Time filter

Source Type

Yang X.,Sun Yat Sen University | Liu Q.,CAS Institute of Geology and Geophysics | Liu Q.,Function Laboratory for Marine Geology | Yu K.,CAS South China Sea Institute of Oceanology | And 5 more authors.
Physics of the Earth and Planetary Interiors | Year: 2016

High-resolution paleomagnetic secular variation (PSV) records bear great information of dynamics processes of the Earth's geomagnetic field, and can be further used for inter-profile correlation and for dating sediments. However, effects of changes in the depositional environment on PSV records have not been fully determined. This study constructed Holocene PSV records for the gravity piston core (ZSQD34) obtained from the northern South China Sea. Rock magnetic and Transmission Electron Microscope (TEM) results indicate that single (SD) and PSD domain magnetites are the main carrier of the natural remanent magnetization. Comparable to the records derived from the freshwater lakes and the modeling results, we observed that direction curves from these two environments of contrasting salinity content are rather consistent. The direction curves are independent of the constructed salinity. However, the gradual increasing trend of relative intensity since about 5 kyr might be related to the decreasing sea surface salinity. Furthermore, on the centennial and millennial time scale, the relative intensity and salinity show some positive relation, suggesting a potential contribution of salinity to the paleomagnetic relative intensity recording processes. © 2016 Elsevier B.V. Source

Wang X.,Huazhong Agricultural University | Wang X.,University of Wyoming | Zhu M.,University of Wyoming | Koopal L.K.,Wageningen University | And 8 more authors.
Environmental Science: Nano | Year: 2016

The structure and magnetic properties of nano-sized (1.6 to 4.4 nm) ferrihydrite samples are systematically investigated through a combination of X-ray diffraction (XRD), X-ray pair distribution function (PDF), X-ray absorption spectroscopy (XAS) and magnetic analyses. The XRD, PDF and Fe K-edge XAS data of the ferrihydrite samples are all fitted well with the Michel ferrihydrite model, indicating similar local-, medium- and long-range ordered structures. PDF and XAS fitting results indicate that, with increasing crystallite size, the average coordination numbers of Fe-Fe and the unit cell parameter c increase, while Fe2 and Fe3 vacancies and the unit cell parameter a decrease. Mössbauer results indicate that the surface layer is relatively disordered, which might have been caused by the random distribution of Fe vacancies. These results support Hiemstra's surface-depletion model in terms of the location of disorder and the variations of Fe2 and Fe3 occupancies with size. Magnetic data indicate that the ferrihydrite samples show antiferromagnetism superimposed with a ferromagnetic-like moment at lower temperatures (100 K and 10 K), but ferrihydrite is paramagnetic at room temperature. In addition, both the magnetization and coercivity decrease with increasing ferrihydrite crystallite size due to strong surface effects in fine-grained ferrihydrites. Smaller ferrihydrite samples show less magnetic hyperfine splitting and a lower unblocking temperature (TB) than larger samples. The dependence of magnetic properties on grain size for nano-sized ferrihydrite provides a practical way to determine the crystallite size of ferrihydrite quantitatively in natural environments or artificial systems. © The Royal Society of Chemistry 2016. Source

Oliva-Urcia B.,CSIC - Pyrenean Institute of Ecology | Oliva-Urcia B.,Autonomous University of Madrid | Munoz A.,University of Zaragoza | Larasoana J.C.,Instituto Geologico Y Minero Of Espana Igme | And 8 more authors.
Geologica Acta | Year: 2016

Environmental magnetic proxies were analyzed in a relatively monotonous, ~25.3m thick alluvial sedimentary sequence drilled in the Añavieja Basin (NE Spain). Results from the core AÑ2 suggest that the concentration dependent magnetic parameters mainly reflect variations in the content of detrital magnetite, sourced in the catchment rocks and soils of the basin, via changes in the dynamics of alluvial fans due to temperature changes in the northern hemisphere during the Late Pleistocene. The correspondence between the magnetic proxies and the temperature variations in the North Atlantic region (NGRIP curve) indicates that higher (lower) concentrations and finer (coarser) magnetite grains coincide with warm (cold) periods. We propose that during cold periods, a sparser vegetation cover favored the incoming of higher energy runoff bearing coarser sediments to the basin that are relatively impoverished in magnetite. In contrast, during warm periods, the wider distribution of the vegetation cover associated with the lower runoff energy lead to finer, magnetite-richer sediment input to the basin. Maghemite, presumably of pedogenic origin, appears to be present also in the studied alluvial sediments. Further studies are necessary to unravel its palaeoclimatic significance. © 2016, Universitat de Barcelona. All rights reserved. Source

