Paleosol Samples Research Articles

The Difference in Shear Behavior and Strength between Loess and Paleosol and Their Prediction of Unsaturated Strength

In recent decades, loess landslide events have attracted increasing attention in the South Jingyang tableland. To elucidate the mechanical mechanism of landslide initiation in the region, this work collected undisturbed loess and paleosol samples taking from the Q2 strata in the South Jingyang tableland. A range of direct shear tests were carried out to explore the strength evolution law of shear zone soil subjected to a varying initial moisture content. In addition, soil water characteristic curves (SWCCs) were also charted and used for predicting the unsaturated shear strength. The findings show that the basic physical properties of the paleosol are different from those of loess due to their different pedogenic environments. The normal stress level and initial moisture content jointly determine whether the shear behavior is strain hardening or strain softening. The shear strength and strength parameters evidently diminish with an increasing initial moisture content, and cohesion contributes to the vast majority of strength attenuation. Paleosol samples possess higher values in shear strength and strength parameters than loess samples due to their stronger inter-particle cementation. The predictive formulas of unsaturated shear strength for undisturbed loess and paleosol are proposed, respectively, based on the Vanapalli model, and the calculated values of the strength prediction model are in perfect agreement with the experimental values.

Zircon U-Pb, whole-rock Rb-Sr and K-Ar ages of metamorphosed and metasomatized paleosol at the base of the Paleoproterozoic Aravalli Supergroup, NW India: A two-billion-year record of tectono-thermal events

The Precambrian paleosols are good proxies for deciphering paleoredox conditions. They are also good indicators of thermal events that affect the metamorphic terrane. The Rb-Sr isochron ages of the Precambrian paleosols are good indicators of the timing of alkali metasomatism. This work presents zircon U-Pb, whole-rock Rb-Sr isochron, and K-Ar ages of a metamorphosed and metasomatized paleosol occurring at the base of the Paleoproterozoic Aravalli Supergroup, Northwestern India, to constrain the time of major thermal events. The Aravalli paleosols (sericite schists) have witnessed a complex history of metamorphism and metasomatism. The LA-ICP-MS U-Pb ages of zircons from the two paleosol samples are 2586 ± 45 Ma and 2793 ± 21 Ma, similar to the zircon ages for the parent Ahar River Granite. Five paleosol samples define an Rb-Sr isochron age of 1394 ± 22 Ma with (87Sr/86Sr)i of 0.7493 ± 0.0034 (2σ) and an MSWD of 3.9, pointing to the timing of the latest alkali metasomatism affecting the paleosols. The whole-rock K-Ar ages of sericite-rich paleosol samples range from 940 ± 30 to 873 ± 25 Ma, corresponding with the emplacement of Erinpura Granite. Thus, the Aravalli paleosol recorded more than two billion years of thermal events in its multi-geochronometers. The correlation between Rb-Sr isotope compositions and REE contents in paleosol samples suggests the REE mobility during K-metasomatism.

Characterization of Pennsylvanian Paleosols in Indiana with a Special Reference to Rare Earths (REE) and Lithium

