Isotope Excursion Research Articles

Pushing the boundary: A calibrated Ediacaran-Cambrian stratigraphic record from the Nama Group in northwestern Republic of South Africa

The Nama Group exposed on the Neint Nababeep Plateau along the Orange River in northwestern Republic of South Africa is now recognized as an expanded record of the Ediacaran-Cambrian transition that provides opportunity for an integrated stratigraphic approach in examining the geochemical and biologic evolution across this fundamental geologic boundary at unprecedented resolution. U-Pb zircon geochronology by the CA-ID-TIMS method on six intercalated volcanic ash beds in the Nama Group (from the Huns Member to the Nomtsas Formation) at this locality is used to construct a high-resolution, Bayesian, age-stratigraphic model, which allows a direct temporal calibration of the biostratigraphy and carbon isotope record from 539.63 ± 0.15 Ma to 537.95 ± 0.28 Ma (2σ internal errors). Across the border in the Witputs subbasin of southern Namibia, ash beds at the base of Nudaus Formation and within the Nasep Member yielded new U-Pb ages of 545.27 ± 0.11 Ma and 542.65 ± 0.15 Ma, respectively. Our combined geochronology reveals the detailed depositional history of the Nama Group at a regional scale, suggesting that a relatively low sediment accumulation rate in the Kuibis Subgroup and the lower Schwarzrand Subgroup was followed by accelerated sedimentation in the upper Schwarzrand Subgroup. This is consistent with a pattern of exponential increase in subsidence typical of foreland basins. Some of the observed chemostratigraphic trends throughout the Nama Group could relate to a shift from a seawater-buffered to a sediment-buffered regime of early marine diagenesis driven by this increase in sedimentation rate.Occurrences of soft-bodied erniettomorphs, calcified body fossils, and trace fossils within the Neint Nababeep Plateau are broadly consistent with known global biostratigraphic ranges. However, we document the youngest radioisotopically calibrated occurrences of Ediacaran-type fossils, which stratigraphically overlap with large and complex bilaterian ichnofossils, between 539.18+0.17/−0.26 Ma and 538.30+0.14/−0.14 Ma. Yet, the index fossil Treptichnus pedum remains undocumented from this section, and we suggest that its first regional occurrence may be younger than these strata. Despite relatively continuous and high rates of carbonate sedimentation across the Ediacaran-Cambrian boundary (as currently recognized), the upper Nama Group of the Neint Nababeep Plateau does not preserve the characteristic negative carbon isotope excursion observed within other basal Cambrian successions. One possible explanation for its absence is that this chemostratigraphic marker is not ubiquitous in all carbonate depositional environments. Alternatively, the basal Cambrian carbon isotope excursion, and perhaps the Ediacaran-Cambrian boundary as defined by the first appearance of Treptichnus pedum, might be >1 m.y. younger than currently recognized, postdating 538 Ma and, thus, suggesting a more condensed early Cambrian radiation. Difficulties in determining with confidence the first appearance datum of the index fossil Treptichnus pedum in the Nama Group highlight the challenge of a global biostratigraphic definition for the base of Cambrian and underscore the necessity of an integrated stratigraphic and radioisotope geochronologic approach to understand the tempo and patterns of environmental and biologic evolution across the Ediacaran-Cambrian boundary.

Enhanced hydrological cycle during Oceanic Anoxic Event 2 at southern high latitudes: New insights from IODP Site U1516

