Nitrogen isotope chemostratigraphy across the Permian–Triassic boundary at Chaotian,Sichuan, South China

Nitrogen isotopic compositions of upper Permian to lowermost Triassic rocks were analyzed at Chaotian in northern Sichuan, South China, in order to clarify changes in the oceanic nitrogen cycle around the Permian–Triassic boundary (P–TB) including the entire Changhsingian (Late Late Permian) prior to the extinction. The analyzed ca. 40 m thick interval across the P–TB at Chaotian consists of three stratigraphic units: the upper Wujiaping Formation, the Dalong Formation, and the lowermost Feixianguan Formation, in ascending order. The upper Wujiaping Formation, ca. 10 m thick, is mainly composed of dark gray limestone with diverse shallow-marine fossils such as calcareous algae and brachiopods, deposited on the shallow shelf. In contrast, the overlying Dalong Formation, ca. 25 m thick, is mainly composed of thinly bedded black mudstone and siliceous mudstone containing abundant radiolarians, deposited on the relatively deep slope/basin. Absence of bioturbation, substantially high total organic carbon contents (up to 15%), and abundant occurrence of pyrite framboids in the main part of the Dalong Formation indicate deposition under anoxic condition. The lowermost Feixianguan Formation, ca. 5 m thick, is composed of thinly bedded gray marl and micritic limestone with minor fossils such as ammonoids and conodonts, deposited on the relatively shallow slope. δ¹⁵N_TN values are in positive values around +1 to +2‰ in the upper Wujiaping Formation implying denitrification and/or anammox in the ocean. δ¹⁵N_TN values gradually decrease to −1‰ in the lower Dalong Formation and are consistently low (around 0‰) in the middle Dalong to lowermost Feixianguan Formation. No clear δ¹⁵N_TN shift is recognized across the extinction horizon. The consistently low δ¹⁵N_TN values suggest the enhanced nitrogen fixation in the ocean during the Changhsingian at Chaotian. Composite profiles based on previous and the present studies demonstrate the substantial δ¹⁵N variation on a global scale in the late Permian to earliest Triassic; a systematic δ¹⁵N difference by low and high latitudes is particularly clarified. Although the enhanced nitrogen fixation throughout the Changhsingian at Chaotian was likely a regional event in northwestern South China, the composite δ¹⁵N profiles imply that the sea area in which fixed nitrogen is depleted has gradually developed worldwide in the Changhsingian, possibly acting as a prolonged stress to shallow-marine biota.     

Middle Permian organic carbon isotope stratigraphy and the origin of the Kamura Event

Large carbon cycle perturbations associated with the Middle Permian (Capitanian) mass extinction have been widely reported, but their causes and timing are still in dispute. Low resolution carbon isotope records prior to this event also limit the construction of a Middle Permian chemostratigraphic framework and global or local stratigraphic correlation, and hence limit our understanding of carbon cycle and environmental changes. To investigate these issues, we analyzed the ¹³C_org values from the Middle Permian chert-mudstone sequence (Gufeng Formation) in the Lower Yangtze deep-water basin (South China) and compared them with published records to build a chemostratigraphic scheme and discuss the underlying environmental events. The records show increased δ¹³C_org values from late Kungurian to early Guadalupian, followed by a decrease to the late Wordian/early Capitanian. The early-mid Capitanian was characterized by elevated δ¹³C_org values suggesting the presence of the “Kamura Event”: an interval of heavy positive values seen in the δ¹³C_carb record. We propose that these heavy Capitanian δ¹³C values may be a response to a marked decline in chemical weathering rates on Pangea and associated reduction in carbonate burial, which we show using a biogeochemical model. The subsequent negative δ¹³C excursion seen in some carbonate records, especially in shallower-water sections (and in a muted expression in organic carbon) coincide with the Capitanian mass extinction may be caused by the input of isotopically-light carbon sourced from the terrestrial decomposition of organic matter.     

