Paleosol Samples Research Articles

Source region analyses of the morainal detritus from the Grove Mountains: Evidence from the subglacial geology of the Ediacaran–Cambrian Prydz Belt of East Antarctica

The Grove Mountains are the inland exposures of the Prydz Belt in East Antarctica. Although the 550–500Ma orogenic event was recognized as the latest major magmatic–metamorphic activity in the Prydz Belt, its subduction–collision origin was not confirmed until the discovery of high-pressure (HP) mafic granulite erratic boulders in the glacial moraines from the Grove Mountains. Because no HP metamorphic bedrock is exposed in this area, an understanding the regional geology required a thorough study of the morainal debris mineralogy and detrital zircon U–Pb chronology. Detrital zircon U–Pb age histograms show 550–450Ma, 900–800Ma, and 1100–1000Ma modes from three morainal deposits and one paleosol samples. The oldest ages were 2300 to 2420Ma. Detailed electron probe microanalyses (EPMA) for the detrital mineral grains were compared with the minerals from the nearby exposed bedrock. The mineral chemistry indicates that the exposed bedrock in the Grove Mountains was not the sole source for morainal materials. This new U–Pb zircon geochronology and microprobe mineral data support the previous interpretation that the 550–500Ma tectonic activity was the final collisional event that formed the Prydz Belt and amalgamated East Antarctica.

The kinetics of solid-state isotope-exchange reactions for clumped isotopes: A study of inorganic calcites and apatites from natural and experimental samples

Carbonate clumped-isotope geothermometry is a tool used to reconstruct formation or (re)equilibration temperatures of carbonate bearing minerals, including carbonate groups substituted into apatite. It is based on the preference for isotopologues with multiple heavy isotopes (for example, ^13^C^16^O~2~^18^O^2−^ groups) to be more abundant at equilibrium than would be expected if all isotopes were randomly distributed amongst all carbonate groups. Because this preference is only a function of temperature, excesses of multiply substituted species can be used to calculate formation temperatures without knowledge of the isotopic composition of water from which the mineral precipitated or other phases with which it may have equilibrated. However, the measured temperature could be modified after mineral growth if exchange of isotopes amongst carbonate groups within the mineral has occurred through internal isotope-exchange reactions. Because these exchange reactions occur through thermally activated processes, their rates depend on temperature and increase at higher temperatures. Thus internal isotope-exchange reactions could lead to effective re-equilibration at high temperatures, overprinting the original temperatures recorded during mineral growth. We measured clumped-isotope temperatures in carbonate bearing minerals (including apatites) from several carbonatites to constrain the kinetics of these internal isotope-exchange reactions. We observe two key trends for clumped-isotope temperatures in carbonatites: (*i*) clumped-isotope temperatures of apatites and carbonate-bearing minerals decrease with increasing intrusion depth and (*ii*) apatites record lower clumped-isotope temperatures than carbonate minerals from the same intrusion. We additionally conducted heating experiments at different temperatures to derive the temperature dependence for the rate constants that describe the alteration of clumped-isotope temperatures with time in calcites and apatites. We find that calcites exhibit complex kinetics as has been seen in previous studies. To quantify these results, we constructed a model that incorporates both diffusion of isotopes through the crystal lattice and isotope-exchange reactions between adjacent carbonate groups. We tested this model through comparison to previous measurements of optical calcites and brachiopods and to samples with known cooling histories and find that the model is able to reasonably capture kinetic data from previous experiments and the observed clumped-isotope temperatures of calcites assuming geologically reasonable cooling rates. A similar model for apatite over-predicts the observed clumped-isotope temperatures found in natural samples; we hypothesize this discrepancy is the result of annealing of radiation damage in our experiments, which lowers the diffusivity and rate of isotope exchange of carbonate groups compared to damaged natural samples. Finally, we constructed models to explore how heating can alter recorded clumped-isotope temperatures. Our model predicts that samples change in clumped-isotope temperatures in two stages. The first stage changes the recorded clumped isotope temperatures by \<1 °C if held at 75 °C for 100 million years and by up to ∼40 °C if held at 120 °C, but the clumped-isotope temperatures does not reach ambient values through this low-temperature mechanism. A second, slower change becomes effective at temperatures above 150 °C and can take the measured clumped-isotope temperature up to the true ambient temperature. This result implies that old (hundreds of million years) samples that have only experienced mild (\<100-125 °C) thermal histories could exhibit small but measurable (order 10 °C) changes in their clumped-isotope temperatures. We compared this heating model to clumped-isotope measurements from paleosol samples from the Siwalik Basin in Nepal, which were buried up to 5 km and then rapidly exhumed to the surface; these samples often do not give reasonable surface temperatures. The modeled temperatures agree with measured temperatures of these samples, suggesting that partial re-equilibration during shallow crustal burial is responsible for their elevated clumped-isotope temperatures.

