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A 40 cm deep Sphagnum-dominated peat monolith from Bolton Fell Moss in Northern England was systematically investigated by lipid molecular stratigraphy and compound-specific δ13C and δD analysis using gas chromatography (GC), GC-mass spectrometry (GC-MS), GC-combustion-isotope ratio-MS (GC-C-IRMS) and GC-thermal conversion-IRMS (GC-TC-IRMS) techniques. 210Pb dating showed the monolith accumulated during the last ca. 220 yr, a period encompassing the second part of Little Ice Age. While the distributions of lipids, including n-alkan-1-ols, n-alkan-2-ones, wax esters, sterols, n-alkanoic acids, α,ω-alkandioic acids and ω-hydroxy acids, display relatively minor changes with depth, the cooler climate event was recorded in the concentrations of n-alkanes and organic carbon, CPI values of n-alkanes and n-alkanoic acids, and the ratio of 5-n-alkylresorcinols/sterols. Superimposed on the fossil fuel effect, the relatively cooler climate event was also recorded by δ13C values of individual hydrocarbons, especially the C23 n-alkane, a major compound in certain Sphagnum spp. The δD values of the C29 and C33 n-alkanes correlated mainly with plant composition and were relatively insensitive to climatic change. In contrast the C23 n-alkane displayed variation that correlated strongly with recorded temperature for the period represented by the monolith, agreeing with previously reported deuterium records in tree ring cellulose spanning the same period in Scotland, Germany and the USA, with more negative values occurring during the second part of Little Ice Age. These biomarker characteristics, including the compound-specific δ13C and δD records, provide a new set of proxies of climatic change, potentially independent of preserved macrofossils which will be of value in deeper sections of the bog where the documentary records of climate are unavailable and humification is well advanced. [Copyright &y& Elsevier]

Quantitative reconstructions of the depth to water table (DWT) of ombrotrophic (rain-fed) peatlands are important for understanding the palaeohydrological responses of peatlands to past climate changes. This understanding can provide insights into projecting peatlands future variability and evolution. However, the postglacial DWT reconstruction of peatlands in China is challenging due to complications of the atmospheric circulation system, the scarcity of hydrological proxies, and the site-specific nature of hydrological signals. Here we present a postglacial quantitative DWT reconstruction based on the analysis of fossil phytoliths from the Dajiuhu Peatland, central China. The reconstructions were based on a phytolith-DWT calibration model using a weighted averaging partial least-squares regression analysis of peatland topsoil calibration datasets. Three shallow DWT (wet) periods at 13,000–11,500 cal yr BP, 9,600–7,500 cal yr BP, and 3,000 cal yr BP-present, and two extended deep DWT (dry) periods at 11,500–9600 cal yr BP and at 7,500–3,000 cal yr BP are found based on the cluster analysis of phytolith assemblages and reconstructed DWT changes. These five documented hydrological periods are consistent with regional precipitation reconstructions from independent records in the middle Yangtze Valley (MYV). We interpret these changes as mostly reflecting changes in ENSO at various timescales. An amplified ENSO forced a southward Western Pacific Subtropical High and caused the persistence of the Meiyu Front in the mid-lower Yangtze Valley, consistent with the intense rainfall periods in our study region. Our results indicate that phytolith records are a reliable and sensitive proxy for the calibration of water table models and the quantitatively palaeo-DWT reconstruction of peatlands and reveal a remarkable link between the local hydrological variations and the coupled atmospheric-oceanic circulation, which is significant for the prediction of future hydrological changes in the Asian monsoon region under the background of global warming. • Quantified postglacial DWT of Dajiuhu Peatland based on phytoliths. • The DWT of Dajiuhu Peatland response to the regional precipitation. • Recognized a wetter YD, early and late Holocene and a drier mid-Holocene in the MYV. • Demonstrated a spatial 'tripole' mode of palaeohydrology in EASM regions of China. • Proposed the ENSO-related hydrological changes at millennial to annual timescales. [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]

