A diagenetic control on the Early Triassic Smithian–Spathian carbon isotopic excursions recorded in the marine settings of the Thaynes Group (Utah,USA)

In the aftermath of the end‐Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian‐negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian–Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ¹³C_carb variations obtained here reproduce the known Smithian δ¹³C_carb‐negative excursion. However, the δ¹³C signal of the bulk organic matter is invariant across the SSB and variations in the δ³⁴S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ¹³C_carb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.     

Stepwise and large-magnitude negative shift in δ13Ccarb preceded the main marine mass extinction of the Permian–Triassic crisis interval

Large perturbations to the global carbon cycle occurred during the Permian–Triassic boundary mass extinction, the largest extinction event of the Phanerozoic Eon (542 Ma to present). Controversy concerning the pattern and mechanism of variations in the marine carbonate carbon isotope record of the Permian–Triassic crisis interval (PTCI) and their relationship to the marine mass extinction has not been resolved to date. Herein, high-resolution carbonate carbon isotope profiles (δ¹³C_carb), accompanied by lithofacies, were generated for four sections with microbialite (Taiping, Zuodeng, Cili, and Chongyang) in South China to better constrain patterns and controls on δ¹³C_carb variation in the PTCI and to test hypotheses about the temporal relationship between perturbations to the global carbon cycle and the marine mass extinction event. All four study sections exhibit a stepwise negative shift in δ¹³C_carb during the Late Permian–Early Triassic, with the shift preceding the end-Permian crisis being larger (> 3‰) than that following it (1–2‰). The pre-crisis shifts in δ¹³C_carb are widely correlatable and, hence, represent perturbations to the global carbon cycle. The comparatively smaller shifts following the crisis demonstrate that the marine mass extinction event itself had at most limited influence on the global carbon cycle, and that both Late Permian δ¹³C_carb shifts and the mass extinction must be attributed to some other cause. Their origin cannot be uniquely determined from C-isotopic data alone but appears to be most compatible with a mechanism based on episodic volcanism in combination with collapse of terrestrial ecosystems and soil erosion.     

Permian and early Triassic isotopic records of carbon and strontium in Australia and a scenario of events about the Permian‐Triassic boundary

The negative shift in δ¹³C values of carbonate carbon at the Permian/Triassic boundary is one of the better documented geochemical signatures of a mass extinction event. The similar negative shift in δ¹³C values in organic carbon from Permian/Triassic boundary marine sediments in Austria and Canada is shown to occur also in marine and non‐marine sediments from Australian sedimentary basins. This negative shift in δ¹³C values is used to calibrate Australian sections lacking diagnostic faunal elements identifying the Permian/Triassic boundary. The minimum in the carbonate ⁸⁷Sr/⁸⁶Sr seawater curve from carbonates across the Guadalupian/Ochoan Stage boundary, mainly from North America, is shown to occur also in brachiopod calcite mainly from the Bowen Basin of eastern Australia, hence providing a second calibration point in the Australian sedimentary record. These two geochemical events support a model of a runaway greenhouse developing about the Permian/Triassic boundary; this is inferred to have contributed to the end‐Permian mass extinction.     

Bioturbation and directionality in Earth's carbon isotope record across the Neoproterozoic–Cambrian transition

