Chenglong Deng

Mineral magnetic properties of loess/paleosol couplets of the central loess plateau of China over the last 1.2 Myr

[1] We have conducted a multiparameter investigation of 15 loess-paleosol couplets (S0/L1 to S14/L15) from the Jiaodao section in the central loess plateau of China using environmental magnetic approaches coupled with soil science techniques. The magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis. High-temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the Lishi Formation to the Holocene black loam, possibly indicating a decrease in weathering intensity. This may reflect a long-term increase in aridity and/or a general long-term cooling trend of the interior of the Asian continent from 1.2 Ma to the present. Several samples display wasp-waisted hysteresis loops. These are most pronounced in moderately enhanced paleosols, less pronounced in the practically unaltered loess, and subdued in the well-developed paleosols, but wasp waistedness reappears in the most developed paleosols. This wasp-waistedness sequence suggests that the composition, concentration, and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess-paleosol sequence, but each factor has a different effect at different stages of pedogenesis.

Variability of the temperature-dependent susceptibility of the Holocene eolian deposits in the Chinese loess plateau: A pedogenesis indicator

Abstract We used a partial heating/cooling method (Van Velzen and Dekkers, 1999) for the analysis of representative samples of the Holocene loess of the Chinese loess plateau, in order to investigate magnetic mineralogical changes during thermal treatment. The temperature-dependence of susceptibility (TDS) results show significant alteration of magnetic phases during heating and cooling, and provide further evidence that magnetite and maghemite are the dominant ferrimagnetic minerals in the Holocene eolian deposits of the loess plateau. The TDS measurements suggest that the degree of the thermally-induced alteration is closely related to pedogenesis, which is a function of climate, and thus the alteration itself could be a useful climate indicator. Our TDS results along a NW-SE transect in the loess plateau suggest that stronger pedogenesis results in higher content of maghemite and greater susceptibility decrease during thermal treatment. This behavior seems to indicate that the final product of pedogenic magnetite in Chinese loess and paleosols is maghemite, which plays an important role in the enhancement of the magnetic susceptibility of Chinese eolian deposits.

Paleoclimatic significance of the temperature-dependent susceptibility of Holocene Loess along a NW-SE transect in the Chinese Loess Plateau

We have conducted mineral magnetic investigations of samples of modern dune sand, pristine loess and present-day loess along a NW-SE transect from the northern to the central loess plateau in China. The methods used include the temperature-dependence of susceptibility (TDS) before and after citrate-bicarbonate-dithionite (CBD) treatment as well as X-ray diffraction (XRD) analysis. The XRD analyses demonstrate that magnetite and hematite both exist in the Chinese loess-paleosol sequence and its modem source area, but the TDS measurements show that magnetite is the predominant contributor to magnetic susceptibility. Maghemite is present in the pristine loess and the present-day loess due to pedogenesis. The pedogenic processes that produce the maghemite are closely linked to paleoclimate, and for this transect, precipitation appears to be the most important climatic variable. We suggest that TDS can be used as a useful method for the analysis of pedogenesis and climatic change.

Magnetism of intermediate hydromaghemite in the transformation of 2‐line ferrihydrite into hematite and its paleoenvironmental implications

[1] A maghemite-like phase referred to here as ‘‘hydromaghemite’’ was obtained as an intermediate product in the hydrothermal transformation of phosphated 2-line ferrihydrite into hematite. In this study, we used magnetic and non-magnetic (e.g., X-ray Absorption Near Edge Structure, XANES) techniques in combination to characterize a series of the intermediate products obtained in the aging of phosphated ferrihydrite (P/Fe atomic ratio = 0.03) at 150C for 120 days. Particle size calculated from both the specific surface area and average unblocking temperature increased with time. XANES spectra revealed the presence of some tetrahedrally coordinated iron, which is consistent with the formation of hydromaghemite (the dominant magnetic phase in the intermediate products). Thus grain size in newly formed hydromaghemite particles increased with time from the initial values in the superparamagnetic region to others in the single-domain region. Further transformation of hydromaghemite into hematite, which was complete by day 120, was probably due to hydromaghemite becoming unstable relative to hematite when the surface to volume ratio fell below a given threshold. The relationships between pedogenically produced maghemite and hematite contents in various soils and paleosols suggest that the ferrihydrite ! hydromaghemite ! hematite transformation may constitute a major pathway accounting for the magnetic enhancement in many soils.

