Dissolved inorganic carbon (DIC) in ocean water is a major sink of fossil fuel derived CO2. Carbon isotopes in DIC serve as tracers for oceanic water masses, biogeochemical processes, and air-sea gas exchange. We present a timeseries of surface DIC δ13C and Δ14C values from 2011 to 2022 from Newport Beach, California. This is a continuation of previous timeseries (Hinger et al. 2010; Santos et al. 2011) that together provide an 18-year record. These data show that DIC Δ14C values have declined by 42‰ and that DIC δ13C values have declined by 0.4‰ since 2004. By 2020, DIC Δ14C values were within analytical error of nearby clean atmospheric CO2 Δ14C values. These long-term trends are likely the result of significant fossil fuel derived CO2 in surface DIC from air-sea gas exchange. Seasonally, Δ14C values varied by 3.4‰ between 2011 and 2022, where seasonal δ13C values varied by 0.7‰. The seasonal variation in Δ14C values is likely driven by variations in upwelling, surface eddies, and mixed layer depth. The variation in δ13C values appears to be driven by isotopic fractionation from marine primary producers. The DIC δ13C and Δ14C values record the influence of the drought that began in 2012, and a major upwelling event in 2016.

We discuss present and developing techniques for studying radiocarbon in marine organic carbon (C). Bulk DOC (dissolved organic C) Δ14C measurements reveal information about the cycling time and sources of DOC in the ocean, yet they are time consuming and need to be streamlined. To further elucidate the cycling of DOC, various fractions have been separated from bulk DOC, through solid phase extraction of DOC, and ultrafiltration of high and low molecular weight DOC. Research using 14C of DOC and particulate organic C separated into organic fractions revealed that the acid insoluble fraction is similar in 14C signature to that of the lipid fraction. Plans for utilizing this methodology are described. Studies using compound specific radiocarbon analyses to study the origin of biomarkers in the marine environment are reviewed and plans for the future are outlined. Development of ramped pyrolysis oxidation methods are discussed and scientific questions addressed. A modified elemental analysis (EA) combustion reactor is described that allows high particulate organic C sample throughput by direct coupling with the MIniCArbonDAtingSystem.

The first ultraviolet photochemical oxidation (UVox) extraction method for marine dissolved organic carbon (DOC) as CO2 gas was established by Armstrong and co-workers in 1966. Subsequent refinement of the UVox technique has co-evolved with the need for high-precision isotopic (Δ14C, δ13C) analysis and smaller sample size requirements for accelerator mass spectrometry radiocarbon (AMS 14C) measurements. The UVox line at UC Irvine was established in 2004 and the system reaction kinetics and efficiency for isolating seawater DOC rigorously tested for quantitative isolation of ∼1 mg C for AMS 14C measurements. Since then, improvements have been made to sampling, storage, and UVox methods to increase overall efficiency. We discuss our progress, and key UVox system parameters for optimizing precision, accuracy, and efficiency, including (1) ocean to reactor: filtration, storage and preparation of DOC samples, (2) cryogenic trap design, efficiency and quantification of CO2 break through, and (3) use of isotopic standards, blanks and small sample graphitization techniques for the correction of DOC concentrations and Fm values with propagated uncertainties. New DOC UVox systems are in use at many institutions. However, rigorous assessment of quantitative UVox DOC yields and blank contributions, DOC concentrations and carbon isotopic values need to be made. We highlight the need for a community-wide inter-comparison study.

We report marine dissolved organic carbon (DOC) ∆14C from seawater collected from the North central Pacific Ocean (NCP) in 2015. These measurements show DOC ∆14C values averaged –235±5‰ (n=3) in the mixed layer (24–81 m) and –544±5‰ (n=5) in the deep water (1500–5139 m). A comparison of these data with two previously published DOC ∆14C profiles from the NCP in 1985 and 1987 reveals that deep DOC ∆14C values have decreased. We discuss several possible mechanisms that could cause such a shift in DOC ∆14C values, including spatial inhomogeneity and temporal variability due to changes in the dissolution and ∆14C value of surface derived particles in the deep sea. We find that forthcoming profiles of DOC ∆14C results from the NCP will determine the primary mechanisms controlling deep DOC ∆14C distributions, and changes over the past three decades.

Radiocarbon (14C) in dissolved inorganic carbon (DIC) was measured for water samples collected from six deep stations in the Kuroshio Extension (KE) region in the northwestern North Pacific in April–May 2015. Vertical profiles of Δ14C-DIC indicate that bomb-produced 14C was present from the surface to ~1500 m water depth. Large variations in Δ14C-DIC values (300‰) were observed at 500 m water depth among the stations and the differences were likely controlled by transport and mixing dynamics of different water masses in the region. The major Pacific western boundary currents, such as Kuroshio and Oyashio and regional mesoscale eddies, could play important roles affecting the observed Δ14C-DIC variability. The depth profiles of both Δ14C-DIC and DIC concentrations can be predicted by the solution mixing model and can be used as conservative tracers of water mass movement and water parcel homogenization in the ocean.

