Low Carbon Isotope Values Research Articles

Characteristics of dissolved pores and dissolution mechanism of zeolite-rich reservoirs in the Wuerhe Formation in Mahu area, Junggar Basin

The reservoir in the Wuerhe Formation in the Mahu Sag, northwestern Junggar Basin, China, exhibits complex dissolution and cementation related to zeolite. The source and mechanism of diagenetic fluids are crucial in studying the reservoir genesis. Thus we investigated the key reservoirs fluids related to the zeolite and discussed their significance in the zeolite-rich reservoir of the Permian Wuerhe Formation in the Mahu Sag. Based on thin sections and electron microscope observations of rock samples and analyses of physical properties, C-O isotopes, and major elements, it is found that the reservoir underwent mainly two stages of fluid-related dissolution and cementation processes, in which the hydrocarbon-bearing fluid played the primary role in forming the high-quality reservoir. Dissolution pores are the most important storage space, and zeolite cement is the most important dissolution mineral. The geochemical characteristics of zeolite and calcite cement indicate the presence of two diagenetic fluids. The iron-rich calcite and orange-red heulandite is related to early diagenetic fluids with high iron content and higher carbon isotope values, whereas the calcites, with high manganese content and lower carbon isotope values, are formed by late acidic organic diagenetic fluids related to oil and gas activities. The hydrocarbon-bearing fluids form different spatial diagenetic zones, including the dissolution zone, buffer zone, and cementation zone, and the dissolution zone near the oil source fault is the main site of zeolite dissolution. The late fluid has the characteristics of multi-stage activity, which makes the spatial zoning expand gradually, resulting in multiple superpositions of dissolution and cementation and increasing the complexity and heterogeneity of the reservoir diagenesis. This study expands the understandings of the dissolution activities of different fluids in zeolite-rich reservoirs and also has reference significance for dissolution activity of hydrocarbon fluid in other types of reservoirs.

Progressively released gases from fluid inclusions reveal new insights on W-Sn mineralization of the Yaogangxian tungsten deposit, South China

The gentle stepwise crushing technique permits progressive extraction of gases from large, irregular, secondary fluid inclusions (SFIs) along micro-cracks to small, regular, liquid-rich primary fluid inclusions (PFIs). Previous studies have verified this general gas release pattern in different hydrothermal minerals based on the agreements between 40Ar/39Ar ages of the released gases from different crushing stages and the most likely corresponding geological events dated with other methods; and based on the excellent agreements between gas chemistry measured by QMS (quadrupole mass spectrometer) during stepwise crushing and the expected characteristics of SFIs and PFIs in the same samples acquired using Raman spectroscopy. In this study, we applied this QMS-based gentle stepwise crushing technique on ore and gangue minerals from the world-class Yaogangxian tungsten deposit in South China. Gases released from PFIs in cassiterite, wolframite, and quartz from ore-bearing veins show high CH4, N2, C3H8, and low CO2, C4H10 contents, low CO2/CH4 (≪1) and high N2/Ar (327–861 for ore minerals and 60.7–943 for quartzs from ore veins) ratios, along with multiple organic gaseous species. Their compositions resemble those of gases released from PFIs in quartzs from the Yaogangxian granites which represent magmatic-hydrothermal fluids that exsolved from the granitic magma. However, compared with these exsolved magmatic-hydrothermal fluids, ore mineral PFIs have relatively high He contents which indicate contributions from non-magmatic crustal fluids. This is consistent with high Ca2+, low F− concentrations, and low K+/Na+ (0.21–0.32) ratios observed in fluids in the quartz samples, as well as their relatively low water hydrogen isotopes (δD-H2O = −102 to −59‰). These chemical characteristics, coupled with their magmatic sulfur (δ34S-molybdenite and arsenopyrite = −0.2–1.4‰), methane hydrogen (δD-CH4 = −58 to −42‰), and fluid oxygen (δ18O-H2O = 3.9–5.1‰) isotopic compositions, indicate both magmatic and non-magmatic crustal components in the mineralizing fluids. Based on these observations and the low methane carbon isotope values (δ13C-CH4) in the studied quartz samples, we propose that fluid-rock interactions between the exsolved magmatic-hydrothermal fluids and the metasedimentary wall rocks played an important role in promoting W-Sn precipitation in this ore deposit. This study demonstrates the power of combining QMS-based stepwise crushing technique with traditional geochemical techniques in investigating ore-forming processes.

Limited evidence of C4 plant consumption in mound building Macrotermes termites from savanna woodland chimpanzee sites.

