δ13C Chemostratigraphy Research Articles

Astronomical Calibration of the Ocean Anoxic Event 1b and Its Implications for the Cause of Mid‐Cretaceous Events: A Multiproxy Record

AbstractThe timing and duration of oceanic disturbances linked to Oceanic Anoxic Event (OAE) 1b, as well as the mechanisms driving anoxia and carbon burial during this period, remain subjects of debate. We conducted cyclostratigraphic analyses on magnetic susceptibility (MS) and elemental Ti and Fe series within the upper Aptian‐lower Albian interval of the Poggio le Guaine core in the Umbria‐Marche Basin, Italy. This interval provides a detailed sedimentary record, supported by variations in magnetic mineral content, Ti, Fe, and significant global shifts in the δ13C curve. Orbital control of MS, Ti, and Fe suggests a duration of 2.84 Myr for the OAE 1b event. Our chronostratigraphic analysis reveals ages of 114.07 ± 0.12 Ma for 113/Jacob, 113.28 ± 0.12 Ma for Kilian, 112.49 ± 0.12 Ma for the central age of the Monte Nerone cluster, 111.70 ± 0.12 Ma for Urbino, and 111.28 ± 0.12 Ma for Leenhardt sub‐events. Stable δ13C chemostratigraphy correlations enable the transfer of tiepoints across various sedimentary basins. Key features of the carbon isotope curves were identified, named, and dated using astrochronology. Our findings suggest that the organic‐rich layers associated with the OAE 1b event exhibit distinct characteristics influenced by several factors, including a warm climate driven by volcanic CO2 emissions, heightened precipitation, intense weathering, and marine transgressions. These factors amplify orbital forcings on paleoclimate changes, leading to oceanic‐atmospheric disturbances that promote deoxygenation and carbon burial during OAE 1b.

Late Jurassic paleoenvironmental reconstruction based on stable oxygen isotopes in bulk carbonates from the Qiangtang Basin, eastern Tethys

A composite eastern Tethyan oxygen and carbon isotope curve reveals major climate changes through the Late Jurassic. Despite significantly lower values and different amplitudes in scale, the δ18O data from whole-rock carbonates present fluctuant temperature results similar with well-studied composite δ18O curves of diagenetically-screened biogenic calcites, and are possibly acceptable as a paleotemperature proxy. The paleoclimate trends imply a cool global Callovian-Oxfordian transition, a mid-Oxfordian warming, a late Oxfordian cooling, a turbulent Kimmeridgian climate, a warm earliest Tithonian and rapid early Tithonian cooling event. The climate shift from the earliest Tithonian warmth to the middle early Tithonian cool climate was up to 8 °C decrease in some regions. These paleoclimate changes are greatly consistent with the eustatic sea-level changes, biological evolutions and paleoatmospheric CO2 reconstructions, and are recorded by coeval carbon isotope perturbations relating to the organic carbon accumulations in marine sediments. Coupled δ18O and δ13C chemostratigraphy evidence that higher temperatures lead to more rapid continental weathering, increased nutrient-rich runoff into the oceans, and intensified marine productivity, resulting in increased organic‑carbon burial and more positive δ13C values in limestones. The decrease in primary productivity and burial rates during cooling periods is, in turn, commonly accompanied by low δ13C values.

Carbonate δ13C chemostratigraphy of the Hunjiang Group in North China and a tentative stratigraphic framework for the Meso-Neoproterozoic transition

A robust stratigraphic framework for the Meso-Neoproterozoic transition is necessary for a better understanding of evolutionary and environmental changes during this critical geological time interval. Given the limited availability of radiometric and biostratigraphic data from late Mesoproterozoic to early Neoproterozoic strata, chemostratigraphic correlations have been used to build this framework. To contribute to Proterozoic chemostratigraphy, here we report new carbonate carbon isotope data from the Hunjiang Group in the Baishan area of Jilin Province in North China. The carbon isotope data (average 3.00 ‰ ± 1.15 ‰, 1 SD), together with existing detrital zircon constraints and microfossils, confirm a late Mesoproterozoic to early Neoproterozoic age of the Hunjiang Group. Using the dynamic time warping algorithm, we then constructed a global chemostratigraphic framework for the Meso-Neoproterozoic transition based on δ13Ccarb data from the North China, São Francisco, and Congo cratons. This exercise confirms the generally narrow range of δ13Ccarb fluctuations in the late Mesoproterozoic but also substantiates a negative δ13Ccarb excursion of notable magnitude (∼9 ‰) and of potential global extent at ca. 1 Ga. This negative excursion, known as the Majiatun excursion, likely marks the Meso-Neoproterozoic transition and the onset of the dynamic carbon cycle in the Neoproterozoic Era.

