Isotope Excursion Research Articles

Biostratigraphy of the Small Shelly Fossils From the Upper Maidiping Formation (Terreneuvian) at the Fandian Section, Sichuan Province, South China

Small shelly fossils (SSFs) are a useful and critical tool for subdivision and correlation of the Cambrian Terreneuvian stratigraphy. The Maidiping Formation in southern Sichuan is a well-known Terreneuvian lithologic unit with rich SSFs. Here, the upper Maidiping Formation at the Fandian section, an important supplementary section of the classic Maidiping section, is studied for understanding its SSF assemblages and stratigraphic implication. There are 26 genera, 24 species, and 3 undetermined species recovered from the Fandian section, andWatsonella crosbyiwas discovered from this section for the first time. Two SSF biozones of the Yangtze Platform,i.e., the possibleParagloborilus subglobosus–Purella squamulosaAssemblage Zone (? Zone II) and theW. crosbyiAssemblage Zone (Zone III), can be recognized. Comparing with the Maidiping section, the Fandian section has a relatively successive SSF biostratigraphy and carbon isotope stratigraphy in the upper Maidiping Formation. The SSF Zone II (?) and Zone III from the Fandian section can be, respectively, correlated to the same zones from the upper Maidiping Formation of the nearby Maidiping section (only Zone III), the upper Zhongyicun and Dahai members of the Zhujiaqing Formation in eastern Yunnan, the top Kuanchuanpu Formation in southern Shaanxi and northern Sichuan, and the beds 4 and 5 (or units 3 and 4) of the Yanjiahe Formation in western Hubei. The almost cosmopolitan distribution and nearly synchronous earliest appearance ofW. crosbyistrengthen the applicability of its first appearance data (FAD) to define the base of Cambrian Stage 2 in South China (Xiaotanian) as well as in other continents. The FAD ofW. crosbyijust around the onset of the ZHUCE carbon isotope excursion at the Fandian section also further reinforces the utility of the onset of ZHUCE as an auxiliary maker for defining the base of the Xiaotanian in South China.

Biostratigraphically-controlled Darriwilian (Middle Ordovician) δ13C excursions in North China: Implications for correlation and climate change

A continuous, 545 m thick carbonate succession through the Darriwilian Bei'anzhuang and Machiakou (Majiagou) formations in the Tao-112 drillcore in the Ordos Basin, North China was sampled for an integrated study of conodont biostratigraphy (48 samples) and carbon isotope chemostratigraphy (510 samples). Two internationally correlatable conodont biozones were recognized in the drillcore, namely the Histiodella holodentata-Tangshanodus tangshanensis Biozone and the Plectodina onychodonta Biozone in stratigraphically ascending order. Constrained by the conodont biostratigraphy, two carbon isotopic anomalies can be distinguished, including the short-term LDNICE (Lower Darriwilian Negative Isotopic Carbon Excursion) in the lower part of the Bei'anzhuang Formation, and the long-lasting MDICE (Middle Darriwilian Isotopic Carbon Excursion) spanning the upper part of the Bei'anzhuang Formation and much of the Machiakou Formation. Moreover, the present high-resolution carbon isotope record allows the recognition of two peaks of the MDICE, the stratigraphically lower, small peak MDICE-1 and the follow-up, long-lived peak MDICE-2. The Darriwilian carbon isotope excursions in North China can be comparable to the δ13Ccarb trends described in coeval successions in Baltoscandia, North America, South Korea, and other regions of China, suggesting their significance for global correlations and providing an integrated and high-resolution chemo-biostratigraphic framework for sedimentary records of Darriwilian age in North China. Based on widely accepted driving mechanisms of global carbon-cycle perturbations, here we put forth an inference on the climatic pattern of the Darriwilian that a short warm period causing the LDNICE in the early Darriwilian was followed in turn by a moderate global cooling event resulting in the MDICE during the middle and late Darriwilian. Carbon cycle processes of the Middle and Late Ordovician indicate that there was a clear pre-Hirnantian cooling step, with gradually enlarged alternating relative warmer and rather cooler interludes, culminating in the ‘Hirnantian’ ice age.

