Organic Carbon Isotope Data Research Articles

Upper Ordovician marine red limestones, Tarim Basin, NW China: A product of an oxygenated deep ocean and changing climate?

Abstract Marine red beds (MRB) have been commonly interpreted as products of bottom water oxygenation (or ventilation) and coeval perturbations of the global carbon cycle during greenhouse climates. The Sandbian (Upper Ordovician) successions at Dawangou and Nanyigou in NW Tarim, China are characterized by deep-shelf nodular red limestones. Using ultraviolet-visible-near infrared (UV-VIS-NIR) diffuse reflectance spectroscopy (DRS), field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray spectrometer (EDS), and paired inorganic and organic carbon isotope systematics, this study attempts to reveal the origin of these MRBs and coeval carbon cycling during deposition. Both the DRS and FESEM results consistently show that these red limestones are exclusively impregnated with amorphous to poorly crystalline, submicron ferric oxides (mostly hematite), constituting the dominant components of the red pigment. Hematite aggregates (or lumps) occur either in the calcite crystal interstices and pores or within the lattice structure of calcite crystals, implicating early diagenetic or syndepositional origins. Paired organic and inorganic carbon isotope data through the MRB successions demonstrate a time-specific Sandbian positive carbon isotope plateau (~1 to 2‰ VPDB) in both fractions as reported elsewhere around the world. This may have intitially resulted from enhanced burial of organic matter, then a drawdown of atmospheric pCO2 levels, and subsequent climate cooling. This scenario agrees with the decreasing isotope difference (Δ13C = δ13Ccarb−δ13Corg; ~26‰VPDB on average) in the Sandbian MRBs relative to the underlying strata (~29‰ VPDB) from both sections. A cooling climate could have accelerated oceanic ventilation rates and led to increased oxygenation of deep-shelf water masses, eventually facilitating the widespread deposition of MRBs near continental margins. The persistence of a cool climate and the expansion of oxygenated deep-shelf water masses onto the shelf that initiated during the Darriwilian may have stimulated the rapid biodiversification of marine organisms during the Sandbian.

A continental record of the Carnian Pluvial Episode (CPE) from the Mercia Mudstone Group (UK): palynology and climatic implications

The generally arid Late Triassic climate was interrupted by a wet phase during the mid-Carnian termed the Carnian Pluvial Episode (CPE). Quantitative palynological data from the Mercia Mudstone Group in the Wessex Basin (UK) reveal vegetation changes and palaeoclimate trends. Palynostratigraphy and bulk organic carbon isotope data allow correlation to other Carnian successions. The palynostratigraphy indicates that the Dunscombe Mudstone is Julian and the lowest part of the overlying Branscombe Mudstone Formation is Tuvalian. The Aulisporites acme characterizing the CPE in Tethyan successions and the Germanic Basin is missing in the UK. The quantitative palynological record suggests the predominance of xerophyte floral elements with a few horizons of increased hygrophytes. A humidity signal is not seen owing to the dry climate in central Pangea. Also, the signal might be masked by the overrepresentation of xerophyte regional pollen and the predominance of xerophyte hinterland flora. The bias towards regional pollen rain is enhanced by the potential increase in continental runoff related to seasonally humid conditions and differences in pollen production rates and transport mechanisms. The vegetation of British CPE successions suggests a more complex climate history during the Carnian, indicating that the CPE is not recognized by the same changes everywhere. Supplementary material: A detailed lithological log of the Strangman's Cove (Devon) outcrop with the description of the Mercia Mudstone Group lithostratigraphical units, a description of the laboratory techniques, seven photoplates with selected spores and pollen grains, a list of all identified palynomorphs, Excel sheets with the palynological and palynofacies counts, bulk organic carbon isotope ratios and total organic carbon values of the Strangman's Cove outcrop are available at https://doi.org/10.6084/m9.figshare.c.4138085

