Search

Atmospheric CO 2 starvation, aridity, fire and warm season precipitation have all been proposed as major contributors to C 4 plant expansion during the Late Miocene. However, the driving factors responsible for the distribution of C 4 plants in the early and mid-Miocene still remain enigmatic. Here we report pedogenic carbon and oxygen isotope data (δ 13 C pedo , δ 18 O pedo ), along with magnetic susceptibility (MS) results, from the Zhuang Lang drilling core on the Chinese Loess Plateau (CLP). Elevated δ 13 C pedo values (>−5‰) signal a prominent C 4 expansion and substantially increased δ 18 O pedo and MS values indicate enhanced Asian summer monsoon (ASM) precipitation. Both of these conditions are observed during the Mid-Miocene Climatic Optimum (MMCO), 14.5–17 million years ago. The marked increase in C 4 plants, associated with warm temperatures and increased precipitation, strongly suggests the control of an enhanced ASM on C 4 expansion on the CLP during the MMCO. This finding contrasts with the late-Miocene C 4 expansion associated with cooling and drying conditions observed in low latitudes and argues for regionally specific control of C 4 plant distribution/expansion. [ABSTRACT FROM AUTHOR]

The ongoing global warming is altering hydrothermal patterns and eco-environments. However, it is unclear how they would respond under a natural warming scenario. Terrestrial mollusks in loess-paleosol sequences could serve as natural biological archives to explore this issue. In this study, terrestrial mollusk records from two loess-paleosol profiles on the southeastern Chinese Loess Plateau (CLP) were analyzed. Our results show that the cold-aridiphilous species decreased and the overall biodiversity increased gradually during the last deglaciation (∼19.0–11.5 ka BP), which was accompanied with the shift from cold-dry to warm-humid climate. Moreover, the mollusk communities reacted to Heinrich event 1 (H1) cooling. In the early-middle Holocene (∼11.5–5.5 ka BP), mollusk communities were dominated by the thermo-humidiphilous species, characterized by stable community composition, increased species richness and evenness with warm-humid climate. Thereafter, there was a noticeable decrease in species diversity and increase in the cold-aridiphilous species in the mid-late Holocene (∼5.5–0 ka BP), associated with a gradual shift towards a cold-dry climate. On the whole, it is noteworthy that the count of thermo-humidiphilous species increased remarkably at ∼16 ka BP, which is synchronous with a significant increase in the Northern Hemisphere summer insolation. Interestingly, this change of terrestrial mollusks preceded the formation of the Holocene paleosol at ∼8 ka as indicated by the pronounced increase in magnetic susceptibility and decrease of mean grain size. This suggests that the substantial East Asian summer monsoon (EASM) precipitation lagged the onset of deglacial warming on the southeastern CLP. Furthermore, this delay may be attributed to the negative feedback of North Atlantic meltwater, which caused a decline in the intensity of Atlantic Meridional Overturning Circulation (AMOC) and inhibited the rapid enhancement of EASM precipitation during the last deglaciation. Altogether, this provides crucial geological background evidence for exploring the potential regional hydrothermal and biodiversity responses to global warming in the near future. • Paleoclimate and eco-environment on the southeastern Chinese Loess Plateau since the last deglaciation were reconstructed. • Mollusk biodiversity responded to climate fluctuations by adjusting community stability, richness, and evenness. • The substantial East Asian summer monsoon precipitation lagged the onset of deglacial warming for ∼8 ka. [ABSTRACT FROM AUTHOR]

