Thomas Stevens

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.

Loess plateau storage of northeastern Tibetan plateau-derived Yellow River sediment

Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the rivers upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO2 and erosion records.

Sedimentation and diagenesis of Chinese loess : Implications for the preservation of continuous, high-resolution climate records

Chinese loess has been extensively utilized to produce continuous and high-resolution climate records of the late Cenozoic. Such work assumes uninterrupted loess deposition and limited diagenesis. Here, closely spaced optically stimulated luminescence (OSL) dates are used to characterize the Holocene and Late Pleistocene sedimentation histories of three sites across a NW-SE transect of the Chinese Loess Plateau. The results suggest that sedimentation is episodic at subglacial-interglacial time scales, with rates rapidly varying within units and between sites. Unconformities, noneolian deposition, and mixing of sediments also appear to be common. Existing understanding of loess deposition therefore requires reexamination, while previous reconstructions of rapid climate change, not dated using absolute methods, should be regarded with caution. Loess deposits may still yield detailed climate records from specific high-sedimentation-rate strata, and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating. The rapid changes in sedimentation presented here indicate the East Asian Monsoon has the capacity to vary on millennial scales.

Fluvial landscapes of the Harappan civilization

The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.

Optical dating of abrupt shifts in the late Pleistocene East Asian monsoon

Chinese loess is regarded as one of the most detailed and complete terrestrial archives of late Cenozoic climate change. However, high-resolution optically stimulated luminescence (OSL) dates presented here reveal that the suborbital chronological framework of Chinese loess used in many previous climate reconstructions requires reassessment. Chronological uncertainty of as much as 10–15 k.y. for the late Pleistocene is largely a result of the widespread use of nonradiometric dating techniques that fail to account for site-specific depositional conditions associated with loess emplacement and diagenesis. OSL-based age models that account for these processes are used to examine detailed records of past sedimentation, as well as grain size and magnetic susceptibility proxies for late Pleistocene East Asian monsoon variation. Abrupt shifts in monsoon proxies occur over 10 2 –10 3 yr time scales, potentially forced by a variety of factors and influenced by site location and site-specific changes in sedimentation.

Assessing the provenance of loess and desert sediments in northern China using U-Pb dating and morphology of detrital zircons

Chinese loess is regarded as one of the most detailed and long-term archives of climate on land. However, there is still significant controversy over the deposit9s origin, limiting interpretation of the sedimentological and paleoclimatic mechanisms responsible for its emplacement. Here this is addressed through morphology and the first laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb ages of detrital zircons from loess (last glacial age; northern Loess Plateau). These are compared to zircon U-Pb age spectra from desert and sandy lands surrounding the Loess Plateau. Surface samples were taken from the Tengger and Mu Us deserts, as well as the Horqin and Otindag sandy lands. The results demonstrate that zircon U-Pb ages can discriminate between potential source areas and highlight both similarities and differences in age spectra for the desert and sandy land samples. Most significantly, the loess age spectrum shows no single affinity to any of these regions and exhibits zircon ages associated with granitoid rocks representing tectonic events in both west and east northern China. Furthermore, and in contrast to proximal deserts, a significant proportion of zircons from the loess show affinities with rocks cropping out in the Qilian Mountains. The euhedral form of many of these grains further suggests direct transport from these crystalline source rocks, in contrast to previously hypothesized production or storage in deserts. Thus, dust-transporting storms tracked from the west during the last glacial maximum, although this does not explain all the zircon variability and implies multiple sources and storm-track variation over the depositional period.

Loess in the Vojvodina region (Northern Serbia): An essential link between European and Asian Pleistocene environments

Loess in the Vojvodina region (Northern Serbia) : an essential link between European and Asian Pleistocene environments

Pacific freshening drives Pliocene cooling and Asian monsoon intensification

The monsoon is a fundamental component of Earths climate. The Pliocene warm period is characterized by long-term global cooling yet concurrent monsoon dynamics are poorly known. Here we present the first fully quantified and calibrated reconstructions of separate Pliocene air temperature and East Asian summer monsoon precipitation histories on the Chinese Loess Plateau through joint analysis of loess/red clay magnetic parameters with different sensitivities to air temperature and precipitation. East Asian summer monsoon precipitation shows an intensified trend, paradoxically at the same time that climate cooled. We propose a hitherto unrecognized feedback where persistently intensified East Asian summer monsoon during the late Pliocene, triggered by the gradual closure of the Panama Seaway, reinforced late Pliocene Pacific freshening, sea-ice development and ice volume increase, culminating in initiation of the extensive Northern Hemisphere glaciations of the Quaternary Ice Age. This feedback mechanism represents a fundamental reinterpretation of the origin of the Quaternary glaciations and the impact of the monsoon.

Shift of large-scale atmospheric systems over Europe during late MIS 3 and implications for Modern Human dispersal

Understanding the past dynamics of large-scale atmospheric systems is crucial for our knowledge of the palaeoclimate conditions in Europe. Southeastern Europe currently lies at the border between Atlantic, Mediterranean, and continental climate zones. Past changes in the relative influence of associated atmospheric systems must have been recorded in the region’s palaeoarchives. By comparing high-resolution grain-size, environmental magnetic and geochemical data from two loess-palaeosol sequences in the Lower Danube Basin with other Eurasian palaeorecords, we reconstructed past climatic patterns over Southeastern Europe and the related interaction of the prevailing large-scale circulation modes over Europe, especially during late Marine Isotope Stage 3 (40,000–27,000 years ago). We demonstrate that during this time interval, the intensification of the Siberian High had a crucial influence on European climate causing the more continental conditions over major parts of Europe, and a southwards shift of the Westerlies. Such a climatic and environmental change, combined with the Campanian Ignimbrite/Y-5 volcanic eruption, may have driven the Anatomically Modern Human dispersal towards Central and Western Europe, pointing to a corridor over the Eastern European Plain as an important pathway in their dispersal.

Relating the Astronomical Timescale to the Loess–Paleosol Sequences in Vojvodina, Northern Serbia

In this study the first astronomical time scale for loess-paleosol sequences of Vojvodina region, northern Serbia is presented astronomical timescale for the loess–paleosol sequences of the Vojvodina region, northern Serbia. The sequence is the longest and most detailed orbitally tuned European loess record, comparable to Asian sequences to the east. Magnetic susceptibility (MS) records from two continuous loess–paleosol sequences in Vojvodina have been used to construct the timescale, with the aim of investigating climatic and environmental evolution and variability over the last million years. The 47.3-m-thick Mosorin (MO) section covers the time interval between marine isotope stages (MIS) 1 and 15, while the lower part of the Stari Slankamen (SS) section covers the time frame prior to MIS 16. The MS records were tuned to June 65°N insolation over the period between 0 and approximately 1 million years. The new timescale suggests older than expected ages for a number of the magnetic polarity boundaries, consistent with lock-in depth offsets reported for other loess sequences. Spectral analyses of the stacked MS variations indicate that climatic dynamics are dominated mainly by the changes in orbital eccentricity and subdominantly by obliquity and precession bands, over the past 1 million years.