Lianwen Liu

RAPID AND QUANTITATIVE MEASUREMENT OF HEMATITE AND GOETHITE IN THE CHINESE LOESS-PALEOSOL SEQUENCE BY DIFFUSE REFLECTANCE SPECTROSCOPY

The long, continuous deposition of dust in the Chinese loess plateau offers a unique opportunity to study the nature of Fe oxide formation in a wide range of climatic conditions. A technique to obtain quantitative estimates of the concentration of hematite and goethite in loess and paleosol samples is reported. Experiments using diffuse reflectance spectroscopy on sets of laboratory mixed and natural loess and paleosol samples show that it is possible to obtain rapid and quantitative estimates of the absolute concentration of hematite and goethite in the Chinese loess sediments. Typical loess and paleosol samples were deferrated using the CBD procedure to produce a natural matrix material to which hematite and goethite in known weight percentages were added to produce a set of calibration standards. Spectral violet, blue, green, yellow, orange, red and brightness of standards were calculated from the reflectance data and served as independent variables for a multiple linear regression analysis. The effect of changing matrix from loess to paleosol was overcome by including a variety of different loess and paleosol samples in the regression equations. The resulting calibration equations provide estimates of wt.% hematite and goethite and have correlation coefficients >0.93. The total measured hematite and goethite concentrations exhibited consistent variations with CBD extractable iron. Tests of the equations for buffering changes in matrix composition were run with samples of varying mineralogical composition (calcite, illite, etc.) and demonstrated that the equations are well buffered for changes in matrix composition from loess to paleosol.

Mid-Pliocene Asian monsoon intensification and the onset of Northern Hemisphere glaciation

The late Pliocene onset of major Northern Hemisphere glaciation (NHG) is one of the most important steps in the Cenozoic global cooling. Although most attempts have been focused on high-latitude climate feedbacks, no consensus has been reached in explaining the forcing mechanism of this dramatic climate change. Here we present a key low-latitude climate record, the high-resolution Asian monsoon precipitation variability for the past fi ve million years, reconstructed from South China Sea sediments. Our results, with supporting evidence from other records, indicate signifi cant mid-Pliocene Asian monsoon intensifi cation, preceding the initiation of NHG at ca. 2.7 Ma ago. This 1.4-million-year-long monsoon intensifi cation probably enhanced monsoon-induced Asian continental erosion and chemical weathering and in the process left fi ngerprints in marine calcium isotopes. Furthermore, increased rock weathering and/or organic carbon burial probably lowered the contemporary atmospheric CO 2 and may have triggered the NHG onset.

Characterization of the Chinese loess–paleosol stratigraphy by whiteness measurement

Abstract Changing color is one of the most striking features of loess–paleosol sequences in central China: the loess is light yellow and the interstratified paleosols are reddish-brown. Until a few years ago, conventional color description of loess sediments was made using a qualitative visual method, the Munsell system. To better understand the character of the color changes and to quantify results, whiteness and magnetic susceptibility were compared from four loess–paleosol sequences. Results show that the two parameters are highly correlated in the four sequences studied. Grain-size analysis and chemical extraction experiments indicate that the concentration and species of fine-grained iron oxides are major factors determining color changes in loess and paleosols. Whiteness and latitude values from the last glacial–interglacial sequence exhibit a linear relationship suggesting that whiteness will be a useful tool for reconstructing past atmospheric circulation, especially the East Asian summer monsoon. Whiteness and magnetic susceptibility in the Luochuan profile over the past 0.9 Myr display a strong correlation indicating that whiteness can be also used to reveal a long climatic history from Chinese loess–paleosol sequences.

