Low δ13C Values Research Articles

Carbon and oxygen isotope microanalysis of calcite in the Permian Kupferschiefer system, Saale subbasin, eastern Germany

The Kupferschiefer district in Central Europe contains some of the world's highest-grade sediment-hosted stratiform Cu (SSC) deposits. In the Saale subbasin (eastern Germany), high-grade sulfides formed via replacement of calcite cement in the continental sandstones of the uppermost Rotliegend (S1), the overlying organic-rich marine mudstones of the Kupferschiefer (T1) and the Zechstein Limestone (Ca1) units. The spatial distribution of the calcite cement, therefore, had a fundamental role to play in the Cu mineralizing system. In this study, we investigate the origin of the calcite cement (and crosscutting calcite veins) using detailed petrography (cathodoluminescence, CL; scanning electron microscopy, SEM), major element chemistry (electron probe microanalyzer, EPMA), and secondary ion mass spectrometry (SIMS) microanalyses of δ13C and δ18O values in drill core samples (n = 47) from the Saale subbasin. The calcite cement in the S1, T1 and Ca1 has a similar CL response and major element chemistry, suggestive of a common origin. Overlapping δ13C and δ18O values in calcite cement in samples from the S1 and T1 in the Sangerhausen and Wallendorf drill cores also suggest that the calcite cement was derived from fluids of similar composition. The low δ13C values of calcite cement in samples from the S1 (−13‰ to 4.3‰, VPDB) and T1 (−10‰ to 0.7‰) indicates carbonate alkalinity was sourced mainly from seawater-derived fluids and the oxidation of organic matter. The wide range of δ18O values in the calcite cement in the S1 (∼18‰ to 31‰, VSMOW) and T1 (∼ 22‰ to 31‰) samples suggest they are derived from pore fluids with a chemical composition influenced by early diagenetic alteration of detrital clasts, mainly dissolution of volcanic rock fragments, with minor contributions from the influx of meteoric waters and evaporated seawater. The negative δ13C values (down to −15‰) in calcite veins from the T1 and Ca1 indicate sources of carbonate alkalinity derived from organic matter degradation. Our data demonstrate that no isotopic hydrothermal alteration haloes can be inferred from the δ13C and δ18O values in calcite cement associated with the high-grade sulfide mineralization. The lack of systematic isotopic variability in the calcite cement likely indicates the mineralizing fluid flux or temperature was not sufficient to overprint the background sources of isotopic variability, which may help to explain the modest size of SSC deposits in this part of the Kupferschiefer district.

Long-Term Camelid Husbandry and Agricultural Intensification in the Southern Nasca Region, Peru: Insight from Faunal Isotopes

Abstract We examined stable isotopes (δ13C, δ15N, and δ34S) of camelid, cavid, and cervid remains from Upanca, an archaeological site located in the Southern Nasca Region on the south coast of Peru. Occupation at the site began in the Middle Archaic (around 3200–3000 BC) and continued through the Nasca period (AD 100–650). Remains predating 2500 BC show low δ13C and δ15N values, whereas remains after this time show increasing and especially more variable isotopic values. We interpret this pattern as marking both a process of agricultural intensification and camelid husbandry diversification. Agricultural intensification began first with C3 plants in fertilized fields, beginning around 2200 cal BC, followed by an increasing use of C4 plants (maize, kiwicha, or both), particularly after 800 cal BC. By the beginning of the first millennium, people were using a diverse range of strategies to raise llamas and alpacas, including feeding them wild or cultivated C3 plants, feeding them cultivated C4 plant foods, mixing C3 and C4 plant foods, foddering some in natural coastal environments, and acquiring still other camelids by hunting wild stocks (guanaco, vicuña). Data also suggest that cavids were consuming at least some C4 products after 1000 cal BC and that the use of C4 plants increased over time.

