Negative δ13C Shift Research Articles

Regime shift of skeletal δ13C after 1997/1998 El Nino event in Porites coral from Green Island, Taiwan

The 1997/1998 El Niño event caused mass coral bleaching and mortality in many tropical and subtropical regions, including corals on Green Island, Taiwan, in the northwestern Pacific Ocean. This study analyzed coral carbon isotope ratios (δ13C), oxygen isotope ratios (δ18O), and Sr/Ca ratios for 29 years, including the 1997/1998 El Niño period, to examine how high water temperature events are recorded in coral geochemical indicators. Sr/Ca ratios in coral skeletons from Green Island show the lowest peak, means the highest temperature during the 1997/1998 El Niño period. However, we couldn’t observe high-temperature events on δ18O. Furthermore, a negative δ13C shift was observed after El Niño events. The regime shift of δ13C might have been caused by temporal bleaching and/or a decrease in symbiotic algae due to high water temperature stress under the continuous decrease in δ13C in DIC due to the Suess effect.

Planktic foraminifer response and paleoceanographic changes across the Danian – Selandian interval in the southern Tethys (Elles section, Central Tunisia)

The Danian – Selandian interval experienced several paleoceanographic changes with two prominent carbon cycle perturbations: the Dan-C2 and Latest Danian Event (LDE). We present integrated planktic foraminifer biostratigraphy, carbon isotope stratigraphy, and paleoceanographic interpretations from the Danian-Selandian at Elles, central Tunisia. Whereas the response of the planktic foraminifers to these events has been widely explored in deep sea sites, this shallow Tethyan shelf setting is significant for evaluating the expression of these global events in proximal environments. Both the Dan-C2 and LDE events are distinguished in our δ13Cbenthic record by biostratigraphically-constrained negative carbon isotope excursions. Recognition of these reliably dated excursions enables assessment of bioevent synchroneity. The lowest occurrence of Subbotina triloculinoides was diachronous at Dan-C2, but the highest occurrence of Praemurica was broadly synchronous at the onset of the LDE. Subsequent lowest occurrence of Igorina albeari shows latitudinal diachroneity between ocean basins. The planktic foraminifer assemblage composition indicates mesotrophic and stratified conditions in the early Danian, warming and increasing oligotrophy during the P2 Zone, peak oligotrophy and enhanced stratification across the LDE, shallowing-induced disruption of the upper water column niches in the latest Danian, and recovery with high diversity in the early Selandian. The Dan-C2 excursion shows a small 0.3‰ negative δ13C shift coinciding with increased abundance of low oxygen tolerant biserial Chiloguembelina, suggesting slightly stressed conditions. In contrast, the larger 0.6‰ LDE excursion coincides with the extinction of Praemurica and proliferation of Morozovella, reflecting an oligotrophic surge with warming in the surface ocean.

Kungurian (Cisuralian) conifers and environmental changes: a negative δ13C shift in the flora of Tregiovo (Northern Italy)

The Le Fraine fossil locality, near the Tregiovo village (Trento Province, northern Italy) yields two of the best documented Kungurian (early Permian) plant fossil assemblages of eastern palaeoequatorial Pangea. Both plant assemblages (Tregiovo A and B) are dominated by walchian and voltzian conifers but differ in the relative abundance of the major plant groups. Analyses of the δ13C of the bulk organic carbon throughout the entire stratigraphic succession of Tregiovo show a negative trend towards the upper part of the succession. Several conifer taxa are recognized and taxon-specific δ13C analyses of the conifer remains in both plant fossil assemblages shows the same trend in all taxa. This correlation between the negative shift of the bulk organic carbon and the taxon-specific δ13C analysis shows that the negative δ13C shift recorded along the stratigraphic section is not the result of change in relative abundance within the plant assemblages but instead has an external cause, likely the δ13C values of atmospheric CO2. These results agree well with a negative shift observed in other Kungurian continental successions (e.g., China, South Africa, Australia), supporting a global perturbation in the carbon cycle. The negative shift in δ13C could very well record the last deglaciation phases of the Late Paleozoic Ice Age and the rise of the atmospheric pCO2.

