Paleosol Profiles Research Articles

Paleosols record dry and humid paleoenvironments during the Upper Pleistocene in the Brazilian Pantanal

Information on Quaternary landscapes and paleoenvironmental changes in the interior of Brazil are scarce. One of such areas, particularly sensitive to these variations, is the modern tropical Pantanal wetland, whose alluvial deposits record paleoenvironmental and paleoclimatic changes during the late Quaternary. Based on detailed pedological and sedimentological descriptions, stimulated optical luminescence dating and analytical data we described a well-exposed sedimentary succession that crops out in slope terraces of the Aquidauana River, located in the southern area of the Pantanal. We interpret environmental changes that have occurred since the beginning of the Upper Pleistocene, and which affected sedimentation and biogeochemical processes. Basal deposits comprise cross-stratified sands, which record active fluvial sedimentation from ∼106 to ∼70 ka, a period of probable increased river discharge. Fluvial channel facies are overlaid by pedogeneized floodplain deposits, in which we recognized a pedocomplex composed of a sequence of three late Pleistocene truncated paleosol profiles and a Holocene soil profile. The two lower paleosols are Vertisols with mottling, slickensides, rhizolites and septarian glaebules of carbonate. These characteristics indicate prolonged drought periods under semi-arid to arid climates ca. ∼70 and ∼30 ka. The upper paleosol profile is a Planosol (Ultisol) with block structure, waxiness, iron glaebules, and manganese films, pointing to prevailing humid climates during the terminal Pleistocene (since ∼30 ka). Covering this truncated Planosol is a layer of fine to medium-grained Holocene sands, with development of a horizon A of an Arenosol (Entisol). These paleoenvironmental changes correlate with regional changes in precipitation deduced from speleothems in nearby plateaus and in southeastern Brazilian sites.

Climate influence on zinc isotope variations in a loess–paleosol sequence of the Chinese Loess Plateau

Zinc (Zn) is an essential micronutrient, and its isotopes are increasingly applied to understand trace metal biogeochemical cycling. Atmospheric deposition from anthropogenic or natural origins can perturbate the nutrient supply globally. However, our understanding on Zn isotope variations in natural mineral dust and during dust–climate interaction is still rare. This study presents Zn isotope data and leaching experiments with ammonium acetate buffering solution (pH = 5) on a loess–paleosol profile (S0–L3) from the Yimaguan section in the central Chinese Loess Plateau. The loess–paleosol sequence records the contributions from Asian mineral dust accumulation and chemical weathering, and the response to glacial–interglacial climate changes. Bulk loess–paleosol samples display limited Zn/Al (3.5 × 10-4 – 4.5 × 10-4) and δ66Zn variations (0.17–0.30‰). The mean δ66Zn of paleosol (0.19 ± 0.01‰, 2SD/√n, n = 11) is slightly lower than that of loess (0.24 ± 0.01‰, 2SD/√n, n = 13). Correlations with magnetic susceptibility and grain size distribution suggest that bulk δ66Zn variations between loess and paleosol are not sensitive to glacial–interglacial source fluctuations or wind sorting, but could result from loess pedogenesis.The leachable Zn fractions have high δ66Zn values (0.80–1.29‰) and display good positive correlations with Ca, Sr, Mg, Fe and Mn (r > 0.7), suggesting that carbonates are enriched in heavy Zn isotopes. Although Zn contents in carbonates are low (<3%) and dissolution of carbonates alone cannot explain the bulk Zn/Al variations (∼7% of Zn loss), significant carbonate weathering and biotic activities during loess pedogenesis could modify soil conditions (pH) and provide dissolved bicarbonate ions and organic ligands, which may facilitate the transport of heavy Zn isotopes by soil liquids. Conversely, the bulk δ66Zn of loess developed during glacial times were not modified by the low degree of chemical weathering, and thus the mean value of 0.24‰ for loess can be regarded as the average composition of the upper continental crust.