Bulk particle size distribution and magnetic properties of particle‐sized fractions from loess and paleosol samples in Central Asia

AbstractRecent studies demonstrate that particle size separation based on gravitational settling and detailed rock magnetic measurements of the resulting fractionated samples constitutes an effective approach to evaluating the relative contributions of pedogenic and detrital components in the loess and paleosol sequences on the Chinese Loess Plateau. So far, however, similar work has not been undertaken on the loess deposits in Central Asia. In this paper, 17 loess and paleosol samples from three representative loess sections in Central Asia were separated into four grain size fractions, and then systematic rock magnetic measurements were made on the fractions. Our results demonstrate that the content of the &lt;4 μm fraction in the Central Asian loess deposits is relatively low and that the samples generally have a unimodal particle distribution with a peak in the medium‐coarse silt range. We find no significant difference between the particle size distributions obtained by the laser diffraction and the pipette and wet sieving methods. Rock magnetic studies further demonstrate that the medium‐coarse silt fraction (e.g., the 20–75 μm fraction) provides the main control on the magnetic properties of the loess and paleosol samples in Central Asia. The contribution of pedogenically produced superparamagnetic (SP) and stable single‐domain (SD) magnetic particles to the bulk magnetic properties is very limited. In addition, the coarsest fraction (&gt;75 μm) exhibits the minimum values of χ, χARM, and SIRM, demonstrating that the concentrations of ferrimagnetic grains are not positively correlated with the bulk particle size in the Central Asian loess deposits.

Geochemical Characteristics of Eolian Deposits on the Eastern Margin of the Tibetan Plateau and Implications for Provenance

AbstractEolian dust deposition is intimately related to atmospheric circulation and environmental setting of the source region, and therefore is an invaluable tool for studying the evolutionary history of atmospheric circulation patterns and paleoclimatic change. Identifying the provenance of any eolian deposit is crucial not only for reconstructing the paleoenvironmental history of the dust source region, but also for understanding the paleoclimatic significance of various indices. Loess and paleosol samples from the Garze region on the eastern margin of the Tibetan Plateau (TP) were analyzed for their elemental (major and trace elements) and isotopic (Sm‐Nd) geochemistry and compared with those of Northern Chinese (NC) loess formed at the same age. The results show that the geochemical compositions of the Garze loess and paleosol samples are similar to those eolian deposits on the Chinese Loess Plateau (CLP), and also resemble the average UCC. This indicates that the eolian deposits on the eastern margin of the TP were derived from well‐mixed sedimentary protoliths that had undergone numerous upper crustal recycling processes, just as with the CLP loess deposits. However, compared with NC loess, the Garze samples have higher ΣREE, Li, Rb, Zr, Cs, Hf and Bi concentrations, higher TiO2/Al2O3, Hf/Nb, La/Nb, Th/Nb and lower K2O/TiO2, Zr/Hf, Ba/Rb ratios. From the Sm‐Nd isotopic geochemistry, εNd(0) and 147Sm/144Nd values of Garze loess and paleosol samples are clearly lower than the NC loess. The higher Bi, Zr and Hf concentrations are relevant to the widely distributed acid‐magmatic rocks in this region, whereas the higher contents of Li, Cs, Rb are attributed to the high background values of the TP. The geochemical characteristics of the Garze loess and paleosol samples further prove that the local glacial and other Quaternary detrital sediments are predominantly the contributors for the eolian deposits on the eastern margin of the TP. Stable element concentrations and their ratios in the Garze loess and paleosol samples formed at different times have relatively greater variation ranges in comparison with the NC loess, indicating that the source regions for eolian deposits have been unstable since the late Early Pleistocene. We attribute the instability of dust sources to variable earth surface conditions and the changeable TP winter monsoon in direction and intensity, which are in turn related to the uplift of the TP.