Middle and Late Jurassic record of sea-level, sequence development, and carbon-isotope fluctuations, Tethyan Adriatic Carbonate Platform, Croatia

The Adriatic Platform, Croatia is a large isolated carbonate platform that developed under a warm greenhouse climate interspersed with cooler episodes in western Tethys. However, the δ13C record and high-resolution sequence stratigraphy of the Middle and Upper Jurassic portion generally is poorly known. The ~1900 m thick (30 Myr duration) mainly Middle to Upper Jurassic succession of the platform was studied bed-by-bed to track the fluctuations of δ13C, facies stacking into bundles, superbundles, and sequences and potential sea-level changes within ongoing studies of evidence of astronomical forcing of the units. The study interval consists of cyclic subtidal intervals (upper Toarcian-lower Aalenian, lower Callovian, Oxfordian, lower Kimmeridgian, and lower Tithonian) interspersed with highly cyclic peritidal units (upper Bajocian-lower Bathonian, upper Kimmeridgian, and upper Tithonian-basal Berriasian). Ages were constrained by biostratigraphy and δ13C chemostratigraphy. The absolute δ13C values obtained from the shallow-marine bulk carbonate matrix are similar to those from coeval pelagic carbonates and exhibit similar trends, are characterized by overall little data spread (<2‰), and generally show little resetting associated with disconformities and/or burial. This rock-buffered record likely preserved at least relative marine δ13C values with several major excursions, including the BjBaE (Bajocian-Bathonian), CaOxBE (Callovian-Oxfordian boundary), EOxE, and MOxE (Early and Middle Oxfordian). The studied interval provides a detailed record of Milankovitch induced platform-flooding and exposure at scales from ~1Myr to ~20 kyr. The study suggests that detailed research of similar thick Jurassic isolated platform successions could provide valuable records of δ13C, climate, and sea-level changes over relatively long geological intervals.

Assessing Milankovitch forcing in disconformity‐prone cyclic shallow‐water carbonates, Upper Jurassic (Kimmeridgian), Adriatic Platform, Croatia

AbstractMost Upper Jurassic studies of astronomical forcing have focused on deeper‐water sections which are relatively continuous. An Upper Jurassic (Kimmeridgian) section on the greenhouse Adriatic Carbonate Platform, Croatia, was studied to determine if astronomical forcing can be recognized in a 5.8 ± 0.1 Myr duration, disconformity‐prone shallow platform succession. The succession consists of metre‐scale subtidal parasequences intermixed with peritidal parasequences, and intermittent subaerial breccias at sequence boundaries. Ages were constrained by biostratigraphy and δ13C chemostratigraphy, and most sequence boundaries appear to match those of the coastal onlap curve of Haq (2018). Logged sections were converted into depth–rank time series and parasequence–thickness time series. Accumulation rates were statistically evaluated for the rank series against an astronomical‐forcing model, and compared with long‐term accumulation rates (thickness divided by time). The statistical rates were used to select the ca 100 kyr eccentricity cycle to tune the series. Spectral analysis showed peaks at ca 400 kyr (superbundles) and ca 100 kyr (bundles), along with obliquity (38 kyr and 27 kyr) and precessional (18−22 kyr) cycles (parasequences). The Kimmeridgian sequences are ca 400 kyr, ca 800 kyr and ca 1.1 Myr duration. Sequence scale (0.4 to 1.2 Myr) stratigraphic completeness based on statistical accumulation rates versus long‐term rates is ca 60%. This study estimates ca 1 Myr missing time in parasequences stacked into superbundles and 1.6 Myr in four major sequence boundaries. Given that the Kimmeridgian was the hottest time of the Middle and Late Jurassic, aquifer eustasy may have influenced the timing of sequence boundaries, although documented late Kimmeridgian cooling could have triggered a glacio‐eustatic component.

Controls on carbonate platform architecture and reef recovery across the Palaeozoic to Mesozoic transition: A high‐resolution analysis of the Great Bank of Guizhou

