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Direct evidence of intense chemical weathering induced by volcanism is rare in sedimentary successions. Here, we undertake a multiproxy analysis (including organic carbon isotopes, mercury (Hg) concentrations and isotopes, chemical index of alteration (CIA), and clay minerals) of two well-dated Triassic–Jurassic (T–J) boundary sections representing high- and low/middle-paleolatitude sites. Both sections show increasing CIA in association with Hg peaks near the T–J boundary. We interpret these results as reflecting volcanism-induced intensification of continental chemical weathering, which is also supported by negative mass-independent fractionation (MIF) of odd Hg isotopes. The interval of enhanced chemical weathering persisted for ~2 million years, which is consistent with carbon-cycle model results of the time needed to drawdown excess atmospheric CO2 following a carbon release event. Lastly, these data also demonstrate that high-latitude continental settings are more sensitive than low/middle-latitude sites to shifts in weathering intensity during climatic warming events. The work shows that volcanic-related elevated continental chemical weathering could have played a significant role in global environmental perturbations during the Triassic-Jurassic mass extinction. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution stratigraphic records through the Permian–Triassic boundary (PTB) interval of the global stratotype section and point (GSSP) at Meishan, Zhejiang Province, China reveal that the PTB crisis was not a single, abrupt catastrophe. A bed-by-bed analysis of environmental and biotic changes makes clear that the crisis can be resolved into two discrete episodes, each consisting of three stages: A) unstably oscillating conditions, B) peak crisis conditions, and C) ameliorating conditions. The first crisis episode commenced in Bed 23, peaked in Beds 24e–26, and ameliorated in Beds 27 and 28, while the second crisis episode commenced in Bed 29, peaked in Beds 34–38, and ameliorated in Beds 39 and higher. The macroscopic mass extinctions happened not at the beginning, nor the end of each cycle, but at times when the crisis or perturbation of environments began. These extinction events do not show detectable feedbacks to concurrent environmental changes. In each episode, cyanobacteria proliferation postdated the macroscopic extinction while proliferation of green sulfur bacteria predated the environmental crisis. Causational analysis between environmental and microbial changes show that geomicrobial functional groups exercised pronounced effects on the marine C–N–S cycles and ocean redox conditions during the PTB crisis. It is possible thus that the microbial crises played an important role in strengthening or evening triggering the environmental crisis. [Copyright &y& Elsevier]

The article focuses on a study which examined the mass extinction event in the Paleo-Tethys Ocean or the Kamura event. Samples of fresh carbonate were collected from the Shangsi and Laibin Sections in China. Results of the study showed variations in the carbon isotope composition in the Laibin Section. The study also found that the high positive plateau of carbon isotope during the Capitanian period did not occur in the Tethyan Ocean.

Abstract: Lipid biomarker abundances and δ 13C values were determined across the Permian–Triassic (P–Tr) boundary at Meishan of southern China. The δ 13C values of n-alkanes showed large (∼9‰) fluctuations, which suggest major episodic changes in oceanographic conditions during faunal mass extinctions. Environment-related biomarker ratios, including pristane to phytane ratios (Pr/Ph), gammacerane to C31 homohopane ratios (γ/C31HP) and C27 18α(H)-22,29,30-trinorneohopane to C27 17α(H)-22,29,30-trinorhopane ratios (Ts/Tm), vary extensively throughout the section, with values typically associated with anoxic conditions coinciding with maximum δ 13C values. In particular, both faunal mass extinction horizons (beds 25 and 28) are characterised by biomarker ratios consistent with anoxic conditions and elevated n-alkane δ 13C values. The records of environment-related biomarkers and n-alkane δ 13C values clearly signify multiple environmental perturbations in association with faunal mass extinctions. [Copyright &y& Elsevier]

Molecular stratigraphic analyses using gas chromatograph-mass spectrometry have been performed in the upper section (S0, L1, S1) of the Yuanbo loess–paleosol sequences in northwest China, with a record extending from the last interglaciation through the present interglaciation. The CPI (Carbon Preference Index) values of both n-alkanols and n-alkan-2-ones display variations between loess deposits and paleosols, showing a correlation with the magnetic susceptibility record, an indicator of the East Asian summer monsoon. The observed variations in the indexes in relation to changes in lithology/paleoclimate are proposed to result from microbial degradation of higher plant lipids in the paleosols. The CPI values of n-alkanes, n-alkanols, and n-alkan-2-ones are negatively correlated with δ13C of bulk organic matter. The correlations suggest that the observed glacial–interglacial variations of δ13C data in the loess stratigraphy reflect the relative importance of the contribution of paleovegetation compared with microorganisms (including both the degradation and the addition of organic matter) and allochthonous loess/soil parent materials. It is thus necessary to evaluate the contributions of the latter two before the paleovegetation can be reconstructed based on the δ13C analysis of bulk organic matter in some loess–paleosol sequences of the Chinese Loess Plateau. [Copyright &y& Elsevier]