Temporal and spatial variations of high-resolution strontium, carbon, and oxygen isotopic chemostratigraphy at the end-Permian crisis boundary in South China

Variable degrees of strontium, carbon, and oxygen isotopic perturbations related to the end-Permian mass extinction (EPME) were recorded from the Latest Permian (i.e., the Lopingian stage prior to the extinction event) to the Earliest Triassic recovery (i.e., the Induan stage), while the underlying causes for their temporal and spatial variations and marine paleoenvironmental indications were poorly understood. In this study, integrated high-resolution 87Sr/86Sr, δ13C, and δ18O proxies are presented across the Wuchiapingian/Changhsingian and Permian/Triassic boundaries within the stratigraphic framework calibrated with high-resolution conodont biostratigraphy and high-precision geochronology in three South China sections, i.e., the Liangfengya, the Meishan, and the Shangsi sections. High-resolution strontium isotopes revealed that the water-mass segregation and accelerated precipitation rate of strontium probably resulted in local heterogeneity and provincial features of 87Sr/86Sr values at short-time interval less than 1 Myr. This provincialism of 87Sr/86Sr values can serve high-resolution chemostratigraphy. Integrated δ13C and 87Sr/86Sr isotopes from the studied sections at the Wuchiapingian/Changhsingian boundary (WCB) revealed that spatial heterogeneity with respect to drift magnitudes and peak values of δ13C negative excursion was not associated with oceanic stratification or high terrestrial input, while the releasing of 13C-depleted CO2 by deep-marine volcanism could be a plausible cause. Moreover, integrated δ18O and 87Sr/86Sr isotopes indicated that the “cooling event” occurring at the WCB was a provincial event, which was influenced by water depth and marine volcanism. The integrated high-resolution 87Sr/86Sr, δ13C, and δ18O proxies confirmed that the oceanic stratification was an ultimate cause for shallow-to-deep-water δ13C gradient at the Permian/Triassic boundary (PTB), but having no link to paleoproductivity increase. The positive excursion of 87Sr/86Sr postdated negative shift of δ13C by ∼ 40 Kyr across the PTB from the studied intervals. This temporal chemostratigraphic variation revealed that terrestrial influx was not a major cause for negative excursion of δ13C. The shallow-to-deep-water 87Sr/86Sr gradient was observed across the PTB in the studied sections, which was probably caused by water segregation resulting from oceanic stratification and distances from high- or low-radiogenic strontium sources supplied by terrestrial input or marine volcanism.

Depositional evolution, sequence stratigraphic framework and its response to relative sea-level change, the Middle and Lower Ordovician carbonate system on outcrops, north-western margin of Tarim Basin

Middle and Lower Ordovician series carbonate platform deposits develop well in the north-western Tarim Basin, Kalpin and Bachu outcrop. An integrated analysis of the outcrop, hand specimen, thin slice and geochemical data allows for interpreting the sequence framework, depositional architecture and the response to sea-level changes. The Middle and Lower Ordovician Yingshan and Yijianfang Formations comprise 2 composite sequences (CS1 and CS2) bounded by regional or major unconformities, 5 third-order sequences (Sq1–Sq5) defined by subaerial exposure unconformities or facies changing and 13–15 fourth-order sequences classified by the outcrop depositional cycle change. Twelve microfacies and seven microfacies associations are identified using hand specimens and thin sections. The primary depositional facies include platform margin reef–bank complexes, open platform intraclast or bioclast shoal, intra-shoal and restricted platforms shoal-dolomitic or intra-clastic tidal flats. The research indicates that the carbonate platform evolved from arid and semiarid ramps, restricted platform and dolomitic tidal flat (CS1) to the open and rimmed platform margins (CS2) and eventually was drowned at the end of the Middle Ordovician. The sea-level changes have been studied using the Fischer plot method, showing two sets of long-term shallow–deep–shallow trends. Large-scale transgressions occurred at the lower parts of Sq4, with a significant negative shift of δ13C and low-energy mudstone or wackestone depositional architectures. The Fischer plot, lithofacies cycle and the geochemical indicators correlate well and are comparable with Haq's global sea-level changes. Sea-level changes dominate the sequence framework and depositional evolution, and dual factors of sea-level change and tectonic subsidence constrain local Sq3-HST. The high-frequent fluctuation of sea-level changes has a noticeable control effect on the development of fourth-order depositional cycles of carbonate deposits.