Integrated graptolite-conodont biostratigraphy and organic carbon chemostratigraphy of the Llandovery of Kallholn quarry, Dalarna, Sweden

A revised graptolite and conodont biostratigraphy together with new organic carbon isotope data for the middle Aeronian to lower Telychian (Llandovery, lower Silurian) of Kallholn quarry, central Sweden, are presented. The base of the section is marked by an unconformity between the Ordovician Boda Limestone and Aeronian shales with limestone nodules belonging to the Pribylograptus leptotheca Biozone. These are overlain by strata assigned to the lower Lituigraptus convolutus Biozone. The Stimulograptus sedgwickii Biozone, previously recorded from the section, is absent and marked by an unconformity which is overlain by radiolarian-rich graptolitic shales belonging to the Stimulograptus halli Biozone. The overlying Telychian strata are assigned to the Spirograptus guerichi, Spirograptus turriculatus and Streptograptus crispus biozones. Conodonts indicate that the boundary between the Distomodus staurognathoides Biozone and Pterospathodus eopennatus Superzone lies within the Sp. turriculatus Biozone, as it does also in the Ohesaare core, Estonia. The δ13Corg record shows mostly minor fluctuations but with a distinct twin-peaked positive excursion in the upper Sp. turriculatus Biozone, close to the boundary with the overlying Streptograptus crispus Biozone. The excursion in the Kallholn section may correlate with the earliest positive shift in δ13C values at the onset of the excursion associated with the Valgu Event or it may represent a new excursion (provisionally named the Kallholn excursion) preceding this. Many more bentonites (36) occur in the section than previously recorded – geochemical studies are needed to confirm whether the Osmundsberg bentonite (which may or may not be equivalent to the “O” bed in Estonia and Latvia) is present.

Obliquity pacing of the hydrological cycle during the Oceanic Anoxic Event 2

The Oceanic Anoxic Event 2 (OAE2, ca. ∼93.5 Ma) represents a major phase of environmental change during the Mesozoic, and is associated with a pronounced positive excursion in the carbon-isotope record. Short-term climate oscillations within the OAE2 are recorded as Milankovitch cycles, which have been used to establish a precise temporal framework for the OAE2. However, few studies discuss the sedimentary expression of Milankovitch cycles during the OAE2, and its paleoenvironmental implications. Here we present carbonate and organic-carbon isotope data from a biostratigraphically well-dated, organic-rich OAE2 interval in a sedimentary succession outcropping in the Briançonnais Domain at Roter Sattel (Fribourg Prealps, Switzerland). We sampled the OAE2 interval (4.28 m) for proxies of detrital sediment quantification (Al, Ti, magnetic susceptibility) at ultra-high resolution (1 cm). Time-series analysis of multiple detrital proxies permits the construction of an orbital timescale for the OAE2 based, for the first time, on the stable 173 kyr (s3–s6) obliquity modulation cycle. The resulting OAE2 orbital timescale at Roter Sattel is consistent with previous timescales obtained in the Western Interior Basin, and in southern Tibet (China). Our cyclostratigraphic results show an unusually strong obliquity signal during the initiation of OAE2. Previous studies have demonstrated that the onset of OAE2 was associated with magmatic pulses coupled with increases in atmospheric pCO2, followed by an overall, gradual drawdown of CO2. Accordingly, we suggest that detrital input during the OAE2 was the result of intensified continental weathering related to magmatic activity, the substantially release of greenhouse gases, and an accelerated hydrological cycle modulated by obliquity cycles.