This study characterizes Pennsylvanian paleosols from coal-bearing strata in Indiana, with the main emphasis on the abundance and distribution on rare earths (REE) and lithium (Li). Eleven locations in southwest Indiana were selected for this study, targeting paleosols of both the Carbondale Group and the Raccoon Creek Group. Most paleosols were directly underlying coal beds. Lithologically, paleosols range from grey mudstones to greenish-gray claystones and occasional siltstones. In the Carbondale Group, the thickest paleosol horizons occur under the Danville and the Houchin Creek Coal Members, with their thickness reaching close to 10 feet. In the Raccoon Creek Group, the paleosol under the Wise Ridge Coal appears to be thickest (4.6 to 11.4 feet) and best developed. Total REE content in paleosol samples ranges from 138.8 ppm to 728.55 ppm, with an average range of 208.3 ppm (Colchester paleosol) to 342.9 ppm in the paleosol under the Upper Block Coal. Light REE dominate over heavy REE, and the variation in the light REE is responsible for the largest variations of the total REE. The largest amount of the heavy REE (61 ppm) occurs in the paleosol under the Lower Block Coal, whereas the paleosol under the Upper Block Coal has the largest variation in the heavy REE content (49 to 148 ppm). REE patterns for paleosol samples show very weak to no fractionation among light, medium, and heavy REE, with no major anomalies present. The majority of paleosols have REE values that are roughly representative of the upper continental crust. Only some paleosols are slightly enriched and these are paleosol samples under the Danville Coal and the Upper Block Coal. Lithium content in the paleosol samples shows a variation from 21 ppm to 530 ppm. On average, the Wise Ridge Coal is richest in Li (249.3 ppm), followed by Viking B Coal (190.8 ppm). The coals of the Brazil and Staunton Formations have higher Li content than the younger coals. Although this study has not uncovered large concentrations of REE or lithium, the wide range of concentrations and potential ease of their extraction from these clay-rich rocks imply that the paleosols deserve further examination, and the current study can be used as a screening guide for more focused investigation.

Testing less-conventional methods to date a late-pleistocene to Holocene eruption: Radiocarbon dating of paleosols and 36Cl exposure ages at Pelado volcano, Sierra Chichinautzin, Central Mexico

Despite their significance for estimating hazards and forecasting future activity, dating young volcanic deposits and landforms (<50,000 yrs old) remains a challenge due to the limitations inherent to the different isotopic chronometers used. The Trans-Mexican Volcanic Belt is one of the most active and populated continental arcs worldwide, yet its temporal pattern of activity is poorly constrained. Such deficiency is particularly problematic for the Sierra Chichinautzin Volcanic Field (SCVF) that is located at the doorstep of Mexico City and Cuernavaca and is hence a major source of risk for these cities. Existing ages for this area derive mostly from either radiocarbon on charcoal, which is rare and may be contaminated, or 40 Ar/ 39 Ar on rock matrix, which is poorly precise for this time period and rock type. Here, we focus on the Pelado monogenetic volcano, which is located in the central part of the SCVF and erupted both explosively and effusively, producing a large lava shield and a widespread tephra blanket. This unique eruptive event was previously dated at ∼12 calibrated (cal) kyrs BP, using radiocarbon dating on charcoal from deposits related to the eruption. To test alternative dating approaches and confirm the age of this significant eruption, we applied two less conventional techniques, radiocarbon dating of bulk paleosol samples collected below the complete tephra sequence at nine sites around the shield, and in-situ 36 Cl exposure dating of two samples of an aphyric lava from the base of the shield. Radiocarbon paleosol ages span a continuous time interval from 13.2 to 20.2 cal kyrs BP (2σ), except for one anomalously young sample. This wide age spread, along with the low organic contents of the paleosols, may be due to erosive conditions, related to the sloping topography of the sampling sites and the cool and relatively dry climate of the Younger Dryas (11.7–12.9 ka), during which the Pelado eruption probably occurred. The two 36 Cl-dated lava samples have consistent ages at 1σ analytical errors of 15.5 ± 1.4 ka and 13.2 ± 1.2 ka, respectively, yielding an average age of 14.3 ± 1.6 ka for this lava flow. The high full uncertainty in 36 Cl ages (24%) is due to high rock Cl content. We conclude that paleosol radiocarbon dating is useful if numerous samples are analyzed and climatic and relief conditions at the time of the eruption and at the sites of tephra deposition are considered. The 36 Cl dating technique is an alternative method to date volcanic eruptions, as it gave consistent results, but in the specific case of Pelado volcano, the high Cl content in the analyzed rocks increases the age uncertainties. • Catalogue of recent eruptions in Trans-Mexican Volcanic Belt is incomplete. • 14 C on paleosol and 36 Cl dating are used on a previously dated monogenetic volcano. • New ages are coherent with previous 14 C charcoal age. • Spread in paleosol ages likely due to erosive conditions at time of eruption. • 36 Cl ages are statistically consistent but poorly precise, due to high Cl in rock.

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.

