High-resolution Carbon Isotope Research Articles

Mid-Pleistocene aridity and landscape shifts promoted Palearctic hominin dispersals

Population expansions and contractions out of and into Africa since the early Pleistocene have influenced the course of human evolution. While local- and regional-scale investigations have provided insights into the drivers of Eurasian hominin dispersals, a continental-scale and integrated study of hominin-environmental interactions across Palearctic Eurasia has been lacking. Here, we report high-resolution (up to ∼5-10 kyr sample interval) carbon isotope time series of loess deposits in Central Asia and northwest China, a region dominated by westerly winds, providing unique paleoecological and paleoclimatic records for over ~3.6 Ma. These data, combined with further syntheses of Pleistocene paleontological and archaeological records and spatio-temporal distributions of Eurasian eolian deposits and river terraces, demonstrate a pronounced transformation of landscapes around the Mid-Pleistocene Climate Transition. Increased climate amplitude and aridity fluctuations over this period led to the widespread formation of more open habitats, river terraces, and desert-loess landscapes, pushing hominins to range more widely and find solutions to increasingly challenging environments. Mid-Pleistocene climatic and ecological transitions, and the formation of modern desert and loess landscapes and river networks, emerge as critical events during the dispersal of early hominins in Palearctic Eurasia.

Revisiting the mid-Pleistocene transition ocean circulation crisis.

The mid-Pleistocene transition (MPT) [~1.25 to 0.85 million years ago (Ma)] marks a shift in the character of glacial-interglacial climate (1, 2). One prevailing hypothesis for the origin of the MPT is that glacial deep ocean circulation fundamentally changed, marked by a circulation "crisis" at ~0.90 Ma (marine isotope stages 24 to 22) (3). Using high-resolution paired neodymium, carbon, and oxygen isotope data from the South Atlantic Ocean (Cape Basin) across the MPT, we find no evidence of a substantial change in deep ocean circulation. Before and during the early MPT (~1.30 to 1.12 Ma), the glacial deep ocean variability closely resembled that of the most recent glacial cycle. The carbon storage facilitated by developing deep ocean stratification across the MPT required only modest circulation adjustments.

High-resolution oceanic anoxic event 2 (OAE2) records from the north of eastern Tethys and evidence for short-term sea regression and wildfire at its early phase

Oceanic Anoxic Event 2 (OAE2), occurring at the Cenomanian-Turonian boundary (CTB; 93.9 Ma), is a representative global event of carbon cycling fluctuations and transient climate anomalies. OAE2 was initially discovered and intensively studied in the trans-Atlantic and western Tethys Oceans; however, research on the eastern Tethys Ocean, especially the vast northern regions, is inadequate. Further, there is still considerable controversy regarding the triggering mechanisms and early environmental changes of OAE2. In this study, we present high-resolution carbonate carbon isotopes (δ13Ccarb), total organic carbon, and mercury (Hg) concentration records from the northern part of the eastern Tethys Ocean during the OAE2 interval, sampled at the ZK20-1 drillcore section of the Kukebai Formation in the western Tarim Basin, China. Following the global classification scheme for carbon isotope excursion (CIE) at the CTB, the carbon isotope change curve of this study is classified into six stages (C1-C6), and three substages (C3a, C3b, C3c) in the C3 stage. Sedimentary features reveal a short-term sea-level regression in the Tarim Basin during the onset stage (C2) of OAE2, which likely involved vast regions of the eastern Tethys Ocean and even the Pacific region. The mercury (Hg) concentration data reveal increasing Hg levels in the lower part of the Kukebai Formation, mainly corresponding to the onset stage (C3) of the OAE2, but it is significantly lower than that in other typical volcanogenic Hg anomalies. We preliminarily speculated that the intensified wildfire activity at the onset stage of OAE2 potentially accelerate the Hg extracted from the terrestrial soil or organic matter into the offshore area. This is also supported by the evidence of wildfire and Hg anomalies from the Western Interior Seaway (WIS) of North America.