Mixing of sediments by moving animals becomes apparent in the trace fossil record from about 550 million years ago (Ma), loosely overlapping with the tail end of the extreme carbonate carbon isotope δ¹³C_carbonate fluctuations that qualitatively distinguish the Proterozoic geochemical record from that of the Phanerozoic. These Precambrian‐scale fluctuations in δ¹³C_carbonate (PSF‐δ¹³C_carbonate) remain enigmatic, due to their high amplitude and inclusion of global‐scale negative δ¹³C_carbonate values, below anything attributable to mantle input. Here, we note that different biogeochemical‐model scenarios plausibly explaining globally synchronous PSF‐δ¹³C_carbonate converge: via mechanistic requirements for extensive anoxia in marine sediments to support sedimentary build‐up of ¹³C‐depleted carbon. We hypothesize that bioturbation qualitatively reduced marine sediment anoxia by exposing sediments to oxygenated overlying waters, which ultimately contributed to decreasing the carbon cycle's subsequent susceptibility to PSF‐ δ¹³C_carbonate. Bioturbation may also have reduced the quantity of (isotopically light) organic‐derived carbon available to contribute to PSF‐ δ¹³C_carbonate via ocean crust carbonatization at depth. We conduct a comparative modelling exercise in which we introduce bioturbation to existing model scenarios for PSF‐ δ¹³C_carbonate: expressing both the anoxic proportion of marine sediments, and the global organic carbon burial efficiency, as a decreasing function of bioturbation. We find that bioturbation's oxygenating impact on sediments has the capacity to prevent PSF‐ δ¹³C_carbonate caused by authigenic carbonate precipitation or methanogenesis. Bioturbation's impact on the f‐ratio via remineralization is partially offset by liberation of organic phosphate, some of which feeds back into new production. We emphasize that this study is semiquantitative, exploratory and intended merely to provide a qualitative theoretical framework within which bioturbation's impact on long‐term, first‐order δ¹³C_carbonate can be assessed (and it is hoped quantified in more detail by future work). With this proviso, we conclude that it is entirely plausible that bioturbation made a decisive contribution to the enigmatic directionality in the δ¹³C_carbonate record, from the Neoproterozoic–Cambrian boundary onwards.     

Ecologically and geologically relevant isotope signatures of C,N, and S: okenone producing purple sulfur bacteria part I

Purple sulfur bacteria (PSB) are known to couple the carbon, nitrogen, and sulfur cycling in euxinic environments. This is the first study with multiple strains and species of okenone‐producing PSB to examine the carbon (C), nitrogen (N), and sulfur (S) metabolisms and isotopic signatures in controlled laboratory conditions, investigating what isotopic fractionations might be recorded in modern environments and the geologic record. PSB play an integral role in the ecology of euxinic environments and produce the unique molecular fossil okenane, derived from the diagenetic alteration of the carotenoid pigment okenone. Cultures of Marichromatium purpuratum 1591 (Mpurp1591) were observed to have carbon isotope fractionations (¹³ε_{biomass – CO2}), via RuBisCO, ranging from ?16.1 to ?23.2‰ during exponential and stationary phases of growth. Cultures of Thiocapsa marina 5653 (Tmar5653) and Mpurp1591 had a nitrogen isotope fractionation (¹⁵ε_{biomass – NH4}) of ?15‰, via glutamate dehydrogenase, measured and recorded for the first time in PSB. The δ³⁴S_VCDT values and amount of stored elemental sulfur for Mpurp1591 cells grown autotrophically and photoheterotrophically were dependent upon their carbon metabolic pathways. We show that PSB may contribute to the isotopic enrichments observed in modern and ancient anoxic basins. In a photoheterotrophic culture of Mpurp1591 that switched to autotrophy once the organic substrate was consumed, there were bulk biomass δ¹³C values that span a broader range than recorded across the Late Devonian, Permian–Triassic, Triassic–Jurassic, and OAE2 mass extinction boundaries . This finding stresses the complexities in interpreting and assigning δ¹³C values to bulk organic matter preserved in the geologic record.     

Isotopic evidence of environmental changes during the Devonian–Carboniferous transition in South China and its implications for the biotic crisis

