New sedimentary evidence reveals a unique history of C4 biomass in continental East Asia since the early Miocene

Pyrogenic carbon (PyC) and n-alkane data from sediments in the northern South China Sea reveal variations in material from C4 plants in East Asia over the last ~19 Ma. These data indicate the likely presence of C4 taxa during the earliest part of the record analysed, with C4 species also prominent during the mid and late Miocene and especially the mid Quaternary. Notably the two records diverge after the mid Quaternary, when PyC data indicate a reduced contribution of C4 taxa to biomass burning, whereas plant-derived n-alkanes indicate a greater abundance of C4 plants. This divergence likely re ects di erences in the predominant source areas of organic materials accumulating at the coring site, with PyC representing a larger source area that includes material transported in the atmosphere from more temperate (relatively cooler and drier) parts of East Asia. Variations in the relative abundances of C3 and C4 taxa appear to be linked to a combination of environmental factors that have varied temporally and geographically and that are unique to East Asia. A major expansion of C4 biomass in warmer subtropical parts of eastern Asia from ~1 Ma and particularly from ~0.4 Ma is later than other parts of the world

Archaeal lipid-inferred paleohydrology and paleotemperature of Lake Chenghai during the Pleistocene-Holocene transition

Over the past decades, paleoenvironmental studies in the Indian summer monsoon region have mainly focused on precipitation change, with few published terrestrial temperature records from the region. We analysed the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) in the sediments of Lake Chenghai in southwest China across the Pleistocene-Holocene transition, to extract both regional hydrological and temperature signals for this important transition period. The lake level was reconstructed from the relative abundance of crenarchaeol in isoGDGTs (%cren) and the crenarchaeol'/crenarchaeol ratio. The %cren-inferred lake level identified a single lowstand (15.4-14.4 ka cal BP), while the crenarchaeol'/crenarchaeol ratio suggests a relatively lower lake level between 15.4-14.4 and 12.5-11.7 ka cal BP, corresponding to periods of weakened ISM during the Heinrich 1 and Younger Dryas cold event. A filtered TetraEther indeX consisting of 86 carbon atoms (TEX86 index) revealed that lake surface temperature was similar to present-day values during the last deglacial period and suggests a substantial warming of similar to 4 degrees C from the early Holocene to the mid-Holocene. Our paleotemperature record is generally consistent with other records in southwest China, suggesting that the distribution of isoGDGTs in Lake Cheng-hai sediments has potential for quantitative paleotemperature reconstruction

Influence of environmental parameters on the distribution of subfossil chironomids in surface sediments of Bosten lake (Xinjiang, China)

The distribution of larval subfossil chironomids in surface sediment samples obtained from Bosten lake was analysed, and ordination methods were used to identify the influences of physical and chemical parameters on the abundance and diversity of chironomids. A total of 18 chironomid taxa was identified across the 32 samples, 15 of which showed minimum abundance of >1% and were present in more than one site. Chironomus plumosus-type, Microchironomus and Tanytarsus glabrescens-type were the most abundant of the 18 chironomid taxa identified. The distribution of the main chironomid species within Bosten Lake exhibited spatial heterogeneity. T. glabrescens-type was found to dominate Huangshuiwan, in the northwest part of Bosten lake, which had higher salinity and organic content. Microchironomus was mainly distributed in the Kaidu river mouth, in the southwest part of Bosten lake, whereas C. plumosus-type was more abundant in the deep centre, suggesting the importance of water chemistry, water dynamics and water depth on chironomid distribution. The relationship of chironomid distribution with environmental parameters was assessed using redundancy analysis, and the results revealed that water depth, Secchi depth and salinity were the predominant factors influencing chironomid distribution; these factors could explain the chironomid variances of 18.3, 5.3 and 5.0%, respectively. Our surface sediment-derived data provide a baseline for the analysis of future environmental variations of the lakes in the arid regions of northwest China, which are threatened by impacts of global climate change and regional water management

Glacial-interglacial vegetation changes in northeast China inferred from isotopic composition of pyrogenic carbon from Lake Xingkai sediments

Understanding the changes in monsoon intensity and ecosystem response at different timescales is crucial for the well-being of humans, yet the paleoclimatic interpretation of stable carbon isotope (δ13C) values from northeast China records is debatable. In this study, reported δ13C data from 76 surface soils in northeast China are compiled, and a δ13C record of pyrogenic carbon (δ13CPyC) from Lake Xingkai in northeast China since the last interglacial period is presented. The aim was to investigate the orbital timescale environmental implication of geological δ13CPyC data for northeast China. The results showed a distinct increase in δ13C values of surface soils, which correlated with increasing temperature of the warmest month. Higher temperature favored the expansion of C4 plants, while precipitation had only a weak correlation with δ13C values of surface soils in the region. On an orbital timescale, the δ13CPyC record from Lake Xingkai generally reflected paleovegetation change, suggesting that the abundance of C4 plants was relatively high during the warm periods, changing to almost purely C3 plants during the cold periods. Both modern and geological analysis suggest that the climatic factor determining the δ13C in northeast China was temperature of the warmest month. This is similar to the situation for mid-latitudes such as the Chinese Loess Plateau, in contrast to low latitudes such as southern China