To achieve a sufficient understanding of the spatial dynamics of terrestrial climate variability, new proxies and networks of data that cover thousands of years and run up to the present day are needed. Here we show the first Gram-negative bacterial 3-hydroxy fatty acid (3-OH-FA) based temperature and hydrological records from any paleoclimate archive globally. The data, covering the last 9 ka before present (BP), are generated from an individual stalagmite, collected from Heshang Cave, located on a tributary of the Yangtze River, central China (30°27′N, 110°25′E; 294 m). Our results indicate a clear early-to-middle Holocene Climatic Optimum (8.0–6.0 ka BP) followed by a long-term monotonic cooling and increasing variability over the last 0.9 ka BP. The hydrological record shows two relatively long wet periods (8.8–5.9 ka BP and 3.0–0 ka BP) and one relatively dry period (5.9–3.0 ka BP) in central China. We show that 3-OH-FA biomarkers hold promise as independent tools for paleoclimate reconstruction, with the potential to deconvolve temperature and hydrological signals from an individual stalagmite. [ABSTRACT FROM AUTHOR]

Understanding the origin and evolutionary history of drought events is of great significance, providing critical insight into future hydrological conditions under the changing climate. Due to the scarcity of drought proxies from northern China, the occurrence and underlying mechanisms of the drought events remains enigmatic on longer timescales. Here we utilize microbial lipid proxies to reconstruct significant drought events over the last four ice age terminations in the southernmost section (Weinan section) of the Chinese Loess Plateau. The abundance of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) relative to bacterial branched GDGTs, measured by R i/b and BIT indices, is diagnostic of enhanced drought conditions. The R i/b (and BIT) indices are stable and low (high) throughout most of the loess section spanning the last 350 thousand years, but they do exhibit sharp transient peaks (valleys) during the intervals associated with the four ice age terminations, and especially Terminations II and IV. These enhanced drought events are, non-intuitively, associated with a significant decrease in the relative abundance of C 4 plants, inferred by a decrease in the carbon isotope composition of bulk organic matter. Although the microbial records show some consistency with the Weinan grain size profiles, indicative of Eastern Asian winter monsoon variability, they also show some apparent difference. In fact, some features of the microbial records exhibit strong similarities with marine sediment planktonic foraminiferal δ 13 C records from the western Pacific warm pool, which reflect ENSO-like changes during glacial terminations. Therefore, enhanced droughts immediately before the interglacial warming in northern China could be explained, at least in part, by teleconnections in tropical ocean–atmosphere circulation via shifts in the Intertropical Convergence Zone (ITCZ) and associated Jet Stream over the Asian continent. According to our microbial biomarker data, these enhanced megadroughts are apparently different, both in terms of severity and causal mechanism, from the more commonly discussed dry conditions observed during glacial periods. [ABSTRACT FROM AUTHOR]

The recently proposed global and local calibrations for the mean annual air temperature (MAT) reconstruction on the basis of 5- and 6-methyl brGDGTs have rarely been applied to the Chinese LPS yet, leaving the applicability of these calibrations unclear. Here, we used the improved chromatography method to analyze 198 loess-paleosol samples from the Weinan section in the southern CLP for the past 350 kyr. The 6-methyl brGDGTs comprise a major proportion of total brGDGTs, pointing to alkaline conditions for most soil samples from the Weinan LPS. The decoupled profile variation of MBT′ and MBT′ suggests their response to different enviromental factors, possibly soil pH (or soil moisture) and temperature, respectively. This discrimination further corraborates that temperature and monsoonal precipitation were not in phase during the last four deglaciations on the CLP. Temperature estimates for the six calibrations tested show similar trends but remarkably differ in amplitudes. The soil moisture appears to affect the global MBT′/CBT calibration, the global MAT calibration (a calibration based on the multiple linear regression) and the Chinese local SSM (Stepwise Selection Method) calibration, resulting in a significant underestimation of late Holocene temperature. In contrast, the dry climate has no effect on the global MBT′ calibration. Of the six calibrations, only the Chinese local SSM calibration and global MAT calibration produce temperature variation amplitude over the past 350 ka that is consistent with other independent proxy data. The recently proposed local calibrations based on 5- and 6-methyl brGDGTs for North China yield the amplitude of temperature changes that is much larger than the results determined by other approaches. All the six calibrations have their own weakness in the MAT reconstruction, due partly to the inconsistency between the brGDGT distribution in the Weinan LPS and the modern soils used to establish these calibrations. [ABSTRACT FROM AUTHOR]

