosols) and provide insight into the degree of weathering intensity; 2) frequency-dependent magnetic susceptibility can constrain nanoscale magnetic contributions, including some with possible biogenic/bacterial origin, but its applicability to indicate the degree of pedogenesis is limited; and 3) the vertical distribution of low field magnetic susceptibility, i.e., the pattern of magnetic susceptibility curves, seems to work well in the indication of the balance between the sedimentary and pedogenic environment. Analysis of magnetic susceptibility curves may contribute to the identification of Martian paleosols, the characterization of the transition period between the soil-forming and subsequent sedimentary periods (Noachian-Hesperian and Hesperian-Amazonian transitions) and the identification of climate cycles; thus, these may be used as a frame of reference for evaluating paleoclimatic changes on Mars to e.g., the Noachian warm Mars and”Snowball Mars” periods.
The results also suggest that the time dependence of the magnetic enhancement of paleosols seems to be nonlinear compared to the length of the pedogenic period itself, and magnetic susceptibility may work as a relative chronometric parameter, which can help to constrain the duration of pedogenic alteration and soil formation on Mars.
•The relationship between soil development indices and basic magnetic proxies is examined.•The potential of on-site astropedological magnetic measurements on Mars is evaluated.•Terrestrial interglacial-glacial analogue Martian periods are discussed as future mission targets.
Rhizoliths are organosedimentary structures produced by plants and are an important proxy for paleoenvironmental reconstructions because the differences in the morphology of rhizoliths and their ...rooting depths can be used to interpret soil humidity regimes, overall biomass at the soil surface, and the behavior of the water table. With the objective of advancing knowledge on rhizoliths, a study was performed on the macromorphological and micromorphological characterizations of the rhizoliths preserved in the thirty-one paleosols of the Marília Formation in Minas Gerais, southeastern Brazil. Based on the topological and architectural attributes of rhizoliths, and on their possible functional adaptations, such as mycorrhizal associations, three different types of rhizoliths are described in this study and represent at least three different plants: rhizohaloes, root casts and rhizocretions. Well-developed paleosols with nodular horizons contain the higher density of long root casts, indicating a seasonally dry environment, because the root casts present two meters in length. These adaptations are associated with an exploitation of a lowered water table during most times of the year. The higher density of rhizoliths indicates an area with agglomerated arboreous arbustive vegetation cover. Paleosols with chalky horizons also show a high density of root casts, indicating a landscape with high plant density that is adapted to drained soils, but with a seasonally elevated water table or the influence of ephemeral flows, given the proximity to the flow channels. The poorly drained paleosols with redoximorphic features show the lowest density of rhizoliths, thus indicating an incipient vegetation in response to near-surface water table and waterlogged soils in a distal floodplain. The rhizogenic laminar horizons have developed near fluvial channels with frequent sedimentary input, resulting in a sequence of laminar horizons composed of rhizocretions that are separated by sandy deposits or poorly developed pedogenic horizons. The micromorphological analysis on the rhizogenic laminar calcretes showed biogenic features, such as alveolar septal structures, calcified filaments, microcodium, calcified cells and pisoliths. Despite the scarce paleobotanical findings in the Marília Formation, the results obtained indicate an ecosystem that was composed of plants varying from herbaceous to arboreous, with an environment similar to modern semiarid shrublands.
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•The architecture and geometry of rhizoliths indicate different types of paleosols.•The preservation of rhizoliths are linked to the paleosols characteristics.•A link between the type of rhizolith and paleotopography was established.
Key evidence of human occupation in Africa during the Middle Palaeolithic (Middle Stone Age MSA) is available from the south and east of the continent, where semi-arid climate prevails. Rare evidence ...of MSA human occupation in the humid tropical region was recently reported from Equatorial Guinea. To identify if paleolithic human occupation occurred in the tropical forest, the stratigraphy of the recently discovered archaeological site “Mabewele I” was analyzed using a paleopedological approach. The properties evaluated along a vertical profile are: granulometry, magnetic susceptibility (K), free iron extractable with sodium dithionite (Fed), total organic carbon (TOC), chemical composition by X-ray fluorescence (XRF), X-ray diffraction mineralogy (XRD), ternary plot of the three main oxides (SiO2-Al2O3-Fe2O3), phytoliths and micromorphology. The predominant fraction is sand, with a significant clay content (> 30 %). The K and the Fed presented low values, as well as the TOC. XRF showed few variations along the sequence and XRD showed that the clays are mainly kaolinites, with a very low component of vermiculites. The phytoliths correspond to tropical vegetation with few changes in the vegetal composition between the deep and superficial zones. A charcoal from the lower part of the profile was dated between 12.57-12.24 ka cal BP, while the main artifact horizon was located at the bottom. The analytical results appear contradictory: on the one hand, there is evidence of intense chemical weathering under a humid tropical climate, but the clay cutans in the pores are very scarce and incipient and the iron nodules are mostly anorthic. This indicates that the sequence is composed of pedosediments associated with a high environmental dynamism, with short periods of erosion-sedimentation-pedogenesis. The results allow us to propose that humans inhabited the tropical forest during the MSA and that erosion-sedimentation processes could be related to anthropization processes.