Eustatic sea-level fall and global fluctuations in carbonate production during the Carnian Pluvial Episode

In this paper, sea-level fluctuations during the Carnian Pluvial Episode (CPE) are investigated. A revision of published data from multiple successions worldwide indicates a sea-level drop that occurred in different geodynamic settings after the onset of the first of multiple carbon-isotope perturbations that characterize the CPE. New stable isotope data, zircon U-Pb geochronology, carbonate petrology, conodont and foraminifer biostratigraphy from the Carnian of the Sichuan Basin and comparison to the well-dated coeval successions of the Dolomites allow pinpointing with unprecedented precision this sea-level fall and determine that it occurred after the onset of the first, but prior to the third negative δ13C shift of the CPE. These lines of evidence indicate that such sea-level oscillation was eustatic. Facies analysis and sequence stratigraphy of units deposited during the ensuing sea-level rise in the Sichuan and Dolomites, further show that a Tethys-wide crisis of microbial carbonate production and drowning of carbonate platforms were followed by a recovery of marine calcification, widely testified by the deposition of oolitic bodies. Whereas a Tethys-wide recovery of microbial carbonate production is documented at the end of the Carnian, this increase in chemical calcification occurred earlier, at the beginning of the Tuvalian, and suggest that global transformations in carbonate systems coincident with the CPE were complex and share commonalities with other times in the geological record when a similar evolution was linked to ocean acidification.

Influences of summer monsoon variations on the terrigenous influx, bioproductivity and early diagenetic changes in the southwest Bay of Bengal during Late Quaternary

The sediment core (MGS11/02), collected from the lower boundary of the Bay of Bengal (BoB) oxygen minimum zone (890 m) is studied to understand the influences of summer monsoon variations on the terrigenous sediment influx and its relation to bioproductivity and early diagenetic changes in the southwest BoB during the Late Quaternary. The high content of terrigenous controlled metals increased, kaolinite/chlorite increased, and K/Rb decreased after 19 cal kyr BP, indicating a simultaneous increase in the terrigenous influx and intensity of chemical weathering during this period. The variations in summer monsoon and land influx observed in the core sediments are also reflected in the sources of organic matter. High values of δ13C observed at 40 to 19 cal kyr BP suggests higher influence of marine organic matter and/or presence of C4 terrestrial plant. The negative shift of δ13C between 19 and 6 cal kyr BP could be the result of the change in terrestrial plant biomass to C3 photosynthesis and increased marine organic matter. Rock magnetic properties and redox-sensitive metals distribution suggests the prevalence of reduced environment during 52 to 19 cal kyr BP and oxic condition during 19 cal kyr BP to present. The low sedimentation during glacial period allowed sufficient time for pyritization which resulted in the low magnetic signals during 52 to 20 cal kyr BP and led to reducing conditions in the sediment interval. The δ15N values in the core sediments are suggesting the absence of denitrification in the water column during last 52 cal kyr BP. Previous studies suggest that the BoB is a geochemical ‘tipping point’ because the slight changes in water column denitrification by anthropogenic or climatic impact can accelerate BoB nitrogen contribution to the global nitrogen budget. This study indicates the absence of water column denitrification signals during last 52 cal kyr BP even though drastic climatic changes happened in the BoB in the Late Quaternary.