Stable isotope analysis is an increasingly used molecular tool to reconstruct the diet and ecology of elusive primates such as unhabituated chimpanzees. The consumption of C4 plant feeding termites by chimpanzees may partly explain the relatively high carbon isotope values reported for some chimpanzee communities. However, the modest availability of termite isotope data as well as the diversity and cryptic ecology of termites potentially consumed by chimpanzees obscures our ability to assess the plausibility of these termites as a C4 resource. Here we report the carbon and nitrogen isotope values from 79 Macrotermes termite samples from six savanna woodland chimpanzee research sites across equatorial Africa. Using mixing models, we estimated the proportion of Macrotermes C4 plant consumption across savanna woodland sites. Additionally, we tested for isotopic differences between termite colonies in different vegetation types and between the social castes within the same colony in a subset of 47 samples from 12 mounds. We found that Macrotermes carbon isotope values were indistinguishable from those of C3 plants. Only 5 to 15% of Macrotermes diets were comprised of C4 plants across sites, suggesting that they cannot be considered a C4 food resource substantially influencing the isotope signatures of consumers. In the Macrotermes subsample, vegetation type and caste were significantly correlated with termite carbon values, but not with nitrogen isotope values. Large Macrotermes soldiers, preferentially consumed by chimpanzees, had comparably low carbon isotope values relative to other termite castes. We conclude that Macrotermes consumption is unlikely to result in high carbon isotope values in either extant chimpanzees or fossil hominins.

Source rock characterization and oil-to-source rock correlation of a Cambrian -Ordovician fold-and-thrust belt petroleum system, western Newfoundland

Abstract In this study, we use biomarker and isotope data to address the complexities of petroleum system analysis in fold-thrust systems, specifically related to hydrocarbon generation and migration, such as variable thermal maturities of source rocks, complex migration pathways, and mixing. Source rocks, extracts, and oil samples are taken from Paleozoic rocks along the Appalachian structural front in western Newfoundland. Oil seeps along this fold-and-thrust belt have motivated episodic exploration efforts over the last 150 years. However, economic development has been unsuccessful to date, in part because of the complex nature of oil-to-source relationships. Pyrolysis analyses identify promising source intervals in Lower Ordovician (Floian) formations, with an excellent source potential containing type I/II organic matter (TOC up to 9.35%, HI up to 840). A second good source interval, identified within the late Cambrian (Furongian) continental slope and rise sediments, contains type II/III organic matter (TOC up to 2.34%, HI = 380). Source rock samples from outcrops are marginally mature to mature with Tmax values between 436 and 447. Geochemical analyses of source rock extract imply a clastic shale-dominated source rock with a minor contribution from carbonate source rocks. Chemometric analyses yield 3 extract groups. Groups 1 and 2 contain lower Ordovician samples with organic matter derived from oxidized micro-plankton with varying amounts of Gleocapsomorpha prisca. Extract group 3 originates from Cambrian samples and shows evidence of a bacterial-derived organic matter. Similar genetic relationships among 10 oil samples from natural oil seeps and abandoned well sites are indicated by high-resolution biomarker analyses. Seven oil samples originated from Ordovician source rocks that produced high API oils with low bisnorhopane/hopane, tricyclic/hopane ratios, and high sterane concentrations, supporting interpretation of a source containing algal-derived organic matter. Three oils originated from Cambrian shales. These high API oils show high bisnorhopane/hopane and tricyclic/hopane ratios, low sterane concentrations, and low carbon isotope values, all indicative of organic matter derived from bacteria, generated over a wide range of maturities (0.98–1.26 %Ro). Thermal maturity-sensitive biomarkers and naphthalene ratios show that source rocks generated oils with a wide range of maturities related to their dipping character in the imbricated thrust stack. The Cambrian source generated oils that locally experienced secondary cracking during Acadian inversion, whereas the Ordovician source generated oil with low maturities after the Acadian inversion. Oil operaters exploring fold-and-thrust belt petroleum systems may expect extreme changes of oil properties (for example, API gravity) in spacially restricted areas.

Status, kinship, and place of burial at Early Bronze Age Bab adh-Dhra': A biogeochemical comparison of charnel house human remains.

