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The long-standing assumption that aboveground plant litter inputs have a substantial influence on soil organic carbon storage (SOC) and dynamics has been challenged by a new paradigm for SOC formation and persistence. We tested the importance of plant litter chemistry on SOC storage, distribution, composition, and age by comparing two highly contrasting ecosystems: an old-growth coast redwood (Sequoia sempervirens) forest, with highly aromatic litter, and an adjacent coastal prairie, with more easily decomposed litter. We hypothesized that if plant litter chemistry was the primary driver, redwood would store more and older SOC that was less microbially processed than prairie. Total soil carbon stocks to 110 cm depth were higher in prairie (35 kg C m-2) than redwood (28 kg C m-2). Radiocarbon values indicated shorter SOC residence times in redwood than prairie throughout the profile. Higher amounts of pyrogenic carbon and a higher degree of microbial processing of SOC appear to be instrumental for soil carbon storage and persistence in prairie, while differences in fine-root carbon inputs likely contribute to younger SOC in redwood. We conclude that at these sites fire residues, root inputs, and soil properties influence soil carbon dynamics to a greater degree than the properties of aboveground litter.

Societal Impact Statement Over half of Earth's human population lives in urban areas where pollution from fossil fuel combustion threatens urban air quality and public health. The isotopic composition of plant tissues can be used as a tool to estimate local variation in the amount of fossil fuel–derived carbon dioxide. We present a case study that examines this variation against the backdrop of historic discriminatory policies in Saint Paul, Minnesota, USA. Data suggest higher loads of atmospheric fossil carbon in neighborhood greenspaces that are situated closer to large roads, which often correspond with historical policies. Evidence of unequal fossil fuel combustion, and potentially pollution load, may precipitate changes in urban management or pollution mitigation strategies. [ABSTRACT FROM AUTHOR]

Cold seeps, where geofluids containing methane and other hydrocarbons originating from the subseafloor seeps through the sediment surface, play important roles in the elemental and energy flux between sediment and seawater. These seep sites often harbor communities of endemic animals supported by chemolithoautotrophic bacteria, either through symbiosis or feeding. Despite these animal communities being intensively studied since their discovery in the 1980's, the contribution of carbon from seep fluid to symbiotic microbes and subsequently host animals remains unclear. Here, we used natural-abundance radiocarbon to discern carbon sources: the ambient bottom water or the seeping geofluid. The 14C concentrations were measured for vesicomyid clams, a parasitic calamyzine polychaete, and a siboglinid tubeworm species from four different cold seep sites around Japan. We found most vesicomyid clams exhibiting 14C concentrations slightly lower than that of the ambient bottom water, suggesting up to 9% of C for chemolithoautotrophy originates from geofluid DIC. The different extent of fluid contribution across species may be explained by different routes to incorporate DIC and/or different DIC concentrations in the geofluid at each seep site. Stable nitrogen isotopic compositions further suggested N incorporation from geofluids in these clams, where the burrowing depth may be a key factor in determining their δ15N values. The siboglinid tubeworm showed a clear dependency for geofluid DIC, with a contribution of > 40%. Our results demonstrate the effectiveness of 14C analyses for elucidating the nutritional ecology of cold seep animals and their symbionts, as was previously shown for hydrothermal vent ecosystems. [ABSTRACT FROM AUTHOR]

The emergence of the Middle Palaeolithic, and its variability over time and space are key questions in the field of prehistoric archaeology. Many sites have been documented in the south-eastern margins of the Massif central and the middle Rhône valley, a migration path that connects Northern Europe with the Mediterranean. Well-dated, long stratigraphic sequences are essential to understand Neanderthals dynamics and demise, and potential interactions with Homo sapiens in the area, such as the one displayed at the Maras rock shelter ("Abri du Maras"). The site is characterised by exceptional preservation of archaeological remains, including bones dated using radiocarbon (14C) and teeth using electron spin resonance combined with uranium series (ESR/U-series). Optically stimulated luminescence was used to date the sedimentary deposits. By combining the new ages with previous ones using Bayesian modelling, we are able to clarify the occupation time over a period spanning 200,000 years. Between ca. 250 and 40 ka, the site has been used as a long-term residence by Neanderthals, specifically during three interglacial periods: first during marine isotopic stage (MIS) 7, between 247 ± 34 and 223 ± 33 ka, and then recurrently during MIS 5 (between 127 ± 17 and 90 ± 9 ka) and MIS 3 (up to 39,280 cal BP). [ABSTRACT FROM AUTHOR]

Based on calibrated radiocarbon ages of terrestrial gastropod shells (Succineidae, Discus , Stenotrema , Webbhelix), the chronology of Peoria Silt (loess) deposition in the Central Lowlands is updated. These taxa provide reliable ages (within ~0.2 ka), based on historical shell dating, shell-organic age comparisons, and stratigraphic consistency. A compilation of 53 new and 36 published Peoria Silt shell ages (calibrated), from 12 localities, date from 30.0 to 17.4 ka. Proximal (fossiliferous) loess from 10 sections had mean loess accumulation rates of 0.6–2.2 mm/yr. Study sites along the upper Mississippi, Illinois, to mid-Mississippi, and Ohio-Wabash Valleys suggest Peoria loess accumulated from ~27 to 15 ka, ~29 to 18 ka, and ~30 to 18 ka, respectively. The cessation age for Peoria Silt, based on surface extrapolations, is ~1–6 ka earlier than some prior Illinois estimates, even assuming slower loess accumulation in the modern solum. Younger loess in northwestern Illinois likely reflects, in part, Superior and Des Moines Lobe glacial-meltwater sediment, and Iowan Erosion Surface inputs to the upper Mississippi Valley, after the Lake Michigan Lobe receded. Furthermore, stronger winds, drier conditions, and reduced vegetation cover in valley deflation areas may have favored higher accumulation rates and later loess deposition in northwestern relative to southeastern areas. [ABSTRACT FROM AUTHOR]

In the last decade Bayesian analysis of radiocarbon dates has become commonplace for archaeologists grappling with regional chronologies. Here we present Bayesian analysis for radiocarbon dates from Moche cultural contexts from the north coast of Peru with the aim to understand the duration of the phenomenon and to compile all known dates in a single location for future analysis and use. The analysis demonstrates that the long-standing Moche ceramic sequence does not provide a perfect proxy for tracking the timing of all social interactions. However, our results show general agreement with traditional relative Moche chronologies. Finally, our study shows that Moche was a shorter cultural phenomenon than has been previously argued, with the current data indicating that it began between the late 4th and early 6th centuries CE and lasted until the 9th century CE. [ABSTRACT FROM AUTHOR]

In 2013 archaeological radiocarbon databases covering 13,000–7000 14C BP for nine countries in South America, including Peru, were published in a special issue of Quaternary International. The past decade has seen new field research and radiocarbon ages contributed to the Peruvian archaeological record, two updates to the southern hemisphere calibration curve, and research papers drawing from South American radiocarbon databases to explore region- and continent-scale patterns of human-environment interaction, settlement dynamics, and paleodemography. This paper provides an update to the Peruvian archaeological radiocarbon database for the Late Pleistocene to early Middle Holocene. The new database (version 2) includes 493 radiocarbon dates spanning 20,000 to 7000 14C BP. No radiocarbon dates >13,000 14C BP meet basic archaeological criteria. First, I review the structure of Peru's early archaeological radiocarbon record and highlight several research and landscape taphonomic biases. Second, I examine distributions of archaeological sites through time, beginning with basic site and date frequencies and progressing to Bayesian analysis of aggregated radiocarbon dates. Third, applying established criteria for well-dated archaeological events, I identify some unresolved problems in early Peruvian radiocarbon chronologies, with the goal of outlining priorities for future archaeological and chronometric research. [ABSTRACT FROM AUTHOR]

In this article, we propose an approach to reconstruct the timing of human activity at Points Cave, an Upper Paleolithic rock art site located in the middle of the Ardèche River Gorge (Rhône valley, France), based on the dating and characterisation of a stalagmite containing soot. Points Cave ('Grotte aux Points' in French), also called the 'little sister of Chauvet Cave', is famous for its parietal art including a series of dots made of palm prints. A large number of stalagmites formed in the cave during the last 500 ka. However, quarrying of the cave floors during historic times led to the partial destruction of the sedimentary deposits, and many of the stalagmites were found lying on the floor. In particular, one of them (STM‐18‐04) showed the presence of at least four dark layers in cross‐section, which appeared as possible remnants of fire‐related activities in the cave. Despite being present at the same site, no other specific link between STM‐18‐04 and the rock art has been documented. This stalagmite, however, allows us to identify phases of human presence, located at the cave entrance. To do so, we performed a series of analyses to determine its period of growth and the nature of the dark layers that it contains. Scanning electron microscopy and Raman microspectroscopy confirmed that the dark layers include soot, and uranium‐series dating indicated that the fire events occurred, respectively, around 14,200–14,100 and 12,500 years ago, in agreement with the radiocarbon ages of charcoal specimens recovered from the excavation areas nearby. We thus highlighted phases of human activity at the site during the Upper Magdalenian and/or beginning of the Epipaleolithic. By comparing our results with the regional paleoclimatic record, the soot layers trapped in the stalagmite appear to be synchronous with two cold periods, likely the Older Dryas and the Younger Dryas. [ABSTRACT FROM AUTHOR]