Comparison between Raman spectra of carbonaceous material and carbon isotope thermometries in low-medium grade meta-carbonates: Implications for estimation of metamorphic temperature condition

The changes in the physicochemical conditions of the carbonaceous material during progressive metamorphism in metapelitic rocks is widely used as a geothermometer with the aid of Raman spectroscopy. However, the application of this technique in carbonate rocks has not been established yet. Here, we compare Raman spectroscopy of carbonaceous material thermometry and carbon isotope thermometry in low- to medium-grade metacarbonate rocks from the Archean Chitradurga Schist Belt in the Dharwar Craton, India. The carbonates in the lowermost Bababudan Group have metamorphosed under lower amphibolite facies metamorphic conditions giving consistent estimates for both Raman spectra of carbonaceous material thermometry (460–592 °C) and carbon isotope thermometry (450–560 °C). Contrastingly, in the Vanivilas Formation, the carbonaceous material with very fine flaky morphology in the grain boundary has slightly lower crystallinity, when compared to the coarse-grained ones near the vein boundary. Nevertheless, the carbon isotope thermometry estimated a lower temperature around 400 °C. The inconsistencies between the temperature estimates are ascribed to the pervasive post-metamorphic aqueous hydrothermal fluid infiltration indicated by the presence of numerous criss crossing quartz veins. This is also corroborated by the lowering of the oxygen isotope ratio of the carbonates near the vein boundary by 2.3‰, but without much difference in the carbon isotope ratios. The coarsening of the carbonaceous material near the vein boundary signifies fluid assisted recrystallisation, that enhanced the crystallinity as evidenced in the Raman spectra. Moreover, the negative δ13C shift in the carbonaceous material (c. –8.5 to –13‰) was possibly due to recrystallization following partial CO2 degassing. In the Chitradurga Schist Belt, the Bababudan Group and the Vanivilas Formation have regionally metamorphosed under lower amphibolite facies condition, possibly related to the widespread granitic intrusion at c. 2.61 Ga and later affected by hydrothermal event at c. 2.5 Ga.

Valanginian cold/warm climatic oscillation and synsedimentary tectonic interaction for drowning the carbonate platform of Southern Tethys (Sicily)

Valanginian is a deeply investigated time interval due to the climatic, environmental and oceanographic changes that characterised it, globally recorded in sedimentary successions.During the Valanginian, an oscillation between warm and cold climatic phases is generally accepted as causing change in sedimentation, especially along the carbonate platform successions. In this context the so-called “Weissert event” occurred.The analysed stratigraphic interval consists of a Valanginian–Hauterivian package of two black metallic crusts (hardgrounds) and condensed pelagites, interbedded in the Lower Cretaceous carbonate shallow-water succession outcropping in the Palermo Mountains (NW Sicily). Integrating facies, structural, mineralogical and geochemical analysis, including analysis of δ13C and δ18O isotope records, the interaction between synsedimentary tectonics and climatic changes, as causing palaeoenvironmental and palaeoceanographic changes in the Lower Cretaceous Panormide carbonate platform, has been envisaged.A regional extensional tectonic event in the Southern Tethyan margin was detected at the Berriasian/Valanginian boundary, as responsible for the tectonic fragmentation of the Mesozoic Panormide carbonate platform in various fault-blocks. Locally, tectonic subsidence caused drowning of some of the fault-blocks. Before the recovery of the benthic carbonate factory in the Barremian, condensed pelagites and mineralised (Fe–Mn) crusts developed during the alternation of warm and cold climatic pulses.The positive–negative excursions of the Valanginian–Hauterivian δ13C pattern of the Colombrina section provide solid evidence for the chronostratigraphic assignment of the study interval to the drowning of many carbonate platforms that preludes the onset of the “Weissert event”. In detail, the drastic negative shift of δ13C in the late early Valanginian mineralised crust reveals massive release of isotopically light carbon to the atmosphere/hydrosphere reservoirs from a possible excess of volcanogenic CO2 or methane release from clathrate dissociation. Combining sedimentological data with the increase of the δ18O isotope records, two cold climate pulses are envisaged. The first one is assigned to the Berriasian/Valanginian boundary and led to the formation of the lower mineralised crust. The second and more evident positive shift of δ18O is assigned to the earliest late Valanginian, during the sedimentation of the laminated pelagites deposited immediately after the upper mineralisation event. These cold events that are comparable with the coeval cooling pulses recorded at a global scale highlight icehouse interludes during the Lower Cretaceous greenhouse mode.