Paleovegetation and paleo-pedogenic properties from the upper Miocene Coffee Ranch fossil site in the North American Great Plains

Modern physical and chemical soil properties can favor or exclude C 3 and C 4 plants, yet little is known regarding these relationships from deep-time records that track the evolution and expansion of C 4 vegetation. In this study, we used a multi-proxy approach to reconstruct vegetation (C 3 vs. C 4 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 C 4 herbivory in the Great Plains of North America. Local proportion of C 4 biomass was assessed using stable carbon isotope ratios of calcium carbonates (δ 13 C cc ) and organic matter (δ 13 C om ) analyzed on four different paleosol types, freshwater tufa, and reworked carbonate nodules in fluvial channel lags. Using a Monte Carlo uncertainty analysis, we interpret δ 13 C cc (range = −8.5 to −5.2‰ VPDB) and δ 13 C om values (range = −25.9 to −24.2‰ VPDB) to be consistent with C 4 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 δ 13 C values, contrary to some modern mixed C 3 -C 4 biomes. Using late Miocene CO 2 and paleoclimate model reconstructions, we argue that conditions were at or near crossover thresholds favoring C 4 over C 3 photosynthesis in the Great Plains despite the low abundance of C 4 vegetation across the paleolandscapes. This study presents evidence that abiotic factors that select for C 4 plants in modern systems—high growing season temperature, low atmospheric CO 2 , salinity-sodicity, and soil texture—were less influential in the late Miocene than biotic factors (i.e., ecological feedbacks) that suppressed C 4 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. • C 4 biomass was estimated using δ 13 C analyses on carbonates and bulk organic matter. • A Monte Carlo analysis was used to assess uncertainty in C 4 biomass estimates. • Low abundance of C4 plants suggests biotic control on plant community composition.

Cooler Equatorial Climate in the Late Lopingian Estimated from Paleosols Developed on Emeishan Basalts

Paleosols that developed on large igneous province flood basalts provide a valuable opportunity to quantitatively reconstruct the terrestrial paleoclimate during a given period. This study uses a newly constructed basaltic soil–climate function to obtain terrestrial paleoclimatic data following the termination of the Emeishan basalt eruption in the late Lopingian (∼253 Ma). The constructed relationships are mean annual temperature of 32.81e−0.4166X (R2=0.55, standard error [SE]=4.6°C) and mean annual precipitation of −875.5ln(X)+1792 (R2=0.69, SE=328 mm), where X=SiO2/(Al2O3+Fe2O3) (the Saf index). We collected samples from nine paleosol profiles developed on the last-phase Emeishan basalts and conducted field observations as well as micromorphological, mineral composition, and geochemical analyses. On the basis of these paleosols, we reconstructed the regional mean temperature and precipitation to be 18.9°C±4.6°C and 1546±328 mm, respectively. The regional climate was cooler than those in modern equatorial regions of similar latitudes. The reduced CO2 concentration in the atmosphere resulting from rapid weathering of Emeishan basalts in a tropical climate could have played a dominant role in causing the regional cool terrestrial paleoclimate following the termination of the Emeishan basalt eruption.

Middle Jurassic climate oscillations from paleosol records of the Sichuan Basin, SW China

The Middle Jurassic is characterized by major plate tectonic changes, variable atmospheric CO2 concentrations (pCO2), and climate oscillations in both marine and terrestrial realms. However, the limited evidence from the terrestrial sedimentary records hampers a comprehensive understanding of global climate state in this period. Well-exposed strata of the Shaximiao Formation in the Sichuan Basin, Southwest China, preserve successions of paleosols that offer the possibility of evaluating the terrestrial paleoclimatic changes during the Middle Jurassic (Bajocian to Callovian). A total of 151 paleosol profiles belonging to four paleosol types (Entisol-like, Inceptisol-like, Aridisol-like and argillic Inceptisol-like paleosols) were identified and characterized. The quantitative paleoclimate reconstructions based on the bulk geochemistry of paleosol horizons and depths of carbonate nodules indicate a generally semiarid–subhumid climate alternating with arid–humid and cool/warm–temperate climates during this time interval. This dynamic climate terminated in a dry, subhumid, humid/perhumid and superhumid moisture regime characterized by steppe/wet or rain forest floral provinces. We suggest that these climatic fluctuations may have been related to global geodynamic (e.g., the fragmentation of Pangea, “megamonsoonal” circulation, and bolide impacts) and regional tectonic uplift (e.g., the uplift of the Longmenshan Mountains in the South China Block) during the Middle Jurassic. The pCO2 estimates suggest relatively low pCO2 levels within the range of ∼100 ppm–∼890 ppm during the Bajocian to Callovian. Our results coincide with the threshold for the initiation of glaciation and may help to explain the occurrence of cool intervals in the generally warm Jurassic. These pCO2 fluctuations, which coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere, are possibly attributable to global geological events (e.g., wildfire events, eustatic sea-level oscillations, and the transient development of glaciations) in the Middle Jurassic.

