ALBERT

Description: On the Tibetan Plateau, applications of dD and d18O values in paleoclimate studies tend to be complicated due to multiple processes influencing isotopic compositions in paleoclimatic archives. In this study, isotopic compositions of modern waters in the eastern Qaidam Basin on the northeastern Tibetan Plateau, and dD values of n-fatty acids (n-FA dD) from a sediment core at Hurleg Lake were systematically analyzed to infer hydroclimate controls during the Holocene. The modern water isotopic results show a major contribution of snowmelt water originating from high-elevation mountains to the north of the Qaidam Basin via river and groundwater discharge, and the importance of evaporation in affecting lake water budget in this region. n-C26 FA dD values tend to be more negative at millennial-scale warm-dry periods during the Holocene, and vice versa, opposite to what is commonly expected. Assisted with modern water isotopic results, we infer amplified contribution of snowmelt water to the soil water around this open lake system at warm-dry periods. Meanwhile, changes in n-C16 FA dD values at Hurleg Lake reflect the evolution of isotopic compositions of lake water, thus we use the isotopic difference between n-C26 and n-C16 FA (dDC16-C26) to infer hydroclimate and evaporation variations in this region. Based on our data, relatively low n-C26 FA dD and n-C16 FA dD values at 10-6 cal ka BP indicate large contribution of snowmelt water into the lake during the Holocene Climate Optimum. After 6 cal ka BP, changes in evaporation became the major control on lake hydrology and led to larger fluctuations of dDC16-C26. Our study highlights the importance of systematic analysis on modern processes before using stable isotopes for paleoclimate reconstructions, and demonstrates that dD difference between long-chain and short-chain n-FA might be an effective way to better understand the controlling factor of hydrological variations in a climatic complex region like the Tibetan Plateau.

Description: Independent inference of the Asian winter monsoon (AWM), albeit achieved at several sparse sites, has reached no consensus for its variability through the Holocene. A sediment core from the northern coast of the South China Sea (SCS) was utilized to analyze organic biomarkers at (bi‐)decadal resolution, unveiling how SCS oceanic conditions fingerprint the AWM signal. Generally, alkenone sea surface temperature (SST) record, resembling the temporal structures of integrated tropical SST over the past 7500 years, shows abnormal cool temperatures (up to ~4°C) during the Little Ice Age (LIA) and between ~1200 and 2500 yr BP, when windborne terrigenous hopane compounds experienced considerable increases superimposed on a general increasing trend. Our results, together with augmented SST gradient between the SCS coast and open ocean, consistently suggest AWM strengthening toward the late Holocene. An intensified AWM during cold intervals like the LIA would have provided strong positive feedback to enhance coastal cooling.

Description: Orbital resolution thermal histories over the last 4 Ma at ODP Site 1143 in the tropical western Pacific are reconstructed using alkenone paleothermometry. The temperature profile is characterized by a steady state of ~29 °C with fluctuations 〈 1 °C before 2.7 Ma and by a strong oscillating state from 2.7 Ma, largely due to cooling by up to 4 °C from ~29 °C in interglacial to 26 °C in glacial intervals. This implies a relative warm and stable surface hydrography during the early and mid Pliocene in this region influenced by the warm pool before temperature decreases in responding to global cooling and the formation of the distinct glacial stages since the late Pliocene. Therefore, the smaller SST gradient between tropical eastern and western Pacific and between southern and northern South China Sea before the late Pliocene indicates a super sized Pliocene Pacific warm pool, while the larger SST gradient since then marks progressively intensification of the zonal Walker circulation and meridional Hadley circulation, representing the monsoon circulations in the region. The intensification of the Walker and Hadley circulations over the tropical Pacific may also have helped on the onset of glaciations and subsequent deglaciations during the late Pliocene and Pleistocene.