Positive Excursion Research Articles

Organic carbon isotope stratigraphy of Devonian-Carboniferous boundary sections in the Rhenish Mountains

The boundary between the Devonian and the Carboniferous has been drawn using ammonoids, conodonts and miospores, while geochemical data have only rarely been used. The sampling of eight sections (Aprath, Apricke, Effenberg, Wocklum, Stockum, Drewer WJ, Drewer WA, Marsberg) at the northern margin of the Rhenish Mountains demonstrates the potential of the isotope ratio of organic carbon to regionally correlate sections in different facies areas. With the help of the carbon isotopes, a distinct stratigraphic succession is documented, which is characterised by several positive and negative excursions; this succession was recorded in all examined sections. The carbon isotopes can therefore make a significant contribution to the stratigraphic subdivision of sections and complement the biostratigraphic methods.

Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco

The Ocean Anoxic Event II (OAE2) was a significant global event associated with a positive carbon isotope excursion, occurring from the Late Cenomanian to the Early Turonian (C/T). It has been commonly associated with enhanced organic carbon preservation. Carbon isotopic analysis carried out over the C/T interval in the Agadir Basin, Morocco, integrated with previous biostratigraphic data, allows a refined correlation of the OAE2 interval. Detailed sedimentary facies analysis identifies ten lithofacies that record the transition from shallow marine environments in the Late Cenomanian to relatively deeper conditions in the Early Turonian. The variation in lithofacies can be correlated to relative sea level changes that show a correlation to the global eustatic curve.The OAE2 interval comprises dark grey mudstones beds that display low total organic carbon (TOC) values. Trace element and facies analysis suggest dilution from high detrital influx, along with oxic water conditions and low productivity. OM-rich black mudstones are identified in post-OAE2 Early Turonian strata. Trace element analysis suggests this increase in organic matter accumulation was related to increased sea surface productivity and oxygen-depleted bottom water conditions, which facilitated organic matter preservation. Deposition of OM-rich black mudstones is widely reported during the global Early Turonian marine transgression, suggesting the very high sea level was a major control on organic matter generation and preservation.

Animal abundance and redox conditions during the Furongian Cambrian SPICE event, western Utah

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is recognized as a global perturbation of the carbon cycle, with δ13C excursions (+4–5‰) found worldwide. The SPICE occurred in the Furongian Epoch of the Cambrian Period (∼500 Ma) and is unique among Paleozoic positive Carbon Isotope Excursions (CIEs) for its magnitude and global correlation, and has been well studied, though its causative mechanism is still debated. The Lawson Cove and House Range sections of western Utah are two localities where the SPICE was first reported and are relatively fossiliferous and thick, allowing us to document shifts in skeletal abundance and diversity through point counts of thin sections coupled with δ13Ccarb, δ13Corg, TOC, and Hg measurements through both sections. The concurrent brief disappearance of trilobites at the onset of the SPICE in both sections, aligned with the Marjumiid-Pterocephaliid biomere boundary, supports a shared mechanism for the δ13C spike and the extinction event. The greatest abundance of fossils was found during the rising limb of the SPICE, reaching, or exceeding 49% of points per slide in both sections. The highest SPICE δ13C values correlated with a brief absence of fossil material in the Lawson Cove section. The House Range recorded a prolonged depauperate stretch during the peak δ13C values of the SPICE, correlated with a facies change at the transition into the Johns Wash Limestone Member of the Orr Formation. The differences seen between the sections highlight the impact of local processes in recording the SPICE event.

Deglacial volcanism and reoxygenation in the aftermath of the Sturtian Snowball Earth.

The Cryogenian Sturtian and Marinoan Snowball Earth glaciations bracket a nonglacial interval during which Demosponge and green-algal biomarkers first appear. To understand the relationships between environmental perturbations and early animal evolution, we measured sulfur and mercury isotopes from the Datangpo Formation from South China. Hg enrichment with positive Δ199Hg excursion suggests enhanced volcanism, potentially due to depressurization of terrestrial magma chambers during deglaciation. A thick stratigraphic interval of negative Δ33Spy indicates that the nonglacial interlude was characterized by low but rising sulfate levels. Model results reveal a mechanism to produce the Δ33S anomalies down to -0.284‰ through Rayleigh distillation. We propose that extreme temperatures and anoxia contributed to the apparent delay in green algal production in the aftermath of the Sturtian glaciation and the subsequent reoxygenation of the iron-rich and sulfate-depleted ocean paved the way for evolution of animals.