The Early Bronze Age (EBA; ca. 3,600-2000 BCE) of the southern Levant underwent considerable transformation as agro-pastoral communities began to utilize their land more intensively, constructing larger, fortified towns prior to site abandonment at the end of the third millennium. At the site of Bab adh-Dhra' in Jordan, the dead of the Early Bronze (EB) II-III (ca. 3,100-2,500 BCE) period were communally interred within charnel houses, but important disparities between these structures and their contents may be reflective of ownership and use by particular extended kin groups whose activity patterns, subsistence strategies, and even social status may have differed from one another. Subsequently, we hypothesized that differences in mobility and dietary intake may differentiate tomb groups from one another. Dental enamel from 31 individuals interred in three different Early Bronze Age charnel houses (A56, A22, A55) at Bab adh-Dhra', Jordan were analyzed for strontium, oxygen, and carbon isotope values. Strontium isotope ratios (range: 0.70793-0.70842) possessed medians that did not differ statistically from one another, but had ranges that exhibited significant differences in variance. Carbon isotope values ( = -13.2 ± 0.5‰, 1σ) were not significantly different. General similarities in human isotopic signatures between EB II-III charnel houses A22 and A55 suggest that their activities were likely similar to one another and agree with findings from excavated domestic spaces with little archaeological evidence for economic, social, or political differentiation. More variable strontium isotope ratios and lower carbon isotope values from A22 could reflect a greater involvement with pastoralist practices or regional trade, including the consumption of more 13 C-depleted foods, while those in A55 may have led a more sedentary lifestyle with greater involvement in cultivating orchard crops. All charnel houses contained nonlocal individuals likely originating from other Dead Sea Plain sites with no EB II-III cemeteries of their own, supporting the idea that extended kin groups throughout the region returned to Bab adh-Dhra' to bury their dead.

Glacial melting pulses in the Antarctica: Evidence for different responses to regional effects of global warming recorded in Antarctic bivalve shell (Laternula elliptica)

Abstract Meltwater history of the Antarctic bivalve Laternula elliptica in Maxwell Bay, King George Island near the Antarctic Peninsula was reconstructed during the shell growth. High resolution trace elemental and stable isotopic compositions along the aragonite outer part of the shell together with growth bands shows that the shell lived for 9 years with distinct annual cycles. Also oxygen and carbon isotope values reveal the local meltwater history in Antarctic Peninsula region. More negative oxygen isotope values than the predicted equilibrium values clearly show that oxygen isotope depletion is due to lower salinity of seawater by glacial melting. This is also confirmed by the similar trend of low carbon isotope values as well as monitored sea surface salinity values. Comparing δ18O values of previous results using the same bivalve species, more negative values from the Antarctic Peninsula (Maxwell Bay) during the austral winter than from East Antarctica (Syowa Coast and Ross Sea) suggests that perennial glacial melting influenced seawater δ18O composition near the peninsula. Also, more negative and variable bivalve δ18O values during austral summer indicate that meltwater pulses fluctuated greatly in the study area. Distinctively different trends in bivalve δ18O profiles between the Antarctic Peninsula and East Antarctica may reflect differential responses to regional warming with regard to the recent global warming over the past few decades.

Conodonts and Carbon Isotopes during the Permian-Triassic Transition on the Napo Platform, South China

Two Permian-Triassic boundary (PTB) sections (Pojue and Dala) are well exposed in an isolated carbonate platform (Napo Platform) on the southwestern margin of the Nanpanjiang Basin, South China. These sections provide an insight into the transition across the PTB and a detailed investigation of the conodont biostratigraphy and inorganic carbon isotopes is presented. The PTB at the Pojue Section is placed at the base of Bed 10B (a dolomitized mudstone found below a microbialite horizon), defined by the first occurrence of Hindeodus parvus. At the Dala Section, four conodont zones occur. They are, in ascending order, the Hindeodus parvus Zone, Isarcicella staeschei Zone, Isarcicella isarcica Zone and Clarkina planata Zone. Comparison with the Pojue Section suggests the PTB at Dala also occurs at the base of dolomitized mudstone below a microbialite horizon, although the first occurrence of Hindeodus parvus is near the top of a microbialite bed: an occurrence that is also seen in other platform sections. The succeeding microbialite beds developed during the ongoing PTB mass extinction phase. This time was characterized by low carbon isotope values, and a microbialite ecosystem that provided a refuge for selected groups (bivalves, ostracods and microgastropods) that were likely tolerant of extremely high temperatures.

Speleothem evidence for C3 dominated vegetation during the Late Miocene (Messinian) of South Africa

During the Late Miocene, Africa experienced a number of ecological transitions including the spread of C4 grasslands, the expansion of the Sahara Desert, the Messinian Salinity Crisis and a number of mammalian migrations and expansions, including the origin of the hominin clade. A detailed understanding of the relationship between environmental change and hominin evolution is hampered by the paucity of data available from terrestrial localities, especially in southern Africa. Here, we present a stable isotope and trace element record from a speleothem from the South African cave site of Hoogland. Uranium-lead dating and magnetostratigraphy places the speleothem within the Messinian Age (7.25–5.33 Ma) of the Late Miocene, making it the oldest known cave deposit from the region near the UNESCO Fossil Hominids of South Africa World Heritage Site (locally known as the “Cradle of Humankind”). Low carbon isotope values indicate a predominantly C3 vegetation in the vicinity of the cave throughout the period of speleothem growth. It is not possible to determine if this represents a C3 grassland or a C3 woodland, but it is clear that an equivalent C3-rich environment has yet to be found during the Messinian of east Africa. We conclude that the C4 grass expansion occurred millions of years later in South Africa than it did in eastern Africa, and that this vegetation shift should be considered when comparing African vegetation change with the late Miocene hominin fossil record.

