Global and Planetary Change最新文献

Despite the economic and scientific importance of Precambrian phosphorites, our understanding of the mechanism leading to their formation remains limited, including for the largest phosphogenic episode in the late Neoproterozoic. To improve our understanding of Precambrian phosphorite formation, we combined sedimentology, petrography, and elemental, and Fe-C isotopic analyses to study the two main phosphorite beds (the lower and upper phosphorite beds) in the Ediacaran Doushantuo Formation, Zhangcunping area, South China. The phosphorites consist mainly of granular textures characterized by densely packed grains, some of which are coated with secondary phosphate growth. However, there are notable differences in the mineralogy, microfossil assemblages, and elemental contents of the two beds. The lower phosphorites have no Ce anomaly, and relatively low Y/Ho ratios and positive δ⁵⁶Fe values (0.04–0.30 ‰, average of 0.19 ‰). In contrast, the upper phosphorites have negative Ce anomalies and higher Y/Ho ratios and near-zero δ⁵⁶Fe values (−0.29–0.19 ‰ (average of −0.01 ‰). These observations suggest that the lower phosphorites formed in anoxic-suboxic environments, whereas the upper phosphorites formed in relatively oxygenated environments. The δ¹³C_carb values of the phosphorites range from −3.97 ‰ to 1.71 ‰ (average of −1.56 ‰), and are lighter than values in dolostones (−0.52 ‰ to 4.39 ‰, average of 2.02 ‰). This suggesting that formation of the Doushantuo phosphorites was influenced by degradation of organic matter in an ocean with high primary productivity. The lower phosphorites, which were also regulated by Fe redox pumping, have positive δ⁵⁶Fe values, along with the presence of pyrite framboids and iron oxides, suggesting deposition near the Fe-redox boundary where extensive Fe cycling. The upper phosphorites show positive correlations between Mn and Fe, and Mn/Fe and P₂O₅, suggesting formation near the Mn boundary with extensive Mn cycling, primarily mediated by Mn redox pumping. Sedimentological observation indicate that primary phosphates were concentrated into granular phosphorites by winnowing processes following primary precipitation. Accordingly, we propose a refined model for Precambrian phosphorite formation in which degradation of organic matter, Fe and Mn pumping, and physical reworking of deposits co-evolve and interact within a dynamic Precambrian redox environment. Our model provides a reasonable explanation for the distribution of global phosphorite deposits throughout geological history.

The increasing frequency of extreme Heat Waves (HWs) has generated significant societal impacts in recent years. This study used different observational datasets to investigate the HW characteristics over India during the post-El Niño spring and early summer seasons (April to June; AMJ). Analysis suggests that HW days are more prevalent over India, predominantly increased in south-central and northwest India, during the decaying phase of strong El Niño years. It is found that anomalous anticyclone circulation accompanied by high pressure extending from the Western North Pacific region towards the Bay of Bengal and India is responsible for enhanced HW days and intensity during the AMJ of strong El Niño decay years. This anomalous anticyclone-induced downdraft reduces the specific humidity in the lower troposphere, leading to decreased cloud cover over India. As a result, shortwave radiation is enhanced at the surface, which causes abnormal HWs over India. During the decaying phase of strong El Niño years, the HW days over India contributed to an increase in the frequency of Discomfort Index hours (above 28 °C), maximum temperatures exceeding 40 °C (hours per day), and Universal Thermal Climate Index days above 38 °C and 46 °C during the spring and early summer months, especially in the East Coast, central, and northwestern parts of India. Thus, proper prediction of large-scale atmospheric circulation over the Indo-western Pacific region during El Niño can help to predict the HW conditions three months in advance. This would help to implement suitable adaptation measures and put into practice strong mitigation policies to limit the increased risk of such events during AMJ of El Niño decay years.

We investigated the reproducibility of the observed seasonal march of the East Asian summer monsoon (EASM) in the climate models participating in Phase 6 of the Coupled Model Inter-comparison Project (CMIP6). Overall, the multi-model ensemble of 24 CMIP6 models captured the major characteristics of the seasonal march of the monsoon, but large intermodel diversity was seen. Most of the models simulated a much weaker pre-summer rainy season over South China in May, with the main rainband shifting north. We paid special attention to the Meiyu season in June and the North China rainy season in July, which varied greatly among individual models. The diversity of the seasonal march of the monsoon from June to July in the CMIP6 models is largely modulated by the simulated evolution of the western North Pacific subtropical high (WNPSH), which is closely tied to the sea surface temperature in both the western North Pacific and tropical Atlantic Ocean. Weaker warming in the western North Pacific and stronger warming in the tropical Atlantic favors strong air-sea interaction and the resultant realistic WNPSH, which brings more water vapor to support abundant rainfall, thus resulting in a more realistic seasonal march of the EASM rainfall in the CMIP6 models.