Stratigraphic architecture of the Tethyan Cenomanian-Turonian succession and OAE2 in the Dokan Area, Kurdistan Region, northeast Iraq

This study provides a detailed examination of a condensed Cenomanian-Turonian (C-T) succession of two sections (Dokan Dam and Khalakan) in the Kurdistan Region, Northeastern Iraq, based on biostratigraphy (calcareous nannofossils and planktic foraminifera), carbon and oxygen isotope geochemistry, and facies analysis. The C/T boundary in this region is characterized by a hiatus noticeable due to the absence of the Globigerinelloides benthonensis and Dicarinella hagni subzones and the lack of positive carbon isotope excursion (CIE) peak b during the Oceanic Anoxic Event 2 (OAE2). Positive CIEs in our studied succession include three peaks (a, c, and d) from the four established positive global CIE peaks (a, b, c, and d). Peak a is recognized at the upper part of the Cenomanian Dokan Formation, while peak b is missing within the hiatus across the C/T boundary. Peak c is found within the lowest Turonian above the Gulneri/Dokan formational boundary, and peak d appears at the base of the middle Turonian Kometan Formation. Twelve microfacies and four submicrofacies types are identified and grouped by depositional paleoenvironments into five facies associations, ranging from peritidal to basinal settings. Three incomplete third-order depositional sequences (DSs) and their transgressive-regressive cyclic stacking pattern have been identified in the Cenomanian-Turonian succession studied herein. These DSs are associated with hiatuses, suggesting tectonic control combined with a global sea-level fall. Hiatuses and extreme condensation recorded in the latest Cenomanian-early Turonian successions on many Neo-Tethys platforms resulted from platform exposure, subsequent drowning, and a warming climate.

First record of Oceanic Anoxic Event 2 in the Eastern Carpathians: implications for chemostratigraphic and biostratigraphic correlations

This study aims to investigate a marlstone and claystone succession located at the southern end of the Eastern Carpathians, a region where oceanic anoxic event 2 (OAE2) has not been pointed out so far. Toward the upper half of this succession, a 17-cm-thick black shale was identified. The investigated depositional interval lies within the late Cenomanian–early Turonian, encompassing the UC3d up to UC7 nannofossil zones. The δ13C values fluctuated between 2.06‰ and 3.89‰, showing a positive isotope excursion that was assigned to OAE2. The δ13C isotope curve displays the following intervals: pre-excursion, first build-up, trough, second build-up, plateau, and post-excursion. Within the second build-up interval of OAE2, a substantial shift in CaCO3 values, accompanied by high concentrations of total organic carbon and a significant decline in the abundance and diversity of calcareous nannofossil assemblages, was observed. The nannofossil turnover related to OAE2 climax revealed predominance of Watznaueria barnesiae and temporary disappearance from the record of surface-water higher fertility taxa, such as Biscutum constans, Zeugrhabdotus erectus, and Discorhabdus ignotus. Above OAE2, peaks of Eprolithus floralis, followed by increased abundance of Eiffellithus turriseiffelii and Nannoconus spp., were identified. In the lower part of the studied succession (i.e., the upper Cenomanian UC3d nannofossil subzone), during the pre-excursion characterized by low δ13C values and less negative δ18O values, a small group of nannofossils more related to mid- and high-paleolatitudes, such as Crucibiscutum salebrosum, Repagulum parvidentatum, and Seribiscutum primitivum, is present, always showing a low abundance.

Upper Carnian to Rhaetian (Upper Triassic) conodont and carbon-isotope stratigraphy of the Csővár borehole succession, Hungary

