Hanchao Jiang

Decadal- to centennial-scale East Asian summer monsoon variability during the Medieval Climate Anomaly reconstructed from an eastern Tibet lacustrine sequence

Abstract Instrumental data suggest changes in the intensity of the East Asian Monsoon system over the past century, possibly in response to anthropogenic climate change. To understand modern observations and explore past variations in East Asian summer monsoon (EASM) strength, we conducted grain size, geochemical, and pollen assemblage studies on a lacustrine sediment sequence from an earthquake-dammed paleolake on the eastern Tibetan Plateau. The chronology, generated from eight optically stimulated luminescence and two pollen concentrate radiocarbon dates, indicates deposition of the lacustrine sequence between 600 and 1250 C.E. Fine grain sizes and low arboreal pollen percentages are associated with regional aridity (790–916, 1020–1080, 1125–1150 C.E.) and a weak EASM, whereas coarser grain sizes and higher arboreal pollen percentages are associated with increased precipitation and a stronger EASM (1090–1125, 1160–1230 C.E.). Although observed variations in our paleodata are predominantly driven by climate, the sequence is also influenced by regional tectonics, as evident from seismites, a ~90-year hiatus (917–1004 C.E.) during a period of regional seismicity, and an abrupt increase in regional sedimentation rates. Human disturbance is also observed to increase during weak EASM intervals. On decadal to millennial scales, our paleodata are highly correlated with reconstructions of EASM strength from northeastern China and sea surface temperature reconstructions from the tropical Pacific Ocean, indicating that the Medieval Climate Anomaly was associated with a strong EASM and prolonged La Niña-like state. Our data also suggest decadal-scale EASM variability associated with solar intensity, but an inconsistent response suggests additional complexity in EASM forcing. The inverse relationship between modern EASM weakening with anthropogenic warming, and a strong EASM during the warm Medieval Climate Anomaly, suggests that the complexity of the decadal to centennial-scale EASM response may be related to changes in the mean state of the tropical Pacific Ocean.

A Preliminary Study on the Soft-Sediment Deformation Structures in the Late Quaternary Lacustrine Sediments at Tashkorgan, Northeastern Pamir, China

This study identified soft–sediment deformation structures (SSDS) of seismic origin from lacustrine sediments in the late Quaternary paleo–dammed lake at Tashkorgan, northeastern Pamir. The observed deformation structures include sand dykes, liquefied diapir and convolute structures, gravity induced SSDS, and thixotropic pillar and tabular structures. We conducted a preliminary study on the morphology, formation and trigger mechanisms of pillar and tabular structures formed by liquefaction of underlying coarse sand and thixotropy of the upper silty clay. The regional tectonic setting and distribution of lacustrine strata indicate that the most probable trigger for the SSDS in lacustrine sediments was seismic activity, with an approximate earthquake magnitude of M>6.0; the potential seismogenic fault is the southern part of the Kongur normal fault extensional system. AMS C dating results indicate that the SSDS were formed by seismic events occurring between 26050±100 yrBP and 22710±80 yrBP, implying intense fault activity in this region during the late Pleistocene. This study provides new evidence for understanding tectonic activity and regional geodynamics in western China.

A continuous 13.3-ka record of seismogenic dust events in lacustrine sediments in the eastern Tibetan Plateau

Lacustrine sediments on the eastern Tibetan Plateau (TP) contain a wealth of information on local and regional tectonic activity. High-resolution grain-size and magnetic susceptibility measurements were conducted on the 23.4-m-thick Lixian lacustrine sedimentary sequence spanning from 19.3 to 6.0 ka, revealing 70 prehistoric seismic events on the eastern TP. The seismic events caused intermittent increases in source materials that endowed the samples of an individual event layer with a gradual fining trend along the C = M line on a C (one percentile)-M (median diameter) plot. Grain-size distribution and end-member modeling imply that dust particles of <20 μm in size were transported primarily by long-term suspension, while medium to coarse silt and sand were transported primarily by short-term suspension, such as aeolian transport constrained by local topography. Provenance analysis based on U-Pb zircon ages indicates that dust particles generated by earthquakes at Lixian had no effect on dust deposition at Xinmocun and Diaolin, and vice versa. These prehistoric seismic events, revealed by variations in grain size and magnetic susceptibility, thus provide invaluable information on the long-term behavior of local seismic activity.