Euxinic conditions and high sulfur burial near the European shelf margin (Pieniny Klippen Belt, Slovakia) during the Toarcian oceanic anoxic event

Abstract The Toarcian oceanic anoxic event (T-OAE; 183 Ma) was accompanied by severe biotic and geochemical perturbations that are considered as some of the most severe of the Mesozoic era. Nevertheless, paleoceanographic conditions during the T-OAE in the Tethyan domain remain poorly understood owing to the relatively low number of marine successions from this area investigated so far. In this study, we report new geochemical, palynological and pyrite framboid size data from Zazriva, northwestern Slovakia, a Lower Toarcian marine succession recording organic-rich sedimentation at the margin of the NW-European shelf. The lowermost Serpentinum ammonite Zone records high total organic carbon (TOC) contents (2 to 5 wt%) and very low organic carbon isotope values (from −30 to −32‰) characteristic of the T-OAE negative carbon isotope excursion in coeval sites. The size of the pyrite framboids, the high S contents independent on TOC, low sulfur isotope values (δ34S

The Albian to Turonian carbon isotope record from the Shilaif Basin (United Arab Emirates) and its regional and intercontinental correlation

AbstractThe sedimentary record of the Arabian Shelf offers a unique opportunity to study the Cretaceous (Albian–Turonian) greenhouse climate from a palaeoequatorial perspective. In particular, hemipelagic to pelagic carbonate successions from the extensive Shilaif intra‐shelf basin have the potential to produce an excellent record of carbon cycle perturbations during this interval. This study presents a 269 m thick chemostratigraphic (carbonate δ13C and δ18O) record from the Middle Albian to Early Turonian of central Abu Dhabi (United Arab Emirates), representing over 14 Myr of uninterrupted carbonate sedimentation. The Mauddud to Shilaif formations represent outer ramp to basinal intra‐shelf carbonates with variations from laminated organic‐rich to clean bioturbated intervals. Isotopic evidence of the latest Albian Anoxic Event (Oceanic Anoxic Event 1d), Middle Cenomanian Event I and the Cenomanian–Turonian Anoxic Event (Oceanic Anoxic Event 2) are confirmed and biostratigraphically calibrated by means of calcareous nannofossils. The carbon isotope record allows correlation with other regional records and well‐calibrated records across the Tethyan Ocean and represents a significant improvement of the chronostratigraphic framework of the United Arab Emirates (Shilaif) and Oman (Natih) intra‐shelf basins. The study further confirms that low carbon isotope values corresponding to the two source rock intervals in the Shilaif Formation clearly precede the isotopic expressions of Oceanic Anoxic Event 1d and Oceanic Anoxic Event 2.

Stable carbon isotopes of sedimentary kerogens and carbonaceous macrofossils from the Ediacaran Miaohe Member in South China: Implications for stratigraphic correlation and sources of sedimentary organic carbon

Ediacaran stratigraphic correlation depends on the integration of paleontological and δ13Ccarb data. However, these data often come from different lithologies (e.g., shales vs. carbonates), making it difficult to directly link biostratigraphic and chemostratigraphic markers. Ediacaran successions in South China consist of mixed carbonates and shales, offering an opportunity for integrated biostratigraphic and chemostratigraphic investigations. Black shales of the Ediacaran Miaohe Member in the Yangtze Gorges area contain macroalgae and Ediacara-type macrofossils, both preserved as carbonaceous compressions. The Miaohe Member has been traditionally regarded as an equivalent to Member IV of the uppermost Doushantuo Formation that records part of the Shuram negative δ13Ccarb excursion in South China. Thus, the Miaohe Member may have the potential to clarify the stratigraphic relationship between Ediacara-type fossils and the Shuram excursion. Recently, however, the Miaohe Member has been partially correlated with the Shibantan Member of the Dengying Formation (the “A” correlation) or partially with Member IV (the “Z” correlation). These correlations have different implications for the relationship between the Miaohe Member and the Shuram excursion. Here we test these two correlations using δ13C data of sedimentary kerogen or total organic carbon (δ13CorgTOC), taking advantage of the distinct δ13CorgTOC values between the Shibantan Member and Member IV. Our data show that δ13CorgTOC of the Miaohe Member is more similar to that of Member IV than to Shibantan Member, providing tentative support for the “Z” correlation. In addition, we also analyzed δ13Corg of individual carbonaceous compression fossils (δ13Corgtaxon), which have lower carbon isotope values than TOC. The data suggest that oxygenic photosynthesizers (e.g., macroalgae, cyanobacteria, and eukaryotic phytoplankton) were not the only contributors to sedimentary TOC. Methylotrophs, photoautotrophs, and chemoautotrophs living in anoxic waters or at the oxycline may have fixed recycled CO2 derived from organic carbon remineralization, thus contributing a significant amount of 13C-depleted organic carbon to sedimentary kerogen in the Miaohe Member.