Molecular carbon isotope variations in core samples taken at the Permian–Triassic boundary layers in southern China

Stable carbon isotope composition (δ¹³C) of carbonate sediments and the molecular (biomarker) characteristics of a continuous Permian–Triassic (PT) layer in southern China were studied to obtain geochemical signals of global change at the Permian–Triassic boundary (PTB). Carbonate carbon isotope values shifted toward positive before the end of the Permian period and then shifted negative above the PTB into the Triassic period. Molecular carbon isotope values of biomarkers followed the same trend at and below the PTB and remained negative in the Triassic layer. These biomarkers were acyclic isoprenoids, ranging from C₁₅ to C₄₀, steranes (C₂₇ dominates) and terpenoids that were all significantly more abundant in samples from the Permian layer than those from the Triassic layer. The Triassic layer was distinguished by the dominance of higher molecular weight (waxy) n-alkanes. Stable carbon isotope values of individual components, including n-alkanes and acyclic isoprenoids such as phytane, isop-C₂₅, and squalane, are depleted in δ¹³C by up to 8–10‰ in the Triassic samples as compared to the Permian. Measured molecular and isotopic variations of organic matter in the PT layers support the generally accepted view of Permian oceanic stagnation followed by a massive upwelling of toxic deep waters at the PTB. A series of large-scale (global) outgassing events may be associated with the carbon isotope shift we measured. This is also consistent with the lithological evidence we observed of white thin-clay layers in this region. Our findings, in context with a generally accepted stagnant Permian ocean, followed by massive upwelling of toxic deep waters might be the major causes of the largest global mass extinction event that occurred at the Permian–Triassic boundary.     

Late Permian and Early Triassic evolution of the Northern Indian margin: carbon isotope and sequence stratigraphy

Abstract

The Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (para- conformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin (landward of the shoulder) is well developed in the Pottawar- Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressive- regressive (T-R) second order cycles, two in the late Permian and one in the early Triassic. The break-up of the rift occurred during the second cycle (late Dzhulfian).

In the Salt Range area, these three T-R cycles have been subdivided in eight third order sequences, five sequences for the upper Permian and three for the lower Triassic.

At the end of Permian, hiatuses, gaps and local erosion of part of the margin are direct consequences of a first order relative sea-level fall; this is also the time of the largest extinction event of the Phanerozoic that deeply affected the carbonate productivity and the stratal patterns. With the following worldwide sea-level rise, a rapid and large scale transgression occurred in the early Triassic, well dated and recorded on the whole margin. High rate thermal subsidence gave way to generalized pelagic deposits about 2 My after the transgression.

Profiles of whole rock inorganic carbon and oxygen isotopes from Guryul Ravine and Palgham sections in Kashmir, Nammal Gorge and Landu sections in Trans Indus Ranges (Pakistan), Thini Chu section in Kali Gandaki Valley, Central Nepal are presented in connection with the sequence stratigraphic analysis. The upper Permian record of high positive δ¹³C values are closely correlated with the second order T-R cycles and the third order sequences. The results presented in this study confirm the drastic drop of δ¹³C from the high positive values that characterised the upper Permian to lower values in the lower Triassic time. Stratigraphic correlation problems in the lower Triassic using carbon isotope geochemistry are briefly discussed. A positive δ¹³C excursion of 4–5% near the Smithian - Spathian substages boundary is observed for the first time. The δ¹⁸O values of samples from all the sections display major variations suggesting that the oxygen isotope record has been significantly affected by meteoric diagenesis, deep burial diagenesis or/and monsoon signature.     

二叠纪-三叠纪之交的古海洋水化学条件变化证据--以贵州织金剖面为例

基于贵州织金剖面的二叠系-三叠系界线附近的岩石薄片鉴定及古生物化石识别,划分出了2种碳酸盐岩沉积相、2种亚相和5种微相;有机质碳同位素组成测试结果表明晚二叠世时期有机质的δ13C_org值（-27.7‰）向早三叠世时期的δ13C_org值（-28.3‰）逐渐偏负,反映了有机质类型由高等植物向低等微生物变化的过程;全岩的δ13C_rock值由晚二叠世（-24.2‰）向早三叠世（-20.9‰）变大,反映了二叠纪末期生物大灭绝事件造成了海洋生物的消失,进而引起12C在海洋中强烈富集;Fe-_HR/Fe-_T、Fe_py/Fe-_HR以及微量元素V/（V+Ni）与V/（V+Cr）比值大小,反映了二叠纪末期该区域的海水为氧化环境或弱还原环境,而三叠纪初期海水处于还原与铁化环境,在盆地内部没有发生硫化作用;V/Ni的值从晚二叠世的0.97~1.97向早三叠世的1.67~7.33明显变大,表明水体逐渐加深的过程,该结论与沉积微相变化相符;在生物演化上,从二叠纪末期的有孔虫-藻屑-三叶虫生物组合演变到三叠纪初期小型介壳-微生物组合,反映了该时期中国南方统一的两幕式生物灭绝与演化样式。     