Reduced Efficiency of Magnetotaxis in Magnetotactic Coccoid Bacteria in Higher than Geomagnetic Fields

Magnetotactic bacteria are microorganisms that orient and migrate along magnetic field lines. The classical model of polar magnetotaxis predicts that the field-parallel migration velocity of magnetotactic bacteria increases monotonically with the strength of an applied magnetic field. We here test this model experimentally on magnetotactic coccoid bacteria that swim along helical trajectories. It turns out that the contribution of the field-parallel migration velocity decreases with increasing field strength from 0.1 to 1.5 mT. This unexpected observation can be explained and reproduced in a mathematical model under the assumption that the magnetosome chain is inclined with respect to the flagellar propulsion axis. The magnetic disadvantage, however, becomes apparent only in stronger than geomagnetic fields, which suggests that magnetotaxis is optimized under geomagnetic field conditions. It is therefore not beneficial for these bacteria to increase their intracellular magnetic dipole moment beyond the value needed to overcome Brownian motion in geomagnetic field conditions.

Paleomagnetic and mineral magnetic investigation of the Baicaoyuan loess‐paleosol sequence of the western Chinese Loess Plateau over the last glacial‐interglacial cycle and its geological implications

Closely spaced paleomagnetic and mineral magnetic studies were conducted on the Baicaoyuan (BCY) loess-paleosol sequence since the last interglacial in the western Chinese Loess Plateau. The multiproxy records of mineral magnetic parameters clearly indicate loess-paleosol alternations with the fine structure of pedostratigraphy over the last glacial-interglacial cycle. However, the paleomagnetic results reveal the absence of any short-lived geomagnetic excursions in the BCY section. This absence could be interpreted by two alternative mechanisms, such as magnetic overprinting and discontinuous sedimentation. The absence of the Mono Lake and Laschamp excursions in the loess unit L1 is probably due to the viscous overprinting of the remanence carried by coarse-grained magnetite of eolian origin and/or to the episodic nature of loess accumulation during the last glacial period. However, discontinuous sediment accumulation may more probably lead to the absence of the Blake excursion in the last interglacial soil S1 as well as of the Mono Lake and Laschamp excursions in the last glacial loess L1 at the BCY section. Together with previously published paleomagnetic results from other sections over the Loess Plateau, the findings probably further indicate the discontinuous nature of loess accumulation in the western Loess Plateau, where sedimentation of the last interglacial soil may be episodic at the timescale equivalent to the duration of the Blake geomagnetic excursion (4–6 ka), much longer than the timescale for episodic loess sedimentation in the last glacial loess of both the western and eastern Loess Plateau due to the reduced sediment accumulation rate compared to that during the last glacial period. Thus, sediments with relatively high sedimentation rate in some parts of the western Loess Plateau do not consistently guarantee the faithful recording of high-frequency geomagnetic variations.

The site of Shuitangba (Yunnan, China) preserves a unique, terminal Miocene fauna

NINA G. JABLONSKI,* DENISE F. SU, LAWRENCE J. FLYNN, XUEPING JI, CHENGLONGDENG, JAY KELLEY, YUGUANG ZHANG, JIYUN YIN, YOUSHAN YOU, and XIN YANG; Department of Anthropology, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A., ngj2@psu.edu; Department of Paleobotany and Paleoecology, Cleveland Museum of Natural History, 1 Wade Oval Drive, University Circle, Cleveland, Ohio 44106, U.S.A., dsu@cmnh.org; Peabody Museum of Archaeology and Ethnology, Harvard University, 11 Divinity Avenue, Cambridge, Massachusetts 02138, U.S.A., ljflynn@fas.harvard.edu; Yunnan Institute of Cultural Relics and Archaeology, 15-1, Chunmingli, Chunyuan Xiaoqu, Kunming, Yunnan 650118, China, jxping@public.km.yn.cn; Key Laboratory for Paleobiology, Yunnan University, No. 2 Cuihu Beilu, Kunming, Yunnan 650091, China; State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Science, 19, Beitucheng Xilu Street, Beijing 100029, China, cldeng@mail.iggcas.ac.cn; Institute of Human Origins and School of Human Evolution and Social Change, Arizona State University, PO Box 874101, Tempe, Arizona 85287-4101, U.S.A., jkelley.iho@asu.edu; Beijing Museum of Natural History, Beijing 100050, China; Yunnan State Land Resources Vocational College, Kunming, Yunnan 650217, China; Zhaotong Institute of Cultural Relics, No. 47, Qingnianlu, Zhaotong, Yunnan 657000, China; Zhaoyang Museum, No. 141, Xueshenglu, Zhaotong, Yunnan 657000, China

