Average δ13C Research Articles

Terrigenous or not? δ13C reveals the origin of the retene and dehydroabietic acid methyl ester

Retene and dehydroabietic acid methyl ester (DAME) are biomarkers linked mostly to terrigenous organic matter. However, their aquatic origin from algae and/or cyanobacteria has been discussed in the past few years. The complex origin of retene and DAME can lead to misinterpretation during biomarker interpretation, which can be minimalised with the support of isotopic analyses. This study contains the results of thirty-two compound-specific isotope analyses (CSIA) of δ13C performed on retene and forty-six CSIA analyses of δ13C performed on DAME. Obtained δ13C results were compared with simple biomarker results to tie isotopic data with the origin of organic matter. Pure aquatic retene and DAME were present within the Middle Cambrian and Early Ordovician samples, as the first appearance of plants was noted in the Middle Ordovician.CSIA was performed on a wide spectrum of the Cambrian-Silurian, Late Triassic-Middle Jurassic, and Neogene samples. Cambrian-Silurian strata include aquatic-origin retene and DAME, as evidenced by their comparatively low carbon isotopic signature. The average δ13C for retene and DAME within Cambrian-Silurian strata was equal to −31.7 ‰ and − 30.8 ‰, respectively. The Late Triassic-Middle Jurassic strata were dominated by terrigenous organic matter, resulting in higher δ13C values in comparison with the Cambrian-Silurian strata. The average δ13C for retene and DAME within the Late Triassic–Middle Jurassic strata was equal to −25.2 ‰ and − 26.3 ‰, respectively. The average δ13C for retene and DAME within the Neogene strata was equal to −27.3 ‰ in both cases, as these strata also contain a significant input of terrigenous organic matter. Some Neogene samples containing strong aquatic organic matter admixture are characterised by a lower δ13C for retene and/or DAME, oscillating around −28.0 ‰ to −30.0 ‰ for both compounds. Despite possible terrigenous input within some Silurian samples, the δ13C for retene and DAME still shows values typical for aquatic origin. These results suggest that retene and DAME within the Silurian strata should not be linked with non-vascular plants or that, due to their affinity with algae and cyanobacteria, the early plants were characterised by a relatively low δ13C composition, characteristic for phytoplankton organic matter. δ13C of retene and DAME were compared with δ13C of C16-C30n-alkanes and stigmastane, which supported the thesis about isotopic differences in aquatic and terrigenous retene and DAME.

Sourcing and long-range transport of particulate organic matter in river bedload: Río Bermejo, Argentina

Abstract. Fluvial transport of organic carbon from the terrestrial biosphere to the oceans is an important term in the global carbon cycle. Traditionally, the long-term burial flux of fluvial particulate organic carbon (POC) is estimated using river suspended sediment flux; however, organic carbon can also travel in river bedload as coarse particulate organic matter (POMBed). Estimates of fluvial POC export to the ocean are highly uncertain because few studies document POMbed sources, flux, and evolution during long-range fluvial transport from uplands to ocean basins. This knowledge gap limits our ability to determine the global terrestrial organic carbon burial flux. In this study we investigate the flux, sources, and transformations of POMBed during fluvial transport over a ∼1300 km long reach of the Río Bermejo, Argentina, which has no tributary inputs. To constrain sourcing of POMBed, we analyzed the composition and stable hydrogen and carbon isotope ratios (δ2H, δ13C) of plant wax biomarkers from POMBed at six locations along the Río Bermejo and compared this to samples of suspended sediment, soil, leaf litter, and floating organic debris (POMfloat) from both the lowland and headwater river system. Across all samples, we found no discernible differences in n-alkane average chain length or nC29 δ13C, indicting a common origin for all sampled POMBed. Leaf litter and POMfloat nC29 δ2H values decrease with elevation, making it a useful proxy for POMBed source elevation. Biomarker δ2H values suggest that POMBed is a mix of distally derived headwater and locally recruited floodplain sources at all sampling locations. These results indicate that POMBed can be preserved during transport through lowland rivers for hundreds of kilometers. However, the POMBed flux decreases with increasing transport distance, suggesting mechanical comminution of these coarse organic particles and progressive transfer into the suspended load. Our provisional estimates suggest that the carbon flux from POMBed comprises less than 1 % of the suspended load POC flux in the Río Bermejo. While this represents a small portion of the river POC flux, this coarse, high-density material likely has a higher probability of deposition and burial in sedimentary basins, potentially allowing it to be more effective in long-term CO2 drawdown relative to fine suspended particles. Because the rate and ratio of POMBed transport versus comminution likely vary across tectonic and climatic settings, additional research is needed to determine the importance of POMBed in the global carbon cycle.

