δ13Corg Values Research Articles

First record of early Aptian Oceanic Anoxic Event 1a from the Paris Basin (France) - Climate signals on a terrigenous shelf

In 2013, Andra (French National Radioactive Waste Management Agency) drilled boreholes to the south-east of the Paris Basin, to characterise Aptian and Albian clayey formations, including the ‘Argiles à Plicatules’ Formation dated as early Aptian. One of these boreholes intersected this formation with an excellent recovery allowing detailed biostratigraphy (ammonites), sedimentology, clay mineralogy, isotope geochemistry (δ13Corg) and Rock-Eval analyses to be performed. The base of the formation corresponds to transgressive dark-grey silty clays with iron oolites and plant debris indicating a coastal environment evolving up-section to upper offshore environments. Higher in the succession, clays with less than 4% CaCO3 and less than 0.8% of organic matter were deposited in lower offshore environments. The occurrence of ammonites can be used to draw a biostratigraphic scheme, and in particular, to recognise the deshayesi and furcata ammonite Zones.The clay mineral assemblages are composed of illite and kaolinite associated with minor amounts of chlorite, vermiculite and smectite. The δ13Corg values range between −24 and −25‰ except in a particular interval characterised by a prominent negative excursion of about −3‰, that is recognised worldwide and precedes Oceanic Anoxic Event 1a (OAE1a). This interval is also characterised by an abundance of kaolinite and the absence of smectite suggesting an acceleration of the hydrological cycle and enhanced runoff and hydrolysing conditions over the emerged landmasses just before and during the onset of OAE1a. This climate change is consistent with isotope and palynological data indicating warm and humid climate conditions before and during the onset of OAE1a. Surprisingly, the interval corresponding to OAE1a is not significantly enriched in organic matter and its lithology is not different from the rest of the core suggesting that Tethyan anoxic water masses did not reach the relatively shallow epicontinental environments of the Paris Basin.

Termination of Organic-Rich Accumulation of the Oceanic Anoxic Event 2 in the Deep-Water Carpathian Basins Based on Carbon Stable Isotope Data

Organic-rich black shales intercalated with green radiolarian shales and bentonites, 2.2 m thick, represent an equivalent of the Bonarelli Level in the Outer Carpathian deep-water succession. Carbon stable isotope data from four sections in the Outer Carpathians show that termination of organic-rich accumulation, related to the oceanic anoxic event 2 (OAE2), occurred at the same time in this part of the Western Tethys. The excellent marker of this event is a first horizon of Fe–Mn layer (nodules), a few centimeters thick, directly covering the youngest black shale layer of the Bonarelli-equivalent Level, which is regarded as the regional chronohorizon. The youngest succession of the organic-rich shales, ca. 30 cm thick, corresponds to the latest Cenomanian interval of δ13Corg values displaying a negative trend, which represents a terminal part within the OAE2 carbon isotope excursion. A deep negative shift which ends this falling trend, close to the Cenomanian–Turonian boundary in δ13C curves from many sections around the world, is not visible in the Outer Carpathian successions. The reason for this was the long period of stratigraphic condensation, which is reflected in the ferromanganese sediments of this area.

The Ediacaran ‘Miaohe Member’ of South China: new insights from palaeoredox proxies and stable isotope data

AbstractThroughout the Ediacaran Period, variable water-column redox conditions persisted along productive ocean margins due to a complex interplay between nutrient supply and oceanographic restriction. These changing conditions are considered to have influenced early faunal evolution, with marine anoxia potentially inhibiting the development of the ecological niches necessary for aerobic life forms. To understand this link between oxygenation and evolution, the combined geochemical and palaeontological study of marine sediments is preferable. Located in the Yangtze Gorges region of southern China, lagoonal black shales at Miaohe preserve alga and putative metazoans, including Eoandromeda, a candidate total-group ctenophore, thereby providing one example of where integrated study is possible. We present a multi-proxy investigation into water-column redox variability during deposition of these shales (c. 560–551 Ma). For this interval, reactive iron partitioning indicates persistent water-column anoxia, while trace metal enrichments and other geochemical data suggest temporal fluctuations between ferruginous, euxinic and rare suboxic conditions. Although trace metal and total organic carbon values imply extensive basin restriction, sustained trace metal enrichment and δ15Nsed data indicate periodic access to open-ocean inventories across a shallow-marine sill. Lastly, δ13Corg values of between −35‰ and −40‰ allow at least partial correlation of the shales at Miaohe with Member IV of the Doushantuo Formation. This study provides evidence for fluctuating redox conditions in the lagoonal area of the Yangtze platform during late Ediacaran time. If these low-oxygen environments were regionally characteristic, then the restriction of aerobic fauna to isolated environments can be inferred.