Methane-flow system within the Nyegga pockmark field, offshore mid-Norway

We investigated fluid seepage within the Nyegga pockmark field (600–900 m water depths) off mid-Norway from Remotely Operated Vehicle dives at the so-called CNE sites (CNE01 to CNE17). The seafloor morphology of some of these features corresponds to pockmarks and adjacent ridges, with the latter being the focus of present seepage activity. These structures are underlain by chimneys above a gas-charged zone with, in some cases, a substantial body of hydrate-invaded sediment (down to 1.3 s in two-way travel time at CNE03). Present-day methane-rich fluid seepage through the seabed is indicated by chemosynthetic fauna, in particular Siboglinidae polychaetes (Oligobrachia haakonmobiensis webbi and Sclerolinum contortum), microbial mats and associated Rissoidae gastropod (Alvania sp.) grazers, and confirmed by measured in situ bottom-water methane anomalies, up to 2,130 nL/L. No free-gas bubble emissions were observed or acoustically identified. The presence of authigenic carbonates reveals past seepage with very low δ13C values (down to −58‰) indicating that the major source of carbon was methane carried by the venting fluids. The ages of major periods of methane venting are provided by vesicomyid bivalve shells (Isorropodon nyeggaensis) present in two sedimentary layers, 14,930 and 15,500 14C yr BP (ca. 17,238 and 17,952 cal yr BP), respectively, corresponding to the time of Melt Water Pulse IA. The seafloor morphology and pattern of seepage -chemosynthetic fauna and microbial mat distribution and dissolved methane concentration-are remarkably heterogeneous. Pore-water chemistry profiles in a gravity core taken only 40 m from major seepage sites indicate no seepage and anaerobic methane oxidation at a sub-bottom depth of about 2 m. Present-day seepage from the studied pockmark-chimney fluid-flow system charged with gas hydrate is dominated by the advection of methane solution in pore water. Some of this methane could result from the dissolution of hydrate in the chimney, most of which would have formed during an earlier period (post-LGM times) of history of the chimney, when it was venting free gas. However, the presence of free gas beneath this chimney is probably why the water entering the chimney is already saturated with methane and the process of hydrate formation in the chimney continues today.

Fibrous dolomite formation at a Miocene methane seep may reflect Neoproterozoic aragonite-dolomite sea conditions

Fibrous dolomite widely formed in Neoproterozoic marine sedimentary environments, but apparently disappeared in the Phanerozoic. Here, fibrous dolomite is recognised in a Miocene methane seep limestone (Marmorito, Italy) by synchrotron X-ray diffraction, length slow crystal optics and primary zonation under cathodoluminescence, which is unexpected. Low δ13C values and their negative correlation with MgCO3 contents indicate a formation driven by highly alkaline pore waters and catalysis of dissolved sulphide generated by sulphate-driven anaerobic oxidation of methane. Cementing cavities of reefal carbonate, Neoproterozoic fibrous dolomite might have formed under sulphate-reducing conditions like Quaternary reef microbialites. Since the cavities of Neoproterozoic reefs were restricted microenvironments, the formation of fibrous dolomite was possibly favoured by catalysis similar to its Miocene seep counterpart. Our findings reinforce the concept of penecontemporaneous dolomite formation by sulphide catalysis and contribute to our understanding of the environmental conditions of the Neoproterozoic.

Theory and classification of mass extinction causation.

Theory regarding the causation of mass extinctions is in need of systematization, which is the focus of this contribution. Every mass extinction has both an ultimate cause, i.e. the trigger that leads to various climato-environmental changes, and one or more proximate cause(s), i.e. the specific climato-environmental changes that result in elevated biotic mortality. With regard to ultimate causes, strong cases can be made that bolide (i.e. meteor) impacts, large igneous province (LIP) eruptions and bioevolutionary events have each triggered one or more of the Phanerozoic Big Five mass extinctions, and that tectono-oceanic changes have triggered some second-order extinction events. Apart from bolide impacts, other astronomical triggers (e.g. solar flares, gamma bursts and supernova explosions) remain entirely in the realm of speculation. With regard to proximate mechanisms, most extinctions are related to either carbon-release or carbon-burial processes, the former being associated with climatic warming, ocean acidification, reduced marine productivity and lower carbonate δ13C values, and the latter with climatic cooling, increased marine productivity and higher carbonate δ13C values. Environmental parameters such as marine redox conditions and terrestrial weathering intensity do not show consistent relationships with carbon-cycle changes. In this context, mass extinction causation can be usefully classified using a matrix of ultimate and proximate factors. Among the Big Five mass extinctions, the end-Cretaceous biocrisis is an example of a bolide-triggered carbon-release event, the end-Permian and end-Triassic biocrises are examples of LIP-triggered carbon-release events, and the Late Ordovician and Late Devonian biocrises are examples of bioevolution-triggered carbon-burial events. Whereas the bolide-impact and LIP-eruption mechanisms appear to invariably cause carbon release, bioevolutionary triggers can result in variable carbon-cycle changes, e.g. carbon burial during the Late Ordovician and Late Devonian events, carbon release associated with modern anthropogenic climate warming, and little to no carbon-cycle impact due to certain types of ecosystem change (e.g. the advent of the first predators around the end-Ediacaran; the appearance of Paleolithic human hunters in Australasia and the Americas). Broadly speaking, studies of mass extinction causation have suffered from insufficiently critical thinking-an impartial survey of the extant evidence shows that (i) hypotheses of a common ultimate cause (e.g. bolide impacts or LIP eruptions) for all Big Five mass extinctions are suspect given manifest differences in patterns of environmental and biotic change among them; (ii) the Late Ordovician and Late Devonian events were associated with carbon burial and long-term climatic cooling, i.e. changes that are inconsistent with a bolide-impact or LIP-eruption mechanism; and (iii) claims of periodicity in Phanerozoic mass extinctions depended critically on the now-disproven idea that they shared a common extrinsic trigger (i.e. bolide impacts).