Carbon isotopic record of a platform-to-basin transect through the Permian Reef Complex (Guadalupian) in the Delaware Basin of Texas and New Mexico

The Guadalupian Permian Reef Complex of the Delaware Basin is one of the most studied carbonate reef systems of the Paleozoic. Despite extensive work on the carbonate sedimentology, sequence stratigraphy, and diagenetic history of the Delaware Basin, a high-resolution carbonate carbon isotope record along a platform to basin transect for the Capitanian (264.3–259.5 Ma, the youngest age of the Guadalupian Epoch) does not yet exist. The carbon isotopic record of the Delaware Basin is important because 1) it allows us to test hypotheses about controls on the carbon isotope proxy, 2) it provides constraints on how well modern carbonate platforms like the Great Bahama Banks serve as analogues for ancient carbonate settings, and 3) these types of restricted basins likely played an important role in Permian carbon cycling and the Capitanian extinctions.In this study we present 493 new Capitanian carbonate carbon isotopic values paired with a detailed sedimentological and sequence stratigraphic framework from the platform, slope, toe of slope, and deep basin of the Delaware Basin. The bulk of the new δ13C values fall within the range of previously reported unaltered carbonates from the basin, suggesting that these results record primary environmental processes and were not significantly altered by diagenetic overprinting. With this dataset, we test hypotheses about sources of carbon isotopic variability in shallow carbonate platforms. Our results indicate that in the Delaware Basin there are no systematic and resolvable depth or lateral gradients in carbon isotopic values, that δ13C values do not vary as a function of grain type, and that there is no resolvable relationship between carbon isotopic composition and sea level change. However, we do document statistically significant differences in δ13C distributions among facies associations which we attribute to the isotopic evolution of an upwelling water mass due to direct precipitation of mud along the slope. Our results support the idea that increasing carbon isotopic values through the Capitanian were driven by increased organic carbon burial in restricted basins.

A mid-Cretaceous carbon isotope reference curve for the SE Neo-Tethys region (Zagros, W Iran)

Stable carbon isotope chemostratigraphy and the associated reference curves have proved to be valuable tools for stratigraphic calibration and long-distance correlations. Nonetheless, the reference curves covering the Cretaceous have mostly been produced from localities located within the W Neo-Tethys. In order to establish a reference curve for the SE Neo-Tethys margin, an Aptian–Albian hemipelagic carbonate-rich succession deposited along the Zagros Basin (SW Iran) was examined for planktic foraminifera biostratigraphy, carbon isotope stratigraphy, and sedimentological characteristics. Eight biozones including Globigerinelloides ferreolensis, Globigerinelloides algerianus, Hedbergella trocoidea, Paraticinella bejaouaensis, Muricohedbergella planispira, Ticinella (T.) primula, Biticinella breggiensis, and Pseudothalmanninella (P.)ticinensis as well as two subzones enclosing T. praeticinensis and P. subticinensis assign the strata to the early late Aptian–late Albian age. A carbon isotope record constrained by the planktic foraminifera zonation scheme shows good concordance with existing composite reference curves from the W Neo-Tethys region. Several carbon isotope excursions (CIEs) characterizing global marine events including Oceanic Anoxic Event (OAE) 1a, OAE1b, OAE1c and associated sub-events can be identified in the studied section. Deoxygenated bottom waters associated with the formation of OAE1a and OAE1b are indicated by distinct lithological signatures including enrichment in pyrite and glauconite, nodular and stratiform chert beds, as well as enhanced organic matter contents. We integrate the here-studied interval with a previously published upper Albian−Turonian succession from the same outcrop section, forming a composite, stratigraphically well-constrained section. This results in a high-resolution carbon isotope record for the SE Neo-Tethys margin, considered to represent an expedient reference Aptian–Turonian curve for this region. Correlation of this new curve with previously published records from the Middle East has helped to address ambiguities regarding the stratigraphic positions of the early/late Aptian and Aptian/Albian boundaries identified by previous studies.

Carbon and oxygen isotopes in mummified wood reveal warmer and wetter winters in the Siberian Arctic 3000 years ago

Paleoclimate reconstructions from the Holocene are important for defining baseline conditions in order to interpret and contextualize the effects of modern climate change. Such records are particularly lacking for Siberia, a region that represents ~ 50% of the Arctic. In addition, the majority of proxy-based paleoclimate reconstructions for the Holocene represent mean annual conditions, and few quantify winter temperature, which is particularly important for predicting the effects of global warming in Arctic environments. Here we provide the first quantitative proxy reconstruction of precipitation and temperature for both summer and winter for 3000 years ago via novel high-resolution intra-annual carbon and oxygen isotope measurements across annual growth rings of fossil wood mummified within the permafrost of far northeastern Siberia. We found that the site experienced greater precipitation year-round (~ 10% increase in summer and ~ 30% increase in winter), cooler summer temperatures, and warmer winter temperatures, compared with today. Our findings indicate that warmer winter temperatures (+ 3.0 °C above early twentieth century values) in the Arctic 3000 years ago drove higher mean annual temperature by up to 1 °C, despite the existence of cooler summers, a similar phenomenon to what is observed within today’s Arctic environments, and past intervals of extreme global warmth.