The Cambrian Drumian carbon isotope excursion (DICE) in the Keping area of the northwestern Tarim Basin, NW China

The middle Cambrian is an important period for the decline of metazoans and the resurgence of microbialites. Based on the analysis of carbon and oxygen isotopes, detrital zircon UPb dating, and geochemical analysis, the middle Cambrian DICE (Drumian carbon isotope excursion) event was identified and its relationship with microbialites and sea-level changes was discussed in the Tarim Basin. The DICE event has a negative shift in δ13C from 0‰ to −3.2‰. About 15 m below the DICE, the youngest age of the detrital zircon was 504.5 Ma, which was consistent with the age of the DICE event. Stromatolites are abundant and agnostoid trilobite Ptychagnostus atavus is absent in the Keping area. The DICE event corresponds to the occurrence of hemispherical stromatolites in the shallow-water platform. Lithologic association and terrigenous clastic index show that the marine transgression and condensation layer occurred during the DICE event, which may be synchronized with organic matter oxidation when stratified water masses overturned during transgression.

Facies changes and a major negative δ13C shift suggest the base of Mila Formation as the likely base of the Miaolingian Series, Alborz Mountains, northern Iran

Facies changes and a major negative δ13C shift suggest the base of Mila Formation as the likely base of the Miaolingian Series, Alborz Mountains, northern Iran

Oxygen and carbon stable isotope records of the Lochkovian-Pragian boundary interval from the Prague Basin (Lower Devonian, Czech Republic)

Climate changes close to the Lochkovian-Pragian Event are still widely discussed. Carbonate δ18O and δ13C and conodont apatite δ18O from medial to distal carbonate ramp sediments were analysed to provide further stable isotope data from the stratotype area in the Prague Basin. The uppermost Silurian to lower Emsian δ18O trends are put into an updated conodont biostratigraphy framework. Carbonate δ18O and δ13C are discussed in the context of facies-dependent diagenesis. The δ13C pattern measured from different sections enables the correlation of intervals with development of different facies inside the Prague Basin. Positive δ13C shifts are often coupled with trends of decreasing computed gamma-ray (CGR) values interpreted as regressions, whereas negative δ13C shifts commonly coincide with opposite trends in CGR. The Lochkovian-Pragian boundary interval coincided with an increase in carbonate δ13C and an increase in the δ18O of conodont apatite and carbonate, which we interpret as consequence of climate cooling. Generally, the Lochkovian was warmer than the Pragian and earliest Emsian.

Millennial-scale ocean redox and δ13C changes across the Permian–Triassic transition at Meishan and implications for the biocrisis

The Permian–Triassic extinction was the largest Phanerozoic mass extinction event, yet its ultimate causes remain unresolved. Ocean redox conditions, global warming, and carbon cycle perturbations owing to massive volcanism and their inter-relationships are keys to resolve the origins of this event. We here assess such relationships using a method to obtain an instantaneous stratigraphic response from high-resolution (millennial-scale) records of carbonate-δ13C and pyrite framboid size distributions across the Permian–Triassic boundary (PTB) at Meishan, China. We report two gradual long-term negative δ13C shifts before and after the PTB. Late Permian ocean redox conditions varied through three successive oxic–dysoxic, dysoxic–euxinic, and high-frequency euxinic stages. Oxic and high-frequency euxinic events occurred in the early Griesbachian and middle Griesbachian, respectively. The short-term euxinia events were not associated with the negative δ13C excursions. Siberian volcanism could have produced large light-carbon inputs, global warming, and low-oxygen conditions at intermediate ocean depths, but it may not directly drive high-frequency euxinia across the PTB. Long-term dysoxia and occasional euxinia well before the PTB stressed mobile macroorganisms and caused gradual extinction before the major extinction event. High-frequency latest Permian euxinia reduced overall biotic adaptability, and caused the initial rapid mass extinction. Increasing temperatures and other deleterious environment factors then reinforced the extinction process. Oxic conditions contributed to biotic recovery immediately after the mass extinction in the early Griesbachian. Further millennial-scale dysoxic/euxinic alternations in the middle Griesbachian would have prolonged Early Triassic biotic recovery.

