Isotopic biosignatures in carbonate‐rich,cyanobacteria‐dominated microbial mats of the Cariboo Plateau,B.C.

Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO₂ below equilibrium and δ¹³C_DIC values up to +6.0‰ above predicted carbon dioxide (CO₂) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ¹³C_carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ¹³C values reflected the balance between photosynthetic ¹³C‐enrichment and heterotrophic inputs of ¹³C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ¹³C_carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ¹³C_carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of ¹³C‐depleted DIC. δ¹³C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ¹³C_org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ¹³C_inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO₂‐limited photosynthesis, suggesting that Precambrian δ¹³C_org values of ~?26‰ do not necessitate higher atmospheric CO₂ concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats.     

Stable isotopic (δ13C and δ18O) signatures of biogenic calcretes marking discontinuity surfaces: a case study from Upper Cretaceous carbonates of central Dalmatia and eastern Istria,Croatia

Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their δ¹³C and δ¹⁸O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative δ¹³C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the δ¹³C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO₃ with CO₂ released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of δ¹³C (from ?13.1 to ?8.2 ‰ Vienna PeeDee Belemnite standard, VPDB) and δ¹⁸O (from ?10.1 to ?6.1 ‰ VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the δ¹³C values of biogenic calcretes with typical Microcodium aggregates (?13.1 to ?12.3 ‰ VPDB) at the ?ibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO₂. These values are expected for authigenic pedogenic carbonates formed under the influence of C₃ plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO₂. Such low δ¹³C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The δ¹³C values (?4.4 to ?3.6 ‰ VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the ?ibenik locality are more negative than the δ¹³C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall δ¹³C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions.     

Stromatolite records of environmental change in perennially ice-covered Lake Joyce,McMurdo Dry Valleys,Antarctica

Calcite-rich columnar stromatolites grew in perennially ice-covered Lake Joyce in the McMurdo Dry Valleys, Antarctica, during a period of environmental change associated with rising lake level. Stromatolite calcite contains carbon and oxygen isotope records of changes to microbial activity in response to variable light environments and water chemistry through time. The stromatolites grew synchronously with correlative calcite zones. The innermost (oldest) calcite zone has a wide range of δ¹³C_calcite values consistent with variable photosynthetic effects on local DIC ¹³C/¹²C. Subsequent calcite zones preserve a progressive enrichment in δ¹³C_calcite values of approximately + 2.6‰ through time, with δ¹³C_calcite values becoming less variable. This enrichment likely records the removal of ¹²C by photosynthesis from the DIC reservoir over decades, with photosynthetic effects decreasing as light levels became lower and more consistent through time. Mean δ¹⁸O_calcite values of the innermost calcified zone were at least 1‰ lower than those of the other calcified zones (t test p-level < 0.001). The significant difference in δ¹⁸O_calcite values between the innermost and other calcified zones could be a product of mixing source waters with different isotopic values associated with the initiation of lake stratification associated with rising lake level. Overall, Lake Joyce stromatolites record significant lateral variability in relative photosynthetic rate and long-lived lake water stratification with microbial modification of the DIC pool. Such processes provide criteria for interpreting microbial activity within polar paleolake deposits and may shed light on variability in lake environments associated with changing climate in the McMurdo Dry Valleys.     

Stable‐isotope comparisons between embryos and mothers of a placentatrophic shark species

Stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopes of Atlantic sharpnose shark Rhizoprionodon terraenovae embryos and mothers were analysed. Embryos were generally enriched in ¹⁵N in all studied tissue relative to their mothers' tissue, with mean differences between mother and embryo δ¹⁵N (i.e. Δδ¹⁵N) being 1·4‰ for muscle, 1·7‰ for liver and 1·1‰ for cartilage. Embryo muscle and liver were enriched in ¹³C (both Δδ¹³C means = 1·5‰) and embryo cartilage was depleted (Δδ¹³C mean = ?1·01‰) relative to corresponding maternal tissues. While differences in δ¹⁵N and δ¹³C between mothers and their embryos were significant, muscle δ¹⁵N values indicated embryos to be within the range of values expected if they occupied a similar trophic position as their respective mothers. Positive linear relationships existed between embryo total length (L_T) and Δδ¹⁵N for muscle and liver and embryo L_T and Δδ¹³C for muscle, with those associations possibly resulting from physiological differences between smaller and larger embryos or differences associated with the known embryonic nutrition shift (yolk feeding to placental feeding) that occurs during the gestation of this placentatrophic species. Together these results suggest that at birth, the δ¹⁵N and δ¹³C values of R. terraenovae are likely higher than somewhat older neonates whose postpartum feeding habits have restructured their isotope profiles to reflect their postembryonic diet.     