AbstractCarbonate platforms spanning intervals of global change provide an opportunity to identify causal links between the evolution of marine environment and depositional architecture. This study investigates the controls on platform geometry across the Palaeozoic to Mesozoic transition and yields new stratigraphic and palaeoenvironmental constraints on the Great Bank of Guizhou, a latest Permian to earliest Late Triassic isolated carbonate platform in the Nanpanjiang Basin of south China. Reconstruction of platform architecture was achieved by integrating field mapping, petrography, biostratigraphy, satellite imagery analysis and δ13C chemostratigraphy. In contrast to previous interpretations, this study indicates that: (i) the Great Bank of Guizhou transitioned during Early Triassic time from a low‐relief bank to a platform with high relief above the basin floor (up to 600 m) and steep slope angles (preserved up to 50°); and (ii) the oldest‐known platform‐margin reef of the Mesozoic Era grew along steep, prograding clinoforms in an outer‐margin to lower‐slope environment. Increasing platform relief during Early Triassic time was caused by limited sediment delivery to the basin margin and a high rate of accommodation creation driven by Indosinian convergence. The steep upper Olenekian (upper Lower Triassic) slope is dominated by well‐cemented grainstone, suggesting that high carbonate saturation states led to syndepositional or rapid post‐depositional sediment stabilization. Latest Spathian reef initiation coincided with global cooling following Early Triassic global warmth. The first Triassic framework‐building metazoans on the Great Bank of Guizhou were small calcareous sponges restricted to deeper water settings, but early Mesozoic reef builders were volumetrically dominated by Tubiphytes, a fossil genus of uncertain taxonomic affinity. In aggregate, the stratigraphic architecture of the Great Bank of Guizhou records sedimentary response to long‐term environmental and biological recovery from the end‐Permian mass extinction, highlighting the close connections among marine chemistry, marine ecosystems and carbonate depositional systems.

The δ13C chemostratigraphy of Ordovician global stage stratotypes: geochemical data from the Floian and Sandbian GSSPs in Sweden

ABSTRACTThe δ13C chemostratigraphy of five of the seven Ordovician global stages has been published previously but no such data have been available from the Floian GSSP and most of the Sandbian GSSP in Sweden. This lack of information has now been remedied by isotope data obtained from series of closely spaced shale samples collected from the Floian stratotype at Diabasbrottet in Västergötland and the Sandbian stratotype at Fågelsång in Scania. Although the bases of these stages cannot be precisely tied to levels of conspicuous δ13C excursions, that of the Floian Stage, which is marked by the appearance of the graptolite Tetragraptus approximatus, is between the closely spaced excursions named LTNICE and BFICE. The base of the Sandbian Stage, which is defined as the appearance level of the graptolite Nemagraptus gracilis, is just below a negative excursion previously known as the “Upper Kukruse Low”, which is nowadays known as the LSNICE. The relations between chemostratigraphy and graptolite and conodont biostratigraphy in the Swedish GSSPs and some coeval key sections in Baltoscandia, China, and America are briefly discussed. It is concluded that the data at hand indicate that there is good regional agreement in these relations.

Evidence of SPICE-related anoxia on the Laurentian passive margin: Paired δ13C and trace element chemostratigraphy of the upper Conasauga Group, Central Appalachian Basin

This study reports data from the Upper Cambrian Conasauga Group and overlying Copper Ridge Formation of the Central Appalachian region, eastern U.S.A. Geochemical, isotopic and petrographic analysis of core material was carried out to constrain the extent of marine anoxia and to characterize sediment fluxes on the carbonate platform and continental shelf of Laurentia contemporaneous with the Steptoean Positive Carbon Isotope Excursion (SPICE), a Late Cambrian global marine anoxic event. Carbonate rocks (primarily dolostone) record a positive δ13Ccarb excursion starting in the middle Nolichucky Formation, reaching its peak (+4.3‰) at the boundary between the Maynardville and Copper Ridge formations. Strontium isotope ratios in the dolostone units are only slightly offset from the expected Cambrian seawater values, suggesting minimal post-diagenetic disturbance of isotopic and trace element systematics. Selective leaching of carbonate units reveal upward decreasing trends in dissolved redox-sensitive trace metals (e.g., U, Ni, V) indicative of drawdown from regional and global marine anoxia during the peak of the Late Cambrian SPICE event.

Astronomical pacing of relative sea level during Oceanic Anoxic Event 2: Preliminary studies of the expanded SH#1 Core, Utah, USA