Composition of clay-fraction organic matter in Holocene paleosols revealed by advanced solid-state NMR spectroscopy

Identifying which components of organic matter are preserved over millennia in paleosols is important for understanding the stabilization mechanisms of soil organic matter (SOM) and in evaluating the portion of the terrestrial carbon stock that might be attributed to paleosols. Using advanced solid-state nuclear magnetic resonance (NMR) techniques, we examined the chemical composition of clay-associated SOM in a chronosequence of four paleosols and one modern soil collected from a site in the eastern Great Plains in Kansas, USA. The NMR spectra of clay SOM of the paleosols indicated the presence of OCH and CH2 moieties, which represent polysaccharides and lipids, respectively. The paleosol SOM also showed an appreciable peptide component, as indicated by signals of NCH and NCO. As in the modern soil, lignin residues in the paleosols contributed little to the SOM, and condensed aromatics contributed modestly to the clay SOM. The Roberts Creek paleosol samples were exceptions. Their NMR spectra included a considerable nonprotonated aromatic carbon component, reflecting fused aromatic rings and suggesting the presence of clay-fraction charcoal residues, probably originating from paleofires. In the clay SOM of both the paleosols and the modern soil, the polysaccharides and peptides indicated by the NMR spectra are consistent with the presence of microbially derived peptidoglycan and perhaps chitin residues. We conclude that lipids, charcoal, and polysaccharides, probably in the form of microbial residues, are likely to be the dominant SOM components in the paleosol clay fractions.

Re–Os isotopic records in Pleistocene loess–paleosol sequences from the Yili Basin, northwestern China

The Re–Os isotope system is a potentially powerful tool for obtaining information on source regions and transportation processes of aerosols such as mineral dust. Re–Os isotopic systematics were studied in two Pleistocene loess–paleosols exposed at Kuerdenengbulake and Zeketai in the Yili Basin, northwestern China. In the late Pleistocene Malan Loess, 187Os/188Os ratios (0.90–1.45) varied inversely with Os abundances (28–61pg/g). In the middle Pleistocene Lishi Loess section, fluctuations in Os abundances (25–50pg/g) were accompanied by a limited range of variation in 187Os/188Os ratios (generally 1.21–1.40). Four samples of Malan Loess from the Kuerdenengbulake section had higher Os concentrations and less radiogenic 187Os/188Os, coupled with higher zoisite/tourmaline ratios but uncorrelated with abundances of other minerals, paleomagnetic parameters, grain-size characteristics, major-element composition, or Rb–Sr and Sm–Nd isotopic systematics. The correlation between Os and zoisite/tourmaline ratio suggests that the source material of the loess changed when these samples were deposited, but this change was not evident in the other data in this study. Depth profiles show how Re–Os isotopic compositions varied with age of deposition. By characterizing Os abundances and 187Os/188Os ratios of late Pleistocene loess–paleosol samples in the Yili Basin, local paleoclimatic changes occurring over intervals of several thousand years could be identified that had not been found by traditional paleoclimatic proxies.When the 143Nd/144Nd ratio alone cannot distinguish the provenance of global eolian sediments, a combination of 143Nd/144Nd and 187Os/188Os can sometimes be effective. However, for the Yili Basin's loess–paleosol, the range of 187Os/188Os overlapped with ratios in eolian sediments from other areas, offering no improvement over 143Nd/144Nd alone in this respect.

Abrupt and high-magnitude changes in atmospheric circulation recorded in the Permian Maroon Formation, tropical Pangaea