A total of 125 samples from core from the Csővár borehole (north-central Hungary) were investigated for conodont bio- and carbon-isotope chemostratigraphy. This 620 m thick continuous pelagic carbonate succession comprises strata from the upper Carnian to upper Rhaetian, including both, the Carnian-Norian and the Norian-Rhaetian stage boundaries. The mass occurrence of the Metapolygnathus communisti conodont group followed by the occurrence of Primatella gulloae mark the base of the Norian in the lowermost part of the section, which allows good correlation with the Pizzo Mondello section (Sicily, Italy). The lower Norian conodonts of the core belong predominantly to the genus Ancyrogondolella. Subdivision of the middle Norian interval is hampered by the lack of fully adult specimens. The upper Norian shows a greater diversity than the rest of the Norian, and the rich assemblages give an insight into the evolutionary transition from Misikella hernsteini to Mi. posthernsteini (sensu stricto). The first occurrence of Mi. ultima in the core changes the previous views of the thickness of the upper Rhaetian in the Csővár area and emphasizes the necessity of re-calibration of the isotope stratigraphic results of previous studies. The carbon-isotope results show distinct fluctuations and trends. Some of them are similar to literature data, others are different. It can be stated from the new data that a ∼ 2 ‰ long-term δ13C background decline throughout the Norian is most probably global in extent. Negative carbon-isotope excursions occur in Norian substages: (upper) Alaunian, lower Sevatian, middle Sevatian and upper Sevatian. Distinct positive δ13C excursions are present across the Alaunian-Sevatian boundary and the Norian-Rhaetian boundary, and the climax of the latter is located in the lower part of the posthernsteini Zone. This last isotope event is particularly important because at the Global Stratotype Section and Point (GSSP) candidate for the base of the Rhaetian stage at Pignola (Southern Apennines, Italy) a distinct negative δ13C excursion has been reported for this stratigraphic level.

The Sommerodde (Telychian, Silurian) positive carbon isotope excursion: why is its magnitude so variable?

The Sommerodde positive organic carbon isotope excursion (SOCIE) within the Oktavites spiralis graptolite Biozone (Telychian, Silurian) was first identified in the Sommerodde-1 core, Bornholm, Denmark, where it is the largest positive excursion within the Upper Ordovician–lower Silurian part of the core. Other published occurrences of the SOCIE are discussed here, including new δ 13 C org data from the Jabalón River section, Corral de Calatrava, central Spain, where the SOCIE is only a very minor positive excursion. Very unusually, the SOCIE is best developed in deeper water settings, contrary to the typical pattern of declining excursion magnitude offshore. In the Sommerodde-1 core (Bornholm), and where it has been tentatively identified in the Vežaičiai-2 core (Lithuania), the SOCIE is developed in pale, organic-poor mudstones. It is considered likely that the magnitude of the SOCIE has been enhanced in the Sommerodde-1 core record by a change in organic matter composition in the deep-marine environment that did not have such a significant effect in shallower marine environments. Supplementary material: A table of organic carbon isotope data from the Jabalón River section, Corral de Calatrava, central Spain is available at https://doi.org/10.6084/m9.figshare.c.6769514 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Seawater carbon and strontium isotope variations in the eastern Tarim Basin during the Cambrian: Implications for connection with the open ocean

Cambrian was a critical period in Earth's history, marked by rapid diversification of early animals and significant changes in oceanic conditions. Marine sediments, particularly carbonates, have been used to study variations in the global carbon cycle, seawater redox, weathering input, etc. In search of new archives of Cambrian paleo-seawater chemistry, we conducted an integrated study of Cambrian C and Sr isotope stratigraphy using the carbonates of the Xingdi section in the Kruqtagh region of the eastern Tarim Basin. Based on reported paleontological data, the C isotope record is comparable to the Cambrian global seawater C isotope variation, providing a feasible stratigraphic framework for the Cambrian strata in the eastern Tarim Basin. A sharp δ13C increase at the top of the Xidasan Formation probably corresponds to the recovery of the Redlichiid–Olenellid Extinction C isotope Excursion. Additionally, a negative C isotope excursion in the lower part of the Mohuosan Formation and a positive C isotope excursion at the bottom of the Tuersaketagh Formation likely correspond to DrumIan Carbon isotope Excursion and Steptoean Positive Carbon Isotope Excursion, respectively. However, the Sr isotope curve in the eastern Tarim Basin decoupled from global seawater Sr isotope variations during Cambrian Series 2–3 until Furongian, indicating that the eastern Tarim Basin was not well connected with the open ocean until Furongian. It also highlights a significant impact of paleogeographical connections of basins with the open ocean on their seawater Sr isotopic signal.

