Evolving Earth最新文献

Phase equilibria modeling becomes a widely accepted tool to constrain the P-T conditions experienced by the metamorphic terrain because of its advantage over the earlier methods e.g., conventional methods and petrogenetic grids. In this study, garnet stability in the mafic granulite has been examined in response to bulk composition and P-T conditions. Karimnagar granulite terrain (KGT) consists of garnet-free mafic granulite along with garnet-bearing metapelite and silica deficient Mg-Al granulite as an enclave within granite gneiss. Mafic granulite consists of amphibole, orthopyroxene, clinopyroxene, and plagioclase with a minor modal amount of biotite and quartz. Mafic granulite has experienced three distinct metamorphic stages: (a) prograde stage defined by inclusions of minerals like amphibole and quartz within the orthopyroxene and augite, (b) peak stage characterized by coarse grain association of amphibole, orthopyroxene, augite, plagioclase, and minor quartz, (c) the retrograde stage illustrated with coronal growth of amphibole over orthopyroxene and augite. The phase equilibria diagrams were calculated in the Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O₂ (NCKFMASHTO) model system using bulk rock compositions. Our results show that mafic granulite of the study area has achieved the highest P-T condition of 7 kbar and 800 °C and followed a clock-wise metamorphic trajectory. The calculated P-T pseudosection for two samples indicates the stability of garnet varies between 7.5 and 9.5 kbar at 800 °C. The calculated P-Mo pseudosection (at 800 °C) illustrates the lower stability limit of garnet further goes down with the increase in Al₂O₃ and FeO in the bulk composition.

The Himalayas possess a distinctive topography owe to the dynamic interplay of tectonic activity, geological erosion and sedimentation, glacial processes, and climatic influences over the millions of years. The rugged, steep terrain and poor land management make it more prone to water erosion, negatively impacts the soil, affecting the goods and services supported by the soil ecosystems. Traditional methods used in soil erosion assessment face limitations when dealing with topographically complex hillslopes. The use of Fallout Radionuclide (FRN) -¹³⁷Cs provides a feasible alternative for measurement of soil erosion in the region with such topography. However, there is lack of ¹³⁷Cs-based soil erosion studies in the north-west Himalayas. Pine (Pinus roxburghii) is the predominant forest type in the Himalayas, offering numerous benefits to both natural ecosystems and human beings. In this study, we selected a typical steep hillslope covered with pine forest in the Himalayas for soil erosion assessment. The study measured ¹³⁷Cs reference inventory of 1409 Bq m⁻² in the landscape. Importantly, the concentration of ¹³⁷Cs along the hillslope positions showed a significant variation attributed to topographic variability. Topographic factors, such as the slope shape and gradient, were identified as the major governing parameters of soil erosion in the hilly and mountainous region. The net soil erosion rate over hillslope positions revealed highest at upper hillslope followed by ridge, middle and valley hillslope positions. The net soil erosion rate under the pine forest ranged from 8.0 to 14.6 t ha⁻¹ yr⁻¹, with an average rate of 9.9 t ha⁻¹ yr⁻¹. Erosion rate over the hillslope positions were found in accordance to the soil loss tolerance limit (SLTL) except for the upper hillslope, indicating it as critical slope position requires to adopt suitable conservation measures. The study signifies the role of the forest in mitigating soil erosion and, in turn, conserving soil resources. The findings provide crucial insights and guidance to land managers and decision-makers, emphasizing the necessity of conserving and restoring forests in the Himalayas.

The Narmada River originates in eastern Madhya Pradesh near Amarkantak (200 40′ N, 81 46′ E), travels through Madhya Pradesh, Maharashtra, and Gujarat before emptying into the Gulf of Cambay in the Arabian Sea, close to Baruch in Gujarat. Numerous fossil discoveries in the region include various mammals, reptiles, fishes, amphibians, and molluscan shells. In addition to being a rare mid-continental location in the Old World, the Narmada Valley has traditionally served as a line delineating Northern and Southern India. This valley has produced countless animal fossils as well as a few human fossils. This has been a crucial route for the movement of both faunal migrations from the north to the south and the transmission of the monsoons from the southeast to the northwest. The present work examines the crystallization index of the faunal fossils from three (Barmanghat,Talayyaghat,Devakachar) localities in the Central Narmada valley falling within the region of Madhya Pradesh. Fossil samples were subjected to X-Ray diffraction in order to calculate the Crystallization index as a direct indicator of the degree of fossilization. The materials of Narmada river valley have been studied extensively by previous scholars; however, this study is first attempt to study the fossils using scientific technique such as XRD.