Biogeochemical controls on black shale deposition during the Frasnian-Famennian biotic crisis in the Illinois and Appalachian Basins, USA, inferred from stable isotopes of nitrogen and carbon

The Frasnian-Famennian biotic crisis is marked by two distinct intervals known as the Lower and Upper Kellwasser Events (KWEs) that in many locations are associated with deposition of organic-rich shales. Sedimentary nitrogen and carbon isotopes offer insight into the biogeochemical processing of nutrients, production of organic matter, and palaeoceanographic conditions during the KWEs. Here we present new bulk nitrogen (δ15Nbulk) and organic carbon (δ13Corg) isotope data from the Late Devonian Appalachian and Illinois Basins (AB and IB), with a focus on intervals encompassing the KWEs. Black shales from the IB and AB, including the KWEs, are 15N-depleted (−1.0–+2.0‰) and have significantly lower δ15Nbulk than interbedded grey shales (+0.5–+4.0‰), a trend consistent with many instances of black shale deposition in the Phanerozoic. Organic carbon isotopes exhibit the broad, positive excursions (~+3.5‰ from background) that are typical of the KWEs globally. Superimposed over these positive excursions in δ13Corg are sharp decreases of up to ~3.0‰ within the black shale beds, to as low as −30.5‰. The pattern of δ15Nbulk and δ13Corg values suggests that the depth of the chemocline and the degree of water-column stratification exert a primary control on both δ15Nbulk and δ13Corg during black shale deposition. In the context of the Frasnian-Famennian biotic crisis, the oscillating redox state and changing temperatures would have likely placed extreme stress on organisms within the marine environment of the AB and IB and may potentially have been a contributing factor to diversity loss over this time period.

Utility of organic carbon isotope data from the Salina Group halite (Michigan Basin): A new tool for stratigraphic correlation and paleoclimate proxy resource

Long-term global carbon isotope records (δ13Ccarb and δ13Corg) for the Silurian have been largely derived from unrestricted openmarine carbonates and shales. Here, we demonstrate how organic carbon harvested from halite-dominated evaporite deposits in a restricted intracratonic basin can be used to produce a carbon isotope record. Inorganic and organic carbon isotope data were generated and compared from four subsurface cores from the Silurian Michigan Basin, representing unrestricted carbonate and restricted evaporite/carbonate deposition. The δ13Ccarb and δ13Corg records exhibit a number of long-term trends and major carbon isotope excursions (CIE) that are correlated with the globally identified Ireviken, Mulde, and Linde events. These data provide temporal and stratigraphic constraints in rocks where paleontological data are sparse or absent. They also potentially highlight the effect of enhanced local evaporation on isotope fractionation. This new technique for generating a long-term organic carbon isotope profile from Silurian halite sequences, which can be correlated to the global curve, is of broad interest to the geoscience and paleoclimate science communities. These data not only provide a valuable tool for understanding the chronostratigraphic framework within an evaporative interior basin, but they also provide a rare temporal link between periods of prolonged evaporite deposition and events of known paleoclimate change.

Early Late Permian coupled carbon and strontium isotope chemostratigraphy from South China: Extended Emeishan volcanism?

Carbon isotope compositions of carbonates (δ13Ccarb) document a new 3.5‰ CIE toward lower values concomitant with an Emeishan Large Igneous Province (ELIP)-related drowning event (Mapojiao Event, southern Guizhou) during the early Wuchiapingian. Organic carbon isotope data (δ13Corg) have a 2‰ shift toward higher values across the drowning event, showing decoupling with the δ13Ccarb evolution. Rock-Eval and palynofacies analyses suggest an elevated flux of terrestrial organic matter (OM) during the drowning episode. Therefore, the decoupling between δ13Ccarb and δ13Corg is best explained by the mixing of different organic carbon pools in the δ13Corg curve. Strontium isotope data (87Sr/86Sr) also show a transient shift from 0.70715 to 0.70694 associated with this early Wuchiapingian carbon isotope excursion (CIE), which is superimposed on the late Permian prolonged global rising trend. This short-lived 87Sr/86Sr excursion is best interpreted as an enhanced hydrothermal flux related to a short pulse of ELIP-related volcanism.Comparison of the Mapojiao Event with other C isotope records from South China and other parts of Tethys reveals substantial discrepancies. Moreover, older Capitanian CIEs concomitant with ELIP-related drownings are also not laterally reproducible in time. The local distribution of CIEs across Guadalupian-Lopingian (G-L) interval indicates the influence of local factors such as bathymetry and increased burial rate of terrestrial OM, rather than perturbations in the global carbon cycle to due volcanism. This prevents the C isotope record to be used as a global correlation tool during Capitanian and Wuchiapingian times. Comparison of palaeobiodiversity changes with perturbations in C isotope compositions across the G-L interval shows that only the oldest CIE was associated with a modest extinction event as documented in South China. Hence, this study undermines any systematic coupling and causal relation between extinctions and iterative perturbations of the C isotope record during the G-L interval.