The Oceanic Anoxic Event 2 (OAE 2) represents one of the most profound global environmental disturbances of the Mesozoic Era, which was associated with a positive carbon isotope excursion due to widespread organic carbon burial. However, the character, evolutionary history, and exact driving mechanisms of OAE 2 are still much debated, exacerbated by the lack of records from climate-sensitive southern high latitudes. Here we present a multi-proxy record of a stratigraphically continuous succession spanning the late Cenomanian to early Turonian at the International Ocean Discovery Program Site U1516 in the Mentelle Basin off southwest Australia, which was located at southern high latitudes (~ 60–62°S) during the Late Cretaceous. Sedimentary records at Site U1516 allow the first detailed insights into source-to-sink processes in the Mentelle Basin and associated paleoenvironmental reconstruction in the southern Indian Ocean during OAE 2. Rare earth element abundances and clay-bound neodymium isotopes of the siliciclastic sediment fractions indicate that southwest Australia was the main detrital source of sediment at Site U1516 during OAE 2, with volcanogenic sediments representing only a minor component. A major provenance shift is observed across OAE 2, indicating an increased sediment contribution from distant sources (i.e., Yilgarn Craton and Albany-Fraser Orogen) relative to proximal sources (i.e., Perth Basin and Leeuwin Block) in southwest Australia, interpreted as reflecting a major reorganization of the drainage system due to enhanced fluvial runoff. Based on these findings, we infer that the OAE 2 interval showing the most prominent δ13C excursion was associated with an intensification of the hydrological cycle in the high-latitude southern hemisphere, with potential impacts on regional ocean chemistry and marine productivity. In particular, we suggest that enhanced terrigenous input and riverine nutrient export likely played a key role in controlling siliceous productivity at nearby ocean margins. As a consequence, the intense burial of organic matter (i.e., black shale) at Site U1516, which occurred episodically throughout OAE 2, possibly resulted from the combination of enhanced riverine-driven marine productivity and increased burial efficiency associated with the development of anoxic/euxinic bottom water conditions.

High-resolution carbon isotope chemostratigraphy of the middle Cambrian to lowermost Ordovician in southern Scandinavia: Implications for global correlation

Continuous and biostratigraphically well-constrained carbon isotope (δ13Corg) profiles through the upper Miaolingian (middle Cambrian) to Tremadocian (Lower Ordovician) part of the Alum Shale Formation in four drill cores from southernmost Scandinavia are described. The sections record seven Furongian (upper Cambrian) excursions/spikes, of which three appear valuable as auxiliary markers for identifying global stage boundaries. The Steptoean Positive Carbon Isotope Excursion (SPICE) event with a magnitude of up to 2.8‰ is located mainly within the Olenus Superzone. The onset, as specified here, coincides with the end-Marjuman extinction and is marked by a minor negative excursion (up to −1.6‰) right below the Miaolingian–Furongian boundary. The end of the SPICE event approximates the upper boundary of the Olenus Superzone and has potential for correlation of the Paibian–Jiangshanian boundary. The Top of Cambrian Excursion (TOCE) with a magnitude of about −1.6‰ straddles the Parabolina lobata–Parabolina heres megalops zonal boundary in Baltoscandia. This chemostratigraphic excursion, considered as a synonym of the Hellnmaria-Red Tops Boundary excursion (HERB), represents a secondary marker for correlating the base of Stage 10, if defined by the FAD of Eoconodontus notchpeakensis. A positive spike at the Cambrian–Ordovician boundary, here named COBS, recognized in the stratotype section at Green Point (Canada; δ13Ccarb magnitude +5.3‰), Scandinavia (δ13Corg magnitude up to +2.2‰) and elsewhere, appears as a valuable supplementary marker for recognizing this important boundary. The described δ13Corg profile from the Albjära-1 core provides a standard reference for Furongian carbon isotope chemostratigraphy in Baltica, refines the correlation between Baltica and other palaeocontinents, and indicates new possibilities for pinpointing international stage boundaries within the Furongian based on carbon isotope records.

Integrated bio- and chemostratigraphy of the Cretaceous – Paleogene boundary interval in the Zagros Basin (Iran, central Tethys)