Changes in rainfall seasonality inferred from weathering and pedogenic trends in mid-Miocene paleosols of La Tatacoa, Colombia

The authigenic clay mineral assemblage of paleosol profiles and trends in their bulk geochemical evolution provide insight into the intensity of past chemical weathering regimes. These climate-sensitive weathering products can be used to interpret regional changes of the soil moisture regime, which in turn reflects upon the nature of rainfall seasonality (e.g., unimodal vs. bimodal). A conspicuous color change in the overbank deposits and paleosols of the mid-Miocene Honda Group of La Tatacoa, Colombia, is suggestive of a climatic transition which affected the fauna of La Venta - a mid-Miocene fossiliferous site and significant component of the South American Land Mammal Age (SALMA) classification. This study aims to determine how the paleopedology of this region records the climatic transition that modified the annual patterns of rainfall and soil moisture, forcing a change from the gray-colored paleosols hosted by the La Victoria Formation (~13.6 to ~13.1 Myr) to the red-beds of the Villavieja Formation (~13.1 to 12.0 Myr). In the La Victoria profile, the pedogenic fraction of the clay assemblage indicates that vermiculite, chlorite/HIV mixed-layered clays and high concentrations of base cations (Ca2+, Mg2+, Na+) developed in a poorly drained paleosol under reducing (low Eh) and above-neutral pH conditions. By contrast, the presence of kaolinite, goethite and hematite in the red-beds paleosols implies more oxidizing conditions (high Eh), whereas pedogenic vermiculite and smectite suggest a pH close to or mildly below neutral. A common detrital source (parent material) is inferred for both sets of paleosols. However, more intense chemical weathering towards aluminium rich clay in the Villavieja red-bed fossil soils indicates a change in soil humidity from a poorly drained soil environment of a seasonally flooded wetland system to an alternating regime of humidity and drought on a better drained floodplain. The variation in soil moisture is consistent with a change in seasonality from a unimodal rainfall regime in the case of the La Victoria paleosols to bimodal conditions in the case of the paleosols of the Villavieja Fm. After the establishment of a fully glaciated Antarctica and the global climate transition into the Mi-4 glacial stage at ~13.1 Ma, an increased rate of continental weathering along the paleotropical fringe could have been caused by a stronger temperature gradient between the equator and South Pole, shifting the Intertropical Convergence Zone northward and the rainfall patterns with it.

Upper Cretaceous (Coniacian-Santonian) dinosaur nesting colony preserved in abandoned crevasse splay deposits, Wi Island, South Korea

Dinosaur egg-bearing deposits of the Upper Cretaceous (Coniacian-Santonian) Wido Volcanics at Wi Island, South Korea, were studied from a taphonomic perspective to improve understanding of nesting behavior. Based on facies, bedset geometry, and other sedimentological characteristics, the section can be classified into three facies associations: (1) ephemeral fluvial channels (FA I), (2) crevasse splay lobes (FA II), and (3) floodplains (FA III). FA I is characterized by single- and multi-storey channel fills enclosed within FA III, while FA II consists of horizontally-stratified sandstone and calcareous mudstone reflecting weakly developed paleosol profiles. The ephemeral fluvial channels are locally developed at 160 m distance from the main egg locality, suggesting that the nesting site was relatively protected from levee failure and favorable for fossil preservation. Isolated eggs and reworked eggshell fragments of three different types (Propagoolithus widoensis, Reticuloolithus acicularis, and Aenigmaoolithus vesicularis) are frequently found together. The most common type, P. widoensis is preserved as clutches and characterized by numerous closely-spaced pore canals, indicating that the eggs were buried during oviposition, increasing preservation potential. Such in situ clutches occur in at least nine horizons and some occur closely spaced (<1 m apart), lateral to crevasse splay lobe successions (FA II). It suggests that the egg-layers of P. widoensis preferred this facies as a nesting ground for extended periods. FA II was deposited at a higher paleo-elevation than FA III, which may have benefited nesting dinosaurs, allowing them to easily detect predators (FA III) while being less frequently exposed to flood hazards.

Development of an argillic horizon in polygenetic paleosols in the Marília Formation (Maastrichtian), Brazil: Precautions for paleoenvironmental interpretation

