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1.
The early Aptian abrupt carbon isotope excursion in marine carbonate and sedimentary organic matter reflects a major perturbation in the global carbon cycle. However, until now almost all the evidences of this event came from marine deposits. Here we present organic-carbon isotope (δ13Corg) data from the non-marine Jehol Group in western Liaoning, China. The lacustrine δ13Corg curve is marked by a relative long-lasting δ13Corg minimum followed by two stages of positive δ13Corg excursions that are well correlated with contemporaneous marine records. The carbon isotope correlation shows that the lacustrine strata of the Jehol Group were deposited at the same time of the early Aptian Oceanic Anoxic Event (OAE1a). The relative long-lasting δ13Corg minimum supports the hypothesis that volcanic CO2 emission may have played the main role in triggering the negative δ13C excursion and global warming at the onset of this event. In addition, the onset of δ13Corg minimum is concomitant with the radiation of the Jehol Biota, implying that the evolutionary radiation of the Jehol Biota may have been closely related with the increase in atmospheric CO2 and temperature.  相似文献   

2.
Large carbon cycle perturbations associated with the Middle Permian (Capitanian) mass extinction have been widely reported, but their causes and timing are still in dispute. Low resolution carbon isotope records prior to this event also limit the construction of a Middle Permian chemostratigraphic framework and global or local stratigraphic correlation, and hence limit our understanding of carbon cycle and environmental changes. To investigate these issues, we analyzed the 13Corg values from the Middle Permian chert-mudstone sequence (Gufeng Formation) in the Lower Yangtze deep-water basin (South China) and compared them with published records to build a chemostratigraphic scheme and discuss the underlying environmental events. The records show increased δ13Corg values from late Kungurian to early Guadalupian, followed by a decrease to the late Wordian/early Capitanian. The early-mid Capitanian was characterized by elevated δ13Corg values suggesting the presence of the “Kamura Event”: an interval of heavy positive values seen in the δ13Ccarb record. We propose that these heavy Capitanian δ13C values may be a response to a marked decline in chemical weathering rates on Pangea and associated reduction in carbonate burial, which we show using a biogeochemical model. The subsequent negative δ13C excursion seen in some carbonate records, especially in shallower-water sections (and in a muted expression in organic carbon) coincide with the Capitanian mass extinction may be caused by the input of isotopically-light carbon sourced from the terrestrial decomposition of organic matter.  相似文献   

3.
During the Ordovician, huge biological revolutions and environmental changes happened in Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, but the cause of these events remains controversial. Herein, we conducted a paired carbon isotopic analysis of carbonate (δ13Ccarb) and organic matter (δ13Corg) through the Ordovician in the Qiliao section on the Yangtze platform of South China. Our results showed that the δ13Ccarb trend of the Qiliao section can be correlated with local and global curves. The δ13Corg trend seems is less clear than the δ13Ccarb trend for stratigraphic correlations, but some δ13Corg positive excursions in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to the global carbon cycle during the Ordovician. Several known δ13Ccarb and δ13Corg negative and positive excursions have been recognised in this study, including the early Floian Negative Inorganic Carbon (δ13Ccarb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon (δ13Ccarb) Excursion, the mid-Darriwilian Inorganic Carbon (δ13Ccarb and δ13Corg) Excursion (MDICE), and the early Katian Guttenberg Inorganic Carbon (δ13Ccarb and δ13Corg) Excursion (GICE). These positive excursions and a smooth decline trend of δ13Corg values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO2 and then global cooling.  相似文献   