Unprecedented 34S‐enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in ³⁴S relative to ³²S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ³⁴S_pyrite values are up to +132‰V‐CDT and with a total range of 186‰. The lightest δ³⁴S_pyrite values (?54‰) suggest very large fractionation during MSR from an initial sulfate with δ³⁴S values (δ³⁴S_sulfate,0) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ³⁴S_pyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120 ± 15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ³⁴S_pyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture‐specific δ³⁴S_pyrite variability and overall average δ³⁴S_pyrite values (+11 to +16‰) lower than the anticipated δ³⁴S_sulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb–Sr dating and U/Th–He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ¹³C values in cogenetic calcite suggest that the superheavy δ³⁴S_pyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR‐related S‐isotope systematics, particularly regarding formation of large fractions of ³⁴S‐rich pyrite.     

Local paleoenvironmental controls on the carbon‐isotope record defining the Bitter Springs Anomaly

Large magnitude (>10‰) carbon‐isotope (δ¹³C) excursions recorded in carbonate‐bearing sediments are increasingly used to monitor environmental change and constrain the chronology of the critical interval in the Neoproterozoic stratigraphic record that is timed with the first appearance and radiation of metazoan life. The ~10‰ Bitter Springs Anomaly preserved in Tonian‐aged (1000–720 Ma) carbonate rocks in the Amadeus Basin of central Australia has been offered as one of the best preserved examples of a primary marine δ¹³C excursion because it is regionally reproducible and δ¹³C values covary in organic and carbonate carbon arguing against diagenetic exchange. However, here we show that δ¹³C values defining the excursion coincide with abrupt lithofacies changes between regularly cyclic grainstone and microbial carbonates, and desiccated red bed mudstones with interbedded evaporite and dolomite deposits, recording local environmental shifts from restricted marine conditions to alkaline lacustrine and playa settings that preserve negative (?4‰) and positive (+6‰) δ¹³C values, respectively. The stratigraphic δ¹³C pattern in both organic and carbonate carbon recurs within the basin in a similar way to associated sedimentary facies, reflecting the linkage of local paleoenvironmental conditions and δ¹³C values. These local excursions may be time transgressive or record a relative sea‐level influence manifest through exposure of sub‐basins isolated by sea‐level fall below shallow sills, but are independent of secular seawater variation. As the shallow intracratonic setting of the Bitter Springs Formation is typical of other Neoproterozoic carbonate successions used to construct the present δ¹³C seawater record, it identifies the potential for local influences on δ¹³C excursions that are neither diagenetic nor representative of the global exogenic cycle.     

The role of <Emphasis Type="Italic">Salvelinus</Emphasis> in contemporary studies of evolution,trophic ecology and anthropogenic change

The aim of this study was to better understand the relations between carbon and oxygen stable isotope values of ambient water, mollusc shells, macrophytes and their carbonate encrustations, commonly used in palaeolimnological studies. Water, molluscs and macrophytes were sampled from the littoral zone in Lake Lednica, NW Poland. The influence of carbon species assimilated during photosynthesis and the net intensity of photosynthesis resulting from the size of charophyte species and the density of their stands were postulated to be the most important factors causing the species-specific δ¹³C values of charophyte thalli and encrustations. It was suggested that photosynthetic activity of charophytes affected not only the δ¹³C values of charophyte encrustations but also mollusc shells by changing δ¹³C values of DIC within charophyte stands. In addition, incorporation of metabolic carbon into the shell was proposed as the main cause of both the ¹³C depletion of mollusc shells relative to δ¹³C values of DIC and the species-specific δ¹³C values of shells. Mollusc shells were precipitated at the isotope equilibrium or close to the equilibrium with δ¹⁸O values of lake water. Charophyte encrustations were found to be ¹⁸O depleted due to the kinetic isotope effects during intense photosynthesis and thus fast precipitation of the calcite.     