The Gubbio Revisited project, initiated to reinterpret the archaeological evidence collected during the 1980s Gubbio Project, primarily by a conversion from a paper to a digital record, has revealed significant insights into the evolving settlement patterns and religious expression in the Gubbio valley in Central Italy. This reanalysis of the survey evidence underscores the rhythms of settlement and ritual practice from the Neolithic through the Bronze and Iron Ages, into Roman times. Key excavations in the 1980s at Monte Ingino, Monte Ansciano, San Marco Romano, and San Marco Neolitico added details not only of settlement activity but also of embedded ritual, evidenced by material culture including pottery, faunal remains, and votive offerings. The foundation myth of indigenous religious practices, even amidst Roman influence, is documented through the Iguvine Tables alongside the introduction of new cults, showcasing a blend of local and imperial religiosity, a common feature in the Roman world. This research enriches the understanding of Gubbio's historical and cultural landscape, emphasizing the demographic rhythms of the valley alongside the integral role of ritual in its societal evolution. [ABSTRACT FROM AUTHOR]

This paper examines newly acquired and previously published carbon and nitrogen isotope values in bone collagen from 58 dogs (Canis familiaris) dated to the Mesolithic–Neolithic transition. Using the Bayesian mixing model FRUITS, we estimate the marine or freshwater dietary fractions. These estimates, together with a radiocarbon‐based Bayesian statistical model, have allowed us to calculate the freshwater reservoir age for selected Danish regions. The Ertebølle and Funnel Beaker cultures display different feeding traditions, and stable isotope values of dogs cannot be used as a direct proxy for reconstructing human diet, as the foodstuffs appear to have been subject to some deliberate differentiation. [ABSTRACT FROM AUTHOR]

As part of a larger project identifying and directly radiocarbon dating Late Pleistocene megafaunal remains in British Columbia (B.C.), Canada, we have confirmed the identity of many newly identified mammoth (Mammuthus sp.) specimens (n = 32) from Vancouver Island in Southwestern B.C. We undertook radiocarbon dating on all specimens and were able to obtain dates (due to preservation) on 16 of these remains, including redating a previously dated mammoth using newer radiocarbon extraction methods. The mammoth dates span a wide range, from >47 500 to 18 000 radiocarbon years B.P. (uncalibrated). These later new dates support other lines of evidence for portions of Vancouver Island remaining ice-free until well into the Fraser Glaciation. [ABSTRACT FROM AUTHOR]

Extensive reservoir construction has fragmented more than 70% of the world's rivers, significantly impacting river connectivity and carbon cycling. However, the response of riverine dissolved organic matter (DOM) to reservoir influence and its potential downstream effects remains unclear. In this study, we employed multiple analytical techniques, including Fourier transform ion cyclotron resonance mass spectrometry, radiocarbon dating, and environmental factor analysis, to investigate the dynamic changes in DOM and its controlling factors under different hydrological management regimes in the LongTan Reservoir, the largest reservoir in the Pearl River, which is the second largest river in China by water discharge. Our results indicate that the molecular diversity of riverine DOM is reduced in the reservoir. Oxygen‐rich and heteroatomic compounds, such as those containing nitrogen, sulfur, and phosphorus, are preferentially removed through enhanced photo‐ and biodegradation processes in the reservoir, particularly during the storage period. This leads to DOM that is enriched with oxygen‐poor compounds and shows a biodegraded Δ14C value downstream. This study highlights that the composition of riverine DOM is significantly altered by the reservoir, but these effects could potentially be mitigated by optimizing the outlet location. Plain Language Summary: The construction of dams has significantly impacted rivers worldwide, fragmenting them and disrupting the flow of water and carbon. To understand how substances in rivers, particularly dissolved organic matter (DOM), are affected by dams and their potential impact on downstream ecosystems, we studied changes in DOM and the factors controlling these changes in the largest reservoir on the Pearl River in China, the second largest river in the country by water discharge. We found that certain compounds, such as those containing nitrogen, sulfur, and phosphorus, are preferentially removed by natural processes in the reservoir. This phenomenon results in DOM with persistent properties that hinder its participation in downstream biogeochemical processes. Although the reservoir significantly alters riverine DOM, our results suggest that the impact of the reservoir could be mitigated by optimizing the water outlet location. Key Points: Primary production and degradation processes in the river will be significantly enhanced by the reservoirThe reservoir will preferentially remove oxygen‐rich and heteroatomic compounds in the riverThe impact of the reservoir on downstream systems is potentially controlled by establishing an ideal outlet location [ABSTRACT FROM AUTHOR]

Organic carbon (OC) association with soil minerals stabilizes OC on timescales reflecting the strength of mineral-C interactions. We applied ramped thermal oxidation to subsoil B horizons with different mineral-C associations to separate OC according to increasing temperature of oxidation, i.e. thermal activation energy. Generally, OC released at lower temperatures was richer in bioavailable forms like polysaccharides, while OC released at higher temperatures was more aromatic. Organic carbon associated with pedogenic oxides was released at lower temperatures and had a narrow range of 14C content. By contrast, N-rich compounds were released at higher temperatures from samples with 2 : 1 clays and short-range ordered (SRO) amorphous minerals. Temperatures of release overlapped for SRO minerals and crystalline oxides, although the mean age of OC released was older for the SRO. In soils with more mixed mineralogy, the added presence of older OC released at temperatures greater than 450°C from clays resulted in a broader distribution of OC ages within the sample, especially for soils rich in 2 : 1 layer expandable clays such as smectite. While pedogenic setting affects mineral stability and absolute OC age, mineralogy controls the structure of OC age distribution within a sample, which may provide insight into model structures and OC dynamics under changing conditions. This article is part of the Theo Murphy meeting issue Radiocarbon in the Anthropocene.

The permafrost region has accumulated organic carbon in cold and waterlogged soils over thousands of years and now contains three times as much carbon as the atmosphere. Global warming is degrading permafrost with the potential to accelerate climate change as increased microbial decomposition releases soil carbon as greenhouse gases. A 19-year time series of soil and ecosystem respiration radiocarbon from Alaska provides long-term insight into changing permafrost soil carbon dynamics in a warmer world. Nine per cent of ecosystem respiration and 23% of soil respiration observations had radiocarbon values more than 50‰ lower than the atmospheric value. Furthermore, the overall trend of ecosystem and soil respiration radiocarbon values through time decreased more than atmospheric radiocarbon values did, indicating that old carbon degradation was enhanced. Boosted regression tree analyses showed that temperature and moisture environmental variables had the largest relative influence on lower radiocarbon values. This suggested that old carbon degradation was controlled by warming/permafrost thaw and soil drying together, as waterlogged soil conditions could protect soil carbon from microbial decomposition even when thawed. Overall, changing conditions increasingly favoured the release of old carbon, which is a definitive fingerprint of an accelerating feedback to climate change as a consequence of warming and permafrost destabilization. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Abstract Cold seeps, where geofluids containing methane and other hydrocarbons originating from the subseafloor seeps through the sediment surface, play important roles in the elemental and energy flux between sediment and seawater. These seep sites often harbor communities of endemic animals supported by chemolithoautotrophic bacteria, either through symbiosis or feeding. Despite these animal communities being intensively studied since their discovery in the 1980’s, the contribution of carbon from seep fluid to symbiotic microbes and subsequently host animals remains unclear. Here, we used natural-abundance radiocarbon to discern carbon sources: the ambient bottom water or the seeping geofluid. The 14C concentrations were measured for vesicomyid clams, a parasitic calamyzine polychaete, and a siboglinid tubeworm species from four different cold seep sites around Japan. We found most vesicomyid clams exhibiting 14C concentrations slightly lower than that of the ambient bottom water, suggesting up to 9% of C for chemolithoautotrophy originates from geofluid DIC. The different extent of fluid contribution across species may be explained by different routes to incorporate DIC and/or different DIC concentrations in the geofluid at each seep site. Stable nitrogen isotopic compositions further suggested N incorporation from geofluids in these clams, where the burrowing depth may be a key factor in determining their δ15N values. The siboglinid tubeworm showed a clear dependency for geofluid DIC, with a contribution of > 40%. Our results demonstrate the effectiveness of 14C analyses for elucidating the nutritional ecology of cold seep animals and their symbionts, as was previously shown for hydrothermal vent ecosystems.