Geochemical, magnetic, sedimentologic, and U-Pb detrital zircon data from loessitepaleosol couplets within the Maroon Formation of Colorado (western U.S.) record abrupt and high-magnitude changes in atmospheric circulation in western tropical Pangaea during Early Permian (Wolfcampian) time. The relative quartz grain size increases by 50%–100% in loessite compared to superjacent paleo sols. Quartz grain size correlates inversely with bulk magnetic susceptibility (r 2 = 0.89); susceptibility values from paleosols are two times higher than from underlying loessite. This pattern is interpreted to refl ect high-frequency shifts in dust-transporting wind intensity. Loessite represents semi-arid times when dusttransporting winds were stronger, depositing coarser sediment, whereas paleosols record accumulation of fisediment marked by pedogenic enhancement of magnetic susceptibility within a wetter, less windy climate. Both major- and trace-element geochemistry differ between the loessite and superjacent paleosols. The paleosols are more phyllosilicate rich, likely tied to differences in grain size. However, a multivariate analysis of log-transformed trace-element data suggests that the thicker loessite units exhibit a shift in provenance compared to paleosols and thinner loessite units. U-Pb ages of detrital zircons also differ between loessite and paleosols. Four couplets were analyzed; all loessite samples contain at least one grain with U-Pb ages (1) <300 Ma and (2) 760–940 Ma, whereas no paleosol sample contains these ages. The fi rst population refl ects coeval arcs fringing western Pangaea and eastward transport in a westerly wind regime. The second population is more enigmatic but may refl ect erosion of Proterozoic rift-related volcanics, located along the western edge of Pangaea or located in northern Canada. In addition, the loessite samples contain a lower ratio (0.69) of Neoproterozoic grains (760–570 Ma) to Paleozoic grains (500–300 Ma) compared to paleosols (1.32). Both loessite and paleosol samples contain zircons with U-Pb ages between 1360–925 Ma and 1800–1610 Ma. The Neoproterozoic, Paleozoic, and 1360–925 Ma (Mesoproterozoic) grains were sourced from the Appalachian-Ouachita orogen to the east to southeast, whereas the 1800–1610 Ma (Paleo proterozoic) grains refl ect local basement uplifts (Ancestral Rocky Mountains). Overall, the combined geochemical and detrital zircon data indicate changes in provenance tied to abrupt and repeated changes in the direction of dust-transporting winds. The loessite-paleosol pairs exhibit a nested stratigraphy with a thick paleosol capping a thick loessite followed by thinner loessites grading into thinner, and less well-developed, paleosols. Our data suggest that thick loessite intervals accumulated in a monsoonal climate with strong, seasonal westerly winds coupled with semi-arid conditions, whereas the superjacent thick paleosol records more humid climate and a provenance derived primarily from the east. The muted sedimentologic, magnetic, and geochemical differences exhibited in the series of thinner loessitepaleosol couplets likely refl ect muted climate change. In this model, the thick loessitepaleosol couplet represents a period of major glaciation and the subsequent interglacial, respectively, in which a more elliptical Earth orbit accentuates climatic extremes. The thinner couplets likely represent more circular Earth orbits during times of advancing glacial conditions when climatic extremes were muted. The inferred depositional duration of the Maroon Formation indicates these

Geochemical Characteristics of Eolian Deposits in the Chengdu Plain of Sichuan Province and the Implications for Provenance

AbstractIn this study Chengdu Red Earth (CRE) from the Chengdu Plain (CP), Sichuan province, was analyzed for its elemental (major and trace elements) and isotopic (Sm‐Nd) geochemistry and compared with Pleistocene loess and paleosol samples from the Chinese Loess Plateau (CLP) in Northern China. The geochemical composition of CRE is similar to north China loess, and also resembles the average UCC. This indicates that CRE, as loess deposits in Northern China, was derived from well‐mixed sedimentary protoliths that have undergone numerous upper crustal recycling processes. However, obvious differences in the geochemical characteristics of CRE and the north China loess are also revealed in our results. For chemically stable elements, CRE has higher Ti, Zr, Hf and lower σREE, Ba contents in comparison with loess samples from the CLP. Further analysis shows that CRE has higher TiO2/Al2O3, SiO2/Al2O3, Ba/Rb and lower Ce/Yb, Eu/Yb, LaN/YbN and σLREE/σHREE ratios. In Sm‐Nd isotopic geochemistry, Sm and Nd content and the εNd(0) value in CRE are significantly higher than those in north China loess. The higher TiO2 content in CRE coincided with a high background concentration of Ti in the Sichuan Basin and the surrounding regions. The lower σREE and higher Sm, Nd, σNd(0) values are related to the wide distribution of basalt in the southwest Sichuan Basin. The elemental and isotopic geochemistry of CRE indicates that eolian materials in the CP predominantly come from the Sichuan Basin and the surrounding regions, which differs from loess deposits in the CLP.

Molecular characterization of organic matter in Canadian Arctic paleosols for paleoecological applications

The preservation of soil organic matter (SOM) in loess–paleosol sequences from the Klondike goldfields in the Canadian Arctic has recorded several Quaternary glacial–interglacial cycles. Both non-targeted and targeted molecular organic geochemical techniques are employed to provide an additional approach to confirm and expand on existing reconstructions of late Quaternary environments in this region. Solid-state nuclear magnetic resonance (NMR) techniques performed on 31 paleosol samples formed during marine isotope stage (MIS) 2 and MIS 4 showed similar SOM characteristics. However solution-state NMR of the base-soluble extracts detected a large amount of vascular plant inputs and preservation. Biomarker analysis revealed a large predominance of constituents from higher plants such as long chain n-alkyl lipids, hydroxyl n-alkyl lipids, sterols and terpenoids. Vegetation source proxies based on the distribution of n-alkanes were inconclusive, however when combined with additional targeted extractions of hydrolysable lipids and lignin-derived phenols, it was revealed that the major vegetative sources originated from woody and non-woody angiosperms. In addition, a novel thiazole–sulfur biomolecular structure in a paleosol from the Goldbottom Creek, 14C dated to 25.3kyr before present (BP), was observed by solution-state NMR. This type of molecule has not been previously detected in base-soluble extracts of SOM. Although the origin and preservation of this structure is unknown, a mechanism based on microbial biomolecular intermediates is proposed. The spectra also contained large resonances attributed to plant cuticle functional groups providing further evidence of plant-derived material preservation. Collectively, this comprehensive dataset agrees with paleoecological hypotheses derived from physical morphology and plant macrofossils suggesting that these molecular methods may be applied as an alternative technique for Arctic paleoclimate studies as well as identification of molecular constituents not previously identified in modern soils.

