Stable Carbon Isotope Research Articles

The Validity of Optical Properties as Tracers of Terrigenous Dissolved Organic Carbon During Extensive Remineralization in Coastal Waters

AbstractTerrestrial dissolved organic carbon (tDOC) is significant for coastal carbon cycling, and spectroscopy of chromophoric and fluorescent dissolved organic matter (CDOM, FDOM) is widely used to study tDOC cycling. However, CDOM and FDOM are often amongst the more labile components of tDOC. Because few studies have compared spectroscopy to measurements of both bulk tDOC concentration and tDOC remineralization, it remains unclear how accurately CDOM and FDOM actually trace tDOC in coastal waters when tDOC undergoes extensive remineralization. We collected a 4‐year coastal timeseries in Southeast Asia, where tropical peatlands provide a large tDOC input. A carbon stable isotope mass balance shows that on average 53% of tDOC was remineralized upstream of our site, while 74% of CDOM was bleached. Despite this extensive tDOC remineralization and preferential CDOM loss, optical properties could still reliably quantify tDOC. CDOM spectral slope properties, such as S275–295, are exponentially related to tDOC; these are highly sensitive tDOC tracers at low, but not at high, tDOC concentrations. Other properties are linearly related to tDOC, and both specific ultraviolet absorbance (SUVA254) and DOC‐normalized fluorescence intensity may be suitable to quantify tDOC over a wider range of concentrations. However, the optical properties did not show consistent changes with the extent of tDOC remineralization. Our data support the validity of CDOM and FDOM spectroscopy to trace tDOC across coastal gradients even after the majority of tDOC has been remineralized, but they also show that these measurements may not provide direct information about the degree of natural tDOC processing.

A long-term study of stable isotope ratios of fingernail keratin and amino acids in a mother-infant dyad.

To evaluate the potential of compound-specific isotope analysis of amino acids (CSIA-AA) for investigating infant feeding practices, we conducted a long-term study that compared infant and maternal amino acid (AA) nitrogen isotope ratios. Fingernail samples were collected from a single mother-infant dyad over 19 months postpartum. Carbon and nitrogen stable isotope ratios were measured in the bulk keratin of the fingernail samples. Selected samples were then hydrolyzed and derivatized for compound-specific nitrogen isotope analysis of keratin AAs. As in previous studies, infant bulk keratin nitrogen isotope values increased during exclusive breastfeeding and fell with the introduction of complementary foods and eventual cessation of breastfeeding. Infant trophic AAs had elevated nitrogen isotope values relative to the mother, while the source AAs were similar between the mother and infant. Proline and threonine appeared to track the presence of human milk in the infant's diet as the isotopic composition of these AAs remained offset from maternal isotope values until the cessation of breastfeeding. Although CSIA-AA is costly and labor intensive, it appears to hold potential for estimating the duration of breastfeeding, even after the introduction of complementary foods. Through the analysis of a full suite of AAs, it may also yield insights into infant physiology and AA synthesis.

Chironomids regulate long‐chain polyunsaturated fatty acid levels independent of lake nutrient or dissolved organic carbon concentrations

Chironomids are keystone primary benthic consumers with semi‐aquatic life cycles. They support aquatic and terrestrial consumers at higher trophic levels by conveying dietary nutrients, such as fatty acids. In this study, we combined field sampling and laboratory experiments to examine the effects of environmental parameters, including diet, on fatty acid composition and metabolism in chironomid larvae and imagines. Results from 53 lakes showed that lake size, depth, dissolved organic carbon (DOC) concentrations, and trophic state had only marginal effects on the content of long‐chain polyunsaturated fatty acids (LC‐PUFA) in chironomids. Compound‐specific stable hydrogen isotope analyses confirmed that chironomids actively bioconvert dietary fatty acid precursors to LC‐PUFA in all lake types, independent of nutrient or DOC concentrations. Moreover, fatty acid‐specific stable carbon isotope data indicated that the diet of chironomids was subsidized, particularly in oligotrophic lakes in spring, by terrestrial C18 fatty acid precursors that were converted to LC‐PUFA. Data from feeding experiments further confirmed that decreased dietary availability of LC‐PUFA enhanced the conversion of dietary short‐chain precursors to LC‐PUFA. These results suggest that chironomids are PUFA regulators that can sustain LC‐PUFA levels under varying environmental conditions. Furthermore, our results indicate that they bioconvert terrestrial low‐quality material to high‐quality resources, which, via chironomid emergence, support terrestrial food webs. Chironomids are abundant and widespread, and thus, the trophic transfer of LC‐PUFA can have significant implications for the fitness and production of upper trophic level consumers in both aquatic and terrestrial ecosystems.