First record of Oceanic Anoxic Event 2 in the Eastern Carpathians: implications for chemostratigraphic and biostratigraphic correlations

This study aims to investigate a marlstone and claystone succession located at the southern end of the Eastern Carpathians, a region where oceanic anoxic event 2 (OAE2) has not been pointed out so far. Toward the upper half of this succession, a 17-cm-thick black shale was identified. The investigated depositional interval lies within the late Cenomanian–early Turonian, encompassing the UC3d up to UC7 nannofossil zones. The δ13C values fluctuated between 2.06‰ and 3.89‰, showing a positive isotope excursion that was assigned to OAE2. The δ13C isotope curve displays the following intervals: pre-excursion, first build-up, trough, second build-up, plateau, and post-excursion. Within the second build-up interval of OAE2, a substantial shift in CaCO3 values, accompanied by high concentrations of total organic carbon and a significant decline in the abundance and diversity of calcareous nannofossil assemblages, was observed. The nannofossil turnover related to OAE2 climax revealed predominance of Watznaueria barnesiae and temporary disappearance from the record of surface-water higher fertility taxa, such as Biscutum constans, Zeugrhabdotus erectus, and Discorhabdus ignotus. Above OAE2, peaks of Eprolithus floralis, followed by increased abundance of Eiffellithus turriseiffelii and Nannoconus spp., were identified. In the lower part of the studied succession (i.e., the upper Cenomanian UC3d nannofossil subzone), during the pre-excursion characterized by low δ13C values and less negative δ18O values, a small group of nannofossils more related to mid- and high-paleolatitudes, such as Crucibiscutum salebrosum, Repagulum parvidentatum, and Seribiscutum primitivum, is present, always showing a low abundance.

Constraining the sources and cycling of dissolved inorganic carbon in an alpine river, eastern Qinghai-Tibet Plateau

It is generally acknowledged that riverine dissolved inorganic carbon (DIC) behaviors play a critical role in global carbon cycling and hence have an impact on climate change. However, little is known about the intricate DIC dynamics under various meteorological conditions in the alpine areas. Here, we investigated DIC biogeochemical processes in the Bailong River catchment, eastern Qinghai-Tibet Plateau (QTP), by combining measurements of major ions, stable and radioactive isotopic compositions of DIC (δ13CDIC and Δ14CDIC), and physiographic parameters in the Bailong River catchment. Statistics and stoichiometry analyses suggest that multiple biogeochemical processes could affect carbon cycling in the Bailong River catchment. The “old” DIC with low Δ14C values (−472.4 ± 127.8 ‰, n = 3) and stoichiometry analysis of dissolved ions showed clear evidence that carbonate weathering is primarily responsible for water chemistry in the upstream (elevation >2000 m). However, upstream samples showed that δ13CDIC increased between 5 ‰ and 11 ‰ from the theoretical mixing line, concomitant with increasing pH and decreasing pCO2, suggesting that isotopic fractionation of DIC due to CO2 outgassing may be the primary cause of the increased δ13CDIC values. Additionally, the higher Δ14C values (−285.4 ± 123.5 ‰, n = 12) in the downstream region below 2000 m suggest that allochthonous modern carbon had a great impact on DIC variations. The presence of younger DIC may have important implications for the interpretation of inorganic carbon age in downstream rivers. Our study demonstrates that physiographic conditions can regulate DIC behaviors, which can improve estimations of carbon yield and comprehension of global carbon cycle.