Tracing the origin of loess in Hungary with the help of heavy mineral composition data

Heavy mineral composition of the fine sand fraction in the Hungarian loess sediments provides important data about the closer sources of the loess in the Carpathian Basin. Quantitative detrital heavy mineral compositions of 216 loess, loess-like sediment, and paleosol samples were compared to each other, and to recent fluvial sands and different Cenozoic sands and sandstones of Hungary using statistical methods: cluster analysis, principal component analysis and discriminant analysis.Usually, garnet is the most frequent detrital heavy mineral in the studied loess sections, followed by chlorite, magnetite, epidote, and amphiboles. Old Loess and Young Loess have similar compositions, indicating uniform or similar sources. Based on the similarities, the garnet-rich loess sediments could originate from aeolian reworking of the floodplain sediments of the Danube and other rivers of Transdanubia, and the local Cenozoic sands of the uplifting Transdanubian Central Range and Transdanubian Hilly Region. The biotite- and tourmaline-rich loess in South Transdanubia, and the loess with high pyroxene content in the North Hungarian Range, had some material from nearby sources: weathered granitoids and volcanic rocks, respectively, or sediments of local rivers, which carried detritus of these rocks. The new results presented in this paper confirm numerous earlier ideas about the origin of loess in the Carpathian Basin, but also suggest newly identified sources of the loess particles at several locations.

Characterizing and quantifying iron oxides in Chinese loess/paleosols: Implications for pedogenesis

To accurately decode the paleoclimatic and paleoenvironmental significance of magnetic properties in Chinese loess/paleosols, neoformation of magnetic iron oxides via pedogenesis as well as the relationship between ferrimagnetic (maghemite and magnetite) and antiferromagnetic minerals (hematite and goethite) of both detrital and pedogenic origins should be well determined. To resolve this problem, magnetic iron oxides from a contiguous loess–paleosol sequence (lower part of the loess unit L1, paleosol unit S1, and the upper part of L2) at Luochuan, Center of the Chinese Loess Plateau, were well characterized and quantified by using an integrated approach including chemical dissolution, diffuse reflectance spectroscopy and rock magnetism methods. Results showed that these magnetic minerals have distinct coercivity and dissolution behavior, which is highly grain size-dependent. More specifically, the paleosol is enriched with nano-sized pedogenic hematites with a medium coercivity of ~130mT, which are also preferentially dissolved in a weak reductive environment. In contrast, the coarse-grained micro-sized detrital hematites with a medium coercivity of ~1T are dominant in loess. In addition, these coarse-grained detrital hematites are more stoichiometric than their pedogenic counterparts as evidenced by the higher band position on the diffuse reflectance spectrum. Goethite can be well distinguished from hematite by its higher coercivity (4–7T) at low temperature (10K). The consistent dissolution behavior of goethite for both loess and paleosol samples in citrate–bicarbonate–dithionite solution indicates that goethite could be less relevant to the pedogenic processes. The reductive dissolution of pedogenic maghemite lags the dissolution of pedogenic hematite and fine-grained goethite, which suggests that average grain size of pedogenic maghemite is relatively larger than those of pedogenic hematite and fine-grained goethite. Detrital magnetite is hard to dissolve and ~50% of its initial remanence survived after dissolution. On the basis of these contrasting behaviors, pedogenic hematite, fine grained goethite and maghemite content was further quantified. Our results suggest that single domain pedogenic hematite was produced after initial formation of pedogenic maghemite. This provides direct evidence for the recently proposed pathway in which ferrihydrite is transformed into a transient maghemite-like phase before its final transformation into hematite.

14C Dating of Soil Organic Carbon (SOC) in Loess-Paleosol Using Sequential Pyrolysis and Accelerator Mass Spectrometry (AMS)

The chemical extraction of soil organic carbon (SOC) fractions from soils often does not produce satisfactory results for radiocarbon dating. In this study, a sequential pyrolysis technique was investigated. The soil was pyrolyzed at temperatures of 200, 400, 600, and 800 °C to partition organic carbon into pyrolytic volatile (Py-V) and pyrolytic residue (Py-R) fractions. The preliminary results show that the 14C dates of both fractions become progressively older as the pyrolysis temperature is increased. In addition, the ages of the Py-V fractions are consistently younger than the corresponding Py-R fractions extracted at the same temperature. Experimental results of known-age paleosol samples indicate that the Py-V fractions obtained between 600 and 800 °C yield the most reliable ages. This technique provides a new approach to improve the accuracy of 14C dating of loess-paleosol sequences.