The Devonian–Carboniferous (D–C) transition coincides with the Hangenberg Crisis, carbon isotope anomalies, and the enhanced preservation of organic matter associated with marine redox fluctuations. The proposed driving factors for the biotic extinction include variations in the eustatic sea level, paleoclimate fluctuation, climatic conditions, redox conditions, and the configuration of ocean basins. To investigate this phenomenon and obtain information on the paleo-ocean environment of different depositional facies, we studied a shallow-water carbonate section developed in the periplatform slope facies on the southern margin of South China, which includes a well-preserved succession spanning the D–C boundary. The integrated chemostratigraphic trends reveal distinct excursions in the isotopic compositions of bulk nitrogen, carbonate carbon, organic carbon, and total sulfur. A distinct negative δ¹⁵N excursion (~−3.1‰) is recorded throughout the Middle Si. praesulcata Zone and the Upper Si. praesulcata Zone, when the Hangenberg mass extinction occurred. We attribute the nitrogen cycle anomaly to enhanced microbial nitrogen fixation, which was likely a consequence of intensified seawater anoxia associated with increased denitrification, as well as upwelling of anoxic ammonium-bearing waters. Negative excursions in the δ¹³C_carb and δ¹³C_org values were identified in the Middle Si. praesulcata Zone and likely resulted from intense deep ocean upwelling that amplified nutrient fluxes and delivered ¹³C-depleted anoxic water masses. Decreased δ³⁴S values during the Middle Si. praesulcata Zone suggests an increasing contribution of water-column sulfate reduction under euxinic conditions. Contributions of organic matter produced by anaerobic metabolisms to the deposition of shallow carbonate in the Upper Si. praesulcata Zone is recorded by the nadir of δ¹³C_org values associated with maximal △¹³C. The integrated δ¹⁵N-δ¹³C-δ³⁴S data suggest that significant ocean-redox variation was recorded in South China during the D–C transition; and that this prominent fluctuation was likely associated with intense upwelling of deep anoxic waters. The temporal synchrony between the development of euxinia/anoxia and the Hangenberg Event indicates that the redox oscillation was a key factor triggering manifestations of the biodiversity crisis.     

Carbon reservoir perturbations induced by Deccan volcanism: Stable isotope and biomolecular perspectives from shallow marine environment in Eastern India

The Deccan Traps in Western India is hypothesized to have caused significant fluctuations in climatic condition and organic matter (OM) productivity across the Cretaceous-Paleogene Boundary (K/PgB). The periodic release of large amounts of volatiles into the atmosphere is thought to drive these changes. Yet, direct impact of volcanism on the carbon cycle and ecosystem remains relatively unconstrained. For the first time, we attempt to trace changes in both marine and terrestrial carbon reservoirs from pre- and intervolcanic sedimentary units (infra- and inter-trappeans respectively) from Rajahmundry, ~1500 km SE of main eruption sites in Western India. Molecular level characterization of OM and stable isotope composition of carbonates (δ¹³C_carb), bulk OM (δ¹³C_org), and n-alkane (δ¹³C_alk and δD_alk) have been analysed to provide a chemo-stratigraphic framework. In Rajahmundry, high CO₂ concentration estimated from infra-trappean carbonate nodule is synchronous with the onset of the Deccan Traps and the Late Maastrichtian warming episode. Impact of the warming event is reflected in Rajahmundry from a major shift in the terrestrial ecosystem. Marine OM production also seems to have been low throughout the infra-trappean. A steady decrease in δ¹³C_carb values, increase in mortality rates and dwarfism in invertebrates immediately below the first volcanic units in Rajahmundry suggest stressed conditions from eruption in the western part of India ~40–60 kyrs prior to K/PgB. A significant increase in heterotrophic activity is observed after the volcanic deposits in Rajahmundry that seems to have controlled the marine carbon reservoir for a maximum of ~200 kyrs after the boundary. Advent of pteridophytes, increase in carbon content and positive shifts in δ¹³C_carb and δ¹³C_alk values in the upper inter-trappean units mark the onset of recovery in terrestrial and marine environments. Overall, our results suggest significant perturbations in the carbon reservoir as a consequence of the Deccan eruption.     

Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ¹³C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in δ¹³C between carbon pools along the plant axis and in respired CO₂. Little is known about the within-plant variation in δ¹³C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ¹³C of water soluble organic matter (δ¹³C_WSOM) of leaves, phloem and roots, as well as dark-respired δ¹³CO₂ (δ¹³C_res) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs—Halimium halimifolium and Rosmarinus officinalis—and a woody invader—Acacia longifolia). Spatial patterns in δ¹³C_WSOM along the plant vertical axis and between respired δ¹³CO₂ and its putative substrate were clearly species specific and the most δ¹³C-enriched and depleted values were found in δ¹³C of leaf dark-respired CO₂ and phloem sugars, ~?15 and ~?33 ‰, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ¹³C_WSOM and δ¹³C_res varied by up to 4 ‰ between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ¹³C signatures to trace changes in photosynthetic discrimination.     