A study of paleosols with argillic horizons found in the Marília Formation associated with Maastrichtian in the Cretaceous is presented herein. This study aims to investigate the polygenetic processes involved in argillic horizon formation under paleoclimatic conditions from arid to semiarid in the Brazilian Cretaceous and considers morphological soil attributes, such as evidence of cementation, macromorphological data (horizons, matrix color, structure, clay coatings, nodules, and bioturbations), micromorphological data (coarse/fine patterns, b-fabric, clay coatings, infillings, and nodules), and water table oscillation. Seven representative profiles of paleosols were selected in the São Paulo state, Brazil, and soil samples of lithified paleosols were collected to determine their macromorphological–micromorphological and chemical properties. Clay illuviation, carbonate accumulation, and gleization were dominant processes in the paleosols. Clay illuviation was identified from clay coatings along grain and ped surfaces. Carbonates occurred as nodules and rhizoliths and as coatings on peds. Red mottles, gray rhizohaloes with red oxidation rims, oxidic nodules, an iron-depleted matrix, and iron oxide depletion hypocoatings confirmed gleization. The argillic horizon polygenesis in the Marília Formation is usually interpreted as evidence of high-moisture events under arid/semiarid climates or climate change toward moister conditions than arid/semiarid. Thus, argillic horizon paleosols in the sedimentary records of the Marília Formation are not necessarily reliquial characteristics of wet paleoenvironments but are associated with changes in dominant granulometry, types of macroaggregates–microaggregates, carbonate accumulation in horizons, and/or water table oscillation. Thus, the presence of Bt horizons does not necessarily imply wet conditions with changes in the depositional system, as proposed by some authors.

Climate and ecology in the Rocky Mountain interior after the early Eocene Climatic Optimum

Abstract. As atmospheric carbon dioxide (CO2) and temperatures increase with modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration as today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwestern Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. Using this well-preserved basin deposited during a period of tectonic and paleoclimatic interest, we employ multiple proxies to study trends in provenance, parent material, weathering, and climate throughout 1 million years. The Blue Rim escarpment exposes approximately 100 m of the lower Bridger Formation, which includes plant and mammal fossils, solitary paleosol profiles, and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New 40Ar / 39Ar ages from the middle and top of the Blue Rim escarpment constrain the age of its strata to ∼ 49.5–48.5 Myr ago during the “falling limb” of the early Eocene Climatic Optimum. We used several geochemical tools to study provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions, including temperature, precipitation (both from paleosols), and the isotopic composition of atmospheric CO2 from plants found in the floral assemblages. Results from paleosol-based reconstructions were compared to semi-co-temporal reconstructions made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608–1167 mm yr−1) and temperature (10.4 to 12.0 ∘C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section and represented a highly preserved event later in time. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical-like forest (i.e., paratropical) throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene Climatic Optimum. The reconstructed atmospheric δ13C value (−5.3 ‰ to −5.8 ‰) closely matches the independently reconstructed value from marine microfossils (−5.4 ‰), which provides confidence in this reconstruction. Likewise, the isotopic composition reconstructed matches the mantle most closely (−5.4 ‰), agreeing with other postulations that warming was maintained by volcanic outgassing rather than a much more isotopically depleted source, such as methane hydrates.

Verification of regional-to-global scale environmental factors in paleosol stable carbon isotope ratios through the lower Permian succession of north-central Texas, U.S.A.

Upper Paleozoic (~300 Ma) rocks in the Midland Basin of Texas (USA) record a transition from icehouse (glaciated) to greenhouse (non-glaciated) conditions in low latitudes, western Pangea. This work revisits the stratigraphic interval that records stable carbon isotopic compositions of paleosol materials from previous studies and uses newly collected paleosol samples to test the reproducibility of paleosol stable carbon isotopic values, and assesses paleoclimate and paleoatmospheric pCO 2 changes across the global climate transition. Thirty upper Wolfcampian (Nacona Formation) to upper Leonardian (Clear Fork Group) paleosol profiles were measured and described in detail. Detailed petrographic analysis of paleosol carbonate nodules was used to identify the best representatives of pristine pedogenic microcrystalline calcite cements and separate them from later diagenetic or altered calcite cements. The best candidates for pedogenic micritic calcite cement were drilled, and organic-rich fractions of acid-treated residues co-existing within carbonate nodules were collected. The δ 13 C values of 23 paired, pristine pedogenic micritic calcite samples (δ 13 C cc ) and associated, occluded organic matter from carbonate nodules and calcareous rhizoliths (δ 13 C om ) range from −6.5‰ to −3.3‰ and −25.3‰ to −20.5‰, respectively. The stratigraphic trends in paleopedogenic micritic calcite δ 13 C cc values are essentially no different from results published in previous studies, which supports the reproducibility of paleosol carbonate as a reliable proxy for the carbon isotopic composition that reflects the regional-to-global paleoclimatic and paleoenvironmental trends. However, δ 13 C om values measured here are consistently more negative than previously reported for fossilized plant compressions preserved within strata associated with these paleosol profiles ( Montañez et al., 2007 ). We suggest that the coexisting organic matter from acid-treated residues in paleosol carbonate is a better material to assess original soil δ 13 C om values that was contributing toward soil CO 2 than the fossilized components of above-ground, photosynthesizing vascular plant parts as was utilized in previous studies. The isotopic differences between the paleosol micritic calcite δ 13 C cc values and associated organic-rich-residue δ 13 C om values were used in conjunction with a two-component CO 2 mixing model to yield atmospheric pCO 2 estimates. The estimates are different and with opposite excursions, ranging from 1.7 to 2.7 times pre-industrial atmospheric levels (PIAL) and from 2.3 to 4.2 times of PIAL, when applying the relationships of modern soil pCO 2 (S z )-mean annual precipitation (MAP) and Sz-pedotype, respectively, to assess the S z of paleosols. The different approaches result in different assumed S z of paleosols at the time of calcite crystallization and lead to lower estimates of atmospheric pCO 2 than previously reported. These results suggest that the icehouse-to-greenhouse climate transition was associated with much lower pCO 2 thresholds than previously thought. • The carbon isotope compositions of pedogenic calcite from microsamples are repeatable. • The carbon isotope compositions of pedogenic calcite can record regional processes. • Only limited increase in atmospheric pCO 2 across icehouse-to-greenhouse transition.