Abstract The emergence of the Middle Palaeolithic, and its variability over time and space are key questions in the field of prehistoric archaeology. Many sites have been documented in the south-eastern margins of the Massif central and the middle Rhône valley, a migration path that connects Northern Europe with the Mediterranean. Well-dated, long stratigraphic sequences are essential to understand Neanderthals dynamics and demise, and potential interactions with Homo sapiens in the area, such as the one displayed at the Maras rock shelter (“Abri du Maras”). The site is characterised by exceptional preservation of archaeological remains, including bones dated using radiocarbon (14C) and teeth using electron spin resonance combined with uranium series (ESR/U-series). Optically stimulated luminescence was used to date the sedimentary deposits. By combining the new ages with previous ones using Bayesian modelling, we are able to clarify the occupation time over a period spanning 200,000 years. Between ca. 250 and 40 ka, the site has been used as a long-term residence by Neanderthals, specifically during three interglacial periods: first during marine isotopic stage (MIS) 7, between 247 ± 34 and 223 ± 33 ka, and then recurrently during MIS 5 (between 127 ± 17 and 90 ± 9 ka) and MIS 3 (up to 39,280 cal BP).

This interdisciplinary investigation of the human remains of Balduin Helm, one of the most important abbots of the Fürstenfeld monastery, provides novel information on this historic individual. This is particularly interesting since Balduin, during the early 18th century, was involved in the renovation of this large Bavarian monastery. Metastatic prostate cancer was found as evidenced by multiple mixed osteolytic–osteoblastic bone lesions in all available vertebral bodies, fragments of both os coxae, and isolated metastases in skull bones. Distribution, radio- and histomorphology, and especially the immunohistochemical detection of prostate-specific antigens in those metastases definitively confirm this diagnosis. Further investigation, especially by stable isotope analysis, showed a balanced high-level diet with considerable contribution from animal protein and significant freshwater fish. These additional findings suggest a significant radiocarbon reservoir effect as an explanation for a “too old” radiocarbon date. Finally, the obviously high-level protein diet may have contributed to the tumorigenesis which caused the death of the abbot at an advanced age.

Abstract Frozen water is the most widespread type of ice present in ice caves and forms ice stalagmites and stalactites as well as floor ice, which is often several meters thick. Organic macroremains are commonly rare in this type of cave ice, which makes it difficult to establish a chronology and severely limits the use of such ice deposits as paleoenvironmental archives. Here, the chronology of such ice deposits in the inner part of the glaciated Eisriesenwelt, one of the world’s largest ice caves located in the European Alps of Austria, is determined by a combination of radiocarbon and 230Th dating of cryogenic calcite. The data suggest that this cave ice has formed over the last three millennia, with a marked increase in the average accumulation rate during the thirteenth century, coinciding with the onset of the Little Ice Age in the Alps. Data from a second site closer to the entrance suggests that large parts of this tourist cave were likely ice-free during the Medieval Warm Period and that a substantial part of the ice is probably a relic of the Little Ice Age. The current warming has already penetrated deeper into the cave than during the Medieval Warm Period, although air exchange during the warm season is restricted by a door at the cave entrance.

The human colonization of the Canary Islands represents the sole known expansion of Berber communities into the Atlantic Ocean and is an example of marine dispersal carried out by an African population. While this island colonization shows similarities to the populating of other islands across the world, several questions still need to be answered before this case can be included in wider debates regarding patterns of initial colonization and human settlement, human-environment interactions, and the emergence of island identities. Specifically, the chronology of the first human settlement of the Canary Islands remains disputed due to differing estimates of the timing of its first colonization. This absence of a consensus has resulted in divergent hypotheses regarding the motivations that led early settlers to migrate to the islands, e.g., ecological or demographic. Distinct motivations would imply differences in the strategies and dynamics of colonization; thus, identifying them is crucial to understanding how these populations developed in such environments. In response, the current study assembles a comprehensive dataset of the most reliable radiocarbon dates, which were used for building Bayesian models of colonization. The findings suggest that i) the Romans most likely discovered the islands around the 1st century BCE; ii) Berber groups from western North Africa first set foot on one of the islands closest to the African mainland sometime between the 1st and 3rd centuries CE; iii) Roman and Berber societies did not live simultaneously in the Canary Islands; and iv) the Berber people rapidly spread throughout the archipelago. [ABSTRACT FROM AUTHOR]

Trace elements and stable isotope ratios in otoliths have been used as proxies for the migration history of teleosts; however, their application in oceanic fishes remains limited. This study reports the first use of radiocarbons in otoliths to evaluate the horizontal migration histories of an oceanic fish species, the walleye pollock Gadus chalcogrammus. We conducted radiocarbon analyses of three stocks sourced from Hokkaido, Japan. The radiocarbon concentrations from the outermost portion of the otoliths from the Japanese Pacific, Northern Japan Sea (JS), and Southern Okhotsk Sea (OS) stocks were in general agreement with the seawater radiocarbon concentration of the sampling region, suggesting that pollock of all three stocks generally inhabited the within the sea region where each pollocks were sampled throughout their life cycle. However, the radiocarbon signals also provided some indications that some JS and OS stocks may be migrating between different sea regions. The proposed novel approach of reconstructing the individual migration history of marine fish using radiocarbon in otoliths may help examine fish migration with a higher temporal and spatial resolution that could not be achieved by trace elements and stable isotope ratios. [ABSTRACT FROM AUTHOR]

This interdisciplinary investigation of the human remains of Balduin Helm, one of the most important abbots of the Fürstenfeld monastery, provides novel information on this historic individual. This is particularly interesting since Balduin, during the early 18th century, was involved in the renovation of this large Bavarian monastery. Metastatic prostate cancer was found as evidenced by multiple mixed osteolytic–osteoblastic bone lesions in all available vertebral bodies, fragments of both os coxae, and isolated metastases in skull bones. Distribution, radio- and histomorphology, and especially the immunohistochemical detection of prostate-specific antigens in those metastases definitively confirm this diagnosis. Further investigation, especially by stable isotope analysis, showed a balanced high-level diet with considerable contribution from animal protein and significant freshwater fish. These additional findings suggest a significant radiocarbon reservoir effect as an explanation for a "too old" radiocarbon date. Finally, the obviously high-level protein diet may have contributed to the tumorigenesis which caused the death of the abbot at an advanced age. [ABSTRACT FROM AUTHOR]

As tropical glaciers recede in response to global warming, glaciated watershed resilience is threatened by the loss of these important water reservoirs. In the Cordillera Occidental, Peru, we investigate the importance of glacial meltwater to mountain groundwater recharge and subsequent discharge to streams surrounding the rapidly retreating Coropuna ice cap. Using stable isotopes, solute concentrations, and radiocarbon activity, our results suggest that in remote ungauged systems, glacial input can be determined via geochemical analysis and a simple mixing model approach. We find that the proportion of glacial meltwater contribution to groundwater and surface waters is spatially heterogeneous. Differences between drainages are attributed to the degree of melting of headwater glaciers and the routing of waters through volcanic strata. These results provide evidence that glacial meltwater recharge is highly variable even within the same mountain block. Furthermore, the effects on streamflow will be disproportionate as the watersheds approach an ice-free state. [ABSTRACT FROM AUTHOR]

Water‐logged peatlands store tremendous amounts of soil carbon (C) globally, accumulating C over millennia. As peatlands become disturbed by human activity, these long‐term C stores are getting destabilized and ultimately released as greenhouse gases that may exacerbate climate change. Oxidation of the dissolved organic carbon (DOC) mobilized from disturbed soils to streams and canals may be one avenue for the transfer of previously stored, millennia‐aged C to the atmosphere. However, it remains unknown whether aged peat‐derived DOC undergoes oxidation to carbon dioxide (CO2) following disturbance. Here, we use a new approach to measure the radiocarbon content of CO2 produced from the oxidation of DOC in canals overlying peatland soils that have undergone widespread disturbance in Indonesia. This work shows for the first time that aged DOC mobilized from drained and burned peatland soils is susceptible to oxidation by both microbial respiration and photomineralization over aquatic travel times for DOC. The bulk radiocarbon age of CO2 produced during canal oxidation ranged from modern to ~1300 years before present. These ages for CO2 were most strongly influenced by canal water depth, which was proportional to the water table level where DOC is mobilized from disturbed soils to canals. Canal microbes preferentially respired older or younger organic C pools to CO2, and this may have been facilitated by the use of a small particulate organic C pool over the dissolved pool. Given that high densities of canals are generally associated with lower water tables and higher fire risk, our findings suggest that peatland areas with high canal density may be a hotspot for the loss of aged C on the landscape. Taken together, the results of this study show how and why aquatic processing of organic C on the landscape can enhance the transfer of long‐term peat C stores to the atmosphere following disturbance. [ABSTRACT FROM AUTHOR]

