Xianyu Huang

Changes in the global carbon cycle occurred as two episodes during the Permian–Triassic crisis

Coeval records of ocean, atmosphere, and terrestrial change are crucial to understanding the pattern and causes of global mass extinction across the Permian-Triassic boundary (PTB). However, relationships among changes in different settings remain largely unclear, primarily due to the challenges associated with the correlation among disparate records. Here we compare marine carbon isotopic records with marine and terrestrial environmental and biotic events recorded in sediments from the Meishan PTB section of south China. Time-scaled carbonate carbon isotopes exhibit two gradual major shifts across the PTB at Meishan, and these are duplicable elsewhere around the Tethys Ocean. The two shifts are associated with two episodes of enhanced terrestrial weathering indicated by an increased abundance of 13 C-enriched moretanes relative to hopanes and an elevated abundance of black carbon fragments. Key marine events previously reported for the PTB, including photic zone euxinia, faunal mass extinction, and cyanobacterial expansion, also occur as two episodes, coinciding with both of the progressive shifts to negative δ 13 C values and enhanced weathering. The temporal sequence of the duplicable events suggests that the biotic crisis was a consequence of prolonged and episodic changes in the marine and continental systems, and argues against an extraterrestrial impact as the main cause.

Concordant monsoon-driven postglacial hydrological changes in peat and stalagmite records and their impacts on prehistoric cultures in central China

Asian monsoon records are widely documented, but specific proxies of monsoonal rainfall are limited. We present here two new independent proxy records from peatland and stalagmite archives that indicate a high degree of concordance between monsoon-driven hydrological changes occurring since the last deglaciation in a broad region of central China. The wet periods elevated the water table in the Dajiuhu peatland, as recorded by reduced mass accumulation rates of hopanoids, biomarkers for aerobic microbes, confirmed by molecular phylogenic analyses. The hopanoid-based reconstruction is supported by the first report of the environmental magnetism parameter ARM/SIRM (anhysteretic remanent magnetization / saturation isothermal remanent magnetization; ratio of fine magnetic particles to total ferrimagnetic particles) in a stalagmite from Heshang Cave in central China. Heavy rainfall resulted in the enhanced transport of coarse particles to the cave and thus low ARM/SIRM values in the stalagmite. The hydrological conditions inferred from the two records reveal three relatively long wet periods in central China: 13–11.5 k.y. ago, 9.5–7.0 k.y. ago, and 3.0–1.5 k.y. ago. Archaeological evidence for the hydrological impacts on regional populations comes from the observation that temporal shifts among six distinctive cultures of the Neolithic Period to the Iron Age in central China occurred during wet periods or flood episodes. Spatiotemporal distributions of >1600 prehistoric settlement sites correlate with the proxy-inferred fluctuating hydrological conditions, with enhanced flooding risk forcing major relocations of human settlements away from riparian zones.

Paleotemperature variability in central China during the last 13 ka recorded by a novel microbial lipid proxy in the Dajiuhu peat deposit

The Asian summer monsoon is a very important climatic component affecting the land ecosystem on the eastern Asian continent. Here we assess microbe-derived lipid biomarker evidence from a well-dated peat core from Dajiuhu to reconstruct paleotemperature changes in central China through the last 13 ka. The branched fatty alcohol ratio BNA15, which is defined as the relative contribution of branched C15 fatty alcohols over their straight-chain homolog, shows a positive correlation with air temperature (R= 0.83, n=11, p<0.001) in an altitude transect at Shennongjia Mountain, central China. This correspondence suggests that the microbial activities associated with branched fatty alcohol synthesis are sensitive to differences in temperature. The BNA15 sequence in the Dajiuhu peat deposit shows a trend similar to the paleotemperature record derived from pollen results over the last 13 ka, further supporting that BNA15 is a sensitive proxy of paleotemperature. Absolute temperatures estimated from BNA15 values of modern surface peats are about 3–4°C lower than the modern annual mean air temperature in the peatland, which may result from the influences of factors other than temperature or from the different microbial communities in the mountain soils used to calibrate the BNA15 acidic peats. Fluctuations in the continuous 13 ka BNA15-derived record of relative temperature change from the Dajiuhu peat core imply that solar activity is the dominant cause for most cold events at multicentennial to submillennial timescales.