4.
As an indicator for terrestrial paleovegetation, the stable isotopic composition of total organic matter (δ13Corg) in loess sediments has been widely used for paleoclimatic reconstruction in western Europe, the Great Plains of North America and the Chinese Loess Plateau (CLP). However, little is known about the variation and paleoclimatic significance of the loess δ13Corg in arid Central Asia (ACA). We report δ13Corg data from an Axike (AXK) loess/paleosol profile from the eastern Ili Basin, eastern Central Asia. Along the profile, the δ13Corg values were more negative in the paleosol layers observed in the field and were confirmed by environmental magnetic proxies and a higher concentration of total organic carbon (TOC), consistent with results for western Europe and the northwestern CLP. Our results demonstrate that the loess δ13Corg in this region documents mainly the response of δ13C of locally predominant C3 plants to paleoclimatic variation, especially paleoprecipitation. Our results also suggest that the loess δ13Corg values in the area have the potential for quantitative paleoprecipitation reconstruction on the basis of detailed δ13Corg results from modern plants and surface soils in the future.  相似文献   

5.
The Cenomanian–Turonian boundary was characterized by distinctive positive carbon isotope excursions that were related to the formation of widespread oceanic anoxia. High-resolution geochemical proxies (TOC, CaCO3, δ13Corg, and δ13Ccarb) obtained from bulk rock, planktic foraminifers, and inoceramids from four marine marlstone-dominated stratigraphic sections in the Western Canada Sedimentary Basin (WCSB) were used to establish a regional carbon isotope stratigraphic framework and to investigate paleoenvironmental variability in four different depositional settings. Compared to background δ13Corg, (<−27‰) and δ13Ccarb (<2‰) values which were correlative to stable isotope excursions during Oceanic Anoxic Event (OAE) II worldwide, the δ13Corg (>24‰), and δ13Ccarb (>4‰) derived from inoceramid prisms in the studied sections within WCSB, were elevated during the Late Cenomanian–Early Turonian. During this interval, TOC and CaCO3 values which increased sporadically to >40% and 7%, respectively, were not consistent enough to be used for stratigraphic correlations. Based on the δ13Corg excursions, two bentonite beds were regionally correlated across this portion of the Western Interior Seaway (WIS). The eruption associated with the “Red” bentonite occurred approximately coeval with the maximum δ13Corg-excursion during OAE II in the Neocardioceras juddii Zone, whereas the “Blue” bentonite coincides with the termination of OAE II in the latest Watinoceras devonense zone. During the Late Cenomanian–Early Turonian in the WCSB, benthic foraminifers were sparse or totally absent, indicating the existence of fully anoxic bottom-water conditions. Planktic foraminifera were common in the well-oxygenated surface waters. A benthic oxic zone characterized by several agglutinated species occurs in the eastern part of the WSCB at the beginning of OAE II in the Sciponoceras gracile zone. The termination of the OAE II in the WCSB coincides with the first occurrence of small ammonites (Subprionocyclus sp.) in the western part of the basin.  相似文献   

6.
The carbon isotope composition (δ13C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ13C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C27 and C29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C29) similar to that of terrestrial C4 plants. Thus, the use of δ13C values of sedimentary C27 and C29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C4 plants. For sedimentary C31, however, the contribution from submerged plants was minor, so that the δ13C values for C31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ13C values of sedimentary C27 and C29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ13C values of sedimentary long chain n-alkanes (C27, C29 and C31) may carry different environmental signals. While the δ13C values of C31 were a reliable proxy for C4/C3 terrestrial vegetation composition, the δ13C values of C27 and C29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.  相似文献   

7.
The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member δ13Ccarb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of 13C-depleted organic matter. δ13Corg values average –36.7‰, consistent with Neoarchean marine units reported from elsewhere, but significantly less 13C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.  相似文献   