The Arctic is experiencing rapid warming, which among other processes results in increasing erosion of coastal permafrost and the release of ancient organic carbon (OC) into the Arctic Ocean, which in turn might result in greenhouse gas emissions following its decomposition. Supply of terrigenous organic matter to the ocean affects near‐shore nutrient concentrations and the composition of microbial communities—highlighting the need to understand the fate of permafrost‐derived carbon in this fragile ecosystem. We incubated material from coastal Yedoma permafrost for 85 days in seawater collected during the Arctic Century expedition. During this experiment, 2.8 ± 1.4% of OC from coastal Yedoma was respired to CO2. Radiocarbon analysis revealed that 88 ± 15% of the released CO2 originated from ancient material (∼40,000 years), indicating that degradation of permafrost OC reintroduces old carbon into the short‐term carbon cycle. Hence, the permafrost climate feedback may be enhanced in the coming decades when coastal erosion accelerates. Additionally, 0.9 ± 0.3% of Yedoma OC was leached as dissolved OC. The observed net production of inorganic nitrogen during the incubation could potentially provide a negative feedback by stimulating primary production. Bacterial community analysis showed a succession of primary responders to biolabile substrates (e.g., Psychrobacter and Colwellia) followed by secondary consumers of less biolabile substrates (e.g., Maribacter and Pseudohongiella), plus a potential establishment of permafrost associated‐bacteria on particles. Overall, our data show that OC input from thawing permafrost stimulates bacterial dynamics, with likely implications for regional biogeochemical cycles and the Earth's climate. Plain Language Summary: The Arctic coast, primarily composed of permanently frozen deposits (permafrost), is experiencing accelerated thawing and erosion due to rising temperatures, four times the global average. As a result, substantial amounts of ancient organic carbon (OC), previously stored in ice‐rich deposits like Yedoma, can be released into the adjacent sea. In our 85‐day incubation experiment, approximately 3% of the OC from ancient permafrost was transformed into CO2 when added to seawater. Remarkably, the released CO2 has an age of around 40,000 years (equivalent to carbon stored in permafrost since the last ice age). This process, termed permafrost carbon feedback, involves ancient permafrost contributing previously freeze‐locked carbon to the modern carbon cycle, intensifying climate change. Microbial communities responsible for organic matter degradation originated from both seawater and the ancient Yedoma itself, suggesting "reactivation" of long‐frozen microbes. The study highlights the impact of collapsing permafrost coastlines on regional biogeochemistry and ecology, with potential repercussions for the global carbon cycle and climate. Key Points: ∼3% of the initial Yedoma organic carbon was respired by microbial communities to CO2 and dissolved inorganic carbon (DIC) over 85 days88 ± 15% of CO2 and DIC released during the incubations originates from ancient organic matter (∼40,000 years old)Seawater and ancient Yedoma microbes increased in abundance during incubation, suggesting both communities metabolize Yedoma organic matter [ABSTRACT FROM AUTHOR]

Anthropogenic perturbations from fossil fuel burning, nuclear bomb testing, and chlorofluorocarbon (CFC) use have created useful transient tracers of ocean circulation. The atmospheric 14C/C ratio (∆14C) peaked in the early 1960s and has decreased now to pre‐industrial levels, while atmospheric CFC‐11 and CFC‐12 concentrations peaked in the early 1990s and early 2000s, respectively, and have now decreased by 10%–20%. We present the first analysis of a decade of new observations (2007 to 2018–2019) and give a comprehensive overview of the changes in ocean ∆14C and CFC concentration since the WOCE surveys in the 1990s. Surface ocean ∆14C decreased at a nearly constant rate from the 1990–2010s (20‰/decade). In most of the surface ocean ∆14C is higher than in atmospheric CO2 while in the interior ocean, only a few places are found to have increases in ∆14C, indicating that globally, oceanic bomb 14C uptake has stopped and reversed. Decreases in surface ocean CFC‐11 started between the 1990 and 2000s, and CFC‐12 between the 2000–2010s. Strong coherence in model biases of decadal changes in all tracers in the Southern Ocean suggest ventilation of Antarctic Intermediate Water was enhanced from the 1990 to the 2000s, whereas ventilation of Subantarctic Mode Water was enhanced from the 2000 to the 2010s. The decrease in surface tracers globally between the 2000 and 2010s is consistently stronger in observations than in models, indicating a reduction in vertical transport and mixing due to stratification. Plain Language Summary: The ocean contains many dissolved gases that can be measured by sampling ocean water from ships. Some of these dissolved gases are changing over time because their atmospheric concentrations are changing. By measuring these changes in the ocean, we can learn about ocean transport and mixing, which is an important factor regulating climate change. We describe ocean measurements from the past three decades of radiocarbon in dissolved inorganic carbon, that was affected by nuclear bomb testing, and of chlorofluorocarbon gases, CFCs, that were banned in the 1990s due to their destructive effects on the ozone layer. The measurements show how their oceanic distributions have changed, as a result of their atmospheric histories and the patterns of ocean uptake and transport. By comparing the measurements with ocean models, we can identify biases in the models that affect how well future climate change can be predicted. In the most recent decade, the presence of all three tracers is shown to be decreasing in the upper ocean, a reversal of the preceding decades. Also in the most recent decade, the decrease in surface tracers is stronger in the observations than the models, suggesting a change to upper ocean mixing and stratification. Key Points: Recent trends in upper ocean Δ14C, pCFC‐11, and pCFC‐12 are negative, reflecting their decreasing atmospheric trendsIncreases in ∆14C are only observed in a few places over 2000–2010s, showing oceanic bomb 14C uptake has stopped and reversedModel‐data Δ14C and chlorofluorocarbon differences could be consistent with decadal ventilation changes and large‐scale stratification or model errors [ABSTRACT FROM AUTHOR]

Sea-level rise was a key factor changing environments during the Early Holocene in Northwest Europe. It affected Mesolithic hunter-gatherer communities by inundating large areas in the current North Sea, commonly referred to as Doggerland. In this paper we present novel inundation models for the southern North Sea providing visualisations of lateral inundation driven by sea-level rise and relate it to frequency analysis of radiocarbon dates from archaeological sites. These improve on previous studies that relied on bathymetric data, which includes post-inundation overprints of Holocene sedimentation and erosion, and thus significantly underestimates the timing of inundation in some areas. We constructed a paleoDEM (a composite elevation grid of the top of the Pleistocene) for the eastern part of the southern North Sea; and sea level surfaces that combine relative sea-level curves from glacio-isostatic adjustment models optimised for Britain and southern Scandinavia respectively. We corrected our paleoDEMs for tectonic background basin subsidence, and in the inundation modelling account for pre-compaction elevation of peat in coastal areas. We evaluated the impact of these model components on our results and describe the possible inundation history of Doggerland. We suggest earlier inundation than predicted by previous models, showing significant area loss around 10.5–10 ka cal BP. Palaeogeographic changes are compared with archaeological radiocarbon data using a dates-as-data approach. Composite Kernel Density Estimate and permutation tested Summed Probability Distributions are used as a proxy for the visibility, nature and intensity of human activity. Results indicate key periods of growth and decline recorded in the dataset, as well as regional differences in growth rate, some correlating with inundation phases. Chiefly, we find elevated growth rates around 10.5–10 ka in northwest Germany and the Netherlands, contemporaneous with the abovementioned phase of extensive area loss, and moreover, with changes in culture and practices on Early Mesolithic sites. However, the spatiotemporal distribution of archaeological data is significantly influenced by accessibility and preservation of sediments of a certain age. We discuss the importance of inundation modelling and sediment data in understanding how landscape taphonomy affects archaeological patterning, especially in dates-as-data approaches. [ABSTRACT FROM AUTHOR]

