Blue carbon ecosystems, such as saltmarshes, play a vital role in mitigating climate change by sequestering atmospheric carbon dioxide and storing it as buried organic carbon for centuries to millennia. While there are international methodologies for generating blue carbon credits through coastal wetland restoration, their application in Aotearoa New Zealand is limited due to insufficient data on saltmarsh carbon stocks, accumulation rates and the processes governing long-term carbon preservation. To quantify these metrics, we examined 45 sediment cores collected from five saltmarsh sites in Aotearoa New Zealand. The cores were analysed for elemental composition, stable isotopes, and lipid biomarkers. These data were collected using a range of techniques, including X-ray fluorescence (XRF), Ramped-Pyrolysis Oxidation-Accelerator Mass Spectrometry (RPO-AMS), and Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS). Results show high variability in soil organic matter properties, carbon stocks (41.3 +/- 9.4 to 92.3 +/- 66.2 Mg C ha-1; mean +/- SE), and accumulation rates (0.46 +/- 0.02 to 1.53 +/- 0.09 Mg C ha-1 yr-1; mean +/- SE). Stable isotope and lipid biomarker results indicate substantial contributions from saltmarsh vegetation to the organic carbon pool. Results suggest that plant-derived organic carbon is preserved in the oldest basal sediments. Our findings highlight that spatial variability must be considered when conducting carbon assessments in saltmarsh ecosystems. Further research is required to determine the environmental drivers that influence long-term carbon storage and to improve the accuracy of blue carbon assessments in Aotearoa New Zealand.

Bamboo, a perennial grass species, exhibits rapid growth rates surpassing many native trees, offering substantial potential for atmospheric carbon capture and subsequent sequestration into durable products. Despite this promise, the carbon sequestration capacity of bamboo forests and its variability under different land management practices and environmental conditions remain underexplored. This study examines carbon sequestration in a representative bamboo forest in Anji, eastern China, employing a novel observation-based approach utilizing multiple atmospheric tracers (CO₂, CO, and ¹⁴C-CO₂) measurements to attribute fluxes accurately. The study also includes regular biomass inventory to be able to compare CO2 fluxes between two approaches. Departing from conventional inventory-based estimates of carbon emissions and uptakes, observations-based method yields detailed insights into individual carbon-cycle processes within bamboo ecosystems and identifies the most effective tracers for quantifying regional CO₂ fluxes. Leveraging high-resolution atmospheric CO₂ observations, coupled with advanced modeling systems and analytical tools—including machine learning techniques to reconstruct and correct prior Net Ecosystem Exchange (NEE) fluxes for the bamboo forest—we derive carbon fluxes while accounting for variations in management strategies and environmental factors. These findings enhance our understanding of bamboo's role in global carbon mitigation, informing sustainable forestry practices and climate policy. This work highlights the transformative potential of tracer-based methodologies for precise, scalable carbon flux assessments in managed ecosystems.The study is supported by the Quadrature Climate Foundation (Grant No. 01-21-000133).

Hydroxyl (OH) is the atmosphere's main oxidant removing most pollutants including methane. Its short lifetime prevents large-scale direct observational quantification. Abundances inferred using anthropogenic trace gas measurements and models yield conflicting trend estimates. By contrast, radiocarbon monoxide (14CO), produced naturally by cosmic rays and almost exclusively removed by OH, is a tracer with a well-understood source. Here we show that Southern-Hemisphere 14CO measurements indicate increasing OH. New Zealand 14CO data exhibit an annual-mean decrease of 12 ± 2% since 1997, whereas Antarctic measurements show a December-January decrease of 43 ± 24%. Both imply similar OH increases, corroborating our own and other model results suggesting that OH has been globally increasing during recent decades. Model sensitivity simulations illustrate the roles of methane, nitrogen oxides, stratospheric ozone depletion, and global warming driving these trends. They have substantial implications for the budgets of pollutants removed by OH, and especially imply larger than documented methane emission increases.

