Yuxin He

A 1700-year n-alkanes hydrogen isotope record of moisture changes in sediments from Lake Sugan in the Qaidam Basin, northeastern Tibetan Plateau

We present a hydrogen isotopic record of long-chain n-alkanes in Lake Sugan to reconstruct regional moisture changes in the last 1700 years at a sampling resolution of 25 years. The δD values decreased by over 30‰ from the ‘Medieval Warm Period’ (MWP; c. ad 600–1500) to the ‘Little Ice Age’ (LIA; c. ad 1500–1850), and indicated that the moisture pattern included a relatively humid climate before the MWP, becoming drier overall in the MWP, wetter in the LIA, and then tending towards drought in the post-industrial era. In the study region, Chenopodiceae shrubs were more abundant in dry climates than in wet. Meanwhile, δD values of shrubs were more positive than those of grasses. Therefore, we suggest that the vegetation type (shrub or grass), which depends on moisture changes, is the controlling factor for δD variations in Lake Sugan.

Biomarker-based reconstructions of Holocene lake-level changes at Lake Gahai on the northeastern Tibetan Plateau

Holocene hydrological changes in regions dominated by the westerlies significantly differ from those by the Asian summer monsoon. The high-elevated northeastern Tibetan Plateau, located in between, is likely influenced by the interactions of both circulation systems. Here, we attempt to use biomarkers, n-alkanes and alkenones, to reconstruct Holocene lake-level changes at Lake Gahai in the Qaidam Basin. We choose a sediment core drilled at the lake shore, where biomarkers would be sensitive to lake-level changes. The n-alkane records show high average chain length (ACL), high carbon preference index (CPI), and low proportion of aquatic macrophyte (Paq) values at 7–2 kyr (thousand calibrated years ago) with peaked values around 6 kyr, whereas low ACL, low CPI, and high Paq values occurred after 2 kyr and before 7 kyr. No alkenones were detected at 7–2 kyr, suggesting that lake level at this period was incapable of constantly reaching to the coring site. Therefore, combined results provide unambiguous evidence of relatively low lake level at 7–2 kyr, probably lowest at ~6 kyr. Holocene lake-level changes in this marginal region thus display a different pattern from either of the core regions dominated by the westerlies (anti-phase) and the Asian summer monsoon (out-of-phase). We suggest that in the arid marginal region, temperature-induced evaporation could significantly affect regional hydrological balance, resulting in the discrepancy with the Holocene long-term precipitation decreasing trend in Asian monsoon-dominated regions.

Changes in the radiocarbon reservoir age in Lake Xingyun, Southwestern China during the Holocene.

Chronology is a necessary component of paleoclimatology. Radiocarbon dating plays a central role in determining the ages of geological samples younger than ca. 50 ka BP. However, there are many limitations for its application, including radiocarbon reservoir effects, which may cause incorrect chronology in many lakes. Here we demonstrate temporal changes in the radiocarbon reservoir age of Lake Xingyun, Southwestern China, where radiocarbon ages based on bulk organic matter have been reported in previous studies. Our new radiocarbon ages, determined from terrestrial plant macrofossils suggest that the radiocarbon reservoir age changed from 960 to 2200 years during the last 8500 cal a BP years. These changes to the reservoir effect were associated with inputs from either pre-aged organic carbon or 14C-depleted hard water in Lake Xingyun caused by hydrological change in the lake system. The radiocarbon reservoir age may in return be a good indicator for the carbon source in lake ecosystems and depositional environment.

Solar Imprints on Asian Inland Moisture Fluctuations over the Last Millennium

Solar irradiance changes are thought to play an important role in natural climate variability. How the hydrological conditions were affected by solar irradiance in westerly-controlled arid central Asia (ACA) on decadal/centennial timescales remains poorly understood because of the lack of high-quality records. Here, we integrate 1.2-year-resolution x-ray fluorescence (XRF) scanner-derived carbonate accumulation estimates with 6-year-resolution biomarker and magnetic records in a well-preserved shoreline core from Lake Manas, northwestern China, to reconstruct lake level fluctuations and potential solar imprints over the last millennium. Besides the generally confirmed cool-wet/warm-dry climate pattern in ACA, our data also consistently show frequent and substantial lake level fluctuations, resembling solar activity changes, especially during the ‘Little Ice Age’. Wavelet spectral analyses of our XRF data indicate strong 8- to 16-year, 64- to 128-year and 128- to 256-year cycles, coinciding with the ~11-year Schwabe cycle, ~70- to 100-year Gleissberg cycle, and the ~200-year Suess-de Vries cycle. We therefore suggest the existence of solar imprints on effective moisture fluctuations in ACA over the last millennium, and the potential occurrence of the Schwabe cycle even during the solar minima.

Onset of frequent dust storms in northern China at ~AD 1100.

Dust storms in northern China strongly affect the living and health of people there and the dusts could travel a full circle of the globe in a short time. Historically, more frequent dust storms occurred during cool periods, particularly the Little Ice Age (LIA), generally attributed to the strengthened Siberian High. However, limited by chronological uncertainties in proxy records, this mechanism may not fully reveal the causes of dust storm frequency changes. Here we present a late Holocene dust record from the Qaidam Basin, where hydrological changes were previously reconstructed, and examine dust records from northern China, including the ones from historical documents. The records, being broadly consistent, indicate the onset of frequent dust storms at ~AD 1100. Further, peaked dust storm events occurred at episodes of high total solar irradiance or warm-dry conditions in source regions, superimposed on the high background of frequent dust storms within the cool LIA period. We thus suggest that besides strong wind activities, the centennial-scale dust storm events over the last 1000 years appear to be linked to the increased availability of dust source. With the anticipated global warming and deteriorating vegetation coverage, frequent occurrence of dust storms in northern China would be expected to persist.

Transient temperature asymmetry between hemispheres in the Palaeogene Atlantic Ocean

During the Late Palaeogene between ~40 and 23 million years ago (Ma), Earth transitioned from a warm non-glaciated climate state and developed large dynamic ice sheets on Antarctica. This transition is largely inferred from the deep-sea oxygen isotope record because records from independent temperature proxies are sparse. Here we present a 25-million-year-long alkenone-based record of surface temperature change from the North Atlantic Ocean. Our long temperature record documents peak warmth (~29 °C) during the middle Eocene, a slow overall decline to the Eocene/Oligocene transition (EOT, ~34 Ma) and high-amplitude variability (between ~28 and 24 °C) during the Oligo–Miocene. The overall structure of the record is similar to that of the deep-sea record, but a distinct anomaly is also evident. We find no evidence of surface cooling in the North Atlantic directly coinciding with the EOT when Antarctica first became cold enough to sustain large ice sheets and subantarctic waters cooled substantially. Surface ocean cooling during the EOT was therefore strongly asymmetric between hemispheres. This transient thermal decoupling of the North Atlantic Ocean from the southern high latitudes suggests that Antarctic glaciation triggered changes in ocean circulation-driven heat transport and influenced the far-field climate response.Northern and Southern hemisphere temperatures were decoupled during the Eocene/Oligocene transition, suggests a sea surface temperature record from the North Atlantic.