An expanded Upper Cretaceous – Lower Paleocene succession, which encompasses the Cretaceous – Paleogene boundary (K – Pg), was studied in detail from the Zagros Basin (southwest Iran, central Tethys). In order to characterize the K – Pg transition, high resolution sampling was performed in the upper part of the Gurpi Formation. The studied interval consists of dark grey marly shales and marls with interbedded limestones. Based on calcareous nannofossils, the studied section is attributed to calcareous nannofossil zones CC26b to NP3 (latest Maastrichtian – early Paleogene). High relative abundances of the nannofossil species Micula staurophora (= Micula decussata) in the Maastrichtian part (CC26) might delineate cool surface water conditions, which are followed by an increase in abundance of Micula murus about 2 m below the K – Pg boundary. Reworked Cretaceous taxa were found at the K – Pg boundary and in the overlying Danian (NP 1–3). These findings hamper a reliable determination of the last occurrence of Cretaceous calcareous nannofossil taxa at the K – Pg boundary. The K – Pg boundary is located in the uppermost part of a positive carbon isotope excursion. Diversity and absolute abundances of calcareous nannofossils show a significant decrease at the K – Pg boundary. A bloom of the dinoflagellate cyst Cervisiella operculata (formerly Thoracosphaera operculata), which is indicating environmental stress, is recorded slightly below the boundary. Throughout the lowermost Danian (NP1), the assemblages are dominated by a bloom of small species like Neobiscutum romeinii, Neobiscutum parvulum, Futyania petalosa and small Cruciplacolithus primus. These taxa indicate stressful, and eutrophic surface water conditions throughout the initial recovery phase. The Dan-C2 and Lower C29n events, recorded in the lower Danian NP1 and lower NP2 biozones, are marked by negative carbon isotope excursions. High relative abundances of Prinsius dimorphosus followed by a Coccolithus pelagicus bloom in the full recovery phase (middle Danian, NP 3), are attributed to a short term cooling phase followed by warm surface water environments recorded along with the highest absolute abundance of calcareous nannofossils.

Astrochronologic calibration of the Shuram carbon isotope excursion with new data from South China

The Shuram carbon isotope excursion (CIE) is the largest negative CIE in geological history that may have recorded dramatic changes in Earth's surface environments, yet its time, duration, and global synchroneity have been debated. In South China, the Shuram CIE has been correlated with the negative CIE of the upper Doushantuo Formation (named as EN3 or DOUNCE). It is previously suggested that the Shuram CIE in South China stratigraphically consists of the upper Member 3 and Member 4, with an ending age of ca. 551 Ma. Recent studies, however, demonstrated that the EN3 or DOUNCE shifts to near zero or positive carbon isotope (δ13C) values within carbonate rocks of the Doushantuo Member 3. To better understand the duration of this CIE and its spatial variability, we have conducted a high-resolution cyclostratigraphic study using magnetic susceptibility (MS) in two sections: the Zhengjiatang section that preserves a complete EN3 in carbonate rocks of the Doushantuo Member 3 and the Wuhe-Gaojiaxi (WG) drillcore in which the EN3 does not shift to positive δ13C values until the end of the Member 4 black shales. In both sections, power spectral analyses of the MS series of the carbonate rocks reveal periodicities that match well with those of the Milankovitch cycles at 570 Ma. The duration of the Shuram CIE is estimated to be 6.1 ± 0.2 Myr by counting the interpreted 405-kyr sedimentary cycles observed in the Zhengjiatang section. Taking the newly reported ReOs age of 567.3 ± 3.0 Ma as the approximate termination age, the Shuram CIE in the best-preserved section in South China is constrained between ca. 573.4 and 567.3 Ma. Regional correlation, however, shows a ≥ 1 Myr stratigraphic hiatus or unconformity at the top of the Doushantuo Member 3 that separates the Shuram CIE from overlying post-Shuram ẟ13C “excursions”. Similar phenomena may exist in the Shuram CIE documented from stratigraphic successions in Oman, Australia, and the Death Valley region of western United States. The results invoke a reevaluation of the stratigraphic range of the Shuram CIE and suggest that the geochemical records documented from the black shales of the Doushantuo Member 4 and its globally correlative intervals may record paleoceanographic conditions millions of years after the Shuram CIE. Comparison of astrochronologically calibrated δ13C profiles reveals significant difference in the shape and amplitude of the Shuram CIE among the sections in South China and globally, which may have been caused by isotopic departure in local depositional environments and/or diagenetic overprints.