AbstractProximal marine strata of the North American Western Interior Basin (WIB) preserve a rich record of biotic turnover during Oceanic Anoxic Event 2 (OAE2; ca. 94 Ma), a pronounced Late Cretaceous carbon cycle perturbation interpreted to reflect global warming, widespread hypoxia, and possible ocean acidification. To develop a more robust synthesis of paleobiologic and geochemical data sets spanning this Earth-life transition, we drilled the 131 m Smoky Hollow #1 Core (SH#1), on the Kaiparowits Plateau of southern Utah, USA, recovering the Cenomanian–Turonian Boundary (CTB) interval in the Tropic Shale Formation. A 17.5 m positive excursion in high-resolution bulk carbon isotope chemostratigraphy (δ13Corg) of SH#1 characterizes the most expanded OAE2 record recovered from the mid-latitudes of the WIB.Depleted values in a paired carbonate carbon isotope (δ13Ccarb) chemostratigraphy cyclically punctuate the OAE2 excursion. These depletions correspond to intervals in the core with a higher degree of carbonate diagenesis and correlate well to an existing sequence stratigraphic framework of flooding surfaces in the shoreface facies of the Markagunt Plateau (∼100 km west). We detect statistically significant evidence for astronomical cycles in the δ13Ccarb data set, imparted by diagenesis at flooding surfaces, and develop a floating astronomical time scale (ATS) for the study interval. Stable eccentricity cycles (405 k.y.) align with stratigraphic sequences and associated trends in sedimentation rate, and short eccentricity cycles (∼100 k.y.) pace nested parasequences. These results confirm an astronomical signal and, therefore, climatic forcing of relative sea level during OAE2 in the WIB. Furthermore, cross-basin correlation of the ATS and expanded δ13C chemostratigraphy of SH#1 suggests that these transgressive-regressive parasequences modulated siliciclastic sediment delivery in the seaway and contributed to deposition of prominent rhythmically bedded CTB units across the WIB, including the Bridge Creek Limestone. The presented approach to analysis of these proximal offshore siliciclastic facies links early diagenetic influences on chemostratigraphy to astronomically modulated sequence stratigraphic horizons, and helps to resolve rates of paleobiologic and paleoenvironmental change during a significant Mesozoic carbon cycle perturbation.

The Cerro Rajón Formation—a new lithostratigraphic unit proposed for a Cambrian (Terreneuvian) volcano-sedimentary succession from the Caborca region, northwest Mexico

A new mappable rock unit, the Cerro Rajón Formation, is proposed for the Cambrian succession of the Caborca region, Sonora, México. Formerly Unit 1 of the Puerto Blanco Formation, the Cerro Rajón Formation is interpreted as a volcano-sedimentary succession deposited along the coast of a passive margin that was impacted by rift-related volcanism. At its proposed type locality, in Cerro Rajón, the Cerro Rajón Formation consists of 270–285 m of tuffaceous conglomerate, metabasalt, mafic tuff, mafic lapillistone, mafic agglomerate, and quartzite with minor siltstone, limestone, and dolostone- and quartzite-dominated conglomerate. The unit contains a major disconformity near its base, where m-to dm-thick conglomerate locally replaces the fine-grained clastics that make up the base of the Cerro Rajón Formation. δ13C chemostratigraphy and biostratigraphy of the Rajón and its bounding strata limits Rajón deposition to the Fortunian Stage of the Terreneuvian Series. Volcanic rocks in the Cerro Rajón Formation are represented by mafic to ultramafic flows, including picrobasalts and metabasalts with hydrothermal alteration characteristics, evidenced by replacement of clinopyroxenes by chlorite, actinolite, and epidote. The mineral paragenesis of these volcanic rocks suggests the succession experienced greenschist grade metamorphism. Basalt geochemistry is consistent with low silica (34.32–48.21%) magmatism with high TiO2 concentrations (3.63–7.52%), related to continental rift volcanism with OIB characteristics. This volcanism could represent the last southern evidence of the rifting process that occurred along the western margin of Laurentia or could be related to volcanic rift deposits further afield.

Turonian Sea Level and Paleoclimatic Events in Astronomically Tuned Records From the Tropical North Atlantic and Western Interior Seaway

AbstractMultiple hiatuses punctuate the Middle to Upper Turonian Substages of the Western Interior Basin (WIB, USA), interrupting an otherwise highly resolved and relatively conformable Upper Cretaceous stratigraphic record. To determine the duration and possible causes of these hiatuses, while simultaneously reconstructing Turonian carbon cycling, we develop a new astronomically tuned carbon isotope (δ13C) chemostratigraphy from time‐equivalent organic carbon‐rich shales of the tropical North Atlantic (Demerara Rise—Ocean Drilling Program Leg 207). A Gaussian kernel smoothing and cross‐correlation technique is also introduced to quantitatively correlate the astronomically tuned Demerara Rise δ13C chemostratigraphy to coeval records globally. This analysis reveals a general consistency among ages for Turonian δ13C excursions from astronomical and radioisotopic timescales. It also indicates that a positive oxygen isotope excursion at Demerara Rise, previously interpreted to signify cooling, is synchronous (~±200 ka) with the Ogbourne Hardground in the English Chalk, the Bass River—Magothy sequence boundary, and an expansive mid‐Turonian hiatus in the WIB. However, the event is not concurrent with maximum regression in the WIB. Trends in δ13C, and additional geochemical data from Demerara Rise (weight percent total organic carbon, weight percent carbonate, and C/N), are consistent with oceanic upwelling conditions modulated by monsoonal winds, which drove significant organic carbon burial instep with several prominent positive δ13C excursions (e.g., Hitchwood #2‐3). Moreover, δ13C and weight percent total organic carbon preserve strong obliquity power (~1 Ma). This suggests that higher cross‐equatorial insolation gradients resulting from changes in Earth's axial tilt intensified monsoonal winds, strongly influencing tropical Atlantic organic carbon burial and Turonian carbon cycling dynamics following Ocean Anoxic Event 2.