Mussel Culture Activities Facilitate the Export and Burial of Particulate Organic Carbon

The recent expansion of shellfish mariculture could significantly impact the ocean carbon cycle and its associated biogeochemical processes. To understand the source and fate of particulate organic carbon (POC), a summer cruise was conducted from September 8 to 10, 2022, at a mussel farm on Gouqi Island and its adjacent areas located in the East China Sea. Parameters included in situ temperature and salinity, contents of dissolved oxygen (DO), suspended particulate matter (SPM), POC, and chlorophyll a (Chl a), as well as the stable carbon isotopic composition (δ13C) of organic matter in particle and sediment samples, which were analyzed to facilitate a comparative assessment of the areas inside and outside the mussel farm. The POM was much fresher (POC/Chl a < 150) inside the farm with little impact from sediment resuspension (lower SPM content, 11.6 ± 6.6 mg/L), while a significant influence of sediment resuspension was found outside the farm (SPM > 20 mg/L, POC/Chl a > 150). A two end-member mixing model showed that 82.0 ± 6.0% of POC originated from marine algae within the farm, much higher than that outside the farming area (66.1 ± 7.8%). Moreover, elevated DO saturation but relatively low Chl a concentration within the farm suggested continuous algae consumption following potential high productivity. The averaged δ13C values were similar among suspended POC, sinking POC, and sedimentary organic carbon within the farm, implying the fast export and burial of POC. This is likely due to the filter-feeding habits of mussels, who ingest fresh POC and then pack it as fecal pellets that rapidly settle into the sediment. This study sheds light on the distribution and sources of POM inside and outside the mussel farm on Gouqi Island, enhancing our understanding of the marine carbon cycle on shellfish farms and providing insights into the underlying biogeochemical processes.

Differential responses in water-use strategies of evergreen (Ligustrum lucidum) and deciduous (Robinia pseudoacacia) trees to tunnel excavation in a subtropical karst trough valley

Tunnel(s) excavation could decrease groundwater levels and soil water storage and consequently change trees growth conditions in karst areas. However, the adaptation mechanisms of different tree species to altered hydrological conditions caused by tunnel(s) construction in karst areas remain unknown. Here, we investigated the water-use strategies of two different tree species (the evergreen Ligustrum lucidum and deciduous Robinia pseudoacacia) in response to tunnel excavation in a typical subtropical karst trough valley by using the sap flow density, stable isotopes (δ13C, δ2H, and δ18O) and MixSIAR models. Tunnel excavation resulted in a 3.7–6.3 % reduction in soil moisture content (SMC) in Zhongliang Mountain Karst Trough (ZMKT) Valley. Both tree species demonstrated reduced transpiration in tunnel-affected valley (TAV) compared to tunnel-unaffected valley (TUAV) (transpiration in TAV was only 77.6 % of TUAV), with evergreen trees exhibiting a smaller decline in transpiration compared to deciduous trees. Furthermore, the proportions of karst groundwater utilized by evergreen and deciduous tree species in TAV were 8.3 % and 5.0 % higher than in TUAV, respectively, with evergreen tree species exhibiting greater absorption of karst groundwater than deciduous tree species. Meanwhile, the average leaf δ13C values and WUEi (intrinsic water-use efficiency) of both tree species in TAV were 0.9 ‰ and 15.7 μmol/mol higher than those in TUAV, respectively, with evergreen trees exhibiting a smaller increase in WUEi compared to deciduous trees. These results indicate that tunneling-induced water stress events may prompt trees to adopt more conservative water-use strategies. Evergreen tree species rely on more karst groundwater absorption to sustain relatively higher transpiration rates, and deciduous trees adapt to severe water stress by increasing their WUEi or shedding leaves to minimize transpiration. Our finding contributes to the evaluation of eco-hydrological processes in karst areas affected by tunnel(s) excavation.

Mercury biomagnification in the food chain of a piscivorous turtle species (Testudines: Chelidae: Chelus fimbriata) in the Central Amazon, Brazil.