Reconstruction of oceanic redox structures during the Ediacaran-Cambrian transition in the Yangtze Block of South China: Implications from Mo isotopes and trace elements

To explore paleo-ocean redox variations and Mo biogeochemistry across the Ediacaran–Cambrian transition (E-C), continuous fresh core shale samples of Ediacaran to Cambrian were collected from a well drilled in the upper slope region of the Yangtze Block in South China and analyzed for δ98/95Mo and δ13Corg values, TOC, as well as the concentrations of trace and rare earth elements. The Mo–U covariations, Th/U and Re/Mo ratios as well as published iron speciation data indicate a redox stratified ocean with oxic surface waters and anoxic/euxinic deep waters prevailing across the E-C transition. During the time of Cambrian Fortunian and Stage 2, the bottom water in the upper slope became oxic due to the deepening oxycline. Afterwards, widespread anoxia and/or euxinia occurred during latest Age 2 and early Age 3. The Mo concentrations and Mo/TOC ratios suggest that the Yangtze Block was well connected with the open ocean during the late Ediacaran, and then became strongly restricted at the beginning of Cambrian. Following a large-scale transgression during Cambrian Age 2, its connection with the open ocean was enhanced again. The negative δ98/95Mo values may reflect preferential adsorption of isotopically light Mo by Mn oxides particles above the chemocline and their subsequent shuttling to euxinic sediments in the redox stratified late Ediacaran ocean; whereas the relatively low δ98/95Mo values during the Cambrian Fortunian and Age 2 indicate the decrease of δ98/95MoSeawater values in the restricted basin. The operation of local Mn particulate shuttles could also be stimulated by inflow of oxic waters into restricted basin during marine transgression, resulting in negative δ98/95Mo values. In conjunction with previous published Mo isotope data, the variations of sedimentary δ98/95Mo values from shallow shelf to deepwater slope regions draw the outline of ‘sandwich’-like model with euxinic waters (e.g., [H2S]aq > 11 μM) sandwiched by oxic surface waters and anoxic deep waters during the E-C transition. The shuttling of isotopically light Mo to euxinic sediments by Mn oxides in the redox stratified Ediacaran-Cambrian ocean may play a role in driving heterogeneous heavy seawater Mo isotopes and variable sedimentary δ98/95Mo values.

Enhanced terrestrial organic matter burial in the marine shales of Yangtze platform during the early Carboniferous interglacial interval

The supply and preservation of terrestrial organic matter (TOM) during early Carboniferous glacial-interglacial intervals in South China were investigated with respect to their roles in paleo-environmental variation and organic matter (OM) accumulation in black shales. The marine redox proxies such as V/Cr, Corg/P ratios, and Mo–U covariation indicate a fully oxic water condition during the earliest Visean, while dominant oxic with occasional suboxic water conditions prevailed during the early to middle Visean. High sedimentary δ15N values (1.9–6.3‰) suggest the existence of enhanced water-column denitrification in the expanded oxygen minimum zones (OMZs) during the earliest Visean, whereas lower δ15N values (avg. 2.6‰) during the early to middle Visean indicate a decreasing water column denitrification in the reduced OMZs. Both the organic matter compositions and the relationship between Corg/N ratios and δ13Corg values indicate a significant contribution of terrestrial OM in the marine sediments, and the low abundance of marine OM is mainly attributed to the low paleoproductivity as evidenced by low P/Ti and Ba/Al ratios. One reason for such a high contribution of TOM is that a large amount of plant debris and wildfire-derived OM was preserved during Tournaisian glacier time and then released and buried rapidly in the marine sediments during the interglacial interval of Visean due to the temperature-driven glacial melting. The rapid transportation of large quantity of TOM from land to ocean may have resulted in remarkable drawdown of atmospheric pCO2 and subsequent cool climate, as evidenced by the decoupled relationship between δ13Corg and δ13Ccarb values. In summary, the sinking of TOM flux during the sea level lowstand and highstand may be the controlling factor for the paleo-environmental variation as well as OM accumulation in the Visean black shales.

