Search

The goal of achieving carbon neutrality in the next 30-40 years is approaching worldwide consensus and requires coordinated efforts to combat the increasing threat of climate change. Two main sets of actions have been proposed to address this grand goal. One is to reduce anthropogenic CO

Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and the majority of such formations are associated with seafloor cold seeps. Here, we document petrographic, rare earth element + yttrium (REE + Y), carbonate clumped isotope temperature (TΔ47), and carbon-isotopic evidence from a shale succession in southern Tibet spanning a ~28 m.y. interval (ca. 113–85 Ma) that coincided with the mid-Cretaceous greenhouse event. At least nine limestone nodule-bearing horizons exhibit seep-associated sedimentary structures, including carbonate fabrics (e.g., micritic crusts, crystal fans, and botryoidal textures) and 13C-depleted isotopic compositions (δ13Ccarb < –32.3‰), which are indicative of methane-derived carbon sources. Along with sedimentary evidence, the patterns of TΔ47–δ13Ccarb–δ18Ocarb support precipitation of these MDCs over a large temperature range. The REE + Y compositions and europium (Eu) anomalies indicate that the release of methane was associated with hydrothermal fluids. Methane may have been derived from both thermogenic and biogenic sources based on the inorganic carbon isotopic signatures of the carbonate. These nodular carbonate horizons document multiple episodes of seafloor methane release during the mid-Cretaceous and represent exceptionally long-lived, active methane seepage. Massive methane release events may have played a role in generating the greenhouse climate of the mid-Cretaceous.

Wetlands play a crucial role in the carbon cycle as they are the largest natural source of methane, a potent greenhouse gas. Changes in wetland hydrology can alter the rate of greenhouse gas release from wetlands and have the potential to alter Earth’s carbon budget. However, the microbial dynamics underpinning these observations are poorly constrained. Here we combine monitoring of environmental parameters and greenhouse gas fluxes with monthly records of microbial phospholipid fatty acid (PLFAs) δ13C values to probe changes in microbial community and biogeochemistry in response to hydrological changes in a monsoon influenced subtropical wetland from central China. Our results show that water table depth is a key factor controlling the microbial community structure, with Gram-negative bacteria and actinobacteria increasing and fungi decreasing during dry and low water table periods. Meanwhile, the δ13C values of specific PLFAs decreased up to 12‰ during dry compared to wet periods. The extent of depletion varied, but PLFAs from Gram-negative bacteria were most depleted in 13C, indicative for a rapid increase in methanotrophy (methane consumption) during these dry periods. Furthermore, the methane emission of the wetland was drastically reduced and even had negative flux values during dry periods, suggesting that the increased methanotrophy led to a reduced methane flux and a temporary shift of the wetland from a methane source to a methane sink. Our results indicate that short-term hydrological variations lead to a rapid response in microbial community and carbon metabolic activity that directly influences wetland carbon dynamics and greenhouse gas emissions.

3-Hydroxy fatty acids (3-OH-FAs), derived from Gram-negative bacterial outer membranes, have received recent attention for their potential as new terrestrial pH and temperature proxies for palaeoclimate studies. Initial studies from altitudinal transects of contemporary soils – correlating bacterial 3-OH-FA compositions to air temperature and pH – have shown promising results. But the geographical extent of recent calibrations is limited. In this study, we analyse 3-OH-FA lipid distributions in 186 globally distributed soil samples to study the environmental factors controlling the relative distribution of the 3-OH-FA isomers. Our sample-set covers a wide range of temperatures (−0.4 to 27 °C) and pH (3.6–9.2). For the global compilation we find that the ratio of anteiso to normal 3-OH-FAs of the C15 or C17 homologues (RAN15 or RAN17) shows a strong linear relationship with mean annual air temperature (MAAT) (R2 = 0.48, p

The policy and practice of ecological restoration and conservation in China obtained some remarkable results. For example, Sphagnum moss growing on abandoned farmland, which was peatland before agricultural use, has rapidly expanded the wetland area in SW China. Microorganisms such as testate amoebae are sensitive to environmental change and thus have been widely used as ecological indicators in various habitats. We analyzed differently aged Sphagnum growing plots on a Sphagnum growing farmland and natural Sphagnum plots in SW China to examine how Sphagnum-dwelling testate amoeba communities and corresponding protozoic silicon (Si) pools respond to ecological restoration practice. We found that abundance, taxon richness, and diversity of testate amoebae were higher in Sphagnum growing farmland plots compared to natural Sphagnum plots. Protozoic Si pools showed an increase with Sphagnum growing time representing increased Si accumulation by idiosomic testate amoeba shells. However, protozoic Si pools were negatively correlated with taxon richness and diversity of testate amoebae. Our results showed that (i) natural Sphagnum plots were not characterized by the expected higher biodiversity of testate amoebae compared to Sphagnum growing plots and (ii) consequently protozoic Si pool quantity in natural Sphagnum plots was less driven by biodiversity of testate amoebae than expected. We concluded our results to underline the value of (i) environmental restoration policy in general and (ii) testate amoeba communities and corresponding protozoic Si pools for Si cycling in restoration areas of peatlands in particular. Based on our results, we recommend a sustainable cultivation of Sphagnum moss and an additional establishment of protected areas, where no Sphagnum harvesting occurs. These protected Sphagnum areas might represent hot spots of undisturbed testate amoeba communities and corresponding protozoic Si pools and thus of microbial Si cycling.

