Magnetic studies of tephra in late Holocene peats and loessic soils from SE Iceland

Magnetic and geochemical properties of five late Holocene peat and loessial soil sections in SE Iceland have been studied. The aims were (i ) to establish a reliable tephrochronology for the sections, using geochemical analysis of tephra shards; (ii) to establish high resolution bulk sample records of mineral magnetic and geochemical properties; (iii) to test the ability of magnetic properties to correlate distal tephra layers; (iv) to assess the possible environmental significance of mineral magnetic time series and (v) to use records of loessial accumulation rates to assess human impact on the environment in SE Iceland. The work has been successful in establishing a reliable tephrostratigraphy for the studied sections. This was achieved using electron microprobe analyses (EMPA) of the geochemistry of tephra shards concentrated using a density separation technique. The results of detailed mineral magnetic measurements suggest that the main magnetic minerals in the tephras are probably ferrimagnetic minerals (e.g. magnetite) and canted anti ferromagnetic minerals (e.g. haematite), with abundant paramagnetic material also present. Cross plots of MrJMs vs. (Bo)c.l(Bo)c and Xfd% vs. XARM/SIRM indicate the main magnetic grain size in the tephras are pseudo-single domain (PSD) and multi domain (MD). The results indicate that the 'fingerprint' of both hysteresis and Curie temperature properties are insufficiently consistent within the tephra layers to provide an obviously sound basis for correlating tephra layers from one profile to the next. However, initial correlation of tephra layers was achieved, using all the measured magnetic parameters, by the use of the multivariate statistical measures of Similarity Coefficient (SC) and Euclidean Distance (ED). Tephra layer correlation was performed first time here using magnetic measurements. This demonstrates that magnetic techniques can potentially assist in the identification and correlation of distal tephra. Iceland offers a unique opportunity to study the role of humans in land degradation processes because of the late and well documented literature of settlements 1100 years ago. In this context, higher sedimentation rates in the upper part of two soils and one peat profile are interpreted as reflecting the effect of Landnam on deforestation and soil degradation

Magnetic studies of tephra in late Holocene peats and loessic soils from SE Iceland

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Seasonality of Response to Millennial‐Scale Climate Events of the Last Glacial: Evidence From Loess Records Over Mid‐Latitude Asia

Abstract Although difficult to resolve, seasonality is an important element in considering paleoenvironmental records and simulated results, and it is therefore critical to address this shortcoming in order to develop more accurate reconstructions of past climate. In this study, climatic proxies with seasonal implications (magnetic susceptibility and mean grain size) are analyzed for several high‐resolution loess sections from mid‐latitude Asia using the Ensemble Empirical Mode Decomposition (EEMD) to detect their response to millennial‐scale oscillations during the last glaciation. In so doing, we are capable to estimate the amplitude and relative contribution of the reconstructed climate components. Combined with an analysis of modern processes, magnetic susceptibility (χ) can be interpreted as representing summer and spring precipitation, while mean grain size (Mz) can be interpreted as representing spring and summer dust activity in the Chinese Loess Plateau (CLP) and southern central Asia (SCA), respectively. Our results show that the spring and summer signals are clearly correlated with Heinrich events, but that the spring signal is more prominent than the summer signal during Dansgaard‐Oeschger (D‐O) oscillations of the last glacial. These results are consistent with modeled simulations. It is proposed that weakening or complete shutdown of AMOC influences the response of seasonal signals to abrupt climate events. The study highlights the need for further high‐resolution climate proxies with robust seasonality indicators in order to develop a deeper understanding of the response of mid‐latitude Asia to rapid climate events

Possible link of an exceptionally strong East Asian summer monsoon to a La Niña-like condition during the interglacial MIS-13