Carbon and Strontium Isotopes of Late Palaeozoic Marine Carbonates in the Upper Yangtze Platform,Southwest China

238 marine carbonate samples were collected from seven sedimentary sections ofthe entire late Palaeozoic (Permian, Carboniferous and Devonian) in the Upper Yangtze Plat-form, southwest China. Based on the absence of cathodoluminescence and very low Mn (gener-ally<50 ppm) contents of the samples, it is thought that they contain information on the orig-inal sea water geochemistry. The results of isotopic analyses of these samples are presented interms of δ~(13)C and ~(87)Sr/~(86)Sr ratios versus geological time. The strontium data, consistent withother similar data based on samples from North America, Europe, Africa and other areas inAsia, support the notion of a global consistency in strontium isotope composition of marinecarbonates. The strontium data exhibit three intervals of relatively low ~(87)Sr/~(86)Sr ratios in thelate Middle Devonian to early Late Devonian, Early Carboniferous and Early Permian, corre-sponding to global eustatic high sea level stands. The lowest ~(87)Sr/~(86)Sr ratio recorded in theLate Permian was probably caused by substantial basalt eruptions in the Upper Yangtze Plat-form at the time. Three corresponding periods of relatively high δ~(13)C values at roughly the samethe intervals were caused by a relatively high rate of accumulation of organic carbon duringsea level rises at these times. The deposition of coal was probably responsible for the increaseof sea water δ~(13)C at other times. The δ~(13)C values drop dramatically near theDevonian/Carboniferous, Carboniferous/Permian and Permian/Triassic boundaries, con-sistent with other similar data, which further support the notion that geological time boundariesare associated with mass extinction and subsequent rejuvenation.     

Major perturbation of ocean chemistry and a 'Strangelove Ocean' after the end-Permian mass extinction

The severe mass extinction of marine and terrestrial organisms at the end of the Permian Period (c. 251 Ma) was accompanied by a rapid (<100 000 years and possibly <10 000 years) negative excursion of c. 3‰ in the δ¹³C of the global surface oceans and atmosphere that persisted for some 500 000 years into the Early Triassic. Simulations with an ocean–atmosphere/carbon-cycle model suggest that the isotope excursion can be explained by collapse of ocean primary productivity, and changes in the delivery and cycling of carbon in the oceans and on land. Model results suggest that severe reduction of marine productivity led to an increase in surface-ocean dissolved inorganic carbon and a rapid, short-term increase in atmospheric pCO₂ (from a Late Permian base of 850 ppm to c. 2500 ppm). Increase in surface ocean alkalinity may have stimulated the widespread microbial and abiotic shallow-water carbonate deposition seen in the earliest Triassic. The model is also consistent with a long-term (>1 Ma) decrease in sedimentary burial of organic carbon in the early Triassic.     

Carbon and oxygen isotopes suggesting deep-water basin deposition associated with hydrothermal events （Shangsi Section,Northwest Sichuan Basin-South China）

Carbon and oxygen isotope records for Shangsi Section in Northwest Sichuan Basin, South China can help investigating depositional environments and processes, including the burial rate and possible contribution of hydrothermal events. Samples from the lower Chihsian Formation show δ13CPDB and δ18OPDB values close to those of typical marine limestone. However, the overlying Permian middle-upper Chihsian, Wujiaping, and Maokou Formation samples reveal negative δ18OSMOW values and strong positive δ13CPDB values. These indicate high biological productivity and rapid burial of organic carbon. Samples from the Dalong Formation present both negative δ13CPDB and negative δ18OPDB values, which are quite different from the underlying Permian strata. These abnormal carbon and oxygen isotope characteristics in the Dalong Formation may suggest that hydrothermal processes contributed to deposition.     