Overestimate of C4 Plant Abundance Caused by Soil Degradation‐Induced Carbon Isotope Fractionation

AbstractThe carbon isotopic composition (δ13C) of soil organic matter (SOM) is a widely used proxy for terrestrial vegetation. However, SOM decomposition can cause C isotope fractionation, which remains poorly constrained in ancient soils thus clouds the interpretations of paleosol‐δ13CSOM records. Here, we report new δ13CSOM records of the Holocene paleosols from the Chinese Loess Plateau and investigate how the decomposition of SOM affects the preserved δ13C signal and the inferred vegetation changes. Our results reveal significant C isotope fractionation, as SOM in bulk paleosol samples are systematically enriched in 13C (up to 3‰) compared with contemporary SOM occluded in calcite nodules, the latter of which are thought to resist degradation. Such fractionation is likely due to the selective preservation of 13C‐enriched microbial biomass by fine‐grained minerals. Previous studies based on paleosol‐δ13CSOM records that underestimate or neglect the 13C‐fractionation related to SOM decomposition probably overestimated the abundance of regional C4 biomass.

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.

Paleoseismic history of the Manisa fault zone, Western Anatolia

The 45-km-long, E-W to NW-SE-striking Manisa Fault Zone, which constitutes the western section of the Gediz Graben, is characterized by a pure normal sense of motion with a minor strike-slip component. Even though there are numerous historical earthquakes have been listed in Western Anatolia Graben System, a few studies have been addressed on seismic sources of the earthquakes so far. According to existing literature, the western segment of the Manisa Fault Zone is well known characterized; however, the eastern segment of it has rarely been addressed. To decipher the Holocene seismotectonic behavior of the eastern segment of Manisa Fault Zone, trench-based paleoseismological analyses for the first time were performed along with it. To constrain the timing and frequency of past earthquakes, and elapse time from the last activation using the optically stimulated luminescence (OSL) and radiocarbon (14C) dating, we collected twenty-six colluvial and paleosol samples from the trenches. The obtained paleoseismic data show that (i) Manisa Fault Zone is responsible for six surface rupturing earthquakes since late Pleistocene-Holocene, which is occurred at 30.6 ± 8.8 ka (E1), 15.0 ± 5.0 ka (E2), 6.6 ± 1.3 ka (E3), 2.9 ± 1.3 ka (E4), 0.8 ± 0.4 ka (E5), and 0.1 ± 0.1 ka (E6), (ii) the recurrences interval of destructive earthquakes on Manisa Fault Zone was found between 0.95 kyr to 3.8 kyr for Holocene, and (iii) the elapsed time since the most recent surface ruptured earthquake on the Manisa Fault Zone is 159 year.

Physico-mechanical properties of thick paleosol in Q1 strata of the Chinese Loess Plateau and their variations during tunnel excavation

To cope with the increasing demand for traffic capacity in the Chinese Loess Plateau, building additional tunnels are required in the thick paleosol strata of Q1 strata. The thick paleosol strata consist of multiple paleoclimatic stratigraphic units, and the behavior and properties of the paleosol in each unit, which are believed to be quite different from those of common loess, remain scarcely investigated. In this work, we focused on the thick paleosol strata in Eastern Gansu of the Chinese Loess Plateau, the physico-mechanical properties of the paleosol in different paleoclimatic stratigraphic units were analyzed, and the relationship between the chemical properties, microstructures, and physico-mechanical properties of the paleosol was discussed. Besides, the water content of the paleosol was measured on site during the tunnel excavation in the thick paleosol strata, and the trends of the physico-mechanical properties of the paleosol during the excavation were also explored. We found that in the thick paleosol strata, the dry density, water content, liquid plastic limit, swelling potential, and mechanical strengths of the paleosol samples from different paleoclimatic stratigraphic units did not show large discrepancies, but obvious differences could be found in the grain size, maximum swelling pressure, and maximum swelling strain. The subtle variation of the water content had modest effects on the contact mode and cementation degree of the particles, but it could considerably influence the hydration film thickness of the clay mineral particles, which explains the distribution of mechanical properties of the paleosol samples. During the tunnel excavation in the thick paleosol strata, the paleosol around the tunnel sustained high and stable mechanical strength, while its swelling pressure and water content showed a consistent trend. The results can provide basic parameters for the stability analysis of the tunnel built in the thick paleosol strata.