Rapid and minimally destructive methods for estimating the endogenous organic content of subfossil bone save time, lab consumables, and valuable ancient materials. Fourier transform infrared (FTIR) spectroscopy is an established method to estimate bone protein content, and portable spectroscopes enable field applications. We review the ability of benchtop and portable FTIR indices to predict %N and %collagen from 137 bone specimens drawn from eight taxa. We also explore associations of these indices with the endogenous DNA content estimated for 105 specimens. Bulk bone elemental abundance and crystallinity index data reflect diagenetic alteration of these specimens, which come from a variety of depositional environments in four countries (Madagascar, Greece, Monaco, and Germany). Infrared (IR) indices from benchtop and portable units perform similarly well in predicting observed sample N content and collagen yields. Samples that include little collagen (0–5 wt%) tend to have similar IR index values, and we present a Bayesian approach for the prediction of collagen yields. Bone type best explains variation in target species DNA content (endogenous DNA being particularly abundant in petrosals), but low IR index values were consistently associated with minimal DNA content. We conclude that, although portable FTIR fails to distinguish collagen preservation among poorly preserved samples, a simple approach with minimal sample preparation can effectively screen bone from a variety of taxa, elements, and environments for the extraction of organics. [ABSTRACT FROM AUTHOR]

Low‐lying islands in tropical regions are vulnerable to near‐term sea‐level rise and hurricane‐induced flooding, with substantial human impact. These risks motivate researchers to elucidate the processes and timescales involved in the formation, growth and stabilization of coastlines through the study of Holocene shoreline dynamics. Little Ambergris Cay (Turks and Caicos Islands) is a low‐lying carbonate island that provides a case study in the nucleation and growth of such islands. This study investigates the sedimentology and radiocarbon chronology of the island's lithified sediments to develop a model for its history. The island's lithified rim encloses a tidal swamp populated by microbial mats and mangroves. Preliminary radiocarbon data supported a long‐standing inference that the island is Holocene in age. This study integrates petrographic, sedimentological and new radiocarbon data to quantify the age of the island and develop a model for its evolution. Results indicate that the ages of most lithified sediments on the island are <1000 cal yr bp, and the generation and lithification of carbonate sediment in this system supports coastline growth of at least 5 cm/year. The lithification of anthropogenic detritus was documented, consistent with other evidence that in recent centuries the lithified rim has grown by rates up to tens of centimetres per year. A unit of mid‐Holocene age was identified and correlated with a similar unit of early transgressive aeolianite described from San Salvador, The Bahamas. It is proposed that this antecedent feature played an important role in the nucleation and formation of the modern island. Results extend an established Bahamian stratigraphic framework to the south‐western extreme of the Lucayan archipelago, and highlight the dynamism of carbonate shorelines, which should inform forward‐looking mitigation strategies to increase coastal resiliency to sea‐level rise. These results inform interpretation of the palaeoenvironmental record of carbonate environments, underscoring their geologically rapid pace of lithification. [ABSTRACT FROM AUTHOR]

The work considers the modelling of nearby supernova (SN) effects on Earth's biosphere via cosmic rays (CRs) accelerated by shockwaves. The rise of the radiation background on Earth resulted from the external irradiation by CR high-energy particles and internal radiation in organisms by the decay of cosmogenic 14C is evaluated. We have taken into account that the CR flux near Earth goes up steeply when the shockwave crosses the Solar System, while in previous works the CR transport was considered as purely diffusive. Our simulations demonstrate a high rise of the external ionization of the environments at Earth's surface by atmospheric cascade particles that penetrate the first 70–100 m of water depth. Also, the cosmogenic 14C decay is able to irradiate the entire biosphere and deep ocean organisms. We analyzed the probable increase in mutation rate and estimated the distance between Earth and an SN, where the lethal effects of irradiation are possible. Our simulations demonstrate that for SN energy of around 1051 erg the lethal distance could be ∼18 pc. [ABSTRACT FROM AUTHOR]

Lake Tennyson's basin and sedimentary record were assessed for multiple branches of paleoenvironmental research. A geophysical investigation of the lake depositional setting was supported by physical and chemical analysis of multiple sediment cores. The basin reaches 60 m depth and several subaerial landforms identified were likely deposited from seismic activity. Deformed sediments and turbidites in gravity cores are attributed to one or more large (pre)historic earthquakes, and episodic grain size changes in sediment cores are related to either seismic and/or climatic/meteorological processes. A hummocky slump deposit located on the western lake bed margin is linked to widespread sediment deformation that was likely caused by movement of the Awatere Fault. Radiocarbon dating constrains the most recent major deformation event between the late thirteenth century CE and mid-twentieth Century. Folded turbidites in the stratigraphy suggest multiple sedimentary deformation episodes occurred during the last millennium. Micro-XRF elemental signatures suggest proxies of past water column mixing (windiness proxy) can be reconstructed via identification of changing redox conditions. Records from Lake Tennyson show potential for several palaeoenvironmental research applications, and combining them with basin morphostratigraphy and land-based palaeoclimate and palaeoseismic evidence can enhance understanding of environmental change in the northern South Island. [ABSTRACT FROM AUTHOR]

The Amazon River mobilizes organic carbon across one of the world's largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems such as the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles with Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kg per second) between the rising and falling stages of the Amazon River mainstem. The C/N ratio and stable and radiocarbon signatures of bulk POC are less variable within the cross‐section at Óbidos and suggest that riverine POC in the Amazon River is predominantly soil‐derived. However, smaller shifts in these compositional metrics with depth, including leaf wax n‐alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross‐sectional surveys at Óbidos highlight the importance of depth‐specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river. Plain Language Summary: The Amazon River transports one of the largest quantities of freshwater organic carbon into the Atlantic Ocean. In this study, we collected suspended particles at different depths within a cross‐section of the Amazon River mainstem during the rising and falling stages of the river's hydrological cycle. We analyzed the organic carbon, nitrogen, grain size, and leaf‐derived compounds in these particles, and integrated water velocity measurements to calculate the quantity of carbon in particle form moving through the river at these two stages. The analyses showed that large, dense particles concentrate with depth in the Amazon River mainstem. The composition of these particles is relatively homogenous, but slight variations in metrics like carbon‐to‐nitrogen ratio, age derived from carbon‐dating, and leaf waxes imply that less degraded sources of organic carbon are found in the deeper and coarser grained particles. Overall, the data suggest that the majority of Amazon River particulate organic carbon comes from a mixture of soil organic carbon washing in from different landscapes and soil depth horizons. A globally significant quantity of this carbon will get buried in the Atlantic Ocean, forming a long‐term carbon sink. Key Points: Amazon River suspended sediments show little variation in organic carbon composition with depth despite hydrodynamic sortingEstimated particulate organic carbon fluxes range from 370 to 540 kg per second during the falling and rising stages, respectivelyThe majority of Amazon River particulate organic carbon exported from the mainstem at Óbidos is soil‐derived [ABSTRACT FROM AUTHOR]

Dams are a globally relevant source of greenhouse gases (GHG), which impairs their function as a source of green energy. High burial rates of organic carbon (OC) in dam sediments may partly or fully offset the emissions. We argue that only the burial of carbon fixed in the timespan of dam operation changes the GHG balance. Here, we took sediment cores from a temperate dam. We analyzed radiocarbon age and OC molecular composition by laser desorption ionization mass spectrometry in the bulk OC and in four chemical extract fractions. The bulk samples contained modern OC, fixed after 1950. However, the extracted OC was of different ages (modern to 1900 years BP). Fractions with OC measured as old (>960 years BP) accounted for 57% of total sediment OC. Correlations of molecular composition with extract age suggest that these older fractions contained insignificant amounts of modern OC. We conclude that a substantial proportion of buried carbon did not originate from the contemporary atmosphere and cannot be offset against recent GHG emissions. Plain Language Summary: Dams are an important source of green electricity. However, it has been known for some time that dams can release large amounts of methane, a potent greenhouse gas. This is due to the fact that dams trap suspended sediment from their tributaries, and reducing chemical conditions in sediments favor the formation of methane. Theoretically, greenhouse gas emissions can be offset by the permanent burial of carbon in sediments, if this carbon is thereby removed from the recent atmosphere. Studying a small dam in Germany, we show that at least 60% of the buried organic carbon was removed from the atmosphere by plants in the catchment, centuries before the dam was constructed. The burial of this aged carbon cannot be offset against recent emissions. This equalizes emissions and burial in the greenhouse gas balance and calls into question the potential of reservoirs to act as sinks of greenhouse gases. Key Points: Organic carbon extract fractions from a reservoir sediment ranged in radiocarbon age from modern to 1,000 years depending on sourcesCompounds with H/O ratios >2.5 correlated with old extract fractions, and compounds with ratios <2.5 correlated with modern fractionsNo more than 40% of sedimentary OC can be offset with GHG emissions with reasonable certainty [ABSTRACT FROM AUTHOR]