Did sea level change drive carbon isotopic trends in the Madison Shelf? Sequence stratigraphy and carbon isotopes in the Mississippian Lodgepole Formation of southwest Montana

The Lower Mississippian Lodgepole Formation of Montana and Wyoming records one of the largest positive carbon isotopic excursions of the Phanerozoic. This globally recognized up to 7‰ increase in δ13Ccarb values occurs across the North American Kinderhookian-Osagean boundary (referred to as the KO excursion). It has been argued to reflect significant organic carbon burial, possibly linked to the onset of the Late Paleozoic Ice Age. Previously proposed correlations between carbon isotopic patterns and the sequence stratigraphic framework within these strata suggests that changes in sea level could have played a significant role in the expression and/or magnitude of the KO excursion in the Madison Shelf. This study explores the relationship between carbon isotopic values and sea level change at multiple scales. To accomplish this, we provide a comprehensive overview of the sedimentological and stratigraphic framework and address uncertainty about the number of sequences in the Lodgepole Formation. Our results support a three-sequence model for the Lodgepole Formation. Based on the number of sequences and the placement of sequence stratigraphic surfaces, we see little evidence of statistically significant correlation between carbon isotopic trends and the sequence stratigraphic framework. We argue that sea level change was not the primary driving mechanism for carbon isotopic trends in the Madison Shelf, nor the KO excursion. Instead, we support models that invoke global ocean anoxia and/or destabilization of the global carbon cycle due to land plants.

Variations in climate habitability parameters and their effect on Earth's biosphere during the Phanerozoic Eon

Essential insights on the characterization and quality of a detectable biosphere are gained by analyzing the effects of its environmental parameters. We compiled environmental and biological properties of the Phanerozoic Eon from various published data sets and conducted a correlation analysis to assess variations in parameters relevant to the habitability of Earth’s biosphere. We showed that environmental parameters such as oxygen, global average surface temperatures, runoff rates and carbon dioxide are interrelated and play a key role in the changes of biomass and biodiversity. We showed that there were several periods with a highly thriving biosphere, with one even surpassing present day biodiversity and biomass. Those periods were characterized by increased oxygen levels and global runoff rates, as well as moderate global average surface temperatures, as long as no large or rapid positive and/or negative temperature excursions occurred. High oxygen contents are diagnostic of biomass production by continental plant life. We find that exceptionally high oxygen levels can at least in one instance compensate for decreased relative humidities, providing an even more habitable environment compared to today. Beyond Earth, these results will help us to understand how environmental parameters affect biospheres on extrasolar planets and guide us in our search for extraterrestrial life.

Nature of Devonian anoxic events based on multiproxy records from Panthalassa, NW Canada

Causes and controls of Middle-Late Devonian anoxic and biotic events are highly debated, and our results from the oceanographically open black-shale succession called the Horn River Group (HRG) provide elucidations. These strata are overall lean in siliciclastics and enriched in authigenic U, Mo, V, and planktogenic organic matter. We focus on three major event horizons within the HRG. Within new conodont constraints, these horizons are correlated to the global Frasnes-Manticoceras, basal punctata, and late punctata events. Two major characteristics of these three events are positive δ13Corg excursions of ~2.0–3.0‰ amplitude and U, Mo, V enrichment, which indicate pulses of severe anoxia within the basin. Trace-metal enrichment horizons are offset from associated isotopic excursions, indicating a time lag of ~80–125 ky between the onset of the δ13C excursion and the onset of severe anoxia. We demonstrate that these events are robust and traceable in off-bank anoxic facies across the entire basin, including remote locations ≥400 km away from the study area. The onsets of three major δ13C excursions are coupled with the thin spikes of elemental mercury and siliciclastic content. Our observations align with the hypothesis that Middle-Late Devonian anoxic events were triggered by pulses of large-scale volcanism through the same pathway as classical Mesozoic oceanic anoxic events. No rigorous evidence exists that land plants could drive shelfal seas into anoxia and mass extinctions.

Enhanced clay formation key in sustaining the Middle Eocene Climatic Optimum

The Middle Eocene Climatic Optimum (around 40 million years ago) was a roughly 400,000-year-long global warming phase associated with an increase in atmospheric CO2 concentrations and deep-ocean acidification that interrupted the Eocene’s long-term cooling trend. The unusually long duration, compared with early Eocene global warming phases, is puzzling as temperature-dependent silicate weathering should have provided a negative feedback, drawing down CO2 over this timescale. Here we investigate silicate weathering during this climate warming event by measuring lithium isotope ratios (reported as δ7Li), which are a tracer for silicate weathering processes, from a suite of open-ocean carbonate-rich sediments. We find a positive δ7Li excursion—the only one identified for a warming event so far —of ~3‰. Box model simulations support this signal to reflect a global shift from congruent weathering, with secondary mineral dissolution, to incongruent weathering, with secondary mineral formation. We surmise that, before the climatic optimum, there was considerable soil shielding of the continents. An increase in continental volcanism initiated the warming event, but it was sustained by an increase in clay formation, which sequestered carbonate-forming cations, short-circuiting the carbonate–silicate cycle. Clay mineral dynamics may play an important role in the carbon cycle for climatic events occurring over intermediate (i.e., 100,000 year) timeframes.