8.
Geochemical and isotopic data for the uppermost 1.2 m of the sediments of the central Santa Monica Basin plain were examined to better understand organic matter deposition and recycling at this site. Isotopic signatures (Δ14C and δ13C) of methane (CH4) and dissolved inorganic carbon (DIC) indicate the occurrence of anaerobic oxidation of CH4 that is fueled by CH4 supplied from a relict reservoir that is decoupled from local organic carbon (Corg) degradation and methanogenesis. This finding was corroborated by a flux budget of pore-water solutes across the basal horizon of the profile. Together these results provide a plausible explanation for the anomalously low ratio between alkalinity production and sulfate consumption reported for these sediments over two decades ago. Shifts in Δ14C and δ13C signatures of Corg have previously been reported across the 20-cm depth horizon for this site and attributed to a transition from oxic to anoxic bottom water that occurred ~350 years BP. However, we show that this horizon also coincides with a boundary between the base of a hemipelagic mud section and the top of a turbidite interval, complicating the interpretation of organic geochemical data across this boundary. Radiocarbon signatures of DIC diffusing upward into surface sediments indicate that remineralization at depth is supported by relatively 14C-enriched Corg within the sedimentary matrix. While the exact nature of this Corg is unclear, possible sources are hemipelagic mud sections that were buried rapidly under thick turbidites, and 14C-rich moieties dispersed within Corg-poor turbidite sections.  相似文献   

9.
To compare natural variability and trends in a developed estuary with human-influenced patterns, stable isotope ratios (δ13C and δ15N) were measured in sediments from five piston cores collected in Chesapeake Bay. Mixing of terrestrial and algal carbon sources primarily controls patterns of δ13Corg profiles, so this proxy shows changes in estuary productivity and in delivery of terrestrial carbon to the bay. Analyses of δ15N show periods when oxygen depletion allowed intense denitrification and nutrient recycling to develop in the seasonally stratified water column, in addition to recycling taking place in surficial sediments. These conditions produced 15N-enriched (heavy) nitrogen in algal biomass, and ultimately in sediment. A pronounced increasing trend in δ15N of +4‰ started in about A.D. 1750 to 1800 at all core sites, indicating greater eutrophication in the bay and summer oxygen depletion since that time. The timing of the change correlates with the advent of widespread land clearing and tillage in the watershed, and associated increases in erosion and sedimentation. Isotope data show that the region has experienced up to 13 wet-dry cycles in the last 2700 yr. Relative sea-level rise and basin infilling have produced a net freshening trend overprinted with cyclic climatic variability. Isotope data also constrain the relative position of the spring productivity maximum in Chesapeake Bay and distinguish local anomalies from sustained changes impacting large regions of the bay. This approach to reconstructing environmental history should be generally applicable to studies of other estuaries and coastal embayments impacted by watershed development.  相似文献   

10.
Carbon isotope (δ13Corg) analyses of non-marine clastic rocks and neritic carbonates and black shales spanning the Silurian/Devonian transition are compared from two richly fossiliferous sequences in Qujing of East Yunnan and Zoige of Sichuan, South China. The two sections, Xishancun and Putonggou sections in South China, reveal positive δ13Corg shifts happening in the Upper Pridoli and Lower Devonian and reaching peak values as heavy as ?25.2‰ (Xishancun) and ?19.9‰ (Putonggou) in the lowermost Lochkovian following the first occurrence of the thelodont Parathelodus and the conodont Icriodus woschmidti woschmidti (only in Putonggou Section and together with Protathyris-Lanceomyonia brachiopod fauna). These results replicate a globally known positive shift in δ13Corg from the uppermost Silurian to the lowermost Devonian. The δ13Corg variations across the Silurian/Devonian Boundary (SDB) at the two sections in South China exhibit a shift in carbon isotopic composition similar to the detailed SDB curves from the borehole Klonk-1 drilled at top of the Klonk Global Standard Stratotype-Section and Point (GSSP) in the Prague Basin, Czech Republic. In addition, four microvertebrate assemblages, including the Liaojiaoshan, Xishancun, Yanglugou and Xiaputonggou assemblages, are recognized from the Silurian/Devonian transition exposed in the Xishancun and Putonggou sections, respectively. The results from both carbon isotope stratigraphy and microvertebrate assemblage sequences suggest that the SDB in South China is located at the base of the Xishancun Formation (between sample QX-20 and sample QX-21) in the Xishancun Section and the lower part of the Xiaputonggou Formation (between sample ZP-09 and sample ZP-10) in the Putonggou Section. The isotopic trend for organic carbon together with the changes of microvertebrate remains across the SDB can offer an approach to a potential correlation of the SDB from different sedimentary facies, which help to correlate the marine with non-marine deposits.  相似文献   