Early-Middle Frasnian (Late Devonian) carbon isotope Event in the Timan-Pechora Basin (Chernyshev Swell, Pymvashor River section, North Cis-Urals, Russia)

Details of the Early-Middle Frasnian boundary interval of the Pymvashor River section (Timan-Pechora Basin, Cis-Urals, in the far north of European Russia) are revealed by biostratigraphically constrained carbonate (δ13Ccarb) and organic carbon (δ13Corg) stable data. The studied interval corresponds to the lower part of the Domanik Formation, which consists of interbedded limestone and shale beds. Organic-rich black shale that lacks bioturbation and benthic organisms indicates an oxygen-depleted depositional environment. Detection of isorenieratene derivatives in organic matter indicates that anoxia was present in the photic zone during deposition. The Pymvashor River section contains δ13Ccarb and δ13Corg isotope records related to the Early-Middle Frasnian isotope Event. The similarity between the Cis-Uralian (this study) and the Chinese δ13Ccarb and δ13Corg oscillations, including the twostep pattern of the recorded major positive excursions, suggests a robust correlation of the Late Devonian Early- Middle Frasnian isotope Event and minor intra-event excursions. Magnitude of variations and values of δ13Corg and δ13Ccarb in the punctata Zone in the Pymvashor River section are minor than those observed in the North American, Polish, and Chinese successions. Such difference may reflect specific variation of the local environments.

Redox fluctuation and δ13Corg-δ34S perturbations recorded in the 1.9 Ga Nuvilik Formation of the Cape Smith belt, Canada

Earth’s surface environment underwent the first major step-change in atmospheric oxygen levels during the Paleoproterozoic Great Oxidation and Lomagundi–Jatuli events. A decrease in atmospheric oxygen levels after the Lomagundi–Jatuli Event has previously been suggested, but details of the environmental transitions at 2.0–1.8 Ga are still debated. Here we report major- and trace-element compositions and δ13Corg and δ34S values for shale samples from a 50-m-thick section of drill-core from the ca. 1.9 Ga Nuvilik Formation of the uppermost Povungnituk Group, Cape Smith belt, Canada. The section comprises a turbiditic sedimentary sequence of well-bedded sandstone–shale alternations accumulated on the continental margin of the Archean Superior Craton. Stratigraphically, the section comprises 4-m-thick graded beds, a fining-upward sequence of predominantly coarse-sandstone to black-shale alternations, and a fine-grained sandstone-dominated sequence. Shale samples in the studied section contain quartz, white mica, pyrite, and pyrrhotite.Mo and U enrichment factors (MoEF and UEF) display increasing trends in the lower part of the section, reaching values of 11.9 and 2.4, respectively, suggesting development of sulfidic (euxinic) benthic water. Subsequent decreases in MoEF and UEF values represent restoration to suboxic states. This redox fluctuation is accompanied by δ13Corg and δ34S perturbations. Concurrent decreases of δ13Corg (−30.6‰ to −33.2‰) and δ34S values (+11‰ to −5‰) suggest isotopically light inorganic carbon and sulfur were supplied to the ocean–atmosphere system. δ34S values increased to +14‰ after the decrease, interpreted as being due to isotopic distillation through enhanced microbial sulfate reduction. The δ13Corg and δ34S perturbations and the overlying ultramafic volcanic sequence of the Chukotat Group (part of the Circum-Superior large igneous province; LIP) suggest that the redox fluctuation recorded in the Nuvilik Formation might be related to the onset of LIP magmatism.

Constraining the conditions of phosphogenesis: Stable isotope and trace element systematics of Recent Namibian phosphatic sediments

Modern phosphogenesis occurs on continental margins influenced by upwelling and high primary productivity. The formation of phosphatic sediments is coupled to global climate fluctuations, biological cycling of phosphorus and local redox conditions. Although the processes involved in phosphogenesis are well described, high-resolution data on the redox and stable isotope systematics in Recent in-situ phosphorites are scarce. In this contribution, we investigate the trace element and sulfur, nitrogen and organic carbon stable isotope composition of Recent in-situ phosphatic sediments off the coast of Namibia. Also, we examine the reliability of different widely used geochemical proxies in phosphatic sediments. Our results suggest a shift from sulfidic to suboxic conditions, coinciding with the maximum in solid calcium phosphate mineral concentration. This shift is accompanied by unidirectional changes in Mo and Re enrichments and TOC abundance. Relatively low pyrite δ34S values (ca −20‰) of phosphatic sediments indicate open system fractionation during phosphogenesis. The initiation of phosphogenesis is also accompanied by negative shifts in sedimentary δ13Corg and δ15N values. Phosphate associated sulfate (PAS) δ34S values are lower than modern seawater sulfate values, suggesting the involvement of chemolithotrophic sulfur oxidation. Our results show a shift in redox conditions from sulfidic to (sub)oxic, coupled with active sulfur cycling are prerequisites for phosphogenesis. Phosphatic sediments show substantial enrichments in U and V highlighting the complexity of using these elements, as well as V/(V + Ni) and V/Cr, as redox proxies particularly in phosphorites and phosphatic sediments.