Moisture conditions during the Younger Dryas and the early Holocene in the middle reaches of the Yangtze River, central China

The last glacial–interglacial climate transition in East Asia was characterized by an intensified monsoon, but knowledge about hydrological responses at a regional scale during this transitional period is limited. Here we provide a novel lipid-based paleohydrological record for this period from the Dajiuhu peat deposit, central China. High water levels and persistent wet conditions during the Younger Dryas (12.5–11.6 ka) and the early Holocene (10.5–9.9 ka) were inferred on the basis of enrichments of sterenes, a group of unstable intermediate diagenetic products derived from biogenic sterols that are most likely preserved under a waterlogged peatland surface. These two wet intervals were accompanied by temperature decreases indicated by an increase in pollen abundance from cold favoring conifer trees and a decrease in total hardwood tree pollen. This cold and wet climate pattern, which is different from the concordant cold-dry pattern documented in north China, was probably caused by the regional influence of the western Pacific subtropical high on the residence time of the Meiyu rainband in the middle and lower reaches of the Yangtze River Valley.

Ecology of testate amoebae in Dajiuhu peatland of Shennongjia Mountains, China, in relation to hydrology

This study investigates the testate amoeba communities of a large peatland in Central China. The ecology and seasonal variability of testate amoeba communities were studied during 2009–2010. Investigation of environmental controls using ordination showed that the relationship between testate amoeba communities and depth to water table (DWT) and pH are extremely weak. The small proportion of variance explained by water table depth here (only 1.9% in the full data) shows that the hydrological control is weaker than we expected in this peatland, and weaker than any study we are aware of using a similar methodology. Attempts to develop species-environment (transfer function) models or identify indicator species for future palaeoecological studies were unsuccessful. Previous large-scale studies of peatland testate amoeba ecology have been largely restricted to Europe and North America and results have been relatively consistent among studies. Our results contrast with this consensus and suggest that at least in minerotrophic peatlands in China testate amoeba communities may be primarily controlled by different environmental variables. In China, testate amoebae have been relatively little studied but may prove to be valuable for a variety of applications in palaeoecology and biomonitoring and much further work is required.

Long chain n-alkanes and their carbon isotopes in lichen species from western Hubei Province: implication for geological records

Five coticolous lichen samples were collected from western Hubei Province of China to analyze the long chain n-alkanes and their carbon isotope compositions. The n-alkanes range in carbon number from C17 to C33 with strong odd-over-even predominance between C21 and C33. Lichens are dominated by n-C29 in the samples of Dajiuhu, Shennongjia Mountain, but by both n-C23 and n-C29 at Qizimei Mountain. This difference may result from the different environmental conditions in these two sites. The δ13C values of long chain n-alkanes in lichen samples show the signature of C3 plants. Based on compoundspecific carbon isotopic values and previous results, we state that alkane homologs >C23 mainly originate from the symbiotic fungi, while symbiotic algae only contribute trace amount of long chain alkanes. Of great interesting is the occurrence of long chain 3-methylalkanes in the Qizimei samples. These anteiso compounds range from C24 to C32, displaying obvious even-over-odd predominance. This study reveals that the association of long chain 3-methylalkanes with n-C23 alkane might be used as proxies to reconstruct the paleoecological implications of lichens in Earth history.

Optimization of acid digestion conditions on the extraction of fatty acids from stalagmites

Lipids in stalagmites have been shown the potential for the paleoclimate reconstruction. However, the low lipid content leads to the difficulty in gaining high resolution lipid record in stalagmites because large mass of samples are required. Previous studies have validated that the acid digestion can improve the yield of lipids, especially fatty acids (FAs) and 3-hydroxy fatty acids (3-OH-FAs). In order to obtain more content of FAs and 3-OH-FAs with limited stalagmite sample weight, we investigate here how the acid digestion conditions (HCl concentration, heating temperature and time duration) could affect the yields of FAs and 3-OH-FAs. Under different concentration of HCl, from 2.0 to 6.0 mol/L, the FAs keep the same step in content variation with 3-OHFAs, and the highest yields of both two appeared under the 3 mol/L HCl. The content of 3-OH-FAs increases positively with the heating temperature from 80°C to 150°C, while FAs showed the highest content at 130°C. Both of FAs and 3-OH-FAs firstly increased to a high content and then decreased as the heating time duration varies from 1.0 to 4.0 h, with the highest yields of both two being at 3.0 h. Consequently, we suggest the optimized acid digestion condition is under 3 mol/L HCl, heating at 130°C for 3 h and 5 g of each stalagmite sample are sufficient for the lipid analysis.