Bonebeds and amber deposits provide a wealth of palaeontological information allowing palaeobiologists to reconstruct ancient ecosystems in great detail. It is a common view that these two sources of data rarely intersect in the fossil record, owing to distinct taphonomic pathways. Nonetheless, the past decades have seen an increasing number of bonebed amber deposits reported and investigated, suggesting that some depositional environments might provide the appropriate conditions for preserving plant material and bones. By reviewing the current knowledge of the taphonomy of amber deposits and that of bonebeds, and through examination of a series of Cretaceous deposits in North America and Europe, we identify a taphonomic window that permits the long-term preservation of both materials. This synthesis allows us to provide some keys for bonebed amber prospecting, which will hopefully lead to additional finds in the field. We also review the main findings of the first comprehensive studies of bonebed amber deposits which focus on the amber assemblage, the chemistry of the fossil resin, and its stable isotopic composition, to demonstrate the utility of combined studies. This approach enables palaeobiologists to reconstruct past forest habitats by (i) exploring the faunal communities (especially, arthropod communities) of the forests, (ii) identifying the dominant source tree, (iii) identifying the ecological conditions, and (iv) characterizing the palaeogeography of the region. These findings show that amber represents a source of complementary data within bonebeds and can help achieving a better knowledge of past terrestrial habitats.

The avulsion scenario of the Brahmaputra has been a debated issue for quite a long time. This study aims to resolve this debate through reconstruction of the avulsion history of the Brahmaputra. We have addressed the processes, timeline and causes of the avulsion. Old maps and images from 1776 to 2023, evidence from contemporary literature and recent studies have been used in our study. We find that the process of creation of the Jamuna was started through capturing the flow of the Brahmaputra by the Jenni during the 1780s. Between 1780 and 1800 the Brahmaputra created two primary avulsion channels upstream and initiated the multi-avulsion process which was completed in the 1880s. The first avulsion channel conveyed the Brahmaputra flow into the Konaie directly and the second avulsion channel diverted the flow captured by the Jenni into the Konaie. Contemporarily, the flow-enriched Konaie switched its flow into the Jenni at downstream through an unnamed channel and the combined flow traveled through the bed of the Jenni before creating the confluence with the Ganges. In the whole process, the Konaie received, inflated and transported the dominant flow of the Brahmaputra and the Jenni became inferior over time. We have introduced a bend migration concept along with local sediment overload to focus on setting up a lower stability threshold of avulsion that led to the eruptive creation of the avulsion channels by a coupled switching mechanism imparted by high monsoonal discharges, avulsion of the Tista into the Brahmaputra and the high-magnitude floods.

Trace fossils are not generally utilized as biostratigraphic indicators due to their long stratigraphic ranges. Despite the use of intricate behavioral traces in the absence of other indicators, existing models like the Precambrian–Cambrian boundary and Cruziana stratigraphy encounter limitations due to crucial data gaps and regional constraints. To surmount these challenges, in this paper, we critically assess established models and present a new framework for Early Paleozoic strata, drawing on trace fossils from the intracratonic basins of Brazil. Our ichnostratigraphic model is calibrated using ichnological data from the Parnaíba, Paraná, and Amazonas basins, including new data. The analysis focuses on trace fossils in strata that are independently dated using chitinozoan, miospore, and acritarch biozonation. Key ichnotaxa, such as Arthrophycus and Cruziana, are identified as prominent indicators of the Llandovery Stage in Brazil. Occurrences of Heimdallia and Musculopodus in the Tianguá Formation also may be used to suggest a Llandovery interval. Notably, Bifungites, found widely across Brazilian basins, emerges as a potential ichnomarker for the Early to mid-Paleozoic interval, with a global presence throughout Cambrian to Mississippian deposits. While current ichnostratigraphic models lack robust calibration with chronostratigraphic or biostratigraphic data, our new proposed model integrates key ichnotaxa, including Bifungites, Climactichnites, Heimdallia, Oldhamia, and Musculopodus, surpassing those pre-existing zonations based on Cruziana and arthrophycids. These ichnotaxa exhibit unique features and narrow temporal ranges, meeting essential biostratigraphic criteria. Although their spatial distribution is somewhat limited, our new model, which is continually evolving with new data, holds promise for enhancing global stratigraphic correlations, particularly where independent age information is available.