The origin of abnormally 13C-depleted organic carbon isotope signatures in the early Cambrian Yangtze Platform

A significant negative organic carbon isotope (δ13Corg) excursion occurred in the lower member of the lower Cambrian Qiongzhusi organic-rich shales in the Sichuan Basin, which is also reported in other contemporaneous strata throughout the Yangtze Platform. In this study, 47 lower Cambrian drill core samples were collected to investigate the possible formation mechanism of this negative δ13Corg excursion using multiple geochemical approaches. Primary paleoproductivity, redox conditions and organic carbon isotopic compositions were determined for these core samples of the Qiongzhusi Formation in the Sichuan Basin. The results suggest that the degree of microbially mediated carbon cycling and its contribution to the total sedimentary organic matter has an important influence on organic carbon isotopic compositions. An extensive transgression during the deposition of the lower member of the Qiongzhusi Formation brought abundant nutrients to primary producers and thus stimulated blooms, which is evidenced by the high primary productivity in the lower member of the Qiongzhusi Formation. In addition, marine transgression also provided sulfide to the seawater. Enhanced primary productivity accelerated the consumption of bottom water oxygen and thus promoted the establishment of euxinic bottom water conditions. The euxinic conditions promoted an anaerobic carbon cycle mediated by sulfate-reducing bacteria and chemoautotrophic bacteria, which resulted in the dissolved inorganic carbon reservoir becoming more depleted in 13C. Once these 13C-depleted carbons are utilized by primary producers, the 13C-depleted carbons will participate in the carbon cycle. This series of processes ultimately resulted in lower δ13Corg values within the lower Cambrian Qiongzhusi organic-rich black shales. A shift to positive and rather stable δ13Corg values in the relatively organic-poor sediments indicates a smaller contribution of 13C-depleted bacterial biomass (e.g., sulfate-reducing bacteria and chemoautotrophic bacteria), which is evidenced by more oxygenated ocean bottom conditions in the study area.

Speculation for quantifying increased C4 plants under future climate conditions: Inner Mongolia, China case study

Alternating conditions from cold to warm periods since the Late Glacial provided a significant contrast for future hydroclimatic variation, as did the corresponding variation in vegetation. To predict the potential variability in C3/C4 biomass under future warmer climatic conditions, we determined the current spatiotemporal distribution of C3 and C4 vegetation by estimating the variability of δ13Corg values in 125 topsoil samples and investigating the geological period variability of C4 plants growing in Inner Mongolia, China. We examined the factors contributing to the growth of C4 plants and the variability in C4 plants for each 1 °C increase in temperature during recent and geological times. The results revealed that the relative abundance of C4 plants in Inner Mongolia shows an increasing trend from the northwest to the southeast. The biomass of C4 plants was found to be positively correlated with temperature, although weakly correlated with precipitation, thereby indicating that temperature is the primary climatic factor determining the growth of C4 plants in this region. Furthermore, we found that in response to each 1 °C increase in temperature, there would be an approximate 2.1% increase in the spatiotemporal variation of present-day C4 plant biomass in Inner Mongolia, whereas during the geological period, there was a corresponding increase of 10.7%. Comparatively, the spatiotemporal variation in present-day C4 biomass on the Chinese Loess Plateau has undergone a 4.0% increase in response to each 1 °C increase in temperature, whereas 6.0% and 7.5% increases in the geological period were estimated based on core samples taken from the Lantian and Xingyang sections, respectively. Interestingly, for each 1 °C increase in temperature during the current growth month of C4 plants, there is a 6.6% increase in C4 biomass, which is essentially consistent with the results obtained for the geological period results (i.e., 6.0% and 7.5% for the Lantian and Xingyang sections, respectively). The findings of this study reveal a distinct contrast in the variation of C4 plants during recent and geological periods, thereby enabling us to quantitatively estimate the potential changes in C4 vegetation in response to projected climate conditions.