INVESTIGATING TROPHIC RELATIONSHIPS OF PINNIPEDS IN ALASKA AND WASHINGTON USING STABLE ISOTOPE RATIOS OF NITROGEN AND CARBON

We measured stable-nitrogen (δ¹⁵N) and stable-carbon (δ¹³C) isotope ratios in muscle and hair from 7 northern fur seals (Callorhinus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea lions (Eumetopias jubatus), and 14 harbor seals (Phoca vitulina) from the Gulf of Alaska and coast of Washington State, in order to contrast dietary information derived from isotopic vs. available conventional dietary studies. Stable-nitrogen-isotope analysis of muscle revealed that harbor seals were enriched over sea lions (mean δ¹⁵N = 18.6‰vs. 17.5‰) which were in turn enriched over northern fur seals (mean δ¹⁵N = 16.6‰). Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Pleurogrammus monopterygius), and large vs. small walleye pollock (Theregra chalcogramma). According to their δ¹⁵N values, adult male Steller sea lions showed a higher trophic position than adult females (mean δ¹⁵N: 18.0‰vs. 17.2‰), whereas adult female northern fur seals were trophically higher than juvenile male fur seals (mean δ¹⁵N: 16.5‰vs. 15.0‰). Each of these observed differences likely resulted from differential reliance on squid or differences in the size range of pollock consumed. Three northern fur seal pups showed higher δ¹⁵N enrichment over adults (mean 17.7‰vs. 15.8‰) due to their reliance on their mother's milk. Stable-carbon isotope measurements of hair revealed a cline toward more negative values with latitude. Segregation in hair δ¹³C between Steller sea lions and harbor seals off the coast of Washington (mean δ¹³C: ?13.6‰vs.?15.0‰) reflected the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable isotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.     

Isotopic enrichment in Rhizoprionodon porosus (Poey, 1861) and Carcharhinus porosus (Ranzani, 1840) embryos

Isotopic values of two Caribbean sharpnose shark Rhizoprionodon porosus litters (Poey, 1861) with two and three embryos and one litter of 11 smalltail shark Carcharhinus porosus embryos showed enriched ¹⁵N and ¹³C compared to their mothers. In R. porosus, embryonic isotope values were 3.06 ± 0.07‰ and 0.69 ± 0.15‰ greater than their mothers' for δ¹⁵N and δ¹³C, respectively, whereas in C. porosus, δ¹⁵N and δ¹³C were 1.79 ± 0.09‰ and 1.31 ± 0.17‰ greater in embryos than their mothers.     

Feeding ecology of pelagic fish larvae and juveniles in slope waters of the Gulf of Mexico

Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used to investigate feeding patterns of larval and early juvenile pelagic fishes in slope waters of the Gulf of Mexico. Contribution of organic matter supplied to fishes and trophic position within this pelagic food web was estimated in 2007 and 2008 by comparing dietary signatures of the two main producers in this ecosystem: phytoplankton [based on particulate organic matter (POM)] and Sargassum spp. Stable isotope ratios of POM and pelagic Sargassum spp. were significantly different from one another with δ¹³C values of POM depleted by 3–6‰ and δ¹⁵N values enriched by 2 relative to Sargassum spp. Stable isotope ratios were significantly different among the five pelagic fishes examined: blue marlin Makaira nigricans, dolphinfish Coryphaena hippurus, pompano dolphinfish Coryphaena equiselis, sailfish Istiophorus platypterus and swordfish Xiphias gladius. Mean δ¹³C values ranged almost 2 among fishes and were most depleted in I. platypterus. In addition, mean δ¹⁵N values ranged 4–5 with highest mean values found for both C. hippurus and C. equiselis and the lowest mean value for M. nigricans during both years. Increasing δ¹³C or δ¹⁵N with standard length suggested that shifts in trophic position and diet occurred during early life for several species examined. Results of a two‐source mixing model suggest approximately an equal contribution of organic matter by both sources (POM = 55%; pelagic Sargassum spp. = 45%) to the early life stages of pelagic fishes examined. Contribution of organic matter, however, varied among species, and sensitivity analyses indicated that organic source estimates changed from 2 to 13% for a δ¹³C fractionation change of ±0·25‰ or a δ¹⁵N fractionation change of ± 1·0‰ relative to original fractionation values.     