A synthetic chronostratigraphy for the upper Campanian–Maastrichtian of the Boreal Realm based on the Danish and German basins

Abstract New carbon isotope results obtained from cores from the Fehmarn Belt (German Basin), along with hitherto unpublished detailed results obtained from brachiopods collected in the 1970s from the Hemmoor section, NW Germany, present the opportunity to revise the holostratigraphy of the Danish Basin and erect a complete chronostratigraphical scheme for the upper Campanian–Maastrichtian of the Boreal Realm based on records from the Danish and North German basins. The correlation of all studied sections is ensured by previously erected high-resolution bulk carbonate carbon isotope records. A number of issues arising from the standard Upper Cretaceous (UC) calcareous nannofossil biozonation scheme for the Boreal Realm are discussed and lead us to propose a new subzonation for the Danish Basin. Carbon isotope records allow correlation to Gubbio, Italy, the Tethyan bio-magnetostratigraphic reference, and to the astronomically calibrated ODP site 762C, Indian Ocean. Revised correlations ensure the application of magnetostratigraphy, a numeric timescale and stacking of 405 kyr cycles to the new scheme, thus erecting a detailed chronostratigraphy for the upper Campanian–Maaatrichtian of the Boreal Realm.

Carbon cycle perturbations and environmental change of the middle permian and Late Triassic Paleo-Antarctic circle

During the middle Permian through the Triassic, Tasmania moved from paleo-latitudes of 78° to 69°S, wedged between Antarctica and Australia, within the paleo-South polar circle. During this time, significant global carbon cycle disturbances triggered major environmental and climatic changes and mass extinction events globally. The Bicheno-5 core from Eastern Tasmania, Australia, provides the opportunity to examine middle Permian and Upper Triassic sediments from the paleo-Antarctic, using high-resolution organic carbon isotope (δ13CTOC) chemostratigraphy, pXRF, and sedimentology, combined with new palynological data integrated with the existing radiometric age model. While there is a significant unconformity in the Upper Permian to the middle Triassic associated with eustatic sea-level fall as a result of regional uplift in eastern Australia, three distinct carbon isotope excursions (CIEs), characterized by negative shifts of up to − 6 ‰ were identified; the middle Permian Guadalupian Carbon Isotope Excursions (G-CIE), the Carnian Pluvial Episode (CPE), and the mid-Norian Event (MNE). These three events highlight a significant climate shift through glacial and interglacial cycles to warmer non-glacial intervals in the Late Triassic, with evidence of the polar record of the Carnian Pluvial Episode and the mid-Norian Event, which are poorly studied in the Southern Hemisphere, specifically within the Paleo-Antarctic circle.

Enhanced primary productivity and perturbation of marine nitrogen cycling during the onset of the Cambrian SPICE event

New high-resolution paired carbonate carbon (δ13Ccarb) and nitrogen (δ15Nbulk) isotope records of the Steptoean Positive Carbon Isotope Excursion (SPICE) from the Laurentian epicontinental platform reveal a transient negative excursion in δ15Nbulk that closely aligns with the onset of the SPICE event and the initiation of the positive δ13Ccarb excursion. The negative excursion in δ15Nbulk identified here likely demonstrates an increase in microbially fixed nitrogen by diazotrophs, or cyanobacteria capable of N2 fixation, in the marine system due to a reduction in bioavailable recycled N through several processes at the onset of the biogeochemical event. Expansion of reducing environments during the initiation of the SPICE event would have liberated phosphorous previously bound to organic matter and/or marine iron oxy-hydroxides and simultaneously promoted an expansion of denitrification in oxygen-poor settings that would have reduced bioavailable N in the marine system. The enhanced delivery of phosphorous to the photic zone would have stimulated primary productivity, which would have also put additional pressure on the marine bioavailable N budget. These feedback systems would have combined to lower the marine N:P ratio such that diazotrophs would become a significant contributor to the total bioavailable N in the marine realm. The data presented here are the first demonstration of this significant perturbation to local marine nutrient cycling during the onset of the SPICE event and provide new insight into what may ultimately be a consistent set of biogeochemical triggers for major Paleozoic biogeochemical events.