The article focuses on a study which examined the use of three international n-alkane standards in compound-specific carbon isotope analysis. A Thermo Finnigan Delta XP mass spectrometer was used to analyze the n-alkane standards for carbon isotope compositions. Results of the study showed that most deviations of the standards have positive excursions. It also found that data obtained from the gas chromatography-combustion-isotope ratio mass spectrometry are of good quality.

The compound-specific stable carbon (δ13C) and hydrogen (δ2H) isotopic compositions of microbial fatty acids have been widely used to trace microbial metabolism across a range of mesophilic environments. However, few studies have combined the δ13C and δ2H values of microbial fatty acids. So far none have determined the δ2H of microbial phospholipid fatty acids (PLFAs) in soils or peats, even though these stable isotope combinations could provide new insights into soil microbial metabolism. Here, we measured the δ13C and δ2H values of microbial PLFAs in top soils from peatlands, meadows, and woodlands in the Dajiuhu basin, central China. We observed a significant (p < 0.05) positive correlation between the δ13C and δ2H of microbial PLFAs across the three habitats studied here, indicating that central metabolic pathways affect both carbon and hydrogen isotopic compositions of microbial PLFAs. Moreover, our stable isotope data consistently indicate a relatively conservative metabolic state, which is dominated by expected heterotrophic metabolism. The exception to these observations is PLFAs associated with methanotrophs, as these appear to have decoupled carbon and hydrogen isotopes, providing an additional tool for tracing methanotrophic signals. Our results suggest that the carbon and hydrogen dual isotopic composition of microbial PLFAs can not only provide cross-validation for microbial metabolism in natural environments, but as a combined tool can also provide new insights into the perturbation of soil microbial community metabolism. [ABSTRACT FROM AUTHOR]

Abstract Floral communities in terrestrial habitats such as steppes are highly sensitive to regional climate changes. Here, we generated carbon and nitrogen isotope profiles for soil organic matter (SOM) in the Weinan loess-paleosol section in order to investigate changes in floral communities on the Chinese Loess Plateau (CLP) and their relationship to climate variation since 75 ka. δ13C SOM at Weinan ranges from −23.6 to −18.0‰ and δ15N SOM ranges from +3.4 to +5.5‰. Modeling based on the δ13C SOM record suggests that the proportion of C4 plants (%C4) has varied from lower during cold intervals (with minima of 24–32% at 73.1 ka, 20.9 ka (LGM), and 0.6 ka) to higher during warm intervals (with maxima of 62–64% at 53.0 ka and 8.2 ka). The δ13C SOM and δ15N SOM profiles are strongly correlated (r = +0.82; p(α) <0.001), suggesting that the N-isotope composition of SOM was also largely controlled by inputs of C4 versus C3 plants, consistent with known patterns of N-isotope variation among modern plants. The %C4 profile exhibits significant covariation with reconstructed mean annual temperatures (MAT) (r = +0.42; p(α) <0.001), mean annual precipitation (MAP) (r = +0.42; p(α) <0.001), and summer monsoon intensity (r = +0.50; p(α) <0.001), suggesting that paleovegetation was controlled by each of these climate variables to a degree. Comparing the Weinan record to SOM profiles across the CLP region, we identified three zones of varying monsoonal influence, with the East Asian Summer Monsoon (EASM) exerting the strongest influence on the southeastern margin of the CLP, and the East Asian Winter Monsoon (EAWM) greater influence on its northwestern margin. This paper provides new insights into deciphering paleovegetation patterns and their links to paleoclimate change on the basis of paired C- and N-isotope records. Highlights • Analysis of δ13C and δ15N of soil organics in Weinan section, Chinese Loess Plateau • δ13C SOM and δ15N SOM profiles exhibit strong correspondence since 75 ka • Both C- and N-isotope compositions controlled mainly by inputs of C3 vs C4 plants • Paleovegetation type strongly influenced by summer monsoon intensity [ABSTRACT FROM AUTHOR]