Nutrient Shift in the Yucatan Basin during the late Early Miocene: Evidence from calcareous nannofossils and stable isotopes at ODP Leg 165 Site 998 Hole A

Paleoceanographic reconstructions in the Caribbean Sea during past climate transitions, namely the Late Oligocene to Early Miocene (up to the Miocene Climatic Optimum and Monterey Excursion) are crucial to understanding the effects of the Eastern Pacific interchange with the Atlantic Ocean. Studies of the planktonic domain in the Yucatan Basin during the Early Miocene are limited, with most Caribbean research focused on reconstructing the series of events that precede the final closure of the Isthmus of Panama in the Early Pliocene. Furthermore, paleoceanography of the Yucatan Basin is largely unrefined during this time, even though it is an important transition for ocean gateways, and development of Atlantic deepwater and Atlantic Meridional Overturning Current (AMOC), driving global ocean circulation. Here we conduct a relative percent abundance analysis of calcareous nannofossils and stable isotopes from bulk sediment of drill cores spanning the Upper Oligocene to Lower Miocene in the central Yucatan Basin, collected by the Ocean Drilling Program Leg 165 at Site 998 Hole A. Nannofossil assemblage data record a shift from a eutrophic to oligotrophic system ∼19.2–17.65 Ma. Eutrophic taxa (Reticulofenestra spp. and Cyclicargolithus floridanus) decline at ∼20.23 Ma, while oligotrophic taxa (discoasters and sphenoliths) increase as stable carbon isotope values decline. The palaeoecological turnover coincides with a possible restriction in the Central American Seaway (CAS) ∼20.7–21 Ma. Population dynamics of the Hole 998 A nannofossil assemblage track with global declines in C. floridanus relative abundance between ∼20.2 to ∼20.5 Ma, followed by a subsequent decrease in Reticulofenestra spp. These data suggest water mass stratification in the Yucatan Basin during the Early Miocene ∼19.2–17.65 Ma, likely due to restriction of the CAS.

The increasing influence of oyster farming on sedimentary organic matter in a semi-closed subtropical bay

Oyster farming activities play a pivotal role in the biogeochemical cycles of coastal marine ecosystems, particularly in terms of sedimentary carbon cycling. To gain deep insights into the influence of expanding oyster culture on the sedimentary carbon cycle, surface sediments were collected from the Maowei Sea, which is the largest oyster farming bay in south China, based on six filed surveys between July 2010 and December 2022. The sediment samples were analyzed for total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ13C and δ15N) to evaluate the inter-annual variations in the source contribution to sedimentary organic matter (SOM). The results revealed that the average contents of sedimentary TOC and TN were 0.67 ± 0.41 % and 0.06 ± 0.03 %, respectively. Fluctuations in the C/N molar ratios ranged from 5.8 to 23.6, with an average of 12.6 ± 2.9, indicating a significant terrestrial input contribution to SOM in the study area. Furthermore, the integration of stable isotope analysis and Bayesian mixing model demonstrated a gradual increase in the mean proportion of shellfish biodeposition to SOM, from 12.0 ± 5.6 % in July 2010 to 21.1 ± 7.3 % in December 2022, consistent with the progressive expansion of oyster aquaculture along this coastal area, thereby emphasizing the substantial influence of oyster farming on SOM composition. With the anticipated expansion of oyster farming scale and production in the future, shellfish biodeposition is expected to assume a more important role in shaping SOM dynamics and sedimentary organic carbon cycling in coastal waters. Overall, this study provided an important perspective for better assessing the impact of expanding mariculture scale on coastal biogeochemical cycles, thereby making valuable contributions to future policy formulation concerning mariculture and ecological conservation.