Coupling of nitrogen biogeochemical cycling and phytoplankton community structure before and after the Late Ordovician mass extinction in South China

The Ordovician–Silurian transition (OST) is a crucial geological interval in Earth history, coinciding with the first of the ‘Big Five’ Phanerozoic mass extinction events (ca. 445 Ma). In addition to the dramatic loss of biodiversity, significant environmental change course such as redox conditions and nutrient availability have been the focus of debate. Previous studies have used isotopes of nitrogen (N) to reconstruct the redox evolution of the paleo-ocean and have identified biological nitrogen fixation as the main mode of nitrogen cycling during the OST. However, low δ15N values, ranging from −2‰ to +1‰, possibly correspond to four different redox conditions. The precise roles of these redox conditions and the spatial distributions of phytoplankton communities under different nitrogen cycles remain elusive. Here we present high-resolution nitrogen isotope, carbon isotope and lipid biomarker datasets from two drilled sections located on a submerged high of the Yangtze Basin to unravel the coupling between nitrogen biogeochemical cycling and phytoplankton community structure across the OST. The results suggest that there was a dominantly biological nitrogen fixation with local aerobic nitrogen cycling in the Yangtze Sea, and the nitrate inventory during the OST was smaller than that in the modern ocean. Particularly, before the Late Ordovician mass extinction (LOME), N-fixing cyanobacteria were likely extensively developed in the photic zone, while eukaryotic algae were limited. After the LOME, sea levels rose and the chemocline was higher than before the LOME. There was probably some NO3− that did not undergo denitrification in the surface ocean, favoring the development of eukaryotic algae. The ecological diversification from prokaryotic cyanobacteria to eukaryotic phytoplankton to eukaryotic zooplankton is likely to be a response to the enhanced biological pump, which generates positive feedbacks among the evolution of eukaryotic phytoplankton, organic matter enrichment, ocean oxygenation and atmospheric oxygen levels.

Accelerated marine carbon cycling forced by tectonic degassing over the Miocene Climate Optimum

Global warming during the Miocene Climate Optimum (MCO, ∼17–14 million years ago) is associated with massive carbon emissions sourced from the flood basalt volcanism and ocean crustal production. However, the perturbation of tectonic carbon degassing on the interaction between climate change and carbon cycle remains unclear. Here, through time-evolutive phase analysis of new and published high-resolution benthic foraminiferal oxygen (δ18O) and carbon (δ13C) isotope records from the global ocean, we find that variations in the marine carbon cycle lead the climate-cryosphere system (δ13C-lead-δ18O) on 405,000-year eccentricity timescales during the MCO. This is in contrast to the previously reported climate-lead-carbon (δ18O-lead-δ13C) scenario during most of the Oligo-Miocene (∼34–6 million years ago). Further sensitivity analysis and model simulations suggest that the elevated atmospheric CO2 concentrations and the resulting greenhouse effect strengthened the low-latitude hydrological cycle during the MCO, accelerating the response of marine carbon cycle to eccentricity forcing. Tropical climate processes played a more important role in regulating carbon-cycle variations when Earth’s climate was in a warm regime, as opposed to the dominant influence of polar ice-sheet dynamics during the Plio-Pleistocene (after ∼6 million years ago).

Obliquity forcing of Late Pliocene–Early Pleistocene aridification in the Qaidam Basin, NE Tibetan Plateau

Knowledge of the variations of aridification during past warm periods can help predict future aridification trends. However, the variation of aridity during the warm Pliocene epoch, a potential analog for the future climate, has not been sufficiently studied due to the lack of high-resolution records. Here, we present high-resolution (∼7 kyr) oxygen and carbon isotope records spanning the Late Pliocene–Early Pleistocene from the Qaidam Basin, northwestern China. We interpret enriched carbonate δ18O value as indicating stronger evaporation, based on the general correlation between the enrichment of carbonate δ18O and an increased sedimentary halite content. Our results reveal that the Qaidam Basin was already an extremely arid environment no later than ∼3.9 Ma, and that aridification was further intensified at ∼2.6 Ma. Interestingly, the Qaidam Basin experienced decreased aridity or climate wetting during the high insolation intervals within the middle Piacenzian warm period, in contrast with the intensified aridity in the semi-arid Chinese Loess Plateau. Strikingly, the Qaidam Basin experienced intensified aridification during high-obliquity intervals, with smaller global ice sheets, during 3.6–2.5 Ma. We attribute this obliquity signal of Asian aridification to the effects of insolation and possibly glacial forcing, via their impacts on regional evaporation and reduced monsoon precipitation. Using the Late Pliocene as an analogue, our findings suggest that future aridification variations in Central Asia during the next ∼100 kyr may be dominated by 40-kyr cycles, under natural climatic forcing.