Mesoproterozoic oxygenation event: From shallow marine to atmosphere

AbstractOxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1%–1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 > 0.1%–1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in δ34SCAS from ~20‰ to ~50‰ and a subsequent decline to ~10‰, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca + Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate.

Spatial heterogeneity in carbonate-platform environments and carbon isotope values across the Paleocene–Eocene thermal maximum (Tethys Himalaya, South Tibet)

The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is a large negative carbon isotope excursion (CIE) that testifies to a massive perturbation of the global carbon cycle and has been considered to be an ancient, deep-time analogue for present and future climate change. However, the environmental and carbon isotopic response to the PETM in shallow-water carbonate platforms has remained largely elusive. This study presents new sedimentological, biostratigraphic, and carbon isotopic data to evaluate the impact of the PETM on a shallow-water carbonate platform from more proximal southern parts of the northern Indian continental margin. Detailed biostratigraphy and carbon isotope data indicate that the PETM occurs 3 m below the boundary between nodular and thin-bedded limestones, is maintained up to the thin-bedded limestone and calcareous marl interval, and is followed by recovery at the base of thick-bedded limestone interval. Microfacies analysis testifies to a regression from open to restricted shallow-marine environments at the transition from PETM onset to PETM core, and shallowing continued through the PETM core. Restricted lagoonal deposition was renewed during PETM recovery. These environmental changes were associated with two major turnovers of shallow-water biota. We infer that the first sudden biotic change at the PETM onset may relate to intensified continental weathering, whereas the second biotic change at PETM recovery may have been caused by sea-level fall. The smaller magnitude of the CIE observed in proximal, shallower-water than in distal, deeper-water environments is ascribed to increased primary productivity due to increased nutrients' supply associated with intensified continental weathering. • A regression at the PETM onset from open to restricted shallow-marine environments, and restored to restricted lagoon at the recovery • The sudden environmental changes across PETM were associated with major changes of shallow-water biota • The smaller magnitude of the CIE observed in shallower-water than in deeper-water strata is ascribed to increased primary productivity

VALANGINIAN RADIOLARIANS OF NE SARDINIA (ITALY) IN THE FRAME OF THE WEISSERT EVENT

This article presents the very rich radiolarian fauna from the S’Ozzastru section of the Siniscola Marl crop-ping out in the northern part of Mt Albo massif (NE Sardinia, Italy). The formation was ascribed to the lower part of the upper Valanginian on the basis of the ammonite fauna indicating the Verrucosum Zone and the nannofossil con-tent suggesting the Zone NK3. Due to the scarcity of pantanelliids and especially of the absence of Cecrops septem-poratus, radiolarian marker of the upper Valanginian, radiolarians can only indicate a Valanginian age. Investigations for integrated litho-, bio-, and chemostratigraphy allowed the identification of the Valanginian Weissert Oceanic An-oxic Event (OAE), testified by a positive carbon isotope excursion (CIE) with values up to 2.98 ‰. Radiolarian study includes not only a determination of the species for biostratigraphic interpretations, but also a revision of the system-atics of the radiolarians occurring in the section. One new family (Crococapsidae), 3 new genera (Apiaromitra, Mi-crosandwichia and Ocellicapsa) and 43 new species are erected. The fauna has some elements in common with the Valanginian faunas from the Mediterranean area and many more with those from Japan, proving a wide circulation between the western and eastern Tethys.

The timing, duration and magnitude of the 8.2 ka event in global speleothem records

Abrupt events are a feature of many palaeoclimate records during the Holocene. The best example is the 8.2 ka event, which was triggered by a release of meltwater into the Labrador Sea and resulted in a weakening of poleward heat transport in the North Atlantic. We use an objective method to identify rapid climate events in globally distributed speleothem oxygen isotope records during the Holocene. We show that the 8.2 ka event can be identified in >70% of the speleothem records and is the most coherent signal of abrupt climate change during the last 12,000 years. The isotopic changes during the event are regionally homogenous: positive oxygen isotope anomalies are observed across Asia and negative anomalies are seen across Europe, the Mediterranean, South America and southern Africa. The magnitude of the isotopic excursions in Europe and Asia are statistically indistinguishable. There is no significant difference in the duration and timing of the 8.2 ka event between regions, or between the speleothem records and Greenland ice core records. Our study supports a rapid and global climate response to the 8.2 ka freshwater pulse into the North Atlantic, likely transmitted globally via atmospheric teleconnections.