Guadalupian (Middle Permian) δ13Corg changes in the Lower Yangtze, South China

The Middle Permian Guadalupian witnessed significant environmental changes in the Phanerozoic such as large-scale sea-level drop, supercontinent Pangaea assembly, and transition from Early Permian glaciation to Late Permian non-glacial intervals. Carbonisotope tracers can provide insights for these environmental changes. The δ13C studies of the entire Guadalupian epoch are rare, and most of them has focused on near the end of Guadalupian and carbon isotopes of inoragnics. Here, we present carbon isotopic compositions of organic matters in the Guadalupian from two sections (Chaohu and Xiaolao) in the Lower Yangtze area, South China. Our results show that δ13Corg profiles in the Guadalupian show a peak in the Roadian and a gradual negative shift from the Roadian to the middle Capitanian. These trends can be matched by δ13C changes of carbonate rocks or organic matter in South China and other places in the world, representing a global carbon cycle signal. The Roadian positive peak was probably due to high productivity which was caused by upwelling during cooling time. The gradual negative shift of δ13C was caused mainly by a decrease of organic matter burial on land and in the ocean, resulting from global sea-level drop and anoxia-caused benthos decline, respectively. The less important causes for the gradual δ13C negative shift are volcanic-gases releasing, decreased mountain belts, and the resultant reduced silicate weathering-consumption of CO2. The gradual negative shift of δ13C coincides with the gradual extinction in the Guadalupian. Therefore, global sea-level drop and marine reducing conditions may be the main causes of the gradual extinction in the Guadalupian.

Investigation of Temporal Apparent C4 Sugar Change in Manuka Honey.

New Zealand manuka honeys are known for their propensity to increase apparent C4 sugar content during storage. Depending on the particular storage regime and the initial content of dihydroxyacetone (DHA) in honey, the ready-to-market product often fails the C4 sugar test because of the above phenomenon. We have used DHA labeled with a radioactive 14C isotope in a set of honeys subject to an incubation experiment. These honeys were analyzed for DHA, methylglyoxal (MG), hydroxymethylfurfural (HMF), apparent C4 sugars, and 14C scintillation counts over a period of 18 months. The major conclusion of this experiment is that neither DHA nor MG is responsible for the δ13C shift in the honey protein extract. There must be some other yet unknown substance of manuka honey, which binds to the protein and causes negative δ13C shift. One identified candidate for such a binding is carbon dioxide.

Origin and characteristics of grain dolomite of Ordovician Ma55 Member in the northwest of Ordos Basin, NW China

The origin of grain dolomite in M55 Member of Ordovician Majiagou Formation in northwestern Ordos Basin was studied by geochemical and petrological tests on core samples. Observation of cores, thin sections and casting thin sections, analysis of cathodoluminescence, X-ray diffraction, microscopic sampling of trace elements, laser sampling δ18O and δ13C, and fluid inclusion homogenization temperature were conducted. The results show that the dolomite is the product of recrystallization of micritic to crystal powder dolomite rather than the product of dolomitization of grain limestone. In the spherical grains are residual gypsum and halite pseudo crystals identical with those in the host micritic dolomite. The spherical particles of dolomite has similar trace elements and δ18O and δ13C characteristics to micritic dolomite. Furthermore, Mn/Sr ratio of the fine—medium dolomite between the dolomite grains is about 5-8, while Mn/Sr ratios of calcite in limestone, micritic dolostone in micritic dolomite, and micritic and powdery dolomite are about 0-2, indicating that the dolomite experienced strong diagenesis. Homogenization temperature of inclusions of fine—medium dolomite is about 148. 19 °C, higher than that of inclusions in micritic to crystal powder dolomite (about 122.60 °C), which also supports the conclusion that the grain dolomite experienced burial diagenesis and negative shift of δ18O and δ13C. The δ18O, δ13C values of micritic to crystal powder dolomite match with the negative migration, but those of calcite in limestone don't. It is of great significance to elucidate the genesis of “dolomite recrystallization” for the prediction of such dolomite reservoirs.