New insights into magnetic enhancement mechanism in Chinese paleosols

Magnetic susceptibility of Chinese loess–paleosol sequences has been extensively used as a reliable paleoclimatic proxy for reconstruction of the Asian summer monsoon system, and its enhancement is usually attributed to neoformation of fine-grained ferrimagnetic particles via pedogenesis, but there is little understanding of either the formation process of such magnetic particles or their actual form and grain sizes. To investigate these problems, scanning electron microscope observations were conducted on magnetic extracts from pristine loess and strongly-pedogenic paleosol samples from Xifeng on the Chinese Loess Plateau. Both magnetic extracts commonly contained coarse-grained detrital magnetite with grain size ranging from a few μm to several tens of μm. However, the magnetic extract from the mature paleosol sample additionally included coarse silicate particles with ultrafine ferrimagnetic inclusions, while that from the pristine loess sample did not. The size of the ferrimagnetic inclusions mainly ranged from tens to hundreds of nanometers. The temperature-dependent susceptibility and hysteresis loops of the paleosol sample, combined with the stratigraphic variations of rock-magnetic parameters, showed that the pedogenic maghemite inclusions should be a potential contributor to magnetic enhancements in Chinese paleosols. The fine-grained ferrimagnetic inclusions have been pedogenically formed by weathering of coarse-grained Fe-bearing silicate minerals. Our proposed magnetic enhancement mechanism is consistent with the accepted rock magnetic result that magnetic enhancement in Chinese paleosols is caused by pedogenic maghemite grains in superparamagnetic and single-domain size regions. The presence of ultrafine ferrimagnetic grains as inclusions within coarse silicate particles is also consistent with Han and Jiang's (1999) significant experimental result that magnetic enhancement in Chinese paleosols is mainly contributed by the >0.5μm sediment fractions.

Spatial and glacial‐interglacial variations in provenance of the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) covers an extensive area over 440,000 km2and provides an unprecedented terrestrial record of Neogene climate. However, it is still unclear whether the provenance of these loess deposits is uniform or contains spatial and temporal differences. Here this is addressed by comparing detrital‐zircon age spectra of typical loess and paleosol samples from three distant sites located at the western, middle, and southeastern parts of the CLP. Our results reveal that the zircon age spectra not only change between loess and paleosol layers but also vary from the western to the eastern CLP, at least during the last glacial cycle. The discrepancies of the zircon age spectra among different sites suggest that the loess provenance of CLP is heterogeneous and spatially variable, although it has been suggested that the mineralogical, elemental and isotopic compositions of loess deposits on CLP are highly homogenous spatially and in glacial‐interglacial cycles.

Eocene vegetation and ecosystem fluctuations inferred from a high-resolution phytolith record

A thorough understanding of the ecosystem factors that can act as drivers of vegetation change, an important factor for projecting impacts of future climate change, requires adequately distinguishing between plant community changes that occur on the local scale or over ecological time scales, and long-term trends. Phytolith assemblages have the potential to provide this information, but little work has been done so far to test their ability to do so in deep time. We examined vegetation patterns inferred from phytoliths (plant silica microfossils) from a single section of the Renova Formation, Timberhills region, Montana, dated to 39.2 ± 3 Ma (late middle Eocene). The section is composed of fl uvial sediments overprinted by paleosols including Alfi sols, Entisols, Inceptisols, and composite paleosols. Phytolith assemblages from 27 paleosol samples were used to reconstruct a high-resolution vegetation history for the area and were compared with paleosol data. Phytoliths are predominantly from forest plants and include tropical elements such as palms (Arecaceae) and gingers (Zingiberales), suggesting a paratropical forest . The high-resolution sampling records heterogeneous vegetation including shifts among closed forest, moderately open forest (forest gaps, edges, and woodland), and grass-dominated habitats. Grasses are interpreted as early open-habitat plants tolerant of drier conditions, but they appear to be replaced by forest vegetation with time, resulting in poor correspondence between paleosol type and phytolith assemblage at some stratigraphic levels, where paleosol morphology indicates a forest ecosystem and the phytolith assemblages indicate a changing grass-forest mixture as a function of depth within the profi les. This is the fi rst study that has used phytoliths to construct a high-resolution record of vegetation from deep time, and we demonstrate that dense sampling and multiproxy approaches to paleo environmental reconstruction are necessary to capture vegetation heterogeneity in time and across microhabitats.