Effects of white-tailed eagle (Haliaeetus albicilla) nestling diet on mercury exposure dynamics in Kopački rit Nature Park, Croatia

The present study assessed for the first time the magnitude and dietary ecological source of total mercury (THg) exposure in a southern population of white-tailed eagles (Haliaeetus albicilla), an apex predator species shown valuable for environmental biomonitoring. This population depends on the Kopački rit Nature Park – the most important breeding site. We assessed THg exposure, using nestling body feathers collected between 2014–2019 (n = 72), and potential dietary ecological sources, proxied by prey remains and stable isotope analysis. Results show THg concentrations vary significantly over the years, though not showing any time trend. Prey remains analysis shows nests with aquatic prey remains to exhibit higher THg concentrations (median: 7.57 μg g−1 dw; min – max: 6.00–13.16 μg g−1 dw) compared to those with terrestrial remains (median: 3.94 μg g−1 dw; min – max: 0.28–12.04 μg g−1 dw) or evidencing a mixed diet (median: 7.43 μg g−1 dw; min – max: 3.38–12.04 μg g−1 dw). Nests with a predominant aquatic diet show elevated lower δ13C and higher δ15N values, indicating agreement between both dietary approaches. The model selection reveals a combination of year and δ15N best explain the variability in feather THg concentrations. Complementing these predictors with a dietary descriptor based on prey remains results in a poorer model fit and lowered explanatory power, similar to sexing the nestlings. The observed body feather THg concentrations (median: 6.99 μg g−1 dw; min − max: 0.27 − 17.16 μg g−1 dw) exceeded putative biogeochemical background levels (5.00 μg g−1 dw) in 71% of the nestlings, though, did not seem to exceed a threshold at which detrimental physiological effects are expected (40 μg g−1 dw). Continued monitoring is warranted as the studied population is likely exposed to a larger cocktail of contaminants while resident-protected bird areas.

A multi-proxy framework to detect insect defoliations in tree rings: a case study on pine processionary

Assessing and reconstructing the impacts of defoliation caused by insect herbivores on tree growth, carbon budget and water use, and differentiating these impacts from other stresses and disturbances such as droughts requires multi-proxy approaches. Here we present a methodological framework to pinpoint the impacts of pine processionary moth (Thaumetopoea pityocampa), a major winter-feeding defoliator, on tree cover (remote-sensing indices), radial growth and wood features (anatomy, density, lignin/carbohydrate ratio of cell walls, δ13C and δ18O of wood cellulose) of drought-prone pine (Pinus nigra) forests in north-eastern Spain. We compared host defoliated (D) and coexisting non-defoliated (ND) pines along with non-host oaks (Quercus faginea) following a strong insect outbreak occurring in 2016 at two climatically contrasting sites (cool-wet Huesca and warm-dry Teruel). Changes in tree-ring width and wood density were analyzed and their responses to climate variables (including a drought index) were compared between D and ND trees. The Normalized Difference Infrared Index showed reductions due to the outbreak of –47.3% and –55.6% in Huesca and Teruel, respectively. The D pines showed: a strong drop in growth (–96.3% on average), a reduction in tracheid lumen diameter (–35.0%) and lower lignin/carbohydrate ratios of tracheid cell-walls. Both pines and oaks showed synchronous growth reductions during dry years. In the wet Huesca site, lower wood δ13C values and a stronger coupling between δ13C and δ18O were observed in D as compared with ND pines. In the dry Teruel site, the minimum wood density of ND pines responded more negatively to spring drought than that of D pines. We argue that multi-proxy assessments that combine several variables have the potential to improve our ability to pinpoint and reconstruct insect outbreaks using tree-ring data.

Killer whales at northern Patagonia, Argentina: Evidence of different foraging groups from stable isotopes

AbstractKiller whales (Orcinus orca) are highly mobile, large marine apex predators that inhabit all oceans. Despite being the most conspicuous top predator, little is known about their ecology along Patagonia, Argentina. Here, we used carbon (δ13C) and nitrogen (δ15N) isotope analysis of bone and dentine collagen from killer whales (n = 13) stranded in northern Patagonia during the period 1970–2014 to assess their trophic ecology. A model‐based clustering analysis identified three distinct groups related with three large marine ecosystems: the Patagonia (P), Subtropical (ST), and Antarctica (A) groups. Group P (n = 8) is characterized by individuals with high δ13C and δ15N values fitting within the isotopic ratios observed over the Patagonian shelf. Group ST (n = 3) is composed of individuals with high δ13C and low δ15N values, similar to those reported for conspecifics in southern Brazil. Group A (n = 2) is composed of individuals with low δ13C and δ15N values, typical from high latitude areas like sub‐Antarctic/Antarctic waters. The finding of different killer whales' isotopic groups over the Patagonian shelf suggests the existence of individuals or groups exploiting different habitats. Our results expand the limited ecological knowledge for the species while presenting the basis to infer more complex ecological hypotheses.