Diagenetic constraints in carbon-isotope stratigraphy of the Upper Jurassic-Lower Cretaceous Vaca Muerta-Quintuco system, Neuquén Basin, Argentina: Implications for a global correlation

The δ13Ccarb profiles of the Vaca Muerta Formation (Neuquén Basin, Argentina), span the Tithonian to lower Valanginian (Jurassic-Cretaceous transition). The investigated interval was sampled from two sections: the Pampa de Tril (PDT, ∼120 m-thick covering the lower Vaca Muerta Formation) and Puerta Curaco (PC, ∼670 m-thick covering the entire Vaca Muerta and Quintuco formations). The succession consists of organic-rich marine carbonates alternating with mudrock beds. A multitechnique screening protocol, including petrography and geochemistry, has been applied to evaluate the degree of preservation of the studied carbonates. The Pampa de Tril and Puerta Curaco sections have δ13Ccarb values that vary from −12.2 ‰VPDB (at the bottom) to 2.9 ‰ VPDB (at the top) and δ18Ocarb values from −13.2 ‰VPDB to −5.3 ‰VPDB. The δ13Ccarb values of the analysed samples, exhibit insignificant correlations with their Mn/Sr (rs = −0.13) and ∑REE (rs = −0.19) counterparts. This supports the preservation of at least near-primary δ13Ccarb compositions that can be implemented to construct a reliable high-resolution carbon-isotope profile for global correlations. The δ13Ccarb profiles of the Vaca Muerta Formation display a negative excursion of ∼8–10 ‰ at the bottom of the formation (Tithonian) which can be correlated with the VOICE (Volgian Isotope Carbon Excursion) event.

Gamma-ray, stable carbon and oxygen isotope chemostratigraphy and sequence stratigraphy of the Lower Mahil Formation (KS-1 Khuff-Equivalent), Northern Oman

This research presents findings from a study focused on the Lower Triassic (Induan) Lower Mahil KS-1 Formation, situated on a homoclinal carbonate platform in Northern Oman. The sequence stratigraphy of this formation is characterized by a considerable thickness variation, slumps, and breccia deposits related to active normal faults coupled with intra-basin growth faults. The main objective was to establish a reliable stratigraphic framework for the Lower Mahil KS-1 Formation by integrating high-resolution carbon isotope data along with high-resolution spectral and total gamma-ray data. To achieve this, whole-rock samples were analyzed for δ13C and δ18O isotopes. Spectral and total gamma-ray records were obtained for the formation. Isotope sampling is conducted every 0.5 m in the Saiq Plateau and Wadi Sahtan sections. Furthermore, spectral gamma-ray measurements were taken at intervals of 10 cm from the logged sections. Within the third-order sequence, the spectral gamma-ray data revealed a distinct sea-level trend, leading to the division of KS1 into two different parts. Five fourth-order depositional sequences were identified by analyzing stable carbon isotopes, uranium, and total gamma-ray profiles. Four of these sequences displayed complete patterns, reflecting transgression and regression phases, while the fifth sequence was incomplete and solely comprised a transgressive phase. An essential outcome of the study is the correlation of the δ13C curve of the Lower Mahil KS-1 Formation with other similar formations around the Tethys region. This correlation indicates that the Lower Mahil KS-1 Formation captures the near-primary signal of carbon isotope variations in coeval seawater. As a result, it holds promise as a reference section for future investigations and studies in this field. Compared to the prior investigation, this study utilizes data with higher precision, capturing spectral gamma-ray measurements at 10 cm intervals and isotope measurements at 50 cm intervals. Furthermore, the study’s focus is confined explicitly to KS1.