Humans permanently occupied the Andean highlands by at least 7 ka.

High-elevation environments above 2500 metres above sea level (m.a.s.l.) were among the planet's last frontiers of human colonization. Research on the speed and tempo of this colonization process is active and holds implications for understanding rates of genetic, physiological and cultural adaptation in our species. Permanent occupation of high-elevation environments in the Andes Mountains of South America tentatively began with hunter–gatherers around 9 ka according to current archaeological estimates, though the timing is currently debated. Recent observations on the archaeological site of Soro Mik'aya Patjxa (8.0–6.5 ka), located at 3800 m.a.s.l. in the Andean Altiplano, offer an opportunity to independently test hypotheses for early permanent use of the region. This study observes low oxygen (δ18O) and high carbon (δ13C) isotope values in human bone, long travel distances to low-elevation zones, variable age and sex structure in the human population and an absence of non-local lithic materials. These independent lines of evidence converge to support a model of permanent occupation of high elevations and refute logistical and seasonal use models. The results constitute the strongest empirical support to date for permanent human occupation of the Andean highlands by hunter–gatherers before 7 ka.

Glendonites track methane seepage in Mesozoic polar seas

During the Phanerozoic, Earth has experienced a number of transient global warming events associated with major carbon cycle perturbations. Paradoxically, many of these extreme greenhouse episodes are preceded or followed by cold climate, perhaps even glacial conditions, as inferred from the occurrence of glendonites in high latitudes. Glendonites are pseudomorphs of ikaite (CaCO3·6H2O), a hydrated carbonate mineral increasingly stable at low temperatures. Here, we show that methane seepage and oxidation provide an overriding control on Mesozoic glendonite formation (i.e., ikaite fossilization). Geochemical and petrological analyses of 33 Early Jurassic to Early Cretaceous glendonites from five sections in Siberia (Russia) reveal that most of their infilling carbonate phases are reminiscent of methane-derived authigenic carbonates. Bulk glendonites and surrounding sediments exhibit exceptionally high and low carbon isotope values (+20‰ to –45‰ VPDB [Vienna Peedee belemnite]), typical for carbon sources linked to methane generation and oxidation. Gas inclusion data confirm the presence of methane and longer-chain hydrocarbon gases, suggesting a thermogenic source for the methane. Glendonite-bearing layers can be traced for hundreds of kilometers, suggesting widespread trapping of methane in the sub-seafloor during the Jurassic. As such, glendonites constitute an unexplored archive for detecting past episodes of methane release and oxidation in polar settings.

Reconstruction of Holocene paleohydrological and sedimentological characteristics of Umm Ar-Rimam basin (northern Kuwait)

The paleohydrological and sedimentological characteristics of a playa lake in northern Kuwait (Arabian Gulf) are reconstructed using sedimentological, geochemical, and isotopic techniques. The sequence consists of up to 8 cycles of S-poor, alluvial sediments capped by a thin organic soil interbedded with gravity-fall calcrete sediments. The succession is locally derived from mainly Quaternary sediments and is regressive with upsection filling of the subsiding basin by cycles of sheetwash flow in response to climatic change. There is no natural, open-water lake water as indicated by low total organic carbon (TOC) data, but the presence of incised calcrete yardangs suggests that more extensive open-water conditions were operative in the past. Stable isotope (δ18O‰ and δ13C‰) values of the authigenic carbonates indicate the following three distinct processes: evaporation, meteoric fluid infiltration, and rapid per-descensum flow (rapid downward movement of water and playa sediment through pipes) through a porous, clastic sequence. Because evaporites are scarce, other factors besides evaporation action control chemical and isotopic compositions of the per-decensum lake fluids. Consequently, the isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge may explain the enriched carbon isotope values for the authigenic carbonates precipitated in the sediments. Hydrologically closed lake water bodies tend to show low negative carbonate oxygen and carbon isotopic signatures. Isotopically negative δ13C values imply a strong input of soil-zone carbon to the groundwater of the top 60 cm of the sediment. Lakes that are hydrologically closed and evaporate or equilibrate with atmospheric CO2 will tend to have low negative δ18O and δ13C values in the carbonates as reported by Talbot (Chem Geol: Isotope Geosci Sect 80(4):261–279, 1990). Biologically active lakes will tend toward lower δ13C of dissolved carbon due to the photosynthetic effects of 12C withdrawal as reported by Dunagan and Driese (J Sed Res 69:772–783, 1999). Increased biological activity during sedimentation may account for low carbon isotope values where plants were abundant, but in shrinkage-dominated systems (those of clay-rich soil subjecting to wet-dry conditions), carbon isotopes will be largely inherited from the calcretic limestones in the land extending landward of the coast and not influenced by coastal processes (known as hinterland), such as Umm Ar-Rimam depression. This basin does not fit the classic shallow playa-type basins of the Arabian Peninsula but rather the recharge playas of the southwestern USA.