Marble trace element and C-O isotope geochemistry of the Paleoproterozoic Jingshan Group, North China: Insights into BIF formation during the Lomagundi-Jatuli Event

The North China Craton, one of the oldest continental blocks in the world, comprises Paleoproterozoic metamorphic sedimentary formations that record the effects of multiple major events in early Earth history, including the Great Oxidation Event (GOE) and its consequent Lomagundi-Jatuli carbon isotope event (LJE). The lack of focused research on the geological record of LJE-equivalent rocks of the Jiaodong Peninsula, Shandong Province, eastern China, has hindered our understanding of the Precambrian atmospheric oxygen evolution recorded by rocks of the North China Craton. This paper considers carbon and oxygen isotope and trace element geochemical data recovered from marble deposits of the Paleoproterozoic Jingshan Group (2.20–1.90 Ga) of the Jiaolai Basin. The investigated strata display a pronounced positive δ13CV-PDB excursion greater than 4.2 ‰. δ13CV-PDB and δ18OV-SMOW values range from −3.7 ‰ to 5.4 ‰ and 4.5 ‰ to 18.7 ‰, respectively. Positive co-variance of carbon and oxygen isotope values of the analyzed Jingshan Group samples is interpreted to reflect the effects of metamorphism and water–rock reactions. Marble deposits of the Jingshan Group experienced recrystallization, metamorphism, and silicification resulting in strong decarbonation. High amphibolite facies metamorphism may have reduced δ13CV-PDB by 3 ‰ to 5 ‰. Thus, estimated original sedimentary δ13CV-PDB values of as great as +8.4 ‰ suggest the existence of a strong positive anomaly that may reflect the global Lomagundi-Jatuli event. Rare earth and other trace element geochemical results of the present study suggest that banded iron formation (BIF) deposits associated with the Jingshan Group marble formed in a redox stratified ocean in the Jiaolai Basin dominated by ferruginous conditions. Moreover, our results suggest that positive carbon isotope anomaly associated with the LJE is largely constrained to sediment deposited in shallow and coastal marine environments.

Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Oceanic anoxic events (OAEs) represent discrete intervals of decreased marine oxygen concentrations often associated with volcanism, enhanced organic carbon burial coupled with positive δ13C excursions, and significant biotic turnover. Cretaceous OAE 2 (ca. 94 Mya) is especially notable for globally-distributed changes in calcareous invertebrate and plankton populations. While the presence of organic-rich facies is consistent with locally anoxic environments in many cases, determining the global extent of anoxia is more problematic. To address this issue, we investigate uranium isotope (δ238U) compositions of upper Cretaceous (Cenomanian-Turonian) open marine platform carbonates from southern Mexico as a proxy for global seawater redox conditions. These data are complementary to previous δ238U studies across OAE 2 in both black shales and pelagic carbonates, which have yielded variable results that reflect both global redox and local depositional processes. In Morelos Formation carbonates, a significant and well-defined negative δ238U excursion down to a nadir of −0.6‰ is recorded over an ∼40 m interval. This is consistent with the expansion of marine anoxia, pointing to an areal extent of anoxic seawater of about 1–10% of the global seafloor (or ∼5 to 50 times the modern value). Importantly, based on biostratigraphically-controlled estimates of sediment accumulation rates, the δ238U anomaly precedes the δ13C excursion by a median of ∼45 to 51 kyr (95th percentile confidence interval, CI) or ∼105 to 120 kyr (95th percentile CI) depending on how the onset of the δ238U anomaly is estimated. These results, along with previously reported thallium isotope and trace metal data, suggest that anoxic expansion preceded carbon cycle perturbation. This observation further increases estimates of the duration of OAE 2, implying that widespread ocean anoxia may have lasted >900 kyr.