The 14 C:C ratio in atmospheric CO 2 (expressed as Δ 14 C) is a powerful tracer of Earth system carbon cycle processes. In the 21st century, spatio‐temporal variations of atmospheric Δ 14 C are mainly the result of anthropogenic fossil CO 2 emissions, but the oceans and terrestrial biosphere also exert significant influence on its variations. Here we present a complete three‐dimensional representation of the impact of 14 CO 2 and CO 2 fluxes on atmospheric CO 2 and Δ 14 C for 2000 through 2012. We compare simulated atmospheric Δ 14 C with approximately 5,000 measurements from both the remote atmosphere and continental areas strongly influenced by fossil CO 2 emissions. These comparisons demonstrate that the spatio‐temporal characteristics of input surface fluxes developed in Part 1 of this study have high fidelity. Based on good model‐observation agreement, we used the model's ability to determine the relative contributions of fossil, oceanic, and terrestrial fluxes to simulated Δ 14 C to help explain the origin of the observed variations. During our study period, the pole‐to‐pole difference in atmospheric Δ 14 C increased, which our analysis indicates results from changes in both fossil and oceanic fluxes. Over the continents, we show that most short‐term variation of Δ 14 C in the PBL results from atmospheric mixing acting on fossil CO 2 fluxes. Overall, the validation of our simulations by comparison with observations demonstrates that we understand the processes affecting atmospheric Δ 14 C at a variety of spatial and temporal scales. This suggests that, especially with an expanded set of measurements, we can use Δ 14 C to better quantify and understand key carbon cycle processes, especially fossil CO 2 emissions.

Submarine canyons transfer substantial amounts of sediment and organic carbon (OC) into the deep ocean, nourishing deep-sea ecosystems and contributing to the global carbon cycle through OC burial and sequestration. Tracking lateral OC transport through submarine canyon systems is challenged by the deep-ocean setting, difficulties with constraining episodic depositional events, and the need to assess the composition and age of marine and terrestrial organic matter. We apply innovative parallel ramped pyrolysis oxidation-accelerator mass spectrometry and pyrolysis-gas chromatography-mass spectrometry with isotope analyses to track OC age and sources in the 2016 Kaik & omacr;ura earthquake-triggered, canyon-flushing event that deposited along >1300 km of a submarine canyon-channel system, offshore Aotearoa New Zealand. Specifically, these techniques allow us to determine the ages, sources, and partitioning of OC within the Kaik & omacr;ura turbidite deposit and test hypotheses of how submarine canyon systems contribute to lateral OC flux and burial. Our results show that, despite considerable canyon floor erosion, substantial amounts of young OC were flushed into the deep sea, with relatively little (similar to 2 %) pre-Holocene OC contributions. Even without a direct connection between rivers and submarine canyons, most (similar to 55 %) of the OC in the Kaik & omacr;ura event bed is from terrestrial sources. However, the deposit also contains substantial amounts (similar to 22 %) of marine-derived OC and similar to 23 % of the material is of unassignable origin. Particle sorting imparts variability on the age and composition of OC within turbidite deposits and along the turbidity current flow path. Terrestrial-derived OC is preferentially older than marine-derived OC and concentrated in coarser particle sizes found more commonly at the deposit base and in proximal settings. Young, marine-derived OC is concentrated at the surface of the deposits and tends to be enriched in finer particle sizes. Such OC partitioning in turbidites supports the relevance of depositional models for predicting and quantifying distribution of OC in deep-sea deposits. Earthquake-triggered, canyon flushing events and resulting turbidites enhance OC burial efficiency and can sequester OC effectively, contributing an important carbon sink to the sedimentary carbon cycle.