Conodont biostratigraphy and carbon isotope chemostratigraphy of the Middle to Upper Ordovician on the western Yangtze Platform, South China

Middle and Late Ordovician strata in the nearshore area of the western Yangtze Platform are investigated. In total, 241 δ13Ccarb samples and 108 conodont samples from the Songliang and Tuanjie sections in the northeastern Yunnan Province were collected for chemostratigraphic and biostratigraphic studies. Altogether, 21 species belonging to 16 conodont genera were recovered from the two sections, among which Ansella jemtlandica, Drepanoistodus basiovalis, Hamarodus brevirameus, and Microzarkodina hagetiana are important age indicators within the Dapingian to early Katian interval. Four carbon isotope shifts are recognized in the studied sections: a negative shift in the early Darriwilian, the middle Darriwilian carbon isotope excursion (MDICE), the early Katian Guttenberg carbon isotope excursion (GICE), and a further positive carbon isotope shift later in the early Katian which is a new record of this event in South China. Integrated carbon chemostratigraphy and conodont biostratigraphy indicate early Darriwilian and early Katian ages for the base and top, respectively, of the Huadan Formation at the Songliang section, and a Darriwilian to early Katian age for the Shihtzupu Formation at the Tuanjie section.

Precipitation stable isotopic signatures of tropical cyclones in Metropolitan Manila, Philippines, show significant negative isotopic excursions

Abstract. Tropical cyclones have devastating impacts on the environment, economies, and societies and may intensify in the coming decades due to climate change. Stable water isotopes serve as tracers of the hydrological cycle, as isotope fractionation processes leave distinct precipitation isotopic signatures. Here we present a record of daily precipitation isotope measurements from March 2014 to October 2015 for Metropolitan Manila, a first-of-a-kind dataset for the Philippines and Southeast Asia. We show that precipitation isotopic variation at our study site is closely related to tropical cyclones. The most negative shift in δ18O values (−13.84 ‰) leading to a clear isotopic signal was caused by Typhoon Rammasun, which directly hit Metropolitan Manila. The average δ18O value of precipitation associated with tropical cyclones is −10.24 ‰, whereas the mean isotopic value for rainfall associated with non-cyclone events is −5.29 ‰. Further, the closer the storm track is to the sampling site, the more negative the isotopic values are, indicating that in situ isotope measurements can provide a direct linkage between isotopes and typhoon activities in the Philippines.

Carbon, oxygen and strontium isotopic and elemental characteristics of the Cambrian Longwangmiao Formation in South China: Paleoenvironmental significance and implications for carbon isotope excursions

The biosphere experienced episodic perturbations during the Neoproterozoic–early Cambrian, accompanied by major fluctuations of the carbon cycle. However, compared to the Ediacaran–early Cambrian, geochemical records of the Cambrian Toyonian Stage (Longwangmiao), which contains the significant Redlichiid–Olenellid Extinction Carbon Isotope Excursion (ROECE), are scarce. To address this data gap, we conducted measurements of carbon, oxygen and strontium isotopes and element contents of a continuous series of carbonate samples from the Longwangmiao Formation of Well HS2, located in the central Sichuan Basin of South China. Our specific aims were to reconstruct the paleoenvironment and to determine the origin of the ROECE. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. The 87Sr/86Sr values of Well HS2, and for sites elsewhere, show that sea level gradually decreased during the Cambrian Toyonian Stage. Additionally, the pronounced negative δ13C excursion in the late Toyonian is globally associated with the ROECE, which is widely thought to have resulted from extinctions caused by a transgression. However, a transgression is not shown in the 87Sr/86Sr profile of Well HS2, which we attribute to local karst formation and erosion caused by frequent regional regressions. The interrelationships among several paleoenvironmental proxies show that continental weathering was climatically controlled, which affected the supply of terrigenous detritus, and that the carbonate platform was semi–restricted. Furthermore, profiles of paleoenvironmental proxies indicate that the environment was warm and humid, favoring high biological productivity and diversity during Interval I (early Toyonian), and that it gradually became colder and drier during Interval II (middle Toyonian). We suggest that by the time of Interval III (late Toyonian), the adverse effects of a cold and arid environment were superimposed on the transgression, triggering the ROECE. Our results provide a reference and possible interpretation for other carbon isotope excursion events.