Cellular dissolution at hypha- and spore-mineral interfaces revealing unrecognized mechanisms and scales of fungal weathering

Fungus-mineral interactions play unparalleled roles in shaping the planet Earth but are underappreciated relative to bacterial influences. Unique to fungus, but largely unknown, are the interfacial processes and extensiveness of hypha- versus spore-mineral interactions given the associated turgor pressure differences and the vast contact areas between mycelia and minerals in the critical zone. Here we examine lizardite [Mg3Si2O5(OH)4] dissolution by single cells of a native fungal strain using confocal laser scanning microscopy, atomic force microscopy, and transmission electron microscopy–energy dispersive X-ray spectroscopy to explore the mechanism, driving force, and magnitude of the interfacial reactions. Results from our inspection showed (1) significant pH reduction in the vicinity of cells upon mineral surface attachment, (2) exclusive Fe loss from the mineral at the cell-mineral interfaces, and (3) destruction of the mineral crystal structure below the area colonized by hyphae but not that by spores. Compared to the results from bulk experiments and at the mineral-water interface, these observations indicate that (1) only attached cells release siderophores and (2) biomechanical forces of hyphal growth are indispensable for fungal weathering and strong enough to breach the mineral lattice. Estimated mineral mass loss at the interface suggests that cellular dissolution can ultimately account for ∼40%–50% of the overall bio-weathering, significantly larger than the previous estimate of ∼1% contribution.

DETECTING CHLORITE IN THE CHINESE LOESS SEQUENCE BY DIFFUSE REFLECTANCE SPECTROSCOPY

Chlorite is one of the most common Fe-bearing minerals and is susceptible to weathering in loess and soils. The conventional method for analyzing chlorite, based on XRD with the Rietveld technique, is quantitative, but very time consuming and expensive. In this paper we develop a new methodology based on diffuse reflectance spectroscopy (DRS) and selective chemical extractions to identify chlorite qualitatively in the Chinese loess sequence and present evidence suggesting that DRS may be used to quantify chlorite content. The spectral signature of chlorite in loess is obscured by Fe oxides, but becomes obvious when they are removed. Changes in the ferrous absorption band near 1140 nm vary consistently with changing chlorite content. Using this spectral feature, DRS can distinguish chlorite contents as small as 1 wt.% in loess sediments. Future possibilities for this method in other soil and sediment types need to be explored.

Distribution and origin of protodolomite from the late Miocene-Pliocene Red Clay Formation, Chinese Loess Plateau

The Pliocene epoch is considered the most recent analog of modern warming because CO2 levels were similar to the present. To explore the carbonate minerals formed in the warmer Pliocene epoch, we studied two continuous sections of the Red Clay Formation on the Chinese Loess Plateau (CLP) by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Stable Isotope Mass Spectrometry. The Red Clay Formation on the CLP exhibits diagnostic FTIR absorption features of calcite and protodolomite. This allowed quantification of the two carbonate minerals by the FTIR method. Using the FTIR method we estimate the average concentration of protodolomite in Bajiazui is 3.6% whereas the Duanjiapo section is 6.0%. Protodolomite occurrence is more consistent and the concentration is higher from similar to 6.5-4.2 Ma B.P. and decreases markedly from 4.2-2.6 Ma B.P. Red Clay protodolomite is depleted in both delta C-13(PDB) and delta O-18(PDB), ranging from -4.1% to -10.4 and from -6.7% to -11.6, respectively, and has a slightly higher delta O-18 value than the calcites. SEM observations show that Red Clay protodolomite is composed of euhedral rhombic crystals that range from 1-20 mu m in diameter, grow into the soil voids and coexist with authigenic calcite and palygorskite. These observations imply that the protodolomite grew in situ and is authigenic from pedogenesis. Dolomitization in the Red Clay sequence appears to be the result of overcoming kinetic barriers. We propose that in the Red Clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to Mg2+ and significantly increasing the Mg/Ca ratio bringing about the formation of protodolomite.