To study the role of biomass burning (BB) in air pollution at upper‐northern Thailand, the source apportionment of size‐resolved carbonaceous aerosols from Chiang Mai was carried out based on the radiocarbon (14C) analysis. The fraction of modern carbon (F14C) was generally decreased with particle size increasing and with the highest and lowest values of 0.90 ± 0.04 and 0.61 ± 0.04, respectively. Elemental carbon, regardless of emission sources, and BB‐derived organic carbon (OCbb) showed unimodal size distribution patterns with peaks at 0.43–0.65 μm. Fossil‐fuel derived‐OC (OCf) displayed a bimodal mode with the major peak at 2.1–10 μm, and the minor one at 0.43–0.65 μm. The biogenic secondary organic aerosols (BSOA) showed a typical fine‐mode unimodal size distribution pattern during the high BB (HBB) season, and a bimodal mode during the low BB season. The BSOA concentration increased by 189% ± 80% due to the interaction with open BB plums during HBB season, which was quantified by a 14C‐involved random forest model. Besides, the concentration of biogenic primary organic aerosols also showed a significant increment during the HBB season, especially in sub‐microns. Our results highlight the critical importance of controlling open fires to reduce air pollutants and the potential exposure risk. Plain Language Summary: Large‐scale biomass burning (BB) activities take place annually in Southeast Asia. BB can emit large amounts of carbonaceous aerosols (CA) thus affecting human health and causing climate change. The particle size, directly affects how deep the particle can penetrate the human body, can be significantly changed by BB. In this study, BB‐derived CA contributed the most to the total concentration of CA. The concentration of BB‐derived CA in PM10 (particles with an aerodynamic diameter of less than 10 μm) was 12 times higher in high BB (HBB) season than in low BB (LBB) season. More specifically, BB‐derived CA in fine (PM1.1) and coarse (PM1.1–10) particles increased by roughly 13 and 5 times in HBB season compared to LBB season, respectively. Besides, BB enhanced the concentration of biogenic secondary organic aerosols by 180%, wherein 65% of the increment was allocated in PM1.1. Our results indicated that BB can not only emit particles or precursors of aerosol particles but also promote the generation of biogenic aerosols. And BB significantly enhanced the loading of CA, particularly in fine particles. Key Points: Fossil‐derived carbonaceous aerosols (CA) showed a bimodal mode size distribution patternNon‐fossil‐derived CA showed a typical fine‐mode unimodal size distribution patternBiomass burning greatly enhanced the production of biogenic secondary organic aerosols particularly in fine aerosols [ABSTRACT FROM AUTHOR]

Frozen water is the most widespread type of ice present in ice caves and forms ice stalagmites and stalactites as well as floor ice, which is often several meters thick. Organic macroremains are commonly rare in this type of cave ice, which makes it difficult to establish a chronology and severely limits the use of such ice deposits as paleoenvironmental archives. Here, the chronology of such ice deposits in the inner part of the glaciated Eisriesenwelt, one of the world's largest ice caves located in the European Alps of Austria, is determined by a combination of radiocarbon and 230Th dating of cryogenic calcite. The data suggest that this cave ice has formed over the last three millennia, with a marked increase in the average accumulation rate during the thirteenth century, coinciding with the onset of the Little Ice Age in the Alps. Data from a second site closer to the entrance suggests that large parts of this tourist cave were likely ice-free during the Medieval Warm Period and that a substantial part of the ice is probably a relic of the Little Ice Age. The current warming has already penetrated deeper into the cave than during the Medieval Warm Period, although air exchange during the warm season is restricted by a door at the cave entrance. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) in the global oceans is an important long‐term carbon sink. Connections between molecular size, reactivity, and isotopic characteristics show that DOC exists on a continuum from biologically reactive to recalcitrant. The driving mechanisms behind the creation and persistence of recalcitrant DOC remain unknown. We show mean recalcitrant DOC (isolated via solid‐phase extraction; SPE‐DOC) δ13C values are 1.3 ± 0.6‰ lower than mean total DOC δ13C between depth ranges 0–200 m and 2–4 km on three GO‐SHIP Repeat Hydrography cruises. Lowest observed δ13C values correlate with low ∆14C and proximity to deep ocean hydrothermal systems. These data support the hypothesis that reworking of DOC through the microbial carbon pump is a key driver of the ocean's long‐term carbon sink. Mass‐balance modeling shows deep‐ocean DOC not captured by SPE is enriched in 13C, highlighting the need for continued research on non‐retained DOC to predict mechanisms that drive ocean carbon storage. Plain Language Summary: The global oceans play an important role in Earth's climate, and dissolved organic carbon (DOC) (produced by surface ocean photosynthesis) has been recognized as a potential long‐term carbon sink in the deep ocean. However, the mechanisms behind the long‐term sink remain unknown. Recent research has shown that a phenomenon called the microbial carbon pump (MCP) may be a driving force in the production of long‐lasting "recalcitrant" DOC via heterotrophic consumption of nitrogen and phosphorus rich biomolecules and subsequent production of recalcitrant DOC as by‐product. The work presented here shows that this phenomenon, previously recognized on regional scales, may be systematic across the global oceans, indicated by our measurements in the Pacific, Indian, and Southern Oceans. Key Points: Solid‐phase extracted dissolved organic carbon has low isotope signatures relative to bulk dissolved organic carbon in three ocean basinsThis research supports the hypothesis that bacteria drive the storage of recalcitrant organic matter through successive reworkingIsotope measurements show hydrothermal vent influence likely captured in deep‐ocean solid‐phase extracted dissolved organic carbon [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) is a major greenhouse gas in the atmosphere and has large impacts on climate change. Its fossil fuel (CO2ff) and biogenic (CO2bio) sources are well investigated, while CO2 emissions from human respiration (CO2hr), a subset of CO2bio, have received less attention. Especially as a source of carbon emissions in densely populated megacities, the role of CO2hr emissions in the carbon cycles was largely neglected. Here we fully characterize the respiratory CO2 emission rates (CERs) of Chinese people for the first time. Using the example of the megacity Beijing in China, we estimate the CO2hr emissions and present a method for quantifying its fraction in the atmospheric CO2 based on radiocarbon (14C) measurements and inventory data sets. The results show that males and females have similar age trends in CERs, but the gender difference is significant, especially between the ages of 20 and 60, the average CERs was 33% higher for males than for females (P < 0.05). The CO2hr emissions were about 22.2 ± 0.6 kt CO2 per day, which was equivalent to 7.5% of daily CO2ff emissions in winter. The proportion is likely to be twice in summer due to the seasonal fluctuations of fossil fuel emissions. More importantly, the respiratory emissions could increase atmospheric CO2 concentration by about 2 ppm, accounting for 14% ± 6% of average CO2bio concentration in winter. This study highlights the importance of human respiration in carbon emissions in megacities and has implications for a better understanding of the regional carbon budget. Plain Language Summary: Human respiration is an important CO2 source in densely populated areas, and its emission can be estimated by the "bottom‐up" method, just like fossil fuel emissions. However, as a key parameter in the estimate of CO2 emissions from human respiration, the variability of the CO2 emission rate is the main source of uncertainty. Another challenge is how to separate it from other atmospheric CO2 sources, which is related to the estimation of real carbon sources and sinks in natural ecosystems. Taking Beijing as a case study, we combined inventory data sets and radiocarbon (14C) measurements to quantitatively evaluate the contribution of human respiratory emission in a megacity in China, and further isolate the emissions from fossil fuels and biogenic CO2 sources. Although human breathing carbon emissions do not belong to the carbon that can be reduced, ignoring the contribution of human respiration would lead to a corresponding overestimation of carbon emissions from fossil fuel or biogenic sources. This pilot study is helpful to arouse people's attention to human respiratory CO2 emissions and inspire the evaluation method of human respiratory emissions. Key Points: We fully characterized the respiratory CO2 emission rates of Chinese people for the first timeAn approach was developed to quantify the respiratory CO2 concentration in the atmospheric CO2Human respiration should be considered in the regional carbon budget and CO2 monitoring [ABSTRACT FROM AUTHOR]

