The anisotropy of magnetic susceptibility (AMS) was investigated in three Chinese Loess Plateau sedimentary sections along a W–E transect. Previously published models assumed that winter monsoons were responsible for the magnetic fabric formation of loess sequences. In our new interpretation, the stronger summer monsoons from the southeast played the major role in magnetic fabric orientation in the studied west and central parts of the Chinese Loess Plateau. The AMS was generated during the rainy summer monsoon when the sedimentary particles including magnetite were rearranged, settled, and fixed. We reconstruct the summer paleomonsoon routes for the last 130 kyr. These winds prevail from SE to NW but appear to be affected by regional topographic factors. Evidence in the world’s ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene-Oligocene boundary (EOB) at 33.9 Ma. I describe and date with magnetostratigraphy a section from the north Junggar Basin (Burqin section). Three fossil assemblages revealed through the EOB (34.8, 33.7, and 30.4 Ma) demonstrate that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and the changes in mammalian communities were accelerated by aridification in central Asia. Paleomagnetic studies of two sections of the northern Junggar Basin, China, are presented from Burqin and Tieersihabahe. Our paleomagnetic results demonstrate counterclockwise tectonic rotations in Burqin and Tieersihabahe (–17.2 ± 9.6° and –11.8 ± 6.1°, respectively) as well as considerable northward latitudinal displacement (12.2 ± 6.5° and 9.7 ± 4.1°, respectively) with respect to Europe. These results are consistent with the motions of contiguous blocks in the same geological time intervals (India, north and south China, Tarim, Amuria, and Kazakhstan). No significant intracontinental shortening or vertical-axis rotation is observed for the Junggar block from 40 Ma to 20 Ma. Our results reveal that the major compression and rotation between Junggar and northern Europe occurred after 20 Ma due to continuous penetration of India into Asia. We interpret the uplift of the Altay Mountains and the formation of the Lake Baikal rift system to be due to such intracontinental compression and relative rotations. / Geophysics

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Understanding the sources of the dust that forms the extensive loess deposits on the Chinese Loess Plateau is crucial for reconstructing atmospheric circulation patterns and dust pathways. Dust sources are however highly debated, mainly due to the low resolution of many records and the often ambiguous nature of geochemical proxies. The widely used concept of elemental ratios as provenance indicators is based on the idea that the relative abundance of immobile elements remains unaffected by chemical weathering, thereby preserving source signals. High-resolution spatial and temporal records of loess composition were determined for two study sites and three lithostratigraphic units using bulk X-ray fluorescence (XRF) analysis, and independently dated using Optically Stimulated (OSL) and post-IR Infrared Stimulated (pIR-IRSL) Luminescence dating. Four elemental ratios were chosen based on element properties of mobility and grain size, linear correlation strength and the existence of lithostratigraphic trends: Ti/Al, Si/Al, K/Al and Fe/Ti. Potential source signals were detected at both sites, at glacial-interglacial transitions as well as within lithostratigraphic units. Source changes detected within lithostratigraphic units show millennial-scale variability, indicating that abrupt shifts in dust provenance are possible. This implies that millennial-scale variability of the East Asian Monsoon may be related to changes in atmospheric dust content.