LIP volcanism (not anoxia) tracked by Cr isotopes during Ocean Anoxic Event 2 in the proto-North Atlantic region

Chromium is a redox sensitive element that exhibits a large range of isotopic compositions in Earth’s surface environments because of Cr(VI)-Cr(III) transformations. This property of Cr has been exploited as a tracer of Earth’s oxygenation history using marine sediments. However, paleoredox applications using Cr are difficult to implement due to its complicated cycling, which creates spatial variability in seawater δ53Cr values. Applications are further hindered by the potential for variability in the major inputs of Cr, such as submarine volcanism, to mask redox processes. Two previous reports of negative excursions in sedimentary δ53Cr values during the middle Cretaceous Ocean Anoxic Event 2 (OAE 2) demonstrate these complications. Observed negative shifts in marine sediments conflict with the positive shifts expected in response to the increased drawdown of isotopically light Cr(III) prompted by the expansion of anoxic depositional sinks. In this study, a marine carbonate succession cored from the Eagle Ford Formation in Texas, USA, in the southern part of the Western Interior Seaway, depicts the negative 1.5‰ δ53Cr excursion occurring in two steps, with the second step reaching peak minimum values indistinguishable from isotopically unfractionated igneous sources. In contrast to published δ53Cr records, each step stratigraphically matches proxy evidence for increased eruption frequency and/or intensity of volcanic activity using combined 187Os/188Os, 87Sr/86Sr and Os concentration proxies previously measured from the same core, supporting higher inputs of volcanically sourced Cr to the oceans as the driver for the negative Cr isotope excursion.

The Ediacaran record of glaciogenic dropstones, diamictites and cap carbonates associated with non-metamorphosed banded iron formations (BIFs) in Iran

The Ediacaran period is marked by global icehouse episode, greenhouse period, increasing levels of oxygen-carbon dioxide, syn-glaciogenic structures-BIFs, typical Ediacaran biota and a marked negative carbon isotope excursion. On this global point, field glaciogenic structures, petrographic textures, Ediacaran biota, U-Pb zircon ages, C-O isotopes and geochemical data record an Ediacaran glaciogenic BIFs event in Iran. The Ediacaran (578–547 or 578–541.8 Ma) glaciogenic sequences in Bafq and Hormuz Island mining districts include submarine syn-glaciovolcanic rhyolitic rocks, rhyolitic tuffs, shaly tuffaceous carbonates, dropstone-bearing jaspilitic BIFs, salt minerals, ferruginous diamictites, and cap carbonates. The presence of polygonal, bullet-shape and flatiron forms of gneiss, greenschist, quartz schist, quartzite, marble and magnetite-hematite clasts in a matrix of jaspilitic magnetite-hematite, muddy sandstones and muddy dolomitic rocks records an Ediacaran glaciogenic event. The dropstones occur as lone-stones, dump-stones and deflect their underlying layers. The diamictites contain similar clasts to those of the dropstones, are gradationally covered by buff to dark-grey cap carbonates ± dropstones and resemble the Ghaub, Chuos and Kingston Peak Formation diamictites. C-O isotopes, REE+Y diagrams, high Y/Ho ratios, positive Eu-Y anomalies and subtle Ce anomaly possibly reflect a possible hydrothermal origin for the cap carbonates. Delta 13C values (−0.42 to −6.33‰) in cap carbonates show similar negative signatures to the Ediacaran Shuram glaciogenic event. Average delta 18O (−8.34‰) in the least-altered cap carbonates gives a precipitation temperature of 252.7 °C in the waning stage of the glaciogenic BIFs-hydrothermal system.