Enhanced regional subduction-related volcanism in the South China craton concurrent with Siberian Traps large igneous province magmatism was a likely contributor to major biotic and environmental stresses associated with the Permian-Triassic boundary (ca. 252 Ma) mass extinction. However, the timing, intensity, and duration of this regional volcanic activity remain uncertain. We analyzed mercury (Hg) concentrations in three widely separated marine sections in the South China craton (Shangsi, Ganxi, and Chaohu) as well as Hg isotopic compositions in one section (Shangsi) from the Upper Permian (Changhsingian) through the lowermost Triassic (Induan) in order to track volcanic inputs. Four mercury enrichment (ME) intervals, dating to the lowermost Changhsingian (ME1), mid–Clarkina changxingensis zone (ME2), upper C. changxingensis to lower C. yini zones (ME3), and latest Permian mass extinction (LPME) interval (ME4), were recognized on the basis of elevated Hg/total organic carbon ratios. These records provide evidence of strong volcanism in the Tethyan region starting ∼2 m.y. before the LPME, whereas only the ME4 event is recorded in extra-Tethyan sections. Mercury isotopes support the inference that pre-LPME Hg peaks were related to regional subduction-related volcanism, and that Hg emissions at the LPME were the result of Siberian Traps large igneous province intrusions into organic-rich sediments. This study demonstrates the feasibility of distinguishing flood-basalt from subduction-related volcanic inputs on the basis of marine sedimentary Hg records.

Late Devonian marine systems were characterized by major environmental perturbations and associated biotic community changes linked to climate change and widespread oceanic anoxia. Here, we provide high-resolution lipid biomarker chemostratigraphic records from the Upper Devonian Chattanooga Shale (Tennessee, USA) to investigate algal-microbial community changes in the southern Illinois Basin that were related to contemporaneous shifts in marine redox (as proxied by trace metals, Fe-species, and Corg/P) and salinity conditions (as proxied by B/Ga, Sr/Ba, and S/total organic carbon). The Frasnian was characterized by dominantly bacterial lipids (high hopane/sterane), near-marine salinity, and a shift from oxic to increasingly reducing conditions in response to increasing organic carbon sinking fluxes. Aryl isoprenoids and aryl isoprenoid ratios reveal that the O2-H2S chemocline was unstable and intermittently shallow (i.e., within the photic zone). The Frasnian-Famennian boundary was marked by a shift in microalgal community composition toward green algal (e.g., prasinophyte) dominance (lower C27 and higher C28 and C29 steranes), a sharp reduction in watermass salinity, and a stable O2-H2S chemocline below the photic zone, conditions that persisted until nearly the end of the Famennian. We infer that changing watermass conditions, especially a sharp reduction in salinity to possibly low-brackish conditions (

Glycerol dialkyl glycerol tetraethers (GDGTs) are widespread but unique membrane23 spanning lipids of many Archaea and some Bacteria. However, their specific biological sources and the associated environmental controls on their distribution remain unclear, especially in lacustrine settings, hindering our understanding of these compounds and their application as environmental proxies. Here we investigated the GDGT distributions across a large bottom water dissolved oxygen (DO) gradient (0.10-7.20 28 mg/L) using surface sediments of Lake Yangzonghai, a warm monomictic lake in southwestern China. We show that the distributions of both branched (br) and isoprenoidal (iso) GDGTs co-vary significantly with bottom water DO concentration. The relative abundances of tetramethylated brGDGTs, 5-methyl penta- and hexamethylated brGDGTs and isoGDGT-0 show a stepwise increase as bottom water DO concentration decreases. On the other hand, the relative abundances of 6-methyl penta- and hexamethylated brGDGTs and crenarchaeol exhibit a stepwise decline with a decrease in bottom water DO concentration. Genetic data indicate these DO-induced changes in GDGT parameters are related to changes in the bacterial and archaeal communities across the oxycline in the lake. For example, the high abundance of isoGDGT-0 in low DO samples coincides with a high abundance of methanogenic archaea and Bathyarchaeota. We propose that brGDGT-producing bacteria might include a diversity of other phyla in addition to the proposed source organism acidobacteria; different groups of anaerobic and aerobic bacteria are likely to contribute to the increased abundance of 5-methyl brGDGTs and 6-methyl brGDGTs in low and high DO zones, respectively. Consequently, the MBT’ and MBT’6ME brGDGT indices display strong correlations with DO concentration. Importantly, the MBT’5ME index is not significantly influenced by changes in DO concentration, suggesting that this index might be more resilient to these impacts and more suitable to reconstruct temperature in lake systems.