Characterization of biotic and abiotic signatures of modern lake sediments of western India, and its palaeo-environmental implications

A multi-proxy study of biotic and abiotic components was conducted on surface sediment samples from six lakes/wetlands located along the western transitional boundary of the contemporary Indian Summer Monsoon (ISM) in the Indian Subcontinent. The primary goal is to assess the suitability of various proxies as representatives of modern vegetation, environmental and climatic conditions. The collected data indicate significant variations in the composition and density of pollen in response to climate-induced and anthropogenic ecological changes throughout the northwest India transect. The palynological studies from eastern Rajasthan shows high forest elements in comparison to western Haryana and western Uttar Pradesh. In addition, the palynological data was juxtaposed with other biotic proxies such as diatom and isotopic studies, along with geochemical proxies and paleomagnetic data of the surface lake sediments. The presence of marker pollen taxa including Cerealia type Poaceae, Chenopodiaceae, and Brassicaceae, allows for distinct recognition of anthropogenic activities throughout the whole transect. The diversity and distribution of diatoms also support the palynological data in response to climate-induced and anthropogenic ecological changes. Furthermore, grain size, geochemistry (TOC/TN ratio with stable carbon isotope), and magnetic susceptibility data offer crucial insights about the sediment's depositional settings and general mineralogical composition. Stable carbon isotope data shows C3 dominance in relatively humid areas and C4 dominance in semi-arid areas, suggesting climate-driven control over sediment organic matter composition. Canonical correspondence analysis (CCA) indicates that biotic variables (pollen, diatom, stable isotopic composition) are significantly controlled by modern precipitation and temperature. Redundancy analysis reveals a significant influence of current average temperature and precipitation on major element oxide variations in surface lake sediments. Therefore, we propose using palynological, stable carbon isotope, diatom data, along with grain size, environmental magnetism, and geochemistry, to establish a multiproxy modern analogue for quantitative palaeoclimatic reconstructions. As a result, this study provides the first modern analogues from a climate-sensitive region that separates the area under ISM influence from an area with meager precipitation in western India.

A Biogeochemical Comparison of three Representative Lakes of Costa Rica

ABSTRACT Lakes are widely distributed across Costa Rica, from coasts to the highest elevation regions, and located in the main terrestrial biomes. However, updated biogeochemical information about the main types of lakes is still lacking. We present comparative biogeochemistry (water chemistry, stable isotopes, and picoplankton) for a coastal lake (Lake Madre de Dios, LMD), a volcanic lake (Lake Barva, LB), and a glacial lake (Lake Ditkevi, LD). Sampling was conducted between February-November 2022 including dry and rainy seasonal conditions. Hydrological and chemical conditions were evaluated using water and carbon stable isotopes, dissolved organic matter (DOM), major ions, and microbiota analysis. Isotopic data (δ18O, δ13CDIC) confirmed lower evaporative losses for the maar and tarn lakes and productivity response to precipitation inputs. Excitation/Emission matrices confirmed the prevalence of fulvic and humic acids in the coastal and glacial lakes, mainly aromatic proteins and soluble microbial by-products in the volcanic lake. Picophytoplankton (PPP, ∼0.2 to 10 μm) was mainly represented by phycocyanin-rich picocyanobacteria (PC-Pcy) events in the three lakes, but maar and tarn lakes had more representation of phycoerythrin (PE-Pcy) events. We confirmed fluctuations in PPP cell abundance in the lakes lower than in temperate lakes. Like in other eutrophic lakes, Pcy also dominated over picoeukaryotes (Peuk) algae abundance. This work seeks to promote adopting an ecosystem approach to study the biogeochemical functioning of tropical lakes using the combination of chemical, hydrological, and biological data and to provide baseline information for future studies (e.g., climate change and pollution impacts) on tropical lakes of Costa Rica.

A review of benthic foraminiferal oxygen and carbon isotopes

Stable isotopes in the calcium carbonate of benthic foraminifera provide important paleoenvironmental information about seawater/sedimentary porewaters that these foraminifera live in. Oxygen isotopes provide essential insights about variations in deep water temperatures and sea-level/ice volume changes, while carbon isotopes provide information about sea-water carbon/nutrient cycling. In this review we look into detail at the direct and indirect mechanisms that contribute to stable oxygen and carbon isotope signals in Rotaliid benthic foraminifera. This includes effects from ontogenetic- and calcification mechanisms, and the impact of methane seeps and post-depositional diagenesis. We conclude our review with an overview of current challenges and provide recommendations for future research endeavors aimed at outstanding knowledge gaps in understanding how these biomineralizers control their stable isotopes.