Temporal and spatial trends in marine carbon isotopes in the Arctic Ocean and implications for food web studies

The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ¹³C) signatures in dissolved inorganic carbon (δ¹³C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ¹³C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ¹³C values in consumers can change as a result of environmentally driven variation in the δ¹³C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ¹³C baseline, with spatial gradients in the δ¹³C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ¹³C‐DIC values (?0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO₂) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ¹³C‐POC values and δ¹³C in arctic marine mammals reflects the anthropogenic CO₂ signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ¹³C‐POC values in the Arctic Ocean and partially explain the δ¹³C values in marine mammals with consideration of time‐varying integration of δ¹³C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.     

Carbon isotopic abundances in Mesozoic and Cenozoic fossil plants: Palaeoecological implications

Carbon isotopic abundances have been measured for more than one hundred samples of fossil plants ranging in age from middle Triassic to late Tertiary. Most of the plant fossils were identified at the specific or generic level and were selected as representing a variety of continental environments, including xeric and humid habitats. Material analysed included numerous fragments of flowers, seeds, fruits, leaves and wood, as well as a single amorphous lignite sample. The analyses performed for the plant fragments indicate relatively constant isotopic compositions during this time interval, with plant δ¹³C values ranging between -28 and -20%. These values are within the range for living terrestrial plants with C₃, photosynthesis, although values more positive than -23% are rare in C₃ plants and typically found in plants growing under environmental stress. Lower δ¹³C values might have been expected owing to the much higher CO₂, levels of the Cretaceous atmosphere that have been inferred from marine carbonates. No fossils with values indicating C₄, photosynthesis were discovered. Fossil plants from inferred mesic environments showed δ¹³C values ranging between -26.7 and -24.1%. Highest δ¹³C values in angiosperms (up to -20.1%) were measured for Late Cretaceous combretaceous flowers from Portugal. Some cheirolepidiaceous conifers from the Early Cretaceous also showed high δ¹³C values. Values measured for Pseudofrenelopsis varians and Glenrosa taxensis were -21.9%, and values of gymnosperm wood, probably of cheirolepidiaceous affinity, were -19.0%. These high values are in accordance with inferred ecological conditions for the fossil plants. They may suggest a tendency for C₄,-like photosynthesis, although the data are equivocal. Higher values (-17.3%) clearly falling outside the C₃, range were, however, obtained from a single lignite fragment of Late Cretaceous (Maastrichtian) age. The nature of this plant fragment is unknown, but the result suggests that C₄-like photosynthesis was present at least in some latest Cretaceous vegetation. A hadrosaurian dinosaur with well-preserved collagen-like organic matter from the same deposit showed δ¹³C values around-16%, which also suggests the presence of CAM or even C₄ plants in the latest Cretaceous. □Carbon isotopic abundances, δ¹³C values, dinosaurs, plants, photosynthetic pathways, Mesozoic.     