The Siberian Traps large igneous province (STLIP) was the likely trigger for the ca. 252 Ma latest Permian mass extinction (LPME), but direct evidence for global volcanic effects on land remains rare. Here, we used mercury (Hg) enrichments, a proxy for ancient volcanic activity, to assess volcanic inputs to two terrestrial Permian-Triassic boundary (PTB) sections that were separated by thousands of kilometers and represent different latitudinal zones—the peri-equatorial Lubei section (South China craton) and the high-latitude (40–60°N) Dalongkou section (Junggar terrane). Both sections exhibit strong Hg enrichment within a discrete (≤40 m) stratigraphic window representing the LPME. At Lubei, this interval is also characterized by negative mass-independent fractionation (MIF) of odd Hg isotopes, consistent with massive volcanogenic and/or terrestrial Hg inputs. These findings are significant in documenting Hg spikes and negative MIF excursions near the PTB in terrestrial sections for the first time, providing evidence of the global influence of the STLIP, as well as in demonstrating at high stratigraphic resolution its synchronicity with the PTB negative carbon-isotope excursion (CIE), supporting a common global cause for these anomalies.

Studies of acid mine drainage (AMD), a typically extremely acidic environment, can provide opportunities to determine how microorganisms adapt to low pH environments by regulating membrane lipid compositions. In this study, to gain an understanding of microbial pH adaptation mechanisms, we investigated the distribution of lipid biomarkers, primarily of microbial origin, in an AMD site at Dexing copper mine, Jiangxi, China. Microbial lipid biomarkers, including glycerol dialkyl glycerol tetraethers (GDGTs), monoalkyl glycerol ethers, fatty acids (FAs), 3-hydroxy fatty acids (3-OH-FAs), sterols, and hopanoids, were detected in surface and 10-cm-deep core sediments. Branched FAs and 3-OH-FAs were found to occur at low levels in the extremely low pH conditions. Similarly, there were fewer methyl moieties in branched GDGTs detected in the AMD, as reflected by the lower methylation degree of branched tetraethers, indicating that a reduction in membrane lipid methyl moieties may be a common strategy whereby bacteria adapt to low pH environments. The presence of only small amounts of crenarchaeol in the AMD suggests that the extremely acidic conditions are inimical to the growth of Thaumarchaeota. In contrast, abundant Euryarchaeota inhabit the AMD and produce an isoprenoid GDGT (isoGDGT) profile dominated by GDGT-2, along with GDGT-5 and GDGT-6, which are rarely detected in non-extreme environments. Furthermore, the GDGT-2/(GDGT-1 + GDGT-3) ratio in the AMD was found to be significantly higher than that in soils and was negatively correlated with pH. These findings indicate that increases in the cyclopentyl rings of isoGDGTs may be favorable for maintaining an osmotic balance across archaeal cell membranes. All the modifications adopted by bacteria and archaea result in more condensed cell membranes.

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.

Bacterial branched and archaeal isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs, respectively) have become important tools for the paleoclimate reconstruction of lacustrine systems. However, it is still unclear how geochemical proxies based on GDGTs, particularly in lake surface sediments, respond to seasonal environmental changes in shallow water lakes, whose sediment cores are increasingly being used for paleoenvironmental reconstruction. In this study, we collected samples from the top 0–2 cm of sediments and water column suspended particulate matter (SPM) at a monthly interval from June 2015 to May 2016 in Lake Donghu, a shallow lake in central China. An updated liquid chromatography method was used to separate the 5- and 6-methyl brGDGTs. A remarkable difference between the lake sediments, SPM, and catchment soils suggested that microorganisms inhabiting the surface sediments contributed a significant proportion of isoGDGTs and brGDGTs to the GDGT pool in the lake sediments. We found a clear seasonality in the concentration and distribution of core lipid (CL) isoGDGTs and brGDGTs in the lake surface sediments, in contrast to the absence of seasonality in brGDGTs in mid-latitude soils reported by previous studies. The intact polar lipid/CL ratios for both isoGDGTs and brGDGTs in the lake surface sediments were significantly higher than those in SPM and catchment soils and were higher in the warm seasons than in winter. This collectively suggested that the turnover of brGDGTs in the lake sediments was faster than that in soils. Furthermore, the GDGT-0/Cren ratio for the lake sediments exhibited a negative correlation with dissolved oxygen in the water column. The cyclisation degree of C5-methylated brGDGTs (CBT5ME) was strongly related to water pH. Seasonal temperature variation clearly impacted the methylation degree of C6-methylated brGDGTs (MBT′6ME) but had no influence on C5-methylated brGDGTs (MBT′5ME), as opposed to the dependence of MBT′5ME on temperature in global soils, peatlands and African lakes. Due to the fast turnover rate and the sensitive response to seasonality, we suggest that the seasonal variation of fossil GDGTs in lake sediments should be considered when developing a calibration using fossil GDGTs in shallow-water lake surface sediments.