Due to their natural history and ecological attributes, turtles are excellent organisms for studies of heavy metal contamination. Turtles have a large geographical distribution, occupy different aquatic habitats, and occupy various trophic levels. The present study investigated mercury bioaccumulation in the carnivorous chelonian Chelus fimbriata (Matamata turtle) and Hg biomagnification in relation to its aquatic food chain in the middle Rio Negro, AM-Brazil. Tissue samples of muscle, carapace and claws were collected from 26 C. fimbriata individuals, as well as collections of autotrophic energy sources found in the turtle's aquatic habitat area. The samples were collected in February-March/2014 and analyzed for THg concentrations and carbon (δ13C) and nitrogen (δ15N) stable isotopes. The highest THg levels were found in claws (3780 ng.g-1), carapace (3622 ng.g-1) and muscle (403 ng.g-1), which were found to be significantly different [F(2.73) = 49.02 p < 0.01]. However, THg concentrations in muscle tissue were below the consumption threshold indicated by the WHO and Brazilian Health Ministry. The average δ13C and δ15N values in Matamata samples were -31.7‰ and 11.9‰, respectively. The principal energy source sustaining the food chain of C. fimbriata was found to be terrestrial shrubs, with smaller contributions from emergent aquatic herbaceous plants and algae, while δ15N values showed its trophic position to be two levels above the autotrophic energy sources. There was a positive correlation between THg and turtle size, while a significant relationship was found between THg and δ15N, showing strong biomagnification in the food chain of C. fimbriata: y = 0.21x + 0.46; r2 = 0.45; p < 0.001, for which the slope presented a value of 0.21.

Characteristics of PM2.5 bounded carbonaceous aerosols, carbon dioxide and its stable carbon isotopes (δ13C) in rural households in northwest China: Effect of different fuel combustion

In order to fully understand the carbon emission from different fuels in rural villages of China, especially in the typical atmospheric pollution areas. The characteristics of carbonaceous aerosols and carbon dioxide (CO2) with its stable carbon isotope (δ13C) were investigated in six households, which two households used coal, two households used wood as well as two households used biogas and liquefied petroleum gas (LPG), from two rural villages in Fenwei Plain from March to April 2021. It showed that the fine particulate matter (PM2.5) emitted from biogas and LPG couldn't be as lower as expected in this area. However, the clean fuels could relatively reduce the emissions of organic carbon (OC) and element carbon (EC) in PM2.5 compare to the solid fuels. The pyrolyzed carbon (OP) accounted more total carbon (TC) in coal than the other fuels use households, indicating that more water-soluble OC existed, and it still had the highest secondary organic carbon (SOC) than the other fuels. Meantime, the coal combustions in the two villages had the highest CO2 concentration of 527.6 ppm and 1120.6 ppm, respectively, while the clean fuels could effectively reduce it. The average δ13C values (−26.9‰) was much lighter than almost all the outdoor monitoring and similar to the δ13C values for coal combustion and vehicle emission, showing that they might be the main contributors of the regional atmospheric aerosol in this area. During the sandstorm, the indoor PM2.5 mass and CO2 were increasing obviously. The indoor cancer risk of PAHs for adults and children were greater than 1 × 10−6, exert a potential carcinogenic risk to human of solid fuels combustion in rural northern China. It is important to continue concern the solid fuel combustion and its health impact in rural areas.

A discussion on carbon and oxygen isotopes of secondary carbonates in loess-paleosol sequences

Although carbon and oxygen isotopic compositions of carbonates are valuable for Quaternary climate reconstruction due to their sensitivity to paleoclimate changes, a comprehensive understanding regarding the formation, factors influencing isotopic variations, and age uncertainties of secondary carbonates in loess deposits remains elusive. This study investigates the microstructure and stable isotopes of secondary carbonates, specifically calcified root cells (CRC) and needle-fiber calcite (NFC), found within the loess deposits of the last glacial period in the northwestern region of the Chinese Loess Plateau. The average δ13C values of soil total organic matter, CRC, NFC, and total inorganic carbonates in the YuanBao sections are −22.1‰, −14.9‰, −7.0‰, −4.6‰, respectively. Radiocarbon dating results reveal that both CRC and NFC were not of synsedimentary origin. The pronounced negativity of the carbon isotopic composition and the microstructures offer insights that CRC formation is closely tied to C3 plants. The synchronous depletion of δ13CCRC and δ18OCRC values during the transition period from the last glacial to the Holocene, underscores the sensitivity of isotopic composition of CRC to local environmental changes, making it a potential candidate for paleo-climate studies. Additionally, CRC formed during the glacial period provides a close approximation to the true sedimentation age. The radiocarbon dating of NFC highlights a significant fluctuation in carbon source translocation depth, resulting in a pronounced temporal and spatial decoupling between NFC formation and the surrounding deposits. Consequently, values of δ13CNFC and δ18ONFC seem to exhibit no significant trend across the limited depth range in this research. We propose that, when using NFC for paleoclimate reconstruction, careful consideration of variations in translocation depths of carbon sources is imperative to prevent misestimations. NFC could potentially be more fitting for broader temporal-scale paleoclimate reconstructions rather than high-resolution investigations.