The Indian Monsoon profoundly impacts the climate, agriculture, and economy of the Indian subcontinent and southwestern China. However, the long-term variability of the Indian summer monsoon (ISM) and its forcing mechanism in the context of the stepwise Pleistocene cooling trend are poorly understood, due to the lack of high-quality terrestrial paleoclimate records. Quaternary loess and paleosol deposits in SW China provide the opportunity to study the long-term evolution of the ISM. Based on analyses of magnetic properties and chemical weathering indices, we found that decreased chemical weathering and pedogenic intensity occurred in SW China since the Middle Pleistocene. Comparison of our results with published paleoclimate records from the Indian Ocean and SW China suggest that substantial ISM weakening occurred after ∼0.7–0.6 Ma, coincident with the Mid-Pleistocene climatic transition. We conclude that decreasing global surface temperatures associated with the major growth of Northern Hemisphere ice sheets caused the Middle Pleistocene weakening of the ISM, leading to changes in precipitation regimes in the southern Asian continent and SW China. Our results suggest that global cooling is strongly linked with the South Asian monsoon climate system, and they provide clues for predicting future climate changes in its area of influence.

The Cryogenian Period (ca. 720–635 Ma) marks a key node in Earth's history, characterized by two global glaciations when the paleo-tropical regions were covered by glaciers, namely the Sturtian and Marinoan Snowball Earth events. Within the Cryogenian, the evolution of primary producers persisted, evidenced by the worldwide discovery of diverse microfossils and the proliferation of algae in the interglacial oceans. The Cryogenian radiation of algae facilitated the emergence and evolution of early eumetazoan animals. However, the paleoredox reconstruction of the interglacial Cryogenian presents more challenges. The open oceans were predominantly and permanently ferruginous throughout the Cryogenian Period, and persistently euxinic conditions developed in restricted basins, such as the Nanhua Basin. Some of the studies suggested a possible fully‑oxygenated interval in the aftermath of the Sturtian glaciation. Therefore, a knowledge gap lies between the biological prosperity and severe habitability in the interglacial oceans. In order to fill this gap, we report a newly discovered interglacial deposition – the Datangpo Formation – of shallow water depth from the Guitouwan section, Shennongjia area, South China. A combination of iron speciation, total organic matter (TOC), and pyrite sulfur isotopes (δ³⁴S_py) analyses are conducted, along with numerical simulations, for reconstructions of paleoredox conditions and paleoproductivity level. Additionally, we collected Fe chemistry, TOC, and pyrite sulfur data from multiple successions of the Datangpo Formation in South China to reconstruct the paleoredox landscape. The spatial TOC comparison suggests that the interglacial interval is marked by a TOC increase at the base, and the Guitouwan succession represents the early deposition during the wake of Sturtian deglaciation. Fe speciation suggests locally (dys)oxic-ferruginous condition in the Shennongjia area, while the δ³⁴S_py systematics indicate a transition from ferruginous to euxinic condition and a sharp increase in primary productivity during the Sturtian deglaciation. With respect to the high δ³⁴S_py values, the observed discrepancy suggests that the iron proxy might be inapplicable during fast environmental changes, especially during the deglaciation episodes. The comparison of Fe chemistry and pyrite sulfur isotopes between different water-depth settings informs a redox-stratified ocean in the Nanhua Basin. The newly discovered interglacial deposition fills the blank in understanding the shallow-water oxygenation in the Sturtian glacial aftermath, recording the transition from (dys)oxic-ferruginous to ferruginous-euxinic and from low to high productivity levels. These findings shed new light on the interpretation of Fe chemistry and δ³⁴S_py in the geological past, and provide a novel perspective on investigating productivity levels.

Fossils of the Ediacara Biota preserve the oldest indisputable evidence of macroscopic animal life. Although new finds continue to expand the near global distribution of such organisms, relatively abundant and diverse populations of these primarily soft-bodied taxa are known from just a handful of localities. More commonly, reports of late Precambrian fossils consist of comparably lower diversity assemblages (less than ∼10 genera), with a limited number of total specimens (less than ∼1000 reported macrofossils over a broad geographic area). Determining the factors responsible for such differences can help to fully appreciate preserved patterns of Ediacaran biodiversity.

The lower member of the Wood Canyon Formation in the Death Valley region of the southern Great Basin, USA contains fossils of the Ediacara Biota. Here we redescribe previously recovered specimens based on new data from 3D surface scans and report several new finds from this unit. Although this allows the addition of two Ediacara taxa (Pteridinium and Charnia?) to the known diversity in the region, most potential macrofauna consist of amorphous, irregular forms with minimal preserved structures wherein an abiotic origin cannot be ruled out. A combination of factors – including variable taphonomic conditions, difficulty of identification and an original depauperate community – contribute to the observed diversity of this assemblage. Many biases that impact this record are either common (e.g., age, preservation requirements, difficulty of recognition) or comparable (e.g., paleoenvironmental and taphonomic controls) to known occurrences of Ediacara macrofauana. Thus, we argue this fossil record at least partially reflects the original community composition, suggesting that fossils of the lower Wood Canyon record a signal of relatively low diversity of the Ediacara Biota in the lead up to the Ediacaran-Cambrian boundary.