Large variability in n-alkane δ13C values in Lake Pa Kho (Thailand) driven by wetland wetness and aquatic productivity

Understanding past climate and environmental conditions depends largely on accurate interpretations of proxy records from a range of environments, including tropical wetlands and lakes. Lipid biomarker analysis can provide important information about the sources of the accumulated organic material, and thus about the environmental information contained therein. Here we use n-alkane distributions and stable carbon isotopes of leaf waxes (δ13Cwax) to identify the sources of organic matter (OM) of a 2000-year long sediment/peat record from Lake Pa Kho (LPK) in northeastern Thailand, and to constrain the mechanisms that cause shifts in δ13Cwax and in δ13C of bulk organic matter (δ13Cbulk). Our results show three main sources of OM: terrestrial plants, aquatic macrophytes and algae. The δ13C values of the long chain n-alkanes, show two distinct groups: C27–C31 and C33–C35n-alkanes, where the δ13C values of C33–C35n-alkanes reflect that of δ13Cbulk. Lower moisture availability on the wetland, known from other sedimentary evidence, was characterized by low carbon isotope values typically seen for C3 plants, whereas greater moisture availability corresponded to higher δ13C values (around −20‰) of C33–C35n-alkanes, resembling a typical C4 plant signal. However, various lines of evidence argue against large shifts between C3 and C4 plant input. Instead, we suggest that the high δ13C values were indirectly caused by higher aquatic productivity during periods of greater moisture availability, decreasing dissolved CO2, but increasing bicarbonate availability caused by higher pH. This caused the dominant macrophytes (e.g., Potamogeton spp.) to shift their carbon source from CO2 to bicarbonate, which has much higher δ13C values. Our results show that the environmental context should be taken into account when interpreting n-alkane δ13C variability as a paleo-environmental/climatic signal as this contains several important variables that need to be disentangled and explained.

Speleogenesis of the Hermannshöhle cave system (Austria): Constraints from 230Th/U-dating and palaeomagnetic analysis

Hermannshöhle is a show cave located near Kirchberg/Wechsel in Lower Austria. Together with three nearby and genetically connected caves, it forms the Hermannshöhlen cave system (HHS). With a length of 5 km, the HHS is the longest cave in the Lower Austroalpine unit. It is arranged as an extreme three-dimensional maze on a ground area of 200 x 200 x 82 m. Speleothems are abundant in this cave and represent the focus of this study. Low carbon isotope values indicate the presence of a soil-covered catchment above the HHS during times of speleothem deposition. 28 samples were dated by the 230Th/U-method and, in combination with palaeomagnetic data from a 5 m-high sediment profile, indicate multiple phases of sediment infill and erosion in the HHS. Although parts of the cave system are nowadays located at or below the level of the nearby Rams brook, they fell dry already at least 125 ka ago. The presence of 540 ka-old speleothems in the middle level demonstrate that this level of the HHS is at least about half a million years old, and the upper level is probably considerably older. A direct correlation between the cave and the modern surface morphology is therefore not possible. The observation that the palaeo-drainage direction in the HHS is perpendicular to the modern surface runoff also demonstrates the occurrence of a major reorganisation of the hydrological regime since the formation of the HHS. The new chronological data allow to constrain the evolution of the HHS as well as its catchment. An average valley incision rate of roughly 100 m/Ma was obtained, which is consistent with studies of other regions in the Alps unaffected by the Pleistocene glaciations. The spatial and temporal distribution of the dated speleothems as well as the lack of corroded flowstone indicate a single major speleogenetic period under phreatic conditions and support a continuous lowering of the groundwater table.