Incorporating lateral variability and extent of paleosols into proxy uncertainty

Paleosols (fossil soils) are valuable records of terrestrial climate and environments, and paleosol-based proxies are commonly used to reconstruct past climates and ecosystems. Results from relatively small outcrops or transects or from single vertical sections are frequently scaled up to represent basin-scale processes and conditions, and reconstructions are relied on for temporal changes in those basins. However, uncertainty arising from limited outcrop extent is not currently considered in the standard application of paleosol-based proxies. To explore uncertainty arising from lateral paleosol heterogeneity, we performed a random subsampling analysis on a newly-collected 2.9 km paleosol transect from SW Wyoming, along with two previously-published paleosols. We demonstrate the importance of sampling multiple paleosol profiles, considering lateral geochemical variability, and focusing on relative rather than absolute changes when outcrop-based uncertainty may require it. • Paleosol-based proxies provide unique records of terrestrial climate. • However, outcrop extent and lateral variability are not typically considered. • We statistically resample 3 paleosol transects (1, 1.5, and 3 km). • Sampling >3 profiles from a paleosol decreases proxy uncertainty. • Relative rather than absolute changes in proxy should be commonly used.

Early-Middle Devonian paleosols and palustrine beds of NW Canada in the context of land plant evolution and global spreads of anoxia

Dozens of subaerial exposure surfaces are assessed in cores from cyclic peritidal carbonates of the Emsian–Eifelian (~410–385 Ma) age. These surfaces range from incipient erosional surfaces to paleokarst profiles and thick (>1 m) calcretic-clayey paleosols. Palustrine carbonates intervening at multiple levels in same strata are the earliest known occurrence of a typical palustrine facies in a coastal carbonate plain environment. None of paleokarst and paleosol profiles contain traces of vascular-plant root penetration, and only palustrine facies exhibit swarms of thin rhizoliths. These findings are within the context of Devonian paleosols on marine carbonate substrates where root traces and laminar calcretes are extremely rare (only 2 reports), and no instances of root penetrations are trackable from unconformities in pre-Givetian carbonate successions. The δ13C and δ18O signatures indicate variable diagenetic reset of isotopic composition. Modest δ13Cvpdb offset towards lighter values is preserved in two thicker paleosols (−3.4 to −8.0‰ in calcretic matrix vs. -1.35 to −6.5‰ in parent limestone). Such offsets are very rare in pre-Late Devonian pedogenic carbonates, and their attribution to plant-derived CO2 is dubious. It is inferred that the land surface in calcimagnesian landscapes remained a regolith or primary desert (i.e., was never colonized by tracheophytes) long after the spread of vascular plants in more favorable wetland settings. Furthermore, the advent of seed reproductive strategy in the latest Devonian and plant adaptations to aridic habitats manifesting in fossil floras only since Pennsylvanian indicate that much of land surface remained within the realm of primary desert long after the afforestation of wetlands. The direct (plant roots, shoots and spores) and indirect (sedimentary features) fossil records of land greening during the Paleozoic are not corroborating the hypothesis holding the primordial embryophytic cover on land accountable for the Late Ordovician atmospheric oxygenation-decarbonization event. It is further argued that the evolution of root systems and plant stature could hardly trigger anoxic events and biotic crises in the Devonian marine realm.