Operation of the boreal peatland methane cycle across the past 16 k.y.

Abstract The role of boreal wetlands in driving variations in atmospheric methane (CH4) concentrations across the last deglaciation (20–10 ka) and the Holocene is debated. Most studies infer the sources of atmospheric methane via ice-core records of methane concentration and its light stable isotopic composition. However, direct evidence for variations in the methane cycle from the wetlands themselves is relatively limited. Here, we used a suite of biomarker proxies to reconstruct the methane cycle in the Chinese Hani peat across the past 16 k.y. We found two periods of enhanced methanogenesis, at ca. 15–11 ka and ca. 10–6 ka, whereas weak methanogenesis characterized the late Holocene. These periods of enhanced methanogenesis relate to periods of high/increasing temperatures, supporting a temperature control on the wetland methane cycle. We found no biomarker evidence for intense methanotrophy throughout the past 16 k.y., and, contrary to previous studies, we found no clear control of hydrology on the peatland methane cycle. Although the onset of methanogenesis at Hani at ca. 15 ka coincided with a negative shift in methane δ13C in the ice cores, there is no consistent correlation between changes in the reconstructed methane cycle of the boreal Hani peat and atmospheric CH4 concentrations.

Impact of early hydrocarbon charge on the diagenetic history and reservoir quality of the Central Canyon sandstones in the Qiongdongnan Basin, South China Sea

The Central Canyon sandstones, located in the Ledong–Lingshui Depression, Qiongdongnan Basin, South China Sea, are the main reservoirs of the deep-water area. The present study aimed to examine the diagenetic processes and reservoir quality of these sandstones. Using an integrated approach of examination of casting thin sections, X-ray diffraction, porosity and permeability measurements, scanning electron microscopy, carbon and oxygen stable isotope analyses, fluid inclusion analyses and electron microprobe analysis, we analysed the diagenetic features of the sandstones and the reservoir quality of the LD30-1 and LS17-2 sandstones, respectively. The results showed that the sandstones consist of lithic arkoses and sublitharenites, the main pore systems consist of primary intergranular pores and secondary pores, and the main cements consist of authigenic quartz, clay minerals, and carbonates. By combining the analyses of burial history and hydrocarbon charging history, we discovered that the LD30-1 sandstone reservoirs and the LS17-2 sandstone reservoirs are quite different. A significant negative shift in δ13C and δ18O (ranging from −4.88‰ to −2.27‰ and from −11.83‰ to −5.96‰, respectively) is evident in the carbonate cements of the LD30-1 sandstones, and oxidation of organic matter had an important role in the occurrence of the carbonate cements. However, relatively high and concentrated δ13C and δ18O values in the LS17-2 sandstones, ranging from −1.76‰ to −0.48‰ and −5.04‰ to −2.95‰, respectively, are closer to the original isotopic compositions. Based on casting thin sections, the abundance of quartz overgrowths in the LD30-1 sandstones is higher than that in the LS17-2 sandstones. The development of each clay mineral in the LD30-1 and LS17-2 sandstones is quite different. The LS17-2 sandstones have higher reservoir quality than the LD30-1 sandstones. Early hydrocarbon charge is an important factor causing the differences of diagenetic minerals and reservoir quality between the LS17-2 sandstones and the LD30-1 sandstones.

