Showing papers by "Xianfeng Wang published in 2020"

On the glacial-interglacial variability of the Asian monsoon in speleothem δ18O records.

Abstract: While Asian monsoon (AM) changes have been clearly captured in Chinese speleothem oxygen isotope (δ18O) records, the lack of glacial-interglacial variability in the records remains puzzling. Here, we report speleothem δ18O records from three locations along the trajectory of the Indian summer monsoon (ISM), a major branch of the AM, and characterize AM rainfall over the past 180,000 years. We have found that the records close to the monsoon moisture source show large glacial-interglacial variability, which then decreases landward. These changes likely reflect a stronger oxygen isotope fractionation associated with progressive rainout of AM moisture during glacial periods, possibly due to a larger temperature gradient and suppressed plant transpiration. We term this effect, which counteracts the forcing of glacial boundary conditions, the moisture transport pathway effect.

Metal Concentrations and Source Apportionment of PM2.5 in Chiang Rai and Bangkok, Thailand during a Biomass Burning Season

Abstract: One of the persistent environmental problems in the provinces of northern Thailand is severe air pollution during the dry season because of open vegetation burning by farmers for land clearance pur

Inconsistency between records of δ18O and trace element ratios from stalagmites: Evidence for increasing mid–late Holocene moisture in arid central Asia:

Abstract: The interpretation of trace element/calcium ratios of speleothems as indicators of local hydroclimatic variability in the vicinity of caves has led to controversy in reconstructing the evolution of...

Climate-forced sea-level lowstands in the Indian Ocean during the last two millennia

Abstract: Sea-level reconstructions over the past two millennia provide a pre-industrial context to assess whether the magnitude and rate of modern sea-level change is unprecedented. Sea-level records from the Indian Ocean over the past 2,000 years are sparse, while records from the Atlantic and Pacific Oceans show variations less than 0.25 m and no significant negative excursions. Here, we present evidence of two low sea-level phases in the Maldives, Indian Ocean, based on fossil coral microatolls. Microatoll growth is constrained by low water levels and, consequently, they are robust recorders of past sea level. U–Th dating of the Maldivian corals identified lowstands at ad 234–605 and ad 1481–1807 when sea level fell to maximum depths of −0.88 m and −0.89 m respectively. These lowstands are synchronous with reductions in radiative forcing and sea surface temperature associated with the Late Antiquity Little Ice Age and the Little Ice Age. Our results provide high-fidelity observations of lower sea levels during these cool periods and show rates of change of up to 4.24 mm yr−1. Our data also confirm the acceleration of relative sea-level rise over the past two centuries and suggest that the current magnitude and rate of sea-level rise is not unprecedented. Two intervals of distinctly lower Indian Ocean sea level during the last two millennia occurred during times of relatively low incoming solar radiation, according to an analysis of U–Th dated coral microatolls in the Maldives.

Fracture dolomite as an archive of continental palaeo-environmental conditions

Abstract: The origin of Quaternary dolomites in continental environments (e.g. karst and lakes) is barely constrained compared to marine dolomites in sedimentary records. Here we present a study of dolomite and aragonite formations infilling young fractures of the ‘Erzberg’ iron ore deposit, Austria, under continental-meteoric and low temperature conditions. Two dolomite generations formed shortly after the Last Glacial Maximum (~20 kyr BP): dolomite spheroids and matrix dolomite. Clumped isotope measurements and U/Th disequilibrium ages reveal formation temperatures of 0–3 °C (±6 °C) and 3–20 °C (±5 °C) for the both dolomite types, and depositional ages around 19.21 ± 0.10 kyr BP and 13.97 ± 0.08 kyr BP or younger, respectively. Meteoric solution and carbonate isotope compositions (δ18O, δ13C and 87Sr/86Sr) indicate the dolomites formed via aragonite and high-Mg calcite precursors from CO2-degassed, Mg-rich solutions. Our study introduces low temperature dolomite formations and their application as a sedimentary-chemical archive. Palaeo-environmental conditions can be recorded in low-temperature continental archives, according to geochemical and isotopic analyses of dolomitic fracture infills at Erzberg, Austria

δ 13 C values in stalagmites from tropical South America for the last two millennia

Abstract: . Due to the many factors controlling δ13C values in stalagmites, complicating their paleoclimatic and paleoenvironmental interpretation, most studies do not present δ13C values, but instead focus mainly on δ18O values. This is also the case for most cave studies from tropical South America, where many new δ18O stalagmite records covering the last millennia were recently published. Here, we test the influence of local hydroclimate, altitude, temperature and changing vegetation types on δ13C values in stalagmites, by employing a new dataset (named δ13C_2k_SA, https://doi.pangaea.de/10.1594/PANGAEA.919050 ) composed of published and unpublished carbon isotope records from various sites in tropical South America. Most locations were dominated by C3 plants over the last two millennia and are characterized by speleothem δ13C values more depleted than −6 ‰. The main factors influencing δ13C values are associated with the local hydroclimate, followed by minor effects from temperature. Most of the isotopic records show a significant correlation between the δ13C and δ18O values, indicating a close relationship between local hydroclimate and atmospheric convective processes related to the South American Monsoon System.

A decadal-resolution stalagmite record of strong Asian summer monsoon from northwestern Vietnam over the Dansgaard–Oeschger events 2–4

Abstract: Previous paleoclimatic studies by Asian cave records have shown that variability in the Asian summer monsoons is well correlated with local summer insolation, North Atlantic climate, and the Greenland stadial-interstadial cycles on millennial to orbital time-scales. However, it remains unclear whether a similar agreement exists on short timescales. With 230Th dates in precision as good as ±62 years, we present a decadal-resolved δ18O record covering 30.0–22.7 thousand years ago (ka) from a stalagmite of Son La Province, northwestern Vietnam as a proxy record of the Asian summer monsoon (ASM). On millennial timescales, our data show strong coherence with Chinese and North Indian speleothem δ18O records, suggesting that the ASM has synchronously responded over a broad region to Dansgaard–Oeschger (D-O) events in the North Atlantic. We observe that the onset timing and structure of D-O 4 varied spatially. The event commenced earlier and less abrupt at sites located with a connection to the modern Intertropical convergence Zone (ITCZ) than sites off the influence from ITCZ. It implies that the tropics may play a critical role in the global climate system. In addition, our decadal-resolution record clearly demonstrated a few centennial events with a duration of ~200 years, which probably correlates with the Suess cycle of solar activity.