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO₂ components—atmospheric CO₂ and CO₂ from in situ oxidation of organic matter. The δ¹³C values measured for the Fe(CO₃)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most ¹³C-rich Fe(CO₃)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO₂ components. δ¹³C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco₂ value of about 1 × PAL, but the scatter in the measured δ¹³C values of organic matter permits a calculated maximum Pco₂ of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO₃)OH in MCGoeth and SAP correspond to soil CO₂ concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO₂ concentrations are similar to modern soils in warm, wet environments.The average δ¹³C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ¹³C value for all profiles of −5.4‰. Thus, the value of Δ¹³C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco₂ and organic matter δ¹³C values were the same for both paleosol types. Furthermore, the atmospheric Pco₂ calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco₂ (permitted by one standard deviation of the organic matter δ¹³C value) of ∼5 × PAL.If, however, measured average δ¹³C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of ¹³C-depleted, labile organic compounds, calculated Permian atmospheric Pco₂ using these ¹³C-enriched organic values would underestimate the actual atmospheric Pco₂ using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO₂ concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO₂.The Fe(CO₃)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in ¹³C relative to Fe(CO₃)OH in goethites from soils in which there are mixtures of two isotopic CO₂ components. Field-relationships and the δ¹³C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO₂ components at the time of goethite crystallization. The three components were probably atmospheric CO₂, CO₂ from in situ oxidation of organic matter and CO₂ from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO₂ components, as recorded by Fe(CO₃)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco₂.     

A paired sulfate-pyrite δS approach to understanding the evolution of the Ediacaran-Cambrian sulfur cycle

An anomalous enrichment in marine sulfate δ³⁴S_SO4 is preserved in globally-distributed latest Ediacaran-early Cambrian strata. The proximity of this anomaly to the Ediacaran-Cambrian boundary and the associated evolutionary radiation has invited speculation that the two are causally related. Here we present a high-resolution record of paired sulfate (δ³⁴S_SO4) and pyrite (δ³⁴S_pyr) from sediments spanning ca. 547-540 million years ago (Ma) from the Ara Group of the Huqf Supergroup, Sultanate of Oman. We observe an increase in δ³⁴S_SO4 from ∼20‰ to ∼42‰, beginning at ca. 550 Ma and continuing at least through ca. 540 Ma. There is a concomitant increase in δ³⁴S_pyr over this interval from ∼ −15‰ to 10‰. This globally correlative enrichment, here termed the Ara anomaly, constitutes a major perturbation to the sulfur cycle. The absolute values of δ³⁴S_pyr reported here and in equivalent sections around the world, require the isotopic composition of material entering the ocean (δ³⁴S_in) to be significantly more enriched than modern (∼3‰) values, likely in excess of 12‰ during the late Ediacaran-early Cambrian. Against this background of elevated δ³⁴S_in, the Ara anomaly is explained not by increased fractionation between sulfate and pyrite (Δδ³⁴S), but by an increase in pyrite burial (f_pyr), most likely driven by enhanced primary production and sequestration of organic carbon, consistent with earlier reports of elevated organic carbon burial and widespread phosphorite deposition.     

The Paleoceanography during the Time with High δ13C of Phanerozoic marine carbonates