A climate perturbation at the Middle –Late Jurassic Transition? Evaluating the isotopic evidence from south-central England

The Jurassic greenhouse is punctuated by short cooling intervals. Some published oxygen isotope records of belemnite rostra and fish teeth have been interpreted to reveal a prominent decrease in seawater temperature during the Late Callovian–Early Oxfordian. This is in part the basis for a proposed an ice age at the Middle-Late Jurassic Transition. In contrast relatively constant oxygen isotope records and therefore seawater temperatures as well as carbon isotope values characterized by significant scatter but showing more positive values during the middle and late Callovian have been reported from elsewhere. This research has constructed a stable isotope stratigraphy (from belemnites, Gryphaea and oysters) principally from the Callovian-Oxfordian interval (from southern England) and integrated these data with existing data to assess the pattern of carbon and oxygen isotopic change. Our marine macrofossil record reveals isotopic patterns that are generally comparable with other European basins. Carbon isotopic trends are consistent with bulk carbonate carbon isotope records displaying systematic fluctuations, the largest of which (Middle Callovian, Calloviense/Jason Zones to Early Oxfordian, Mariae Zone) corresponds to previously identified phases of environmental perturbation. Such a trend may have resulted from enhanced burial and preservation of organic matter, leaving the seawater enriched in the heavy carbon isotope. Inferred cooling, derived from our oxygen isotope data from southern England, occurs within the Late Callovian and the Early Oxfordian (Athleta to Mariae zones). Cooling post-dates this positive carbon isotope excursion. Enhanced carbon burial and atmospheric carbon dioxide draw down may have induced cooling. In this study the analysis of a single region (southern England) allows critical factors controlling the oxygen isotopic composition of the carbonates (e.g. temperature and the oxygen isotopic composition of the solution from which the carbonates precipitated) to be more comparable. If Jurassic polar climates were truly warm such a degree of cooling would undoubtedly lead to cooler polar temperatures – but only with seasonal ice.

The transition from hydrothermal oxic conditions to restricted euxinia in the lower cambrian yurtus formation black shale, Tarim Basin

The Yurtus Formation is generally believed as one of the most potential hydrocarbon source rocks under the Cambrian gypsum-salt bed in the Tarim Basin of China. This formation exhibits unique geochemical characteristics during the Ediacaran-Cambrian transition period. In this study, we investigated the lower and upper black shales from the Yurtus Formation for the high-resolution geochemical analysis in order to further characterize the prevailing redox conditions and sedimentary mechanisms involved in the organic matter accumulation process. During the deposition of the lower black shale with high TOC in Unit2, the high concentrations of phosphorite and barite, and the REE geochemistry manifested a mixture of hydrothermal fluid and seawater. Through in-situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), we explored the element distribution of the drill-core-preserved phosphate nodule, indicating that the phosphogenesis may be related to the involvement of bacteria activities under intermittently oxic bottom waters or consistently free oxygen conditions. Biomarker evidence by the identification of green sulfur bacteria demonstrated the occurrence of photic zone euxinia (PZE), and the trace metal patterns revealed intermittently bottom water oxygenation and stratified ocean which was consistent with the scenario of phosphate nodule formation. In contrast, the upper black shale with relatively low TOC was characterized by a detrital input, with significantly decreasing redox-sensitive trace mental enrichment (UEF, MoEF, VEF) and positive excursion of carbon isotopes, as well as a positive relationship between ∑REE and Al. The present study provides more explicit insights into not only the sedimentary mechanism of the black shale, but also the evolution of the Earth system at the key interval of the early Cambrian.

Extensive sea-floor oxygenation during the early Mesoproterozoic

The Mesoproterozoic ocean was previously proposed to be characterized by low oxygen concentrations that may have resulted in a stasis in eukaryotic evolution. Recent studies reveal several pluses of oxygenation during this period that may have facilitated eukaryote evolution. However, few quantitative constraints on the global oxygenation events have hindered the comprehensive understanding of interactions between eukaryotes and environments. Here we present new molybdenum isotope (δ98/95Mo, simplified as δ98Mo) data coupled with sulfur and carbon isotope compositions and trace element contents from the carbonates and mudstones of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in two-well preserved sections in the North China Craton. The δ98Mo data from the two sections show similar trends in stratigraphic variation, with a prominent positive excursion in the Gaoyuzhuang Member III and peak values reaching up to 2.03‰ and 1.92‰, respectively. We developed a revised isotope mass balance model with high manganese (Mn) (hydr)oxide burial rate due to higher manganese concentration in the Mesoproterozoic oceans. Model calculation suggests that at least more than half of global seafloor (>54%) was covered with oxygenated seawater during the deposition of the Gaoyuzhuang Member III. Our estimation points to a significant oxygenation event in the early Mesoproterozoic with magnitude and extent much higher than previously thought. Further, a simple mass-balance calculation suggests that the atmospheric oxygen level was at least > 4% PAL (present atmospheric level) and likely up to 30% PAL at ca. 1.57 Ga. This oxygenation event, together with other environmental and/or developmental factors, may have facilitated a short and rapid evolution of eukaryotes in the early Mesoproterozoic.