New discovery of late early ordovician graptolites and discussion on stratigraphic sequence of paleozoic sediments in the Dinh Ca - Trang Xa Section, Vo Nhai Disctrict, Thai Nguyen Province, Northeast Vietnam

Sandstone, quartzitic sandstone rhythmically interbedded with siltstone, calcareous siltstone and cherty shale are largely exposed in the Dinh Ca - Trang Xa area, Vo Nhai District, Thai Nguyen Province, NE Vietnam. In previous studies, they were described as Mo Dong or Than Sa Formations (Middle-Upper Cambrian). The graptolite faunas newly discovered from the Dinh Ca Market - Na Mo - Dong Bo cross section are composed of Expansograptus urbanus (Monsen), Expansograptus abnormis (Hsu), Tetragraptus quadribrachiatus (Hall), T. approximatus Nicholson, T. pacificus Ruedemann, T. decipiens T.S. Hall, Schizograptus sp., belonging to the quadribrachiatus Biozone, indicating Floian Stage or late Early Ordovician. This serves as the basis for age determination and for establishing new lithostratigraphic units termed as the Dinh Ca Formation (O1 dc) and the Ba Nhat Member. The stratigraphic sequence of Paleozoic sediments in the Dinh Ca - Trang Xa area is also discussed in this paper.References Bergstöm S. M., Löfgren A., and Maletz J., 2004. The GSSP of the Second (Upper) Stage of Lower Ordovician Series: Diabasbrottet at Hunneberg, Province of Västergӧtland, Southwestern Sweden. Episodes, 27(4), 265-272. Bergstöm S. M., Chen X., Gutierrez-Marco J.C., and Dronov A. V., 2008. The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages to δ13C chemostratigraphy. LETHAI, 42(1), 97-107. Dovjikov, A. E. (ed) et al., 1965. (1971 in Vietnamese). The Geology of the Northern part of Vietnam. Geological Survey of Vietnam, Ha Noi, 583p. Doan Ky Thuy (Ed.), 1976. Geologyof the Lạng Sơn sheet (F-48-XXIII), 1:200.000 scale. Housed in the Central of Geological Information and Archives (in Vietnamese), Ha Noi. Doan Ky Thuỵ (Ed.), 2000. Geology and mineral resources of the Lạng Sơn sheet (F-48-XXIII), 1:200.000 scale. Published by Department of Geology and Minerals of Viet Nam (in Vietnamese with English summary), Ha Noi. Le Duy Bach, 1985. Structure and formation stages of Viet Nam in the tectonic framework of SE Asia. Abstract of PhD Dissertation (in Russian). The National Library of Viet Nam, 31 Trang Thi Street, Ha Noi. Mansuy H., 1908. Contribution à la carte géologiques de l'Indochine. Paléontologie, 73pp., Ha Noi. Nguyen Van Liem, 1978. On the Carboniferous system in North Viet Nam (in Vietnamese). Journal of Biology-Geosciences, 16(3), 78-85. Patte E., 1927. Études géologiques dans l'Est du Tonkin. Bull. Serv. Géol. Indoch., vol. XVI, fasc. 1, 314pp, Ha Noi. Pham Dinh Long (Ed.), 1968. Geology of the Tuyen Quang sheet (F-48-XXII), 1:200.000 scale. Housed in the Central of Geological Information and Archives. Ha Noi (in Vietnamese). Pham Dinh Long, Hoang Xuan Tinh, Phan Viet Ky, 1981. Some main features of regionalgeology of Northeast Viet Nam. Mapping, 48, 4-19, Ha Noi. Pham Kim Ngan, 1975. Some biostratigraphical sections of the Lower Paleozoic of North Viet Nam. Collections of works onstratigraphic studies in Viet Nam, 40-54. Science and Technics Publishing House (in Vietnamese), Ha Noi. Pham Kim Ngan, 1982. Some research results of the Cambrian and Ordovician biostratigraphy in Viet Nam. Geology and Minerals, 1, 7-17. Published by Institute of Geology and Mineral resources (in Vietnamese with English summary), Ha Noi. Saurin E., 1956b. Lexique stratigraphique international. Vol. III, fase, 6a: Indochine. Centre National de la Rech. Scient., 140p. Paris. 1970: published by Science and Technics Publishing House, 222pp. (in Vietnamese), Ha Noi. Tong Duy Thanh, Vu Khuc (Eds.), 2011. Stratigraphic units of Viet Nam.Vietnam National University Publishing House, 526pp, Ha Noi. Tran Van Tri, Ta Hoang Tinh, Phan Son, Le Duc An, 1964. On the Lower Paleozoic sediments in the Than Sa area. Journal of Geology, series A, 37, 6-11 (in Vietnamese with Russian summary), Ha Noi. Tran Van Tri, Vu Khuc (Eds.), 2011. Geology and Earth resources of Viet Nam. Publishing House for Science and Technology, 645pp, Ha Noi. Xu Chen, Strig M. Bergström, Yuan-Dong Zhang and Jun-Xuan Fan, 2009. The base of the Middle Ordovician in China with special reference to succession at Hengtan near Jiangshan, Zhejiang Province, Sothern China. LETHAI, 42, 218-231.