The Antarctic environment is amongst the coldest and driest environments on Earth. The ultraxerous soils in the McMurdo Dry Valleys support exclusively microbial communities, however, 15 million years ago, a tundra ecosystem analogous to present-day southern Greenland occupied this region. The occurrence of ancient soil organic carbon combined with low accumulation of contemporary material makes it challenging to differentiate between ancient and modern organic processes. Here, we explore the additions of modern organic carbon, and the preservation and degradation of organics and lipid biomarkers, in a 1.4 m mid-Miocene age (similar to 14.5-14.3 Ma) permafrost soil column from Friis Hills. The total organic carbon is low throughout the soils (<1 wt %). The near-surface (upper 35 cm) dry permafrost has lower C:N ratios, higher delta C-13(org) values, higher proportion of branched fatty acids with an iso and anteiso configuration relative to n-fatty acids, lower phytol abundance and higher contributions of low-molecular weight homologues of n-alkanes, than the underlying icy permafrost, indicating higher contributions from bacteria-derived organic matter. Conversely, the icy permafrost contains higher molecular weight n-alkanes, n-fatty acids and n-alkanols, along with phytosterols (e.g. sitosterol and stigmasterol) and phytol (and its derivatives pristane and phytane) that are indicative of the contributions and preservation of higher-level plants. This implies that legacy mid-Miocene age carbon in the near-surface soils (ca. 35 cm) has been prone to microbial organic matter degradation during times when the permafrost thawed, likely during relatively warm intervals through the late Neogene (similar to 6.0 Ma) and sporadically during the Holocene (<1 %), when ground summer temperatures were >=+2 degrees C (based on branched glycerol dialkyl glycerol tetraether (brGDGT) temperature reconstructions). Conversely, lipid biomarkers found deeper in the permafrost have been preserved for millions of years. These results suggest that ancient organics preserved in permafrost could underpin significant ecological changes in the McMurdo Dry Valleys under the current warming climate.

The 14C:C ratio in atmospheric CO2 (expressed as Delta 14C) is a powerful tracer of Earth system carbon cycle processes. In the 21st century, spatio-temporal variations of atmospheric Delta 14C are mainly the result of anthropogenic fossil CO2 emissions, but the oceans and terrestrial biosphere also exert significant influence on its variations. Here we present a complete three-dimensional representation of the impact of 14CO2 and CO2 fluxes on atmospheric CO2 and Delta 14C for 2000 through 2012. We compare simulated atmospheric Delta 14C with approximately 5,000 measurements from both the remote atmosphere and continental areas strongly influenced by fossil CO2 emissions. These comparisons demonstrate that the spatio-temporal characteristics of input surface fluxes developed in Part 1 of this study have high fidelity. Based on good model-observation agreement, we used the model's ability to determine the relative contributions of fossil, oceanic, and terrestrial fluxes to simulated Delta 14C to help explain the origin of the observed variations. During our study period, the pole-to-pole difference in atmospheric Delta 14C increased, which our analysis indicates results from changes in both fossil and oceanic fluxes. Over the continents, we show that most short-term variation of Delta 14C in the PBL results from atmospheric mixing acting on fossil CO2 fluxes. Overall, the validation of our simulations by comparison with observations demonstrates that we understand the processes affecting atmospheric Delta 14C at a variety of spatial and temporal scales. This suggests that, especially with an expanded set of measurements, we can use Delta 14C to better quantify and understand key carbon cycle processes, especially fossil CO2 emissions.