Precambrian paleosols on the Great Unconformity of the East European Craton: An 800 million year record of Baltica’s climatic conditions

Meso- and Neoproterozoic paleosols, collected from different areas of the East European Craton: Belarus, Estonia, Lithuania, and Ukraine, offer a chance to examine continental weathering sequences from early to advanced stages of weathering on a variety of different parent materials such as gneisses, granites, gabbros and amphibolites. They were studied using quantitative XRD of the bulk rock, XRD and Mössbauer of the clay fractions, microscopic, geochemical, carbonate stable isotopes and carbonate U-Pb geochronology methods. These paleosol profiles are on average 10 m thick, reddish coloured, and many of them are characterized by a well-developed and well-defined alteration sequence with uppermost horizons approaching the lateritic stage, as indicated by the Chemical Index of Alteration values reaching 90. The dominant type of weathering leads towards kaolinite and a Fe-oxide/hydroxide mineral assemblage through a smectitic intermediate stage. In the paleosol profiles developed on mafic parent materials, dioctahedral smectite is the first weathering product at the base, it dominates in the middle-upper horizons and it later becomes unstable and alters into kaolinite; whereas in paleosols developed on felsic parent materials kaolinite forms already at the initial stage of weathering, as a result of Na-plagioclase dissolution. Kaolinite content in the uppermost horizons reaches 34 wt% in the best developed profile, and Fe-minerals (hematite and goethite) show a clear increasing trend towards the top, reaching 12 wt%. It is likely that uppermost kaolinite-dominated horizons, which are lacking in some profiles, have been eroded. Such paleosol composition and ferric composition of smectites indicate oxidative weathering and are interpreted to represent a warm and humid climate, which seems to have prevailed on the EEC over the Meso- and Neoproterozoic, except well-documented glacial periods. The δ13C signatures of the pedogenic carbonates document microbial processes in the paleosols, which is also indicated by the elevated U/Th values in their top layers. One paleosol was dated with in situ pedogenic calcite U-Pb geochronology at 655 ± 45 Ma (2σ), confirming the estimate based on its stratigraphic position. The Proterozoic paleosol profiles, and dioctahedral smectite in particular, remained unaltered for over 900 Ma until the Paleozoic, when they were affected by low-temperature (<110 °C) diagenesis, evidenced by the presence of K-Ar dated illite–smectite and aluminoceladonite. The maximum and the most common degree of the dioctahedral smectite illitization is 26% S; the %S zonation indicates that the illitizing fluids invaded paleosols from the overlying sediments.

Patterns of geochemical variability across weakly developed paleosol profiles and their role as regional stratigraphic markers (Upper Pleistocene, Po Plain)

Weakly developed paleosols from two distinct interfluve surfaces of Late Pleistocene age provide excellent keys to high-resolution stratigraphic correlation and may serve to trace large-scale genetic packages (systems tract equivalents) across the continental portion of the Po Basin. Twenty-four paleosol profiles from 17 sediment cores were identified and characterized for bulk-geochemical analysis. X-ray fluorescence data were used to trace the degree of weathering. Paleosols, 0.5–1.5 m thick, are pedogenically altered floodplain deposits, developed over time spans of a few thousand of years and mostly partitioned into A-Bk horizons. The most notable paleosol features are dark, organic-matter-rich and carbonate-free mineral surface horizons (A) that overlie bright calcic horizons (Bk) typified by the accumulation of secondary carbonates in the form of pedogenic nodules.Paleosol profiles exhibit a homogeneous geochemical signature that fingerprints a moderate degree of weathering, with little strike- and dip-oriented variability across the different study localities. Plots of Al-normalized calcification and base loss indices against depth reveal systematic increasing values from intensely altered A horizons to underlying Bk horizons. These trends reflect consistent patterns of Ca translocation from surface horizons deeper into the profile, with significant to almost complete Ca removal from A horizons through leaching and accumulation in Bk horizons. Selected trace element ratios (Ba/Sr, Rb/Sr), redox-sensitive trace elements and Zr contents display opposite, up-profile increasing trends that reflect Sr loss in A horizons, with selective Zr concentration in residual minerals.Vertical trends in element ratios are laterally extensive and consistent on a regional basis and represent key pedochemical/stratigraphic markers that can be traced over great distances (tens of kms) throughout the inland portion of the basin. Through quantitative assessment of the degree of weathering, geochemical profiling provides high potential for robust subsurface paleosol correlation that might not be captured by visual core descriptions alone.