One of the largest uncertainties in the terrestrial carbon cycle is the timing and magnitude of soil organic carbon (SOC) response to climate and vegetation change. This uncertainty prevents models from adequately capturing SOC dynamics and challenges the assessment of management and climate change effects on soils. Reducing these uncertainties requires simultaneous investigation of factors controlling the amount (SOC abundance) and duration (SOC persistence) of stored C. We present a global synthesis of SOC and radiocarbon profiles (nProfile = 597) to assess the timescales of SOC storage. We use a combination of statistical and depth‐resolved compartment models to explore key factors controlling the relationships between SOC abundance and persistence across pedo‐climatic regions and with soil depth. This allows us to better understand (i) how SOC abundance and persistence covary across pedo‐climatic regions and (ii) how the depth dependence of SOC dynamics relates to climatic and mineralogical controls on SOC abundance and persistence. We show that SOC abundance and persistence are differently related; the controls on these relationships differ substantially between major pedo‐climatic regions and soil depth. For example, large amounts of persistent SOC can reflect climatic constraints on soils (e.g., in tundra/polar regions) or mineral absorption, reflected in slower decomposition and vertical transport rates. In contrast, lower SOC abundance can be found with lower SOC persistence (e.g., in highly weathered tropical soils) or higher SOC persistence (e.g., in drier and less productive regions). We relate variable patterns of SOC abundance and persistence to differences in the processes constraining plant C input, microbial decomposition, vertical C transport and mineral SOC stabilization potential. This process‐oriented grouping of SOC abundance and persistence provides a valuable benchmark for global C models, highlighting that pedo‐climatic boundary conditions are crucial for predicting the effects of climate change and soil management on future C abundance and persistence. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) is a critical player in the global carbon cycle and a common paleoclimate archive, yet the mechanisms governing its evolution, particularly in subsoil (>30 cm) systems, remain less well understood. To better characterize subsoil SOM dynamics, we investigated the isotopic compositions of SOM in fine (<2 μm) and coarser (>2 μm) grain‐size fractions of the top 2 m of a soil profile developing in eolian sediments on the Chinese Loess Plateau. We found that in the subsoil, SOM in the fine fraction, characterized by lower C/N and higher δ13C, exhibits consistently younger radiocarbon ages than those in the coarser fraction. Such offsets suggest that organo‐mineral interactions facilitate fresh carbon input to the fine fraction rather than preventing SOM decomposition. We then developed a mass‐balance model to interpret the radiocarbon data. The results suggest continual carbon input from the above‐ground biosphere to the 2‐m deep subsoil. The active subsoil organic carbon cycling can lead to significant variations in δ13CSOM under an environment that experienced substantial vegetation changes, resulting in time‐averaged signals when using δ13CSOM for paleoenvironmental reconstructions. Our study underscores the dynamic nature of the subsoil carbon pool, emphasizing its potentially underestimated role in the contemporary carbon cycle. Plain Language Summary: Soil organic carbon (SOC) plays a key role in the global carbon cycle and serves as an important material for studying past climates. However, the processes involved in its breakdown, transformation, and interaction with soil minerals, especially in deep soils, are not fully understood. To gain better understanding of the vertical movement of SOC within soils, we analyzed the radiocarbon activities of SOC in different particle‐size fractions from a 2‐m deep soil profile located on the Chinese Loess Plateau. Based on the radiocarbon data and a mass balance model, our results suggest that the deep soils (30–200 cm) receive significant and continuous input of fresh carbon from the soil surface, which then closely interact with fine‐grained minerals and microorganisms. The transport of biospheric carbon into deep soils highlight an active subsoil SOC pool in the modern carbon cycle. On the other hand, the constantly evolving SOC might modify its original geochemical signals, posing challenges for reconstructing past climates from these soil records. Key Points: Soil organic carbon in a 2‐m soil profile was systematically analyzedSubsoil organic matter is dominated by mineral‐associated microbial productsBiospheric carbon continually fuels the subsoil organic carbon pool [ABSTRACT FROM AUTHOR]

The thawing of ancient organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (CO2, a greenhouse gas), is predicted to amplify global warming. However, the extent to which organic carbon in thawing permafrost soils will be released as CO2 is uncertain. A critical unknown is the extent to which dissolved organic carbon (DOC) from thawing permafrost soils is respired to CO2 by microbes upon export of freshly thawed DOC to both dark bottom waters and sunlit surface waters. In this study, we quantified the radiocarbon age and 13C composition of CO2 produced by microbial respiration of DOC that was leached from permafrost soils and either kept in the dark or exposed to ultraviolet and visible wavelengths of light. We show that permafrost DOC most labile to microbial respiration was as old or older (ages 4,000–11,000 a BP) and more 13C‐depleted than the bulk DOC in both dark and light‐exposed treatments, likely indicating respiration of old, 13C‐depleted lignin and lipid fractions of the permafrost DOC pool. Light exposure either increased, decreased, or had no effect on the magnitude of microbial respiration of old permafrost DOC relative to respiration in the dark, depending on both the extent of DOC oxidation during exposure to light and the wavelength of light. Together, these findings suggest that photochemical changes affecting the lability of permafrost DOC during sunlight exposure are an important control on the magnitude of microbial respiration of permafrost DOC in arctic surface waters. Plain Language Summary: Organic carbon has been frozen in arctic permafrost soils for thousands of years. As these soils warm and thaw, the vast stores of ancient organic carbon they contain can amplify global warming if the carbon is converted to carbon dioxide (a greenhouse gas) in both dark soils and when drained into sunlit lakes and streams. However, quantifying the carbon dioxide produced from thawed permafrost organic carbon requires understanding the controls on this conversion. Here we show that native permafrost microbes can respire permafrost organic carbon to carbon dioxide both in the dark and after carbon exposure to different wavelengths of light. The carbon dioxide produced was as old as the organic carbon in the soil, suggesting that even ancient carbon can be respired by microbes. This study suggests that respiration by microbes will contribute to the rapid conversion of ancient permafrost organic carbon to carbon dioxide in sunlit arctic surface waters, further amplifying global warming. Key Points: Microbes respire ancient, 13C‐depleted dissolved organic carbon (DOC) from permafrost soils to carbon dioxideEffects of sunlight exposure on the amount of permafrost DOC respired depend on the extent of DOC oxidation by each wavelength of lightSunlight exposure of permafrost DOC can increase or decrease microbial respiration in arctic lakes and streams [ABSTRACT FROM AUTHOR]

Fossil fuel carbon dioxide (CO2) emissions (ffCO2) constitute the majority of greenhouse gas emissions and are the main determinant of global climate change. The COVID-19 pandemic caused wide-scale disruption to human activity and provided an opportunity to evaluate our capability to detect ffCO2 emission reductions. Quantifying changes in ffCO2 levels is especially challenging in cities, where climate mitigation policies are being implemented but local emissions lead to spatially and temporally complex atmospheric mixing ratios. Here, we assess ffCO2 emission patterns associated with pandemic-induced changes to human activity using direct observations of on-road CO2 mole fractions in the Los Angeles (LA) urban area and analyses of the radiocarbon (14C) content of annual grasses collected by community scientists throughout California, USA. With COVID-19 mobility restrictions in place in 2020, we observed a significant reduction in ffCO2 levels across California, especially in urban centers. In LA, on-road CO2 enhancements were 60 ± 16% lower than the corresponding period of 2019 and rebounded to pre-pandemic levels by 2021. Plant 14C analysis indicated ffCO2 reductions of 5 ± 10 ppm in 2020 relative to pre-pandemic observations in LA. However, ffCO2 emission trajectories varied substantially by region and sector as COVID-related restrictions were relaxed. Further development of these techniques could aid efforts to monitor decarbonization in cities, especially in developing countries without established CO2 monitoring infrastructure.

The Southern Ocean regulates atmospheric CO2 and Earth's climate as a critical region for air‐sea gas exchange, delicately poised between being a CO2 source and sink. Here, we estimate how long a water mass has remained isolated from the atmosphere and utilize 14C/12C ratios (Δ14C) to trace the pathway and escape route of carbon sequestered in the deep ocean through the mixed layer to the atmosphere. The position of our core at the northern margin of the Southern Indian Ocean, tracks latitudinal shifts of the Southern Ocean frontal zones across the deglaciation. Our results suggest an expanded glacial Antarctic region trapped CO2, whereas deglacial expansion of the subantarctic permitted ventilation of the trapped CO2, contributing to a rapid atmospheric CO2 rise. We identify frontal positions as a key factor balancing CO2 outgassing versus sequestration in a region currently responsible for nearly half of global ocean CO2 uptake. Plain Language Summary: The Southern Ocean is a key region for the inhalation and exhalation of carbon dioxide, responsible for absorbing nearly half of the total amount of atmospheric c taken up by the modern global ocean. Today, regions of communication between the deep ocean and atmosphere are bounded by oceanographic fronts. During cold glacial periods, when sea ice cover expanded in the Southern Ocean, these regions were shifted northward. Our data shows that this northward movement pinched together the fronts in the Southern Indian Ocean, restricting glacial ocean‐atmosphere gas exchange, allowing CO2 to accumulate in the deep ocean. We find that at the end of the glaciation, the fronts returned southward and expanded, allowing transfer of the stored CO2 into intermediate and surface waters before escaping to the atmosphere. This trapping and releasing mechanism driven by frontal shifts is important for understanding how changes in Southern Ocean dynamics can affect the ability of the oceans to take up the additional atmospheric CO2 produced from human activities. Key Points: Indian Ocean Antarctic Intermediate Water was a pathway for ventilation of a deglacial carbon reservoirLatitudinal shifts in the Southern Ocean fronts contributed to glacial carbon storage and deglacial ventilationIntermediate waters of the Southern Indian Ocean were more than 2,000 years older in the deglaciation than the glacial and Holocene [ABSTRACT FROM AUTHOR]