The Southern Ocean plays a key role in regulating global climate and acting as a carbon sink. This region, defined as south of 35 degrees S, is accountable for 40 % of all oceanic anthropogenic CO2 uptake and 75 % of ocean heat uptake between 1861 and 2005. However, the strength of the Southern Ocean sink (air-sea CO2 flux) is variable - weakening in the 1990s and strengthening again in the 2000s. Typical methods of constraining the flux must grapple with two opposing forces: outgassing of natural CO2 and uptake of anthropogenic CO2. Reconstructions of atmospheric radiocarbon (Delta 14CO2) from Southern Hemisphere tree rings may be a viable method of observing the one-way outgassing flux of natural CO2, driven by Southern Ocean upwelling. Here we present 280 tree-ring Delta 14C measurements from 13 sites in Chile and Aotearoa / New Zealand from 1980 to 2017. These measurements dramatically expand the dataset of Southern Hemisphere atmospheric Delta 14CO2 records. We use these records to analyze latitudinal gradients in reconstructed atmospheric Delta 14CO2 across the Southern Ocean. Tree rings from Aotearoa / New Zealand's Motu Ihupuku / Campbell Island (52.5 degrees S, 169.1 degrees E) show Delta 14CO2 was on average 3.1 +/- 3.3 parts per thousand lower than atmospheric background, driving a latitudinal gradient among Aotearoa / New Zealand sites between 41.1 and 52.5 degrees S, whereas samples from similar latitudes in Chile do not exhibit such a strong gradient. We demonstrate that the gradient is driven by the combination of CO2 outgassing from the Antarctic Southern Zone (ASZ) and atmospheric transport to the sampling sites.

Accurate national-scale greenhouse gas source and sink estimates are essential to track climate mitigation efforts. Inverse models can complement inventory-based approaches for emissions reporting by providing independent estimates underpinned by atmospheric measurements, yet few nations have developed this capability for carbon dioxide (CO2). We present results from a decade-long (2011-2020) national inverse modelling study for New Zealand, which suggests a persistent carbon sink in New Zealand's terrestrial biosphere (-171 +/- 29 Tg CO2 yr-1). This sink is larger than expected from either New Zealand's Greenhouse Gas Inventory (-24 Tg CO2 yr-1) or prior terrestrial biosphere model estimates (-118 +/- 22 Tg CO2 yr-1; Biome-BGCMuSo and CenW). The largest differences are in New Zealand's South Island, in regions dominated by mature indigenous forests, generally considered to be near equilibrium, and certain grazed pasture regions. Relative to prior estimates, the inversion points to a reduced net CO2 flux to the atmosphere during the autumn/winter period. The overall findings of this study are robust with respect to extensive tests to assess the potential biases in the inverse model due to transport error, prior biosphere, ocean and fossil fuel estimates, background CO2, and diurnal cycles. We have identified CO2 exchange processes that could contribute to the gap between the inverse, prior and inventory estimates, but the magnitude of the fluxes from these processes cannot entirely explain the differences. Further work to identify the cause of the gap is essential to understand the implications of this finding for New Zealand's inventory and climate mitigation strategies.

Sea ice in the Ross Sea plays a critical role in the formation of dense water masses, ice sheet stability, and air‐sea gas exchange, and also supports unique ecosystems. However, its seasonal and spatial variability makes it challenging to include in model simulations. To address this, new sea ice records that extend beyond the satellite era and include periods of climate change are essential. This new sediment record from Moubray Bay, northwest Ross Sea, reconstructs environmental conditions between ∼11,300 and ∼10,900 cal yr BP—a time of rapid retreat of marine‐based ice sheets and coastal glaciers in the region. The diatom assemblage is dominated by three taxa: Fragilariopsis curta , Corethron pennatum , and Chaetoceros resting spores. Variations in their relative abundances reveal changes in wind strength, water column structure, and sea ice concentration and duration. Between ∼11,300 and ∼11,200 cal yr BP, environmental conditions are characterized by a stabilized water column due to fresh meltwater influx, and weaker winds, which resulted in shorter sea ice duration and reduced winter sea ice concentration. This continued after ∼11,200 cal yr BP but stronger winds linked to deepening of Amundsen Sea Low‐like circulation triggered short‐term water column stratification. Sea ice concentration and duration increased after ∼11,100 cal yr BP driven by cooling of the sea surface by stronger southerly winds. Concurrent changes in early Holocene marine and terrestrial climate records from the Ross Sea indicate a shift in atmospheric circulation during the early Holocene.