Char and polycyclic aromatic hydrocarbons (PAHs), which can serve as proxies for wildfire frequency and/or intensity, are widely present in Permian-Triassic boundary (PTB) strata, providing information on volcanism or on climatic control of fuel availability (linked to vegetation type and density) or combustibility (linked to aridity). Here, we present evidence to separate temporal from geographic signals of climate impact on wildfire during the Permian-Triassic (P-Tr) transition. Geographically, char and pyrolytic PAHs for wildfire are concentrated in temperate and tropical regions and show a connection with forest distribution as reflected by occurrences of Upper Permian coals. Temporally, the subtropical terrestrial Dalongkou section of northwestern China provides evidence of two intervals of increased pyrolytic PAHs of combustion origin. The first interval was linked to climatic hyperwarming during the latest Permian, whereas the second interval was associated with rapid climatic drying, as recorded by spore-to-pollen ratios (S/P), during the P-Tr transition. This record suggests that rapid climate change during the P-Tr transition increased wildfire frequency. A climate link is also shown by turnovers of ostracods, conchostracans, and charophytes in conjunction with greater wildfire frequency. The trigger for enhanced wildfires and biotic changes is inferred to have been volcanically induced climatic warming and aridification. Our data thus document important links between wildfires, climate change, volcanism, and mass extinction during the P-Tr transition. • Pyrolitic PAHs document increased wildfire frequency during the P-Tr transition. • Wildfire distribution strongly correlated to tropical/temperate forested regions. • Two intervals of enhanced wildfire frequency coincided with end-Permian biocrisis. • Increased wildfire linked to volcanically induced climate warming and aridification. • Rapid climate change during end-Permian contributed to increase wildfire frequency. [ABSTRACT FROM AUTHOR]

Changes in marine productivity and redox conditions during the end-Ordovician (Hirnantian) glaciation and Ordovician–Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus , Dicellograptus complexus , and Paraorthograptus pacificus , were deposited under euxinic conditions at both localities, as shown by high Mo EF , U EF , and δ 98 Mo values. A major sea-level regression during the Hirnantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Hirnantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high Mo EF , U EF , and δ 98 Mo values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct. The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by δ 13 C carb , δ 13 C org , and δ 34 S py records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Hirnantian Isotopic Curve Excursion) δ 13 C carb excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Hirnantian glaciation were probably due to crossing of tipping points in the Late Ordovician climatic–oceanic system. [ABSTRACT FROM AUTHOR]

Abstract: In order to investigate how lipids in cave water respond to seasonal climate change, drip water samples were collected from 2006 to 2008 in Heshang Cave, central China for fatty acid analysis. These lipids are abundant in the drip water. Their compositions are dominated by lower-molecular-weight nC16:0, nC18:0 and nC14:0 acids, together with mono-unsaturated nC18:1, nC16:1 and nC14:1. Analysis of one water sample revealed marked differences between the dissolved and particulate fractions. The dissolved fraction contains total fatty acids one order of magnitude higher than that of the particulate fraction. The distributional patterns of the fatty acids suggest that microbes living in the overlying soils and/or groundwater system contribute most fatty acids to the drip water. This 2-a monitoring experiment reveals that the abundance of mono-unsaturated fatty acids relative to the saturated homologues (nC16:1/nC16:0 and nC18:1/nC18:0) relate inversely to the changes of synchronous external air temperature. Higher values occur under cold conditions (winter/spring), while lower values appear in warm intervals (summer). Further studies are needed to elucidate the dynamic processes by which the external temperature affects fatty acids in drip water and to confirm the potential application of fatty acid ratios such as nC16:1/nC16:0 and nC18:1/nC18:0 in paleotemperature reconstructions. [Copyright &y& Elsevier]