A probabilistic analysis of meteorically altered δ13C chemostratigraphy from late Paleozoic ice age carbonate platforms

Abstract The stratigraphic expression of meteoric diagenesis in carbonates is a glimpse into the weathering, fluid transport, and biological productivity of the ancient near-surface terrestrial environment. To infer this environmental information, we use a probabilistic approach to merge an isotope-based reactive transport model with chemostratigraphic data from carbonates that were subaerially exposed during an expansion of the late Paleozoic ice age in the middle Carboniferous. The rate of carbon flowing through the carbonate platform relative to the rate of carbon reacting between mineral and fluid phases controls the length scale, curvature, and magnitude of the diagenetic carbon isotope profiles. The ratio of advection to reaction is determined for seven stratigraphic sections, and the advection rate is used to estimate the minimum carbonate weathering associated with each profile. These carbonate weathering rates extrapolated over the expansive shallow carbonate platforms of the middle Carboniferous indicate that glacioeustatic fall may have caused a 20%–50% increase in the dissolved CaCO3 flux to the ocean. The complex feedbacks among carbonate weathering and accumulation, atmospheric pCO2, glacioeustasy, and passive margin subsidence may have played an unexplored role in the glacial-interglacial climate dynamics of the late Paleozoic ice age.

A high-resolution, continuous δ13C record spanning the Ordovician–Silurian boundary on Anticosti Island, eastern Canada

One of the best-exposed and most complete stratigraphic records from paleotropical areas spanning the Ordovician–Silurian (O–S) boundary is located on Anticosti Island, eastern Canada. Our study is the first one to sample strata superbly exposed at low tide along the west coast of Anticosti Island, thus providing a previously unexploited, nearly complete stratigraphic interval (∼300 m) at the O–S boundary for δ13C chemostratigraphy. A new high-resolution δ13C curve with more than 500 data points spaced at every ∼0.5 m has been produced, rectifying important pitfalls of previously published δ13C curves (i.e., low sampling resolution, variable sampling intervals, stratigraphic gaps). This new high-resolution δ13C curve displays a lower and an upper positive Hirnantian Isotope Carbon Excursion (HICE) recognized elsewhere around the globe. The ascending limb of the lower HICE starting from baseline values of +0.5‰ corresponds to the upper 20 m of the late Katian Vauréal Formation, but δ13C peak values of +2.5‰ occur in the lower part of the Hirnantian Ellis Bay Formation. In spite of a δ13C record segmented by a few stratigraphic hiatuses, the upper HICE, with its peak values of +4.5‰, is well recorded in the upper part of the Hirnantian Ellis Bay Formation. When compared with sections from around the globe, our δ13C curve displays a distinct long-term trend with a long sustained lower HICE followed abruptly by the upper HICE and return to baseline values of +0.5‰ prior to the Rhuddanian. The continued descending isotopic trend well into the Becscie Formation suggests that the O–S boundary may occur at a higher stratigraphic level (up to 30 m) than previously interpreted. Active subsidence combined with moderate initial water depths prior to the Hirnantian were key factors controlling the deposition of a thick O–S sedimentary succession with a few hiatuses on Anticosti Island and capturing a comprehensive, reliable δ13C record across this interval.

Evolution of the northern Tethyan Helvetic Platform during the late Berriasian and early Valanginian

