Youbin Sun

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Glacial-Interglacial Indian Summer Monsoon Dynamics

Indian summer monsoon changes during the Pleistocene were influenced by dynamic effects originating in both hemispheres. The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.

Amplitude and timing of sea-surface temperature change in the northern South China Sea: Dynamic link to the East Asian monsoon

Magnesium/calcium (Mg/Ca) ratios of foraminiferal shells from a sediment core from the northern South China Sea, a semi-enclosed basin in the western tropical Pacific, document variations in sea-surface temperature (SST) during the past 145 k.y. Glacial SSTs were 4 °C colder than interglacial SSTs. During the last deglaciation, most of the warming was accomplished in a single abrupt step after continental ice-sheet decay had already begun, but warming and ice-sheet demise were nearly synchronous during the penultimate deglaciation. Abrupt SST changes of the past 15 k.y. were apparently synchronous with events in East Asian monsoon rainfall, suggesting that variations in monsoon winds and their influence on surface circulation of the western Pacific exerted a strong control on northern South China Sea SSTs. We suggest that this link persisted for the previous 130 k.y., during which time orbital-scale 2-3 °C SST changes and several small (≤2 °C) abrupt SST events occurred in the northern South China Sea. The similar timing of northern South China Sea SST, on a benthic δ 1 8 O time scale, to a well-dated speleothem record from eastern China suggests that the demise of ice sheets associated with the penultimate deglaciation did not precede Northern Hemisphere summer insolation increase. Our results suggest that surface waters had higher δ 1 8 O values during times of strong summer monsoon than during times of weak monsoon, likely reflecting a redistribution of 1 8 O-depleted rainfall from land during times of strong summer monsoons, to the western Pacific during times of weaker summer monsoons.

Tracing the provenance of fine-grained dust deposited on the central Chinese Loess Plateau

Eolian dust deposits in north China provide an excellent means of determining past variations in continental paleoclimate and atmospheric circulation. However, debate still exists on which deserts in east Asia are the dominant sources of Chinese loess and whether the dust provenance has shifted significantly at different time scales. Here we present new constraints on the provenance of fine-grained dust deposited on the central Chinese Loess Plateau (CLP) by combining electron spin resonance signal intensity and crystallinity index of fine-grained quartz contained in samples from two loess-paleosol sequences. Our results show that the fine-grained dust deposits on the CLP originate mainly from the Gobi desert in southern Mongolia and the sandy deserts in northern China (primarily the Badain Juran and Tengger deserts), rather than from the Taklimakan desert in western China, at least during the last climatic cycle. The dominant source of fine-grained dust varied significantly, from southern Mongolia during cold periods, to northern China during warm periods. The glacial-interglacial provenance fluctuations are strongly coupled with changes in the intensity of the near-surface northwesterly winter monsoon.

Southern Hemisphere forcing of Pliocene δ18O and the evolution of Indo-Asian monsoons

monsoon phase relationships consistent with direct orbital forcing across the entire Indo-Asian region, including marine and terrestrial proxies from the Chinese Loess Plateau, the South China Sea, and the Arabian Sea. Strong Pliocene summer and winter monsoons were in phase with one another, strengthened at obliquity minima and precession minima; the summer monsoon was also strengthened at precession maxima, yielding a semiprecession spectral signal. Strong Pliocene monsoons at orbital extremes indicate a direct response to fast physics processes including sensible heating and cooling of the Asian landmass and, for the summer monsoon, the export of latent heat from the southern Indian Ocean. As Northern Hemisphere ice volume grew into the Pleistocene, the timing of strong winter and summer monsoons drifted apart becoming influenced by the combined effects of fast physics and slow physics (ice volume) variables. The phase of strong winter monsoons shifted toward ice maxima, and the phase of strong summer monsoons shifted toward ice minima.

Last deglaciation in the Okinawa Trough: Subtropical northwest Pacific link to Northern Hemisphere and tropical climate

[1] Detailed deglacial and Holocene records of planktonic delta O-18 and Mg/Ca - based sea surface temperature (SST) from the Okinawa Trough suggest that at similar to 18 to 17 thousand years before present (kyr B. P.), late spring/ early summer SSTs were approximately 3 degrees C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.