Use of carbon-13 and carbon-14 natural abundances for stream food web studies

We review the use of stable carbon isotope ratios (δ¹³C) and radiocarbon natural abundances (Δ¹⁴C) for stream food web studies. The δ¹³C value of primary producers (e.g., periphytic algae, hereafter periphyton) in streams is controlled by isotopic fractionation during photosynthesis and variable δ¹³C of dissolved CO₂. When periphyton δ¹³C differs from that of terrestrial primary producers, the relative contribution of autochthony and allochthony to stream food webs can be calculated. Moreover, the variation in periphyton δ¹³C can reveal how much stream consumers rely on local resources because each stream habitat (e.g., riffle vs. pool, open vs. shaded) usually has a distinctive δ¹³C. However, periphyton δ¹³C often overlaps with that of terrestrial organic matter. On the other hand, periphyton Δ¹⁴C is less variable than δ¹³C among habitats, and reflects the Δ¹⁴C of dissolved CO₂, which could be a mixture of “aged” (Δ¹⁴C < 0 ‰) and “modern” (Δ¹⁴C > 0 ‰) carbon. This is because the Δ¹⁴C is corrected by its δ¹³C value for the isotopic fractionation during photosynthesis. Recent studies and our data indicate that many stream food webs are supported by “aged” carbon derived from the watershed via autochthonous production. The combined use of δ¹³C and Δ¹⁴C allows robust estimation of the carbon transfer pathway in a stream food web at multiple spatial scales ranging from the stream habitat level (e.g., riffle and pool) to watershed level (autochthony and allochthony). Furthermore, the Δ¹⁴C of stream food webs will expand our understanding about the time frame of carbon cycles in the watersheds.     

Carbon isotopic composition of lipid biomarkers from an endoevaporitic gypsum crust microbial mat reveals cycling of mineralized organic carbon

Microbial mats that inhabit gypsum deposits in ponds at Guerrero Negro, Baja California Sur, Mexico, developed distinct pigmented horizons that provided an opportunity to examine the fixation and flow of carbon through a trophic structure and, in conjunction with previous phylogenetic analyses, to assess the diagenetic fates of molecular δ¹³C biosignatures. The δ¹³C values of individual biomarker lipids, total carbon, and total organic carbon (TOC) were determined for each of the following horizons: tan‐orange (TO) at the surface, green (G), purple (P), and olive‐black (OB) at the bottom. δ¹³C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ¹³C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO₂ fixation. δ¹³C_TOC of the underlying G was 3‰ greater than that of TO. The most δ¹³C‐depleted acetogenic lipids in the upper horizons were the cyanobacterial biomarkers C₁₇ n‐alkanes and polyunsaturated fatty acids. Bishomohopanol was 4 to 7‰ enriched, relative to alkanes and intact polar fatty acids (IPFA), respectively. Acyclic C₂₀ isoprenoids were depleted by 14‰ relative to bishomohopanol. Significantly, ?[δ¹³C_TOC ? δ¹³C_∑IPFA] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that ¹³C‐enriched residual organic matter accumulated at depth. The permanently anoxic P horizon was dominated by anoxygenic phototrophs and sulfate‐reducing bacteria. P hosted an active sulfur‐dependent microbial community. IPFA and bishomohopanol were ¹³C‐depleted relative to upper crust by 7 and 4‰, respectively, and C₂₀ isoprenoids were somewhat ¹³C‐enriched. Synthesis of alkanes in P was evidenced only by ¹³C‐depleted n‐octadecane and 8‐methylhexadecane. In OB, the marked increase of total inorganic carbon δ¹³C (δ¹³C_TIC) of >6‰ perhaps indicated terminal mineralization. This δ¹³C_TIC increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of ¹³C‐depleted methane from the mat.     

Revisiting marine redox conditions during the Ediacaran Shuram carbon isotope excursion

The Neoproterozoic carbonate record contains multiple carbon isotope anomalies, which are the subject of intense debate. The largest of these anomalies, the Shuram excursion (SE), occurred in the mid-Ediacaran (~574–567 Ma). Accurately reconstructing marine redox landscape is a clear path toward making sense of the mechanism that drives this δ¹³C anomaly. Here, we report new uranium isotopic data from the shallow-marine carbonates of the Wonoka Formation, Flinders Ranges, South Australia, where the SE is well preserved. Our data indicate that the δ²³⁸U trend during the SE is highly reproducible across globally disparate sections from different depositional settings. Previously, it was proposed that the positive shift of δ²³⁸U values during the SE suggests an extensive, near-modern level of marine oxygenation. However, recent publications suggest that the fractionation of uranium isotopes in ferruginous and anoxic conditions is comparable, opening up the possibility of non-unique interpretations of the carbonate uranium isotopic record. Here, we build on this idea by investigating the SE in conjunction with additional geochemical proxies. Using a revised uranium isotope mass balance model and an inverse stochastic carbon cycle model, we reevaluate models for δ¹³C and δ²³⁸U trends during the SE. We suggest that global seawater δ²³⁸U values during the SE could be explained by an expansion of ferruginous conditions and do not require a near-modern level of oxygenation during the mid-Ediacaran.     