The subsistence strategy transformation of the Yangshao culture (6900-4800BP) in the Guanzhong Area and Western Henan based on new faunal materials from the Miaodigou site

While the significance of the Yangshao culture in early China is widely recognized, systematic and detailed analysis and discussion of its subsistence strategy are lacking, especially for faunal materials. This paper examines the new and crucial faunal remains that include 3,400 identified specimens from the Miaodigou site unearthed in 2002. At that site, pig husbandry was predominant and wild animals were occasionally hunted as supplemental resources. In combination with published research including zooarchaeology, paleoethnobotany, and carbon and nitrogen stable isotope studies of 29 other sites mainly from the Guanzhong area and western Henan, we argue that subsistence strategy underwent a comprehensive and profound transformation during the middle period of Yangshao culture, namely Miaodigou period, as crop production and pig husbandry flourished in a wide range of areas. Domestic pigs accounted for over 80% of mammals, reaching an unprecedented proportion in the Neolithic age and manifesting a high level of consistency in feeding and slaughtering, and foxtail millet replaced broomcorn to become the most important crop at all sites, both of which demonstrate the breadth and depth of this transformation. The tension between the deterioration of climate, the decrease of surrounding wildlife resources due to excessive development and the continuous growth of the population may have contributed to this transformation. This study enriches our understanding of human-environment interactions in the embryonic stage of Chinese civilization and triggers further thoughts on how culture, society, the environment and subsistence were entangled in the development of civilization.

Relationship between fish traits and the methane‐derived carbon contribution to fish in a shallow lake

Abstract Although the transfer of methane‐derived carbon (MDC) to benthic macroinvertebrates such as chironomid larvae and oligochaetes has been well documented, knowledge of the transfer of MDC to fish is limited. We investigated the MDC contribution to 28 fish species through their growth stages and examined the relationship between fish traits including body size, and MDC contribution to fish within species in Lake Kasumigaura, Japan. We used carbon stable isotope ratios and a two‐source mixing model (methane‐oxidation bacteria [MOB] and particulate organic matter) to estimate the MDC contribution to fish. The MDC contribution to individual fish ranged from 0% to 21.5%. Anguilla japonica showed the highest MDC contribution (mean 3.8%). Fish species were classified into four groups according to two criteria: (a) whether the correlation between the MDC contribution and fish size (within species) was significantly negative (negative vs. not significant; ns); and (b) whether the 90th percentile of MDC contribution was 0% or not. The mean MDC contribution was highest (2.18% ± 2.93%, 12 species) for the group with (a)ns/(b)>0%, including the high contribution species such as Abbottina rivularis (mean 5.5%) and Biwia zezera (mean 9.8%). This group (12 species) included relatively high numbers of benthic habitat use (eight species) and omnivores (nine species). The mean MDC contributions were almost zero for the group with (a)ns/(b)=0% (seven species), which included only two species of benthic habitat use and one species of omnivores. The mean MDC contributions were intermediate for the other groups with (a)negative/(b)>0% (1.30% ± 1.17%, seven species) and (a)negative/(b)=0% (0.10%, two species). The negative correlation between fish size within species and MDC contribution in groups with (a)negative/(b)>0% and (a)negative/(b)=0% indicated that ontogenetic habitat and dietary shifts induced changes in MDC transfer. Our results suggest that 21 out of 28 fish species were involved in the MDC transfer in a shallow lake, but variation in MDC transfer could be explained by fish traits (trophic guild and habitat use). Although the mean MDC contribution to all fish species was low (0.61% ± 1.97%), the roughly estimated annual consumption rate of fish on MOB corresponded to approximately 10% of the MOB production in the sediment, suggesting that fish contribute to methane dynamics from the perspective of biogeochemical cycles.

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.