The Black Angel deposit, Greenland: a Paleoproterozoic evaporite-related Mississippi Valley-type Zn–Pb deposit

The Paleoproterozoic Mârmorilik Formation in the Karrat basin of West Greenland hosts the Black Angel Zn–Pb deposit. Chlorine-rich scapolite, zones with vuggy porosity and quartz nodules in the ore-bearing marble are herein interpreted to represent metamorphosed, vanished, and replaced evaporites, respectively. Mineralization is closely associated with anhydrite with δ34S values (5.2–12.6‰) broadly comparable to published values for Paleoproterozoic seawater sulfate. Considering the fundamental attributes of the mineralization and host sequence, a Mississippi Valley-type (MVT) model is the most obvious explanation for mineralization. Overlying the ore-bearing sequence are organic-rich semipelites and massive calcitic marbles, which may have served as seals for hydrocarbon or reduced sulfur and acted as chemical traps for deposition of the sulfidic ore. The Mârmorilik Formation contained an interlayered sulfate-rich evaporite-carbonate sequence, a common setting for MVT deposits in the late Neoproterozoic and Phanerozoic, but unique among the few known MVT deposits in the Paleoproterozoic. This ca. 1915 Ma evaporite-carbonate platform is younger than sulfate evaporites deposited during and immediately after the ca. 2220–2060 Ma Lomagundi carbon isotope excursion and records a significant seawater sulfate level during a time interval when it was assumed that it had been too low to form extensive evaporite deposits. Therefore, MVT and clastic-dominated (CD) Zn–Pb deposits in the geological record might progressively fill the apparent gap in marine sulfate evaporites and provide unique insights into Proterozoic seawater sulfate level. Considering the sequence of tectonic events that affected the Karrat basin, the mineralization took place between Nagssugtoqidian collision (< 1860 Ma) and Rinkian metamorphism (ca. 1830 Ma).

Cadmium isotopes in Late Ediacaran–Early Cambrian Yangtze Platform carbonates – Reconstruction of bioproductivity in ambient surface seawater

Cadmium (Cd) isotope signatures (expressed as δ114Cd values) in seawater and in suitable modern and ancient sediments have been proposed as a useful tracer for paleobioproductivity and for constraining Cd as a micronutrient. This study contributes to the calibration of Cd isotope compositions in ancient shallow-water carbonates and proposes a combination with chromium (Cr) isotopes to link bioproductivity and ocean redox condition. We analysed 64 carbonate samples from the Jiulongwan, Gaojiaxi-Yanjiahe and Xiaotan sections of the Yangtze Platform, South China, covering the Shuram negative carbon isotope excursion (SCE) and Ediacaran-Cambrian transition (ECT), for Cd concentrations [Cd] and Cd isotopes. The results of this study show δ114Cd values ranging from −0.9‰ to +0.6‰, averaging −0.20 ± 0.65‰ (2σ, n = 56) and [Cd] with substantial variations from 0.003 μg/g to 2.78 μg/g. Using the experimental fractionation factor for Cd into calcite (−0.45‰), we calculate an average ambient surface seawater δ114Cd during the entire studied period of +0.24 ± 0.65‰, largely covering today's surface and deep ocean water compositions. The δ114Cd of ambient seawater during the SCE is reconstructed to an average value of +0.48 ± 0.40‰. This elevated signature possibly reflects the result of increased nutrient upwelling and concomitant enhanced bioproductivity levels in the upper photic surface waters during this period, leading to an increased oxygenation of the surface water. The carbonates deposited during the post-Shuram-Wonoka interval reveal a strong correlation between [Cd] and [Zn] and a correlation between degree of negative Ce anomalies and positive δ114Cd excursions, which point to strongly oxidized surface waters and concomitant biotic removal of isotopically light Cd isotopes into phytoplankton. It remains unclear why surface waters during the Late Ediacaran—Early Cambrian did not exhibit more positively fractionated Cd isotope signatures as expected from elevated bioproductivity during this transitional period.