11.
J. A. BEIER 《Sedimentology》1987,34(6):991-998
Two caliche profiles from a Pleistocene carbonate dune on San Salvador Island, Bahamas, were examined by petrographic and geochemical analysis. Profile A is an immature buried caliche profile characterized by caliche pisolites, a friable crust and abundant Cerion. Profile B is a more well-developed caliche profile at the top of the dune which contains abundant pisolites, rhizomorphs, laminated calcrete, a breccia and abundant Cerion. Geochemical changes in caliche profiles relative to the host rock are an increase in Al2O3, Fe2O3 and total organic carbon, a decrease in Mg and Sr, and a decrease in δ13Ccarb, δ18Ocarb and δ13Corg. The magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora. Abundance of calcified filaments and needle-fibre crystals in profile A attests to the importance of microbial processes in the early development of caliche profiles. Biogenic structures are largely destroyed in profile B due to recrystallization, but indirect evidence of biological activity is retained in the form of carbon isotope values.  相似文献   

12.
Carbon isotope ratio (δ13Corg) values of organic matter in lake sediments are commonly used to reconstruct environmental change, but the factors which influence change are varied and complex. Here we report δ13C values for sediments from Erlongwan maar lake in northeast China. In this record, changes in δ13C cannot be explained by simple changes in aquatic productivity. Instead, values were likely influenced by differences in the ratio between planktonic and benthic algae, as indicated by the remains of diatoms. This is because the variation of δ13Corg in algae from different habitats is controlled by the thickness of the diffusive boundary layer, which is dependent on the turbulence of the water. Compared with benthic algae, which grow in relatively still water, pelagic algae are exposed to greater water movement. This is known to dramatically reduce the thickness of the boundary layer and was found to cause even more severe δ13C depletion. In Erlongwan maar lake, low values were linked to the dominance of planktonic diatoms during the period commonly known as the Medieval Warm Period. Values gradually increased with the onset of the Little Ice Age, which we interpret as being driven by an increase in the proportion of benthic taxa, due to effect of the colder climate. The increase in planktonic diatoms at the end of the Little Ice Age, linked to higher temperature and a reduction in ice cover, resulted in a further decline in δ13Corg.  相似文献   

13.
We use the evolution of river sediment characteristics and sedimentary Corg from the Himalayan range to the delta to study the transport of Corg in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and Corg content in order to evaluate the controls on Corg loading. Contributions of C3 and C4 plants are estimated from Corg stable isotopic composition (δ13Corg). We use the evolution of δ13Corg values from the Himalayan range to the delta in order to study the fate of Corg during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar Corg content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on Corg loading. The general correlation between the total Corg content and Al/Si ratio indicates that Corg loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ13Corg values. In bed sediments, δ13Corg values are compatible with a dominant proportion of fossil Corg derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ13Corg values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ13Corg values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ13Corg values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic Corg coming from the Himalaya is oxidised and replaced by floodplain Corg. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain.  相似文献   