Origins of sediments and fluids in submarine mud volcanoes off Tanegashima Island, northern Ryukyu Trench, Japan

Mud volcanoes are topographic features through which over-pressurized sediments are intruded upward and erupted, transporting fluids and sediments from the deep subsurface to the surface/seafloor. To understand material and biogeochemical cycling through mud volcanoes, we investigated the origins of sediments and fluids erupted by four submarine mud volcanoes off Tanegashima Island, along the northern Ryukyu Trench: MV#1–3 and MV#14. We estimated the ages of the source sequences of the mud volcano sediments to be middle to late Miocene based on nannofossils and 10Be ages. The sediments were characteristically enriched in illite. The similar mineral compositions and vitrinite reflectance values (0.42%–0.45%) of sediments erupted by the mud volcanoes strongly suggest that each is rooted in the same source sequence. Pore waters had Cl− concentrations ∼40% that of seawater and were proportionally enriched in 18O and depleted in D, indicating the addition of freshwater from the dehydration of clay minerals. However, the smectite and illite contents (<40%) in the clay size fraction ruled out in situ smectite dewatering as the cause of the pore water Cl− dilution. Thus, fluids derived from clay dewatering must have originated from deeper than the source sequence of the mud volcano sediments. Vertical Cl− profiles indicate that the upward fluid advection rate and eruption frequency decrease from MV#3 to MV#2, MV#1, and MV#14 (MV#14 being dormant). At the active mud volcanoes, the C1/C2 ratios (<100) and methane δ13C values (−56‰ to −42‰) indicate that hydrocarbon gases are mostly derived from the thermal decomposition of organic matter in deep sediments where the in situ temperature exceeds 80°C. At the dormant MV#14, high C1/C2 ratios (700–4,000) and low methane δ13C values (ca. −75‰) suggest the limited supply of thermogenic methane and the subsequent shallow methanogenesis. Because vitrinite reflectance values indicate that the source sediments are too immature to produce thermogenic hydrocarbons, the hydrocarbon gases, like the fluids derived from clay dewatering, were probably supplied from deeper than the source strata. The supply of deep fluids into the source sequence might be a universal phenomenon in subduction zones, and may play an important role in mud volcanism and associated biogeochemical cycling.

Effect of marine sediment on the phase partitioning and isotopic content of riverine DOC

AbstractRivers discharge significant quantities of dissolved organic carbon (DOC) to the ocean, yet biomarker and isotope studies suggest that terrigenous DOC makes up only a small amount DOC in the ocean. One of the removal pathways proposed for riverine DOC is sorption to marine sediments. This process is chemically selective, but whether sorption alters the isotopic composition of riverine DOC is unknown. Because there is isotopic variability across different organic compound classes, sorptive removal of DOC could also alter the isotopic composition of DOC. As a first step in addressing this question, we examined phase partitioning and isotopic composition of a riverine DOC standard in the presence of marine sediment particles. In a series of controlled experiments, the standard was mixed with marine sediment in 35‰ NaCl solution, then separated into particulate and dissolved phases for analyses of mass, δ13C, and ∆14C of organic carbon (OC). Across a range of sediment OC to DOC mass ratios (from < 0.1 to ~ 3), we found that: (1) sediment sorbed 0.8 μg OC per mg of sediment; and (2) DOC compounds with higher ∆14C and lower δ13C values relative to the bulk DOC was preferentially removed from solution. In effect, mixing a riverine DOC standard with marine sediment resulted in increased ∆14C and decreased δ13C of the DOC that remained in solution. These results show that sorption of DOC to sediment can alter the isotopic content of riverine DOC.

What can hippopotamus isotopes tell us about past distributions of C4 grassy biomes on Madagascar?