Insights into the Devonian–Carboniferous transition and Hangenberg Event from δ13Ccarb and 87Sr/86Sr chemostratigraphy of shallow platform carbonate strata of northwestern Colorado

ABSTRACTPaired chemostratigraphic and biostratigraphic data suggest that the Devonian–Carboniferous boundary and the Hangenberg extinction event are recorded in the Coffee Pot Member of the Dyer Formation of the White River uplift region of northwestern Colorado. The Hangenberg isotopic excursion interval occurs in biostratigraphically depauperate shallow platform micritic dolostone and limestone representing the aculeatus–?ultimus–kockeli? Zone. The Hangenberg interval strata have δ13Ccarb values up to 7.69‰, and locally contain ooids, stromatolites, and other microbial structures. In three sections, there is a short-lived &amp;gt; 4‰ “pre-Hangenberg” positive excursion that is associated with the presence of detrital dolomite siltstone. The main Hangenberg isotopic signature exists in overlying strata, and is replicated in four different sections of the Dyer Formation across an area ~300 km2. In each section, the excursion interval is preceded by, and in one location is interrupted by, evidence of meteoric diagenesis and/or emergence characterized by macroscopic dissolution features and pronounced negative δ13Ccarb values (to ca. -6‰). Conodont 87Sr/86Sr ratios through the Dyer Formation dip to nearly 0.708 during the onset of the Hangenberg Event, consistent with the brachiopod based 87Sr/86Sr nadir from the same interval in Missouri, France, and Germany. The least-radiogenic trendline through the Dyer 87Sr/86Sr record matches the global minimum 87Sr/86Sr trendline through the Famennian–Tournaisian transition and infills several gaps in the global record.

The Guttenberg δ13C excursion (GICE) and paleoclimate change recorded in the early Late Ordovician carbonate succession of the Ordos Basin, Northern Chinas

The well-known Guttenberg isotopic carbon excursion (GICE) is a global palaeoceanographic event in the early Katian, Late Ordovician. Here, based on analysis of the carbon oxygen isotopes and K-bentonite characteristics, we documented the evidence of the GICE from the Upper Ordovician carbonate succession in the Ordos Basin, Northern China. The results revealed a positive δ13C excursion occurred in the Jinsushan Formation. Based on SHRIMP U-Pb dating of the K-bentonites and biostratigraphic constraints, the carbon isotope excursion of X1 (in Xilingggou section) and Z1 (in Zhaolaoyu section) identified in the area can be regarded as local expressions of the GICE. It is the first time that the GICE was identified in the Ordos Basin. Compared with the GICE profiles from the Dolly Ridge, North America and the Mehikoorma Drill Core from Europe, the X1 profile exhibits a similar feature with a positive δ13C excursion of 2.11‰. While the Z1 profile shows a two-peaks curve with a positive shift up to 5.61‰, representing one of the largest shifts in δ13C for the GICE so far. Considering that the X1 and Z1 profiles started immediately above the K-bentonites, and the GICE in North America and Europe were also in close association with marked bentonites, we argued that the carbon cycle perturbation depicted by the GICE and its equivalents were genetically related to increased volcanic eruptions in the early Late Ordovician, which may have led to a climate cooling and enhanced organic burial, thus resulted in the positive δ13C excursion.

Linkage of the late Cambrian microbe-metazoan transition (MMT) to shallow-marine oxygenation during the SPICE event