14.
Stable carbon isotopic composition of organic matter (δ13Corg) and compound-specific δ13C values of biomarkers from 15 lacustrine source rocks were analyzed to identify the original paleoenvironment and source organisms. The δ13C values of hopanes (δ13Chop) ranged from −68.7‰ to −32‰ and exhibit strongly 13C-depleted values in the lower part of Member 1 of the Nenjiang Formation (K2n1, up to −68.7‰), suggesting an origin from predominantly methanotrophic bacteria. 13C-enriched δ13CGa values and significantly 13C-depleted δ13Chop in K2n1, which coincide with water stratification and an intermittent anoxic photic zone, represents a shallow chemocline. The presence of an intermittent anoxic photic zone, which means that the anoxia expanded into the euphotic zone, is beneficial for OM preservation and results in high values of TOC and HI in this section. However, the absence of gammacerane and 13C-enrichment of δ13Chop in Member 2 of Nenjiang Formation (K2n2) reflect a deeper chemocline, corresponding to relatively oxidizing conditions and low values of TOC and HI. Moreover, the negative correlation of TOC vs δ13Corg and HI vs δ13Corg reflects the control of OM formation by sedimentary environments rather than productivity in the water column. Thus, the depth of the chemocline not only controls the abundance of OM but also affects the development of the microbial community, such as chemoautotrophic bacteria in the deep chemocline and chemoautotrophic and methanotrophic bacteria in the shallow chemocline. Moreover, δ13CGa and δ13C values for 4-methyl steranes are related to water salinity, with a higher salinity accompanied by 13C-enrichment in gammacerane and 4-methyl steranes.  相似文献   

15.
Subaerial exposure and oxidation of organic carbon (Corg)-rich rocks is believed to be a key mechanism for the recycling of buried C and S back to Earth's surface. Importantly, processes coupled to microbial Corg oxidation are expected to shift new biomass δ13Corg composition towards more negative values relative to source. However, there is scarcity of information directly relating rock chemistry to oxidative weathering and shifting δ13Corg at the rock-atmosphere interface. This is particularly pertinent to the sulfidic, Corg-rich alum shale units of the Baltoscandian Basin believed to constitute a strong source of metal contaminants to the natural environment, following subaerial exposure and weathering. Consistent with independent support, we show that atmospheric oxidation of the sulfidic, Corg-rich alum shale sequence of the Cambrian-Devonian Baltoscandian Basin induces intense acid rock drainage at the expense of progressive oxidation of Fe sulfides. Sulfide oxidation takes priority over microbial organic matter decomposition, enabling quantitative massive erosion of Corg without producing a δ13C shift between acid rock drainage precipitates and shale. Moreover, 13C enrichment in inorganic carbon of precipitates does not support microbial Corg oxidation as the predominant mechanism of rock weathering upon exposure. Instead, a Δ34S = δ34Sshale − δ34Sprecipitates ≈ 0, accompanied by elevated S levels and the ubiquitous deposition of acid rock drainage sulfate minerals in deposited efflorescent precipitates relative to shales, provide strong evidence for quantitative mass oxidation of shale sulfide minerals as the source of acidity for chemical weathering. Slight δ15N depletion in the new surface precipitates relative to shale, coincides with dramatic loss of N from shales. Collectively, the results point to pyrite oxidation as a major driver of alum black shale weathering at the rock-atmosphere interface, indicating that quantitative mass release of Corg, N, S, and key metals to the environment is a response to intense sulfide oxidation. Consequently, large-scale acidic weathering of the sulfide-rich alum shale units is suggested to influence the fate and redistribution of the isotopes of C, N, and S from shale to the immediate environment.  相似文献   

16.
This paper investigates the stable isotopic composition from late Pleistocene–Holocene (~ 13 to ~ 10.5 cal ka BP) shells of the land snail Helix figulina, from Franchthi Cave (Greece). It explores the palaeoclimatic and palaeoenvironmental implications of the isotope palaeoecology of archaeological shells at the time of human occupation of the cave. Modern shells from around the cave were also analysed and their isotopic signatures compared with those of the archaeological shells. The carbon isotope composition of modern shells depicts the consumption of C3 vegetation. Shell oxygen isotopic values are consistent with other Mediterranean snail shells from coastal areas. Combining empirical linear regression and an evaporative model, the δ18Os suggest that modern snails in the study area are active during periods of higher relative humidity and lower rainfall δ18O, probably at night. Late glacial and early Holocene δ18Os show lower values compared to modern ones. Early Holocene δ18Os values likely track enhanced moisture and isotopic changes in the precipitation source. By contrast, lower late glacial δ18O could reflect lower temperatures and δ18Op, compared to the present day. Shell carbon isotope values indicate the presence of C3 vegetation as main source of carbon to late glacial and early Holocene snails.  相似文献   