Abstract: The peat sequence at Hani in northeastern China accumulated over the past 16calkyr in a percolation mire in which rain water and ground water seeped through the peat system. The molecular compositions of n-alkanes, n-alkanols, and n-alkanoic acids extracted from the Hani peat sequence reveal different responses to the progressive evolution of climate and changes in the nature of the peat-forming vegetation. Long chain length components that originate from the waxy coatings of subaerial vascular plants dominate the n-alkane distributions throughout the Hani peat sequence. The paleoclimate integrity of these biomarker molecules appears to be well preserved. Most of the n-alkanol distributions are similarly dominated by long chain components that indicate their origins from subaerial plants. In contrast, n-alkanoic acid distributions are dominated by secondary components that record the importance of post-depositional microbial activity in this peat sequence, which evidently can be extensive in a percolation mire. Elevated n-alkane P aq values and C23/C29 ratios, which are both molecular proxies for water-loving plants, record an especially moist local climate in the Bølling-Allerød (14.5 to 12.9ka), Younger Dryas (12.9 to 11.5ka), and Pre-Boreal (11.5 to 10.5ka) portions of the Hani peat sequence. Depressed P aq values and C23/C29 ratios and larger n-alkane average chain length values indicate that the Holocene Climatic Optimum (10.5 to 6ka) was a period of warmer climate with lower effective precipitation, which contrasts with evidence of wetter climates in most of East Asia. [Copyright &y& Elsevier]

Abstract: Lipid biomarkers extracted from a peat core obtained from the northeast edge of the Tibetan Plateau have provided a 13 kyr record of regional climate change. A period of warmer climate corresponding to the Holocene Optimum between 9ka and 5ka is evident in greater alteration of n-alkanoic acids and n-alkanols and in the appearance of biomarkers indicative of greater microbial activity. Concomitant increases in local precipitation that raised the water level of the peat bog are recorded by increases in the proportions of C23 and C25 n-alkanes that are mainly derived from submerged/floating plants. Alternations in submergent and emergent plant n-alkane proxies reveal multi-decimeter scale rises and falls in the bog water level that relate to variation in precipitation on the Tibetan Plateau during the Holocene. Major low water level events occurred around 12.6, 11.3, 10.7, 9.4, 8.3, 6.0, 4.5, 3.1, 2.1 and 1.0ka. These events correlate well with cold dry events recorded in same region from other climatic proxies that reflect millennial-scale Holocene instability in the Asian monsoon system. Our biomarker results also indicate that peat accumulation and preservation on the Tibetan Plateau are more sensitive to variability in monsoonal precipitation than in annual temperature. [Copyright &y& Elsevier]

• Warm-dry and cold-wet hydroclimate modes were showed according to 18,000-year temperature and DWT changes in Dajiuhu Basin. • Peatland ecology sensitively responded to palaeo-hydroclimatic changes. • More bulliform phytoliths induced by exaggerated plant leaf water loss occurred during warm-dry mid-Holocene. • Plant water regulation, climate, and irradiance showed a synchronous ~1000-yr cycle. Peatland ecosystem processes are strongly influenced by hydrology linked to climate change. However, how transpiration, as major water loss pathway in closed peatland, responds to climate change and then regulates ecosystem water balance that remains poorly understood. Here, we reported an 18,000-year fossil phytolith record from an herbaceous community-dominated mountain peatland in central China to reconstruct climate changes and vegetation evolution and reveal the interactions between regional climate changes and peatland ecosystem. The abundance of bulliform phytoliths—a silicified type of cells that regulate the movements of plant leaves to reduce light exposure and transpiration rate—shows close correlations with the Holocene climate variations with higher bulliform abundance in response to warm-dry climate (11,500–9600 cal yr BP; 7500–3000 cal yr BP) and lower values associated with cold-wet climate (13,000–11,500 cal yr BP; 3000 cal yr BP-present). Spectral analysis reveals that bulliform abundance and reconstructed climate vary with a major ~1000-year periodicity during the Holocene, suggesting a possible causal relationship to solar activity. A high bulliform abundance and warm-dry climate correspond with enhanced solar activity, and vice versa. We interpret that the resultant leaf water deficit resulted from greater light interception, high temperature and drought promoted the production of bulliform cells to fold leaf for water conservation, which is an effective protective mechanism of peatland grasses from the environmental stress. These results expand our understanding of how grass-dominated peatland plant water regulation recorded by bulliform phytoliths responds to solar radiation and local hydroclimate during geological period. [ABSTRACT FROM AUTHOR]