Constraining the formation of authigenic carbonates in a seepage‐affected cold‐water coral mound by lipid biomarkers

Cold‐water coral (CWC) mounds are build‐ups comprised of coral‐dominated intervals alternating with a mixed carbonate‐siliciclastic matrix. At some locations, CWC mounds are influenced by methane seepage, but the impact of methane on CWC mounds is poorly understood. To constrain the potential impact of methane on CWC mound growth, lipid biomarker investigations were combined with mineralogical and petrographic analyses to investigate the anaerobic oxidation of methane (AOM) and authigenic carbonate formation in sediment from a seep‐affected CWC mound in the Gulf of Cadiz. The occurrence of AOM was confirmed by characteristic lipids found within a semi‐lithified zone (SLZ) consisting of authigenic aragonite, high‐magnesium calcite and calcium‐excess dolomite. The formation of high‐Mg calcite is attributed to AOM, acting as a lithifying agent. Aragonite is only a minor phase. Ca‐excess dolomite in the SLZ and upper parts may be formed by organoclastic sulphate reduction, favouring precipitation by increased alkalinity. The AOM biomarkers in the SLZ include isoprenoid‐based archaeal membrane lipids, such as abundant glycerol dibiphytanyl glycerol tetraethers (GDGTs) dominated by GDGT‐2. The δ¹³C values of GDGT‐2, measured as ether‐cleaved monocyclic biphytanes, are as low as ?100‰ versus V‐PDB. Further, bacterial dialkyl glycerol diethers with two anteiso‐C₁₅ alkyl chains and δ¹³C values of ?81‰ are interpreted as biomarkers of sulphate‐reducing bacteria. The lipid biomarker signatures and mineralogical patterns suggest that anaerobic methane‐oxidizing archaea of the ANME‐1 group thrived in the subsurface at times of slow and diffusive methane seepage. Petrographic analyses revealed that the SLZ was exhumed at some point (e.g. signs of bioerosion of the semi‐lithified sediment), providing a hard substrate for CWC larval settlement. In addition, this work reveals that AOM‐induced semi‐lithification likely played a role in mound stabilization. Lipid biomarker analysis proves to be a powerful tool to disentangle early diagenetic processes induced by microbial metabolisms.     

Global calibration of ecological models for planktic foraminifera from coretop carbonate oxygen-18

A global dataset of coretop planktic foraminiferal δ¹⁸O_c is combined with a global database of seawater δ¹⁸O_w observations and ocean climatologies to determine robust optimum parameters for an ecological model for six commonly measured species and varieties. These parameters consist of the temperature ranges, optimum temperatures, depth habitat, and the amount of secondary calcification for Globigerinoides ruber (white), G. ruber (pink), Neogloboquadrina pachyderma (l), N. pachyderma (r), Globigerinoides sacculifer, and Globigerina bulloides. This approach produces ecological models and temperature ranges consistent with previous work, and manages to reproduce the coretop oxygen-18 carbonate values remarkably well. The standard error of modelled values of coretop calcite globally and for all species is 0.53‰, compared to an error of 1.2‰ when assuming annual average mixed layer equilibrium calcite.     

Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China

The foliar stable N isotope ratio (δ¹⁵N) can provide integrated information on ecosystem N cycling. Here we present the δ¹⁵N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ¹⁵N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ¹⁵N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more ¹⁵N enriched. Our results show that foliar δ¹⁵N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO₃ ^? had low δ¹⁵N (?11.4 to ?3.2 ‰) and plant NO₃ ^? uptake could not explain the negative foliar δ¹⁵N values (NH₄ ⁺ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH₄ ⁺ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH₃/NH₄ ⁺. The variation in foliar δ¹⁵N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ¹⁵N and the enrichment factor (foliar δ¹⁵N minus soil δ¹⁵N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ¹⁵N between primary and secondary forests.     

Stable isotope enrichment in laboratory ant colonies: effects of colony age,metamorphosis, diet,and fat storage

Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ¹³C and δ¹⁵N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ¹³C, but not δ¹⁵N; larvae, pupae, and workers collected at 75 days were slightly depleted in ¹³C relative to collections at 15 days (Δδ¹³C = ?0.27‰). Metamorphosis had a significant effect on δ¹⁵N, but not δ¹³C; tissues of each successive developmental stage were increasingly enriched in ¹⁵N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ¹³C, but treatment with a non‐polar solvent resulted in enriched δ¹⁵N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using ¹⁵N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.     

Environmental insights from high‐resolution (SIMS) sulfur isotope analyses of sulfides in Proterozoic microbialites with diverse mat textures

In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ³⁴S) variability over small spatial scales (~50‰ over <4 mm), providing information about microbial sulfur cycling within different ecological niches in the mat. In the geological record, the location of pyrite formation, overprinting from mat accretion, and post‐depositional alteration also affect both fine‐scale δ³⁴S patterns and bulk δ³⁴S_pyrite values. We report μm‐scale δ³⁴S patterns in Proterozoic samples with well‐preserved microbial mat textures. We show a well‐defined relationship between δ³⁴S values and sulfide mineral grain size and type. Small pyrite grains (<25 μm) span a large range, tending toward high δ³⁴S values (?54.5‰ to 11.7‰, mean: ?14.4‰). Larger pyrite grains (>25 μm) have low but equally variable δ³⁴S values (?61.0‰ to ?10.5‰, mean: ?44.4‰). In one sample, larger sphalerite grains (>35 μm) have intermediate and essentially invariant δ³⁴S values (?22.6‰ to ?15.6‰, mean: ?19.4‰). We suggest that different sulfide mineral populations reflect separate stages of formation. In the first stage, small pyrite grains form near the mat surface along a redox boundary where high rates of sulfate reduction, partial closed‐system sulfate consumption in microenvironments, and/or sulfide oxidation lead to high δ³⁴S values. In another stage, large sphalerite grains with low δ³⁴S values grow along the edges of pore spaces formed from desiccation of the mat. Large pyrite grains form deeper in the mat at slower sulfate reduction rates, leading to low δ³⁴S_sulfide values. We do not see evidence for significant ³⁴S‐enrichment in bulk pore water sulfide at depth in the mat due to closed‐system Rayleigh fractionation effects. On a local scale, Rayleigh fractionation influences the range of δ³⁴S values measured for individual pyrite grains. Fine‐scale analyses of δ³⁴S_pyrite patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ³⁴S_pyrite records.     

Effects of charring on the carbon isotopic composition of grass (Poaceae) epidermis

Charred modern grass epidermis preserves the carbon isotopic composition of the parent plant photosynthetic pathway. Fifty-nine modern grasses and sedges were collected in lowland western Uganda. All charred epidermal samples from C₄ grasses or sedges preserve a carbon isotopic value within the range typical for C₄ plants (−17 to −10‰), and charred epidermal fragments from C₃ plants have carbon isotopic values between −30 and −26‰. The process of charring results in a slightly enriched carbon isotopic signature (−11.9‰ mean charred value as compared to −12.8‰ mean unaltered grass tissue value). δ¹³C measurements of replicate samples from the same plant vary within 1–2‰, yet all values for the same plant stay within the expected values for the photosynthetic pathway of the plant. δ¹³C measurements on >180-μm charred grass epidermal fragments extracted from surface sediment samples from three lakes on the lowland western Ugandan landscape confirm the predominant lowland C₄ grass input (δ¹³C=−16 to −19‰). These results demonstrate the utility of using carbon isotopic analysis of charred grass epidermis to reconstruct C₃ vs. C₄ grassland assemblages on the landscape. Furthermore, such downcore δ¹³C profiles can be used to highlight key zones of C₃ vs. C₄ grass change for which taxonomic analysis of fossil grass epidermis could provide more detailed information regarding grassland community composition.     