Habitat Partitioning in the Marine Sector of Karst Subterranean Estuaries and Bermuda's Marine Caves: Benthic Foraminiferal Evidence

Karst subterranean estuaries (KSEs) are created from the two- and three-way mixing of saline groundwater, rain, and oceanic water in the subsurface on carbonate landscapes, and this hydrographic framework promotes unique physical processes, biogeochemical cycling, and biological communities. Here we provide evidence that the source and quantity of particulate organic matter (POM) that is delivered to the benthos strongly correlates to benthic habitat partitioning in the oxygenated marine sectors of KSEs. A dataset of benthic foraminifera at 128 different locations from several large flooded cave systems in Bermuda were compiled and evaluated against common environmental characteristics (e.g., tidal exposure, substrate particle size, bulk organic matter, C:N, total organic carbon, and δ13Corg). Benthic areas receiving more carbon isotopically depleted organic matter sources (mean δ13Corg values < −23.2‰, C:N ratios >11), most likely from the terrestrial surface and some marine plankton, were dominated by Trochammina inflata, Bolivina spp., and Helenina anderseni. In contrast, benthic areas receiving more carbon isotopically enriched organic matter sources (mean δ13Corg values > −21.6‰, C:N ratios <10), most likely from marine plankton transported through marine cave openings cave from adjacent coastal waters, were dominated by Spirophthalmidium emaciatum, Spirillina vivipara, Patellina corrugata, and Rotaliella arctica. The benthic foraminifera most distal from any cave entrances were dominated by taxa also known from the deep-sea (e.g., Rotaliella, Spirophthalmidium) in sediment with the lowest bulk organic matter content (mean: 6%), or taxa that prefer hard substrates and are potentially living attached to cave walls (Patellina, Spirillina). While physical groundwater characteristics (e.g., salinity, dissolved oxygen) are expected drivers of benthic ecosystems in KSEs, these results suggest that POM source, quantity, and delivery mechanisms (e.g., groundwater-seawater circulation mechanisms, terrestrial flux) play an important role in benthic habitat partitioning and the spatial variability of biogeochemical cycles in the oxygenated marine sector of KSEs.

Intensive Wildfire Associated With Volcanism Promoted the Vegetation Changeover in Southwest China During the Permian−Triassic Transition

Palaeo-wildfire, which had an important impact on the end Permian terrestrial ecosystems, became more intense in the latest Permian globally, evidenced by extensive occurrence of fossil charcoals. In this study, we report abundant charcoals from the upper part of the Xuanwei Formation and the Permian–Triassic transitional Kayitou Formation in the Lengqinggou section, western Guizhou Province, Southwest China. These charcoals are well-preserved with anatomical structures and can be classified into seven distinctive types according to their characteristics. Organic carbon isotopic analyses of both bulk rocks and charcoals show that the δ13Corg values in the Kayitou Formation are notably more negative than those in the Xuanwei Formation, with a negative excursion of 4.08‰ immediately above the volcanic ash bed in the middle of the uppermost coal bed of the Xuanwei Formation. Charcoals with high reflectance values (Romean = 2.38%) are discovered below the ash bed. By contrast, the reflectance values (Romean = 1.51%) of the charcoals in the Kayitou Formation are much lower than those of the Xuanwei Formation, indicating the palaeo-wildfire types have changed from crown fires to surface fires, which was probably due to the retrogression of vegetation systems during the extinction. Based on the above evidence, we suppose that palaeo-wildfires became more frequent and more severe since the climate became drier during the latest Permian in Southwest China, and the eventual vegetation changeover of the terrestrial ecosystems in Southwest China could be caused by volcanism.

The impact of volcanic activity on the deposition of organic-rich shales: Evidence from carbon isotope and geochemical compositions

The organic-rich black shales from the Late Ordovician and early Silurian were identified as essential source rocks for shale gas exploration and development in South China. To investigate the influence of volcanic activity on the deposition of organic-rich shales in the Wufeng and Longmaxi formations, this study presents the carbon isotope and geochemical compositions of the Upper Ordovician-lower Silurian succession of the Yangtze Platform. The organic-rich Wufeng and lower Longmaxi shales are characterized by isotopically negative carbon shifts; however, obviously high δ13Corg values are shown in the organic-poor shales of the upper Longmaxi Formation. The negative δ13Corg excursion is attributed to 12C-depleted CO2 sourced from volcanic activity. Furthermore, the positive δ13Corg excursion in the upper Longmaxi Formation occurs because a large amount of 12C was removed from the ocean-atmosphere system through continuous burial of organic carbon. The biogenic silica (Sibio) contents and (Cu + Ni)/Al ratios in the lower Longmaxi Formation display higher values than those in the upper Longmaxi Formation, suggesting that the primary productivity gradually decreased from the Wufeng Formation to the Longmaxi Formation. This study proposes that high primary productivity was related to accelerated chemical weathering and release of micro-nutrients. The former emphasizes that high pCO2 and globally high temperatures can enhance chemical weathering, so that more nutrients are transported from continents to oceans (Yan et al., 2010, 2019). The latter highlights that volcanic material input can act as fertilizer by releasing many nutrients. Moreover, the values of UEF and MoEF are higher in the Wufeng and lower Longmaxi formations than in the upper Longmaxi Formation, suggesting that the oxygen concentration increased upward through the section. Given the positive correlations between indicators of productivity and redox conditions, the constantly high primary productivity would consume much O2, resulting in anoxic conditions in the Wufeng and lower Longmaxi formations. Overall, this study considers that volcanic activity-driven primary productivity is the first-order control for organic matter (OM) enrichment.