Insolation driven biomagnetic response to the Holocene Warm Period in semi-arid East Asia

The Holocene Warm Period (HWP) provides valuable insights into the climate system and biotic responses to environmental variability and thus serves as an excellent analogue for future global climate changes. Here we document, for the first time, that warm and wet HWP conditions were highly favourable for magnetofossil proliferation in the semi-arid Asian interior. The pronounced increase of magnetofossil concentrations at ~9.8 ka and decrease at ~5.9 ka in Dali Lake coincided respectively with the onset and termination of the HWP, and are respectively linked to increased nutrient supply due to postglacial warming and poor nutrition due to drying at ~6 ka in the Asian interior. The two-stage transition at ~7.7 ka correlates well with increased organic carbon in middle HWP and suggests that improved climate conditions, leading to high quality nutrient influx, fostered magnetofossil proliferation. Our findings represent an excellent lake record in which magnetofossil abundance is, through nutrient availability, controlled by insolation driven climate changes.

Biomineralization and magnetism of bacterial magnetosomes

Magnetosomes of magnetotactic bacteria are of great interest in understanding biomineralization and possible links between organisms and geomagnetic field. Fossil magnetosomes are ubiquitous in marine and lake sediments and may significantly contribute to magnetic signals. In this review, we firstly introduce some characteristics of magnetotactic bacteria, followed by considering recent progress in magnetosome formation, magnetic measurements, and identification of bacterial magnetites in bulk sediments as well as their paleoenvironmental implications. Finally, we briefly discuss potential future breakthroughs in magnetosome studies and its applications.

Archaeointensity results spanning the past 6 kiloyears from eastern China and implications for extreme behaviors of the geomagnetic field

Significance The geomagnetic field is an intriguing fundamental physical property of the Earth. Its evolution has significant implications for issues such as geodynamics, evolution of the life on the Earth, and archaeomagnetic dating. Here, we present 21 archaeointensity data points from China and establish the first archaeointensity reference curve for eastern Asia. Our results record rarely captured extreme behaviors of the geomagnetic field, with an exceptionally low intensity around ∼2200 BCE (hitherto the lowest value observed for the Holocene) and a “spike” intensity value dated at ∼1300 ± 300 BCE (either a precursor to or the same event as the Levantine spikes). These anomalous features of the geomagnetic field revealed by our data will shed light on understanding geomagnetic field during the Holocene. Variations of the Earth’s geomagnetic field during the Holocene are important for understanding centennial to millennial-scale processes of the Earth’s deep interior and have enormous potential implications for chronological correlations (e.g., comparisons between different sedimentary recording sequences, archaeomagnetic dating). Here, we present 21 robust archaeointensity data points from eastern China spanning the past ∼6 kyr. These results add significantly to the published data both regionally and globally. Taking together, we establish an archaeointensity reference curve for Eastern Asia, which can be used for archaeomagnetic dating in this region. Virtual axial dipole moments (VADMs) of the data range from a Holocene-wide low of ∼27 to “spike” values of ∼166 ZAm2 (Z: 1021). The results, in conjunction with our recently published data, confirm the existence of a decrease in paleointensity (DIP) in China around ∼2200 BCE. These low intensities are the lowest ever found for the Holocene and have not been reported outside of China. We also report a spike intensity of 165.8 ± 6.0 ZAm2 at ∼1300 BCE (±300 y), which is either a prelude to or the same event (within age uncertainties) as spikes first reported in the Levant.