High-resolution isotopic record of C4 photosynthesis in a Miocene grassland

The origin and spread of grasslands is one of the key evolutionary events in the Cenozoic, which is characterized by long-term cooling and drying climatic conditions. One way to examine the relationship between vegetation and climate is to study the isotopic composition of organic carbon preserved in paleosols. Paleosols that preserve organic matter in the form of carbonaceous root traces provide direct evidence of the proportion of C3 to C4 biomass that grew in the soil, in contrast with pedogenic carbonate δ13C values, which may also reflect complicating factors including mixing from atmospheric δ13C in low productivity ecosystems. A high temporal and spatial resolution reconstruction of past ecosystems was derived from thirty-five paleosols in a thirty-four meter section of the Sixmile Creek Formation at Timber Hills, Montana (USA) that was deposited during the Miocene (10.2–8.9Ma ago). Phytoliths were extracted from paleosol samples to compare vegetation assemblages to inferences based on isotopic compositions, with both proxies giving similar results. Isotopic results from organic matter indicate both a small component of C4 photosynthetic plants locally prior to their regional expansion to dominance in the late Miocene through the early Pleistocene, and large variation in the abundance of C4 plants (0–25%) in this ecosystem both laterally and on a 100Kyr timescale. In contrast, pedogenic carbonate δ13C values from this site indicate a high proportion of C4 photosynthesis that is at odds with both phytolith and δ13Corg results, suggesting that the carbonate values are biased by diagenesis or diffusion of atmospheric CO2, and that a similar issue may impact previous paleovegetation reconstructions based on pedogenic carbonates. Quantitative reconstructions of mean annual temperature and mean annual precipitation indicate little local variability through time and that the fluctuations in C4 proportion were not climatically driven. Instead, the variable proportion of C4 photosynthesis is best explained by ecosystem-scale variables such as succession, fluvial avulsion, and fire regime.

Discovery and study of silver sulfate mineral in S5 from the eastern suburb of Xi’an

The paleosol samples from the fifth layer of the loess profile at Renjiapo in the eastern suburb of Xi’an are observed and analyzed using electron microscope and energy spectrum. Minerals such as AgSO4 and molybdenum, which are rare to find and can indicate typical dry climate environment, are found in this layer of paleosol. Secondary mineral is usually granular form of ellipsoidal and crystallization, and has the characteristics of chemical precipitating crystallization of apertures and fracture. Molybdenum minerals have the characteristics of colloidal substances. There are two kinds of secondary minerals. One is silver sulfate mineral and the other is silver oxide mineral. The movement of secondary silver, molybdenum and cobalt minerals, new clay mineral, Fe2O3 and Al2O3 indicates that S5 has experienced strong chemical weathering and mineral dissolution during its development. Silver, molybdenum, and cobalt can be released from primary minerals. During that period, the precipitation was abundant in Xi’an where soil reached an acidity stage of chemical weathering. At the later development stage of paleosol in the lowest part of S5, warm and wet monsoon climate had changed to dry and non-monsoon climate. In the period of the formation of AgSO4, which is easier to dissolve than CaSO4, a dry and non-monsoon climate was present in the Guanzhong Plain. Strong evaporation resulted in the accumulation of SO42− in the soil water solution and the formation of AgSO4. At that time, summer monsoon of East Asia was weak and did not cross Qinling Mountains to reach Guanzhong Plain. And at that time, the precipitation in Xi’an was less than 300 mm, and it was drier then in Xi’an than at present in Lanzhou.

Coleoptera and microbe biomass in Antarctic Dry Valley paleosols adjacent to the Inland Ice: Implications for Mars

Bulk paleosol samples collected from a Middle to Early Miocene moraine in the New Mountain area of the Dry Valleys, Antarctica, yielded Coleoptera exoskeletons and occasional endoskeletons showing considerable diagenetic effects along with several species of bacteria, all lodged in a dry-frozen but salt-rich horizon at shallow depth to the land surface. The till is at the older end of a chronologic sequence of glacial deposits, thought to have been deposited before the transition from wet-based to cold-based ice (∼15Ma), and hence, entirely weathered in contact with the subaerial atmosphere. It is possible, though not absolutely verifiable, that the skeletons date from this early stage of emplacement having undergone modifications whenever light snowmelt occurred or salt concentrations lowered the freezing temperature to maintain water as liquid. Correlation of the Coleoptera species with cultured bacteria in the sample and the likelihood of co-habitation with Beauveria bassiani found in two adjacent, although younger paleosols, leads to new questions about the antiquity of the Coleoptera and the source of N and glucose from chitinase derived from the insects. The skeletons in the 831 section may date close to the oldest preserved chitin (Oligocene) yet found on Earth. While harsh Martian conditions make it seemingly intolerable for complex, multicellular organisms such as insects to exist in the near-surface and subaerially, life within similar cold, dry paleosol microenvironments (Cryosols) of Antarctica point to life potential for the Red Planet, especially when considering the relatively diverse microbe (bacteria and fungi) population.