Xia P.,State Oceanic Administration | Meng X.,State Oceanic Administration | Meng X.,Function Laboratory for Marine Geology | Li Z.,East China Normal University | And 3 more authors.
Organic Geochemistry | Year: 2015

Located at the interface of terrestrial and marine ecosystems, mangroves are particularly sensitive to environmental change. They provide a sedimentary sink for organic carbon, whereby cores can provide detailed records of mangrove species. We aimed to trace the history of mangrove development over the past 150years in Yingluo Bay, SW China. Sedimentation rates (avg. 0.32 and 0.37cm/year) were calculated on the basis of ln(210Pbex) vs. mass depth, and offset the rate of relative sea level rise (0.22-0.24cm/year), leading to a seaward expansion of new mangrove habitats. Chemical tracers (δ13Corg and C:N) and an isotope mixing model were utilized to trace the contribution of mangrove-derived organic matter (MOM). Changes in the relative abundance of pollen from mangrove plants was used to compensate for diagenetic alteration of the stable isotope values and potential overlaps in isotope values for different sources of organic matter. The result of Pearson correlation analysis showed that the MOM was moderately positive correlated with total mangrove pollen, indicating that stable carbon isotopes and mangrove pollen provide similar information for tracing mangrove ecosystems. Based on results from this study, compositional changes in mangrove communities could be divided into two main stages: a degradation period (1870-1930 AD) and a flourishing period (1930-2011 AD), corresponding to colder temperature and warmer temperature, respectively. Owing to the location being far away from any industrial area and human activity, temperature may be a key factor for mangrove development. © 2015 Elsevier Ltd. Source

Yang S.,Key Laboratory of Coastal Wetland Biogeosciences | Yang S.,Function Laboratory for Marine Geology | Yang S.,CAS Qingdao Institute of Oceanology | Li J.,CAS Qingdao Institute of Oceanology | And 9 more authors.
Quaternary International | Year: 2015

Eighty-six seafloor surface sediment samples from the western Bohai Sea were analyzed palynologically to understand the distribution pattern and transport paths of pollen and spores. The results reveal that the pollen assemblages are dominated by arboreal pollen, which accounts for an average of 52% and mainly includes Pinus, deciduous Quercus, Carpinus, Betula, Nitraria, Castanea, and Ulmus. The pollen percentages of herbaceous taxa reach an average of 25%, mostly represented by Chenopodiaceae, Artemisia, Gramineae, Liliaceae, Polygonum, Typha, and Cyperaceae. The fern spores are mostly Selaginella, Osmunda, Triletes, and Monoletes. The distribution characteristic of pollen and spores implies that the pollen assemblages correspond well with the watershed vegetation. Variation in the pollen assemblages in different parts of the marine area could reflect differences in local vegetation, especially the vegetation along the inflowing rivers. The spatial distribution of pollen assemblages further suggests that the discharge from the Yellow River and Luanhe River has a great contribution to the pollen inputs into the western Bohai Sea. Pollen concentrations are lower in the nearshore sea area (water depth<20. m) due to the combined effects of dynamic sedimentary environment and inflowing rivers. By contrast, pollen concentrations show higher values in the deeper waters of fine sediments near estuaries of Yellow River and Luanhe River and the sea area north of 39.5°N, reaching a maximum concentration of 7000grains/g. Based on the distribution characteristics of pollen and spores, the PCA analysis results of the distribution of the dominant pollen taxa confirm that pollen grains and spores deposition into the western Bohai Sea was primarily affected by the hydrodynamic condition or water sorted effect (including river flows and ocean currents), and secondarily through aeolian transportation. Non-arboreal pollen were mainly carried by fluvial input and deposited in the nearshore shallow water area. Arboreal pollen dominated by Pinus were mainly transported by winds and ocean currents, and fern spores mainly by river flows and ocean currents. The latter two types were mostly concentrated in relatively deeper water area. © 2015 Elsevier Ltd and INQUA. Source

Discover hidden collaborations