Global and local drivers of the Ediacaran Shuram carbon isotope excursion

The Ediacaran Shuram carbon isotope excursion (CIE) follows the regional Gaskiers glaciation and occurs before the appearance of macroscopic animal fossils. Previous interpretations for the Shuram CIE have proposed global perturbations to Earth's carbon cycle accompanied by significant climatic and environmental change. These studies assume that carbonate carbon isotopes through the Shuram CIE record the composition of dissolved inorganic carbon in seawater. Through a compilation of new geochemical, sedimentological, and stratigraphic data from five localities on four separate paleocontinents, we find that all of the analyzed Shuram CIE successions share general similarities including a common transgressive-regressive sequence that is coincident with similar shifts in a host of geochemical proxies (δ13C, δ18O, δ44/40Ca, Sr/Ca, Mn/Sr, U/Ca). Despite these broad similarities between sections, there are important differences in the proxies between water depths that may be linked to changes in carbonate diagenesis. Specifically, we find the excursion's magnitude is largest in slope environments where diagenesis is sediment-buffered and smaller in shallow-water settings dominated by fluid-buffered diagenesis. The transgression that accompanied the Shuram CIE is consistent in amplitude (∼10-100s m) and duration (>1 My) with eustatic fluctuations driven by plate reorganization. These coupled geochemical and stratigraphic observations argue against meteoric and burial diagenesis as drivers for the excursion; instead, they are best explained by global changes to the locus and intensity of photosynthetic primary productivity in shallow-water environments and do not necessarily require large changes in global climate or marine redox.

Cloudina aggregates from the uppermost Dengying Formation, Three Gorges area, South China, and stratigraphical implications

Numerous issues remain concerning palaeobiology, palaeoecology, and biostratigraphy of Cloudina, an iconic tubular organism of the Ediacaran period. In this study, accumulations of Cloudina are described from the topmost bed of the Dengying Formation in the Baiguotang section of the eastern Three Gorges area, South China. The massive occurrence of Cloudina refutes any possibility of setting the Ediacaran-Cambrian boundary (E-CB) beneath the lithological transition between Dengying and Yanjiahe formations in this area. The age of the Cloudina aggregates is younger than 540.6 Ma based on a concordia 206Pb/238U age of 540.6 ± 3.4 Ma obtained from underlying upper part of the Baimatuo Member (Baiguotang section), which is consistent with the 539 Ma E-CB reported from southern Namibia. Combining fossil evidence provided by Asteridium–Heliosphaeridium–Comasphaeridium acritarchs and Anabarites trisulcatus–Protohertzina anabarica small shelly fossil assemblages and the Basal Cambrian Carbon isotope Excursion (BACE), we tentatively place the E-C boundary within the lower part of the Yanjiahe Formation, close to the BACE excursion located in the lower part of the Yanjiahe Formation in the southern Huangling Anticline.

Middle Oxfordian carbon cycle perturbation expressed in the Smackover Formation, Gulf of Mexico

Abstract The Middle–Late Jurassic opening of the central Atlantic and proto-Caribbean seaways connecting the Pacific with the Tethys Ocean combined with rising sea level led to a major oceanographic and climatic reorganization conducive to the development of widespread carbonate platforms and diverse reefs. The timing of this shift in carbonate production and opening of the Tethys Ocean is well-constrained from outcrops in Europe and western Asia and marked by a notable positive carbon isotope (δ13C) excursion in the middle Oxfordian Gregoryceras transversarium ammonite Zone, termed “MOxE”. However, the temporal constraints from the western arm of the circum-global seaway through the proto-Caribbean and Gulf of Mexico are not well resolved. Presented here are the first paired organic and carbonate carbon-oxygen isotope records from the Smackover Formation, Gulf of Mexico, that record a positive δ13C isotope excursion that is correlated to the middle Oxfordian transversarium Zone. These data are the first record of the MOxE in the Western Hemisphere and provide temporal constraints on the opening of the Gulf of Mexico. Furthermore, the existence of similar δ13C profiles from European basins strongly suggests a coeval response of the carbon cycle to the opening of the central Atlantic and proto-Caribbean seaways connecting the Pacific with the Tethys Ocean.