Lipids can survive long geological intervals within sediments and provide a unique tool that allows the reconstruction of past organismic diversity and environmental conditions. The lipids discussed in this review include all substances produced by organisms that are insoluble in water but extractable by organic solvents. Lipid biomarkers refer to both functionalized biolipids as well as their hydrocarbon derivatives in geological materials that contain diverse information about biotic sources and environmental conditions. In spite of diagenetic and catagenetic alteration, lipid biomarkers commonly preserve the hydrocarbon structure of their biotic counterparts and have been found in rocks up to 1.6 billion years in age. These features have promoted the use of lipid biomarkers in many fields, including petroleum geology, paleoclimatology, oceanography, meteorology, geobiology and environmental science. Here, we (i) review the use of lipid biomarker records for the reconstruction of environmental conditions in deep time, including climatic conditions (temperature), sedimentary environments (redox, salinity and chemical composition) as well as catastrophic terrestrial events (soil erosion and wildfire), and (ii) generate new insights into environmental perturbations during the Permian-Triassic transition based on investigation of lipid biomarkers. We further propose that the ratio of dibenzothiophene to phenanthrene (DBT/P) in marine carbonates may be a robust proxy for seawater sulfate concentrations in deep time. Our compiled DBT/P records show substantial variations in seawater sulfate levels through Earth history that are consistent with the results of other proxies. We discuss the future outlook for application of lipid biomarker records to deep-time environmental research.

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. δ13CSOM at Weinan ranges from −23.6 to −18.0‰ and δ15NSOM ranges from +3.4 to +5.5‰. Modeling based on the δ13CSOM 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 δ13CSOM and δ15NSOM profiles are strongly correlated (r = +0.82; p(α)

Recent time-series from sediment traps show abnormally high chlorophyll-a concentrations and primary productivity in the oligotrophic central South China Sea (SCS), especially during wintertime. Here we present new insights from compound-specific hydrogen isotopic analysis of leaf wax n-alkanes and Sr-Nd isotope compositions extracted from four basin-wide surface sediment transects. We find that the deepest surface sediments in the central basin contain the most depleted n-alkane hydrogen isotopes, suggesting inputs from higher latitude soils in northern China. This is supported by the Sr-Nd isotope compositions of the same surface sediments. We propose that aeolian dust is transported by the winter monsoon and might fertilize the phytoplankton bloom in the central SCS. This process may have been enhanced in ancient times when the winter monsoon was stronger, driving both vertical mixing and dust transport to the central basin.

Summary Here, we explore the use of bacterial and archaeal lipid carbon and hydrogen isotopic compositions to explore carbon cycling processes in modern settings and in response to ancient warming events. Investigations of Holocene peat and Eocene lignites reveal both stability over long time-scales but rapid, transient responses to climate perturbations.

Summary we separate the air temperature and hydrological records extending back six million years (Ma) on the basis of lipid biomarkers preserved in a terrestrial drill core (XA3 drill core) in North China, East Asia.

Summary 3-Hydroxy fatty acids (3-OH-FAs), derived from Gram-negative bacterial outer membranes, have received recent attention for their potential as both: a) terrestrial temperature and pH palaeoclimate proxies; and b) marine sea-surface temperature proxies. However, the distribution and performance of 3-OH-FA based proxies at a global scale and in lake sediments remains largely unknown. This is a critical gap in knowledge as lake sediments are the most common terrestrial archive for paleoclimate. Here we explore (including the application of machine learning tools) 3-OH-FA lipid distributions in 186 globally distributed soils. We also investigate co-located lake sediments and catchment soils in China and the USA. We find that the ratio of anteiso to normal 3-OH-FAs of the C15 or C17 homologues (RAN15 or RAN17) shows a significant linear relationship with mean annual air temperature (MAAT) in global soils. However, in lakes these do not appear to be suitable for temperature calibration. Analysis of gene community composition reveals that Gram-negative bacterial populations from lake sediments are distinct from both soils and marine sediments (but are closer to the former). We find strong relationships between alternative 3-OH-FA based proxies and MAAT in lakes. Thus 3-OH-FA geochemical tools continue to show promise for palaeoclimate reconstruction.