Carbon dioxide (CO2) is the single largest contributor to anthropogenic radiative forcing, with 70% of global CO2 emissions originating from urban areas. New Zealand has set ambitious greenhouse gas emission reduction targets, and its largest city, Auckland, will play a key role in achieving those reductions as it houses over 25% of the national population. To meet reduction targets, it is vital to understand current emissions and monitor the impact of implemented policies (e.g., planting trees). For these reasons, we are developing the first observation-constrained, urban-scale emission estimation framework for Auckland. This work is part of the New Zealand CarbonWatch-NZ project that also includes emission estimation at the national scale.A new and developing atmospheric observation network is operated in and around Auckland to measure CO2, 14CO2, CO, CH4, and COS. The combination of trace gases is useful in distinguishing between source sectors, especially biosphere from anthropogenic fluxes. This is important for Auckland: a green city with a year-round growing season. High-resolution bottom-up emission estimates have been developed specifically for anthropogenic (Mahuika-Auckland) and biospheric (UrbanVPRM) CO2 fluxes in Auckland. We combine bottom-up estimates and atmospheric CO2 observations in an inverse emission estimation framework that includes atmospheric transport simulations with the Lagrangian NAME-III model, driven by meteorological data from the 333-m horizontal resolution Auckland Numerical Weather Prediction model. Use of such high-resolution meteorological data is unique and helps interpret atmospheric measurements in the heterogeneous landscape of Auckland, especially when combined with our high-resolution bottom-up estimates. Finally, we explore the value and difficulties of including the full diurnal cycle of CO2 data. The resulting emission product will be a policy-relevant instrument that can help evaluate and meet New Zealand’s emission reduction targets.

The Ross Ice Shelf buttresses ice draining from both East and West Antarctica and its collapse could accelerate the loss of inland ice sheets, rapidly raising sea level. Documenting the location, timing and rate of past glacial retreat can help reveal processes driving rapid mass loss, informing projections of ice sheet responses to a warming climate. Here, we present a record of mid-Holocene ice retreat from the southwestern Ross Sea using facies succession and paired ramped pyrolysis oxidation 14C/210Pb chronology. This record shows rapid ice shelf retreat from 6.9-5.4 cal kyr BP, coeval with thinning of adjacent outlet glaciers. Our findings reconcile earlier discrepancies in terrestrial and marine reconstructions, and indicate that synchronous grounding line retreat from west of Ross Island to the Siple Coast at ~7-6.2 cal kyr BP was likely driven by warm-water incursions, a process active in parts of Antarctica today. Methods used to date a network of marine sediment cores reveal that rapid retreat of the Ross Ice Shelf was contemporaneous with the lowering of nearby outlet glaciers, implicating warm ocean waters as a driver of Antarctic deglaciation.

A two-year (March 2021 to February 2023) continuous atmospheric CO2 and a one-year regular atmospheric (CO2)-C-14 measurement records were measured at the northern foot of the Qinling Mountains in Xi'an, China, aiming to study the temporal characteristics of atmospheric CO2 and the contributions from the sources of fossil fuel CO2 (CO2ff) and biological CO2 (CO2bio) fluxes. The two-year mean CO2 mole fraction was 442.2 +/- 16.3 ppm, with a yearly increase of 4.7 ppm (i.e., 1.1 %) during the two-year observations. Seasonal CO2 mole fractions were the highest in winter (452.1 +/- 17.7 ppm) and the lowest in summer (433.5 +/- 13.3 ppm), with the monthly CO2 levels peaking in January and troughing in June. Diurnal CO2 levels peaked at dawn (05:00-07:00) in spring, summer and autumn, and at 10:00 in winter. C-14 analysis revealed that the excess CO2 (CO2ex, atmospheric CO2 minus background CO2) at this site was mainly from CO2ff emissions (67.0 +/- 26.8 %), and CO2ff mole fractions were the highest in winter (20.6 +/- 17.7 ppm). Local CO enhancement above the background mole fraction (Delta CO) was significantly (r = 0.74, p < 0.05) positively correlated with CO2ff in a one-year measurement, and Delta CO:CO2ff showed a ratio of 23 +/- 6 ppb/ppm during summer and winter sampling days, much lower than previous measurements and suggesting an improvement in combustion efficiency over the last decade. CO2bio mole fractions also peaked in winter (14.2 +/- 9.6 ppm), apparently due to biomass combustion and the lower and more stable wintertime atmospheric boundary layer. The negative CO2bio values in summer indicated that terrestrial vegetation of the Qinling Mountains had the potential to uptake atmospheric CO2 during the corresponding sampling days. This site is most sensitive to local emissions from Xi'an and to short distance transportation from the southern Qinling Mountains through the valleys.

