Sumiko Tsukamoto

Asian summer monsoon instability during the past 60,000 years : magnetic susceptibility and pedogenic evidence from the western Chinese Loess Plateau

Abstract The 28 m high-resolution Shajinping loess section in Lanzhou on the western Chinese Loess Plateau records a 60 ka, millennial summer monsoon variation. The record shows that Asian summer monsoons have rapid episodic pulse enhancements spanning only ca. 1–2 ka in high-frequency domain and having sub-Milankovitch cycles of progressive weakening in low frequency domain in the last glaciation. Soil formation seems to occur with a surprisingly fast response to these summer monsoon enhancements, resulting in weakly or moderately developed paleosol sequences. Both the pattern and timing of the summer monsoon enhancements show that they can be correlated to most major warm (Dansgaard–Oeschger) episodes and long-term cooling (Bond) cycles of the North Atlantic climatic records, indicating a possible teleconnection between tropic oceanic air masses and the North Atlantic climatic system. But differences exist for the transition of MIS 2/3 and the Holocene, where extraordinarily heavy dust-input events and fairly variable climatic fluctuations occur for the former and latter, respectively. A westerlies-swing model is proposed to interpret this link.

OSL of tephric loess and volcanic quartz in Japan and an alternative procedure for estimating De from a fast OSL component

Abstract Parameters of OSL components of quartz in volcanic ashes and in tephric loess from Japan were determined, and their characteristic in terms of recuperation was investigated. As a result, the medium and slow1 components, which were the main components in the volcanic quartz, were also found in the quartz in tephric loess, while the OSL of quartz in tephric loess was dominated by fast component. This suggests that the medium and slow1 components in tephric loess are probably coming from volcanic quartz. All the components in volcanic quartz appeared to give strong recuperation after preheating. In order to date tephric loess, the separation of fast component was essential, because only negligible recuperation was observed in the fast component. The equivalent dose ( D e ) of tephric loess decreased significantly with the illumination time, while recuperation increased with illumination time. A plot of D e versus recuperation indicated that D e decreased exponentially as recuperation increased. Since the fast component is free from recuperation, the extrapolation of the exponential curve to the point that recuperation equals zero gives D e , which entirely utilizes fast component. The agreement of the OSL ages with independent ages using the corrected D e values provides strong evidence for the validity of the correction method.

Timing of past glaciations in Kanchenjunga Himal, Nepal by optically stimulated luminescence dating of tills

Optically stimulated luminescence (OSL) was used to date glacial sediments to elucidate the ages of past glacial advances in Kanchenjunga Himal, eastern Nepal. Seven glacial sediments samples were collected from four different moraines, and quartz grains were extracted from samples. However, the luminescence from feldspars existing as minor contamination and/or microinclusion in the quartz grains was so bright it masked the quartz signal. After etching the fraction 20 min extra with HF, the natural infrared stimulated luminescence (IRSL) signal, which indicates the existence of feldspar, almost disappeared. Equivalent dose (DE) values were estimated by the single aliquot regenerative-dose protocol, using two different quartz fractions: fraction A, which was etched 20 min extra, and fraction B, which was prepared by the standard procedure still had a strong feldspar signal. Although the natural IRSL intensity for fraction A was so weak, the regenerated IRSL was relatively strong and not negligible for most of the aliquots. Such aliquots were omitted because the sensitivity of the feldspar is much higher than quartz and the difference in the sensitivity can cause DE underestimation. Out of 42 aliquots from four samples for fraction A, only six aliquots from two samples were remained. The DE values for single aliquots were uniformly distributed for fraction B. Moreover, the calculated OSL ages for fractions A and B from the two samples agreed with each other. It is suggested that both quartz and feldspars grains or microinclusions in quartz grains were likely to have been bleached, while glacial debris has been transported on the glacier. The resulting OSL ages suggest that the glaciers expanded at 5–6, 8–10, and 20–21 ka in the Kanchenjunga Himal. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.