A possible Norian–Rhaetian boundary in the high-latitude continental Junggar Basin indicated by the δ 13 C org record

Abstract In contrast to the Triassic–Jurassic boundary, there is no consensus on the definition and age of the Norian–Rhaetian boundary (NRB), which hampers the global correlation of Rhaetian strata and thus interpretation of the patterns and driving mechanism of the end-Triassic mass extinction. Recent works show that a significant negative carbon isotope excursion (N-CIE) occurs in the NRB interval ( c. 205.7 Ma), which probably provides a physical marker for the NRB. However, no such records have so far been reported from high-latitude continental deposits. Here we present high-resolution bulk organic carbon isotope data from the Upper Triassic continuous lacustrine-to-fluvial deposits in the Haojiagou section of the high-latitude continental Junggar Basin in northwestern China. A significant N-CIE is shown at an age of c. 205 Ma, which is most probably the expression of the NRB N-CIE in the Junggar Basin owing to their very close occurrence times. The NRB is thus probably along the N-CIE in the Haojiagou section, and a short Rhaetian stage is then supported. The Late Triassic climate in the high-latitude Junggar Basin was consistent with the global changes in climate, characterized by a long-sustained cooler and humid climate interrupted by the late Norian and latest Rhaetian warm events.

Fingerprinting eukaryotic metabolism across the animal kingdom using position-specific isotope analysis (PSIA) 13 C/ 12 C measurements

Despite differences in their overall metabolism, eukaryotes share a common mitochondrial biochemistry. We investigated how this fundamental biochemistry supports overall metabolism using a high-resolution carbon isotope approach, position-specific isotope analysis. We measured carbon isotope 13 C/ 12 C cycling in animals, focusing on amino acids that are formed in mitochondrial reactions and are most metabolically active. Carboxyl isotope determinations for amino acids showed strong signals related to common biochemical pathways. Contrasting isotope patterns were measured for metabolism associated with major life history patterns, including growth and reproduction. Turnover of proteins and lipids as well as gluoconeogensis dynamics could be estimated for these metabolic life histories. The high-resolution isotomics measurements fingerprinted metabolism and metabolic strategies across the eukaryotic animal kingdom, yielding results for humans, ungulates, whales, and diverse fish and invertebrates in a nearshore marine food web.

Constraining the Triassic–Jurassic boundary carbon cycle perturbations using high-resolution δ 13 C org records from the Haojiagou section in northwestern China

Abstract Large magnitude and rapid negative carbon isotope (δ 13 C) excursions in the Triassic–Jurassic boundary (TJB) interval have been proposed as critical evidence for the hypothesis that the emplacement of the Central Atlantic Magmatic Province (CAMP) triggered global carbon cycle perturbations and the end-Triassic mass extinction (ETE). However, the pattern and timing of δ 13 C variations during the ETE remain poorly constrained. Here I present high-resolution organic carbon isotope (δ 13 C org ) records from a terrestrial TJB succession continuously exposed along the Haojiagou valley on the southern margin of the Junggar Basin, northwestern China. The δ 13 C chemostratigraphic correlations, combined with the palynological data, indicate that the position of the TJB should be placed at the base of bed 53. Importantly, the results show that two rapid negative carbon isotope excursions at the end Triassic roughly coincide with two episodes of variations in the palynological records. This observation supports the hypothesis that two pulses of biotic crisis during the ETE may have been triggered by two major episodes of CAMP magmatic activity. In addition, the data in this study support an early start for CAMP magmatic activity in the early Rhaetian, which may have led to a global carbon cycle perturbation and the related biotic crisis or turnover.

A first high-resolution carbon isotope stratigraphy from the Boreal (NW Germany) for the Berriasian to Coniacian interval—implications for the timing of the Aptian–Albian boundary