Extensive primary production promoted the recovery of the Ediacaran Shuram excursion

Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history. However, the main biogeochemical controls that ultimately drove this recovery have yet to be elucidated. Here, we report new carbon and nitrogen isotope and concentration data from the Nanhua Basin (South China), where δ13C values of carbonates (δ13Ccarb) rise from − 7‰ to −1‰ and δ15N values decrease from +5.4‰ to +2.3‰. These trends are proposed to arise from a new equilibrium in the C and N cycles where primary production overcomes secondary production as the main source of organic matter in sediments. The enhanced primary production is supported by the coexisting Raman spectral data, which reveal a systematic difference in kerogen structure between depositional environments. Our new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and suggest that blooms of oxygenic phototrophs modulated the recovery from the most negative δ13Ccarb excursion in Earth history.

Geochemical indications for the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2) hyperthermals in terrestrial sediments of the Canadian Arctic

Abstract During the late Paleocene to early Eocene, clastic fluvial sediments and coals were deposited in northern high latitudes as part of the Margaret Formation at Stenkul Fiord (Ellesmere Island, Nunavut, Canada). Syn-sedimentary tectonic movements of the Eurekan deformation continuously affected these terrestrial sediments. Different volcanic ash layers occur, and unconformities subdivide the deposits into four sedimentary units. Rare vertebrate fossils indicate an early Eocene (Graybullian) age for the upper part of the Stenkul Fiord outcrop. Here, we present carbon isotope data of bulk coal, related organic-rich mud and siltstones, a plant leaf wax-derived alkane, and additional plant remains. These data provide a complete carbon isotope record of one stratigraphic section with defined unconformity positions and in relation to other Eurekan deformation features. A previously dated ash layer MA-1 provided a U-Pb zircon age of 53.7 Ma and is used as a stratigraphic tie point, together with a discrete negative carbon isotope excursion found above MA-1 in a closely sampled coal seam. The excursion is identified as the likely expression of the I-1 hyperthermal event. Based on our isotope data that reflect the early Eocene dynamics of the carbon cycle, this tie point, and previous paleontological constraints from vertebrate fossils, the locations of the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2) hyperthermals and their extent along the complete section are herein identified. Within the intervals of the PETM and ETM-2 hyperthermal events, increasing amounts of clastic sediments reached the site toward the respective end of the event. This is interpreted as a response of the fluvial depositional system to an intensified hydrological system during the hyperthermal events. Our study establishes an enhanced stratigraphic framework allowing for the calculation of average sedimentation rates of different intervals and considerations on the completeness of the stratigraphic record. As one of the few high-latitude outcrops of early Eocene terrestrial sediments, the Stenkul Fiord location offers further possibilities to study the effects of extreme warming events in the Paleogene.

Pridoli (Silurian) to Lochkovian (Early Devonian) conodonts and the Silurian-Devonian boundary interval in the Decatur Limestone and Ross Formation in the Western Valley of Tennessee, USA

The timing of the facies transition from the Decatur Limestone to the overlying Rockhouse Limestone Member of the Ross Formation in western Tennessee was evaluated using conodont faunas and the delta 13C carb curve. The facies transition corresponds to conodont extinctions associated with the Klonk Oceanic Event and with the Klonk isotopic excursion, which span the Silurian-Devonian boundary interval. The interpreted position of the Silurian-Devonian boundary is placed near the top of the Decatur, above the conodont extinction level and in the ascending limb of the isotopic excursion. The lowest occurrences of the Devonian species Caudicriodus murphyi and C. anitae are slightly higher, at the top of the Decatur, and both species characterize the Rockhouse Limestone Member. The conodont succession across the Silurian-Devonian boundary is like that reported from sections in West Texas and Oklahoma and represents a regional biological response to the Klonk Event in the southern part of the Appohimchi Subprovince of the Appalachian Province, one that is also represented by comparable changes in brachiopod and ostracode faunas. Correlation into the northern part of the Appohimchi Subprovince is best accomplished using the delta 13C carb curve. The western Tennessee conodont fauna includes two closely related species of Zieglerodina, Z. zellmeri and Z. planilingua, as well as distinctive forms of Pseudooneotodus.