Elemental (CNS) and isotopic (δ13C and δ15N) characterization of local sources to atmospheric dust at Trombay, Mumbai, using elemental analyzer–isotope ratio mass spectrometry

Particulate matter (PM) in the air has significant implications for human health and the environment, particularly in urban areas with high concentrations. This study focuses on the isotopic (δ13C and δ15N), elemental (CNS) as well as the total carbon to total nitrogen ratio (TC/TN) characterization of PM in Trombay, Mumbai. Monthly atmospheric dust samples were collected from 2016 to 2020 during the premonsoon and postmonsoon seasons. The average elemental concentrations for premonsoon samples were N (1.73% ± 0.25%), C (7.38% ± 0.72%), S (0.84% ± 0.13%), and TC/TN ratio (4.38 ± 0.86). For postmonsoon samples, the average elemental concentrations were N (1.47% ± 0.185%), C (6.95% ± 1.04%), S (0.84% ± 0.13%), and TC/TN ratio (4.80 ± 0.94). The average δ13C values for premonsoon and postmonsoon seasons were −24.31 ± 1.15‰ and −24.56 ± 1.20‰, respectively. The average δ15N values for premonsoon and postmonsoon seasons were 5.22 ± 1.92‰ and 4.57 ± 1.32‰, respectively. The results showed variations in elemental concentrations between the seasons, with higher values observed in premonsoon samples which can be attributed to increased dust generation from land and sea. Overall, this study provides insights into the local sources and characteristics of PM in the Trombay area and highlights the importance of long-term monitoring to understand the seasonal variations in PM composition.

Inter- and intrapopulation resource use variation of marine subsidized western fence lizards

Marine resource subsidies alter consumer dynamics of recipient populations in coastal systems. The response to these subsidies by generalist consumers is often not uniform, creating inter- and intrapopulation diet variation and niche diversification that may be intensified across heterogeneous landscapes. We sampled western fence lizards, Sceloporus occidentalis, from Puget Sound beaches and coastal and inland forest habitats, in addition to the lizards’ marine and terrestrial prey items to quantify marine and terrestrial resource use with stable isotope analysis and mixing models. Beach lizards had higher average δ13C and δ15N values compared to coastal and inland forest lizards, exhibiting a strong mixing line between marine and terrestrial prey items. Across five beach sites, lizard populations received 20–51% of their diet from marine resources, on average, with individual lizards ranging between 7 and 86% marine diet. The hillslope of the transition zone between marine and terrestrial environments at beach sites was positively associated with marine-based diets, as the steepest sloped beach sites had the highest percent marine diets. Within-beach variation in transition zone slope was positively correlated with the isotopic niche space of beach lizard populations. These results demonstrate that physiography of transitional landscapes can mediate resource flow between environments, and variable habitat topography promotes niche diversification within lizard populations. Marine resource subsidization of Puget Sound beach S. occidentalis populations may facilitate occupation of the northwesternmost edge of the species range. Shoreline restoration and driftwood beach habitat conservation are important to support the unique ecology of Puget Sound S. occidentalis.

Temporal and spatial variation in the food web structure in artificial reef areas and adjacent habitats in Haizhou Bay

Artificial reef is regarded as an effective tool for restoring habitats and supporting the marine ecosystem. While the energy flow and material exchange in the food web of artificial reef ecosystem still remain vacant and should be further understood. In this study, stable isotopic characteristics (δ13C and δ15N) were used to identify the trophic niches and potential carbon sources of consumers (fish and invertebrates) in artificial reef areas and adjacent habitats in Haizhou Bay (the estuary area (EA), the aquaculture area (AA), the artificial reef area (AR), the natural area (NA) and the comprehensive effect area (CEA)) in spring and autumn. The results showed that the average δ13C for all consumers decreased from − 16.99 ± 2.07‰ to − 21.81 ± 1.66‰, and the average δ15N ranged from 11.37 ± 0.35‰ to 14.69 ± 0.64‰ in all seasons. A SIBER model revealed highly similar trophic niche among habitats in autumn, and lower overlap in spring. The SIMMR model showed that phytoplankton and sedimentary organic matter (SOM) were the major contributors to consumers, and particulate organic matter (POM) accounted for less contribution. In AA and NA phytoplankton and SOM generally contributed fairly equally to most fish, while single SOM sources were major contributors to nearly all the species in AR and CEA. Our findings reveal that ARs can serve as seascape mosaics to increase fish community diversity because of their high variation in the δ13C value and the trophic niches from EA to the NA. Preliminary multidisciplinary studies are needed for the mitigation of effects before any implementations deployed to the offshore waters that influence habitat connectivity, especially some habitat–specific migratory species.