AbstractThe Early Cretaceous period is characterized by widespread carbonate production in tropical and subtropical epicontinental seas, which was modulated by changes in sea‐level, detrital and nutrient fluxes, and the global carbon cycle. As a result, carbonate platforms were sensitive recorders of environmental change, which often anticipated global environmental perturbations. A good example is provided by the northern Tethyan carbonate platform, which is presently preserved in the central European Helvetic Alps. There, the latest early to late Valanginian Weissert episode of global change, which is defined by the first important positive shift in δ13C records of the Cretaceous, is expressed by a prolonged, stepwise drowning phase. In this contribution, a detailed reconstruction of palaeoenvironmental change before and during the Weissert episode is provided based on three representative sections of the Helvetic platform. The sections are placed along a deepening transect and correlated by means of ammonite and microfossil biostratigraphy, sequence stratigraphy and δ13C chemostratigraphy. In a first phase of palaeoenvironmental change during the latest Berriasian, photozoan carbonate production was stopped by a major and hitherto undetected drowning episode, which was followed by a phase of renewed carbonate production by heterozoan biota. This phase was linked to major sea‐level rise, a change to a more humid climate and strong regional subsidence associated with tectonic block tilting. During the Valanginian, the circulation of nutrient‐enriched sea waters prevented a return to oligotrophic conditions and two further drowning episodes occurred, which are both documented by condensed phosphate‐rich beds and dated as middle early Valanginian and late Valanginian to early Hauterivian. The exact causes of the three‐step deterioration in carbonate production are not established but a link to episodic volcanic activity is likely, eventually related to the formation of the Paranà‐Etendeka large igneous province.

Estimating the impact of early diagenesis on isotope records in shallow-marine carbonates: A case study from the Urgonian Platform in western Swiss Jura

The poor preservation of sedimentological and geochemical evidence often hampers the identification of subaerial exposure surfaces in carbonate platform successions. Such discontinuities may represent significant time gaps, justifying the quest for diagnostic indications. We investigated a lower Hauterivian to upper Barremian sedimentary succession from Switzerland to highlight the stratigraphic impact of meteoric diagenesis on platform carbonates. Petrographic observations revealed the presence of five generations of calcitic cements. Spot measurement of carbon and oxygen stable isotope compositions (δ13C and δ18O values, respectively) measurements relate them to specific diagenetic environments: selected blocky calcite cements exhibit an enrichment in light isotopes (12C, 16O), indicative of meteoric eogenesis, whereas very negative δ18O values link the last phase of cementation to mesogenesis. Meteoric calcitic cements formed during karstification of the top of the upper Barremian succession; burial diagenetic phases overlap these eogenetic phases, which thus cannot be related to recent telogenesis. We estimated the ratio of early meteoric versus burial cements within the studied succession: its stratigraphic repartition reveals that lower Aptian eogenesis influenced the geochemistry of platform carbonates as deep as 45m below the exposure surface. In this interval, negative whole-rock δ13C values do not reflect contemporaneous variations of the δ13C curves documented in sections devoid of strong diagenetic impact. Such an impact is a function of the amount of meteoric cement filling pore space as well as of the primary carbon isotope composition of the carbonate sediments and of the meteoric cement. The vertical influence of a karst event is not restricted to the first metres directly below the exposure surface. Although meteoric diagenesis may not significantly alter microfacies, it may strongly perturb isotope systems. Building onto this case study, the application of δ13C chemostratigraphy to platform carbonates can only be performed with great caution and after a careful examination of diagenetic features.

Climate‐modulated sequence development in a tropical rift basin during the Late Palaeocene to Early Eocene super greenhouse Earth

AbstractSequence developments in rift basins are considered to be influenced largely by tectonics and to a lesser extent by eustatic sea‐level and climate. Studies indicate that in passive margin basins, climate can mask the effects of tectonics and eustasy by modulating the sediment supply. It is, however, less understood how the sedimentary sequence in rift basins might respond to strong climatic fluctuations where tectonic pulses generate rapid accommodation space. Here a case study has been provided to assess the effect of climate vis à vis sea‐level and tectonics on sequence development in the Cambay rift basin, western India, during the Early Palaeogene (Late Palaeocene to Early Eocene) super greenhouse globe. Facies analysis of this shale–lignite sequence suggests deposition in a lagoon/bay, developed over the Deccan Trap basement. Detailed sequence stratigraphic analysis using basin‐wide representative composite sections, marker lignite seam, event bed and high‐resolution carbon isotope (δ13C) chemostratigraphy suggest an overall transgressive motif. Among the three prominent Early Eocene eustatic highstands, only the one at ca 53·7 Ma is expressed by the thickest coal accumulation throughout the basin. Expression of the other sequence stratigraphic surfaces is subdued and can be due to the overall finer grain size of the sediment or local variation in the subsidence rate at different fault‐controlled mini‐basins. Enigmatic presence of a maximum flooding surface coinciding with the 53·7 Ma climate event (Eocene Thermal Maximum 2), manifested by negative carbon isotope excursion, indicates possible influence of climate over and above tectonics in developing the rift sequence. Qualitative rainfall variation assessed using the magnitude of the carbon isotope excursion and pollen abundance show that a relatively dry/low precipitation climatic phase during the Eocene Thermal Maximum 2 hindered the siliclastic supply to the basin. Thus, it has been inferred that climate‐induced high siliciclastic supply possibly enhanced the autocyclic reorganization and hindered the development of the key sequence stratigraphic surfaces across the basin during climate extremes.

