Genming Luo

Climate warming in the latest Permian and the Permian-Triassic mass extinction

High-resolution oxygen isotope records document the timing and magnitude of global warming across the Permian-Triassic (P-Tr) boundary. Oxygen isotope ratios measured on phosphate-bound oxygen in conodont apatite from the Meishan and Shangsi sections (South China) decrease by 2‰ in the latest Permian, translating into low-latitude surface water warming of 8 °C. The oxygen isotope shift coincides with the negative shift in carbon isotope ratios of carbonates, suggesting that the addition of isotopically light carbon to the ocean-atmosphere system by Siberian Traps volcanism and related processes resulted in higher greenhouse gas levels and global warming. The major temperature rise started immediately before the main extinction phase, with maximum and harmful temperatures documented in the latest Permian (Meishan: bed 27). The coincidence of climate warming and the main pulse of extinction suggest that global warming was one of the causes of the collapse of the marine and terrestrial ecosystems. In addition, very warm climate conditions in the Early Triassic may have played a major role in the delayed recovery in the aftermath of the Permian-Triassic crisis.

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.

Isotopic Evidence for Massive Oxidation of Organic Matter Following the Great Oxidation Event

Analysis of two-billion-year-old rocks reveals an extreme carbon-cycle disruption after atmospheric oxygen increased. The stable isotope record of marine carbon indicates that the Proterozoic Eon began and ended with extreme fluctuations in the carbon cycle. In both the Paleoproterozoic [2500 to 1600 million years ago (Ma)] and Neoproterozoic (1000 to 542 Ma), extended intervals of anomalously high carbon isotope ratios (δ13C) indicate high rates of organic matter burial and release of oxygen to the atmosphere; in the Neoproterozoic, the high δ13C interval was punctuated by abrupt swings to low δ13C, indicating massive oxidation of organic matter. We report a Paleoproterozoic negative δ13C excursion that is similar in magnitude and apparent duration to the Neoproterozoic anomaly. This Shunga-Francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen-rich atmosphere.

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.

Enhanced nitrogen fixation in the immediate aftermath of the latest Permian marine mass extinction

The carbon (δ 13 C org ) and nitrogen (δ 15 N) isotopic compositions of bulk organic matter were analyzed in two high-resolution Permian-Triassic transitional sections containing microbialite in south China. The results from these shallow-marine sections show that an abrupt negative shift in δ 15 N, from ∼+3‰ to ∼0‰, occurred immediately after the latest Permian mass extinction (LPE) in both sections, concurrent with a distinct negative shift in δ 13 C org . The persistently low values of δ 15 N suggest that, following the LPE, microbial nitrogen fixation became the main source of biologically available nitrogen in the Nanpanjiang Basin and perhaps over a broader region of the eastern Paleotethys Ocean. Enhanced N fixation is probably indicative of the prevalence of stratified anoxic water masses characterized by intense denitrification and/or anaerobic ammonium oxidation at the time. Perturbation of the marine nitrogen cycle might have contributed to high temperatures following the main marine mass extinction through the release of the greenhouse gas N 2 O. The sharp declines in δ 15 N and δ 13 C org may be ascribed to an abrupt change in shallow-water microbial communities, which differed in composition from contemporaneous deep-water communities.

Microbial response to limited nutrients in shallow water immediately after the end-Permian mass extinction.

