Shucheng Xie

Two episodes of microbial change coupled with Permo/Triassic faunal mass extinction

Microbial expansion following faunal mass extinctions in Earth history can be studied by petrographic examination of microbialites (microbial crusts) or well-preserved organic-walled microbes. However, where preservation is poor, quantification of microbial communities can be problematic. We have circumvented this problem by adopting a lipid biomarker-based approach to evaluate microbial community changes across the Permo/Triassic (P/Tr) boundary at Meishan in South China. We present here a biomarker stratigraphic record showing episodic microbial changes coupled with a high-resolution record of invertebrate mass extinction. Variation in the microbial community structure is characterized by the 2-methylhopane (2-MHP) index (a ratio of the abundance of cyanobacterial biomarkers to more general bacterial biomarkers). Two episodes of faunal mass extinction were each preceded by minima in the 2-MHP index, followed by strong maxima, likely reflecting microbial responses to the catastrophic events that caused the extinction and initiated ecosystem changes. Hence, both cyanobacterial biomarker and invertebrate fossil records provide evidence for two episodes of biotic crisis across the P/Tr boundary.

n-Alkane distributions in ombrotrophic mires as indicators of vegetation change related to climatic variation

Lipid analysis of modern bog vegetation revealed n-alkane distributions for Sphagnum species displaying enhanced abundances of lower chain length homologues (C21–C25). Other plants examined revealed typical higher plant distributions (C29 or C31 maxima). Investigation of a 40 cm peat profile from Bolton Fell Moss, Cumbria, UK showed a varying abundance of the n-C23 homologue down core which appear to be related to vegetation changes, which are presumed to occur as a result of climate variation.

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.

Palaeoclimate records in compound-specific δD values of a lipid biomarker in ombrotrophic peat

Compound-specific δD values recorded by means of gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS) of the biomarker n-alkane (n-tricosane; n-C23) representative of the dominant Sphagnum species in a 40 cm peat profile from Bolton Fell Moss, Cumbria, UK, correlate with vegetation changes in the past >200 years (age depth model based on 210Pb dating). The bog vegetation is sensitive to climate change correlating with the global scale cooler period of the later 19th and early 20th centuries. The correlation with meteorological records suggests compound-specific δD values of lipid biomarkers have potential for use as a new climate proxy.

Microbialites and global environmental change across the Permian-Triassic boundary: a synthesis

Permian-Triassic boundary microbialites (PTBMs) are thin (0.05-15 m) carbonates formed after the end-Permian mass extinction. They comprise Renalcis-group calcimicrobes, microbially mediated micrite, presumed inorganic micrite, calcite cement (some may be microbially influenced) and shelly faunas. PTBMs are abundant in low-latitude shallow-marine carbonate shelves in central Tethyan continents but are rare in higher latitudes, likely inhibited by clastic supply on Pangaea margins. PTBMs occupied broadly similar environments to Late Permian reefs in Tethys, but extended into deeper waters. Late Permian reefs are also rich in microbes (and cements), so post-extinction seawater carbonate saturation was likely similar to the Late Permian. However, PTBMs lack widespread abundant inorganic carbonate cement fans, so a previous interpretation that anoxic bicarbonate-rich water upwelled to rapidly increase carbonate saturation of shallow seawater, post-extinction, is problematic. Preliminary pyrite framboid evidence shows anoxia in PTBM facies, but interbedded shelly faunas indicate oxygenated water, perhaps there was short-term pulsing of normally saturated anoxic water from the oxygen-minimum zone to surface waters. In Tethys, PTBMs show geographic variations: (i) in south China, PTBMs are mostly thrombolites in open shelf settings, largely recrystallised, with remnant structure of Renalcis-group calcimicrobes; (ii) in south Turkey, in shallow waters, stromatolites and thrombolites, lacking calcimicrobes, are interbedded, likely depth-controlled; and (iii) in the Middle East, especially Iran, stromatolites and thrombolites (calcimicrobes uncommon) occur in different sites on open shelves, where controls are unclear. Thus, PTBMs were under more complex control than previously portrayed, with local facies control playing a significant role in their structure and composition.

Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo-Triassic biotic crisis

Widespread but episodic deposition of cyanobacterial mats occurred during the 5 m.y. Permo-Triassic biotic crisis, providing a critical record to decipher the long-term stressful environmental conditions and thus a linkage with the killer. Here we document the timing and duration of these blooms, indicated by lipid biomarkers and microbialites, throughout South China and compare them to the timing of volcanic activity. The initial cyanobacterial bloom has the longest duration and is associated with a prominent Eu anomaly in southwestern China, where the most intensive volcanism has been proposed to occur, suggesting a causal relationship. Subsequent cyanobacterial blooms in the Early Triassic are associated with known volcanic events in South China and also with the most voluminous Siberian flood basalts, and with the largest negative carbon isotope excursions. Thus, it appears that volcanic activity has had a significant impact on microbial development in South China. South China volcanism around the Permian-Triassic boundary (PTB) could have played a much larger role in biotic changes of both bacterial and calcified faunal ecosystems than previously thought. The well-known Siberian volcanism is proposed herein, on the basis of the current compilation of dating data, to protract the Early Triassic faunal recovery rather than to cause the end-Permian extinction.

Microbial lipid records of highly alkaline deposits and enhanced aridity associated with significant uplift of the Tibetan Plateau in the Late Miocene

Saline alkaline sediments and soils are widespread in arid and semiarid regions, but their occurrence in ancient dry periods remains unknown due to the lack of a suitable proxy. On the basis of investigations of modern Chinese soils with a wide pH range of 3.5–9.1, we suggest that the microbial lipid ratio R i/b , i.e., the abundance ratio of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) to bacterial branched GDGTs, indicates the presence of drought-induced alkaline deposits in terrestrial settings. The R i/b is invariant in modern soils with pH 600 mm, but it increases sharply at higher pH values and lower mean annual precipitation ( i/b ratio in a fluviolacustrine section in the Zhada basin of the southwestern Tibetan Plateau, covering the time period 9.2–2.6 m.y. ago. The R i/b ratio remains relatively stable in most intervals but exhibits maxima in some horizons, indicative of the occurrence of severe drought and alkaline deposits in the basin catchment. These occur in fluvial sediments deposited 9 m.y. ago, a critical time with respect to the intensification of the East Asian and Indian monsoons, and the significant uplift of the plateau that has previously been associated with enhanced aridity in Central Asia.

Rapid oxygenation of Earth’s atmosphere 2.33 billion years ago

Continuous multiple sulfur isotope profiles from South African rocks pinpoint the Great Oxygenation Event in the geologic record. Molecular oxygen (O2) is, and has been, a primary driver of biological evolution and shapes the contemporary landscape of Earth’s biogeochemical cycles. Although “whiffs” of oxygen have been documented in the Archean atmosphere, substantial O2 did not accumulate irreversibly until the Early Paleoproterozoic, during what has been termed the Great Oxygenation Event (GOE). The timing of the GOE and the rate at which this oxygenation took place have been poorly constrained until now. We report the transition (that is, from being mass-independent to becoming mass-dependent) in multiple sulfur isotope signals of diagenetic pyrite in a continuous sedimentary sequence in three coeval drill cores in the Transvaal Supergroup, South Africa. These data precisely constrain the GOE to 2.33 billion years ago. The new data suggest that the oxygenation occurred rapidly—within 1 to 10 million years—and was followed by a slower rise in the ocean sulfate inventory. Our data indicate that a climate perturbation predated the GOE, whereas the relationships among GOE, “Snowball Earth” glaciation, and biogeochemical cycling will require further stratigraphic correlation supported with precise chronologies and paleolatitude reconstructions.

Lipid distributions in loess-paleosol sequences from northwest China

Molecular stratigraphic analyses using GC/MS have been performed on the top section of the Jiuzhoutai loess-paleosol sequences near Lanzhou in northwest China, with a record extending from the second-last interglacial through the present interglacial. The various molecular fossils identified from the second-order loess and paleosol layers include C14–C35n-alkanes, C8–C30n-alkanoic acids, C12–C30n-alkanols and C23–C33n-alkan-2-ones. The CPI (carbon preference index) values and the ratio of C29/C31n-alkan-2-ones display alternations between loess deposits and paleosols. They show a significant correlation with the loess magnetic susceptibility, an indicator of the East Asian summer monsoon. The observed variations of the ketone indices in relation to the change of lithology are proposed to result from microbial re-working of higher plant inputs in the paleosols.

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.