Societal Impact StatementToday, expansive C4 grassy biomes exist across central, western, and northern Madagascar. Some researchers have argued that the island's now‐extinct pygmy hippopotamuses belonged to a megaherbivore grazing guild that maintained these grasslands prior to human arrival. However, the chemistry of hippo bones indicates that C4 grasses were only a minor part of hippo diet. This, in turn, suggests that C4 grasses were present but not widespread when hippos were alive and that grasses expanded only after Malagasy people shifted from hunting and foraging to agropastoralism approximately 1000 years ago. These results have important implications for environmental reconstructions and biodiversity management.Summary Extinct hippopotamuses (Hippopotamus spp.) were part of Madagascar's megaherbivore guild. Stable carbon (δ13C) and nitrogen (δ15N) isotopes in radiometrically dated bone collagen track spatial and temporal variation in diet and habitat. If hippos helped maintain C4 grassy biomes, then they should have regularly consumed C4 grasses, which have high δ13C values. However, if expansive C4 grassy biomes are anthropogenic, then forests would have been more extensive in the past, and hippos would have predominantly consumed C3 plants with low δ13C values. Nitrogen isotopes can clarify foraging habitat (moist or dry). We assessed δ13C and δ15N values for hippos from different ecoregions of Madagascar and compared these with data for extinct herbivorous lemurs from the same ecoregions. We further explored the effects of wet/dry transitions on isotopic trends for hippos from the central highlands and spiny thicket ecoregions. Carbon isotopes suggest (1) limited C4 consumption by hippos in the central highlands, dry deciduous forest, and succulent woodland ecoregions; and (2) moderate consumption of C4 resources in the spiny thicket. Nitrogen data indicate that hippos foraged in wetter habitats than sympatric lemurs in all regions. Malagasy hippos did not regularly graze C4 grasses in dry, open habitats, even in regions blanketed by C4 grassy biomes today. Malagasy grasses are adapted to grazing and fire, but these are likely ancient adaptations that accompanied grasses when they initially spread to Madagascar. C4 grassy biomes were spatially limited in extent in the past and only expanded after the Late Holocene introduction of domesticated ungulates.

Stable isotopic and stomach content analyses reveal changes in the trophic level and feeding habit of large-head hairtail (Trichiurus lepturus) in the northern South China Sea

The feeding habit of large-head hairtail (Trichiurus lepturus) in the northern South China Sea was investigated through isotopic and stomach content analyses. The isotopic features of the hairtail at the same body size differed among regions, with the fish in coastal waters presenting higher δ15N and δ13C values compared to those in the open sea, indicating different trophic levels (TL), food habits, and isotopic baselines. According to the partial correlation of water depth with δ15N values, the sampling stations were divided into three regions based on the depth of water: coastal (20–40 m), near coastal (60–80 m), and open sea (100–200 m) regions. In the coastal region, the hairtail from stations affected by the Pearl River plume exhibited lower δ15N and δ13C values. The stomach content analysis indicated different feeding habits of the hairtail from different regions. The hairtail in the coastal and near coastal waters fed more on fish and less on crustaceans compared to the hairtail in the open sea. The relationship between δ15N and fish size exhibited two contrary patterns. First, the δ15N values increased with increasing preanal length in the hairtail sampled from the water depth of 30–40 m in section F (in fish with preanal length < 200 mm) and those samples from the water depth of 100–200 m. This finding reflected an ontogenetic shift in diet and TL. However, the δ15N values tended to decrease with the increasing preanal length of the hairtail samples collected from the water depth of 30–40 m in section F (fish with a preanal length of ~200–300 mm). These findings suggested that under the conditions of insufficient availability of high-quality prey, the larger hairtail fed more on low-TL prey to compensate for the increase energy demand, arising due to growth, which led to the observed decrease in δ15N values.

Experimental precipitation of cryogenic carbonate

Cryogenic cave carbonates (CCC) represent a specific type of speleothem precipitating from freezing water in caves. Over the past decade, considerable progress has been made concerning cryogenic calcite petrography, crystallography and geochemistry. Uncertainties remain, however, as the cave waters from which ancient cryogenic calcites form are not preserved. The present study provides an experimental approach to re-calculate the isotopic composition of the precipitating solution using CCC data. Calcium-rich bicarbonate water prepared by bubbling CO2 gas with a very low δ13C value (∼ −35 ‰) into the water was cooled under controlled laboratory conditions to account for the water chemistry changes during freezing. The experiments yielded cryogenic calcite and vaterite precipitating at temperatures of +1, +0, −0.5, −0.7, −1 and − 2 °C, with complete freezing occurring at −0.7, −1 and − 2 °C and partial freezing at −0.5 °C. The δ18Owater and δ13CDIC values, pH, electrical conductivity and alkalinity were recorded before and after the experiments. Our work documents an increase in pH, suggesting CO2 degassing eventually reaches supersaturation with calcite, leading to CaCO3 precipitation. Calcite precipitation rates are lower at the longer experiment (> 45 days). This is further confirmed by the decreasing calcite saturation index (SIcc) with time. Carbon isotope analyses of the water and carbonates revealed kinetic effects during rapid freezing. A large increase in Δ13CDIC values of the water (up to about 30 ‰) was found between the start and end of the experiments. Reasons may include CO2 degassing with increasing time leading to a markedly 13C-enriched solution. Based on our experimental data, the cryogenic crystal morphotypes are related to the precipitation sequence. Sharp-edged rhombohedra (calcite) precipitate first. Subsequently, spherulitic (vaterite) morphotypes precipitate from almost completely frozen water characterised by a high SIcc. This first experimental work dealing with cryogenic carbonate precipitates and their stable isotope composition sheds light on the origin of these peculiar speleothems and provides constraints to interpret morphologies and isotopic compositions of ancient CCCs.