Changes in fire regimes on the Chinese Loess Plateau since the last glacial maximum and implications for linkages to paleoclimate and past human activity

A high-resolution black carbon (BC) record from 27.5 kyr BP to present was reconstructed using a chemical oxidation method on loess and paleosol samples from the Lijiayuan section of the Chinese Loess Plateau. The black carbon mass sedimentation rates (BCMSR) and carbon isotopic record reveal a paleofire history and its relationship with climate and vegetation changes at the study site. The BCMSR record was decomposed into two components: background BCMSR and the BCMSR peaks. The background BCMSR represents regional fires and shows high fire activities occurred contemporaneous with the Younger Dryas, Older Dryas, Heinrich events and Greenland stadials as registered in the loess grain size record. This suggests a rapid response of regional fires on the Loess Plateau to abrupt climate changes. Spectral analysis of background BCMSR showed two meaningful periodicities of 1620 and 1040 years, close to the cyclicity of the East Asian monsoon as recorded in the stalagmite δ 18O record in Central China. This indicates a tight control of millennial scale wet–dry changes in the monsoonal climate on regional fires on the Loess Plateau. By contrast, the BCMSR peaks are considered to reflect local fire episodes. The occurrences of local fires were more frequent during the last glacial period, with a maximum frequency of ~ 6 episodes/1000 years during the Last Glacial Maximum (LGM) (22.3 to 14.6 kyr BP), when the climate was drier and more continuous grassy fuels existed on the landscape. During the last glacial–interglacial transition (LGIT) period (14.6 to 11.0 kyr BP), fire frequency was largely reduced due to an increase in precipitation and more woody vegetation. If the LGIT period is taken as an analog for the projected near future, then future global warming alone may not produce large wildfires in northwestern China. Wildfires remained infrequent during the early-to-middle Holocene. Biomass burning increased after 4.0 kyr BP, when the climate became drier and land-use was more intensive. BC carbon isotope ratios may well reflect changes in the vegetation being burnt (i.e., grasses versus trees), yielding results consistent with the associated pollen data in the region.

Geochemical characteristics of the eolian deposits in southern China, and their implications for provenance and weathering intensity

The loess–paleosol sequences in the middle-lower reaches of the Yantgze River are valuable geological records of Quaternary environments that bear information about climate change for the subtropical zone of southern China. Type sections containing eolian deposits in this region mostly include the upper loess–soil sequence referred to as Xiashu Loess Formation, the middle Vermiculated Red Soil (VRS) and the underlying fluvial deposits or bedrock. In this study, Xiashu loess, paleosol formed in the Xiashu loess, and the VRS samples from Xuancheng (XC) and Jiujiang (JJ) sections were analyzed for their elemental (major and trace elements) and isotopic (Sm–Nd) geochemistry and compared with Pleistocene loess and paleosol samples from the Chinese Loess Plateau (CLP) in northern China. The results show an overall similarity in geochemical characteristics between the southern China loess and the average UCC, indicating that the dust materials were derived from well-mixed sedimentary protoliths which had undergone numerous upper crustal recycling processes. In comparison with the loess of northern China, the eolian deposits in southern China have higher TiO 2, Zr, Hf and lower Ba concentrations, higher TiO 2/Al 2O 3, U/Pb, Th/Pb and lower Ba/Rb, Zr/Hf, Eu/Eu ⁎, Ce/Yb, Eu/Yb ratios, and lower ε Nd(0) values with relatively greater range. These features coincide with the complicated source rock compositions in the middle-lower Yangtze River, which suggests local sources for the eolian deposits in this region. The eolian deposits in southern China have experienced the intermediate K removal weathering stage, Ca, Sr, Na, Mg and K are the main mobile elements in the weathering process. In comparison with the Xiashu Loess Formation, the VRS has greater depletion of Na, Mg, K and Mn, indicating a stronger weathering intensity or perhaps a longer period of weathering.