Ammonite biostratigraphy and organic carbon isotope chemostratigraphy of the early Aptian oceanic anoxic event (OAE 1a) in the Tethyan Himalaya of southern Tibet

The early Aptian oceanic anoxic event (OAE 1a) is well known in the western Tethys, the North Atlantic and the Pacific Ocean, but has not been reported in the eastern Tethys to date. In this paper, we present bulk organic carbon isotope data and ammonite biostratigraphy of a lower Aptian succession from the Gucuo area (southern Tibet). These findings document the occurrence of the OAE 1a for the first time from the eastern Tethys. The studied sequence can be attributed to the D. forbesi and D. deshayesi ammonite zones of the lower Aptian. The δ13Corg data can be correlated with published early Aptian carbon isotope records from the western Tethys and the Pacific. A distinctive negative carbon isotope excursion of 2.4‰ in the upper part of the section corresponds to segment C3 of the OAE 1a, and the following positive excursion correlates to segment C4. The absolute values of the carbon isotope ratio in the Gucuo area are higher than those of known sections in the western Tethys and equatorial Pacific. We suggest that diagenetic alteration is the major cause of the higher absolute values in the Gucuo area.

Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum

Fixed nitrogen is an essential nutrient for eukaryotes. As N2 fixation and assimilation of nitrate are catalyzed by metalloenzymes, it has been hypothesized that in Mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses, restricting eukaryotes to near-shore environments and limiting their evolutionary innovation. To date this hypothesis has only been tested in the Belt Supergroup (∼1.4Ga), with results that support an onshore-offshore nitrate gradient as a potential control on eukaryote ecology. Here we present bulk nitrogen and organic carbon isotopic data from non-isochronous cross-basinal facies across the Bangemall (∼1.5Ga) and the Roper (∼1.4–1.5Ga) basins to better understand the extent and variability of onshore-offshore nitrogen isotope gradients in the Mesoproterozoic. Both basins show an average ∼1–2‰ enrichment in δ15Nbulk from deep to shallow facies, with a maximum range from −1‰ offshore to +7.5‰ onshore. Unlike the Belt basin, the Bangemall and Roper basins show some offshore δ15Nbulk values that are enriched beyond the isotopic range associated with biological N2 fixation alone. This suggests a mixture of aerobic and anaerobic metabolisms offshore. In shallow waters, where δ15Nbulk enrichment peaks, an aerobic nitrogen cycle was evidently operating. Even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the Bangemall and Roper basins, our data are consistent with a lateral gradient in nitrate availability within the photic zone, with higher concentrations in near-shore environments than offshore. The variability in δ15Nbulk values in each depositional environment and the consistently low δ15Nbulk values from Mesoproterozoic units compared to the Paleoproterozoic and Neoproterozoic suggest that nitrate concentrations in the global ocean were likely low. This trend is now seen in all three Mesoproterozoic basins so far examined, and contrasts with the Paleoproterozoic and Neoproterozoic where nearly all δ15Nbulk data plot above the N2 fixation window. Thus, we propose that the Mesoproterozoic ocean was characterized by a nitrate minimum, with the lowest concentrations in offshore environments. This inference is consistent with a Mesoproterozoic O2 decline following a temporary Paleoproterozoic O2 peak, and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near-shore environments, delaying their rise to ecological dominance until the Neoproterozoic Era.