Isotope-based inferences of the trophic niche of short-finned pilot whales in the Webbnesia

Knowledge of predator-prey interactions is key in ecological studies and understanding ecosystem function, yet this is still poorly explored in the deep-sea environment. Carbon (δ13C: 13C/12C) and nitrogen (δ15N: 15N/14N) stable isotope ratios of a deep-diving species, the short-finned pilot whale (Globicephala macrorhynchus), were used to explore knowledge gaps on its ecological niche and foraging habitats in the Webbnesia marine ecoregion (Tenerife Island, n = 27 animals vs. Madeira, n = 31; 500 km apart) where animals display distinct levels of site fidelity. Specifically, we tested whether intraspecific isotopic variation results from differences between geographic areas (due to possible foraging plasticity between regions), sexes, and/or years (2015–2020) using Generalized Linear Models. In general, significant differences (p < 0.05) were found in the stable isotope profiles of pilot whales between the two archipelagos, which were also reflected in their isotopic niche. The higher mean and wider range of δ15N values in Tenerife suggest that pilot whales consume prey of higher trophic levels and more diverse than Madeira. The higher mean and wider range of δ13C values in Madeira suggest that in that island, pilot whales rely on prey from more diverse habitats. There was significant variation between some years, but not between sexes. Finally, we discuss pilot whales' foraging strategies worldwide and infer the reliance on benthic or benthopelagic food sources in the Webbnesia.

Simultaneous onboard analysis of seawater dissolved inorganic carbon (DIC) concentration and stable isotope ratio (δ13C‐DIC)

AbstractDissolved inorganic carbon (DIC) and its stable carbon isotope (δ13C‐DIC) are valuable parameters for studying the aquatic carbon cycle and quantifying ocean anthropogenic carbon accumulation rates. However, the potential of this coupled pair is underexploited as only 15% or less of cruise samples have been analyzed for δ13C‐DIC because the traditional isotope analysis is labor‐intensive and restricted to onshore laboratories. Here, we improved the analytical precision and reported the protocol of an automated, efficient, and high‐precision method for ship‐based DIC and δ13C‐DIC analysis based on cavity ring‐down spectroscopy (CRDS). We also introduced a set of stable in‐house standards to ensure accurate and consistent DIC and δ13C‐DIC measurements, especially on prolonged cruises. With this method, we analyzed over 1600 discrete seawater samples over a 40‐d cruise along the North American eastern ocean margin in summer 2022, representing the first effort to collect a large dataset of δ13C‐DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μmol kg−1 for the DIC concentration and 0.03‰ for the δ13C‐DIC value (1σ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μmol kg−1 between CRDS and the coulometry‐based results. The cross‐validation of δ13C‐DIC in the deep‐ocean data exhibited a mean difference of only −0.03‰ ± 0.07‰, emphasizing the consistency with historical data. Potential applications in aquatic biogeochemistry are discussed.

Minimally Invasive Vacuum-Aided Extraction Technique for the Lipid Analysis of Historic Parchment.

Parchment is an ancient writing support formed from dehaired animal skins. Its manufacture comprises a series of liming and scraping steps before being stretched and dried under tension. Historical parchment represents a valuable source of cultural heritage which, until now, has limited investigations to noninvasive analyses to infer ink composition, degradation, or physical changes over time. We highlight the prospect of the molecular and isotope compositions of animal lipids from parchment as an untapped record of its production and the animal's diet and environment. We report a minimally invasive, total lipid extraction aided by a vacuum for historical parchments. The quantitative and qualitative compositions of lipid extracts obtained using this method are compared with those obtained using invasive sampling for nine sacrificial membranes dated 1765-1825 CE. This extraction method is then applied to membranes from the Chancery Parliament Rolls (1814-1820 CE) held by The National Archives, UK to obtain lipids and derive taxonomic and dietary information using their stable carbon isotope compositions. This novel vacuum-aided extraction allows, for the first time, animal lipids to be obtained from parchment minimally invasively, paving the way for dietary and paleoclimate studies using this well-dated and common material.

A pilot study on the isotopic characterization of feeding habits of Diptera in a tropical rain forest

Flies (Diptera) are among the most diverse groups of insects and are known to utilize various food resources, including plants, detritus, microbial tissues, and fresh and dead animal tissues. However, their feeding habits in the field remain poorly understood. We conducted a pilot study to apply stable nitrogen (N) and carbon (C) isotope techniques to examine the feeding habits of flies in a tropical rain forest in Sarawak, Malaysia. The fly samples comprised 13 families and >18 species. The results showed significant differences in nitrogen and carbon isotope ratios (δ15N and δ13C) among families and species within a family. The observed pattern is largely consistent with their known feeding habits; flies that use carcasses and carrion as diets (e.g., Sarcophagidae and Calliphoridae) have significantly higher δ15N values than those likely utilizing plant-based diets (e.g., Cecidomyiidae). There were significant differences in δ15N values among the six species of Calliphoridae, which is consistent with insect succession on carcasses. The differences in δ15N may be explained by the use of carrion at different stages of decomposition, because microbial decomposition can lead to the 15N enrichment. Tachinid flies had relatively low δ13C values, reflecting the use of lepidopterans as a host. This pilot study shows that the δ15N and δ13C values of flies provide insights into the diversity of feeding habits of fly communities, which could also serve as an indicator of resource availability in an entire ecosystem.