The carbon cycle of Quebec boreal reservoirs investigated by elemental compositions and isotopic values

Dissolved and particulate organic matter (POM) of three Quebec boreal reservoirs of different ages (Laforge-1, 7 years; Robert-Bourassa, 25 years and Cabonga, 70 years at the time of sampling) and sets of lakes from the same watersheds was analyzed using organic carbon concentrations, C/N and C/P elemental composition, δ¹³C and δ¹⁵N isotopic values. The reservoirs are characterized by lower dissolved organic carbon concentrations with lower C/N ratios and by lower δ¹³C and higher δ¹⁵N in POM. They contain more autochthonous dissolved organic matter and less terrigenous organic matter than the lakes. Some of those characteristics are more pronounced in the younger than in the older reservoirs. The differences can be attributed to two causes: (1) more extended degradation of terrigenous organic matter, caused by an increase in residence time; and (2) differences in food web structure resulting from the phenomenon known as trophic upsurge, in newly flooded reservoirs. The results indicate that some effects of reservoir creation on the carbon cycle are short term perturbations, others however long term features of those reservoirs. The implications of these findings for CO₂ emissions from reservoirs are discussed.     

Upward increase of kerogen δ13C in the Peru Margin Upper Oligocene: possible implications for the Cenozoic evolution of atmospheric CO2

Carbon isotopic composition of predominantly marine kerogen in latest Oligocene mudstones of the Peru Margin ODP 682A Hole shows an about 3.5‰ increase with decreasing age. Py-GC and elemental (C/N ratio) analysis of the kerogen plus sulphur isotopic study together with earlier knowledge on geological setting and organic geochemistry results in a better understanding of depositionary environment and allows to separation of the influence of concentration of water dissolved carbon dioxide (c_e) on kerogen δ¹³C from that of other factors (bacterial degradation, sea surface temperature, DIC δ¹³C, productivity, and admixture of land plant OM). Based on this analysis, the major part of the kerogen shift is considered as a result of the latest Oligocene decrease of marine photosynthetic carbon isotopic fractionation in the Peru Margin photic zone, which in turn possibly reflects a simultaneous drop in atmospheric CO₂ level. Uncertainties in the evaluation of the factors affecting the marine photosynthetic carbon isotopic fractionation and the extent of ocean–atmosphere disequilibrium do not permit calculation of the decrease of the atmospheric CO₂.     

Carbon isotope compositions (δ13C) of leaf,wood and holocellulose differ among genotypes of poplar and between previous land uses in a short‐rotation biomass plantation

The efficiency of water use to produce biomass is a key trait in designing sustainable bioenergy‐devoted systems. We characterized variations in the carbon isotope composition (δ¹³C) of leaves, current year wood and holocellulose (as proxies for water use efficiency, WUE) among six poplar genotypes in a short‐rotation plantation. Values of δ¹³C_wood and δ¹³C_{holocellulose} were tightly and positively correlated, but the offset varied significantly among genotypes (0.79–1.01‰). Leaf phenology was strongly correlated with δ¹³C, and genotypes with a longer growing season showed a higher WUE. In contrast, traits related to growth and carbon uptake were poorly linked to δ¹³C. Trees growing on former pasture with higher N‐availability displayed higher δ¹³C as compared with trees growing on former cropland. The positive relationships between δ¹³C_leaf and leaf N suggested that spatial variations in WUE over the plantation were mainly driven by an N‐related effect on photosynthetic capacities. The very coherent genotype ranking obtained with δ¹³C in the different tree compartments has some practical outreach. Because WUE remains largely uncoupled from growth in poplar plantations, there is potential to identify genotypes with satisfactory growth and higher WUE.     