• A three-year, high-resolution, relatively continuous water vapor isotopic composition was measured in Xi'an. • High-resolution water vapor isotopic data provide the defining criteria for the onset and retreat of the East Asian Summer Monsoon. • A single water vapor source and a large temperature gradient are two prerequisites for temperature effect of precipitation isotopes. Atmospheric precipitation has widely taken part in the formation and accumulation of many geological archives. In these processes, the isotope fractionations are tightly related to the meteorological factors, such as temperature and precipitation amount. Therefore, precipitation isotopes are usually regarded as an effective proxy to reconstruct the paleoclimate and paleoenvironmental changes. However, with the deepening of research, it is found that the temperature effect is not suitable for all regions and time scales, especially in regions with complex moisture sources. Moreover, the applicable mechanism of the temperature effect is still unclear. Here, we chose Xi'an as our study site, because it is located in the transition zone of the East Asian Summer Monsoon (EASM) and the westerlies with relatively complex water vapor sources. Through conducting a three-year, high-resolution, relatively continuous water vapor isotopic composition measurements in Xi'an, we defined the duration of the EASM here, which normally starts in June and is over in September. By separating the δ18O p into monsoon and non-monsoon seasons, we found a significant temperature effect in the non-monsoon seasons, with a correlation coefficient of 0.54, while no temperature effect is observed in the monsoon seasons and the whole year. Our water vapor isotopic results suggested that the establishment of temperature effect of precipitation isotopes should follow two prerequisites: 1. single water vapor source; 2. large temperature gradient. Our results specify the applicable conditions of temperature effect and potentially help us to better use precipitation isotopes to understand the paleo-temperature variations in different regions. [ABSTRACT FROM AUTHOR]

Precipitation isotopic composition variations that are directly recorded in geological samples play an important role in understanding past hydroclimatic changes. In arid regions, the hydrogen isotopes of leaf wax (δD wax) in loess sections can provide records of summer precipitation isotopes. However, previous studies have primarily focused on the orbital scale, and there was still a lack of clear understanding of variations in precipitation isotopes on shorter timescales and their implications for climate change. In this study, we report centennial-resolution loess δD wax records from the central Chinese Loess Plateau (CLP) from the last deglaciation to the early Holocene. We compared the loess optically stimulated luminescence chronology and U-Th ages conveyed by linking stalagmite δ18O and loess δD wax to validate the δD wax method as a potential tool for assessing loess chronology on the suborbital scale. Our findings also indicated that regional precipitation isotopes experienced a depletion of approximately 24‰ for δD and around 3.2‰ for δ18O during the last deglaciation. The overall precipitation isotope amplitude in the central CLP was similar to that of typical East Asian monsoon regions and lower than that of the Indian monsoon and westerly regions. However, the precipitation isotope oscillations on the millennial scale in the central CLP were significantly weaker than those in eastern to central China. The finding affirmed that although the summer precipitation in the CLP was primarily influenced by the East Asian monsoon system at the orbital scale, the northern boundary of the summer rain belt remained a distance away from the interior of the CLP during the last deglaciation. As a result, the short-term oscillations of atmospheric circulation were not strongly reflected in the precipitation isotopes of the central CLP. This result highlighted spatial and temporal differences in precipitation isotope variability between the dominant and marginal regions of the East Asian monsoon system. • The Chinese Loess Plateau (CLP) centennial-resolution δDwax record was assessed. • Similar precipitation isotopes amplitudes in the central CLP and East Asian monsoon regions during the last deglaciation. • Weak millennium-scale oscillations were found in the precipitation isotopes of the central CLP. • The potential application of δDwax for short-scale loess chronology was validated. [ABSTRACT FROM AUTHOR]

Grain-size compositions of lake sediments can provide direct information on changes in transporting mechanism and sedimentary environment. Various grain-size parameters have been employed to reconstruct hydrological conditions within the lake and dust influx from outside the lake. Here we present the grain-size results of a 5-m core retrieved from Lake Qinghai, northeastern Tibetan Plateau in order to address the links between Holocene depositional process and climatic change. Our results indicate that two parameters (skewness and grain-size ratio of 8–50/2–8 μm) are sensitive to hydrodynamic changes in Lake Qinghai, which are further linked to the strength of the Asian summer monsoon. Variations of these two parameters reveal that summer monsoon intensity weakened gradually from early to late Holocene, superimposed by persistent centennial variability with dominant periods at 0.35, 0.23 and 0.12 kyr. Comparison of grain-size variations with solar activities and North Atlantic cooling events reveals that solar forcing likely plays an important role in driving centennial monsoon variability. [ABSTRACT FROM AUTHOR]