Although the trajectory of the marine mass extinction at the ∼252-Ma Permian-Triassic (P-Tr) boundary has been well studied, details of the coeval collapse of terrestrial ecosystems remain murky. Here, we use hydrocarbon biomarker compositions and other geological records (i.e., organic carbon isotopes (δ 13 C org), charcoal abundance, and Hg content) from a tropical peatland succession in southwestern China to reconstruct in detail the history of terrestrial ecosystem collapse during the P-Tr crisis. Our high-resolution hydrocarbon biomarker records reveal that this collapse proceeded in a stepwise manner with increasing intensity as the crisis unfolded. We recognize three discrete crisis stages: Stage I within the uppermost Xuanwei Formation and Stages II and III within the lowermost Kayitou Formation. Stage I, the early crisis stage, is marked by a significant decline in terrestrial biomass (continuing into the later stages), as recorded by reduced C 29 steranes relative to total steranes and a concomitant reduction in the ratio of pristane to phytane (Pr/Ph). Stage II, the main crisis stage, records intensified soil erosion and sediment flux as revealed by rising dibenzofuran (DBF) content and high hopane/sterane ratios, the disappearance of coal seams, a sharp negative shift in δ 13 C org , and peak concentrations of charcoal reflecting increased wildfire incidence. Stage III, the late crisis stage characterized by enhanced soil erosion, corresponds to peak values of Hg and Hg/TOC but no charcoal peak, suggesting intensified volcanism and a return to a humid climate. These stages closely follow temperature records, which show a stepwise rise during the crisis interval, implying that the deforestation process was strongly influenced by punctuated rises in temperature and/or its attendant effects (e.g., climate aridification). This P-Tr transition scenario suggests that global warming can trigger deforestation and reduce terrestrial carbon storage, thus serving as a positive climate feedback, with important implications for present-day climate change. • Three stages of terrestrial crisis are recognized during P-Tr in South China. • Plant biomass was decreased and soil erosion was enhanced during P-Tr crisis. • The crisis of the terrestrial and marine ecosystems can be well correlated. • Rising temperature played a substantial role in the terrestrial crisis. [ABSTRACT FROM AUTHOR]

Terrestrial organic carbon (OC) could be transported to marginal oceans by large rivers and deposited under different hydrological conditions. A mixture of the fresh & lithospheric OC, superimposed by the OC from different rivers, makes it challenging to distinguish sources and contributions of terrestrial OC. Here, we employed multiple geochemical proxies, including the hydrogen isotopic composition of leaf wax (δD wax), the carbon isotopic composition of total organic carbon (δ13C org), TOC/TN ratio, and major element distributions, to investigate the variability in sources of terrestrial OC in the Yangtze River Estuary (YRE) since 1270 CE. Our results revealed that the terrestrial OC deposition in the YRE was lower before 1950 CE and considerably increased after 1950 CE. The δD wax record in core A3 indicates that the OC deposition in the YRE was primarily modulated by the variability of El Niño-Southern Oscillation (ENSO) and East Asia Summer Monsoon. The strengthened El Niño-like state since 1600 CE resulted in increased precipitation in the middle and lower reaches of the Yangtze River basin and in turn, affected the terrestrial OC deposition in the YRE. Several episodes with lower carbon preference index (CPI) values of n -alkanes (CPI < 3) were identified. These episodes were concurrent with the streamflow shift of the ancient Yellow River during the transitions from wet to dry conditions, highlighting the potential contribution of the ancient Yellow River to the aged OC deposition in the YRE. • Geochemical proxies and lipid biomarkers were used to constrain the OC source. • ENSO regulates the organic matter deposition in the Yangtze River estuary. • Extreme lower CPI values reveal the aged OC from the ancient Huanghe River. [ABSTRACT FROM AUTHOR]

Organic matter preserved in speleothems has the potential to reflect the changes of the overlying vegetation and soil ecosystem. Here we report the first near 50-yr-resolution acid-soluble organic matter (ASOM) carbon isotope (δ 13 C ASOM ) sequence derived from a laminated stalagmite (HS-4) spanning the last 9 ky in Heshang Cave, central China. The δ 13 C ASOM values vary between − 25.8‰ and − 22.0‰, with lower values from 9 to 4 ka BP and less negative values from 4 to 0 ka BP. We postulate that the δ 13 C ASOM sequence is mainly controlled by temperature and water balance. Temperature could affect both vegetation physiology and microbial degradation in soil horizons. The influence of temperature on the δ 13 C ASOM is supported by the negative correlation ( r = − 0.48, p < 0.001) between the δ 13 C ASOM record and the paleotemperature record in the nearby Dajiuhu peatland. The water balance can affect the retention time of organic matter in soils. Under drier conditions, the soil organic matter will be retained longer and is more likely to be biologically degraded, resulting in more negative δ 13 C ASOM values. Our results reveal that δ 13 C ASOM in speleothems has the potential to reflect the response of vegetation and/or soil processes to paleoclimate changes. [ABSTRACT FROM AUTHOR]