The Pre-Sturtian negative δ13C excursion of the Dajiangbian formation deposited on the western margin of Cathaysia Block in South China

The Dajiangbian Formation in South China is a siliciclastic-dominated sedimentary succession with low-grade metamorphism deposited on the western margin of the Cathaysia Block, and is capped by a glaciogenic diamictite (the Sizhoushan Formation). The Sizhoushan glaciogenic strata can be attributed to the Jiangkou glacial (Sturtian glacial) episode as they share stratigraphic and lithological similarities with Jiangkou strata in South China. Some carbonate, chert and shale units throughout the upper part of the Dajiangbian Formation were sampled for carbonate carbon isotope (δ13Ccarb) and organic carbon isotope (δ13Corg) analyses. A range of geochemical indices including oxygen isotopes (δ18O) and Mn/Sr (Fe/Sr) ratios suggest that primary carbon isotope values were preserved in the upper Dajiangbian Formation. The upper Dajiangbian Formation shows δ13Ccarb of -0.1‰, upward decreasing towards to -5.4‰. We suggest that the negative δ13C excursion beneath the Sizhoushan diamictite is correlative with the Pre-Sturtian Islay δ13Ccarb anomaly and allows correlation with the global Neoproterozoic isotope stratigraphy. We find that carbonate and organic carbon isotope data of the upper Dajiangbian Formation are coupled, consistent with the δ13Ccarb-δ13Corg pattern observed on multiple continents.

A massive input of coarse-grained siliciclastics in the Pyrenean Basin during the PETM: the missing ingredient in a coeval abrupt change in hydrological regime

Abstract. The Paleocene–Eocene thermal maximum (PETM) is represented in numerous shallow and deep marine sections of the south–central and western Pyrenees by a 2–4 m thick unit (locally up to 20 m) of clays or marly clays intercalated within a carbonate-dominated succession. This unit records a massive input into the Pyrenean Gulf of fine-grained terrestrial siliciclastics, attributed to an abrupt hydrological change during the PETM. However, the nature of such a change remains controversial. Here we show that, in addition to fine-grained deposits, large volumes of coarse-grained siliciclastics were brought into the basin and were mostly accumulated in incised valleys and in a long-lived deep-sea channel. The occurrence of these coarse-grained deposits has been known for some time, but their correlation with the PETM is reported here for the first time. The bulk of the incised valley deposits in the PETM interval are cross-bedded sands and pebbly sands, almost exclusively made of quartz. The criteria for indicting a relation to the PETM include their stratigraphic position between upper Thanetian and lower Ilerdian marine carbonates, organic carbon isotope data, and a high percentage of kaolinite in the clay matrix. The axially flowing deep-sea channel existed throughout Paleocene times in the Pyrenean Basin, within which coarse-grained calciclastic and siliciclastic turbidites were accumulated. This Paleocene succession is capped by thickly bedded quartz sandstones and pebbly sandstones, probably deposited by hyperpycnal flows, which are here assigned to the PETM based on their stratigraphic position and organic carbon isotopic data. The large and simultaneous increase in coarse- and fine-grained terrestrial siliciclastics delivered to the Pyrenean Gulf during the PETM is attributed to an increased intra-annual humidity gradient. During the PETM a longer and drier summer season facilitated the erosion of landscapes, whereas a dramatic enhancement of precipitation extremes during the wet season led to intensified flood events, with rivers carrying greater volumes of both bed and suspended loads. This scenario argues against the possibility that PETM kaolinites indicate a coeval warm and humid climate in northern Spain. Instead, the kaolinite reflects the erosion of thick Cretaceous lateritic profiles developed on the Hercynian basement.