Microbe-metazoan transitions (MMTs), representing a switch from microbe-mediated to metazoan-mediated carbonate production, have been linked to major changes in Earth-surface conditions. The ‘late Cambrian MMT’ (nomen novum), during which microbial reefs were replaced by maceriate and lithistid sponge reefs, coincided with a sharp rise in atmospheric O2 levels attributed to the Steptoean Positive Carbon Isotope Excursion (SPICE) at ~497–494 Ma. However, relationships between atmospheric oxygenation, marine redox conditions, and the MMT have not been thoroughly investigated to date. Here, we conducted paired analyses of carbonate carbon isotopes (δ13Ccarb), sulfur isotopes of carbonate-associated sulfate (δ34SCAS) and pyrite (δ34SPy), pyrite morphologies, and fossil assemblages of two upper Cambrian shallow-marine sections on the North China Platform (Tangwangzhai and Huangyangshan). The sulfur isotopic composition of framboidal pyrite (δ34SFrPy) was calculated from that of total pyrite (δ34STPy) assuming a varied fractionation with euhedral pyrite based on pyrite morphological analyses. Modelling of S-isotope fractionations (Δ34SCAS-FrPy) indicates that contemporaneous seawater had low sulfate concentrations (i.e., [SO42––]sw ~4–6 mM) with locally high spatial heterogeneity during the Rising SPICE (early to middle Paibian), which were the product of massive burial of pyrite (together with organic matter) linked to extensive oceanic anoxia. Immediately following the SPICE, a reduced pyrite burial rate and an increased flux of terrestrially sourced, 34S-depleted sulfate to shallow-marine environments resulted in lower δ34SCAS values. [SO42–]sw rose to a maximum of ~9–16 mM and became more uniform during the Post-SPICE (middle to late Jiangshanian), suggesting a more oxidized condition of seawater and the atmosphere. Rising O2 levels in both the atmosphere and marine environments triggered the late Cambrian MMT and set the stage for the subsequent Great Ordovician Biodiversity Event.

Biostratigraphic and chemostratigraphic record of the Cenomanian/Turonian boundary interval of the Tellian Atlas (Algeria)

A multi-proxy approach integrating data on planktonic foraminifera, calcareous nannofossils, δ13Ccarb, δ18Ocarb and the coiling direction of Muricohedbergella delrioensis was carried out for two sections in the Tellian Atlas (Algeria) both to establish a precise stratigraphic scheme and to precise the climatic events.The Sour el Ghozlane section offers a good record of the Oceanic Anoxic Event 2 (OAE2) in the southern margin of the Tethys, with organic-rich sediments and the continuous presence of planktonic foraminifera, nannofossils and other microfossils. The Cenomanian/Turonian boundary (CTB) in the Sour el Ghozlane section lies in the interval limited at the base by the last occurrence of Helenea chiastia and the first occurrence of Quadrum gartneri respectively located immediately below and above the last peak (C) of the carbon isotope excursion. However, in the Sidi Nadji section, the CTB interval is extremely thin; the last occurrences of Rotalipora cushmani and Thalmanninella greenhornensis coincide with the last occurrences of Corollithion kennedyi, Helenea chiastia, Axopodorhabdus albianus and Rhagodiscus asper and occur in the same level, therefore a hiatus is suspected.Sour el Ghozlane and Sidi Nadji are so far the southernmost sections where the percentage of sinistral M. delrioensis has been investigated. High proportion of left-coiled morphotypes among M. delrioensis indicate transient cooling episodes, including the Plenus Cold Event at the onset of OAE2 and before the warm conditions at the CTB characterized by the dominance of dextral forms.

Laurentide Ice Sheet extent over the last 130 thousand years traced by the Pb isotope signature of weathering inputs to the Labrador Sea

Understanding the history of continental ice-sheet growth on North America, and in particular that of the Laurentide Ice Sheet (LIS), is important for palaeoclimate and sea-level reconstructions. Information on ice-sheet extent pre-dating the Last Glacial Maximum (LGM) is heavily reliant, though, on the outputs of numerical models underpinned by scant geological data. Important aspects of LIS history that remain unresolved include the timing of its collapse during Termination 2, the first time that it expanded significantly during the Last Glacial Cycle, and whether its volume was significantly reduced during marine isotope stage (MIS) 3. To address these issues and more, we present authigenic iron-manganese (Fe–Mn) oxyhydroxide-derived high-resolution records of Pb isotope data and associated rare earth element profiles for samples spanning the past ∼130 kyr from northwest North Atlantic Labrador Sea, IODP Site U1302/3. We use these new data to track chemical weathering intensity and solute flux to the Labrador Sea associated with LIS extent on the adjacent highly radiogenic (high Pb isotope composition) North American Superior Province (SP) craton since the Penultimate Glacial Maximum (PGM). Our new records show that relatively high (radiogenic) values characterise warm marine isotope stages (MIS) 5, 3 and 1 and the lowest (most unradiogenic) values occurred during cold stages MIS 6, 4 and 2. The radiogenic Pb isotope excursion associated with Termination 2 is short-lived relative to the one documented for Termination 1, suggesting that LIS retreat during the PGM was relatively fast compared to the LGM and that its collapse during the last interglacial occurred ∼125 ka. Highly radiogenic inputs to the Labrador Sea during MIS 5d-a, ∼116–71 ka, most likely reflect a spin-up in Labrador Current vigour, incipient glaciation and renewed glacial erosion of high grounds of the eastern SP craton by localised wet-based ice-caps. A large decrease in Pb isotope values towards unradiogenic LGM-like compositions between ∼75–65 ka across the MIS 5/4 transition likely reflects a slow-down in Labrador Current vigour, an increase in subaerial deposition of aeolian dust and a significant advance of the LIS across Hudson Bay caused a strong reduction or even abandonment of Pb sourcing from the SP. The relatively radiogenic Pb isotope composition of bottom-waters bathing our study site during MIS 3, 57–29 ka, is unlikely to support a recently proposed major reduction in LIS extent for this time. Instead, we argue these values are better explained by southern Greenland Ice Sheet retreat, increased chemical weathering of the Ketelidian Mobile Belt and subsequent Pb runoff from Greenland.