Fatty acid isotopic composition in Atlantic pollock is not influenced by environmentally relevant dietary fat concentrations.

The application of fatty acid (FA) isotopic analysis has great potential in elucidating food web structure, but it has not experienced the same wide-spread use as amino acid isotopic analyses. The failure to adopt FA isotopic methods is almost certainly linked to a lack of reliable information on trophic fractionation of FA, particularly in higher predators. In this work, we attempt to address this shortfall, through comparison of FA δ13C values in captive Atlantic pollock (Pollachius virens) liver and their known diets. Since catabolism is likely the main cause of fractionation and it may vary with dietary fat content, we investigated the impact of dietary fat concentration on isotopic discrimination in FA. We fed Atlantic pollock three formulated diets with similar FA isotopic compositions but different fat concentrations (5-9% of diet), representative of the range found in natural prey, for 20weeks. At the conclusion of the study, δ13C values of liver FA were very similar to the FA within the corresponding diets, with most discrimination factors < 1. For all FA except 22:6n-3, dietary fat had no effect on discrimination factors. Only for 22:6n-3 did fish fed the highest fat diet have lower δ13C values than the diet consumed. Thus, these FA-specific discrimination factors can be applied to evaluate diets in marine fish consuming natural diets and will serve as additional and valuable biomarkers in fish feeding ecology.

Interactions between distal epiclastic and bio‐chemogenic sedimentation at the foothills of a mafic alkaline volcano: The case of the Oligocene Doupovské Hory Volcanic Complex (Czech Republic)

AbstractLate Oligocene (ca 25 Ma) volcano‐sedimentary successions exposed on the western periphery of the Doupovské Hory Volcanic Complex reveal a complex sedimentation history influenced in various ways by decay of the alkali basalt volcanic edifice. Weathering of the volcanic rocks supplied abundant reactants that promoted carbonate precipitation in the peripheral palaeolakes—as evidenced by strongly non‐radiogenic 87Sr/86Sr values (0.7038–0.7041). On the other hand, the sediments of the initial shallow lake became deformed by the bulldozing effect of a debris avalanche. The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano. At this stage, surface water influx from the surrounding granites conferred a more radiogenic character (87Sr/86Sr values 0.7046–0.7049) to the calcrete deposits. Fossil assemblages as well as limestone textures suggest significant seasonal water‐level fluctuations, possibly reflecting the alternating rainy and dry‐seasons of a prevalently humid Central‐European Late Oligocene climate. The seasonal drying out of the ponds resulted in significant 18O enrichments. Although the ca 0‰ δ13C values might suggest mixing of atmospheric and volcanic CO2 during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity. The lower δ13C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present. This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non‐radiogenic Sr isotopic ratios (87Sr/86Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates. Contrary to the notion of carbonatites being present when highly non‐radiogenic Sr isotopes are found, these results show that Sr isotopes in carbonates formed in alkali basalt‐sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.

Bacterial symbiont diversity in Arctic seep Oligobrachia siboglinids

BackgroundHigh latitude seeps are dominated by Oligobrachia siboglinid worms. Since these worms are often the sole chemosymbiotrophic taxon present (they host chemosynthetic bacteria within the trophosome organ in their trunk region), a key question in the study of high latitude seep ecology has been whether they harbor methanotrophic symbionts. This debate has manifested due to the mismatch between stable carbon isotope signatures of the worms (lower than -50‰ and usually indicative of methanotrophic symbioses) and the lack of molecular or microscopic evidence for methanotrophic symbionts. Two hypotheses have circulated to explain this paradox: (1) the uptake of sediment carbon compounds with depleted δC13 values from the seep environment, and (2) a small, but significant and difficult to detect population of methanotrophic symbionts. We conducted 16S rRNA amplicon sequencing of the V3-V4 regions on two species of northern seep Oligobrachia (Oligobrachia webbi and Oligobrachia sp. CPL-clade), from four different high latitude sites, to investigate the latter hypothesis. We also visually checked the worms’ symbiotic bacteria within the symbiont-hosting organ, the trophosome, through transmission electron microscopy.ResultsThe vast majority of the obtained reads corresponded to sulfide-oxidizers and only a very small proportion of the reads pertained to methane-oxidizers, which suggests a lack of methanotrophic symbionts. A number of sulfur oxidizing bacterial strains were recovered from the different worms, however, host individuals tended to possess a single strain, or sometimes two closely-related strains. However, strains did not correspond specifically with either of the two Oligobrachia species we investigated. Water depth could play a role in determining local sediment bacterial communities that were opportunistically taken up by the worms. Bacteria were abundant in non-trophosome (and thereby symbiont-free) tissue and are likely epibiotic or tube bacterial communities.ConclusionsThe absence of methanotrophic bacterial sequences in the trophosome of Arctic and north Atlantic seep Oligobrachia likely indicates a lack of methanotrophic symbionts in these worms, which suggests that nutrition is sulfur-based. This is turn implies that sediment carbon uptake is responsible for the low δ13C values of these animals. Furthermore, endosymbiotic partners could be locally determined, and possibly only represent a fraction of all bacterial sequences obtained from tissues of these (and other) species of frenulates.