Sedimentary Record of Glacial Impacts and Melt Water Discharge off the East Siberian Continental Margin, Arctic Ocean

AbstractThe East Siberian Continental Margin is a major constituent of the western Arctic Ocean, ideal for studying the evolutionary history of the Siberian Ice Sheet (SIS) based on its unique geographical location and medium‐depth bathymetry. For this study, the sedimentary core CHINARE‐ARC07‐E25 (E25) was analyzed, revealing nearly continuous stable oxygen isotope and mineralogy records that preserved scenarios of the SIS development. Lithostratigraphic evidence suggests that the stratigraphic range of E25 extends to marine isotope stage (MIS) 7. Stable oxygen isotope analysis reveals multiple strong, light‐oxygen isotopic excursions since the penultimate glacial, accompanied by multiple patterns of ice rafted debris content and mineralogy. Based on these isotopic events, the potential factors causing the excursions and the provenance of drainage were evaluated. It was found that the excursion events recorded in pink‐white carbonate detritus layers could be related to the retreat of the Laurentide Ice Sheet. The most typical events, occurring during the deglaciation periods of MIS6/5e and MIS4/3 to early MIS3, were characterized by involving a mixture of two or more sources, and could be used to trace the activity of the SIS, which has been ignored in previous studies. Findings of this study are expected to advance the understanding of SIS contribution to the western Arctic Ocean basin development, which is important for interpreting global climate change behavior of similar ice sheets in the Arctic region.

Organic carbon accumulation at the northern Gondwana paleomargin (Tunisia) during the Toarcian Oceanic Anoxic Event: Sedimentological and geochemical evidence

During the early Toarcian (Early Jurassic; ~183 Ma) global environmental changes promoted the expansion of oxygen-deficient conditions across vast shelf areas around the globe. Ocean margin deoxygenation favored the preservation of organic matter in shelf sediments and is considered to reflect a Toarcian Oceanic Anoxic Event (T-OAE). Higher-level global drivers controlled temperature fluctuations, sea level evolution, oceanic circulation patterns, marine primary productivity, and stable carbon isotope excursions. Lower-level factors, controlled by individual basin bathymetry and paleogeography, operated on local to regional scale and here regulated organic matter accumulation. Accordingly, the T-OAE has been described as global event with a strong regional character. During the Toarcian, preferentially organic matter-lean sediments have been deposited at the northern Gondwana paleomargin. Enhanced connectivity to the Tethys Ocean and low marine primary productivity are considered to have been major factors preventing the formation of oxygen-deficient conditions and organic matter preservation. We here report on early Toarcian sediments from the Châabet El Attaris Section (Tunisia), located at the northern Gondwana paleomargin that episodically became enriched in organic matter. Bio- and chemostratigraphy both confirm that organic matter-rich sediments preferentially formed during the T-OAE but also predate the T-OAE. Sedimentological and micropaleontological characterization in concert with bulk, molecular and stable isotope geochemical investigations reveal that accumulation of marine organic matter occurred under anoxic, but not under euxinic, conditions. Limitation in the supply of reactive iron regulated organic matter sulfurization indicating that during most of the depositional history the levels of free H 2 S in bottom waters were rather low but limited to the sediment pore water. We conclude that the chemocline was either situated within the sediment or close to the sediment water interface. Spatiotemporally-restricted occurrence of organic matter enriched sediments and anoxic pore waters was promoted by restriction in water column ventilation in a local graben structure. • Organic matter-enriched strata in Tunisia predates but maximizes in the T-OAE. • Organic matter preservation occurred under anoxic but non-euxinic conditions. • Formation of oxygen deficient conditions was promoted by local paleobathymetry and basin restriction.

Carbon isotope excursions during the late Miocene recorded by lipids of marine Thaumarchaeota, Piedmont Basin, Mediterranean Sea