The evolution of ants (Hymenoptera: Formicidae) is increasingly well-understood due to recent phylogenetic analyses, along with estimates of divergence times and diversification rates. Yet, leading ...hypotheses regarding the ancestral habitat of ants conflict with new findings that early ant lineages are cryptic and subterranean. Where the ants evolved, in respect to habitat, and how habitat shifts took place over time have not been formally tested. Here, we reconstruct the habitat transitions of crown-group ants through time, focusing on where they nest and forage (in the canopy, litter, or soil). Based on ancestral character reconstructions, we show that in contrast to the current consensus based on verbal arguments that ants evolved in tropical leaf litter, the soil is supported as the ancestral stratum of all ants. We also find subsequent movements up into the litter and, in some cases, into the canopy. Given the global importance of ants, because of their diversity, ecological influence and status as the most successful eusocial lineage on Earth, understanding the early evolution of this lineage provides insight into the factors that made this group so successful today.
The natural abundances of stable oxygen (18O/16O) and hydrogen isotopes (2H/1H) are valuable proxies of climate changes in the past. Yet, to date no continuous δ18O and only few δ2H records are ...available from loess–paleosol sequences. Taking advantage of a recently developed method based on compound-specific δ18O analyses of hemicellulose sugar biomarkers in soils (Zech and Glaser, 2009. RCM 23, 3522-3532), we here present a first terrestrial δ18O biomarker record from an eolian permafrost paleosol sequence in NE-Siberia that covers the last ~220ka.
The δ18O values of the hemicellulose biomarkers arabinose and xylose range from 22.5 to 32.8‰ and from 21.3 to 31.9‰, respectively, and reveal systematic glacial–interglacial shifts. The modern topsoil and the interglacial paleosols exhibit more positive δ18O values, whereas the glacial paleosols are characterized by more negative δ18O values. This is in agreement with the δ2H record obtained for sedimentary n-alkane leaf wax biomarkers. We present a conceptual model for interpreting the combined δ18O and δ2H biomarker record. Based on this model, we suggest that both our δ18O and the δ2H records primarily reflect the temperature-controlled isotopic composition of paleoprecipitation modified by evaporative isotope enrichment of leaf water during transpiration.
Considering fractionation factors during sugar and n-alkane biomarker biosynthesis allows reconstructing the leaf water isotopic composition and the deuterium excess of the leaf water. The deuterium excess may serve as proxy for evaporative enrichment and allows calculating relative humidity using a Craig–Gordon model. Accordingly, relative humidity in NE-Siberia was higher during marine isotope stage (MIS) 6 compared to MIS 2, 4 and 5d and thus could help explaining the much larger extent of the Late Saalian glaciation compared to the Weichselian glaciations.
Using the Craig–Gordon model, we also calculated δ18O of the plant source water (δ18Osource water), which can be assumed to primarily reflect δ18O of paleoprecipitation. Our 220ka δ18Osource water record is well in agreement with the δ18Odiatom record from Crater Lake El'gygytgyn in NE-Siberia and enables a regional paleoclimate reconstruction and interpretation. Accordingly, summer temperature was periodically warmer than at present during the Weichselian glacial period and there is a strong July insolation forcing of the summer temperature in the extremely continental study area. Overall, our study highlights the great potential of the novel hemicellulose biomarker δ18O method for paleoclimate reconstructions, especially when combined with δ2H analyses of n-alkane lipid biomarkers.
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•We established a δ18O biomarker record from an eolian permafrost paleosol sequence.•Our hemicellulose δ18O record is generally confirmed by the n-alkane δ2H record.•We present a conceptual model for interpreting combined δ18O–δ2H biomarker records.•Relative humidity history is reconstructed using d-excess of calculated leaf water.•The δ18O record suggests a strong insolation forcing on summer temperatures.