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n-alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ13C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).

2.0 Ga orogenic graphite deposits and associated 13C-enriched meta-carbonate rocks from South China Craton: Implications for global Lomagundi event

The Lomagundi (-Jatuli) event, characterized by extremely high positive global inorganic carbon isotope excursion at about 2.2 billion years ago, is pivotal in investigating the causes and consequences of great oxygenation event, inventory and sequestration of carbon on the Earth’s surface, evolution of life, and more profoundly tectonic control on Earth’s environment. However, the reasons that caused the isotopic excursion are not resolved yet. Herein, we report the discovery of meta-carbonate rocks with distinct positive carbon isotopic excursion from the Paleoproterozoic continental collision zone of the Kongling Complex, South China Craton. The δ13CV-PDB values for meta-carbonate rocks show positive values in the range from +5.5‰ to +11.6‰, whereas the δ13CV-PDB values of associated graphite deposits range from −25.8‰ to −9.5‰. Zircon U-Pb-Hf isotopes from zircon-bearing meta-carbonate sample yielded weighted average 207Pb/206Pb age of 2001.3 ± 9.5 Ma, with corresponding εHf(t) range from −7.05 to −3.16, comparable to the values of local 2.9–2.6 Ga basement rocks. Geochemical characteristics of meta-carbonate rocks, such as their rare earth element patterns and the trace element parameters of La, Ce, Eu, and Gd anomalies and Y/Ho ratio, suggest that the carbonate deposition took place in passive continental margin in association with large volumes of organic carbon. The extensive graphite deposits from Kongling Complex in South China Craton, their equivalents in the North China Craton and elsewhere across the globe prove that the burial of 12C-enriched organic carbon has eventually resulted in the global enrichment of 13C in the atmospheric CO2, which is recorded in the marine carbonate rocks. Isotopic mass balance estimates indicate that more than half of the organic carbon was buried during the oceanic closure. Hence, the observed global shift could be directly related to the continent collision event in greater China, thus resolving the long-standing paradox of the Lomagundi global positive carbon isotope excursion. Moreover, the present results suggest that orogenesis play a significant role in sequestration of carbon into the continental crust.

Ediacaran carbon cycling and Shuram excursion recorded in the Tarim Block, northwestern China