East Asian monsoon variability over the last seven glacial cycles recorded by a loess sequence from the northwestern Chinese Loess Plateau

A 180-m-thick loess-paleosol sequence from the northwestern Chinese Loess Plateau was investigated to construct a high-resolution record of the East Asian monsoon variability over the last seven glacial-interglacial cycles. The low-field magnetic susceptibility (?, mass-specific) and the mean grain size are used as proxies for changes in the intensity of the East Asian summer and winter monsoon, respectively. Because of the weaker pedogenesis at the northwestern Chinese Loess Plateau compared to the central Chinese Loess Plateau, our ? and mean grain size records show a muted glacial-interglacial contrast for the Asian summer monsoon but an enhanced contrast for the Asian winter monsoon. Although better resolved, most orbital-scale East Asian monsoon variations captured by our ? and grain size records are similar to those reported from the central Chinese Loess Plateau. Nevertheless, variations in ? exhibit clear precessional cycles in three paleosol layers (i.e., S1, S2, and S3), corresponding with solar insolation maxima. Furthermore, unlike ? records at the central Chinese Loess Plateau where ? is dramatically enhanced at paleosol S5SS1 (corresponding to marine isotope stage 13), our new ? record shows a major enhancement at paleosol S4 (corresponding to marine isotope stage 11), which indicates geographic differences in the timing of local monsoon precipitation in the two regions.

Preferential dust sources: A geomorphological classification designed for use in global dust-cycle models

Received 11 April 2011; revised 28 September 2011; accepted 4 October 2011; published 24 December 2011. [1] We present a simple theoretical land-surface classification that can be used to determine the location and temporal behavior of preferential sources of terrestrial dust emissions. The classification also provides information about the likely nature of the sediments, their erodibility and the likelihood that they will generate emissions under given conditions. The scheme is based on the dual notions of geomorphic type and connectivity between geomorphic units. We demonstrate that the scheme can be used to map potential modern-day dust sources in the Chihuahuan Desert, the Lake Eyre Basin and the Taklamakan. Through comparison with observed dust emissions, we show that the scheme provides a reasonable prediction of areas of emission in the Chihuahuan Desert and in the Lake Eyre Basin. The classification is also applied to point source data from the Western Sahara to enable comparison of the relative importance of different land surfaces for dust emissions. We indicate how the scheme could be used to provide an improved characterization of preferential dust sources in global dust-cycle models.

Eolian evidence from the Chinese Loess Plateau: the onset of the Late Cenozoic Great Glaciation in the Northern Hemisphere and Qinghai-Xizang Plateau uplift forcing

On the basis of a newly-constructed record of magnetic susceptibility (SUS) and the depositional rate change of eolian loess-red clay sequences in the last 7.2 Ma BP from the hea Plateau, together with a cornperison of a record of °18O values from the equatorial East Pacific Ocean and eolian Quartz flux variations fmm the North Pacific Ocean, the evolutiomuy process of the Late Cenozoic Great Glaciation in the Northern Hemisphere can be divided into three stages: the arrival stage around 7.2–3.4 Ma BP, the initial stage at about 3.4—2.6 Ma BP, and the Great Ice Age since 2.6 Ma BP. The evolution of the East Asian monsoon is characterized by paid winter and summer monsoons, and it is basically composed of the initial stage of weak winter and summer monsoons, the transitional stage of simultaneous increase in intensity of winter and summer monsoons, and the prevailing stage of strong winter and week summer monsoons, or weak winter and strong summer monsoons. The Late Cenowic global tectonic uplift, paaicdarly the Qinghai-Xizang Plateau uplift and the associated CO2 concentration variation, controls the dng processes of the onset of Great Glaciation and the long-term changes of East Asian monsoom climate in the Northern Hemisphere to a large extent. The accelerating uplift of the Qinghai-Xizang Plateau between 3.4 and 2.6 Ma BP provided an important driving force to global climiatic change.

Grain size distribution of quartz isolated from Chinese loess/ paleosol

Grain size distribution of bulk loess-paleosol and quartz chemically extracted from the loess/paleosol shows that mean size of the bulk samples is always finer than that of the quartz. The original aeolian depositions have been modified to various degrees by post-depositional weathering and pedogenic processes. The grain size distribution of the isolated quartz should be close to that of the primary aeolian sediment because the chemical pretreatment excludes secondary produced minerals. Therefore, the grain size of the quartz may be considered to more clearly reflect the variations of winter monsoon intensity.