Modern physical and chemical soil properties can favor or exclude C3 and C4 plants, yet little is known regarding these relationships from deep-time records that track the evolution and expansion of ...C4 vegetation. In this study, we used a multi-proxy approach to reconstruct vegetation (C3 vs. C4 biomass) and pedogenic properties (soil alkalinity, salinity, sodicity, and texture) from paleolandscapes at Coffee Ranch, Texas, a site from which fossil horses provide the earliest evidence for C4 herbivory in the Great Plains of North America. Local proportion of C4 biomass was assessed using stable carbon isotope ratios of calcium carbonates (δ13Ccc) and organic matter (δ13Com) analyzed on four different paleosol types, freshwater tufa, and reworked carbonate nodules in fluvial channel lags. Using a Monte Carlo uncertainty analysis, we interpret δ13Ccc (range = −8.5 to −5.2‰ VPDB) and δ13Com values (range = −25.9 to −24.2‰ VPDB) to be consistent with C4 biomass low in abundance and variability at the study site, but with large uncertainties that would be overlooked using simple linear mixing model approaches. Paleo-pedogenic properties were reconstructed using pedotransfer functions and provide evidence of possible salinity and sodicity in two of five paleosol profiles. However, saline-sodic conditions and soil texture were not correlated with δ13C values, contrary to some modern mixed C3-C4 biomes. Using late Miocene CO2 and paleoclimate model reconstructions, we argue that conditions were at or near crossover thresholds favoring C4 over C3 photosynthesis in the Great Plains despite the low abundance of C4 vegetation across the paleolandscapes. This study presents evidence that abiotic factors that select for C4 plants in modern systems—high growing season temperature, low atmospheric CO2, salinity-sodicity, and soil texture—were less influential in the late Miocene than biotic factors (i.e., ecological feedbacks) that suppressed C4 plants prior to their increase in abundance in the Great Plains in the Pliocene.
•Paleosols were studied from the late Miocene Coffee Ranch fossil site.•Two paleosol profiles show possible evidence of saline-sodic conditions.•C4 biomass was estimated using δ13C analyses on carbonates and bulk organic matter.•A Monte Carlo analysis was used to assess uncertainty in C4 biomass estimates.•Low abundance of C4 plants suggests biotic control on plant community composition.
The present work analyzes the monotypic Taenidium barretti ichnofabric developed in abandoned channels and floodplains to understand its paleoecological significance in highly seasonal fluvial ...systems. The data come from the Cretaceous paleosols of the Marília Formation (Bauru Basin, SE Brazil), in which the T. barretti ichnofabric and rhizoliths represent the biotic record. Beetle larvae are considered to be the most likely T. barretti tracemakers in these paleosols. The intensity and recurrence of bioturbation suggest that the windows of opportunity for beetle population growth were very short and controlled by the progressive substrate desiccation after flood cessation. The characteristics of the trace fossil assemblage suggest soil colonization by r-strategic organisms triggered by flooding events that provided the resource inputs necessary for their living during rainy seasons. The results also show that in fluvial systems with high discharge variations, the boundaries between a “pre-desiccation suite” and a “desiccation suite” in the Scoyenia ichnofacies can be diffuse due to the short duration of colonization windows and progressive terrestrialization of floodplains and channels. Therefore, the frequency of flooding events, the lowered water table, and the rapidity of substrate desiccation constrained colonization by other burrowers and may be considered as first-order factors controlling the generation of a monotypic T. barretti ichnofabric. Thus, the monotypic Taenidium ichnofabric in the Scoyenia ichnofacies context is an ichnomarker of brief windows for colonization in highly seasonal environments.
Filled mammal tunnels (krotovinas) are the most common traces in modern grassland soils and trace fossils in loess paleosols. Krotovinas are still insufficiently used in paleogeographic ...reconstructions owing to the absence of a procedure for documenting their features. When their tracemaker can be identified, krotovinas complement body fossil evidence for past geographic ranges of fossorial mammals. Moreover, local palaeoenvironments can be reconstructed more precisely based on the preferred habitat of the identified species. However, the morphology of krotovinas has rarely been reported in detail that would allow their identification.
In this article, we suggest five groups of diagnostic features that should be recorded and assessed for taxonomic identification of krotovinas. (1) Ornamentation on the burrow walls, which can be observed when there is sediment compaction contrast between infill and the host material, provides the most taxonomically specific features. (2) Infill type distinguishes between subterranean forms and other burrowers. (3) Tunnel repair and reburrowing patterns. (4) Motifs representing distinctive architectural elements: shafts, spirals, their connections to horizontal tunnels, the shape of chambers, and the predominant orientation of tunnels (horizontal, inclined, vertical). (5) Diameter, which has to be measured transverse to each tunnel or estimated from oblique sections. The measurements and observations can be compared against a reference collection of burrow plaster casts. Synoptic diagrams are suggested as a method for condensing images of diagnostic features from several sections. We suggest that the presence of krotovinas is one criterion for distinguishing grassland paleosols from humus-rich colluvial deposits.
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