Carbon isotope stratigraphy and correlation of depositional sequences in the Upper Ordovician Ely Springs Dolostone, eastern Great Basin, USA

Shallow water carbonate shelf strata of the Ely Springs Dolostone accumulated on the western Laurentian margin as relative sea level fluctuated during the Late Ordovician icehouse. The Ely Springs Dolostone is divided into four upper Katian depositional sequences and one Hirnantian sequence. Here we present new carbon isotope data from the Ely Springs in the context of the sequence stratigraphic architecture. These data derive from a shelf transect of five stratigraphic sections exposed in the eastern Great Basin, USA and provide records of carbon cycling on the Laurentian margin prior to and during the Hirnantian glacial maximum. The δ13C values range between −2‰ and +4‰ at each section, and chemostratigraphic tie points occur in all sections of the transect. Intrabasinal correlations based on the δ13C records are consistent with correlations derived from sequence stratigraphy and conodont biostratigraphy. The Ely Springs Dolostone carbon isotope curve can also be correlated with the late Katian (Cincinnatian) composite derived from epicontinental deposits of the Laurentian midcontinent. The globally documented Hirnantian positive carbon isotope excursion (commonly referred to as “HICE”) occurs within the uppermost depositional sequence immediately below a regional unconformity coinciding with peak glacial conditions on Gondwana. The late Katian Elkhorn and Waynseville δ13C excursions are also present in the Ely Springs Dolostone carbon isotope record. The peak value of the Hirnantian carbon excursion increases from 2.6‰ to 3.8‰ across the shallow shelf and reaches 7.3‰ in coeval limestones of the Hanson Creek Formation deposited in a deep dysoxic setting. This lateral gradient is not the result of changes to the δ13C of weathered carbon supplied by rivers, as previously proposed; it may be instead related to authigenic carbonate cement precipitation during early diagenesis of the Hanson Creek strata. That these pervasively dolomitized shallow water carbonates retain δ13C signatures useful for regional and global stratigraphic correlation indicates that δ13C chemostratigraphy may be an appropriate correlation tool for dolostones with few biostratigraphically useful fossils.

Coupled sedimentary and δ13C records of late Mississippian platform-to-slope successions from South China: Insight into δ13C chemostratigraphy

Variability in stratigraphic accumulation rates and distribution of stratal hiatuses along with strong endemism of index fossils hinder regional to global stratigraphic correlation of the Visean–Serpukhovian (V–S) boundary interval (late Mississippian) and thus geological inferences regarding the onset of the late Paleozoic ice age. Here we integrate high-resolution δ13C time series with detailed sedimentary facies analysis of late Visean–early Serpukhovian (V–S) carbonate platform-to-slope successions from South China to evaluate the influences of local depositional and diagenetic processes on δ13Ccarb and to address the correlation issue.Analysis of 11 sedimentary facies from 5 outcrop sections indicates a restricted platform setting (Yashui section) dominated by bioclastic wacke-packstone to lime mudstone in which paleokarst developed, and contemporaneous carbonate slope settings dominated by thin-bedded lime mudstones intercalated with slumps and calciturbidites. Based on vertical facies assemblages, three depositional units are recognized, recording a significant sea-level drawdown across the V–S boundary. Multiple negative δ13C excursions (>1‰) can be correlated across the V–S boundary interval in several slope sections (Naqing, Luokun, and Narao sections). Variability in the V–S boundary δ13C record in some sections is interpreted to record truncation physically by submarine erosion by slumps or chemically during karstification. A long-term decrease in δ13C values through the Serpukhovian of the Yashui section likely records local influences on carbon cycling in the restricted platform setting. This negative δ13C trend and associated depositional facies at the Yashui section can be correlated to the Arrow Canyon section, USA, which, together with other coeval global sedimentary and geochemical records, indicate a widespread eustatic drawdown in the late Visean with initial buildup of Gondwanan ice sheets. We conclude that integrated sedimentary facies analysis and δ13C chemostratigraphy can be used for stratigraphic correlation when interpreted within a well-constrained sedimentary and carbon-isotope regional framework.

A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China

Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of δ13C and δ18O values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ13C chemostratigraphy possesses some unique characteristics: (1) stable δ13C values as a whole, but ubiquitous low δ13C anomalies through the Cap Carbonate, (2) increase of the δ13C values from −3 to +5‰ across the C2/C3 boundary, (3) no δ13C anomaly between the C1 and C2 boundary, and (4) presence of an anomalous high δ13C value (+2.3‰) and a faint positive correlation between δ13C and δ18O values in the C1 unit.Evidence of quite low δ13C anomalies (with a nadir of −41‰), ubiquitous negative δ13C anomalies through the Cap Carbonate, and a high δ13C anomaly accompanied with a faint positive correlation between δ13C and δ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ13C values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.