For hundreds of millions of years wildfire has influenced, and been influenced by, plant evolution, biogeochemical cycling, and climate. Wildfire has even been implicated as one of the causative agents of mass extinctions. The deep time geologic record offers demonstrated, but underdeveloped, potential to advance knowledge on the role of wildfire in the Earth system. Herein, we present and discuss the geologic history of wildfire and methods for its reconstruction. We argue that application of the numeric approaches to wildfire reconstruction often used in Quaternary studies would advance understanding of deep time wildfire. Application of numeric methods increases statistical rigour, with the intent of reducing bias and increasing accuracy. For example, numeric methods offer a means to robustly calibrate the provenance and taphonomy of particles used to reconstruct wildfire, and to quantify uncertainties. Statistical methods should be used to assess the fidelity of new chemical proxies of wildfire, such as the types, amounts, distributions, and isotope signatures of polycyclic aromatic hydrocarbons, to represent source area and fuel type.

The end-Permian mass extinction event is traceable across several non-marine basins in Australia. In the Sydney Basin, the lithological succession is characterized by a change from coal seams to mudstones and sandstones, recording a major environmental change following the disappearance of the Permian vegetation. A few millimeter-thick iron-rich ‘rusty’ layer occurs between the uppermost Permian coal seam and the mudstone, a layer that extends laterally across the basin and which has also been documented from coeval successions in Antarctica. This layer is overlain by the <1.5-m-thick Frazer Beach Member, whose basal 10-cm-thick microbreccia bed comprises 99% kaolinite and quartz, and is dated as 252.10 ± 0.06 Ma. The Frazer Beach Member corresponds to the so-called end-Permian ‘Dead Zone’ lacking fossil pollen and leaves. This distinctive member was deposited directly following the extinction of the Permian peat-forming forests.

Here we identify, through X-ray absorption spectroscopy, a drastic redox shift across the extinction interval with increasing amount of reduced Fe-species followed by highly oxidized Fe-species, most resembling Fe(III) complexed with organic matter. Values subsequently normalise in younger samples through the ‘Dead Zone’, attaining only slightly higher redox-levels than before the event. The organically complexed Fe-species in the event bed is consistent with the standard Suwannee River fulvic acid, an acid Fe-complex with iron bound to organic matter, whereas the samples above and below the extinction layer yield spectra predominantly resembling magnetite (Fe₃O₄) mineral phase. We consider that the iron redox fluctuation marking the extinction interval is related to significant environmental changes with accumulation of organic matter following the mass extinction. The highly reduced iron in the extinction layer may relate to methane release from bacterial degradation, or emissions from clathrates. The presence of fulvic acid in the distinct iron-rich extinction layer indicates that an abrupt onset of the process of degradation of plant matter, lipids and calcium hydroxide (CaOH) took place, resulting in this ‘Death layer’. This was followed by millions of years of erosive conditions before new, complex vegetation could establish.

The Arabian Sea hosts a perennial and intense oxygen minimum zone (OMZ) at 150–1200 m depths with O₂ concentrations <0.5 ml/l. It is generally believed that the oxygen-depleted conditions at mid-water depths result from high rate of O₂ consumption due to monsoon-driven productivity generating a high organic matter flux, combined with slow renewal of thermocline waters in the region. With global warming and increasing hypoxia, there is growing interest to better understand the various factors controlling oxygen conditions in the thermocline waters and the impact on the nutrient cycling and climate. In this contribution, we provide an overview of new advances in understanding the basin-wide changes of the OMZ, and highlight new perspectives on the relative roles of ocean and atmospheric circulations in modulating the OMZ intensity through the late glacial-Holocene period. Comprehension of the existing and new proxy records (δ¹⁵N, aragonite preservation, δ¹³C of benthic foraminifera) from the productive western and oligotrophic eastern and north-eastern Arabian Sea provides insights into the regional heterogeneity in basin-wide changes of the OMZ, denitrification and carbonate (aragonite) lysocline, and their links to the seasonal monsoon variability and reorganisation of thermocline circulation. We also highlight the limitations of the existing proxy data to address the important questions of how circulation and chemical properties of intermediate/deep water masses contributing to the Arabian Sea thermocline waters changed in the past. Hence, more detailed proxy data are required to characterise sources of water masses, past changes in their pathways and vertical extents in the Arabian Sea, which are crucial to better constrain the temporal evolution of thermocline ventilation basin-wide.