Shuangwen Yi

Variation of East Asian monsoon precipitation during the past 21 k.y. and potential CO2 forcing

Paleoclimatic research can provide critical insight on causes of change in the East Asian monsoon, which influences the lives of 1.6 billion people today. In this study, we use paleoclimatic indexes from Chinese loess deposits, which have clear climatic implications and are independently dated, to reconstruct the monsoon precipitation since 21 ka. Our results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8–3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation. Our TraCE-21000 modeling experiment supports this interpretation.

Dune mobility and aridity at the desert margin of northern China at a time of peak monsoon strength

Wind-blown sands were mobile at many sites along the desert margin in northern China during the early Holocene (11.5–8 ka ago), based on extensive new numerical dating. This mobility implies low effective moisture at the desert margin, in contrast to growing evidence for greater than modern monsoon precipitation at the same time in central and southern China. Dry conditions in the early Holocene at the desert margin can be explained through a dynamic link between enhanced diabatic heating in the core region of the strengthened monsoon and increased subsidence in drylands to the north, combined with high evapotranspiration rates due to high summer temperatures. After 8 ka ago, as the monsoon weakened and lower temperatures reduced evapotranspiration, eolian sands were stabilized by vegetation. Aridity and dune mobility at the desert margin and a strengthened monsoon can both be explained as responses to high summer insolation in the early Holocene.

Holocene climatic changes revealed by aeolian deposits from the Qinghai Lake area (northeastern Qinghai-Tibetan Plateau) and possible forcing mechanisms

Previous palaeoclimatic studies in the northeastern Qinghai-Tibetan Plateau (NETP) during the Holocene, mainly using lake sediments, have deepened our understanding of the climatic system in this remote region. The timing and forcing mechanisms of climatic change in this region are still controversial, however. Aeolian sand and silt deposits, which are widely distributed in the NETP, can be readily dated by optically stimulated luminescence (OSL) techniques and reveal the sensitive response of the landscape to climatic change. In this study, aeolian sand and loess at six sites around Qinghai Lake were studied to reconstruct millennial-scale climatic changes during the Holocene. Multiproxy data along with 24 OSL age determinations show that low effective moisture and aeolian activity occurred at c. 13 ka, 10—9.1 ka, and 8.9—7.8 ka. Periods of greater effective moisture may have occurred at ~11 ka and 9 ka, and there is evidence for a previously undocumented wet climate at 4—3 ka. These results show that millennial-scale Holocene palaeoclimatic changes in the NETP cannot be explained simply as direct responses to changes in monsoon precipitation forced by summer insolation. We suggest that changes in effective moisture were determined by the balance between monsoon-induced rainfall and evaporation loss (mainly controlled by temperature). Thus, climatic change in the NETP may have been influenced by complex interactions between the monsoon circulation and local convection/evaporation effects, in addition to large-scale change in the Asian monsoon and the westerlies.

IRSL and post-IR IRSL residual doses recorded in modern dust samples from the Chinese loess plateau

Using a set of modern/young (0 to about 200 years old) dust samples collected from the Chinese Loess Plateau the bleachability of IRSL measured at 50°C (IR50) and post-IR50 elevated temperature IRSL (measured at 225°C and at 290°C) is investigated by measuring the apparent (residual) doses recorded by these signals. Doses recorded by quartz OSL are used as a reference. Allowing for differences in dose rates it seems that both IRSL and post-IR IRSL signals yield residual doses that are significantly larger than the doses measured in quartz. These residual doses can be largely explained by thermal transfer caused by preheating. Nevertheless, we advise against the use of a low temperature preheat (<200°C) with IR50 to date loess samples because, as has been reported before, the signal appears to be thermally unstable. In general, we conclude that it may not be advisable to apply post-IR IRSL dating to Chinese loess samples where residuals of up to ∼20 Gy are a significant fraction of the total dose. However, these residuals quickly become unimportant when dating older samples, and this is the age range in which post-IR IRSL dating is likely to be most useful.

Ages for hominin occupation in Lushi Basin, middle of South Luo River, central China.

A newly discovered Paleolithic site in loess deposits in the Lushi Basin, South Luo River, central China, is dated using pedostratigraphic analysis, optically stimulated luminescence (OSL) and magnetostratigraphic analysis. This region is regarded as an important place for hominin occupation and settlement during the early to middle Pleistocene. Results indicate that the archaeological site dates from 600ka to 620ka, reinforcing the view that Homo erectus had occupied a large area of eastern Asia by 620ka. The lithic assemblages of Lushi Basin is a flake and core technology, typical for this time period in north-central China. It may be compared with that at the Zhoukoudian locality 1 in north China and some sites in the Luonan Basin, and provides important data for understanding the behavior and stone tool technology of early Chinese hominins.