Abstract Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC), accompanied by a biosynthetic fractionation factor of ~20‰. Accordingly, the δ13C signature of their diagnostic biomarker crenarchaeol was suggested as a potential δ13CDIC proxy in marine basins if input from nonmarine Thaumarchaeota is negligible. Semi-enclosed basins are sensitive to carbon-cycle perturbations, because they tend to develop thermohaline stratification. Water column stratification typified the semi-enclosed basins of the Mediterranean Sea during the late Miocene (Messinian) salinity crisis (5.97–5.33 Ma). To assess how the advent of the crisis affected the carbon cycle, we studied sediments of the Piedmont Basin (northwestern Italy), the northernmost Mediterranean subbasin. A potential bias of our δ13CDIC reconstructions from the input of soil Thaumarchaeota is discarded, since high and increasing branched and isoprenoid tetraether (BIT) index values do not correspond to low and decreasing δ13C values for thaumarchaeal lipids, which would be expected in case of high input from soil Thaumarchaeota. Before the onset of the crisis, the permanently stratified distal part of the basin hosted a water mass below the chemocline with a δ13CDIC value of approximately −3.5‰, while the well-mixed proximal part had a δ13CDIC value of approximately −0.8‰. The advent of the crisis was marked by 13C enrichment of the DIC pool, with positive δ13CDIC excursions up to +5‰ in the upper water column. Export of 12C to the seafloor after phytoplankton blooms and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification primarily caused such 13C enrichment of the DIC pool.

Carbon isotopic chemostratigraphy of the Ediacaran-Cambrian successions in the northwestern Tarim Craton, NW China: Correlations with Gondwana supercontinent

Among the three cratons (North China, Yangtze, and Tarim) in China, the Tarim Craton during Precambrian is the least known for its evolution due to its broad Phanerozoic cover. The affinity of the Tarim Craton with Gondwana supercontinent during the Cryogenian-Ediacaran interval has long been debated. In this contribution, we report carbon and oxygen isotopes of the Ediacaran-Cambrian siliciclastic‑carbonate from successions in the northwestern Tarim Craton. The results exhibit one pronounced negative and one positive carbon isotopic excursions (CIE) in the Ediacaran sections (N3 and P1), and a distinct negative CIE at the base of the Cambrian (N4). With the constraints of geochronological data, we proposed that the Ediacaran-lower Cambrian chemostratigraphy of the northwestern Tarim basin correlates well with the lower Ediacaran and lower Cambrian from Yangtze, Oman, Australia, Namibia, and Canada basins. N3 in the Tarim Craton possibly represents a new contribution to the Shuram-Wonoka anomaly globally. With these data, together with previously published tectonostratigraphic and paleogeographic reconstructions, we propose that the Tarim Craton may have been involved in the Pan-African Orogeny during the assembly of Gondwana, and was positioned close to the Yangtze, Oman, and Namibia cratons along the northern periphery of the Gondwana supercontinent

Carbon cycle perturbation and mercury anomalies in terrestrial Oceanic Anoxic Event 1b from Jiuquan Basin, NW China

Abstract Oceanic Anoxic Event (OAE) 1b is well documented in western Tethys; however, records in Eurasia are still lacking. Here, we carried out high-resolution organic carbon isotope (δ 13 C org ), total organic carbon (TOC) content and mercury (Hg) concentration analysis of the lacustrine sediments from the Xiagou and Zhonggou formations in the Hanxiagou section of the Jiuquan Basin, NW China. The lacustrine δ 13 C org curve presents three stages of negative excursions above the basalt layer dated at 112.4 ± 0.3 Ma in the lowermost Zhonggou Formation. The three negative δ 13 C org excursions correspond well with the three sub-events (Kilian, Paquier and Leenhardt) of the OAE 1b in the Poggio le Guaine (central Italy), Vocontian Basin (SE France) and St Rosa Canyon (NE Mexico) sections, supporting the record of the terrestrial OAE 1b in the Jiuquan Basin. Five mercury enrichment intervals in Hg/TOC ratios were recognized, indicating that the pulsed volcanism from the Southern Kerguelen Plateau likely triggered OAE 1b. However, the decoupling between negative isotopic excursion shifts and mercury enrichments, signifying another carbon reservoir (with no link to mercury), probably contributed to the global carbon cycle perturbation during the OAE 1b period. Our results provide direct evidence to link OAE 1b and the terrestrial ecosystem in Eurasia.

A bio- and chemostratigraphic search for the Mid-Ludfordian Carbon Isotope Excursion interval in the Ludlow of the Ohesaare core, Estonia

A bio- and chemostratigraphic search for the Mid-Ludfordian Carbon Isotope Excursion interval in the Ludlow of the Ohesaare core, Estonia