Variation of Zr/Rb ratios in the Chinese loess deposits during the past 1.8 Myr and its implication for the change of East Asian monsoon intensity

Most of our knowledge concerning the evolution of the East Asian monsoon (EAM) comes from the Chinese loess-paleosol sequence. Variations in the EAM, as recorded in loess, are highly correlated to the marine δ18O record and show a strong 100 kyr cyclicity during the past 800 kyr. This conclusion is based on both weathering and pedogenesis intensity changes as well as grain size variation in loess-paleosol sequences which usually serves as a proxy for winter monsoon variation. However, grain size distributions are subject to modification by pedogenic processes. In this paper we used the Zr/Rb ratio from the Lingtai section on the Loess Plateau in central China to trace variations in the East Asian winter monsoon for the past 1.8 Myr. Long-term Zr/Rb variations in the Lingtai section exhibit a similar amplitude and frequency to those of mean grain size in bulk samples over the past 1.8 Myr. Comparing the Zr/Rb record and mean grain size record between L1 and L9 shows that a good linear relationship exists in the glacial age loess samples, whereas paleosol samples usually deviate from the linear relationship because of intense pedogenesis during the relatively warm and humid interglacial intervals. Our results indicate that the Zr/Rb ratio reflects the original eolian grain size and may serve as a proxy for the strength of the East Asian winter monsoon. In the Lingtai section over the past 1.8 Myr, the Zr/Rb ratio record displays a very strong 41 kyr obliquity periodicity from 1.8–1.2 Myr B.P., implying that variations of the winter monsoon during this interval are controlled by high-latitude climate. However, during the past 0.8 Myr the Zr/Rb ratio exhibits a stronger precession cycle influence, suggesting a possible low-latitude control on the East Asian monsoon regime during this interval.

Dolomite abundance in Chinese loess deposits: A new proxy of monsoon precipitation intensity

Dolomite of detrital origin in carbonate-rich soil can help indicate the intensity of carbonate dissolution and leaching processes. Using Fourier transform infrared spectroscopy, we generated a high-resolution record of dolomite and calcite contents from eight sections on the Chinese Loess Plateau spanning the last 130 kyr. Based on the relative abundance of calcite and dolomite, four dissolution phases of carbonate minerals related to summer monsoon (SM) variations are identified as follows: (1) coexistence of dolomite and calcite indicating a very weak SM with a mean annual precipitation (MAP)   725 mm.

A chemical index of weathering without effect of wind sorting: Fe/Mg ratios in the acid-insoluble phases of loess deposits

Chemical analysis of acid-insoluble fractions in loess and paleosols shows that concentrations of Fe and Mg were under control of wind sorting and post-depositional weathering-pedogenesis. The former caused Fe and Mg concentrated in the finer grain-size fractions, displaying synchronous variations, while the latter made Fe and Mg separated, leading to Fe retained in the weathered section and Mg leached out. Therefore, Fe/Mg ratios in the acid insoluble fraction of loess and paleosols can eliminate the effect of wind sorting and serve as an excellent proxy record on intensity of weathering-pedogenesis. Based on calculation, leaching percentage of Mg in the paleosol S1 from the Luochuan, Xifeng and Huanxian sections is 15%, 11% and 2%, respectively, and on average 9% for the paleosols S2–S14 from the Luochuan section, with the highest value amounting to 22% in S5-1, suggesting the strongest weathering-pedogenesis.

A seven-million-year hornblende mineral record from the central Chinese Loess Plateau

Previous studies of the late Cenozoic erosion rate have yielded different views—long-term stable rates or a significant increase at climate transitions—leading to uncertainty concerning the hypothesized global erosion rate controlled by either tectonic uplift or climatic changes. Here, we present a seven-million-year hornblende mineral record along the Lingtai section of the Chinese Loess Plateau. By examining the spatial distribution of hornblende minerals in seven desert basins, which are potential loess source areas, we constructed a ratio of hornblende versus total heavy minerals to reflect past changes in physical/chemical weathering strength. Our results demonstrate that the ratio has generally increased since 7 Ma, with three significant shifts recorded at 2.6 Ma, 1.4 Ma and 0.5 Ma linked to the onset, continuation and expansion of the Northern Hemisphere glaciation, respectively. Given that chemical weathering during the diagenetic history produces a trend of smoothly increasing hornblende migrating upwards, the three shifts at these boundaries can be interpreted as changes in the bedrock erosion rate on the northern Tibetan Plateau, which may be related to tectonic uplift events and incision of the Yellow River. Evidence presented here supports the idea of coupling between climate change, tectonic uplift and regional erosion.