Provenance, and environment context of pedogenic carbonates formation from MIS 3 to MIS 1 in the Teotihuacan Valley, Mexico

Pedogenic carbonates are considered important indicators of past climatic changes. Their formation is due to calcite accumulation through evaporation processes in arid or semi-arid regions. Over time, secondary carbonate accumulation results in the development of calcic and petrocalcic horizons in soil profiles. In this study, we conducted an analysis of pedogenic carbonates as records of paleoenvironmental characteristics in the Teotihuacan Valley, Basin of Mexico. Micromorphological, mineralogical, and geochemical characteristics of secondary carbonates from three paleosol profiles (Cerro Gordo 1, Cerro Gordo 2, and Maseca 1) were analyzed. Pedogenic carbonates were characterized by irregular and variable carbonate crystal sizes, inclusions of soil particles, and dark or light micrite and microsparite laminations. Micromorphology and scanning electron microscopy revealed weathered plagioclase minerals as a potential source of calcium for carbonate precipitation. The 87Sr/86Sr ratio of carbonate laminar crusts and nodules from the analyses pedons in the study area also indicated that rocks of the Transmexican Volcanic Belt were the principal source of Ca and Sr for carbonates, while contribution of a far-distance eolian input was minimal. The δ13C isotopic composition evidenced a mixture of C3 and C4 plants during the Pleistocene and a transition to a higher proportion of C4 vegetal cover during the Holocene. This agreed with an increase in modeled paleo-temperature from 10.9 °C to 13.7 °C based on 18O-temperature equations. Our results support those determined for the regional paleoenvironment during the Pleistocene, the Pleistocene-Holocene transition and Mid-Late Holocene.

Paleoenvironmental and paleoclimatic reconstruction of Lower to Upper Cretaceous sequences of the Bauru Basin based on paleosol geochemistry and mineralogical analyses

The Cretaceous climate was characterized by arid and semiarid conditions in different parts of the world. Climatic changes for the Early to Late Cretaceous had been reported by many authors in North America, East Asia, Africa, and South America due to the geological evolution of Laurasia and Gondwana. In South America, the Bauru Basin, located in Brazil, is one of the most critical places to develop paleoclimatic reconstructions due to the presence of well-preserved Cretaceous sedimentary sequences, as well as abundant fossil content and paleosol profiles. The transition between the Lower and Upper Cretaceous in the Bauru Basin is represented by the Santo Anastácio and Araçatuba formations, which are characterized by a succession of sandstones, claystones, and paleosols. The paleosols are well distributed in the basin, providing valuable information about the paleoenvironmental conditions and insights about the tectonic and climatic changes in the Cretaceous Period. Since the 1960's decade, the Cretaceous paleosols of the Bauru Basin have been used as stratigraphic and sedimentological proxies providing paleoenvironmental and paleoclimatic insights, principally for the Late Cretaceous Period. This study, based on macromorphological descriptions, clay mineralogy (X-ray diffraction and scanning electron microscopy - SEM), and geochemical of paleosols, identified environmental and climatic patterns from the Lower and Upper Cretaceous. Results allowed to elaborate maps of paleoclimatic evolution that indicate changes occurred from Lower to Upper Cretaceous sequences of the Bauru Basin. These changes were compared and correlated with other continental basins in West Gondwana and Laurasia around the world, contributing to the understanding of the Cretaceous paleoclimatic context of South America.

Anoxic continental surface weathering recorded by the 2.95 Ga Denny Dalton Paleosol (Pongola Supergroup, South Africa)

Iron mobilization during continental weathering was pervasive before the Great Oxidation Event (GOE) that started at around 2.43 billion years (Ga) ago, due to the soluble nature of reduced iron. However, various geochemical proxies indicate transient oxygenation during deposition of the Mesoarchean (∼2.95 Ga) Pongola Supergroup, South Africa, which suggests that continental weathering could have also occurred under transiently oxic conditions before the GOE. We analyzed trace elemental and Fe, Ti, and Cr isotopic compositions of the ca. 2.95 Ga Denny Dalton paleosol in the Pongola Supergroup to better understand continental weathering and redox conditions in the ancient critical zone, and the nature of geochemical fluxes from the continents to the oceans and marine sediments. Iron isotope systematics are consistent with a model where Fe was released during intense leaching from the paleosol to concentrate in Fe-rich groundwaters in the deeper part of the soil horizon. We show for the first time that Fe isotopic fractionation during Mesoarchean continental weathering was limited, and Fe enrichments and depletions are coupled with those of divalent transition metals, Co, Ni, and particularly Zn. This suggests that Fe redox cycling was not involved in paleosol formation, and Fe2+ was mobilized under anoxic conditions. Chromium isotopes are also unfractionated relative to the parent igneous rock in this paleosol, which precludes removal of isotopically heavy Cr6+ and thus supports anoxic continental weathering. We show that previously reported Cr isotopic fractionation in another Denny Dalton paleosol profile does not follow a Cr6+ leaching trend, but instead scales with Cr enrichment and may reveal Cr enrichment from post-burial fluids. Thus, there is no clear evidence for an oxidative continental weathering during deposition of the Pongola Supergroup. Titanium isotopes are not significantly fractionated in the paleosol, suggesting that continental weathering and erosion in the Archean did not fractionate Ti isotopes. Similarly, Ni/Co and Th/Sc ratios are reasonably conserved, which validates their use as a robust proxy for upper continental crust composition in shales, whereas La/Sc, Cr/Zn and Cr/U ratios are highly variable relative to the provenance composition, which suggests that caution should be used when applying these ratios in shale studies of the ancient upper continental crust composition.