Influence of a chromium reduction treatment in the analysis of organic carbon isotopes in Quaternary coastal plain incised valley sediments

This paper reports bulk organic carbon isotope data from a laboratory study designed to evaluate whether acid chromium reduction (ACR), commonly used to prepare rock/sediment samples for sulfur isotope analysis, significantly fractionates bulk organic carbon isotopes compared with the standard decarbonation (SD) method in Quaternary coastal plain incised valley sediments. In this study, 36 sediment samples obtained from a Quaternary sediment core from the Qiantang River incised valley on the East China coastal plain were tested. The results show differences in the δ13Corg values between the ACR and SD methods for Quaternary incised valley sediments ranging from – 0.97‰ to +0.34‰, with most values (86%) showing a negative bias. Approximately half of the samples yield δ13Corg values that differ by ≥ ± 0.3‰. The carbonate and total organic carbon (TOC) contents of the sediment samples have no significant effects on the bulk organic carbon isotopes derived by ACR. There is also no obvious correlation between the organic carbon loss by ACR and the difference of the δ13Corg values. We believe that ACR loses more 13C-enriched labile organic carbon and/or volatile organic compounds (VOCs) than SD, which leads to a negative bias in the δ13Corg values. The mineral composition was speculated to be related to a significant positive bias in the δ13Corg values by ACR observed in two particular samples and requires further study. Consequently, ACR should be used with caution for sediments containing primarily young labile organic carbon with medium to high TOC contents (TOC ≥ 0.5 wt %), as it may cause a large offset of the δ13Corg values compared with SD. However, ACR may be used for sediments containing organic matter primarily from terrestrial plants with relatively low TOC contents (TOC < 0.5 wt %).

Minor changes in biomarker assemblages in the aftermath of the Cretaceous-Paleogene mass extinction event at the Agost distal section (Spain)

The Cretaceous/Palaeogene Boundary (KPgB) represents one of the five major mass extinctions in Earth's history, and determining the nature of associated environmental change and biotic recovery is critical for understanding the history of life on our planet. To explore that, we examined the distributions of selected biomarkers (n-alkanes, acyclic isoprenoids, steranes and hopanes), organic and carbonate δ13C values, total organic carbon contents and major and trace elements in a distal section spanning the KPgB (Agost, SE Spain). The studied Agost section is an ~32 cm-thick bathyal sequence of marlstones, clays and marly limestones, and high sedimentation rates allow high (cm) resolution analysis. These analyses exhibit sample-to-sample variability, but there are few significant differences between pre- and post-KPgB biomarker assemblages, suggesting a rapid recovery of the non-fossilizing phytoplankton community after the KPgB. Despite the persistence of life, the organic matter assemblage is rather variable through the first 10 kyr after the impact event. This interval is associated with changing terrigenous and petrogenic inputs as well as varying redox conditions as reflected by the enrichment factor of uranium (UEF) vs that of molybdenum (MoEF) as well as biomarker indices (gammacerane and homohopane indices). Moreover, sterane distributions do differ between pre- and post-KPgB sediments. Thus, the KPgB impact did affect environmental conditions and non-fossilizing algal and bacterial communities even in distal sites, but these organisms appear to have rapidly recovered, within 10 kyr after the KPgB.