Previous studies from the central and eastern Chinese Loess Plateau (CLP) showed that the S5-1 paleosol, which corresponding to Marine Isotope Stage (MIS) 13, was the strongest developed during the last million years, suggesting extremely strong East Asian summer monsoon (EASM). However, evidence shows that regional diversity exists in the relative intensity of the S5-1 soil formation. In this study, we first verify systematically the S5-1 soil development across the whole CLP by using numerous loess records. We then examine the spatial variation of the magnetic susceptibility of different loess-paleosol sections to identify the spatial change of the EASM intensity during MIS-13. We also compare the loess records with other monsoon records in China. Our results show that, in central CLP, S5-1 is indeed the strongest developed as suggested by previous studies, whereas in the western CLP, it is weakly developed and the S4 paleosol developed during MIS-11 is the strongest. As compared to MIS-11, the northern front of the EASM during MIS-13 didn’t penetrate into western CLP, but was located more northerly and eastward. Based on sea surface temperature records in the tropical Pacific, we suggest that the temporal and spatial abnormal variation of the EASM during MIS-13 could be related to a strong La Niña or La Niña-like climate condition. Under such condition, the Western Pacific Subtropical High (WPSH) becomes weaker and retreats more northeastward, leading to more precipitation in the mid-east region of northern China but less rainfall in the western inland

The cause of extremely high magnetic susceptibility of the S5S1 paleosol in the central Chinese Loess Plateau

The paleosol unit S5S1, which corresponds to Marine Isotope Stage (MIS) 13, is the most prominent paleosol layer in the central Chinese Loess Plateau (CLP). S5S1 has extremely high magnetic susceptibility, but it remains uncertain whether this is related to climate or to time (the duration of pedogenesis) due to the anomalously low loess deposition rate. To address this question, we investigate the iron mineralogical properties of the S5S1 paleosol at the Xifeng loess section located in the central CLP. We compare the results of S5S1 with other paleosol units, yielding additional insights into the CLP climatic changes during MIS-13. The results show that magnetic enhancement of S5S1 paleosol is stronger than that of the other paleosol units, which is mainly governed by pedogenic produce fine-grained ferrimagnetic minerals; the maximum value of the goethite concentration compared with hematite in the S5S1 paleosol unit is markedly higher than those in the other paleosol units. The prolonged period of pedogenesis is not main controlling factor that leads to the magnetic enhancement of S5S1 because the concentration of SD/SP has hardly changed compared with other paleosols as with the increasing of magnetic susceptibility. Changes in the ratio of pedogenic hematite to goethite, as well as in magnetic enhancement, indicate that the S5S1 paleosol developed under extremely humid conditions. This extremely humid climate is a main cause that leads to the extremely high magnetic susceptibility

Magnetic properties and correlation with heavy metals in urban street dust : a case study from the city of Lanzhou, China

We report results obtained from magnetic and geochemical measurements of 71 street dust samples from four distinct districts (residential, commercial and industrial) in Lanzhou, an industrial city in China. Magnetic properties and the concentrations of 17 elements (As, Ba, Bi, Cr, Cu, Mn, Ni, Pb, Ti, Zn, Fe, Si, Na, Mg, K, Ca, Al) are reported for each sample. Ferrimagnetic mineral concentrations are generally high in Lanzhou street dust, mainly due to grains above Stable Single Domain size. The highest concentrations of magnetic materials and heavy metals are in dusts from the two main industrial districts, Xigu and Chengguan. In the least polluted Anning district, some samples have magnetic properties more probably derived from arid areas to the north of the city. The two main industrial areas also show some differences in magnetic mineral assemblages suggesting that magnetic properties and magnetic-metal correlations may be of use in ascribing heavy metal contamination to distinct sources. Geochemical studies and the significant positive correlation between magnetic concentrations and those for Fe, As, Cu, Mn, Ni, Pb and Zn confirm that much of the heavy metal contamination in the study area is linked to combustion derived particulate emissions. The results confirm that a combined magnetic and geochemical approach can provide useful information on the types, levels and sources of heavy metals in street dust.10 page(s