Chemical and diatom compositions of dating deep-sea sediment core of the Kuril-Kamchatka Trench, northwestern Pacific

For the first time the grain size, contents of major (Al, Ti, Fe, Mn, Ca, Mg, Na, and K) and trace elements (Mo, Ni, and Cu), total organic carbon, and stable carbon isotope were studied in the sediment cores collected at a water depth of approximately 9500 m from the Kuril-Kamchatka Trench. This study determined the factors of sedimentary environment. The quantitative content and species composition of diatoms were also studied. We have estimated the sedimentation rate based on the excess 210Pb data. According to age models, the sedimentation rate was ∼1.64 mm/year. Paleoconstructions were formed on comparing the distribution of chemical and biological parameters in deep-sea basin sediments. The studied sediments were characterized by the high content of the bioproductivity indicators and chemical elements (Fe, Ca, and Cu) corresponding to a period of increased supply of nutrients that stimulated the production of diatoms. Based on the study of chemical indicators and characteristics of nutrient utilization by phytoplankton, a period of outbreak of diatom productivity due to the introduction of volcanic ash into the sediment in the period 1860–1883 was marked. New data will complement existing information and helpful for further understanding of sedimentary geochemical characteristics in this hadal area, and allow us to relate abiotic conditions to productivity.

From swamp to sea: Quantifying terrestrial dissolved organic carbon in a tropical shelf sea using hydrogen isotope ratios

Dissolved organic carbon (DOC) is a key component of coastal biogeochemical cycles, but its composition and reactivity depend on the relative contribution of autochthonous aquatic versus allochthonous terrigenous DOC (tDOC). In complex coastal waters, tDOC is commonly quantified using the bulk DOC stable carbon isotope ratio (δ13CDOC). However, several limitations can hamper the use of δ13CDOC in marine ecosystems, such as (1) the narrow and often overlapping separation of the autochthonous and allochthonous endmembers, and (2) mineralization of tDOC to dissolved inorganic carbon creates a reservoir effect such that autochthonous DOC can acquire a terrigenous-like δ13CDOC. The stable isotope ratio of non-exchangeable hydrogen in the DOC (δ2Hn) has emerged as a new tool that can potentially overcome these limitations: (1) δ2Hn has a large separation between aquatic and terrigenous endmembers (>50‰) and (2) it is not subject to reservoir effects caused by tDOC mineralization. Here, we evaluate the potential of δ2Hn obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to δ13CDOC and chromophoric DOM (CDOM) optical properties. We collected samples at a site in Southeast Asia’s Sunda Shelf that experiences substantial seasonal variation in tDOC input, driven primarily by the monsoon-induced physical advection of peatland-derived tDOC. Over a 1-year monthly time series, the terrigenous fraction of DOC (fterr) determined using δ2Hn of SPE-DOM and δ13CDOC of bulk DOC was well correlated (r2 = 0.42), and there was no significant difference in fterr between the two isotope systems. In fact, δ2Hn displayed slightly stronger correlations with salinity and CDOM optical properties compared to δ13CDOC. Our results indicate that δ2Hn of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than δ13CDOC, and is a viable alternative in settings where δ13CDOC is inadequate.