Biogenic methane and burrowing as important controlling factors in the early diagenesis of permian carbonate rocks in South Sichuan/China

The 400m thick Lower Permian carbonate sequence of South Sichuan/China, which is considered to be a most important gas reservoir rock, exhibits calcitized gas bubbles and dolomitized burrows which are the result of early diagenesis. The fossiliferous, highly algal-enriched limestones are part of the Yangtze Platform and are composed of biomicrites, biosparites and calcarenites of subtidal and intertidal environments which underwent a maximum burial depth of some 6000 m. The calcitized or, rarely, chert-filled gas bubbles represent a morphogenetic series ranging from a slightly brecciated sediment to globular forms and vertically elongated to branched dm-sized vugs; the series contains some internal sediments (lumps, calcarenites) which are identical to the host rock. Vadose influence is missing. Marcasite aggregates precipitated at the beginning of calcite cement formation emphasize the reducing conditions whereas negative δ¹³C values (?0.6 to ?2.5 ‰) indicate the participation of CO₂ and H₂O originating from the decay of organic matter. Calcite cements contain reduced amounts of Na. K, Sr, Ba and Mg but are enriched in Fe and Mn in comparison to the host rock. It is obvious from the data that rising biogenic gases (CH₄, CO₂, NH₃, H₂S) under different partial pressures formed the vugs within an unconsolidated sediment (water content approximately 80%). Burrows parallel to the bedding (typeThalassinoides), which contain biocalcarenites (echinoderm bioclasts) as internal sediment, underwent early diagenetic dolomitization. These dolomites, which rarely developing into dm-thick supratidal beds, document a slight evaporation within the intertidal environments. Surprisingly enough they reveal a slight over abundance of Mg (Ca₄₇Mg₅₃...) which can be explained by an influence of rising formation waters rich in Mg. In comparison to the host rock the dolomites are enriched in Fe, Mn, K, Na, Zn, Pb and As, and exhibit decreased amounts of Ba and Sr as well as slightly higher δ¹³C values. The vugs play an insignificant role in respect to reservoir properties, because of their isolated distribution and early cementation which destroyed the pore spaces. By contrast, the recrystallized early diagenetic dolomitized burrows exhibit intercrystalline pores which are often occupied by migrated bitumen, but they are not economically relevant for the reservoir properties of the whole sequence.     

Use of stable isotopes to understand food webs in Macao wetlands

In this study, components of the food-web in Macao wetlands were quantified using stable isotope ratio techniques based on carbon and nitrogen values. The δ¹³C and δ¹⁵N values of particulate organic matter (δ¹³C_POM and δ¹⁵N_POM, respectively) ranged from ?30.64 ± 1.0 to ?28.1 ± 0.7 ‰, and from ?1.11 ± 0.8 to 3.98 ± 0.7 ‰, respectively. The δ¹³C values of consumer species ranged from ?33.94 to ?16.92 ‰, showing a wide range from lower values in a freshwater lake and inner bay to higher values in a mangrove forest. The distinct dietary habits of consumer species and the location-specific food source composition were the main factors affecting the δ¹³C values. The consumer ¹⁵N-isotope enrichment values suggested that there were three trophic levels; primary, secondary, and tertiary. The primary consumer trophic level was represented by freshwater herbivorous gastropods, filter-feeding bivalves, and plankton-feeding fish, with a mean δ¹⁵N value of 5.052 ‰. The secondary consumer level included four deposit-feeding fish species distributed in Fai Chi Kei Bay and deposit-feeding gastropods in the Lotus Flower Bridge flat, with a mean δ¹⁵N value of 6.794 ‰. The tertiary consumers group consisted of four crab species, one shrimp species, and four fish species in the Lotus Flower Bridge Flat, with a mean δ¹⁵N value of 13.473 ‰. Their diet mainly comprised organic debris, bottom fauna, and rotten animal tissues. This study confirms the applicability of the isotopic approach in food web studies.     