Carbon: freshwater plants

δ¹³C values for freshwater aquatic plant matter varies from ?11 to ?50‰ and is not a clear indicator of photosynthetic pathway as in terrestrial plants. Several factors affect δ¹³C of aquatic plant matter. These include: (1) The δ¹³C signature of the source carbon has been observed to range from +1‰ for HCO₃^? derived from limestone to ?30‰ for CO₂ derived from respiration. (2) Some plants assimilate HCO₃^?, which is –7 to –11‰ less negative than CO₂. (3) C₃, C₄, and CAM photosynthetic pathways are present in aquatic plants. (4) Diffusional resistances are orders of magnitude greater in the aquatic environment than in the aerial environment. The greater viscosity of water acts to reduce mixing of the carbon pool in the boundary layer with that of the bulk solution. In effect, many aquatic plants draw from a finite carbon pool, and as in terrestrial plants growing in a closed system, biochemical discrimination is reduced. In standing water, this factor results in most aquatic plants having a δ¹³C value similar to the source carbon. Using Farquhar's equation and other physiological data, it is possible to use δ¹³C values to evaluate various parameters affecting photosynthesis, such as limitations imposed by CO₂ diffusion and carbon source.     

Microbes,mud and methane: cause and consequence of recurrent Early Jurassic anoxia following the end‐Triassic mass extinction

The end‐Triassic mass extinction (c. 201.6 Ma) was one of the five largest mass‐extinction events in the history of animal life. It was also associated with a dramatic, long‐lasting change in sedimentation style along the margins of the Tethys Ocean, from generally organic‐matter‐poor sediments during the Triassic to generally organic‐matter‐rich black shales during the Jurassic. New core material from Germany provides biomarker evidence of persistent photic‐zone euxinia during the Hettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the Hettangian and the better‐known Toarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the Sinemurian and Pliensbachian. Both the Hettangian and Toarcian events are marked by important changes in phytoplankton assemblages from chromophyte‐ to chlorophyte‐dominated assemblages within the European Epicontinental Seaway. Phytoplankton changes occurred in association with the establishment of photic‐zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the δ¹³C of atmospheric CO₂ and variation in the sources of organic carbon. Regardless of the causes of δ¹³C variability, long‐term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.     

End-Permian stratigraphic timeline applied to the timing of marine and non-marine extinctions

A latest Permian timeline (251.9 Ma) can be constructed from the perspectives of: a global nickel spike attributed to emissions from the coeval Siberian flood-basalt eruptions, the correlative end-Permian marine mass extinction (EPME), a transition from reversed to normal paleomagnetism, and a negative anomaly in δ¹³C_carb and δ¹³C_org. In a number of marine and non-marine localities, this timeline is also correlated (to within ≤30 ky) with palynological evidence for the latest Permian destruction of terrestrial vegetation and the accompanying short-lived global fungal (Reduviasporonites) event. This correlation suggests that devastation in marine and non-marine environments was essentially coeval at a time marked by hyperthermal conditions and anoxic oceans.We utilized this proposed timeline to estimate the relative timing of the extinction of latest Permian vertebrates in the Karoo Basin of South Africa. In several sections in the Karoo, the LAD of the therapsid Dicynodon, is correlated with the proposed timeline. In the Carlton Heights section in the Karoo we estimate that the palynological changes and the fungal event occurred within ≤30 ky of the LAD of Dicynodon. Further sampling in the Karoo and other Permian–Triassic non-marine basins would help to clarify the relative timing of the global marine extinctions, plant devastation and the disappearance of non-marine vertebrates.     

Uncovering the spatial heterogeneity of Ediacaran carbon cycling

Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to ?12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ¹³C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ¹³C_carb and δ¹³C_org) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ¹³C_org are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ³⁴S of carbonate‐associated sulfate (CAS) and increased ⁸⁷Sr/⁸⁶Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ¹³C_org records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O₂ and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ¹³C presented in this study.