Reconstruction of palaeoenvironment and ancient human activities at Obrovac-type settlements (Serbia) using a geochemical approach

This study aims to determine the palaeoenvironmental characteristics and activity patterns of Obrovac-type archaeological sites in Western Serbia, dated to the Late Neolithic/Early Eneolithic period, ∼5th millennium BC. These mound-like sites, enclosed by a wide ditch, that are not known in other parts of the central Balkan area, have long intrigued archaeologists investigating their origin and function over the last few decades.In this study, for the first time, organic-geochemical analysis of paleosol samples from the Obrovac-type sites was applied with the aim of palaeoenvironmental reconstruction. Additionally, organic carbon content and anion analysis of 58 subsoil samples from these settlements were performed to determine the use of space and activity zones.The analysis of biomarkers from selected sites suggests significant plant biodiversity in the Mačva region during the Late Neolithic/Early Eneolithic. Distribution of n-alkanes with the maximum at n-C25 and predominance of C30 hop-22(29)-ene among hopanoids in samples from Obrovac type-sites indisputably indicates that macrophytes are a dominant source of organic matter, implying a marshy and floodplain depositional environment. On the other side, a strong signal of long-chain n-alkanes indicates the input of terrestrial plants into the precursor biomass, confirming that this environment was habitable for the first settlers in this region. Anion-based analysis delineates certain activity zones, demonstrating that Obrovac type-sites manifest rather complex spatial behavior despite their relatively small size and available space.

The influence of ancient soil biota on the precipitation and distribution of pedogenic carbonate in paleosols of the Marília Formation (Upper Cretaceous, Brazil)

In arid and semi-arid climates, soil development is strongly influenced by seasonal patterns and biogenic activity, and often results in pervasive pedogenic horizons consisting of highly concentrated carbonate. In Brazil, such horizons enriched in calcium carbonate are common and relatively well-studied in paleosols of the Upper Cretaceous Marília Formation. However, detailed analyses on the sequence of mineral precipitation phases and the role of organisms in these processes are still lacking. The present work describes eight argillic and calcic horizons characterized by different degrees of carbonate accumulation. Argillic horizons show translocation clay structures and weak to moderate cementation by calcium carbonate. Calcic horizons contain moderate to high carbonate accumulations ranging from powdery coatings to nodular masses of micrite. Carbonate distribution in these horizons is preserved largely as carbonate-cemented macropores produced by such ichnofossils as Planolites, Taenidium, Naktodemasis, Skolithos, Palaeophycus and calcareous rhizocretions. Moreover, stable isotope data, respired soil CO2 function, and cathodeluminescence (CL) of the paleosol samples were used to evaluate soil paleovegetation, paleoproductivity, and the distinctive stages of calcite precipitation. Stable isotopic analysis suggests that paleovegetation cover in the paleosols was predominantly C3-photosynthsizing plants. The relationship between bioturbation indices and soil respiration suggests a positive pattern of paleoproductivity controlled by higher moisture conditions and the increased activity of soil fauna. Finally, CL imaging shows that there were multiple stages of carbonate precipitation with both α-type and β-type fabrics that are interpreted as abiotic and biotic processes during pedogenesis.

Nanosized Authigenic Magnetite and Hematite Particles in Mature‐Paleosol Phyllosilicates: New Evidence for a Magnetic Enhancement Mechanism in Loess Sequences of China