Aeolian process and climatic changes in loess records from the northeastern Tibetan Plateau: Response to global temperature forcing since 30 ka

The response of surface processes to global climatic changes since the last glacial is critical to understanding the mechanism of climatic changes on the Tibetan Plateau. In this study, loess from the northeastern Tibetan Plateau (NETP) was closely spaced dated to provide an independent high-resolution record of dust accumulation processes and millennial-scale climatic changes. Optically stimulated luminescence ages reveal episodes of rapid dust deposition at approximately 12.3 ka, 16 ka, 21–23 ka, 25–28 ka, and possibly also at 30–33 ka. These episodes are broadly correlated with high-latitude and marine records cold climatic events, such as the Younger Dryas, Heinrich event 1, and the Last Glacial Maximum. This correlation implies that dust storms in the NETP represent episodic aridification and wind strengthening, which is ascribed to the southward shift of the polar front and the intensification of the Siberian high-pressure cell that was forced by millennial-scale cooling in high northern latitudes.

Ice-volume-forced erosion of the Chinese Loess Plateau global Quaternary stratotype site

The International Commission on Stratigraphy (ICS) utilises benchmark chronostratigraphies to divide geologic time. The reliability of these records is fundamental to understand past global change. Here we use the most detailed luminescence dating age model yet published to show that the ICS chronology for the Quaternary terrestrial type section at Jingbian, desert marginal Chinese Loess Plateau, is inaccurate. There are large hiatuses and depositional changes expressed across a dynamic gully landform at the site, which demonstrates rapid environmental shifts at the East Asian desert margin. We propose a new independent age model and reconstruct monsoon climate and desert expansion/contraction for the last ~250 ka. Our record demonstrates the dominant influence of ice volume on desert expansion, dust dynamics and sediment preservation, and further shows that East Asian Summer Monsoon (EASM) variation closely matches that of ice volume, but lags insolation by ~5 ka. These observations show that the EASM at the monsoon margin does not respond directly to precessional forcing.A basic requirement for reconstructing past environmental change is accurate understanding of sediment age. Here, the authors show that the interpretation of a benchmark archive in China has been inaccurate, and that ice volume primarily controls desert dynamics, sediment preservation, and precipitation at the site.

A preliminary quantitative reconstruction of precipitation in southern Mu Us sandy land at margin of Asian monsoon-dominated region during late Quaternary

We present the first quantitative estimation of monsoon precipitation during the late glacial-Holocene in the sandy land of northern China, based on organic carbon isotopic composition data from a loess-sand sequence at margin of the Mu Us sandy land. We use the relationship between monsoon precipitation and the carbon isotopic composition of modern soils as an analogue, with a minor modification, to reconstruct precipitation back to c. 47 ka ago. The preliminary results indicate that annual monsoon precipitation was high after 8 ka, with an average of 435 mm; and it decreased during 18 and 8 ka with a mean value of 194 mm. The precipitation value of 47-18 ka varied between the two. We compare the reconstructed precipitation with other records and paleoclimatic modeling results, showing that our record agrees with reconstructions of the monsoon precipitation from other sources, even capturing short climatic events such as the Younger Dryas. We suggest that solar irradiance, high-latitude temperature/ice volume and local evaporation have together modified moistures in the sandy land.

A modified depositional hypothesis of the Hanjiang Loess in the southern Qinling Mountains, central China

Geomorphologically, alluvial deposits in river systems are expected to be older on higher terraces than on the lower terraces. However, loess deposits of aeolian origin may also occur on the surface of terrace systems and as seen in the Chinese Loess Plateau (CLP). Such loess is continuously, rather than episodically deposited. This study presents numerous dating results of loess cover on the Hanjiang River terrace system in the southern Qinling Mountains, an atypical loess deposit belt outside of the CLP. We name this “Hanjiang Loess”. Results indicate that the Hanjiang Loess deposited on the high, middle, and low terraces are also the oldest, old, and most recent, respectively, apparently analogous with alluvial expectations. We thus propose a modified depositional hypothesis, whereby terrace loess reflects the deposition of aeolian loess and other material, subsequent fluvial reworking. This depositional hypothesis should also be applicable to loess deposits on the river terraces in the Xiashu Loess, Quaternary Reticulate Red Clay, and Quaternary Red Clay in southern China, and other atypical loess areas outside of the CLP.

Tectonically-controlled infilling of the eastern Nihewan Basin, North China, since the middle Pleistocene

There has been a significant debate about the nature and causes of the Pleistocene evolution of the Nihewan Basin, North China. We studied the eastern Nihewan Basin sedimentary facies at two main sites, Hutouliang and Donggou. A combination of field observations and measurements of sediment grain-size distribution was used to reconstruct the sequence of sedimentary environments since the middle Pleistocene, and optically-stimulated luminescence measurements were used to date the sediments. Our results indicate that a shallow lake occupied the basin center along the Sanggan River, probably lasting until ~440 kyr ago before disappearing completely ~340 kyr ago. It was succeeded by a phase of fluvial-dominated sediment accumulation which ended ~30 kyr ago. We suggest that the formation of the gorge resulted from the relative uplift of the Niuxin Mountain along the Liulengshan fault ~140 kyr ago. However, since ~30 kyr ago the fault may have become inactive and the river downcutting near Shixia was no longer offset by the relative uplift, which caused a shift from deposition to denudation in the Nihewan Basin from then on. The disappearance of the paleolake ~340 kyr ago may have been the culmination of the ongoing process of basin infilling.