Sclerochronology of the large scallops Gigantopecten latissimus and Pecten jacobaeus in a Pliocene warmer Mediterranean Sea

Here we apply a sclerochronological approach to reconstruct the life-history of two stenohaline bivalves of the family Pectinidae, Gigantopecten latissimus and Pecten jacobaeus from the Pliocene of Italy. The specimens come from the locality of Torrita di Siena (Siena-Radicofani Basin, Tuscany), dated to the late Zanclean-Piacenzian with nannoplankton biostratigraphy and Sr-isotope stratigraphy. After measuring the width of micro-growth increments and verifying that the shells were not diagenetically altered, we sampled them at high resolution for carbon and oxygen stable isotopes. δ13C and δ18O shell values allowed to distinguish between specimens that lived above or below the thermocline. Those influenced by surface waters indicate temperatures (for δ18Oseawater = 1.5‰) with winter minima of 16–18 °C and summer values up to 28–29 °C, close to present temperature conditions in the tropical west-African climate belt. In line with this, we found that the two species had similar seasonal growth patterns, with faster growth during colder months and summer slowdown, a typical adaptation of bivalves of tropical affinity suffering from summer temperature extremes. Despite this similar adaptation, G. latissimus, with large and heavy shells (length up to 30 cm) became extinct around 3.0 Ma, while the smaller P. jacobaeus survived Plio-Pleistocene cooling. Different growth rates between the two species and, therefore, different metabolic costs, might explain such differential response, together with differences in reproduction strategies. Habitat loss and fragmentation, due to the decrease of shelf margins and organogenic substrates caused by cooling and sea level fall, are abiotic factors that could have also contributed to the extinction of G. latissimus. P. jacobaeus adaptation to live across a larger bathymetric range, which indicates the ability to thrive in a wider range of temperatures, most likely played a role in its survival. Further studies, including more specimens across multiple localities, will help verifying these hypotheses.

Contextualizing wild cereal harvesting at Middle Palaeolithic Ghar-e Boof in the southern Zagros

A stratigraphic sequence from Ghar-e Boof, a cave site in Iran, covering a period of c. 80,000–30,000 BP and containing more than 20,000 seed and chaff remains, allows a detailed study of the use of annual seed species of Palaeolithic hunter-gatherer groups and its evolution under the influence of changing environmental conditions. Taxonomic changes in the archaeobotanical assemblage and the stable carbon isotope data of pistachio support a considerable change in environmental conditions over the sequence from MIS 5a to MIS 3. The exceptional dominance of wild ancestors of modern crop species, including glume wheat and large-seeded legumes from Middle Palaeolithic layers AH VI (OSL ranges 72–81 ka BP), coincides broadly with the transition from MIS 5a to MIS 4. With the beginning of MIS 4 these taxa are strongly reduced, corresponding with a strong decrease in global CO2 concentrations and in the Δ13C values of Pistacia khinjuk/atlantica from the site. Wild glume wheat completely disappears after Middle Palaeolithic AH Vb and never reappears at the site. We hypothesize that the Middle Palaeolithic niche that allowed the harvesting and consumption of wild cereals and legumes ended with a destabilization of the vegetation in early MIS 4.

Organic Matter Enrichment in Jurassic Hydrocarbon Source Rocks from the Turpan-Hami Basin: Insights from Organic and Elemental Geochemistry.

Petroleum can be generated by thermal cracking of organic matter within sediments, and the organic matter within sediments plays the dominant role in determining oil and gas generation. Organic matter within sediments is characterized by various sources, such as sapropelic organic matter from algal, microbial, and planktonic organisms and humic organic matter from higher plants. Paleo-productivity, terrestrial influx, and depositional environments could obviously influence the enrichment processes of the organic matter within sediments. Organic and elemental geochemical proxies can investigate the sedimentary process and reflect the enrichment characteristics of organic matter. In this study, hydrocarbon source rocks from the Shuixigou Group were collected from the Taibei Sag of the Turpan-Hami Basin, rock-eval pyrolysis was conducted, and stable carbon isotope composition of organic carbon and major and trace element distributions were measured. Based on this, the type and maturation of organic matter, paleo-productivity, terrestrial influx, and depositional paleo-environments were investigated. The results show that the Jurassic hydrocarbon source rocks are characterized by type III kerogen and are in the oil-window stage of maturation. The depositional paleo-environments of hydrocarbon source rocks in different formations are not remarkably different. The water bodies have freshwater oxidizing environments, and the paleo-climatic characteristics are warm and humid. However, the paleo-productivity of samples from the lower Jurassic Sangonghe Formation is higher than samples from other formations. Overall, the organic matter enrichment in Jurassic hydrocarbon source rocks of the Turpan-Hami Basin could be mutually controlled by the paleo-productivity and depositional paleo-environments. The results of this study could provide theoretical insight into deep petroleum exploration and resource evaluation in the Turpan-Hami Basin.