AbstractMagnetic enhancement of Chinese loess‐paleosol sequences has been used extensively as a proxy for East Asian summer monsoon variations. However, the pedogenic magnetic particles contributing to this magnetic enhancement are difficult to extract, so it is not clear how they formed. In this study, we reveal pedogenic magnetite and hematite using electron microscopy, synchrotron radiation X‐ray diffraction, and rock magnetic methods. First‐order reversal curves indicate that superparamagnetic/single domain/vortex state magnetic properties dominated both loess and paleosol samples. Samples of muscovite and chlorite, which are paramagnetic, have weak spontaneous magnetization. The 1‐ to 10‐μm‐sized fraction of host silicatesis responsible for most of the magnetic enhancement of paleosols. In the paleosol fraction, we found weathered phyllosilicates (muscovite/chlorite), including many elongated submicron to a few microns authigenic magnetite and hematite particles between layers; however, few such interlayer particles were found in phyllosilicates of the loess fraction. The concentration of magnetite/hematite particles within paleosol muscovite/chlorite grains and in aggregates of phyllosilicate fragments is much higher than that of the submicron iron oxides found on silicate surfaces. Interlayer magnetite particles are dominantly prism‐shaped with aspect ratios &gt;~4. The authigenic magnetite must be mainly responsible for the spontaneous magnetization of the muscovites and chlorites and the paleosol magnetic properties. The protective silicates account for the low extraction efficiency and also the near absence of surface oxidation of pedogenic magnetite. Based on our results, we suggest that magnetite/hematite in weathered phyllosilicates contribute significantly to the magnetic enhancement of mature paleosols.

The deficiency of organic matter 14C dating in Chinese Loess-paleosol sample

Abstract In the absence of reliable charcoal, wood or plant macrofossils, organic matter in loess-paleosols is often used for 14C dating. However, organic matter can be contaminated by a variety of carbon sources within the sediments, and this may affect the radiocarbon dating results. Specific chemical pretreatments can be applied to minimize such effects, and comparisons with optically stimulated luminescence (OSL) dates from the same sediments allows us to assess potential open system effects on organic matter. Here we present comparisons of the two geochronometers from loess-paleosol deposits in the Chinese Loess Plateau. Our results show that loess -paleosol samples from the past 25 cal kyr BP generally yield reliable 14C dates, however, beyond this range, specifically ~25–59 cal kyr BP, we observe a steady increase in OSL ages with depth, but no consistent increase in corresponding AMS 14C ages. We suggest that relatively old samples from loess-paleosols should be studied using techniques such as compound-specific radiocarbon dating, ramped pyrolysis or stepped-temperature combustion.

Experimental study on the shear behavior of loess and paleosol based on ring shear tests

Landslide hazards frequently occur in the Chinese Loess Plateau every year, causing devastating casualties, damage to engineering structures, and reduction in land resources. This work presents an experimental investigation of four samples retrieved from Jingyang in the southeast of the Chinese Loess Plateau, using ring shear tests and microscopic observation, in an attempt to explore the shear behavior of loess and paleosol under the larger deformations that are involved in landslide hazards. The results of ring shear tests reveal that the loess and paleosol samples exhibited different strength characteristics. The residual strength envelopes could be assumed to be a straight line, and the overconsolidated state had negligible influence on the residual shear strength. After the shear experiment, shear bands were studied from both macroscopic and microscopic perspectives. Macroscopic observation and mechanical analysis indicate that all samples developed a main horizontal failure plane related to the maximum shear stress, while loess samples also developed en echelon failure planes tilting from the horizontal at angles of 14–18°, which are interpreted as Coulomb slip planes. Scanning electron microscopy reveals that the particles inside shear bands tend to be re-oriented. Particle crushing occurred in the shear process, which was also verified by particle size distribution analysis.