The cross correlation has a wide range of applications in geophysical fields for measuring linear connections or relationships among physical quantities. Nonetheless, there remains a dearth of comprehensive discourse regarding its statistical significance testing, which is crucial for differentiating meaningful outcomes from those merely stemming from fortuity or pure randomness. Conventional theoretical methods for significance testing, commonly used in statistical analysis tools such as SPSS and MATLAB, are only applicable when dealing with idealized circumstances such as white noise. In discretionary application of these methods to analyze geophysical signals with, say, red noise may result in potentially unjustified conclusions. This study aims to develop a comprehensive approach based on the $t$-distribution within a rigorous statistical context, aiming to facilitate significance tests of cross correlation for general signals with specified time shifts or ranges. Extensive Monte Carlo experiments substantiate its robustness, thereby paving the way for accurate and expeditious identification of statistically (hence potentially physically) meaningful correlations in general. As an example, we examine critically the previously purported significant correlations between ENSO (El Niño Southern Oscillation) and global terrestrial water storage variations derived from the GRACE (Gravity Recovery and Climate Experiment) satellite mission, demonstrating that they are subject to questioning in the absence of complete significance testing.

The Totten Glacier of East Antarctica drains a basin containing about 3.5 m of sea-level rise equivalent ice. While Totten Glacier is generally considered somewhat more stable than the rapidly retreating sectors of the West Antarctic Ice Sheet, such as the Amundsen Sea Embayment and Getz Ice Shelf, our understanding of its intricate dynamics and interaction with the Southern Ocean since the Last Glacial Maximum remains incomplete. Situated in a precarious position grounded below sea level, Totten Glacier represents a critical yet enigmatic component of East Antarctic ice dynamics. Its susceptibility to marine ice sheet instability raises concerns, as positive feedback from grounding line retreat could trigger irreversible ice discharge or even complete collapse. A meticulous reconstruction of the retreat history of the Totten Glacier is imperative to comprehensively grasp the glacier's response to present and future climate changes.

Here, we present a beryllium isotope ratio analysis of marine sediment from the continental slope proximal to Totten Glacier, the first such record from this location, supplemented by grain size data from the same sediment core. The results, when evaluated together with nearby proxy records, reveal that the initial deglaciation of the Totten Glacier sector of the East Antarctic Ice Sheet began at ca. 21 ka BP. The rapid deglaciation from ca. 7 ka BP that followed is determined to be caused by the intrusion of modified Circumpolar Deep Water, leading to melting at the grounding zone of the Totten Glacier and causing instability of the glacier system. The southward shift and intensification of Antarctic easterlies may be one of the causes of this intrusion. These results contribute to the body of knowledge regarding the dynamical response of marine-terminating glaciers to climate variability during the last deglaciation.

Global climate change has changed vegetation phenology substantially around the world. However, the necessity of coupling legacy effects with temperature responses in phenological models remains unclear. The objective of this study was to demonstrate that legacy effects [based on day of year (DOY) of phenology events] have substantial positive impacts on crop phenology. Data from 1883 crop×site combinations across Germany and China were analyzed. DOY was found to be a temperature-independent factor for both vegetative (VGP) and reproductive growth periods (RGP) (based on variance inflation factor values). Partial correlation analysis suggested that DOY explained almost the same variability in date of phenology events as temperature. Akaike information criterion showed the cost-effectiveness of coupling DOY with temperature in 71.2% and 59.1% of sites in VGP and RGP, respectively. A model that coupled a linear legacy effect and a temperature response mechanism (LETM) improved fitting efficiency by an average of 57%. LETM was observed to outperform the well-calibrated WOFOST and WE models in VGP and RGP for all crops. Averaged over all crops, root mean square errors for WOFOST, WE, and LETM were 4.0, 3.9, and 3.7 d in VGP, respectively, and are 5.3, 4.6, and 4.0 d in RGP, respectively. Our results verified the necessity of coupling the legacy effect with temperature responses in phenology models. Given that the results were consistent for all of the crops investigated, we believe that our conclusions can apply to other field crops. Results of this study expand the knowledge of crop phenology responses to environment, and are helpful for accurately predicting crop growth and development responses under future global climate change.

Mountains have a significant impact on the transport path of water vapour and local meteorological variations. Therefore, understanding the influence mechanism of mountains on stable isotopes of precipitation is essential. In this study, we analyzed the precipitation stable isotope data within the Westerlies in the Eurasian continent. The results indicate the following: (1) East-west-oriented mountain ranges and plains play a “channelling role” in facilitating the inland penetration of westerly moisture, with this effect being more pronounced in the northern part of the Alps. Conversely, north-south-oriented mountain ranges and plateaus act as a “barrier” to westerly moisture. (2) Mountains influence stable isotopes of precipitation primarily by altering the water vapour movement paths and regulating local meteorological factors. (3) The “altitude effect” is the most common mechanism through which mountains affect local meteorological elements reflected in stable isotopes of precipitation. Furthermore, we anticipate that with global climate warming, the interaction and feedback between mountains and climate will become even more complex. This aspect deserves consideration in future research.