Pattern and timing of the Early Jurassic calcareous nannofossil crisis

abstract Article history:Received 27 April 2014Received in revised form 12 June 2014Accepted 23 June 2014Available online 30 June 2014Keywords:Early ToarcianCalcareous nannofossilsBiocalcification crisisCarbonate production TheToarciancalcareousnannofossilcrisisassociatedwiththeearlyToarcianOceanicAnoxicEvent(T-OAE)intheEarlyJurassicperiodisthoughttorepresentoneofthemostimportantbiocalcificationcrisesduringtheMesozo-ic,occurringsimultaneouslywithaprofounddisturbanceofthecarboncycle.However,thecausesarestillunderdebate, particularly with regard to the pattern, timing of the biocalcification crisis, relative roles of intrinsic andextrinsic processes as drivers of the crisis, and also causal mechanisms of the T-OAE.In this study, a new quantification of Toarcian calcareous nannofossil abundance and size is presented for theSancerre borehole (Paris Basin, France). Beyond the recognition of a severe biocalcification crisis defined bythe major drop in abundance, and the reduction in size of the most important pelagic carbonate producerSchizosphaerella punctulata, for the first time, this study proposes an insight into the pace and timing of thenannoplanktoncrisis.AtSancerre,thecarbonateproductionofthelowerToarciansedimentspreviouslyattribut-ed to obliquity forcing of climate allows to estimate a duration of ~210 kyr for the biocalcification crisis andof ~120 kyr for the shift towards lower carbon isotope values. The onset of the biocalcification crisis marked bya fertility event lasted~60kyr,andthe calciumcarbonate valuesremainedlow for ~150 kyr;thesubsequent re-covery of carbonate and nannoplankton lasted ~60 kyr and N550 kyr, respectively. Additionally, a link betweenthe biocalcification crisis, the seawater palaeotemperature, and the carbon isotope steps can be demonstrated.This covariance provides compelling evidence of fundamental change in the response of the climatic warmingand the carbon cycle systems triggering the biocalcification crisis. These observations indicate that thebiocalcification crisis can be regarded as a direct or indirect consequence of a global warming. Moreover, a defi-ciency of the biological pump is proposed here as a complementary casual mechanism for explaining thenegative carbon-isotope excursion.© 2014 Elsevier B.V. All rights reserved.Contents1. Introduction 572. TheSancerreborehole:geologicalsettingandpalaeogeographyoftheParisBasin 573. Materialandmethods 583.1. Quantificationofnannofossilabundanceandvolume 583.2. Analyticalanalysis 584. Results 594.1. Calcareousnannofossilpreservation 594.2. Stratigraphicdistributionofthecalcareousnannoplankton 594.3. Clusterandprincipalcomponentanalyses 594.4. Evolution of Schizosphaerella size 605. Discussion 605.1. ThecalcareousnannoplanktonresponsetotheT-OAEenvironmentalchanges 605.1.1. Patternsinnannofossilassemblageswithinpre-eventstableenvironmentalconditions 605.1.2. TheT-OAEperturbationasrecordedbynannofossilassemblages 605.1.3. Theenvironmentalrecoveryasrecordedbynannofossilassemblages 605.2. Expressionandevolutionofthebiogeniccarbonateproduction 615.3. AtimescaleforthenannoplanktoncrisisacrosstheT-OAE 61

New insights in the pattern and timing of the Early Jurassic calcareous nannofossil crisis

Abstract The Toarcian calcareous nannofossils crisis associated with the early Toarcian Oceanic Anoxic Event (T-OAE) in the Early Jurassic period is thought to represent one of the most important biocalcification crises during the Mesozoic, occurring simultaneously with profound disturbance of the carbon cycle. However, the causes are still under debate, particularly with regard to the pattern, timing of the biocalcification crisis, relative roles of intrinsic and extrinsic processes as drivers of the crisis, and also causal mechanisms of the T-OAE. In this study, a new quantification of Toarcian calcareous nannofossil abundance and size is presented for the Sancerre borehole (Paris Basin, France). Beyond the recognition of a severe biocalcification crisis defined by the major drop in abundance, and the reduction in size of the most important pelagic carbonate producer Schizosphaerella punctulata , for the first time, this study proposes an insight into the pace and timing of the nannoplankton crisis. At Sancerre, the carbonate production of the lower Toarcian sediments previously attributed to obliquity forcing of climate allows estimating a duration of ~ 210 kyr for the biocalcification crisis and of ~ 120 kyr for the shift towards lower carbon isotope values. The onset of the biocalcification crisis marked by a fertility event lasted ~ 60 kyr, and the calcium carbonate values remained low for ~ 150 kyr; the subsequent recovery of carbonate and nannoplankton lasted ~ 60 kyr and > 550 kyr, respectively. Additionally, a link between the biocalcification crisis, the seawater palaeotemperature, and the carbon isotope steps can be demonstrated. This covariance provides compelling evidence of fundamental change in the response of the climatic warming and the carbon cycle systems triggering the biocalcification crisis. These observations indicate that the biocalcification crisis can be regarded as a direct or indirect consequence of a global warming. Moreover, a deficiency of the biological pump is proposed here, as a complementary causal mechanism for explaining the negative carbon-isotope excursion.