Grassland fire ecology has roots in the late Miocene

That fire facilitated the late Miocene C4 grassland expansion is widely suspected but poorly documented. Fire potentially tied global climate to this profound biosphere transition by serving as a regional-to-local driver of vegetation change. In modern environments, seasonal extremes in moisture amplify the occurrence of fire, disturbing forest ecosystems to create niche space for flammable grasses, which in turn provide fuel for frequent fires. On the Indian subcontinent, C4 expansion was accompanied by increased seasonal extremes in rainfall (evidenced by δ18Ocarbonate), which set the stage for fuel accumulation and fire-linked clearance during wet-to-dry seasonal transitions. Here, we test the role of fire directly by examining the abundance and distribution patterns of fire-derived polycyclic aromatic hydrocarbons (PAHs) and terrestrial vegetation signatures in n-alkane carbon isotopes from paleosol samples of the Siwalik Group (Pakistan). Two million years before the C4 grassland transition, fire-derived PAH concentrations increased as conifer vegetation declined, as indicated by a decrease in retene. This early increase in molecular fire signatures suggests a transition to more fire-prone vegetation such as a C3 grassland and/or dry deciduous woodland. Between 8.0 and 6.0 million years ago, fire, precipitation seasonality, and C4-grass dominance increased simultaneously (within resolution) as marked by sharp increases in fire-derived PAHs, δ18Ocarbonate, and 13C enrichment in n-alkanes diagnostic of C4 grasses. The strong association of evidence for fire occurrence, vegetation change, and landscape opening indicates that a dynamic fire-grassland feedback system was both a necessary precondition and a driver for grassland ecology during the first emergence of C4 grasslands.

Paleovegetation changes accompanying the evolution of a riverine system at the BK paleoanthropological site (Upper Bed II, Olduvai Gorge, Tanzania)

Paleovegetation studies are essential to ecologically frame hominin evolution. The analysis of the phytolith content of 22 paleosol samples collected along the vertical sequence of the channel containing the BK (Bell's Korongo) archaeological levels, provided evidence of the paleovegetation that accompanied the evolution of the riverine system that formed this site 1.353millionyears ago, when climate was particularly arid. The analysis reveals the abundance of forest indicator phytoliths (46% to 92% of the assemblages), rare grass silica short cells and sedges phytoliths (up to 15% of assemblages), and palms (up to 10% of the phytolith assemblages). The vegetation of the BK fluvial system was therefore characterized by abundant woody plants. It did not vary significantly diachronically in the period of time under scrutiny. The alluvial paleovegetation corresponds well to the fluvial dynamic that formed BK, including the presence of sterile samples recovered in areas where the water traction changed the soils frequently. This spot of dense paleovegetation together with the fluvial watercourse (in a paleolandscape likely dominated by sparse/open vegetation) could have been attractive, seasonally, for animals and hominins.

Evolution of C4 plants and controlling factors: Insight from n-alkane isotopic values of NW Indian Siwalik paleosols

The δ13C values of long chain n-alkanes from paleosol samples (n=74) of four NW Indian Siwalik sections have been used to reconstruct the relative abundance of C3-C4 plants, after isotopic characterization of modern plants from the Ganges floodplain considered equivalent to the past Siwalik floodplain. The values from the Naladkhad (11.6–8.9Ma) and Ranital (11.3–7.4Ma) sections of the Kangra sub-basin indicate the presence of ca. 20% C4 plants at ca. 11Ma. The existence of C4 plants at ca.11Ma indicate an early appearance of C4 plants, compared with the published data from the Siwalik regions. The Ranital section shows a patchy occurrence of C4 plants between 11.3Ma and 7.4Ma and the Jabbarkhad section shows (5.2–3.0Ma) a gradual increase in C4 plant abundance. The C4 plant abundance fluctuated between 83% and 0% (5.8–1.3Ma) in the Haripur Khol section of the Subathu sub-basin. Comparison of C4 plant abundance from the four sections shows that each section was characterised by a distinct evolutionary pattern. The δD values of the same n-alkanes from the Kangra and Subathu sub-basins indicate two episodes of Indian summer monsoon intensification at ca. 9Ma and ca. 4Ma. The correlation between δDC29 and δ13CC29 values varied in different sections. It was also observed that the C4 plant abundance variation could be linked to the variable channel/overbank ratio of the studied sections. This observation suggests that Indian summer monsoonal rainfall, type of overbank sediments and moisture content controlled the abundance of C4 plants in the Siwalik floodplain.