Quantifying source effects based on rainwater δ18O from 10-year monitoring records in Southwest China

The stable isotopic composition of rainwater (δ18Op and δDp) has been considered an effective proxy in studying atmospheric circulation and hydrological cycle processes. However, the linkage between variabilities in moisture sources and δ18Op remains poorly understood in Southwest China. Here, we utilized three long-term (10-year) δ18Op records (Beibei, Furong, and Yangkou stations) in Chongqing, Southwest China, from 2010 to 2019 A.D., integrated with a cluster analysis based on the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) to quantitatively demonstrate the close remote coupling of moisture sources and δ18Op on seasonal and interannual timescales. The clustering calculation results indicated that the percentage initial moisture source proportions of the westerlies and Asian summer monsoon have changed significantly at seasonal and interannual timescales, leading to the significant variation in δ18Op in Southwest China. Domination of westerlies and inland moisture contribution resulted in the positive δ18Op excursion in the dry season (November, December, January–April) while prevailing oceanic moisture led to the negative δ18Op excursions in the rainy season (May–October). On the interannual scale, δ18Op and d-excess exhibited a persistent positive/high trend, which is consistent with a decrease/increase in water vapor contributions from the ocean/westerly sources. Furthermore, δ18Op showed a good correlation with the Indian Ocean Dipole index, El Niño-Southern Oscillation, and the South Asian monsoon index, indicating that large-scale circulation patterns may affect regional δ18Op through their influence on moisture transport and convective activity in the source region. The long-term observation confirmed changes in moisture sources largely depend on the Westerly and Indian monsoon dynamics controlled by ocean-atmosphere circulation, supporting the effects of moisture sources on the rainwater isotopic composition.

Organic matter enrichment in Asia's palaeolake controlled by the early and middle Eocene global warming and astronomically driven precessional climate

The sediments deposited in continental lake systems record sensitive palaeoclimate signals and provide better perspectives for understanding rapid palaeoclimate changes and their implications. To better understand the terrestrial record and climate-driven organic matter enrichment mechanism in a low-latitude region, which have rarely been investigated, we studied the Eocene terrestrial records from the Beibuwan Basin, South China Sea (SCS). Cyclostratigraphic analysis was conducted to assess the orbital forcing. Milankovitch-scale astronomical signals were determined by spectral analyses, evolutionary Fast Fourier transform, and sedimentation rates estimation of the natural gamma ray logging data in the depth domain. A floating astronomical time scale (ATS) was constructed based on astronomical tuning that converts the depth domain to the time domain by sedimentation rates. The ATS was calibrated using published dating result of 35.2 Ma from the Eocene to the Oligocene. This ATS constrains the ages of two organic-rich lacustrine petroleum source intervals of the Eocene Liushagang Formation in the Beibuwan Basin at ∼49-47 Ma and ∼40.9–40.2 Ma, respectively. A comparison of the timing of these two source rocks with a published global benthic carbon and oxygen isotope series shows that they correspond to Eocene positive carbon isotope excursions. This finding suggests that organic carbon burial in Eocene lacustrine basins responded to early and middle Eocene Climatic Optimum (i.e., EECO and MECO). We used geochemical data based on X-ray fluorescence core scans for further investigation of the effect of orbital forcing on organic matter enrichment. The high Rb/Sr, K + Ti + Rb and Sr/Ca geochemical ratios and stable Mo and U/Th values all correspond to the precessional minimum, suggesting that the orbital precessional minimum is responsible for a warm and humid climate, and brackish and hypoxic water conditions. Thus, four schematic hypotheses were proposed to explain the effect of the precessional monsoonal climate on organic matter enrichment. This study provides a terrestrial record that documents the mid-Eocene global warming event well and provides evidence that the orbital precessional climate controlled organic matter enrichment in Asia's palaeolake.