Phytolith records of the climate change since the past 15000 years in the middle reach of the Yangtze River in China

Based on 14C dating and core sediments survey, phytolith records are employed to reconstruct paleovegetation and paleoclimate in the Jianghan Plain in the middle reach of the Yangtze River. Phytoliths identified are assigned into 21 well-described morphotypes and divided into four groups (Poaceae, fern, coniferous and broad-leaved). The phytolith assemblages together with warmth index (Iw) are divided into 18 ecological zones, which reflect a complete vegetation history related to climate change in the middle reach of the Yangtze River during the past 15000 years. On the basis of the correlation of phytolith records with the paleoclimatic indicators from stalagmite, peatland, North Atlantic deep-sea sediments, Loess Plateau of Central China, and Arabic Sea sediments, eight climatic phases are identified included Last Glacial Maximum (LGM) (20–14.8 cal kaBP), Last Deglaciation (LDG) (14.8–11.9 cal kaBP), low-temperature phase in the Early Holocene (11.9–8 cal kaBP), Holocene Optimum (8–4.9 cal kaBP), Holocene Katathermal (4.9–1.1 cal kaBP), Medieval Warmth Period (MWP) (1.1–0.7 cal kaBP), Little Ice Age (LIA) (0.7–0.15 cal kaBP), and Modern Warming (0.15 cal kaBP — present). Climatic events such as Bolling-Allerod warm intervals, Older Dryas, Inter-Allerod Cold Period, and Younger Dryas, and eight Holocene Bond events (B1–8) have been identified since the LDG. Our results demonstrate that the evolution of the climate in the research area has a strong link with the Indian Summer Monsoon (SW Monsoon), Asian Summer Monsoon (SE Monsoon), and Holocene events in North Atlantic simultaneously, which might indicate that solar variability affects the Earth surface climate system at the centennial and millennial scales.

Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary

ABSTRACT Aromatic compounds extracted from sedimentary rocks can reflect environmental conditions, organic sources and maturity. The aromatics, identified in association with mass extinction in particular, would provide a signature assisting our understanding of the causes of the biotic crisis. Aromatic hydrocarbons were fractionated from the total lipid extracts of 37 samples taken from the Permian-Triassic boundary (beds 23 to 34) of section B at Meishan ( ), Zhejiang ( ) Province in South China. These aromatics were analyzed by using gas chromatography-mass spectrometry (GC-MS). Main compounds identified include naphthalene, phenanthrenes, fluorene, dibenzothiophene, dibenzofuran, fluoranthene, pyrene and some of their methyl homologues. The indices of methyl phenanthrene distribution fraction indicate the comparable maturity (within the oil window, 0.7%–1.0% of the mean vitrinite reflectance) of the organic matter throughout the whole profile analyzed. The ratio of dibenzothiophene to phenanthrene (DBT/PHN) varies generally at a comparable pace with lithology. Significantly, a gradual decrease of this ratio was observed within bed 24 limestone, which is probably due to the variation of sedimentary environment. This change is in line with the drop in the carbon isotope composition of carbonate, the loss of the Changhsingian reef ecosystem, and the decrease of cyanobacteria abundance within the bacteria population. The coincidence of these records suggests a close relation between the biotic crisis and marine environmental conditions, and these records clearly show the onset of the biotic crisis prior to event bed 25.

Molecular Paleoclimate Reconstructions over the Last 9 ka from a Peat Sequence in South China

To achieve a better understanding of Holocene climate change in the monsoon regions of China, we investigated the molecular distributions and carbon and hydrogen isotope compositions (δ13C and δD values) of long-chain n-alkanes in a peat core from the Shiwangutian (SWGT) peatland, south China over the last 9 ka. By comparisons with other climate records, we found that the δ13C values of the long-chain n-alkanes can be a proxy for humidity, while the δD values of the long-chain n-alkanes primarily recorded the moisture source δD signal during 9–1.8 ka BP and responded to the dry climate during 1.8–0.3 ka BP. Together with the average chain length (ACL) and the carbon preference index (CPI) data, the climate evolution over last 9 ka in the SWGT peatland can be divided into three stages. During the first stage (9–5 ka BP), the δ13C values were depleted and CPI and Paq values were low, while ACL values were high. They reveal a period of warm and wet climate, which is regarded as the Holocene optimum. The second stage (5–1.8 ka BP) witnessed a shift to relatively cool and dry climate, as indicated by the more positive δ13C values and lower ACL values. During the third stage (1.8–0.3 ka BP), the δ13C, δD, CPI and Paq values showed marked increase and ACL values varied greatly, implying an abrupt change to cold and dry conditions. This climate pattern corresponds to the broad decline in Asian monsoon intensity through the latter part of the Holocene. Our results do not support a later Holocene optimum in south China as suggested by previous studies.