High resolution dating of loess on the Chinese Loess Plateau (CLP) using Optically Stimulated Luminescence (OSL) has increased the understanding of past monsoon climate but also raised questions regarding dust mass accumulation rate (MAR), the presence of disturbances or gaps in the sediment record, a possible teleconnection between North Atlantic and East Asian monsoon climate, and whether these events are due to climate variability, local settings, or age model uncertainties. This study undertakes a detailed (<20cm sampling resolution) -OSL investigation of the Lingtai section to create an independent age model using linear regression, to reconstruct monsoon climate changes using magnetic susceptibility(MS) and grain size (GS) proxies, as well as to calculate MAR for the site. The data shows that between 44-15 ka climate changes from a period of strong summer monsoon to a strong winter monsoon. GS data show variable trends attributed to changes in dust storm activity and local environmental conditions. MAR data does not correlate with grain size and is different from other loess records on the CLP. Such differences are either due to local variations or age model uncertainties. No clear correlation with Heinrich events or teleconnection with North Atlantic climate is visible in the records over the Last Glacial period, and hiatuses or gaps are not obviously present at this site. It is suggested that using linear regression for continuous age model construction from the luminescence ages comes with uncertainties due to subjective bias when fitting the lines and determining shifts in the data, especially during MAR calculations. Further studies are needed to ascertain optimal methods for creating age models, and to constrain the reasons behind the variability between different sites and loess records on the CLP. / Högupplöst OSL-datering av loessjordar från CLP har ökat förståelsen av dåtida monsunklimat, men har även lyft frågan gällande MAR, närvaron av avbrott eller småskaliga event i jordsektionerna, möjligheten av en relation mellan de Nordatlantiska och Ostasiatiska monsunklimaten, och om dessa förändringar sker till följd av faktisk klimatvariation, lokala förutsättningar, eller osäkerheter i åldersmodellen. Studien genomförde en detaljerad (<20cm) OSL-undersökning av Lingtai-sektionen för att skapa en oberoende åldersmodell, återge monsunklimatförändringarna genom tillämpning av MS och GS proxies, samt beräkna MAR från platsen. Resultaten påvisar att mellan 44-15 ka förändrades klimatet från en period av stark sommar- till en stark vinter-monsun. Kornstorleks-resultaten påvisar stora variationer vilka kan kopplas till en förändring i sandstormsaktivitet samt de lokala förutsättningarna för deposition av sediment. MAR-resultaten överensstämmer inte med kornstorleksdatan och skiljer sig från den övriga empirin, detta kan bero på lokala variationer i regionen eller osäkerheter i åldermodellen. Det finns ingen klar korrelation mellan "Heinrich events" eller en "teleconnection" i sektionen, och avbrott samt störningar i sektionen är inte förekommande. Avsaknaden av korrelation antyder att användningen av regressionslinjer för att skapa kontinuerliga åldermodeller kommer innebära fortsatta osäkerheter i empirin, speciellt gällande beräking av MAR. Ytterligare studier krävs för att bestämma optimala metoder för att framställa åldersmodeller, samt att utröna orsaken till skillnaden mellan resultat från olika platser på plat

Loess is a well sorted, fine grained, windblown (aeolian) sediment and deposits as a blanket over large areas, reaching from a few centimetres to several hundreds of meters in height and can drape large landscapes in form of mountains. During a more humid and warmer climate, redox conditions are changed in the loess deposits, with greater seasonal moisture creating greater soil formation (pedogenesis), i.e. palaeosol.  The Chinese Loess Plateau is dominated by the East Asian monsoon climate, and consists of an alternating Loess-palaeosol Formation, spanning back to at least the whole Quaternary period and late Pliocene (~2.8 Ma), and overlays a Red Clay Formation ranging into at least Miocene. This deposit covers therefore the onset of the latest Quaternary Ice Age. The Magnetic Susceptibility (MS) is used to investigate the magnetism of a rock which is inferred to be driven by the pedogenetic activity changes, i.e. during a stronger pedogenesis a higher amount of magnetic minerals forms. In turn the MS measurements becomes as a good indicator of the intensity and duration of the East Monsoon climate. In present study, a newly applied technique in palaeoenvironment is used to investigate the reasons for the out-of-phase response by the MS measurements. The study shows that this is due to viscous relaxation. Thus, the ultrafine superparamagnetic mineral magnetite is giving rise to the out-of-phase response and no other mechanisms. / Lössjord är ett välsorterad, finkornigt, vindblåst (eolisk) sediment som avsätts som ett täcke över stora områden, och kan variera från några centimeter till flera hundra meter i höjd och drapera stora landskap i form av berg. Under ett mer fuktigt och varmare klimat förändras de redoxa förhållandena i lössjordarna, då större säsongsfuktighet skapar en större paleosol-bildning, under en så kallad pedogenes. Den Kinesiska Lössjordsplatån domineras av det östasiatiska monsunklimatet och består av en alternerande Lössjord-paleosol Formation, som sträcker sig tillbaka till åtminstone hela kvartära perioden men även sena Pliocen (~ 2,8 Ma), och överlagrar en Rödlera Formation som sträcker sig minst tillbaka till Miocen. Dessa avlagringar täcker därför övergången till början på den kvartära istiden. Den magnetiska susceptibiliteten (MS) används för att undersöka de magnetiska egenskaperna hos avlagringarna som drivs av de pedogenetiska aktiviteterna, dvs under en starkare pedogenes bildas en högre mängd magnetiska mineraler. I sin tur blir därför MS-mätningarna en bra indikator på intensiteten och varaktigheten av östasiatiska monsunklimatet. I denna studie används en nyligen tillämpad teknik inom paleo-miljö för att undersöka orsakerna bakom ur-fas responsen som fås genom MS-mätningar på avlagringarna. Studien visar att detta beror på viskös relaxation. Således är det ultrafina superparamagnetiska mineralet magnetit som ger upphov till ur-fas responsen och inga andra mekanismer.