Carbon isotopic composition of marine carbonates is a record for various important geological events in the process of earth development and evolution. The carbonates of Carboniferous, Permian and Triassic, as the transition from Paleozoic to Mesozoic-Cenozoic have very high ¹³C value. Taking this as the main point, and combined with the oxygen, strontium isotopic composition in carbonates, distribution of carbonate basin area through geologic time, the correlation of carbon isotopic composition of marine carbonates to sea level change, organic carbon burial flux, exchange of CO₂ content in atmosphere and ocean, and long cycle evolution of the earth ecosystems were approached. The results are shown as follows: ①The interval of ¹³C >3‰ during Phanerozoic was concentrated in Carboniferous, Permian and the beginning of Triassic, but the beginning of Triassic was characterized by higher frequency and larger fluctuations in ¹³C value during a short time, whereas the Carboniferous-Permian presented a continuously stable high ¹³C value, indicating a larger amount of organic carbon accumulation in this time interval. Relatively high ¹⁸O values during this time was also observed, showing a long time of glaciations and cold climate, which suggest a connection among rapid organic carbon burial, cold climate, as well as pCO₂ and pO₂ states of atmosphere. ②The over consumption of atmosphere CO₂ by green plants during the time with high ¹³C of seawater forced CO₂ being transferred from ocean to atmosphere for the balance, but the decrease in the seawater amount and water column pressure caused by the global cooling could weaken dissolution capacity of CO₂ in seawater and carbon storage of marine carbonates, and also reduce the carbonate sedimentary rate and decrease the carbonate basin area globally from Devonian to Carboniferous and Permian. During the middle-late Permian carbonate was widely replaced by siliceous sediments even though in shallow carbonate platform, which resulted in the decrease of marine invertebrates, suggesting the Permian chert event should be global. ③The Phanerozoic ⁸⁷Sr/⁸⁶Sr trend of seawater showed a sharp fall in Permian and drop to a minimum at the end of the Permian, indicting input of strontium from the submarine hydrothermal systems (mantle flux). Such process should accompany with a supplement of CO₂ from deep earth to atmosphere and ocean system, but the process associated with widespread volcanism and rises of earth’s surface temperature pricked up the mass extinction during the time of end Permian. ④Cold climate and increase of continental icecap volume, the amalgamation of northern Africa and Laurentia continentals were the main reasons responsible for the sea level drop, but the water consumption result from the significantly increased accumulation of organic carbon should also be one of the reasons for the sea level drop on the order of tens of meters. ⑤The mass extinction at the end Permian was an inevitable event in the process of earth system adjustment. It was difficult for marine invertebrates to survive because of the continuously rapid burial of organic carbon, and of the decrease of sea water amount and its dissolution ability to CO₂. At last, at the end of Paleozoic, the supplement of CO₂ to atmosphere and ocean by widely magma activities resulted in a high temperature of earth surface and intensified mass extinction.     

A carbon‐isotopic study of an end‐Permian mass‐extinction horizon,Bulla, northern Italy: a negative δ13C shift prior to the marine extinction

Palaeontological data from the Permian‐Triassic Bulla section, northern Italy, demonstrate a rapid extinction at this site. This occurs after a negative carbonate carbon‐isotope (δ¹³C_carb) shift, consistent with two other northern Italian sites (Val Badia and Tesero). However, conclusion goes against recent reporting that the extinction occurs before the δ¹³C_carb shift. We agree that the shift occurs after the extinction at Jameson Land, east Greenland (a high latitude palaeolocation). However, all other sections show the shift before, or coincident with, the extinction. We suggest that the simplest explanation is a coeval shift in carbonate carbon‐isotope shifts, and it follows that the extinction was not. This suggests that the end‐Permian extinction crept from region to region. It also suggests that the marine extinction occurred first in high northern latitudes.     

The Buday’ah Formation, Sultanate of Oman: A Middle Permian to Early Triassic oceanic record of the Neotethys and the late Induan microsphere bloom

The Middle Permian to Lower Triassic Buday’ah section, exposed in the Oman Mountains, is the first deep-sea section to be described in the Neotethys. The oceanic sediments were deposited along the southern Tethys margin in the newly formed Hawasina Basin. It is one of the few places where true Tethyan Permian radiolarites are exposed that allow the documentation of CCD evolution through time. The succession begins as oceanic crust pillow basalt with red ammonoid-rich pelagic limestone occurring both above and within inter-pillow cavities; the new occurrence of Clarkina postbitteri hongshuiensis indicates a late Capitanian age for the carbonate. The sharp change to overlying late Capitanian to Changhsingian radiolarite reflects rapid subsidence about 10 Myrs after initial continental breakup that resulted in the formation of the Neotethys Ocean. New conodonts indicate that the Permian-Triassic boundary succession occurs in the first platy lime mudstone beds above a Changhsingian siliceous to calcareous shale unit. The platy lime mudstone beds include an Upper Griesbachian bloom of calcite filled spheres (radiolarians?) that marks a potential world-wide event. New conodonts indicate an early Olenekian age for overlying grey papery limestone that are devoid of both macrofossils and trace fossils indicating that recovery from the Late Permian extinction has not yet progressed within this deep-water environment.δ¹³C_org, isotope values have not been disturbed and they show a negative shift just below the Permian-Triassic transition and a second one at the parvus zone level above. The Buday’ah succession may represent the most distal and probably deepest Permian and Lower Triassic depositional sequence within the basin.     