Holocene vegetation and hydrology variations and their associations with climate changes: a multi-proxy analysis of a sediment core from an alpine basin in the middle Tianshan Mountains

We in this paper present a multi-proxy study of a 186-cm-long lacustrine sediment core (i.e., BY Core) obtained from the Youerdusi Basin in the middle Tianshan Mountains in an attempt to disentangle the causal associations among climate changes, ecological responses and hydrological variations. Palynological, assemblages and the associated biome scores indicate that the upper limit of the forest zone was closer to the Youerdusi Basin with a larger wetland extent in the Youerdusi Basin during the early part (i.e., from ~ 10.5 to ~ 5.8 cal. kyr BP) of the data-covering period (i.e., from ~ 10.5 to ~ 2.4 cal. kyr BP). The upper limit of the forest zone was farther away from the Youerdusi Basin with a smaller wetland extent in the Youerdusi Basin during the late part (i.e., from ~ 5.8 to ~ 2.4 cal. kyr BP). The changes of taiga biome score and the AP% ratio indicate a persistent cooling trend during the data-covering period which is well corroborated by various proxy data from nearby sites and the cooling trend is chronologically consistent with the trend of Northern Hemisphere temperature anomalies that was interpreted to be a delayed response to the summer solar irradiance at 50° N. The tundra biome score and the associated percentages of Cyperaceae in the Youerdusi Basin show two major stages of wetland extent variation. The wetland extent variation in the Youerdusi Basin might have been controlled by precipitation and/or by glacier-melting potential that was in turn controlled by both temperature and the extent of remained ice cover in higher elevations. The δ13Corg values obtained from the BY Core indicate a wetting trend during the data-covering period which is reasonably corroborated by various proxy data from nearby sites. We infer that the wetting trend was causally associated with the Holocene increasing trend of the North Atlantic Ocean SST (sea surface temperature) and also with the Holocene increasing trend of the westerlies’ intensity.

The Middle to Late Cretaceous marine incursion of the Proto-Paratethys Sea and Asian aridification: A case study from the Simao-Khorat salt giant, Southeast Asia

Asian aridification during the Cenozoic was strongly related to moisture flux modulated by sea incursion and the retreat fluctuations of the proto-Paratethys Sea via the Westerlies. However, the extents of the sea incursions are poorly constrained, and there is a lack of sedimentary records beyond the Central Asian basins. The Middle to Late Cretaceous Simao Basin and the Khorat Plateau in Southeast Asia contain thick continental sequences of red beds, evaporites, and aeolian deposits, but whether the depositional environment of the evaporites was marine or continental has been hotly debated. In this study, we compiled multiple sedimentology proxies, biomarkers, element geochemistry, and isotopic geochemistry of the fine-grained sediments of the Mohan section in the Simao Basin. Sedimentological analyses show that this section is composed of subtidal flat fine-grained siliciclastic and carbonate rocks and supratidal flat conglomerates and evaporites. The paleosalinity proxies show that the B concentration and B/Ga and Srcorrected/Ba ratios range from 30.3 to 81.7 ppm, 2.4 to 5.0, and 0.05 to 0.60, respectively, which indicate brackish conditions. The two typical increases in these proxies are consistent with the stratigraphic horizons of the diagnostic marine biomarkers 24-n-propyl- and 24-iso-propylcholestanes, which suggests seawater recharge. The carbonate δ13C and δ18O values (−4.84‰ to −1.75‰ and −8.25‰ to −3.69‰, respectively) and the bulk δ13Corg values (−26.5‰ to −23.22‰) also indicate the influence of seawater. We propose that the seawater likely originated from the proto-Paratethys Sea during a marine incursion. We propose for the first time that the proto-Paratethys seawater incursions extended into the Simao Basin and the Khorat Plateau. These results provide a novel paleogeographic configuration for East Asia during the Middle to Late Cretaceous. The retreat of the proto-Paratethys Sea, the northeast Trade Winds, and the tectonic uplift dramatically decreased the moisture and intensified the extreme aridification in the Simao Basin and the Khorat Plateau.

Post depositional changes of sedimentary organic matter influence chromium speciation in continental slope sediments - A case study.