Stable carbon isotopes and stomatal frequency of Middle Jurassic ginkgophyte fossils from the Turpan basin, northwestern China: Implications for reconstructing paleo-CO2 changes

The Jurassic climate change has attracted a great deal of attention from geologists. However, the investigations on the correlation between carbon isotopes of plant fossils and high CO2 levels in the Mesozoic are scarce. In this study, 16 well-preserved fossilized Ginkgoites and Phoenicopsis leaves were selected from the Xishanyao Formation in the Turpan basin. Two chemical protocols were applied for the pretreatment of stable carbon isotopes: HCl reagent and HCl + HF reagents. The average δ13C values followed the two protocols were –22.8 ‰ and –22.9 ‰, respectively, indicating that HF reagent has no effect on the carbon isotopes of the ginkgophyte fossils. The difference between the δ13C values of Ginkgoites fossils and Phoenicopsis fossils is obvious, which is mainly caused by distinct carbon isotope fractionation between different plant taxa. However, both values show the same δ13C trend in the early Middle Jurassic, i.e., increasing and then decreasing with time. In addition, both stomatal index (SI) and stomatal ratio (SR) methods are used to reconstruct paleo-CO2 levels with average values of 2564 ppmv and 1245 ppmv, respectively, indicating that SR method is more reliable. There was weak correlation (R = 0.23) between δ13C and CO2 values in the early Middle Jurassic which may be due to low response of δ13C values of ginkgophyte fossils to CO2 changes. Therefore, our work emphasizes the caution on using stable carbon isotopes of ginkgophyte fossils to reconstruct high paleo-CO2 concentrations.

Downed deadwood habitat heterogeneity drives trophic niche diversity of soil-dwelling animals

Habitat heterogeneity is one of the important drivers of biodiversity, but the underlying mechanisms are still vague. Increasing habitat heterogeneity often coincides with greater habitat space and/or greater food availability for organisms, but it is unclear whether and how these components are related to habitat selection. Downed dead wood (i.e., dead wood laying on the ground) is an important but understudied component of dead plant matter that creates spatial habitat heterogeneity for soil fauna on the ground. To unravel the role of dead wood for providing niche dimensionality for soil fauna via variance in resource use, we sampled Collembola communities from moss growing on bark and from bark on the logs of two tree species (pedunculate oak and Norway spruce) and from soil next to the logs. We assessed habitat selection of Collembola species combining abundance data with trophic position data using stable isotope ratios (i.e., δ13C and δ15N). Based on the Bray-Curtis dissimilarity, Collembola community composition differed significantly among the three habitats (i.e., moss, bark and soil). Collembola community-weighted mean δ13C and δ15N values also differed among the three habitats (average δ13C values of moss: −25.94‰, bark: −25.30‰, soil: −25.60‰; average δ15N values of moss: −4.80‰, bark: −3.83‰, soil: −5.11‰), partly reflecting isotopic differences of the habitats themselves (δ13C of moss: −29.83‰, bark: −28.26‰, soil: −29.04‰; δ15N of moss: −7.59‰, bark: −4.18‰, soil: −7.84‰). These findings indicate that Collembola depend on food sources derived from the habitats they inhabit at least at the community level. Collembola community-weighted variance of both δ13C and δ15N values ranked as bark > moss > soil. Species-level data suggested that the bark community included both bark-specific food specialists as well as species which likely use bark only as a temporary habitat based on their morphological traits supporting high mobility. These findings together indicate that the trophic niche variation of the Collembola community is larger in bark than in soil. Our results stress the importance of downed dead wood as a source of habitat heterogeneity, which in turn promotes soil animal diversity, likely via heterogeneity in food sources. Furthermore, the presence of dead wood on the ground could have consequences for organic matter processing by the soil animal community through sustaining a larger trophic niche variation even at a small spatial scale.