Analysis of climate and landscape change through the Pennsylvanian and Permian Monongahela and Dunkard Groups, Southeastern Ohio, USA

Paleosols and ichnofossils of the Late Pennsylvanian to Early Permian Monongahela and Dunkard groups of southeastern Ohio provide significant paleoenvironmental and paleoclimatic data related to the Late Paleozoic climatic transition. Forty paleosol profiles from three outcrops in southeastern Ohio were investigated. Methods included the description of paleosol profiles in the field and the analysis of thin sections. Clay mineralogy and bulk geochemistry were used to assess weathering processes and paleoprecipitation. Results, such as the distribution, abundance, and size of physical features including slickensides, pedogenic carbonates, and rhizoliths suggest erratic fluctuations between mildly seasonal wet environments and strongly seasonal dry environments throughout the studied section which are supported by oscillations in calculated paleoprecipitation values. Paleosols of the Monongahela Group are interpreted as mildly to strongly seasonal, wet to wet-dry, sparsely to heavily vegetated fen, dry woodland, clastic marsh, early successional vegetation, and brakeland ecosystems forming on proximal to distal floodplain and shoreline lacustrine environments. Paleosols of the Lower Dunkard Group are interpreted as a mildly to strongly seasonal, wet to wet-dry, sparsely to heavily vegetated, clastic marsh, early successional vegetation, and brakeland ecosystems forming on proximal to distal floodplain and distal levee environments. Variability in climate has profound effects on paleolandscapes. By investigating changes in paleosols through episodes of climatic transition, we can better understand the impact these changes have on soils and soil ecosystems of the past and future.

Stable climate in India during Deccan volcanism suggests limited influence on K–Pg extinction

Large igneous provinces (LIPs) have been temporally correlated to mass extinctions throughout the Phanerozoic, including the emplacement of the Deccan Volcanic Province (DVP; 66.3–65.6 Ma) in western and central India, which has been invoked as either a cause or exacerbating factor in the Cretaceous-Paleogene (K-Pg) extinction. However, relatively little is known about local paleoclimatic and paleoenvironmental responses to volcanism. To investigate the DVP's role as a driver of local environmental change and to provide climatic background for known ecological shifts, new inter-basaltic paleosol profiles at the eastern edge of the DVP are used in conjunction with profiles from the literature to reconstruct paleoclimate and terrestrial environments before and after the K–Pg. These profiles provide a novel opportunity to study the sediments within basalt flows before, during, and after a mass extinction event and in the midst of a LIP emplacement event. Paleoclimate proxies and the Floral Humidity Province proxy reflect little long-term change in either climate or environment across the K–Pg, with stable precipitation values and temperatures accompanied by a constant forest signal. These interpretations are corroborated by macrofloral records and sedimentology from India, which suggest some environmental turnover but generally support a forested, fluvio-lacustrine environment throughout the duration of volcanism. Our results support the possibility of rapid recovery times for terrestrial ecosystems during volcanism and suggest that while DVP eruptions may have exacerbated long-term environmental perturbation, the emplacement of the DVP is not likely to have caused the terrestrial mass extinction at the K–Pg boundary.

STEPPED-COMBUSTION14C DATING IN LOESS-PALEOSOL SEDIMENT

ABSTRACTIn this study, low temperature (room temperature, 400°C LT) and high temperature (400–900°C HT) of bulk organic carbon samples were dated from two loess and paleosol profiles. The results showed that radiocarbon (14C) dates of the LT were younger than HT fractions, indicating effect of younger contamination from overlying layers. The δ13C variation of the HT fraction appears to respond much more sensitively to climate change, and14C ages of HT fraction can produce reasonable14C ages from a younger layer, but it is very difficult to obtain reliable14C ages from older layer as a result of uncomplete removal of young carbon.