Detection sensitivity is a critical characteristic to consider during selection of spectroscopic techniques. However, high sensitivity alone is insufficient for spectroscopic measurements in spectrally congested regions. Two-color cavity ringdown spectroscopy (2C-CRDS), based on intra-cavity pump-probe detection, simultaneously achieves high detection sensitivity and selectivity. This combination enables mid-infrared detection of radiocarbon dioxide (14CO2) molecules in room-temperature CO2 samples, with 1.4 parts-per-quadrillion (ppq, 10-15) sensitivity (average measurement precision) and 4.6-ppq quantitation accuracy (average calibrated measurement error for 21 samples from four separate trials) demonstrated on samples with 14C/C up to ~1.5× natural abundance (~1,800 ppq). These highly reproducible measurements, which are the most sensitive and quantitatively accurate in the mid-infrared, are accomplished despite the presence of orders-of-magnitude stronger, one-photon signals from other CO2 isotopologues. This is a major achievement in laser spectroscopy. A room-temperature-operated, compact, and low-cost 2C-CRDS sensor for 14CO2 benefits a wide range of scientific fields that utilize 14C for dating and isotope tracing, most notably atmospheric 14CO2 monitoring to track CO2 emissions from fossil fuels. The 2C-CRDS technique significantly enhances the general utility of high-resolution mid-infrared detection for analytical measurements and fundamental chemical dynamics studies. [ABSTRACT FROM AUTHOR]

Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30‐year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes ("soilR") and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0–10 cm and 35–60 cm depths relative to a modeled control soil with constant inputs over the 30‐year period. Between 1988 and 2018, the occluded C stock increased and δ13C in all pools decreased, while changes in particulate and mineral‐associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris‐deposited C, and possible microbial scavenging of old and young C in the particulate and mineral‐associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors. [ABSTRACT FROM AUTHOR]

The mechanisms underlying stabilization of soil organic matter (SOM) in vegetated coastal ecosystems, including mangrove forests, are poorly understood, limiting our ability to predict the consequences of disturbances. Here, we introduce density fractionation to mangrove soils to identify the distribution and properties of the functional components of SOM regarding degradation state, stability, and origin, namely, the free low-density fraction (f-LF), mineral-associated LF (m-LF), and high-density fraction (HF). Three soil cores (1 m) were collected in a mangrove forest on Ishigaki Island, Japan, and cut into 10 cm intervals and analyzed. Although HF was the most abundant, the massive production of mangrove fine roots resulted in a high abundance of LFs throughout the cores, which markedly differed from terrestrial soils. Relative abundance of LFs together accounted for 38–66% of total soil C. The m-LF was as abundant as f-LF and 1.6 times higher in relative abundance than the global average of terrestrial soils. The C/N ratios and δ13C values clearly increased with depth in all fractions, which was attributed to the increased contribution from roots. We found a consistent pattern in Δ14C values of density fractions. HF was the oldest with Δ14C between −149‰ and −97‰ followed by m-LF (between −130‰ and −87‰) and then f-LF (between −89‰ and 78‰), suggesting that mineral association may be pivotal in long-term carbon storage in the mangrove mineral soil. Our analysis successfully identified meaningful functional components of mangrove SOM, yet several questions remained unanswered, including a large variability in Δ14C in different cores. Future studies would benefit from a coupled analysis of the quantity and quality of density fractions and geochemical factors in mangrove soils. [ABSTRACT FROM AUTHOR]

Estimation of residence time of groundwater, particularly in regions with inadequate surface waters are very important for formulating sustainable groundwater management policies. We developed a technique for extracting dissolved inorganic carbon (DIC) quantitatively from water for measuring its 14C contents and presented the analytical details here. We also measured stable carbon isotope ratio (δ13C) in soil CO2 and groundwater DIC to correct the groundwater 14C ages. In addition, 14C in soil CO2 were measured for making necessary correction in the initial activity of the recharging water. The corrected 14C contents in the groundwater samples were used to estimate their residence times employing Lumped Parameter Models (LPM), a set of mathematical models to account for the processes that take place during transport from the recharge to the sampling spots. We present a case study focused on the calculation of radiocarbon ages and residence times for a groundwater sample collected from the campus of Physical Research Laboratory in Ahmedabad, Gujarat, India. The study also includes estimations of groundwater residence times using previously measured 14C ages of groundwater samples from Gujarat, India. Various factors controlling the groundwater ages in the LPM and their applicability are discussed. [ABSTRACT FROM AUTHOR]

Accelerator mass spectrometry radiocarbon (AMS 14C) dating was used for determining the age of wedge ice. It has been found that between 11,270 and 6420 cal BP, or the Greenlandian and Northgrippian stages of the Holocene, ice wedges grew syngenetically in sandy deposits with gravel in the Chara River valley. The variations of δ18O values in the ice wedges are about 8‰, from –25.5‰ to –18.8‰. Based on the stable isotope composition of ice wedges, paleotemperature reconstructions revealed that the mean January temperature was as low as –38°C during the coldest periods of the early half of the Holocene and as high as –28°C during the warmer periods. [ABSTRACT FROM AUTHOR]

Observations of radiocarbon (14C) in Earth's atmosphere and other carbon reservoirs are important to quantify exchanges of CO2 between reservoirs. The amount of 14C is commonly reported in the so-called Delta notation, i.e., Δ14C, the decay- and fractionation-corrected departure of the ratio of 14C to total C from that ratio in an absolute international standard; this Delta notation permits direct comparison of 14C/C ratios in the several reservoirs. However, as Δ14C of atmospheric CO2, Δ14CO2 is based on the ratio of 14CO2 to total atmospheric CO2, its value can and does change not just because of change in the amount of atmospheric14CO2 but also because of change in the amount of total atmospheric CO2, complicating ascription of change in Δ14CO2 to change in one or the other quantity. Here we suggest that presentation of atmospheric 14CO2 amount as mole fraction relative to dry air (moles of 14CO2 per moles of dry air in Earth's atmosphere), or as moles or molecules of 14CO2 in Earth's atmosphere, all readily calculated from Δ14CO2 and the amount of atmospheric CO2 (with slight dependence on δ13CO2), complements presentation only as Δ14CO2, and can provide valuable insight into the evolving budget and distribution of atmospheric 14CO2. [ABSTRACT FROM AUTHOR]

Freshwater ecosystems are responsible for a large proportion of global methane emissions to the atmosphere. The radiocarbon (14C) content of this aquatic methane is useful for determining the age and source of this important greenhouse gas. Several methods already exist for the collection of aquatic methane for radiocarbon analysis, but they tend to only sample over short periods of time, which can make them unsuitable for characterizing aquatic methane over longer timespans, and vulnerable to missing short-term events. Here, we describe a new time-integrated method for the collection of aquatic methane that provides samples suitable for radiocarbon analysis, that are representative for periods of up to at least 16 days. We report the results of a suite of tests undertaken to verify the reliability of the method, and the 14C age of aquatic methane from field trials undertaken at sites within Scotland, UK. We believe that this new method provides researchers with a simple approach that is easily deployable and can be used to collect representative time-integrated samples of methane for radiocarbon analysis from a wide range of aquatic environments. [ABSTRACT FROM AUTHOR]

The Eastern Chukotka is considered a unique permafrost region where massive ice bodies are widespread. However, the origin and age of these ice formations are often discussed. The age of the massive ice of Chukotka was established for the first time using AMS 14C dating. It was revealed that three massive ice bodies on the coast of Mechigmen Bay were formed at the end of the Late Pleistocene: a) near the Akkani site, 21,612 to 22,147 cal BP; b) near the Lavrentiya settlement, 27,553 cal BP; and c) near the Lavrentiya settlement, 22,193 cal BP. Stable isotope values in the studied massive ice vary in a rather wide range by about 10‰ for δ18O values (from –14.8‰ to –24.5‰) and about 75‰ for the δ2H values (from –116‰ to –191‰). The studied massive ice bodies are of intrasedimental genesis and formed epigenetically during the final stage of MIS2 (22–27 cal ka BP). [ABSTRACT FROM AUTHOR]