• First evidence of T-J boundary wildfires outside Central Atlantic Magmatic Province. • Lipid profiles document three wildfire episodes during T-J transition in South China. • PPAH index documents provenance from alkyl-PAHs by pyrolytic (combustion) reactions. • Warmer, drier conditions caused by CAMP set stage for TJB wildfires in South China. • Regional aridification related to southward paleo-ITCZ shift was immediate trigger. Emplacement of the Central Atlantic Magmatic Province (CAMP) during the Triassic-Jurassic (Tr-J) transition is believed to have indirectly increased wildfire frequency through climatic warming and drying. To date, evidence for wildfires has come mainly from within the CAMP region. Here, we present biomarker evidence for wildfires in the form of polycyclic aromatic hydrocarbons (PAHs) in two terrestrial Triassic-Jurassic boundary sections in the Sichuan Basin, southwestern China. Pyrolytic PAHs of probable wildfire origin identified in these sections include pyrene, benzofluoranthene, benzo[e]pyrene, indeno[1,2,3-cd]pyrene, benzo[ghi]perylene, and coronene. Sharp increases in the abundances of pyrolytic PAHs normalized to total organic carbon were found during the Rhaetian Stage (R1 and R2) and at the Tr-J boundary. The ratios of pyrolytic PAHs (PPAHs) to methylated homologues document the combustion origin of PPAHs from methylated PAHs during these intervals of increased wildfire frequency. The abundances of oxygenated PAHs with furan structure show the same stratigraphic trend, testifying to enhanced soil erosion or weathering that may have resulted from a reduction of vegetative cover. Although CAMP-induced climate changes probably set the stage for more frequent wildfires in South China, the immediate trigger was probably a southward shift of the paleo-Intertropical Convergence Zone (ITCZ). These observations suggest a complex feedback involving climatic warming, precipitation and evapo-transpiration, vegetation density, and wildfire prevalence during the Tr-J transition. [ABSTRACT FROM AUTHOR]

Stalagmites have become a key archive in Quaternary palaeoclimatic reconstruction due to their ability to yield continuous and undisturbed records, precise and absolute chronologies, and their global terrestrial distribution. Oxygen isotopes are effectively the master proxy for speleothem analysis, but inherently encode a mix of climatic signals. Thus it is tricky to deconvolve the temperature and precipitation signal from speleothems. Bacterial derived 3-hydroxy fatty acids (3-OH-FAs) have potential as independent temperature and pH proxies. By applying the 3-OH-FA based temperature proxy RAN15 and the pH proxy RIAN to the HS4 stalagmite from central China, we recently reconstruct Holocene temperature and hydrological changes, respectively. Now we address the issue of how terrestrial carbon storage feeds back on warm climate states, which is critical for improving global warming projections. Soils may act as a positive feedback on climate if warming increases soil carbon decomposition rates. Conversely, if increases in net primary production (NPP) exceed increases in decomposition, the climate feedback will be negative. Here we present new constraints on the response of terrestrial carbon cycle feedbacks, especially soil respiration, during earlier warm episodes of the early Holocene Climate Optimum (HCO) and Medieval Warm Period (MWP), which have hitherto been poorly constrained due to a lack of proxies (and thus available data). We utilize the first palaeoclimatic application of compound-specific δ13C measurements on n-fatty acid biomarkers extracted from a stalagmite. We resolve a proportional increase in C3 plants in the catchment area during these warmer/wetter intervals. Moreover, we find that heterotrophic soil respiration was highly substrate selective, indicating that NPP outpaced decomposition and the catchment behaved as a carbon sink. Thus, we provide the first palaeoclimate evidence that subtropical mineral soils in warmer/wetter climates can act as a negative climate feedback. [ABSTRACT FROM AUTHOR]