广西田东剖面二叠纪-三叠纪之交的生物绝灭及生态演化

本文以广西田东地区浅海相二叠系-三叠系剖面为研究对象,对二叠纪-三叠纪之交的生物绝灭与浅海生态环境演化进行了深入的探讨。镜下化石鉴定与统计结果表明在P/T界线附近生物物种两次急剧减少,一次为生物大绝灭所致,另一次为生物迁徙所致。另外,本剖面上所研究的物种的数量变化是突变的,而不是渐变的。在残存期内,腹足类生物较有孔虫生物表现出较强的自我恢复和适应环境的能力。整个剖面沉积为一个海浸的过程,其二叠系-三叠系界线之交生态环境发生了很大的变迁。     

Lower Jurassic epicontinental carbonates and mudstones from England and Wales: chemostratigraphic signals and the early Toarcian anoxic event

Sections through Lower Jurassic epicontinental carbonates from Southern Britain (Junction Bed and equivalent) show a positive carbon-isotope excursion (δ¹³C_carbonate), detectable in bulk rock, in the falciferum Zone of the lower Toarcian. Isotopic data from organic matter in more clay-rich sections from Wales and north-east England, together with determinations on belemnite calcite, indicate that highest δ¹³C values are localized in the upper exaratum Subzone of the falciferum Zone. Levels of particular enrichment in organic carbon were developed in the early to mid-exaratum Subzone and hence pre-date this δ¹³C maximum. These phenomena reflect the impact of the early Toarcian oceanic anoxic event in the British area. Similar isotopic trends have been recorded in other Toarcian sections from Tethyan Europe and are interpreted as reflecting the chemistry of sea water. On the assumption of isotopic correlation between the English and Tethyan sections, the δ¹³C maximum would be everywhere dated as latest exaratum Subzone in terms of the north European ammonite scheme. Absolute oxygen-isotope values in carbonates probably reflect both early diagenetic cementation and later temperature-related burial diagenesis, although a palaeotemperature maximum is tentatively identified as characterizing the early falciferum Zone. Subsequent climatic deterioration may have been triggered by drawdown of CO₂, related to regional excess carbon burial during the oceanic anoxic event. Using the positive δ¹³C excursion as a correlative level in sections from different faunal provinces (Britain, Italy and Spain) implies that lower Toarcian zonal stratigraphy is diachronous between northern and southern Europe. There is evidence for partitioning of water masses between the north European shelf and the Tethyan continental margin during the Early Jurassic.     

Carbon and sulfur isotopic fluctuations associated with the end-Guadalupian mass extinction in South China

Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ¹³C_carb, δ¹³C_org), and sulfur isotopic compositions of carbonate associated sulfate (δ³⁴S_sulfate) across the Guadalupian–Lopingian (G–L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ¹³C_carb values show a positive excursion from − 0.45‰ to the peak of 3.80‰ in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to − 2.60‰ in the lowest Heshan formation, then returned to about 1.58‰. Similar to the trends of the δ¹³C_carb values, Δ¹³C_carb–org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean–atmosphere carbon cycle. The first biostratigraphic δ³⁴S_sulfate values during the G–L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to − 37.41‰. These sulfate isotopic records suggest that the ocean during the G–L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G–L biotic crisis.     

Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges,South China

The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed the oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding the co-evolution of the Earth's environment and life. Here, we first report on coupled organic carbon and nitrogen isotope chemostratigraphy from the entire Ediacaran to Early Cambrian period by using drill core samples from the Yangtze Platform, South China. The results show that δ¹⁵N_TN values were high (~ + 6‰) until middle Ediacaran, gradually dropping down to − 1‰ at the earliest Cambrian, then rising back to + 4‰ in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with a relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest that the δ¹⁵N_TN and C/N trends mainly reflect secular changes in nitrogen cycling in the Yangtze Platform. Onset of the observed negative N isotope excursion coincided with a global carbon isotope excursion event (Shuram excursion). Before the Shuram event, the high δ¹⁵N probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the ¹⁵N-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of the Shuram event, both δ¹³C_carb and δ¹⁵N_TN values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by an anomalously low C/N ratio, implying that enhanced respiration resulted in selective loss of carbon as CO₂ with recycled organic nitrogen. After the Shuram event, the δ¹⁵N value continued to decrease despite that δ¹³C_carb rose back to + 4‰. The continued δ¹⁵N drop appears to have coincided with a decreasing phosphorus content in carbonate. This suggests that ocean oxygenation may have generated a more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative δ¹³C_carb event in the Canglanpuan stage of the Early Cambrian is also characterized by carbon isotopic decoupling as well as the low C/N ratio. The results strongly support that the two stages of the decoupled negative δ¹³C_carb excursions reflect a disappearance of a large organic carbon pool in the ocean. The two events appear to relate with the appearance of new metazoan taxa with novel feeding strategies, suggesting a link between ocean oxygenation, nutrient cycling and the appearance and adaptation of metazoans. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions.     