RETRACTED: Pattern and timing of the Early Jurassic calcareous nannofossil crisis

abstract Article history:Received 27 April 2014Received in revised form 12 June 2014Accepted 23 June 2014Available online 30 June 2014Keywords:Early ToarcianCalcareous nannofossilsBiocalcification crisisCarbonate production TheToarciancalcareousnannofossilcrisisassociatedwiththeearlyToarcianOceanicAnoxicEvent(T-OAE)intheEarlyJurassicperiodisthoughttorepresentoneofthemostimportantbiocalcificationcrisesduringtheMesozo-ic,occurringsimultaneouslywithaprofounddisturbanceofthecarboncycle.However,thecausesarestillunderdebate, particularly with regard to the pattern, timing of the biocalcification crisis, relative roles of intrinsic andextrinsic processes as drivers of the crisis, and also causal mechanisms of the T-OAE.In this study, a new quantification of Toarcian calcareous nannofossil abundance and size is presented for theSancerre borehole (Paris Basin, France). Beyond the recognition of a severe biocalcification crisis defined bythe major drop in abundance, and the reduction in size of the most important pelagic carbonate producerSchizosphaerella punctulata, for the first time, this study proposes an insight into the pace and timing of thenannoplanktoncrisis.AtSancerre,thecarbonateproductionofthelowerToarciansedimentspreviouslyattribut-ed to obliquity forcing of climate allows to estimate a duration of ~210 kyr for the biocalcification crisis andof ~120 kyr for the shift towards lower carbon isotope values. The onset of the biocalcification crisis marked bya fertility event lasted~60kyr,andthe calciumcarbonate valuesremainedlow for ~150 kyr;thesubsequent re-covery of carbonate and nannoplankton lasted ~60 kyr and N550 kyr, respectively. Additionally, a link betweenthe biocalcification crisis, the seawater palaeotemperature, and the carbon isotope steps can be demonstrated.This covariance provides compelling evidence of fundamental change in the response of the climatic warmingand the carbon cycle systems triggering the biocalcification crisis. These observations indicate that thebiocalcification crisis can be regarded as a direct or indirect consequence of a global warming. Moreover, a defi-ciency of the biological pump is proposed here as a complementary casual mechanism for explaining thenegative carbon-isotope excursion.© 2014 Elsevier B.V. All rights reserved.Contents1. Introduction 572. TheSancerreborehole:geologicalsettingandpalaeogeographyoftheParisBasin 573. Materialandmethods 583.1. Quantificationofnannofossilabundanceandvolume 583.2. Analyticalanalysis 584. Results 594.1. Calcareousnannofossilpreservation 594.2. Stratigraphicdistributionofthecalcareousnannoplankton 594.3. Clusterandprincipalcomponentanalyses 594.4. Evolution of Schizosphaerella size 605. Discussion 605.1. ThecalcareousnannoplanktonresponsetotheT-OAEenvironmentalchanges 605.1.1. Patternsinnannofossilassemblageswithinpre-eventstableenvironmentalconditions 605.1.2. TheT-OAEperturbationasrecordedbynannofossilassemblages 605.1.3. Theenvironmentalrecoveryasrecordedbynannofossilassemblages 605.2. Expressionandevolutionofthebiogeniccarbonateproduction 615.3. AtimescaleforthenannoplanktoncrisisacrosstheT-OAE 61

Isotopic and REE evidence for the paleoenvironmental evolution of the late Ediacaran Dengying Section, Ningqiang of Shaanxi Province, China

Weakly mineralized taxa are most likely the ancestors of higher organisms that existed during the Cambrian explosion, thereby representing an important yet least understood evolutionary link between simple and complex life. An investigation of the seawater temperature and the carbon cycle during the late Ediacaran, in the time span of the emergence of weakly mineralized taxa, contributes to the deeper understanding of the environmental conditions existing during the early stages of animal skeleton mineralization. Towards this end, we conducted the analyses on the carbon and oxygen isotope composition, petrography, major elements, and trace elements, involving the rare earth elements (REEs) variations in the carbonate rocks of Ningqiang County in Shaanxi Province of China. The samples from the Dengying Formation (Fm.) include the Gaojiashan and Beiwan Members, in ascending order. In addition, the Gaojiashan Member hosts one of the most diverse collections of biota with weakly mineralized skeletons discovered to date. Our data shows that a positive carbon excursion, high δ18O values, positive correlation between δ13C and δ18O values, high REE concentrations, a distinctive MREE-enriched pattern, high Fe concentrations and slightly negative Ce anomalies in the Gaojiashan Member. These evidences indicate a restricted warm sea environment with an influx of river water or groundwater. Conversely, the low carbon isotope values, high δ18O values and its heterogeneity, the absence of correlation between carbon and oxygen, combined with a seawater-like REE distribution pattern in the Beiwan Member suggested that a restrict, shallow, slightly cool and/or arid sea environment condition that was subject to periodic perturbations in the hydrologic balance. Therefore, we surmised that the environmental differences between the Gaojiashan and the Beiwan Members probably resulted in the preservation of a number of animal skeleton fossils in the middle section of the Dengying Fm.