Characterization of particulate matter emissions from internal combustion engines using δ13C values: Impact of engine operation conditions and fuel type on PM10 isotopic composition

Stable carbon isotope ratio of particulate emissions from internal combustion engines (ICEs) is an important characteristic in atmospheric carbonaceous PM source studies. Long-term measurements (2009–2020) of δ13C values of commercial fuels in Lithuania were conducted to evaluate the current isotopic composition of the fuel due to the changing share of renewable energy sources (RES). In this work, we present experimental results on stable carbon isotope ratio (δ13C) in PM10 emitted by different types of ICEs (two-stroke and four-stroke), including on-road transport and non-road mobile machinery. Different driving scenarios were simulated on a test bench (idling, NEDC, different torque, and speed). At idle, the average δ13C(PM10) values of different fuel types were determined: −27.80‰ for gasoline, −28.97‰ for diesel, − 27.89‰ for biofuel-C3, and −16.95‰ for biofuel-C4. Isotopic fractionation during combustion in ICEs depended on fuel type. The magnitude of Δ13C was 3‰ for gasoline, 2‰ for diesel, 1.5‰ for biofuel-C3, and 0.6‰ for biofuel-C4. The δ13C values of ICEs-generated PM10 were found to be dependent on the amount biobased components such as hydrotreated vegetable oil (HVO) to the fuel. However, insubstantial differences were found between the δ13C of PM derived from pure diesel and from an ethanol/diesel blend (E40).

Hydroclimatic influence on the trophic ecology of Atlantic goliath grouper juveniles (Epinephelus itajara) in a tropical estuary using non-lethal methodologies

The Atlantic goliath grouper Epinephelus itajara is the largest species of groupers in the Atlantic Ocean, reaching over 2 m total length and 400 kg. It has an ontogenetic migration, with adults using nearshore and offshore marine areas and juveniles inhabiting estuarine/mangrove areas. Despite of its high ecological relevance and classification as a threatened species for several decades in the IUCN red lists, critical phases of its life cycle, like juveniles inhabiting estuaries, are still poorly investigated. In this study, we evaluated if hydroclimatic changes may affect the trophic ecology of juveniles in a tropical estuary. We described their diet composition, isotopic niche area, food assimilation, and trophic position of juveniles across seasons using two non-lethal techniques: stomach content analysis using a stomach flushing procedure and stable isotope analysis using a dermal biopsy. A total of 87 individuals with TL ranging from 38 to 331 mm had their stomachs analyzed, of which 31 had food content. We identified 11 prey items, shrimp being the most abundant prey group (IRI% = 88.4), followed by fish (6.6), blue crab (2.8), and crab (2.2). A total of 93 specimens had their muscle's carbon (δ13C) and nitrogen (δ15N) stable isotope ratios analyzed (38–332 mm TL). Average δ13C values showed statistically significant changes across seasons, but no significant changes were observed for δ15N. As initially predicted, isotopic niche size of juveniles changed consistently along the entire hydroclimatic cycle, reaching its peak at the end of the wet season. We also observed changes in the proportion of prey assimilation by juveniles between seasons. Shrimp (92.1%) was the most assimilated prey group during the late dry season, whereas tidal crabs (36.7%), blue crab (27.8%), and shrimp (25.1%) were the most important in the late wet season. Average trophic position of juveniles remained similar across wet (3.32 ± 0.10) and dry (3.23 ± 0.11) seasons. These findings provide important information to help decision makers to promote more efficient management and conservation legislations to protect early life stages of the Atlantic goliath grouper in tropical estuaries.

Fossil and non-fossil sources of the carbonaceous component of PM2.5 in forest and urban areas

Atmospheric particulate matter (PM2.5) can damage human health. Biogenic organic compounds emitted from trees may increase the concentration of PM2.5 via formation of secondary aerosols. Therefore, the role of biogenic emissions in PM2.5 formation and the sources of PM2.5 need to be investigated. Dual carbon isotope and levoglucosan analyses are powerful tools to track the sources of total carbon (TC) in PM2.5. We collected a total of 47 PM2.5 samples from 2019 to 2020 inside a pine forest and in urban areas in South Korea. The average δ13C and Δ14C of TC in PM2.5 at the Taehwa Research Forest (TRF) were − 25.7 and − 380.7‰, respectively, which were not significantly different from those collected at Seoul National University (SNU) in urban areas. Contribution of fossil fuel, C3-, and C4- plants to carbonaceous component of PM2.5 were 52, 27, and 21% at SNU, whereas those were 46, 35, and 19% at TRF, respectively. The biomass burning tracer, levoglucosan, was most abundant in winter and correlated with the contribution of C4 plants derived carbon. Results indicate that biogenic aerosols emitted from trees is less likely to be an important source of PM2.5 and that trees can act as a bio-filter to reduce PM2.5.

Investigation of sources and atmospheric transformation of carbonaceous aerosols from Shyamnagar, eastern Indo-Gangetic Plains: Insights from δ13C and carbon fractions

This study reports the chemical characterization of the carbonaceous component of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 μm) collected over a year-long campaign from a regional site in Shyamnagar, West Bengal, in the Indo-Gangetic Plains (IGP), India. The carbonaceous fractions (elemental and organic carbon), mass concentrations, and stable carbon isotopic composition (δ13C value) of aerosols were measured and utilized to characterize the sources and understand the atmospheric processing of aerosols. Cluster analysis, Potential Source Contribution Function (PSCF) modeling, and fire count data were analyzed to decipher the pattern of air masses, source contributions, and extent of burning activities. The PM2.5 mass concentrations were significantly higher during winter (168.3 ± 56.3 μg m−3) and post-monsoon (109.8 ± 59.1 μg m−3) compared to the monsoon (29.8 ± 10.7 μg m−3) and pre-monsoon (55.1 ± 23.0 μg m−3). Organic carbon (OC), elemental carbon (EC), and total carbon (TC) concentrations were also several factors higher during winter and post-monsoon compared to monsoon and pre-monsoon. The winter and post-monsoon experienced the impact of air masses from upwind IGP. On the other hand, long-range transported air masses from the South-West direction dominated during monsoon and pre-monsoon, which are also relatively cleaner periods. The average δ13C during post-monsoon and winter was ∼1‰ higher compared to monsoon and pre-monsoon. The vehicular exhaust and biomass/biofuel burning contributed dominantly in winter and post-monsoon. In comparison, lower δ13C in pre-monsoon and monsoon might be attributed to the dominance of biomass/biofuel combustion. Photochemical-induced aging of the anthropogenic aerosols resulted in a higher δ13C of TC in winter and post-monsoon, whereas the mixing of different local sources in pre-monsoon and monsoon resulted in lower δ13C values. These findings benefit policymakers in strategizing proper and effective management of biomass/biofuel burning in the IGP to minimize air pollution.

Shift in the Mode of Carbon Cycling Recorded by Biomarkers and Carbon Isotopic Compositions in the Yanchang Formation, Ordos Basin: Autotrophy vs Heterotrophy.

Organic-rich shales and mudstones have long been investigated regarding the control of source, environment, climate, etc. on the enrichment of organic carbon. However, little is documented about how autotrophy and heterotrophy influence organic carbon cycling/export. Here, we show molecular and carbon isotopic compositional changes of the shale or mudstone source rocks from the Chang 3 to 7 members of the Yanchang Formation. The Chang 7 member source rocks have higher quality (480-500 mg/g) and total organic carbon (TOC) (15.3% on average) than other member source rocks; the sterane/hopane ratio and the δ13C of organic carbon and kerogen decrease from the Chang 3 to 7 members, but Δδ ([average δ13C of n-C17 + n-C18] - [average δ13C of pristane + phytane]) increases, and no aryl isoprenoids and C40 aromatic carotenoids (e.g., isorenieratane) were observed. These low maturity biomarker features suggest that there were no water stratification, photic zone euxinia (PZE), and no obvious change in the organic matter source, and the water column is generally anoxic. A comparison of the δ13C of Pr and Ph with the δ13C of the n-C17 and n-C18 alkanes reveals a shift in the mode of carbon cycling/export (autotrophy versus heterotrophy) in the Yanchang Formation and that there was dominant heterotrophic bacterial activity or bacterial biomass in the Chang 7 member. The TOC spike in the Chang 7 member may result from boosted carbon cycling/export that improves organic carbon preservation than other members. Possible external forcings on the shift are abundant hydrothermal- or volcanic-derived metal salts as electron acceptors in the palaeowater, which is a reasonable explanation for enhanced heterotrophic bacterial activity. This finding improves our understanding of heterotrophic bacterial activity control on organic matter (OM) preservation and may be a significant supplement for understanding the ecological or environmental forcings in the Yanchang Formation, Ordos Basin.

Biogenetic peloidal micrites within coral skeletons cause geochemical anomalies in reef limestones, Zhongsha Atoll, South China Sea

High-magnesian calcite (HMC) peloidal micrite is a ubiquitous synsedimentary cement observed in abundance in the Holocene cryptic niches of coral intraskeletal pores and borings at Zhongsha Atoll in the South China Sea. In this study, we assessed the petrographic and geochemical effects of these intraclasts during early diagenesis. Peloidal micrites with an average diameter of 20–40 μm were composed of radially arranged rhombic micro-crystals of HMC (∼20 mol% MgCO3) and massive micritic nuclei. Some living bacterial/algal clumps in peloidal micrites are associated with bioluminescence and manifest an organic-rich micro-environments. Fragmented calcified cyanobacterial sheaths, which are components of the peloidal nuclei, further enhance biogenetic carbonate precipitation throughout peloidal micrite formation. The stable isotope characteristics of the peloidal microfabrics (average δ13C of 2.55 ± 0.15‰ and δ18O of −1.64 ± 0.25‰) suggest that photosynthesis and heterotrophic decomposition promoted peloidal micrite precipitation by increasing the carbonate saturation. Plentiful peloidal micrites within the intraskeletal pores also influenced the geochemical signatures of the bulk reef limestone. Owing to the presence of mixed HMC peloidal micrites, the δ13C and δ18O values of the bulk limestone (0.26 ± 1.04 and-3.82 ± 1.02‰, respectively) were elevated and fluctuated widely compared with pristine coral skeletons (−3.43 ± 0.48‰ and − 5.75 ± 0.278‰, respectively). Enriched Mn and depleted Sr contents led to an increase in the Mn/Sr diagenetic proxy. The co-occurrence of HMC peloidal micrites and elevated Mg/Ca, Mn/Sr, δ13C, and δ18O ratios indicate that the HMC peloidal micrites played an essential role in these geochemical variations. We, therefore, propose performing a comprehensive diagenetic evaluation before geochemical analyses of reef limestone. In such cases, selective micro-analysis of pristine areas in altered limestone would yield more robust reconstructions based on corals.

Guano stable isotopic evidence of Anthropocene climate change influence on aridity and vegetation dynamics

A 2.8 m-long core of cave bat guano was obtained from an excavated pit within Bracken Cave (Texas, USA) and analyzed for δ13C and δ15N. Radiocarbon dating of the core indicated that guano deposition occurred between 1974–1989 and 1998–2016 CE. The hiatus between 1989 and 1998 CE was interpreted to be related to documented mining activities during this interval. The average Suess-corrected δ13C values for the entire series (–19.7 ± 1.0 ‰) suggest that insects ingested by the bats had a diet incorporating both C3 and C4 plants. The proportion of each appeared to be dependent on the sensitivity of vegetation to prolonged and frequent droughts. Two documented La Niña events centered around 2008 and 2011 CE occurred contemporaneously with intervals of higher δ13C values. The drought recorded in 2006 may have corresponded with foraging of insects located over C3 vegetation, which resulted in lower δ13C (–22.0 ‰). δ15N values agree well with records of drought and precipitation in Texas, suggesting an aridity control on the local nitrogen cycle. The highest δ15N values occurred during periods of a weaker North American Monsoon (1977 and 1982 CE). Higher δ15N values also occur at 2006, 2009, and 2012 CE, corresponding with years of reduced precipitation locally and throughout the region. This study further demonstrates that guano δ13C and δ15N records can provide paleo-information with respect to climatic influence on the environment in semi-arid regions.

Spatial and seasonal variations in the carbon and lead isotopes of PM2.5 in air of residential buildings and their applications for source identification

To understand isotope distributions of PM2.5 in residential buildings and apply them for source identification, carbon (δ13C) and lead (Pb) isotope ratios in indoor and outdoor air of residential buildings were analyzed. Moreover, factor analysis (FA) was employed to investigate sources, which were compared through isotopic analyses. The average δ13C values of indoor air are −26.94 ± 1.22‰ and −27.04 ± 0.44‰ in warm (August to October) and cold (February to March) seasons, respectively, and the corresponding values for outdoor air are −26.77 ± 0.54‰ and −26.57 ± 0.39‰. The average 206Pb/207Pb (208Pb/207Pb) ratios of indoor air are 1.1584 ± 0.0091 (2.4309 ± 0.0125) and 1.1529 ± 0.0032 (2.4227 ± 0.0081) in warm and cold seasons, respectively, and the corresponding values for outdoor air are 1.1594 ± 0.0069 (2.4374 ± 0.0103) and 1.1538 ± 0.0077 (2.4222 ± 0.0085). Seasonal variation in δ13C values or Pb isotope ratios of indoor air was not significant, and similar results were obtained for outdoor air. Significant differences were not observed between δ13C values or Pb isotope ratios of indoor and outdoor air. Traffic emission is the major contributor to indoor and outdoor PM2.5 based on isotopic analyses; this result was consistent with the results of FA. The δ13C values of indoor air in buildings with poor ventilation conditions were significantly lighter than those of outdoor air. In summary, the spatial and seasonal variations of isotopes were similar in residential buildings, which can be used to identify sources of indoor PM2.5, and ventilation condition is an influencing factor.