Abstract: The Late Paleoproterozoic to Early Mesoproterozoic (from ∼1700 Ma to ∼1300 Ma) was highlighted by the assembly of the Nuna supercontinent, expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes. The isotopic composition of carbonate carbon (δ 13Ccarb) was surprisingly constant during this interval, but little is known about the secular variation in the organic carbon isotopic composition (δ 13Corg). Here we report δ 13Corg data from the latest Paleoproterozoic (>1650 Ma) to Early Mesoproterozoic (∼1300 Ma) succession in North China. The δ 13Corg values range from −25‰ to −34‰, and are dependent on sedimentary facies. In subtidal and deeper environments δ 13Corg values are low and constant, ca. −32‰, but relatively enriched and more variable in shallower intertidal and supratidal environments. We attribute the facies-dependent variation in δ 13Corg to the presence of a shallow chemocline. A probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and/or anaerobic photoautotrophic microbes in relatively deep environments from the latest Paleoproterozoic to Early Mesoproterozoic continental shelf of North China. [Copyright &y& Elsevier]

Abstract: Large perturbations to the global carbon cycle occurred during the Permian–Triassic boundary mass extinction, the largest extinction event of the Phanerozoic Eon (542Ma to present). Controversy concerning the pattern and mechanism of variations in the marine carbonate carbon isotope record of the Permian–Triassic crisis interval (PTCI) and their relationship to the marine mass extinction has not been resolved to date. Herein, high-resolution carbonate carbon isotope profiles (δ13Ccarb), accompanied by lithofacies, were generated for four sections with microbialite (Taiping, Zuodeng, Cili, and Chongyang) in South China to better constrain patterns and controls on δ13Ccarb variation in the PTCI and to test hypotheses about the temporal relationship between perturbations to the global carbon cycle and the marine mass extinction event. All four study sections exhibit a stepwise negative shift in δ13Ccarb during the Late Permian–Early Triassic, with the shift preceding the end-Permian crisis being larger (>3‰) than that following it (1–2‰). The pre-crisis shifts in δ13Ccarb are widely correlatable and, hence, represent perturbations to the global carbon cycle. The comparatively smaller shifts following the crisis demonstrate that the marine mass extinction event itself had at most limited influence on the global carbon cycle, and that both Late Permian δ13Ccarb shifts and the mass extinction must be attributed to some other cause. Their origin cannot be uniquely determined from C-isotopic data alone but appears to be most compatible with a mechanism based on episodic volcanism in combination with collapse of terrestrial ecosystems and soil erosion. [ABSTRACT FROM AUTHOR]

Abstract: The cataclysmic end-Permian mass extinction was immediately followed by a global expansion of microbial ecosystems, as demonstrated by widespread microbialite sequences (disaster facies) in shallow water settings. Here we present high-resolution carbonate carbon (δ 13Ccarb) and carbonate-associated sulfate–sulfur isotope (δ 34SCAS) records from the microbialite in the Cili Permian–Triassic (P–Tr) section in South China. A stepwise decline in δ 13Ccarb begins in the underlying skeletal limestone, predating the main oceanic mass extinction and the first appearance of microbialite, and reaches its nadir in the upper part of the microbialite layer. The corresponding δ 34SCAS, in the range of 17.4‰ to 27.4‰, is relatively stable in the underlying skeletal limestone, and increases gradually from 2m below the microbialite rising to a peak at the base of the microbialite. Two episodes of positive and negative shifts occurred within the microbialite layer, and exhibit a remarkable co-variance of sulfur and carbon isotope composition. The large amplitude of the variation in δ 34SCAS, as high as 7‰ per 100kiloyears, suggests a small oceanic sulfate reservoir size at this time. Furthermore, the δ 13Ccarb and δ 34SCAS records co-vary without phase lag throughout the microbialite interval, implying a marine-driven C cycle in an anoxic ocean with anomalously low oceanic sulfate concentrations. On the basis of a non-steady-state box model, we argue that the oceanic sulfate concentration may have fallen to less than 15%, perhaps as low as 3%, of that in the modern oceans. Low oceanic sulfate concentration likely was the consequence of evaporite deposition and widespread anoxic/sulfidic conditions prior to the main mass extinction. By promoting methanogenesis and a build-up of atmospheric CH4 and CO2, low oceanic sulfate may have intensified global warming, exacerbating the inimical environmental conditions of the latest Permian. [Copyright &y& Elsevier]