Dry early Holocene revealed by sand dune accumulation chronology in Bayanbulak Basin (Xinjiang, NW China)

Understanding the Holocene climate changes (especially moisture variation) and their mechanisms in the semiarid–arid Central Asia (SCA) is very important, as water availability is crucial for sustainable developments in this area. This study presents chronostratigraphy of an aeolian sedimentary sequence from Bayanbulak Basin (Xinjiang, NW China), aiming to infer the Holocene moisture history in the SCA. Luminescence dating technique was selected to construct the age framework of the study section. Since the luminescence signal intensity of quartz from the samples was too low to be detected, feldspar was used as an alternative dosimeter. A newly developed post-infrared (IR) IR-stimulated luminescence (IRSL) (pIRIR) dating protocol on K-feldspar was applied to determine the ages of samples. A set of tests on luminescence characteristics was performed and demonstrate that this method is suitable and the resultant ages are reliable. Combined with the stratigraphic investigation on the sand dune/palaeosol section, the results imply a very dry climatic condition characterized by sand dune accumulation at ~9–8 kyr, and a wet interval of 5–1.6 kyr when a soil layer was formed. This moisture variation pattern is consistent with that inferred from many lacustrine records in the SCA, suggesting a widespread dry period at 11–8 kyr. The present results appear to challenge a new stalagmite δ18O sequence, located nearby to Bayanbulak Basin, which instead suggested a humid early Holocene.

On the applicability of post-IR IRSL dating to Japanese loess

Recent work on infrared stimulated luminescence (IRSL) dating has focussed on finding and testing signals which show less or negligible fading. IRSL signals measured at elevated temperature following IR stimulation at 50°C (post-IR IRSL) have been shown to be much more stable than the low temperature IRSL signal and seem to have considerable potential for dating. For Early Pleistocene samples of both European and Chinese loess natural post-IR IRSL signals lying in the saturation region of the laboratory dose response curve have been observed; this suggests that there is no significant fading in nature. As a contribution to the further testing of post-IR IRSL dating, we have used 18 samples from two Japanese loess profiles for which quartz OSL and tephra ages up to 600 ka provide age control. After a preheat of 320°C (60 s), the polymineral fine grains (4–11 μm) were bleached with IR at 50°C (200 s) and the IRSL was subsequently measured at 290°C for 200 s. In general, the fading uncorrected post-IR IRSL ages agree with both the quartz OSL and the tephra ages. We conclude that the post-IR IRSL signal from these samples does not fade significantly and allows precise and accurate age determinations on these sediments.

Chronology of Cauvery Delta Sediments from Shallow Subsurface Cores Using Elevated-Temperature Post-IR IRSL Dating of Feldspar

Chronology of Cauvery Delta Sediments from Shallow Subsurface Cores Using Elevated-Temperature Post-IR IRSL Dating of Feldspar We present the results of luminescence dating of sediments from two cores from the Cauvery Delta in south-east India. Since all natural quartz OSL signals except one sample were in saturation, the elevated temperature post-IR IRSL protocol for K-feldspar was applied to establish a chronology. Internal dose rates of K-feldspar grains were calculated from the measured internal content of potassium, uranium, thorium and rubidium in the bulk of K-feldspar grains using solution ICP-OES and ICP-MS analysis. A substantial scatter in single-aliquot De values was observed which is most probably due to the effect of incomplete bleaching of fluvial sediments before burial. A minimum age model was applied to extract possible depositional ages. The study revealed that except an upper layer of Holocene sediments (< 5m), the majority of the upper ~50m of Cauvery delta sediments were deposited between marine isotope stage MIS-5 and MIS-10 or older. The feldspar luminescence ages also indicate the existence of a period of non deposition or erosion in the upper part of the cores.

New investigations at Kalambo Falls, Zambia: Luminescence chronology, site formation, and archaeological significance