Independent identification of carbon emission peaks determined from fuel inventories is a challenging goal. Because of the complete depletion of radiocarbon (C-14) in fossil fuel sources, the measurement of atmospheric (CO2)-C-14 has proven to offer a means of achieving this goal. Here, we present a study identifying peak carbon emissions from two Chinese cities using urban tree-ring Delta C-14 time series during 2000-2019. After subtracting background atmospheric Delta C-14 from urban tree-ring Delta C-14 to isolate local Delta C-14 (Delta C-14(local)), we find a minimum in 2010 (-51.1 +/- 4.5 parts per thousand) in Beijing and in 2013 in Xi'an (-52.5 +/- 0.5 parts per thousand). These levels correspond to an urban carbon emission peak in 2010 and in 2013 in the two respective cities. The urban carbon emission peaks are further identified by the declines of the mean absolute interannual rate of decrease of tree-ring Delta C-14 during a period, with the respective values of 3.6 and 6.4 parts per thousand/yr after and before a turning point in Beijing and 3.0 and 6.0 parts per thousand/yr after and before a turning point in Xi'an. This study provides an observation method to identify carbon emission peaks in basin cities.

Accelerating ocean-driven basal melting of Antarctic ice shelves in recent decades has implications for sea level rise and global overturning circulation. Here, we reconstruct oceanographic conditions at the confluence of the Ross Sea and the Southern Ocean by analyzing a multi-proxy Holocene marine sedimentary record collected from Robertson Bay. A ramped pyrolysis oxidation radiocarbon age-depth model provides a timeline for glacial behavior and oceanographic changes over the last 6700 years. The diatom assemblage, magnetic susceptibility, grain size, total organic carbon and nitrogen, trace elements, and bulk delta 13C are used as proxies for changing ocean and glacial conditions, which we interpret in the context of modern oceanographic measurements. Our record shows evidence of persistent ice cover in the northwestern Ross Sea during the Antarctic midHolocene climate optimum (ca. 5 cal kyr BP). Based on this observation, we suggest that meltwater and iceberg discharge associated with ice sheet retreat in the Ross Sea region altered local oceanography during the mid-Holocene. The onset of modern style oceanographic conditions in Robertson Bay occurred at ca. 4 cal kyr BP. Stable late Holocene conditions in are punctuated by a period of enhanced polynya activity and upwelling of nutrient rich Circumpolar Deep Water ca. 0.8 cal kyr BP and an increase in the seasonal duration of sea ice after 0.7 cal kyr BP, during the Little Ice Age. The response of the marine environment in Robertson Bay to midHolocene ice sheet retreat and natural climate variability during the last millennium underscores the sensitivity of the Antarctic ice-ocean interface to projected changes in coming decades.