Abstract: Diploptene is present in four moss species (Sphagnum palustre, Aulacomnium palustre, Polytrichum commune and Hypnum revolutum) and some surface samples collected from the Dajiuhu Peatland in southern China. It exists in enhanced amounts in surface peat samples taken from a site dominated by H. revolutum, the specie that contains the greatest content of diploptene. By comparison of the carbon isotopic values of n-alkanes and diploptene in the four moss species and the peat, we postulate that the moss could be an important contributor of diploptene, especially in environmental settings such as peatlands. [Copyright &y& Elsevier]

C 4 -grasslands are known to have first expanded globally in the Late Miocene, and recent research has hinted at a second expansion phase during the Early Pliocene; however, the geographic extent of this second event and its driving force are debated. In this study, we present organic carbon-isotopic evidence from a high-resolution drillcore in North China spanning the interval from ∼8.0 Ma to the present. A prominent positive δ13C org shift at ∼4.1 Ma marks C 4 -grassland expansion on the North China Plain, and rapid oscillations of >6‰ from ∼3.2 to 2.2 Ma imply large shifts between C 3 - and C 4 -dominated floras. Our δ13C org profile mirrors carbon isotope shifts in loess-red clay deposits of the Chinese Loess Plateau, confirming the regional nature of this event. Furthermore, coeval carbon isotope records from Africa, North America, and South America provide evidence that the Early Pliocene C 4 -grassland expansion was global in scale and temporally distinct from the Late Miocene event. Whereas the Late Miocene event may have been related to aridification and altered fire regimes, this mechanism cannot account for the Early Pliocene expansion owing to contemporaneous increases in humidity at many Northern Hemisphere sites. We hypothesize that C 4 -grassland expansion in the Early Pliocene was triggered by falling atmospheric CO 2 levels, as evidenced by multiple p CO 2 proxy records and supported by quantum photosynthetic efficiency modeling. • Organic carbon isotope records from North China Plain show a significant expansion of C 4 grasslands at ∼4 Ma. • Early Pliocene C 4 grassland expansion also documented from Pakistan, East Africa, and North and South America. • Early Pliocene C 4 grassland expansion probably driven by declining atmospheric p CO 2 , not aridification. • Control by declining atmospheric p CO 2 supported by quantum photosynthetic efficiency modeling. • Contraction of C 4 grasslands in North China since ∼2.2 Ma due to cooling and strengthening of the East Asian Winter Monsoon. [ABSTRACT FROM AUTHOR]

The distribution of archaeal ether lipids is thought to be governed by salinity in hypersaline and marine environments due to differences in their biological sources. However, little is known about their source and relationship with salinity in lacustrine environments. To explore this, we analyzed the distribution of specific archaeal lipids (isoprenoid GDGTs and diethers) of 11 ponds along a salinity gradient (0.8‰ to 279‰) in Inner Mongolia, NE China. Functional gene mcrA and compound-specific stable carbon isotope data were used to explore the biological source(s) of archaeal diether and tetraether lipids. Both lipid- and DNA-based methods showed methanogens to be abundant in ponds, which is consistent with the dominance of GDGT-0 relative to other isoprenoid GDGTs as well as the occurrence of archaeol (C 20 /C 20 DGD), hydroxyarchaeol and the mcrA gene. However, the occurrence of the extended archaeol (C 20 /C 25 DGD) also suggests the presence of Halobacteriales, non-halophilic methanogens and halophilic methanogens. This means that the biological sources of archaeal diethers and tetraethers are complicated. We subsequently evaluated the applicability of TEX 86 temperature and ACE salinity proxies in these settings; however, due to the mixed sources of archaeal diethers and tetraethers, these proxies are not applicable in these ponds and it appears that salinity is not the dominant control on archaeal ether lipid distributions in these saline ponds. [ABSTRACT FROM AUTHOR]