Previous work indicates that a variety of microbes bloomed in the oceans after the end-Permian faunal mass extinction, but evidence is sporadically documented. Thus, the nature and geographic distribution of such microbes and their associations are unclear, addressed in this study using a series of biomarker groups. On the basis of microbial biomarker records of the 2-methylhopane index, evidence is presented for cyanobacterial blooms in both the western and eastern Tethys Sea and in both shallow and deep waters, after the mass extinction. The enhanced relative abundance of C(28) (expressed by the C(28) /C(29) ratio of) regular steranes suggests a bloom of prasinophyte algae occurred immediately after the end-Permian faunal extinction, comparable with those observed in some other mass extinctions in Phanerozoic. Significantly, cyanobacteria and prasinophyte algae show a synchronized onset of bloom in the shallow water Bulla section, north Italy, inferring for the first time their coupled response to the biotic crisis and the associated environmental conditions. However, in Meishan of Zhejiang Province in south China, the bloom declined earlier than in Bulla. The association of increased 2-methylhopane index with a negative shift in the nitrogen isotope composition infers a scenario of enhanced nitrogen fixation by cyanobacteria immediately after the faunal mass extinction. N(2) fixation by cyanobacteria is here interpreted to have provided prasinophyte algae with ammonium in nutrient-limited shallow waters, and thus caused their associated blooms.

No evidence for high atmospheric oxygen levels 1,400 million years ago

Zhang et al. (1) recently proposed atmospheric oxygen levels of ∼4% present atmospheric levels (PAL) based on modeling a paleoenvironment reconstructed from trace metal and biomarker data from the 1,400 Ma Xiamaling Formation in China. Intriguingly, this pO2 level is above the threshold oxygen requirements of basal animals and clashes with evidence for atmospheric oxygen levels <<1% PAL in the mid-Proterozoic (2). However, there are fundamental problems with the inorganic and organic geochemical work presented by Zhang et al. (1).

Uncovering the spatial heterogeneity of Ediacaran carbon cycling

Records of the Ediacaran carbon cycle (635-541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to -12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13 C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13 Ccarb and δ13 Corg ) records from the Ediacaran Doushantuo Formation along a proximal-to-distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13 Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34 S of carbonate-associated sulfate (CAS) and increased 87 Sr/86 Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13 Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near-surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13 C presented in this study.

Prospects for Sterane Preservation in Sponge Fossils from Museum Collections and the Utility of Sponge Biomarkers for Molecular Clocks

Abstract The sponge biomarker hypothesis argues that 24-isopropylcholestanes preserved in Neoproterozoic-age rocks are “molecular fossils” left behind by marine sponges. Despite genetic and geologic support for this hypothesis, 24-isopropylcholestane has never been reported from a sponge body fossil. This lack of direct evidence regarding the source of sponge biomarkers through deep time leaves unanswered questions, such as whether their biosynthesis evolved once in sponges or multiple times across different lineages. In this study, we analyzed 10 sponge fossils from the Yale Peabody Museum of Natural History collections in pursuit of evidence of sterane biomarkers. We failed to recover 24-isopropylcholestane and instead found a near-identical sterane profile across all samples. This result indicates a combination of little to no sterane preservation in the fossils themselves, coupled with anthropogenic hydrocarbon contamination during their collection and storage. However, signals from bacterial biomarkers (hopanes) were more diverse across samples and consistent with a priori expectations, meaning that we cannot rule out the possibility that at least part of the hydrocarbon signal is syngenetic. We suggest that future attempts to extract biomarker hydrocarbons from sponge fossils be performed on freshly collected and specially prepared field samples. Despite the fact that demosponges or their ancestors still present the most likely source of Neoproterozoic 24-isopropylcholestanes, multiple evolutionary scenarios are consistent with current genetic and biomarker evidence: the “sponge biomarker” could represent an evolutionary novelty in demosponges, or a trait that evolved deeper in the animal tree. We therefore continue to affirm the validity of the sponge biomarker hypothesis but caution against using Neoproterozoic 24-isopropylcholestanes as a calibration point for sponges in molecular clocks. Instead, we recommend using it as a reference point for comparison, as scenarios where crowngroup demosponges radiate after the Neoproterozoic remain inconsistent with the geologic record.

Late inception of a resiliently oxygenated upper ocean

The rise of oxygen To understand the evolution of the biosphere, we need to know how much oxygen was present in Earths atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O2 occurred at around 400 million years ago and that O2 levels underwent a step change to near-modern values around 200 million years ago. Science, this issue p. 174 The I/Ca ratio in marine carbonates tracks atmospheric oxygen levels for the past 2.5 billion years. Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.