The largest perturbation of the carbon cycle in geological history, named as the ‘Shuram Carbon Isotope Excursion (SCIE)’, has been widely reported in the Ediacaran Period in different continents over the world. In the Tarim Block, a complete Ediacaran carbonate succession was deposited; however, whether it recorded the SCIE or not still remains controversial, hindering establishment of a reliable stratigraphic framework and understanding of environmental perturbations and evolution during the Ediacaran Period. Here, we present two high-resolution carbon isotopic stratigraphy profiles from the upper Sugetbrak through the Qigebrak formations in the northwestern Tarim Block. Two negative excursions (TN1, TN2), and two positive excursions (TP1, TP2) are identified from both sections and supposed to be of the primary origin. Of these, the TN1 was recorded in a storm-dominated inner-to-middle ramp beginning with the marine transgression. It marks the largest negative excursion with a nadir down to ca. −12‰ which can be correlated to the SCIE of late Ediacaran in terms of its similar magnitude, variation pattern, time constraints and depositional characteristics. Other carbon isotope anomalies can also be correlated with coeval anomalies elsewhere around the world. The temporal coincidence of TN1 (or SCIE) with onset of transgressive, storm-dominated successions suggests that enhanced oceanic upwelling and storm agitations together may have greatly increased the depth of ventilated seafloor and extent/volume of oxygenated DOC pool in shallow marine realms, leading to the extremely δ13C excursion of carbonates. Our new carbon isotopic chemostratigraphic profiles of upper Ediacaran from the Tarim Block thus not only provide a more reliable time stratigraphic framework helpful for documenting depositional history and environment-life evolution, but also promote comparisons with other continental blocks globally.

Paleoenvironmental evolution during the Early Eocene Climate Optimum in the Chicxulub impact crater

The Early Eocene represents the warmest climate during the Cenozoic Era punctuated by transient warming events. Here, we apply for the first time biomarker and stable isotope data to describe the depositional and environmental changes at the actual Chicxulub impact crater [International Ocean Discovery Program (IODP) Site M0077A] during the Early Eocene (∼56–48 Ma) including the Early Eocene Climate Optimum (EECO; ∼53–49 Ma). TEX86-based reconstructions indicate elevated sea surface temperature around 37°C for the entire succession, punctuated by warmer and cooler periods (37.8°C, 35.7°C, respectively). Biomarker and δ13C-org data of the core sediments reveal a shift from an organic-lean lower to an organic-rich upper section of the Early Eocene interval. Embedded in the lower section is a prominent organic-rich black shale layer. The lower section, directly overlying strata deposited upon the Paleocene-Eocene Thermal Maximum (PETM), is organically lean with low biomarker concentrations indicating low primary productivity. In the upper section, higher concentrations of algal and bacterial biomarkers are ascribed to high primary productivity and nitrogen fixation in an epipelagic zone, potentially supported by upwelling. The organic-rich black shale layer exhibits a negative δ13C-org isotope excursion ascribed to recycling of 13C-depleted organic matter via upwelling attributed to a transient hyperthermal event.

A reassessment on the timing and potential drivers of the major seawater 87Sr/86Sr drop in the Ordovician Period: New evidence from conodonts in China

The major drop in marine 87Sr/86Sr records across the Middle-Late Ordovician boundary (Darriwilian-Sandbian) was one of the most dramatic perturbations of the global Sr cycle in Phanerozoic. However, the precise timing of this isotopic event has not been well constrained, partially making it difficult to further clarify the possible drivers of this isotopic event. For this study, we present multi-continental conodont apatite-based Sr-isotope datasets from three well-studied Ordovician carbonate successions in China, i.e., the Huanghuachang section in South China, the Kalpin Shuinichang-Dawangou and the Nanyigou sections in Tarim Basin, northwestern China. Our new 87Sr/86Sr records generally accord with global seawater 87Sr/86Sr secular variation curve and are supposed to be a reflection of coeval primary seawater signatures. Constrained with detailed conodont biostratigraphic and high-resolution δ13C chemostratigraphic framework, the onset of the sharp decline in 87Sr/86Sr ratios was placed within the Histiodella holodentata zone-Eoplacognathus suecicus zone, and approximately synchronous with the initiation of the Middle Darriwilian Carbon Isotope Excursion (MDICE). Numerical modeling reveals that this major 87Sr/86Sr drop could have been generated by a sustainable increase of oceanic hydrothermal (mantle-derived) Sr flux and/or a progressive decrease of continental-derived Sr flux. Multi-proxy analyses further demonstrate that the impact of enhanced oceanic hydrothermal activities accompanying with the accelerating opening of the Rheic and Paleo-Tethys oceans, may have played a central role in driving the major 87Sr/86Sr drop during the Middle-Late Ordovician transition.