High-amplitude changes in sedimentary δ13C characterize the Cretaceous system and have been proven useful for supra-regional chemostratigraphic correlation. In the Cretaceous, these δ13C perturbations indicate large shifts between the global carbon reservoirs that are usually caused by volcanic activity of large igneous provinces, the widespread deposition of thick organic carbon-rich sequences and/or changes in orbital parameters. Here, we present an upper Berriasian to lower Coniacian (c. 142–88 Ma) composite carbon isotope record based on 14 drill cores, 2 outcrops, and almost 5,000 samples. The total record comprises a composite thickness of more than 1,500 m. All cores and successions are located in the larger Hanover area, which represents the depocenter of the North German Lower Saxony Basin in Early to mid-Cretaceous times. In Northern Germany, Boreal Lower Cretaceous sediments are predominantly represented by CaCO3-poor mud and siltstones of up to 2,000 m thickness, which become more carbonate-rich during the Albian–Cenomanian transition and even chalkier in the upper Cenomanian to Coniacian interval. The carbon isotope record reveals a number of global key events, including the Valanginian Weissert Event, the Oceanic Anoxic Events (OAEs) 1a and d, and the Kilian Event (Aptian–Albian boundary, part of OAE 1b). For the early Late Cretaceous, the Mid-Cenomanian Event, the OAE 2 (Cenomanian–Turonian Boundary Event), and the Navigation Event, among others, have been identified. The Kilian Event represents the Aptian–Albian boundary and has been identified herein for the first time in Northern Europe. Based on the evaluation of its relative position to the Vöhrum boundary tuff, we tentatively propose a slightly older age for the Aptian–Albian boundary of c. 113.65 Ma instead of 113.2 Ma. The observed chemostratigraphic events enable a detailed stratigraphic comparison with Tethyan and other Boreal records and associated paleoenvironmental data. Thus, this new detailed chemostratigraphy provides a unique opportunity to potentially overcome many still existing Boreal–Tethyan correlation issues. The presented record can be considered almost complete, albeit a 2-Myr gap during the early Albian is likely, and condensed intervals occur specifically during the lower Aptian.

Carbonate factories and their critical control on the geometry of carbonate platforms (mid-Cretaceous, southern Iran)

Tropical rimmed platforms represent a much more efficient protection from coastal erosion than carbonate ramps. Understanding the controlling factors of platform geometry is therefore crucial to predict the fate of rimmed platforms under global-warming conditions. Here, we present detailed biostratigraphic, sedimentological, geochemical, and high-resolution carbon-isotope analyses of mid-Cretaceous (Albian-Cenomanian) carbonate rocks well exposed in the Khormoj section located in the Zagros Mountains of southern Iran. Sixteen microfacies were identified, allowing us to define the superposition of different platform geometries, a carbonate ramp in the Albian and a rimmed platform in the Cenomanian. Biostratigraphic and carbon-isotope analyses allowed us to identify the OAE1b, OAE1d, and Middle Cenomanian events in the Khormoj section. The paleodepth changes documented by carbonate microfacies testify to two transgressive events that can be correlated with long-term eustatic curves during the ramp stage. The Albian ramp was formed by orbitolinids under high siliciclastic input whereas the Cenomanian rimmed platform was built by rudists during clear water conditions. These two carbonate factories have drastic different carbonate sedimentation modes. The orbitolinid factory is characterized by a low carbonate production rate and fine carbonate grains resulting in ramp geometry, whereas the rudist factory is characterized by a high carbonate production rate and coarse carbonate grains leading to a rimmed platform geometry.

Paleoenvironmental significance of the carbon isotope record across the Cenomanian–Turonian transition and the Oceanic Anoxic Event 2 (OAE2) in the southeastern Neotethys, Zagros, Iran

A high–resolution carbon isotope record of pelagic carbonates (δ13Ccarb) from the Zagros Mountains, Iran, documents a 1.8‰ positive carbon isotope excursion (CIE) in the southeastern Neotethys during the Cenomanian–Turonian transition, corresponding to Ocean Anoxic Event (OAE2). The succession is controlled by biostratigraphy that includes the Rotalipora cushmani, Whiteinella archaeocretacea, and Helvetoglobotruncana helvetica biozones. The CIE is characterized by a positive shift culminating in a first positive peak (at 3.3‰), followed by a 1‰ negative excursion, accompanied by the highest δ18Ocarb values that can be interpreted as the result of a short–lived global cooling, the Plenus Cold Event. A subsequent second positive peak and a plateau of higher δ18Ocarb values are followed by relatively low values in the earliest Turonian akin to pre–excursion values. The biostratigraphic positions of the three diagnostic maximum points, referred to as peaks A, B, and C, within the generally positive CIE indicate a synchronous δ13Ccarb variability worldwide. The average δ13Ccarb values in southeastern Neotethys are generally lower compared to the open–ocean records of the same period. We attribute this offset to a substantial regional regression combined with recycling of isotopically light organic carbon. The diachronous deposition of black shales/mudstones after the onset of positive CIE suggests that its formation strongly depends on local paleogeography and paleoenvironmental factors.