Organic matter sources and flows in tundra wetland food webs.

Arctic lowland tundra is often dominated by wetlands. As numbers and types of these wetlands change with climate warming, their invertebrate biomass and assemblages may also be affected. Increased influx of nutrients and dissolved organic matter (DOM) from thawing peat may alter the relative availability of organic matter (OM) sources, differentially affecting taxa with disparate dependence on those sources. In five shallow wetland types (<40 to 110 cm deep) and in littoral zones of deeper lakes (>150 cm), we used stable isotopes (δ13C, δ15N) to compare contributions of four OM sources (periphytic microalgae, cyanobacteria, macrophytes, peat) to the diets of nine macroinvertebrate taxa. Living macrophytes were not distinguishable isotopically from peat that likely contributed most DOM. Within invertebrate taxa, relative OM contributions were similar among all wetland types except deeper lakes. Physidae snails consumed substantial amounts of OM from cyanobacteria. However, for all other taxa examined, microalgae were the dominant or a major OM source (39-82%, mean 59%) in all wetland types except deeper lakes (20‒62%, mean 31%). Macrophytes and macrophyte-derived peat, likely consumed mostly indirectly as DOM-supported bacteria, ranged from 18‒61% (mean 41%) of ultimate OM sources in all wetland types except deeper lakes (38-80%, mean 69%). Invertebrate consumption of microalgal C may often have involved bacterial intermediates, or a mix of algae with bacteria consuming peat-derived OM. High production of periphyton with very low δ13C values were favored by continuous daylight illuminating shallow depths, high N and P levels, and high CO2 concentrations from bacterial respiration of peat-derived DOM. Although relative OM sources were similar across wetland types except deeper lakes, total invertebrate biomass was much higher in shallow wetlands with emergent vegetation. Impacts of warming on the availability of invertebrate prey to waterbirds will likely depend not on shifts in OM sources, but more on changes in overall number or area of shallow emergent wetlands.

Formation process of Pliocene cold seep carbonates from the southern Western Foothills, Southwestern Taiwan: A synthetical rare earth element and C–O–Sr–Nd isotope study

In this work, the Pliocene dolomitic carbonate concretions and pipes from Chiahsien of the Western Foothills, southwestern Taiwan were studied. We conducted a synthetical analysis of stable carbon and oxygen isotopes, rare earth element (REE) contents, and Sr and Nd isotopes, to trace the compositions and sources of pore fluids from which carbonates have precipitated and reconstruct the carbonate formation processes. The carbonates with low δ13C values (as low as −48.6‰) are promoted by the anaerobic oxidation of biogenic methane. The 87Sr/86Sr ratios of samples reflect the mixing of coeval bottom seawater and seep fluid enriched in 87Sr. The Nd isotopic signatures of studied carbonates indicate multiple Nd sources, including detrital components, Fe–Mn oxyhydroxides, organic matter, and seawater. The MREE-bulge patterns in samples are typical of anoxic conditions, which are supported by Ce anomalies. Our results demonstrate that the seepage intensity played a key role in the formation processes of concretions and pipes by controlling the depth of the sulfate-methane transition zone (SMTZ). The concretions characterized by the lowest δ13C values formed at a deeper depth in sediments under low methane flux, which suffered no/less influence of seawater. The representative pipe with obvious concentric lamination grew in a rim-to-core pattern. In the early stage, the variable flux of highly channelized seepage caused the fluctuation of carbon and oxygen isotopes. During the later stage, the modification of isotopic compositions in the core part were resulted from meteoric alteration. Our work provides insights into the formation process related to seepage evolution in ancient cold seeps on Taiwan Island.