Foraging segregation by sex and age class in the Guadalupe fur seal from Guadalupe Island,Mexico

A proper assessment of the foraging habits of the Guadalupe fur seal (GFS; Arctocephalus townsendi) is a priority to better understand its recovery, in which the potential for intraspecific competition for prey and space resources is expected to lead to segregation. This study aimed to determine the foraging habits of different sex and age classes. A total of 146 GFS fur samples was collected at Guadalupe Island, Mexico (2014–2020) for stable isotopes (δ¹³C and δ¹⁵N) analysis. Isotopic areas were created (SIBER package in R). Significant isotopic differences were observed between classes. Male (3.6‰²) and female (3.0‰²) juveniles had the largest isotopic areas due to a greater foraging dispersion. Adult females showed the lowest mean δ¹⁵N value (16.1‰ ± 0.5‰) due to foraging trips that are mostly performed towards high latitudes. Except for pups, adult and subadult males presented the highest mean δ¹⁵N (17.4‰ ± 0.4‰) and δ¹³C values (−17.0‰ ± 0.8‰) due to a possible higher trophic level and coastal foraging habits, whereas pups presented the highest mean δ¹⁵N value (17.6‰ ± 0.3‰) because of lactation, which reflects their mothers δ¹⁵N signal plus their own enrichment. Our findings suggest a segregation explained by differences in life history, energy requirements, and a possible strategy to avoid competition.     

Miocene methane‐derived carbonates from southwestern Washington,USA and a model for silicification at seeps

Kuechler, R.R., Birgel, D, Kiel, S, Freiwald, A, Goedert, J.L., Thiel, V & Peckmann, J. 2011: Miocene methane‐derived carbonates from southwestern Washington, USA and a model for silicification at seeps. Lethaia, Vol. 45, pp. 259–273. Exotic limestone masses with silicified fossils, enclosed within deep‐water marine siliciclastic sediments of the Early to Middle Miocene Astoria Formation, are exposed along the north shore of the Columbia River in southwestern Washington, USA. Samples from four localities were studied to clarify the origin and diagenesis of these limestone deposits. The bioturbated and reworked limestones contain a faunal assemblage resembling that of modern and Cenozoic deep‐water methane‐seeps. Five phases make up the paragenetic sequence: (1) micrite and microspar; (2) fibrous, banded and botryoidal aragonite cement, partially replaced by silica or recrystallized to calcite; (3) yellow calcite; (4) quartz replacing carbonate phases and quartz cement; and (5) equant calcite spar and pseudospar. Layers of pyrite frequently separate different carbonate phases and generations, indicating periods of corrosion. Negative δ¹³C_carbonate values as low as ?37.6‰ V‐PDB reveal an uptake of methane‐derived carbon. In other cases, δ¹³C_carbonate values as high as 7.1‰ point to a residual, ¹³C‐enriched carbon pool affected by methanogenesis. Lipid biomarkers include ¹³C‐depleted, archaeal 2,6,10,15,19‐pentamethylicosane (PMI; δ¹³C: ?128‰), crocetane and phytane, as well as various iso‐ and anteiso‐carbon chains, most likely derived from sulphate‐reducing bacteria. The biomarker inventory proves that the majority of the carbonates formed as a consequence of sulphate‐dependent anaerobic oxidation of methane. Silicification of fossils and early diagenetic carbonate cements as well as the precipitation of quartz cement – also observed in other methane‐seep limestones enclosed in sediments with abundant diatoms or radiolarians – is a consequence of a preceding increase of alkalinity due to anaerobic oxidation of methane, inducing the dissolution of silica skeletons. Once anaerobic oxidation of methane has ceased, the pH drops again and silica phases can precipitate. □Biomarkers, carbonates, isotopes, methane, Miocene, silicification, Washington.