Integrated stratigraphy and palaeoclimate history of the Carnian Pluvial Event in the Boreal realm; new data from the Upper Triassic Kapp Toscana Group in central Spitsbergen (Norway)

The Late Triassic climate is characterized by arid conditions interrupted by a humid phase known as the Carnian Pluvial Event (CPE). This wet phase is well documented in the Tethyan realm, but evidence from the Boreal realm is scarce. Here we present evidence from quantitative palynology for the CPE from the Kapp Toscana Group on central Spitsbergen integrated with organic carbon isotope data linked to the geomagnetic polarity time scale. Our data reveal an early to mid-Julian-1 age for the Tschermakfjellet Formation. The transition to the Julian-2 is located in the De Geerdalen Formation and the Isfjorden Member is confirmed as mostly Tuvalian-3 in age. The Aulisporites astigmosus pollen assemblage zone that marks the base of the CPE in the Tethys realm at the base of the Julian-2 is assigned to the Julian-1 in the Boreal region. Palaeoclimate proxy data inferred from principal component analysis indicate wetter conditions from the Julian-2 onwards, which is in agreement with the establishment of local swamp vegetation on top of a delta plain. The palaeotemperature curve indicates a period of cooler climate during the early Julian-1 followed by warming during the late Julian-1. Supplementary materials: A list of all identified morphotaxa of pollen, spores and aquatic palynomorphs is available at http://www.geolsoc.org.uk/SUP18879 .

Euxinic conditions recorded in the ca. 1.93 Ga Bravo Lake Formation, Nunavut (Canada): Implications for oceanic redox evolution

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere–ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere–ocean system decreased considerably after ca. 2.06billionyearsago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90millionyears before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event.

Late Permian–earliest Triassic high-resolution organic carbon isotope and palynofacies records from Kap Stosch (East Greenland)

During and after the end Permian mass extinction terrestrial and marine biota underwent major changes and reorganizations. The latest Permian and earliest Triassic is also characterized by major negative carbon isotope shifts reflecting fundamental changes in the carbon cycle. The present study documents a high-resolution bulk organic carbon isotope record and palynofacies analysis spanning the latest Permian–earliest Triassic of East Greenland. An almost 700meter thick composite section from Kap Stosch allowed discriminating 6 chemostratigraphic intervals that provide the basis for the correlation with other coeval records across the world, and for the recognition of basin wide transgressive–regressive events documenting tectonic activity during the opening of the Greenland–Norway Basin. The identification of the main factors that influenced the organic carbon isotope signal during the earliest Triassic (Griesbachian to Dienerian) was possible due to the combination of bulk organic carbon isotope, palynofacies and Rock-Eval data. Two negative carbon isotopic shifts in the Kap Stosch record can be correlated with negative shifts recorded in coeval sections across the globe. A first negative shift precedes the base of the Triassic as defined by the first occurrence of the conodont Hindeodus parvus in the Meishan reference section, and the second one coincides with the suggested Griesbachian–Dienerian boundary. This new organic carbon isotope record from the extended Kap Stosch section from the Boreal Realm documents regional and global carbon cycle signals of the interval between the latest Palaeozoic and the onset of the Mesozoic.

Multiple controls on the paleoenvironment of the Early Cambrian marine black shales in the Sichuan Basin, SW China: Geochemical and organic carbon isotopic evidence

In order to understand the paleoenvironment of the Early Cambrian black shale deposition in the western part of the Yangtze Block, geochemical and organic carbon isotopic studies have been performed on two wells that have drilled through the Qiongzhusi Formation in the central and southeastern parts of Sichuan Basin. It shows that the lowest part of the Qiongzhusi Formation has high TOC abundance, while the middle and upper parts display relative low TOC content. Redox-sensitive element (Mo) and trace elemental redox indices (e.g., Ni/Co, V/Cr, U/Th and V/(V + Ni)) suggest that the high-TOC layers were deposited under anoxic conditions, whereas the low-TOC layers under relatively dysoxic/oxic conditions. The relationship of the enrichment factors of Mo and U further shows a transition from suboxic low-TOC layers to euxinic high-TOC layers. On the basis of the Mo-TOC relationship, the Qiongzhusi Formation black shales were deposited in a basin under moderately restricted conditions. Organic carbon isotopes display temporal variations in the Qiongzhusi Formation, with a positive excursion of δ13Corg values in the lower part and a continuous positive shift in the middle and upper parts. All these geochemical and isotopic criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Qiongzhusi Formation black shales. The correlation of organic carbon isotopic data for the Lower Cambrian black shales in different regions of the Yangtze Block shows consistent positive excursion of δ13Corg values in the lower part for each section. This excursion can be ascribed to the widespread Early Cambrian transgression in the Yangtze Block, under which black shales were deposited.

A preliminary quantitative reconstruction of precipitation in southern Mu Us sandy land at margin of Asian monsoon-dominated region during late Quaternary

We present the first quantitative estimation of monsoon precipitation during the late glacial-Holocene in the sandy land of northern China, based on organic carbon isotopic composition data from a loess-sand sequence at margin of the Mu Us sandy land. We use the relationship between monsoon precipitation and the carbon isotopic composition of modern soils as an analogue, with a minor modification, to reconstruct precipitation back to c. 47 ka ago. The preliminary results indicate that annual monsoon precipitation was high after 8 ka, with an average of 435 mm; and it decreased during 18 and 8 ka with a mean value of 194 mm. The precipitation value of 47-18 ka varied between the two. We compare the reconstructed precipitation with other records and paleoclimatic modeling results, showing that our record agrees with reconstructions of the monsoon precipitation from other sources, even capturing short climatic events such as the Younger Dryas. We suggest that solar irradiance, high-latitude temperature/ice volume and local evaporation have together modified moistures in the sandy land.

Environmental change in the vicinity of the Neolithic wetland settlement Pestenacker (S-Germany) during the last 6600 years

Pestenacker and the adjacent Neolithic wetland sites Unfriedshausen and Pestenacker-Nord in southern Germany belonged to the Altheim culture from around 5750–5350 years cal. BP. In contrast to contemporaneous pile dwellings, the houses of Pestenacker were constructed at ground level in a wetland environment. Only little is known about the environmental and hydrological conditions in the direct vicinity of the settlement and how they changed in time. To find detailed answers to these questions, stratigraphy, sedimentology, microfossils, geochemistry and stable isotopes of a sediment profile immediately to the west of the settlement were studied. An age model comprises five radiocarbon dates and dates the base of the profile to about 6600 years cal. BP. Ostracods and aquatic gastropods were frequent only in the basal part of the profile. Ostracod taxa typical for open-water conditions point to higher humidity especially at 6120–5860 years cal. BP and 5750–5370 years cal. BP. Sedimentology and organic carbon isotope data confirm this observation. Thereafter open-water habitats disappeared, while increasing numbers of non-aquatic gastropods occur and soil formation is indicated by elevated δ15N values. We conclude that during the time of settlement an open water body existed near-by that disappeared shortly after the settlement was abandoned. However, the spatial extent of this water body still remains to be clarified.

New high resolution geochemistry of Lower Jurassic marine sections in western North America: A global positive carbon isotope excursion in the Sinemurian?