Influences of post depositional changes of sedimentary organic matter (SOM) on chromium (Cr) speciation in continental slope sediments from the east and west coast of India are presented in this study. Average concentrations of total sedimentary Cr (CrT) in the slope areas of both the coasts were not elevated and ranged from 56.8 to 163mg·kg-1 (avg. 97.3±28.3mg·kg-1). The geology of adjacent land masses influenced sedimentary Cr concentration in the slope areas. Geochemical fractionation study showed that a major fraction (51.6-159mg·kg-1; avg. 89.6±24.3mg·kg-1) of the total sedimentary Cr was present in residual part of the sediments. Prime hosting phase for non-residual sedimentary Cr was oxidizable binding phase (SOM) (1.4-23.9mg·kg-1; avg. 6.7±6.3mg·kg-1) followed by reducing binding phase (Fe/Mn-oxyhydroxide phase) (0.1-1.7mg·kg-1; avg. 0.7±0.3mg·kg-1). Concentration of Cr (VI) in the slope sediments, varied from 0.2 to 8.3mg·kg-1 (avg. 1.2±1.0mg·kg-1), was also predominantly associated with the SOM. The δ13Corg values of the SOM (ranged from -19.4 to -21.4‰; avg. -20.3±0.5‰) suggests that they were derived from marine sources. Based on molar C/N ratio (ranged from 6.3-12.9; avg. 9.0±2.0), the SOM were classified as labile organic matter. Increasing molar C/N ratio in the studied sediments was used as a proxy to reflect post depositional changes of the SOM. Association of normalized Cr (VI) (with respect to CrT) increased with increasing SOM concentration by 0.008mg·kg-1. One unit increase of sedimentary molar C/N ratio was found to increase normalized Cr (VI) (w.r.t. CrT) by 0.003mg·kg-1 in the sediments. This study suggests that post-depositional changes of SOM may increase Cr binding capacity and control its speciation and mobility in marine sediment system.

Spatiotemporal variations of sedimentary carbon and nitrogen isotopic compositions in the Yangtze Shelf Sea across the Ordovician-Silurian boundary

The Hirnantian Glaciation resulted in a perturbation in global carbon cycling, which is recognized as a positive excursion in the δ13C values of both carbonates and organic matter. To investigate the factors controlling the carbon and nitrogen cycles from the late Ordovician to the early Silurian, the C and N isotopic and geochemical compositions of marine sediments deposited on the Yangtze Platform were measured. The majority of rocks in the Wukemuchang section exhibit high δ15Nbulk values, reflecting aerobic nitrogen cycling in the inner shelf. The global spatial variation in δ15Nbulk values likely reflects the occurrence of nitrate gradients in seawater, implying that nitrate was limited in offshore environments that were characterized by oxic conditions. More eukaryotes may be restricted to the inner shelf environments, where large amounts of bioavailable nitrate were accessible. The limited availability of nitrate may have affected the fossil distributions and the complexity of animal ecosystems, thereby influencing the late Ordovician extinction and early Silurian biotic recovery. The δ13Corg profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentary δ13CDIC values. We suggest that the global variation in δ13Corg values during the late Ordovician was related to the amount of nitrate because productivity is limited when bioavailable N is scarce. In addition to intense carbonate weathering, the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally, driving sedimentary δ13Corg values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted

Geochemical evidence of tropical cyclone controls on shallow-marine sedimentation (Pliocene, Taiwan)

Abstract Shallow-marine sediment typically contains a mix of marine and terrestrial organic material (OM). Most terrestrial OM enters the ocean through rivers, and marine OM is incorporated into the sediment through both suspension settling of marine plankton and sediment reworking by tides and waves under fair-weather conditions. River-derived terrestrial OM is delivered year-round, although sediment and OM delivery from rivers is typically highest during extreme weather events that impact river catchments. In Taiwan, tropical cyclones (TCs) are the dominant extreme weather event, and 75% of all sediment delivered to the surrounding ocean occurs during TCs. Distinguishing between sediment deposited during TCs and that redistributed by tides and waves during fair-weather conditions can be approximated using δ13Corg values and C:N ratios of OM. Lower Pliocene shallow-marine sedimentary strata in the Western Foreland Basin of Taiwan rarely exhibit physical evidence of storm-dominated deposition. Instead they comprise completely bioturbated intervals that transition upward into strata dominated by tidally generated sedimentary structures, indicating extensive sediment reworking under fair-weather conditions. However, these strata contain OM that is effectively 100% terrestrial OM in sediment that accumulated in estimated water depths &amp;lt;35 m. The overwhelming contribution of terrestrially sourced OM is attributed to the dominance of TCs on sedimentation, whereby ∼600,000 TCs are estimated to have impacted Taiwan during accumulation of the succession. In contrast, the virtual absence of marine OM indicates that organic contributions from suspension settling of marine OM is negligible regardless of the preserved evidence of extensive reworking under fair-weather conditions. These data suggest that (1) even in the absence of physical expressions of storm deposition, TCs still completely dominate sedimentation in shallow-marine environments, and (2) the organic geochemical signal of preserved shallow-marine strata is not reflective of day-to-day depositional conditions in the environment.