Evaluating the S-isotope fractionation associated with Phanerozoic pyrite burial

This study examines the sulfur isotope record of seawater sulfate proxies using δ³⁴S and Δ³³S to place constraints on the average global fractionation (Δ³⁴S_py) associated with pyrite formation and burial and the exponent λ that relates variations of the ³⁴S/³²S to variations of the ³³S/³²S. The results presented here use an analysis of the sulfur isotope record from seawater sulfate proxies and sedimentary sulfide to extract this quantity as the arithmetic difference between δ³⁴S of seawater sulfate and contemporaneous sulfide. It also uses an independent method that draws on inferences about the Δ³³S evolution of seawater sulfate to evaluate this further. These two methods yield similar results suggesting that Δ³⁴S_py and λ changed over the course of the Phanerozoic from slightly lower values of Δ³⁴S_py (lower values of λ) in the early Phanerozoic (Cambrian-Permian) to higher values of Δ³⁴S_py (higher values of λ) starting in the Triassic. This change of Δ³⁴S_py and the exponent λ is interpreted to reflect a change in the proportion of sulfide that was reoxidized and processed by bacterial disproportionation on a global scale. The revised record of Δ³⁴S_py also yields model pyrite burial curves making them more closely resemble model evolution curves for other element systems and global sea level curves. It is suggested that possible links to sea level may occur via changes in the area of submerged continental shelves which would provide additional loci for pyrite burial.The slightly different constraints used by the two approaches to calculate this fractionation may allow for additional information to be obtained about the sulfur cycle with future studies. For instance, the correspondence of these results suggests that the inferred variation of ³⁴S/³²S of pyrite is real, and that there is no significant missing sink of fractionated sulfur at the resolution of the present study (such as might be associated with organic sulfur). Burial of organic sulfur may, however, have been important at some times in the Phanerozoic and shorter timescale deviations between results provided by these methods may be observed with higher resolution sampling. If observed, this would suggest either that the record for pyrite (or less likely sulfate) is biased, or that another sink (possibly as organic sulfur) was important during these times in the Phanerozoic.     

Global correlation of basal Triassic layers in the light of the first carbon isotope data on the Permian-Triassic boundary in Northeast Asia

This paper is dedicated to the global correlation of the marine Permian-Triassic boundary layers based on some published and original data on the δ¹³C_org and δ¹³C_carb values obtained for the section along the Suol Creek (Setorym River, southern Verkhoyansk region). The section includes six carbon-isotope intervals readily distinguished in the corresponding curves obtained for several Permian and Triassic reference sections in Eurasia and North America including palleontologically well substantiated sections of central Iran, Kashmir, and South China. This provides grounds for assuming the Permian-Triassic boundary’s position in the Suol Creek section as being close to the carbon-isotope minimum of Interval IV. In the light of new data, the upper part of the Upper Permian Changhsingian Stage in Siberia is proposed to correspond in range to the Otoceras concavum range zone, and the lower substage of the Lower Triassic Induan Stage, to the Tompophiceras pascoei and Wordieoceras decipiens zones. The Otoceras concavum Zone of the Verkhoyansk region in its new understanding is likely correlative with the upper Changhsingian Hypophiceras triviale Zone of Greenland. Carbon-isotope intervals II, III, IV, and V defined in the Permian-Triassic boundary layers of the Verkhoyansk region, which are traceable in several reference sections of Eurasia and North America, evidently coincide with the period of some intensification of the volcanic activity in the initial late Changhsingian and with the first phases of extensive eruption of Siberian trappes in the terminal Changhsingian and initial Induan ages. New data imply the probable survival of some ammonoid species from the superfamily Otoceratoidea after the mass extinction of organisms in the terminal Permian Epoch.     

Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates

Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ¹³C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ¹⁸O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with ¹⁸O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction.