Organic Carbon Stable Isotope Research Articles

Response of mangrove development to paleoclimate variation over the past 3,550 years in Phang Nga Province, Thailand

High-resolution reconstruction of the development of mangrove forests in the late Holocene is the basis for scientifically predicting the fate of mangroves. In this paper, we analyze the contribution of mangrove-derived organic matter (CMOM) from sediment core G in Phang Nga Province, Thailand by using a ternary end-member mixing model based on stable organic carbon isotope (δ13C) and the molar ratio of total organic carbon to total nitrogen (C/N (molar)). The variations of the CMOM in Phang Nga Province over the past ∼3,550 cal yr BP indicate that mangrove forests underwent three periods of flourishing: ∼3,550–2,720 cal yr BP, ∼1,960–1,240 cal yr BP and ∼500–0 cal yr BP, and two periods of degradation: ∼2,720–1,960 cal yr BP and ∼1,240–500 cal yr BP. Of the potential factors that affect mangrove development, relative sea level (RSL) changes and regional hydrological conditions did not have notable effects on mangrove flourishing/degradation. However, paleoclimate variations, especially the air temperature and rainfall controlled by the Indian Summer Monsoon (ISM) were the primary factors. The mangrove development reconstructed by the CMOM in Phang Nga Province, Thailand was extremely synchronized with that in the Yingluo Bay and Qinzhou Bay of Southwest China and on the west coast of India Peninsula. This study provides evidence that mangrove development controlled by the Asian monsoon in tropical Asia has regional synchronism.

Introduction of a New Method for Total Organic Carbon and Total Nitrogen Stable Isotope Analysis of Dissolved Organic Matter in Aquatic Environments

Introduction of a New Method for Total Organic Carbon and Total Nitrogen Stable Isotope Analysis of Dissolved Organic Matter in Aquatic Environments

Late Miocene paleoecology and paleoclimate in the central High Plains of North America reconstructed from paleopedological, ichnological, and stable isotope analyses of the Ogallala Formation in western Kansas, USA

The late Miocene was a critical time in the development of the North American Great Plains marked by paleoclimate-driven biotic change, including faunal turnovers and the spread of C4 dominated grasslands. The large volume of sediment shed from the Rocky Mountain region during this time preserves a record of these transitions, which can be informed by previously undescribed paleosol and trace fossil properties from the Ogallala Formation of the central High Plains. The purpose of this paper is to reconstruct paleoenvironments, paleoclimate, and paleoecology from outcrops of the Ogallala Formation in western Kansas by integrating paleopedology, ichnology, and stable isotope geochemistry. Eleven lithofacies are recognized in the study area, mostly massive to crudely stratified, fine-to coarse-grained sandstone and pebbly gravel with stratigraphically uncommon fine-grained lithofacies restricted to thinly bedded intervals or lens-shaped geometries within the sand-dominated strata. These host five pedotypes: 1) calcic Vertisols developed on overbank fines, 2) Entisols developed on braid bar gravels, 3) Entisols developed on volcaniclastic sediment lenses, 4) calcic Inceptisols developed on coarse sandy channel fills, and 5) calcareous Mollisols developed on fine sandy bedforms and channel fills. We recognize ten ichnogenera within paleosols, including burrows attributed to ants, bees, beetles or hemipterans, vinegarroon-like arthropods, fossorial reptiles, seed caching mammals, and large carnivorous mammals. Organic carbon stable isotopes indicate that the flora consisted entirely of C3 plants, and paleosols and trace-fossil evidence suggest a tree-limited savanna environment with patches of unvegetated soil. Paleoclimate proxies from paleosol and trace fossil properties indicate mean annual air temperatures between 8 °C and 20 °C with seasonal differences of up to 14 °C between mean monthly temperatures of the warmest and coolest months. Mean annual precipitation was likely between 250 mm and 460 mm with a seasonal difference of up to 250 mm between mean monthly precipitation of the driest and wettest months. While hymenopteran tracemakers were active, soil surface temperatures reached at least 30 °C and moisture content was between 10% and 20%. With timing of deposition constrained biostratigraphically to the Barstovian through Hemphillian, our paleoclimate interpretations are most consistent with the period of relative climate stability after Middle Miocene Climate Transition cooling and before Late Miocene Cooling (∼13.8 Ma–7 Ma). The trace-fossil assemblage reveals previously unknown biodiversity among soil arthropods, as well as important trophic connections between the belowground and aboveground components of the terrestrial food web.

Disentangling carcass size and climate effects on soil biogeochemistry during decomposition

When an animal dies the decomposition of its carcass releases multiple compounds that become available to soil microbiota altering soil biogeochemistry. This zone of affected substrate is characterized by significant changes to numerous soil biogeochemical parameters including pH, conductivity, microbial respiration rates, dissolved organic carbon (DOC), C/N ratios, carbon and nitrogen stable isotopes (δ13C and δ15N), and major/trace elemental concentrations. Although the effects of animal decomposition on these parameters have been recorded for various organisms and in a variety of ecosystems several knowledge gaps remain, including: the effect of carcass size; and decomposition effects in prairie ecosystems (i.e., semi-arid climates). the effect of carcass size; and decomposition effects in prairie ecosystems (i.e., semi-arid climates). Here, we investigate how soil biogeochemical parameters are affected by the decomposition of a horse carcass (~660 kg) in a prairie ecosystem in comparison to beavers (~18 kg) in a temperate forest. Three parameters were identified as significantly influenced by carcass size: conductivity, DOC, and microbial respiration rate. Average soil conductivity underneath the horse carcass was three to fifteen times higher than control soils and reached a maximum at 1114 ADD, while maximum soil conductivity values were observed after 160 ADD in beaver-associated soils. Maximum DOC concentrations were observed after 160 ADD in beaver-associated soil (67 ± 40 mg C gdw-1) and after 1114 ADD in horse-associated soil (326 ± 115 mg C gdw-1). Microbial respirations rates were both greater in horse-associated soil and longer-lasting compared to beaver-associated soil. Respiration rates were greatest in two of the three horse-associated soils after 1114 ADD (~733 µg CO2-C gdw-1 day-1), which significantly differs compared to beaver-associated soils (300 ± 90 µg CO2-C gdw-1 day-1 at 160 ADD). Taken together, these results demonstrate that larger carcasses result in a greater release of decomposition products, including C, and greater stimulation of soil heterotrophic communities. Other measured biogeochemical indicators of decay suggest a mixed influence due to carcass size and climate, notably δ15N values of the soil, and major element concentrations. Climate was revealed to be more important in controlling changes to pH and gravimetric moisture than carcass size, with baseline soil conditions and type playing critical roles. Additionally, in semi-arid regions such as western South Dakota, the role of wet-dry cycles on carcasses undergoing decomposition may help to explain the pulses observed with C concentrations and C cycling. This study provides the first direct comparison of soil biogeochemistry associated with the decomposition of two different sized taxa decayed in two different climates. Attempting to normalize climate data using ADD was found to be an imperfect system that does not account for all climate variables affecting decomposition. Unravelling patterns in decomposition can better help to constrain nutrient cycling in modern and ancient ecosystems.

Carbon dynamics of the plant-soil system during vegetation succession in dune-meadow cascade ecosystems in Horqin Sandy Land, China

Understanding the relationship between the plant-soil carbon dynamics and environment is important for the management of arid and semi-arid ecosystems. Under long-term overgrazing and reclamation, the landscape of the Horqin Sandy Land has transformed into a dune-meadow ecotone. However, the effects of landscape evolution on the carbon dynamics in this region remain poorly investigated. This study evaluated changes in plant-soil organic carbon content and stable carbon isotope ratios (δ13C) attributable to vegetation restoration in dune ecosystems and wetland degradation in meadow ecosystems, and evaluate the indicative implications and driving mechanisms of the 13C enrichment in the plant-soil system. The dune ecosystems comprised semimobile dune with 25- and 15-year-old Artemisia Halodendron communities (SMAH1, SMAH2), fixed dune with 30- and 40-year-old Caragana microphylla communities (FCM1, FCM2), semimobile dune with 40-year-old Salix gordejevii community (SMSG), and transitional zone with a 30-year-old artificial Populus forest (TPF) in the gentle lower part. The meadow ecosystems comprised a corn field that transformed from natural meadow over 40 years (MCF) and a natural meadow with Phragmites australis community (MPA). The results show that plant leaves in different landscapes in the cascade ecosystems had various δ13C. And the mechanism of the leaves carbon isotope fractionation with increasing leaf age was significant and consistent, but different vegetation communities had different nutrient utilization strategies during ontogeny. Landscape types and soil depths are the key factors affecting SOC content, its δ13C (δ13Cs) and SOC stock changes. And the overall soil carbon turnover was accelerated with vegetation restoration. Especially in TPF, vegetation restoration is particularly significant for SOC, which is mainly reflected in the upper 20 cm of soil profile. The degradation of MPA to MCF led to loss of SOC content and storage and accelerated the soil carbon turnover rate. The δ13Cs of the dune and meadow ecosystems were positively correlated with organic-rich litter nitrogen content and were significantly negatively correlated with SOC, soil moisture, clay content, and total organic carbon to total nitrogen ratio of organic-rich litter. The revegetation using native and introduced tree and shrub species increased SOC content in dune ecosystems and significantly promoted the degree of its coupling with soil nitrogen and water.

First evidence of fractionation on stable organic carbon isotopes in ancient Egyptian linen textile fibers during degradation

The study of the fractionation of the stable carbon isotopes 13C and 12C through the δ13C value finds applications in various fields of science, from quality control in agribusiness to the study of the geographical origin of materials. At the plant level, this information is useful to prove which photosynthetic cycle a particular plant belongs to: C3, C4 or Crassulacean Acid Metabolism. In this work, the Isotopic Ratio Mass Spectrometry technique was used to shed light and investigate about the degradation due to aging of an ancient linen textile of Egyptian origins, in order to see whether degradation phenomena change the content of stable carbon isotopes, over time. For this purpose, the δ13C values of the ancient sample were compared with contemporary Egyptian and French flax fibers samples. For the first time, it was observed that, contrary to what might have been expected, material degradation due to natural aging contributes to the phenomenon of the isotopic fractionation, due to the increasing in measured δ13C values from contemporary to ancient samples.

Sources of organic matter and carbon stocks in two mangrove sediment cores and surface sediment samples from Qinglan Bay, China

The carbon stocks (Corg stocks) in mangrove sediments and the distribution and source changes of sedimented organic matter in Qinglan Bay are not understood as the mangrove forests decrease. In this paper, we collected two sediment cores in the interior mangrove and 37 surface sediment samples from mangrove-fringe, tidal flat and subtidal habitats and then analysed the total organic carbon (TOC), total nitrogen (TN), and the stable organic carbon isotope (δ13C) and nitrogen isotope (δ15N) in the sediment samples to obtain the organic matter sources and carbon stocks in two different mangrove sediment cores from Qinglan Bay. The δ13C and TOC/TN values showed that mangrove plants and algae were the main sources of organic matter. Relatively high contributions of mangrove plants (>50 %) were distributed in the mangrove areas in Wenchang estuary, the northern region of Bamen Bay and the eastern side of the Qinglan tidal inlet. The enriched δ15N values might be related to anthropogenic nutrient inputs, including increased aquaculture wastewater, human sewage and ship wastewater. The Corg stocks in cores Z02 and Z03 were 357.79 Mg C ha−1 and 265.78 Mg C ha−1, respectively. This Corg stock difference might have been related to the salinity and the benthos activities. The high Corg stock values measured in Qinglan Bay were caused by the mangrove maturity and stand age. The total Corg storage of the mangrove ecosystem in Qinglan Bay was estimated to be approximately 263.93 Gg C. This study contributes to organic carbon stocks and sources of sedimented organic matter in global mangroves.

Short-chain n-alkanes in benthic mats and mosses from the Larsemann Hills, East Antarctica

Variation in leaf colour (green, red and grey) of mosses and lake benthic mats in Antarctica is often linked to water stress and ultraviolet light (UV-B) exposure. Changes in the abundance of organic compounds, such as pectin and phenols, are associated with mechanisms protecting against desiccation and UV radiation. However, the function of n-alkanes, especially against UV radiation, is rarely examined. Here, gas chromatography–mass spectrometry and Fourier-transform infrared spectroscopy analyses were performed to study the variation in n-alkanes in freshwater lake benthic mats and mosses collected from the Larsemann Hills in East Antarctica. Stable isotopes of organic carbon and nitrogen, environmental DNA characterisation and microscopy-based analyses are used to estimate the presence of cyanobacteria, algae and diatoms in moss and benthic mat consortia. Variation in the short-chain (n-C17 to n-C20) versus long-chain (n-C21 to n-C30) n-alkanes in the mosses and benthic mats with their colour were noted. The research links the relative abundance of short-chain n-alkanes to the UV-B exposure and proposes that Antarctic mosses and benthic mats synthesise short-chain n-alkanes for protection against UV-B.

Carbon isotope and biostratigraphic evidence for an expanded Paleocene–Eocene Thermal Maximum sedimentary record in the deep Gulf of Mexico

AbstractIn this study, we present evidence of a Paleocene–Eocene Thermal Maximum (PETM) record within a 543-m-thick (1780 ft) deep-marine section in the Gulf of Mexico (GoM) using organic carbon stable isotopes and biostratigraphic constraints. We suggest that climate and tectonic perturbations in the upstream North American catchments can induce a substantial response in the downstream sectors of the Gulf Coastal Plain and ultimately in the GoM. This relationship is illustrated in the deep-water basin by (1) a high accommodation and deposition of a shale interval when coarse-grained terrigenous material was trapped upstream at the onset of the PETM, and (2) a considerable increase in sediment supply during the PETM, which is archived as a particularly thick sedimentary section in the deep-sea fans of the GoM basin. Despite other thick PETM sections being observed elsewhere in the world, the one described in this study links with a continental-scale paleo-drainage, which makes it of particular interest for paleoclimate and source-to-sink reconstructions.

RECONSTRUCTING KUNGURIAN (CISURALIAN, PERMIAN) TERRESTRIAL ENVIRONMENTS WITHIN A MEGACALDERA IN THE SOUTHERN ALPS (N-ITALY) USING LITHOFACIES ANALYSIS, PALYNOLOGY AND STABLE CARBON ISOTOPES

During the Permian, climate experienced a change from icehouse to greenhouse conditions. Few multidisciplinary studies have investigated Kungurian (late Cisuralian) tropical terrestrial ecosystem and climate changes. Here, we apply an interdisciplinary approach to two alluvial-lacustrine successions of the Athesian Volcanic Group (Southern Alps, northern Italy) deposited in a Kungurian megacaldera during periods of volcanic quiescence. Sedimentological analysis combined with palynofacies studies allowed the reconstruction of the depositional environments. The study of sporomorph assemblages and stable organic carbon isotopes provided information on plant communities and the climate context. Two different depositional environments were present in the megacaldera: one proximal and one more distal with respect to the source, distinguished by a slightly different composition of the sediments, palynofacies and organic carbon isotopes. The plant community in the area was dominated by xeromorphic-hygromorphic taxa. The δ13Corgvalues are comparable to those of other Cisuralian continental organic matter and plants. The stable carbon isotope values show a small variability, which correlates weakly, but significantly, with the abundance of xeromorphic elements. All observations support deposition during semiarid to arid climate conditions, typical of the mid-late Cisuralian in the area.

Reconstruction of landscape and climate of the largest drainage basin in the Ladakh Range, NW Trans Himalaya during the last 7000 years

Landform geomorphology and glacial lake deposits in the largest drainage basin of the Ladakh Range i.e. Chang La-Tangtse basin were studied to infer their palaeoclimatic significance. The grain size, mineral magnetism, percentage loss on ignition (%LOI) and organic carbon stable isotope (δ13C) data in combination with total organic carbon (TOC) supported by AMS 14C (calibrated) dates of a 190 cm long section from the Chang La-Tsoltak palaeolake provides a climatic record since the last ∼ 7075 cal yr BP. The χlf, χARM, and SIRM values suggest that the catchment-derived palaeolake sediments predominantly contain magnetically “soft” minerals like magnetite and maghemite. The δ13C values range between −21 and −24 ‰ with an average of –22 ‰ which suggests a mixed C3-C4 plant signature and water stressed ecosystem. The relatively small variations in the δ13C values of organic matter in the entire lake profile suggest a stable climatic condition. The prominent effect of westerlies is seen between 7075 and 6040 cal yr BP with huge detrital influx at the lake bottom indicating a glacial advancement in the region. The affect of Mid-Holocene warm period is evident at 6040 cal yr BP with the advent of ISM. Paradigm shifts in the proxy values are observed at 5710, 4890, 3435, and 2800 cal yr BP. The influence of westerlies gradually reduces at 2800 cal yr BP. The landscape evolution and the climatic variations in the Trans-Himalaya are primarily governed by westerlies and do not correspond to the Indian monsoon variability records, particularly during the Mid-Holocene Thermal Maxima. Several other regions of the Ladakh Range also record similar climatic variations, indicating that the palaeolake sediments also reflect regional climate variations.

Holocene variations in Lake Titicaca water level and their implications for sociopolitical developments in the central Andes

Holocene climate in the high tropical Andes was characterized by both gradual and abrupt changes, which disrupted the hydrological cycle and impacted landscapes and societies. High-resolution paleoenvironmental records are essential to contextualize archaeological data and to evaluate the sociopolitical response of ancient societies to environmental variability. Middle-to-Late Holocene water levels in Lake Titicaca were reevaluated through a transfer function model based on measurements of organic carbon stable isotopes, combined with high-resolution profiles of other geochemical variables and paleoshoreline indicators. Our reconstruction indicates that following a prolonged low stand during the Middle Holocene (4000 to 2400 BCE), lake level rose rapidly ~15 m by 1800 BCE, and then increased another 3 to 6 m in a series of steps, attaining the highest values after ~1600 CE. The largest lake-level increases coincided with major sociopolitical changes reported by archaeologists. In particular, at the end of the Formative Period (500 CE), a major lake-level rise inundated large shoreline areas and forced populations to migrate to higher elevation, likely contributing to the emergence of the Tiwanaku culture.

The East Gotland Basin (Baltic Sea) as a candidate Global boundary Stratotype Section and Point for the Anthropocene series

The short sediment core EMB201/7-4 retrieved from the East Gotland Basin, central Baltic Sea, is explored here as a candidate to host the stratigraphical basis for the Anthropocene series and its equivalent Anthropocene epoch, still to be formalized in the Geological Time Scale. The core has been accurately dated back to 1840 CE using a well-established event stratigraphy approach. A pronounced and significant change occurs at 26.5 cm (dated 1956 ± 4 CE) for a range of geochemical markers including 239+240Pu, 241Am, fly-ash particles, DDT (organochlorine insecticide), total organic carbon, and bulk organic carbon stable isotopes. This stratigraphic level, which corresponds to a change in both lithology and sediment colour related to early anthropogenic-triggered eutrophication of the central Baltic Sea, is proposed as a Global boundary Stratotype Section and Point for the Anthropocene series.

Geochemistry of the Cretaceous Mowry Shale in the Wind River Basin, Wyoming

The siliceous nature of the Mowry Shale distinguishes it from many of the well-studied organic-rich mudstones of the Cretaceous Western Interior Seaway. Available models of organic enrichment in mudstones rarely incorporate detailed biomarker, bulk organic, inorganic, and mineralogy data. Here, we used these data to evaluate how variations in organic matter source, productivity, dilution, and preservation modulated organic matter accumulation during the deposition of the Mowry Shale, while also demonstrating the benefits of this integrated approach. An organic stable carbon isotope vertical profile for the Mowry Shale is presented to test whether the Mowry Shale was deposited during oceanic anoxic event 1d (OAE 1d), thereby contributing to organic enrichment in the Mowry Shale. Samples were analyzed from three thermally immature drill cores collected from the Wind River Basin, Wyoming to advance our understanding of the conditions that led to the formation of the Mowry Shale. Results from bulk organic and inorganic geochemistry and biomarkers show that redox-driven preservation is closely coupled with stratigraphic changes in organic matter content in intermediate to distal settings of the Mowry Shale. Biogenic silica dilution decouples productivity and organic enrichment, such that the interval that was deposited during the highest productivity is offset from the interval containing maximum organic content. These findings contrast with previous studies that identified productivity as the primary driver of organic enrichment in distal settings of the Mowry Shale. The middle and lower Mowry Shale contain primarily marine organic matter, whereas the relative contribution of terrestrial organic matter increases in the upper Mowry Shale and overlying Frontier Formation as relative sea level declined and the western shoreline prograded eastward. Finally, the organic stable carbon isotope profile of the Mowry Shale together with recent radioisotopic dating suggest that the Mowry Shale postdates OAE 1d by ∼1−2 million years (m.y.), providing another example of temporal offset between organic matter accumulation in the Cretaceous Western Interior Seaway compared to other regions that record global OAEs.

Spatial distribution and environmental significance of modern organic carbon and nitrogen stable isotopes in the source area of the Yellow River: A case study of Gyaring Lake and Ngoring Lake

Organic carbon and total nitrogen isotopes (δ13Corg and δ15N) are useful for identifying sources of organic matter and for reconstructing environmental conditions in paleoecology and paleoclimatology. Gyaring Lake and Ngoring Lake are the two largest freshwater lakes in the source area of the Yellow River (SAYR), which lies in the northeastern the Qinghai–Tibet Plateau. These bodies of water were chosen as our research objects, and we analyzed the carbon and nitrogen isotopes in the surface sediments of the two lakes and in soil samples from around the lakes. The results indicate the consistency and endogenous characteristics of the organic matter sources of the distribution of δ13Corg and δ15N in the surface sediments of Gyaring Lake and Ngoring Lake. The δ13Corg values of the two lakes were significantly positive in areas near the inflow and outflow of the Yellow River. The distribution of δ15N in Gyaring Lake was similar to that of total organic carbon (TOC) and total nitrogen (TN), and its values were relatively higher than those affected by the joint influence of the Yellow River and some small tributaries, while δ15N was more positive in the deep-water area of Ngoring Lake. This study provides reliable and modern data on δ13Corg and δ15N for the long-term paleoenvironment and paleoecological reconstruction of the SAYR in the future.

Spring soil CO2 and CH4 emissions in the Yellow River Delta wetland, China

Climate change has become a global concern. Coastal wetlands play an important role in the global carbon cycle and significantly contribute to the emission and absorption of the greenhouse gases CO2 and CH4. In this study, the temporal and spatial variations of soil CO2 and CH4 emission fluxes, soil total carbon, total organic carbon, and carbon and nitrogen stable isotopes of four vegetation areas (Suaeda salsa, Phragmites australis, Tamarix chinensis, and farmland) and a bare beach in the Yellow River Delta wetland were observed in the spring of 2011–2013. Soil organic carbon δ13C ranged from −25.43–-23.69‰ and soil organic carbon content from 0.311 to 1.42%, and the two were significantly negatively correlated, indicating a low decomposition degree of soil organic matter. The soil CO2 emission fluxes in the study period were carbon emissions. In the area with mainly Tamarix chinensis, the 3-year average CO2 emission flux was the highest, in the area with predominantly Phragmites australisthe lowest. The soil CH4 emission flux in the spring of 2011 was a carbon absorption, whereas it was a carbon emission in the spring of 2012 and 2013. Its 3-year average was the highest in the bare beach area and the lowest in the Tamarix ramosissima area. Based on Pearson correlation analysis combined with RDA and multiple linear regression analysis, we found that the main factors affecting soil CO2 and CH4 emission fluxes may be the fast decomposition rate of soil organic matter, the content of soil carbon and organic carbon, and the low degree of soil organic matter decomposition.

A paleopedological approach to understanding Eocene environmental conditions in southern Patagonia, Argentina

The Eocene Las Flores and Koluel-Kaike formations in southern Patagonia (~48° S, Golfo San Jorge Basin, Argentina) are pedogenically modified fluvial and fluvio-eolian successions, respectively, which document early-middle Eocene environments at mid-paleolatitudes in the Southern Hemisphere. In order to reconstruct the paleoenvironment for the Las Flores and Koluel-Kaike formations, we performed a multiproxy and coordinated study of sediments and paleosols of both units, considering abiotic components. Using detailed sedimentology and paleopedology (macro- and micromorphology), bulk paleosol geochemistry and clay mineralogy, and organic carbon concentrations and stable isotope (δ13Corg) compositions, we show that the Las Flores and Koluel-Kaike paleosols are overall Ultisol-like paleosols, mineralogically and chemically consistent with a high to high-moderate degree of weathering, and developed on different parent materials (sedimentary with sandy and silty texture vs. silty volcaniclastic). Climate proxies and a comparison with modern Ultisols with similar features suggest that these paleosols formed under a broadly tropical-temperate and humid-subhumid climate with distinct seasonality. Overall, these combined data record long-term environmental conditions during the Paleogene (early-middle Eocene), and preserve a record of Eocene terrestrial climate in the Southern Hemisphere. This research is relevant for understanding latitudinal climatic gradients during warm periods like the Eocene, a key knowledge gap for future predictions, and these sites are particularly important because mid-latitude reconstructions in the Southern Hemisphere are the poorest resolved.

Proxy Response Heterogeneity to the Indian Monsoon During Last Millennium in the Himalayan Region

We reviewed the available climate records for the past 2 millennia based on the analyzed sediment and speleothem archives from different regions of South Asia. Speleothem records from the core-monsoon regions of the Indian sub-continent have revealed the Little Ice Age (LIA) as a climatically dry phase, whereas the same from the western and central Himalaya recorded LIA as wet. Moreover, the sediment-derived vegetation proxy records [pollen-spores and stable organic carbon isotope (δ13Corg)] from the western Himalaya also reported LIA as a dry phase. Heterogeneous results by different proxies during LIA enhanced our interest to understand the response of the proxies toward the primary precipitation sources, Indian summer monsoon (ISM) and winter westerly disturbances (WDs), over the Himalaya. We emphasize that in the Himalayan region, the vegetation predominantly responds to the ISM dynamics, whereas speleothem also captures the WD effect.

Carbon-isotope excursions in the Norian stage (Upper Triassic) of the Baoshan terrane, western Yunnan, China

The biostratigraphy and carbon stable isotopes of the Norian Stage (Upper Triassic) are well studied in the western Tethys, but little information is available from the eastern Tethys. Therefore, we studied the Hongyan-B (HYB) section in the Baoshan terrane, western Yunnan Province, SW China, which was located in the eastern Tethys during the Late Triassic. The HYB section was investigated for conodonts, radiolarians, total organic carbon, carbonate carbon and oxygen stable isotopes (δ13Ccarb and δ18Ocarb), and carbon isotopes of organic matter (δ13Corg). The Mockina slovakensis and Mockina bidentata conodont biozones proposed in the HYB section are associated with two radiolarian zones of the Sevatian substage, the Praemesosaturnalis multidentatus and Praemesosaturnalis pseudokahleri zones (TR8A and TR8B Sugiyama zones). This biostratigraphy places the Alaunian-Sevatian substage boundary (middle-upper Norian) at meter 23 of the HYB section. Unfortunately, the δ13Ccarb and δ18Ocarb values suggest that they might have been influenced by diagenesis. However, the δ13Corg record preserves multiple carbon-isotope excursions across the Alaunian-Sevatian boundary that can be correlated with other coeval global sections. The carbon-isotope excursions registered at this middle-upper Norian boundary are perhaps related to the major outgassing of light carbon during the emplacement of Angayucham flood basalts.

Distributions and potential sources of traditional and emerging polycyclic aromatic hydrocarbons in sediments from the lower reach of the Yangtze River, China

This study investigated the current contamination status and potential sources of traditional and emerging polycyclic aromatic hydrocarbons (t-PAHs and e-PAHs) in the sediments across a wide area of the Yangtze River, spanning nine cities. Fifty-seven sediment samples were collected in 2019, from which 15 t-PAHs and 11 e-PAHs were analyzed using GC-MSD. In addition, organic carbon (OC), total nitrogen (TN), and carbon and nitrogen stable isotope ratios (δ13C and δ15N) in sediments were measured to evaluate associations with PAHs contamination. OC, TN, and their stable isotope ratios showed a wide range of site-specific contents and values, indicating high variation in contamination and sources. Concentrations of t-PAHs and e-PAHs in sediments ranged from 0.6 to 200,000 ng g−1 dry weight (dw) and 1.1 to 20,000 ng g−1 dw, respectively. Hotspot sites located in Nanjing (PuKou), Taizhou (JingJiang), and Suzhou (ZhangJiaGang). PAHs contamination reflected land use type and human activity in the surrounding area. Positive matrix factorization (PMF) modeling showed that, on average (n = 57), vehicle emissions were the most dominant contribution (57%), followed by petroleum (28%) and fossil fuel combustion (15%). Sites with high PAHs contamination in sediments were of severe ecological risk. Contributions to the potential risks of PAHs were most significant in the order of dibenz[a,h]anthracene, benzo[b]fluoranthene, and benzo[a]pyrene. The primary origin of these compounds appeared to be fossil fuel combustion. The results of this study are expected to provide useful baseline data on the current contamination status and potential sources of traditional and emerging pollutants in the sediments of the Yangtze River, China.