Fluvial deposits can provide excellent archives of early hominin activity but may be complex to interpret, especially without extensive geochronology. The Stone Age site of Kalambo Falls, northern Zambia, has yielded a rich artefact record from dominantly fluvial deposits, but its significance has been restricted by uncertainties over site formation processes and a limited chronology. Our new investigations in the centre of the Kalambo Basin have used luminescence to provide a chronology and have provided key insights into the geomorphological and sedimentological processes involved in site formation. Excavations reveal a complex assemblage of channel and floodplain deposits. Single grain quartz optically stimulated luminescence (OSL) measurements provide the most accurate age estimates for the youngest sediments, but in older deposits the OSL signal from some grains is saturated. A different luminescence signal from quartz, thermally transferred OSL (TT-OSL), can date these older deposits. OSL and TT-OSL results are combined to provide a chronology for the site. Ages indicate four phases of punctuated deposition by the dominantly laterally migrating and vertically aggrading Kalambo River (∼500-300 ka, ∼300-50 ka, ∼50-30 ka, ∼1.5-0.49 ka), followed by deep incision and renewed lateral migration at a lower topographic level. A conceptual model for site formation provides the basis for improved interpretation of the generation, preservation, and visibility of the Kalambo archaeological record. This model highlights the important role of intrinsic meander dynamics in site formation and does not necessarily require complex interpretations that invoke periodic blocking of the Kalambo River, as has previously been suggested. The oldest luminescence ages place the Mode 2/3 transition between ∼500 and 300 ka, consistent with other African and Asian sites where a similar transition can be found. The study approach adopted here can potentially be applied to other fluvial Stone Age sites throughout Africa and beyond.

Infrared radiofluorescence (IR-RF) dating of middle pleistocene fluvial archives of the Heidelberg Basin (Southwest Germany)

The infrared radiofluorescence (IR-RF) dating technique was applied to eight fluvial samples that were collected from two sediment cores at the Heidelberg Basin located near Viernheim and Ludwigshafen in southwest Germany. Based on the IR-RF derived ages of the samples it was possible to establish a chronological framework for the Mid-Pleistocene fluvial deposits of the Heidelberg Basin. The results allow us to distinguish between four main periods of aggradation. The lowermost sample taken from 100 m core depth lead to an IR-RF age of 643 ± 28 ka pointing to a Cromerian period of aggradation (OIS 17–16). For the Elsterian it is now possible to distinguish between two aggradation periods, one occurring during the Lower Elsterian period (OIS 15) and a second during the Upper Elsterian period (OIS 12–11). For the so called Upper interlayer (or “Oberer Zwischenhorizont” — a layer of organic-rich and finer-grained deposits), the IR-RF results point to a deposition age of around 300 ka, with samples taken directly on top and out of this layer yielding IR-RF ages of 288 ± 19 ka and 302 ± 19 ka, respectively. Hence, the measured IR-RF ages clearly point to a deposition during the Lower Saalian period (OIS 9–8) whereas earlier studies assumed a Cromerian age for the sediments of the Upper Interlayer based on pollen records and also mollusc fauna. The new IR-RF dataset indicates that significant hiatuses are present within the fluvial sediment successions. In particular the Eemian and Upper Saalian deposits are missing in this part of the northern Upper Rhine Graben, as the 300 ka deposits are directly overlain by Weichselian fluvial sediments. It is obvious that time periods of increased fluvial aggradation were interrupted by time periods of almost no aggradation or erosion which should have been mainly triggered by phases of increased and decreased subsidence of the Heidelberg Basin.

A comparison of GLSL, IRSL and TL dating methods using loess deposits from Japan and China☆

Abstract A comparison of ages of loess samples from Japan and China was made utilizing three methods, thermoluminescence (TL) using quartz fine grains, green-light stimulated luminescence (GLSL) using quartz fine grains, and infrared-stimulated luminescence (IRSL) using polymineral fine grains. The GLSL and IRSL ages of the three samples agreed within the error ranges, however, the TL ages are greater than those of GLSL and IRSL. The GLSL ages of the samples were concordant with independent age control given by tephrochronology but the TL ages were overestimated. The degree of overestimation varied with age and location of the samples, which might be connected with age and sediment source differences and the bleaching environment of loess in East Asia.

The Characteristics of OSL Signal from Quartz Grains Extracted from Modern Sediments in Japan

The Characteristics of OSL Signal from Quartz Grains Extracted from Modern Sediments in Japan Quartz grains from sediments in Japan are derived from complex mixtures of sources, including volcanic, plutonic, metamorphic and sedimentary rocks. We have measured the OSL signal of quartz grains from modern coastal sediments derived from different source rocks and compared these characteristics with the likely source. Each sample shows a different combination of various OSL components. It is concluded that the source rock affects the characteristics of the OSL components from quartz grains in Japanese sediments. By comparing the LM-OSL signals from volcanic sources with those from various source rocks, it can be deduced that quartz which has a higher fast component ratio is more suitable for dating. We also conclude that volcanic source areas should be avoided.