Carbon dioxide (CO2) is the single largest contributor to anthropogenic radiative forcing, with 70% of global CO2 emissions originating from urban areas. New Zealand has set ambitious greenhouse gas emission reduction targets, and its largest city, Auckland, will play a key role in achieving those reductions as it houses over 25% of the national population. To meet reduction targets, it is vital to understand current emissions and monitor the impact of implemented policies (e.g., planting trees). For these reasons, we are developing the first observation-constrained, urban-scale emission estimation framework for Auckland. This work is part of the New Zealand CarbonWatch-NZ project that also includes emission estimation at the national scale. A new and developing atmospheric observation network is operated in and around Auckland to measure CO2, 14CO2, CO, CH4, and COS. The combination of trace gases is useful in distinguishing between source sectors, especially biosphere from anthropogenic fluxes. This is important for Auckland: a green city with a year-round growing season. High-resolution bottom-up emission estimates have been developed specifically for anthropogenic (Mahuika-Auckland) and biospheric (UrbanVPRM) CO2 fluxes in Auckland. We combine bottom-up estimates and atmospheric CO2 observations in an inverse emission estimation framework that includes atmospheric transport simulations with the Lagrangian NAME-III model, driven by meteorological data from the 333-m horizontal resolution Auckland Numerical Weather Prediction model. Use of such high-resolution meteorological data is unique and helps interpret atmospheric measurements in the heterogeneous landscape of Auckland, especially when combined with our high-resolution bottom-up estimates. Finally, we explore the value and difficulties of including the full diurnal cycle of CO2 data. The resulting emission product will be a policy-relevant instrument that can help evaluate and meet New Zealand’s emission reduction targets.

ABSTRACT Radiocarbon ( 14 C) dating of sediment deposition around Antarctica is often challenging due to heterogeneity in sources and ages of organic carbon in the sediment. Chemical and thermochemical techniques have been used to separate organic carbon when microfossils are not present. These techniques generally improve on bulk sediment dates, but they necessitate assumptions about the age spectra of specific molecules or compound classes and about the chemical heterogeneity of thermochemical separations. To address this, the Rafter Radiocarbon Laboratory has established parallel ramped pyrolysis oxidation (RPO) and ramped pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) systems to thermochemically separate distinct carbon fractions, diagnose the chemical composition of each fraction, and target suitable RPO fractions for radiocarbon dating. Three case studies of sediment taken from locations around Antarctica are presented to demonstrate the implementation of combined RPO-AMS and Py-GC-MS to provide more robust age determination in detrital sediment stratigraphy. These three depositional environments are good examples of analytical and interpretive challenges related to oceanographic conditions, carbon sources, and other factors. Using parallel RPO-AMS and Py-GC-MS analyses, we reduce the number of radiocarbon measurements required, minimize run times, provide context for unexpected 14 C ages, and better support interpretations of radiocarbon measurements in the context of environmental reconstruction.

ABSTRACT Observations of radiocarbon ( 14 C) in Earth’s atmosphere and other carbon reservoirs are important to quantify exchanges of CO 2 between reservoirs. The amount of 14 C is commonly reported in the so-called Delta notation, i.e., Δ 14 C, the decay- and fractionation-corrected departure of the ratio of 14 C to total C from that ratio in an absolute international standard; this Delta notation permits direct comparison of 14 C/C ratios in the several reservoirs. However, as Δ 14 C of atmospheric CO 2 , Δ 14 CO 2 is based on the ratio of 14 CO 2 to total atmospheric CO 2 , its value can and does change not just because of change in the amount of atmospheric 14 CO 2 but also because of change in the amount of total atmospheric CO 2 , complicating ascription of change in Δ 14 CO 2 to change in one or the other quantity. Here we suggest that presentation of atmospheric 14 CO 2 amount as mole fraction relative to dry air (moles of 14 CO 2 per moles of dry air in Earth’s atmosphere), or as moles or molecules of 14 CO 2 in Earth’s atmosphere, all readily calculated from Δ 14 CO 2 and the amount of atmospheric CO 2 (with slight dependence on δ 13 CO 2 ), complements presentation only as Δ 14 CO 2 , and can provide valuable insight into the evolving budget and distribution of atmospheric 14 CO 2 .