Recognising variations in the carbon isotope compositions of marine organic-rich sedimentary rocks can provide insight into changes in ocean chemistry throughout geological time. Further, identification of global excursions in the carbon isotope record has proved to be valuable as a chronostratigraphic correlation tool.This investigation presents new high-resolution organic carbon isotope data (δCorg13) for marine sediments from 2 regions in North America (Last Creek, British Columbia, Canada and Five Card Draw, Nevada, USA). The carbon isotope profiles demonstrate that there were significant differences between the carbon reservoirs at Five Card Draw and Last Creek, notably in the upper part of the Leslei Zone. The δCorg13 values show a gradual positive CIE (∼2‰) at Last Creek in the upper part of the Leslei Zone. This corresponds to a coeval positive CIE of similar duration in Dorset, UK (upper Turneri Zone; Jenkyns and Weedon, 2013), suggesting that this may be a global marine carbon isotope signature, and likely reflects a widespread increase in primary productivity during the Early Sinemurian. In addition, a brief negative CIE is observed in the uppermost Lower Sinemurian at Last Creek. This negative excursion is not recorded in the Dorset section, suggesting localised upwelling of 12C-rich bottom-waters at Last Creek. Further, the signals identified at Last Creek are not present in coeval sections at Five Card Draw, thus highlighting a significant difference between these localities. Osmium (Os) isotope data (initial 187Os/188Os values) provide a quantitative determination of the contrasting depositional environments of Five Card Draw and Last Creek (at least partially restricted with high levels of continental inundation and open-ocean, respectively). This demonstrates that basinal restriction may act as a major factor that controls isotopic stratigraphic signatures, thus preventing the identification of global or widespread regional excursions.

Determination of multiple organic matter sources in aerosol PM10 from Wrocław, Poland using molecular and stable carbon isotope compositions

The natural and anthropogenic contributions of hydrocarbon groups (aliphatic and aromatic), as well as total organic carbon, in atmospheric PM10 dust (particulate matter <10 μm) collected from Wrocław (SW Poland) were assessed using combined molecular (gas chromatography–mass spectrometry – GC–MS) and stable carbon isotopic (isotope-ratio mass spectrometry – IR-MS) analyses. The PM10 samples were taken in the seasonal sampling program in 2007, and represent air pollution from all months of the year. The δ13C values of the total carbon varied seasonally from −27.6 to −25.3‰. The isotopic mass balance calculations confirmed greater coal burning input, reaching 70.5%, in the heating season and dominant transported sources 47.5% in the vegetative season. The data obtained for the aliphatic fractions: carbon preference index (CPI), carbon number maximum (Cmax), wax n-alkane contents (%WNA), and δ13C values of the aliphatic fractions (−36.6 to −29.4‰), indicated a dominant anthropogenic origin (gasoline/diesel/coal combustion) and a lesser biogenic input (biomass burning and natural organic matter). Petroleum and coal combustion emissions were confirmed by the presence of hopanes and moretanes. The molecular analysis of the concentrations and diagnostic ratios of the polycyclic aromatic hydrocarbons (PAHs) and the δ13C values of the aromatic fractions (−35.4 to −26.8‰) indicated that the main PAH sources were also collectively from combustion of liquid fuels and coal. Based on PAH discrimination diagrams it is also clear that the main organic carbon source is derived from coal, biomass and petroleum combustion in both seasons. However, taking into account the PAH concentrations during the vegetative and heating seasons, coal and biomass burning seem to be their major source. Additionally, the polar organic compounds (mainly levoglucosan) confirmed a significant contribution from biomass burning to the total anthropogenic input. The general conclusion derived from coupling of organic tracer analysis and carbon isotopic data of PM10 was that the total carbon (including insoluble soot) is likely derived from fossil fuel combustion, while the extractable organic matter is a mixture from different sources with significant inputs of biomass burning. We have also shown that dominant organic tracers do not always represent the major input source in aerosol PM and the unresolved part of the organic matter (soot) is important in the carbon budget.

Organic carbon isotope gradient and ocean stratification across the late Ediacaran-Early Cambrian Yangtze Platform

Organic carbon isotope (δ 13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ 13Corg values of the late Ediacaran Dengying Formation range from −29‰ to −24‰. In the deep-water Longbizui section, δ 13Corg values from time-equivalent strata of the Dengying Formation are mostly between −35 and −32‰. These new data, in combination with δ 13Corg data reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δ 13Corg gradient. High δ 13Corg values (>−30‰) occur mostly in shallow-water carbonate rocks, whereas low δ 13Corg values (<−32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ 13Corg variations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ 13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ 13Corg values (>−30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ 13Corg data and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δ 13Corg values (<−32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ 13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.