Regional to global correlation of Eocene–Oligocene boundary transition successions using biostratigraphic, geophysical and geochemical methods

AbstractRegional to global high-resolution correlation and timing is critical when attempting to answer important geological questions, such as the greenhouse to icehouse transition that occurred during the Eocene–Oligocene boundary transition. Timing of these events on a global scale can only be answered using correlation among many sections, and multiple correlation proxies, including biostratigraphy, lithostratigraphy, geochemistry and geophysical methods. Here we present litho- and biostratigraphy for five successions located in the southeastern USA. To broaden the scope of correlation, we also employ carbon and oxygen stable isotope and magnetic susceptibility (χ) data to interpret these sections regionally, and correlate to the Global Boundary Stratotype Section and Point (GSSP) near Massignano in central Italy. Our results indicate that approaching the Eocene–Oligocene boundary, climate warmed slightly, but then δ18O data exhibit an abruptc.+5 ‰ positive shift towards cooling that reached a maximumc.1 m below the boundary at St Stephens Quarry, Alabama. This shift was accompanied by ac.−3 ‰ negative shift in δ13C interpreted to indicate environmental changes associated with the onset of the Eocene–Oligocene boundary planktonic foraminiferal extinction event. The observed cold pulse may be responsible for the final extinction of Hantkeninidae, used to define the beginning of the Rupelian Stage. Immediately preceding the boundary, Hantkeninidae species dropped significantly in abundance and size (pre-extinction dwarfing occurring before the final Eocene–Oligocene extinctions), and these changes may be the reason for inconsistencies in past Eocene–Oligocene boundary placement in the southeastern USA.

Siberian Trap volcanism, global warming and the Permian-Triassic mass extinction: New insights from Armenian Permian-Triassic sections

AbstractPermian-Triassic boundary sections from Armenia were studied for carbon isotopes of carbonates as well as oxygen isotopes of conodont apatite in order to constrain the global significance of earlier reported variations in the isotope proxies and elaborate the temporal relationship between carbon cycle changes, global warming and Siberian Trap volcanism. Carbon isotope records of the Chanakhchi and Vedi II sections show a 3–5‰ negative excursion that start in the Clarkina nodosa (C. yini) conodont Zone (latest Permian) with minimum values recorded in Hindeodus parvus to Isarcicella isarcica conodont zones (earliest Triassic). Sea surface temperatures (SST) reconstructed from oxygen isotopes of conodont apatite increase by 8–10 °C over an extrapolated time interval of ∼39 ka with the onset of global warming occurring in the C. iranica (C. meishanensis) Zone of the latest Permian. Climate warming documented in the Armenian sections is comparable to published time-equivalent shifts in SST in Iran and South China suggesting that this temperature change represents a true global signature. By correlating the Armenian and Iranian section with the radiometrically well-dated Meishan GSSP (Global Stratotype Section and Point) section (South China), the negative shift in δ13C is estimated to have occurred 12–128 ka prior to the onset of global warming. This temporal offset is unexpected given the synchrony in changes in atmospheric CO2 and global temperature as seen in Pleistocene ice core records. The negative δ13C excursion is explained by the addition of emission of isotopically light CO2 and CH4 from thermogenic heating of organic carbon-rich sediments by Siberian Trap sill intrusions. However, the observed time lag in the δ13C and δ18O shifts questions the generally assumed cause-effect relationship between emission of thermogenically produced greenhouse gases and global warming. The onset of temperature rise coincides with a significant enrichment in Hg/TOC (total organic carbon) ratios arguing for a major volcanic event at the base of the extinction interval. Whether global warming was a major factor for the Late Permian mass extinction depends on the duration of the extinction interval. Warming only starts at the base of the extinction interval, but with the extinction encompassing a time interval of 60 ± 48 ka, global climate warming in conjunction with temperature-related stressors as hypoxia and reduced nutrient availability may have been one of the major triggers of the most devastating biotic crisis in Earth history.