17.
Strontium and carbon isotope stratigraphy was applied to a 202 m-thick shallow marine carbonate section within the Late Jurassic Bau Limestone at the SSF quarry in northwest Borneo, Malaysia, which was deposited in the western Palaeo-Pacific. Strontium isotopic ratios of rudist specimens suggest that the SSF section was formed between the latest Oxfordian (155.95 Ma) and the Late Kimmeridgian (152.70 Ma), which is consistent with previous biostratigraphy. The δ13Ccarb values of bulk carbonate range from −0.10 to +2.28‰ and generally show an increasing upward trend in the lower part of the section and a decreasing upward trend in the upper part of the section. A comparable pattern is preserved in the δ13Corg isotope record. Limestone samples of the SSF section mainly preserve the initial δ13Ccarb values, except for the interval 84–92 m, where an apparent negative anomaly likely developed as a result of meteoric diagenesis. Comparing with the Tethyan δ13Ccarb profile, a negative anomaly in the lower SSF section can be correlated with the lowered δ13C values around the Oxfordian/Kimmeridgian boundary. In addition, δ13Ccarb values of the Bau Limestone are generally ∼1‰ lower than the Tethyan values, but comparable with the values reported from Scotland and Russia, located in Boreal realm during the Late Jurassic. This suggests that either the Tethyan record or the other records have been affected by the δ13C values of regionally variable dissolved inorganic carbon (DIC). The Late Jurassic δ13CDIC values are thought to have been regionally variable as a result of their palaeoceanographic settings. This study shows that δ13C chemostratigraphy of the Palaeo-Pacific region contributes to an improved understanding of global carbon cycling and oceanography during this time period.  相似文献   

18.
《Gondwana Research》2014,25(3-4):1276-1282
Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ13Ccarb, δ13Corg), and sulfur isotopic compositions of carbonate associated sulfate (δ34Ssulfate) across the Guadalupian–Lopingian (G–L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ13Ccarb values show a positive excursion from − 0.45‰ to the peak of 3.80‰ in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to − 2.60‰ in the lowest Heshan formation, then returned to about 1.58‰. Similar to the trends of the δ13Ccarb values, Δ13Ccarb–org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean–atmosphere carbon cycle. The first biostratigraphic δ34Ssulfate values during the G–L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to − 37.41‰. These sulfate isotopic records suggest that the ocean during the G–L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G–L biotic crisis.  相似文献   

19.
A sediment core from the Pearl River Estuary (PRE) was analyzed for grain size and organic geochemistry parameters (TOC and δ13Corg). The results showed that high mean grain-size value and increased sand content were correlated with the high TOC and negative δ13Corg. These results indicated high river runoff in the PRE area. Peak river discharge occurred during the periods 1900–1750, 1500–1600, 1400–1200, 1000–900 and 750–600 cal yr BP. The main changes recorded in grain-size distributions, TOC contents, and δ13Corg variations appear to be directly related to monsoon precipitation in the sediment source area. An increased East Asian summer monsoon rainfall (EASM) and/or an enhanced East Asian winter monsoon rainfall could result in the increasing of monsoon rainfall. Typhoon related rainfalls could act as positive influence on precipitation levels. The study of the correlations between the rainfall records and ENSO activities revealed a close relationship between the monsoon rainfall in the PRE and the tropical Pacific variations. The frequent occurrence of ENSO might result in the southern migration of the EASM rain belt and lead to more typhoon-derived rainfall in the PRD during the late Holocene.  相似文献   

20.
The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.  相似文献   

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