Organic geochemical approaches to identifying formation processes for middens and charcoal-rich features

Charcoal is an important component of archaeological sites that can yield fundamental information useful in understanding past human activities. We applied a suite of organic geochemical tools to identify and quantify anthropogenic molecular charcoal fragments in an archaeological site in Panola County, N Texas to determine the provenance of a buried melanic horizon. The site is a small Middle-Late Caddo occupation inhabited principally between ca. 1450 and 1600 AD. We tested three molecular techniques to determine their ability to constrain site formation processes: solid state 13C nuclear magnetic resonance spectroscopy (NMR), lignin free phenols and lignin oxidation products (LOPs) and anhydrosugar analysis. We demonstrated that, in conjunction, the three afforded the identification and quantification of anthropogenic molecular charcoal fragments at this archaeological site.The approach allowed reconstruction of the fire history and vegetation type around the site during the occupation period. The NMR and anhydrosugar analysis pointed to temperature conditions associated with thermal refuse in different contexts. In particular, geochemical signatures in a smudge pit indicated a higher combustion temperature than for the charcoals in the other paleosol samples, suggesting multiple fire uses. The data also showed differential persistence of fire signals in environmental samples, likely as a result of differential decomposition rate and solubility, which may lead to heterogeneous fates of different forms of pyrogenic C in the environment. The techniques expand the tool kit available to archaeologists and soil scientists in detecting the human footprint in soil formation, especially in warm, humid climates where decomposition is rapid.

Plagioclase sub-species in Chinese loess deposits: Implications for dust source migration and past climate change

Plagioclase mineral sub-species in the Lingtai Section in central Chinese Loess Plateau are examined using Mineral Liberation Analyzer techniques, showing that loess and paleosol samples exhibit similar patterns in terms of plagioclase feldspar sub-species content. This suggests that both loess and paleosol units have preserved their primary Ca-bearing plagioclase compositions of loess source regions. Weighted average CaO (%) in Ca-bearing plagioclase lies within a narrow range and is equivalent to the average plagioclase composition for upper continental crust. This fact supports the hypothesis that Chinese loess deposits are the result of a thorough mixing of dust sources. The sum of Ca-bearing plagioclase content exhibits a general increasing trend superimposed by glacial–interglacial oscillations. In combination with observed plagioclase data in the deserts, the variations of Ca-bearing plagioclase minerals might be used as a proxy for dust source migration and climate changes in the loess source regions. Furthermore, linear relationship between lithogenic magnetic susceptibility (MS) component input and contents of Ca-bearing plagioclase in loess units revises a MS proxy for reconstructing paleo-monsoon precipitation history. The revised MS and plagioclase sub-species records help in understanding the mechanism of glaciation across northern Tibetan Plateau.

Geochemistry of a paleosol horizon at the base of the Sausar Group, central India: Implications on atmospheric conditions at the Archean–Paleoproterozoic boundary

A paleosol horizon is described from the contact of the Sausar Group (∼2400 Ma) and its basement (Tirodi Gneiss; >2500 Ma) in Central India. Physical evidence of pedogenesis is marked by the development of stress corrosion cracks, soil peds, corestone weathering and nodular rocks. XRD and SEM-EDX data indicate the presence of siderite, ankerite, uraninite, chlorite, alumino-silicate minerals, ilmenite, rutile and magnetite, in addition to quartz, feldspar and mica. The chemical index of alteration, the plagioclase index of alteration, and the chemical index of weathering show an increasing trend from parent rock to the paleosol and indicate a moderate trend of weathering. The A-CN-K plot indicates loss of feldspars, enrichment in Al2O3 and formation of illite. Different major element ratios indicate baseloss through hydrolysis, clay formation, leaching of some elements, and more precipitation with good surface drainage. The paleosol is depleted in HREE in comparison to the parent rock indicating high fluid-rock interaction during weathering. The paleosol samples show flat Ce and Eu anomalies, low ΣREE, and high (La/Yb)N, indicative of a reducing environment of formation. Reducing condition can also be inferred from the concentration of elements such as V, Co, Cu, Pb, and Zn in the paleosol profile. Although enriched in Fe and Mg, the overall geochemical patterns of the paleosol indicate oxygen deficient conditions in the atmosphere and development by weathering and leaching processes associated with high precipitation and good surface drainage at the time of development of this paleosol during the Archean–Paleoproterozoic transition.