Potential forcings of summer temperature variability of the southeastern Tibetan Plateau in the past 12 ka

Investigating potential forcing mechanisms of terrestrial summer temperature changes from the Asian summer monsoon influenced area is of importance to better understand the climate variability in these densely populated regions. The results of spectral and wavelet analyses of the published chironomid reconstructed mean July temperature data from Tiancai Lake on the SE Tibetan Plateau are presented. The evidence of solar forcing of the summer temperature variability from the site on centennial timescales where key solar periodicities (at 855±40, 465±40, 315±40 and 165±40 yr) are revealed. By using a band-pass filter, coherent fluctuations were found in the strength of Asian summer monsoon, Northern Hemisphere high latitude climate and high elevation mid-latitude (26 °N) terrestrial temperatures with solar sunspot cycles since about 7.6 ka. The two abrupt cooling events detected from the Tiancai Lake record, centered at ∼9.7 and 3.5 ka were examined respectively. Coupled with the paleoclimate modelling results, the early Holocene event (9.7 ka) is possibly linked to an ocean-atmospheric feedback mechanism whereas the latter event (3.5 ka) may be more directly related to external forcing

Early warning of critical transitions in biodiversity from compositional disorder

Global environmental change presents a clear need for improved leading indicators of critical transitions, especially those that can be generated from compositional data and that work in empirical cases. Ecological theory of community dynamics under environmental forcing predicts an early replacement of slowly replicating and weakly competitive “canary” species by slowly replicating but strongly competitive “keystone” species. Further forcing leads to the eventual collapse of the keystone species as they are replaced by weakly competitive but fast‐replicating “weedy” species in a critical transition to a significantly different state. We identify a diagnostic signal of these changes in the coefficients of a correlation between compositional disorder and biodiversity. Compositional disorder measures unpredictability in the composition of a community, while biodiversity measures the amount of species in the community. In a stochastic simulation, sequential correlations over time switch from positive to negative as keystones prevail over canaries, and back to positive with domination of weedy species. The model finds support in empirical tests on multi‐decadal time series of fossil diatom and chironomid communities from lakes in China. The characteristic switch from positive to negative correlation coefficients occurs for both communities up to three decades preceding a critical transition to a sustained alternate state. This signal is robust to unequal time increments that beset the identification of early‐warning signals from other metrics

>32 μm% (GS32) dataset of Holocene and modern surface sediments in Lake Ximenglongtan, southwestern China.

This dataset includes >32 μm% (GS32) data of Holocene and modern surface sediments in Lake Ximenglongtan, southwestern China. The grain size of sediments was measured by the Malvern Mastersizer 3000 laser particle sizer. Before measurement, the organic matter and carbonates in samples are removed using H2O2 and HCl, respectively. All the samples were dispersed in an ultrasonic bath for 15 min after the addition of (NaPO3)6. Each sample was measured three times and the three results needed to be highly similar to be considered qualified. A final average value of three times was obtained to reduce the measurement errors. The GS32 in the lake center area is a sensitive proxy of low lake-level change and is also used to reconstruct the lake-level change across the Holocene in Lake Ximenglongtan, northeastern South Asian monsoon region.</p

The first DNA barcode library of Chironomidae from the Tibetan Plateau with an evaluation of the status of the public databases

Abstract The main aim of this study was to curate a COI barcode library of Chironomidae from the Tibetan Plateau (TP) as an essential supplement to the public database. Another aim is to evaluate the current status of the public database of Chironomidae in aspects of taxonomic coverage, geographic representation, barcode quality, and efficiency for molecular identification, the Tibetan Plateau, China. In this study, 512 individuals of Chironomidae from the TP were identified based on morphological taxonomy and barcode analysis. The metadata of public records of Chironomidae were downloaded from the BOLD, and the quality of the public barcodes was ranked using the BAGS program. The reliability of the public library for molecular identification was evaluated with the newly curated library using the BLAST method. The newly curated library comprised 159 barcode species of 54 genera, of which 58.4% of species were likely new to science. There were great gaps in the taxonomic coverage and geographic representation in the public database, and only 29.18% of barcodes were identified at the species level. The quality of the public database was of concern, with only 20% of species being determined as concordant between BINs and morphological species. The accuracy of molecular identification using the public database was poor, and about 50% of matched barcodes could be correctly identified at the species level at the identity threshold of 97%. Based on these data, some recommendations are included here for improving barcoding studies on Chironomidae. The species richness of Chironomidae from the TP is much higher than ever recorded. Barcodes from more taxonomic groups and geographic regions are urgently needed to fill the great gap in the current public database of Chironomidae. Users should take caution when public databases are adopted as reference libraries for the taxonomic assignment