We report ∆14C measurements of cellulose extracted from near monthly tree ring growth for the 1960s of a white oak that grew in western Oregon, USA. Comparison with ∆14C measurements of atmospheric CO2 reveals that the tree ring ∆14C values were equal to or lower than those in atmospheric CO2 at the time of ring formation. We suggest that the low tree ring ∆14C values during the period 1962–1963 were caused by the presence of an atmospheric front or blocking between subpolar and temperate air masses that delayed the arrival of the bomb signal at the tree’s site.

The standard procedure for storing/preserving seawater dissolved organic carbon (DOC) samples after field collection is by freezing (–20°C) until future analysis can be made. However, shipping and receiving large numbers of these samples without thawing presents a significant logistical problem and large monetary expense. Access to freezers can also be limited in remote field locations. We therefore test an alternative method of preserving and storing samples for the measurement of DOC concentrations ([DOC]), stable carbon (δ13C), and radiocarbon (as ∆14C) isotopic values via UV photooxidation (UVox). We report a total analytical reproducibility of frozen DOC samples to be [DOC]±1.3 µM, ∆14C±9.4‰, and δ13C±0.1‰, comparable to previously reported results (Druffel et al. 2013). Open Ocean DOC frozen versus acidified duplicates were on average offset by ∆DOC±1.1 µM, ∆∆14C± –1.3‰, and ∆δ13C± –0.1‰. Coastal Ocean frozen vs. acidified sample replicates, collected as part of a long-term (380-day) storage experiment, had larger, albeit consistent offsets of ∆DOC±2.2 µM, ∆∆14C±1.5‰, and ∆δ13C± –0.2‰. A simple isotopic mass balance of changes in [DOC], ∆14C, and δ13C values reveals loss of semi-labile DOC (2.2±0.6 µM, ∆14C=–94±105‰, δ13C=–27±10‰; n=4) and semi-recalcitrant DOC (2.4±0.7 µM, ∆14C=–478±116‰, δ13C=–23.4±3.0‰; n=3) in Coastal and Open Ocean acidified samples, respectively.

Annual radiocarbon from a massive Porites lutea coral collected from Hon Tre Island, Vietnam, South China Sea (SCS) was analyzed over a ~100-yr-long period from AD 1900 to 1986. The pre-bomb results from 1900–1953 show a steady Δ14C value of –54.4±1.8‰ (n=60). These values are similar to coral records located in the central and southern SCS and from Indonesian waters, but are lower than those from Japan. Following the input of anthropogenic bomb 14C, our results show a sharp increase in Δ14C from 1960, reaching a peak value of 155.3‰ in 1973. The Hon Tre Island post-bomb Δ14C values are lower than those of other corals located in the SCS and Japan, but higher compared to those in the Indonesian Seas. This study infers a seasonal input of upwelled water depleted in 14C from the deeper SCS basin that originates from the tropical Pacific via the Luzon Strait. The bifurcation of the North Equatorial Current feeds the surface and intermediate currents in the SCS and Makassar Strait region. However, unlike the Makassar site, this study’s coral Δ14C does not receive lower 14C water from the South Pacific Equatorial Current. The Vietnam record therefore represents a unique oceanographic position, reflecting the seasonal influence of older, deeper SCS waters that upwell periodically in this area and have modified the surface waters locally in this region over the last 100 yr.

We report results of duplicate measurements of dissolved organic carbon concentrations ([DOC] as μM) and Δ14C in seawater in order to assess the total uncertainty of reported [DOC] and Δ14C measurements via the UV oxidation method. In addition, pure International Atomic Energy Agency standards analyzed over a 4-yr period are evaluated. We find the total uncertainty of Δ14C measurements to be ±4.0. However, in samples that were thawed, subsampled, and refrozen prior to UV oxidation, the total uncertainty of Δ14C measurements is on the order of ±10. The [DOC] measurements of these samples were also higher by 1.8 ± 0.3 μM. The reason(s) for these documented increases in total [DOC] and Δ14C uncertainty are unclear. In order to minimize any effects on sample measurements, we recommend the following: 1) regular monitoring of duplicate samples, DOC standards, and diluent (Milli-Q™) water for Δ14C and [DOC] measurements; 2) thawing a sample immediately and only once prior to UV oxidation; 3) avoid the use of DOC-leachable plumbing (such as PVC pipe) in both water systems feeding and within Milli-Q systems; and 4) consider the use of DOC-free diluents (i.e. pre-UV oxidized Milli-Q or sample water).

Extraneous carbon (Cex) added during chemical processing and isolation of black carbon (BC) in environmental matrices was quantified to assess its impact on compound specific radiocarbon analysis (CSRA). Extraneous carbon is added during the multiple steps of BC extraction, such as incomplete removal of solvents, and carbon bleed from the gas chromatographic and cation columns. We use 2 methods to evaluate the size and Δ14C values of Cex in BC in ocean sediments that require additional pretreatment using a cation column with the benzene polycarboxylic acid (BPCA) method. First, the direct method evaluates the size and Δ14C value of Cex directly from the process blank, generated by processing initially empty vials through the entire method identically to the treatment of a sample. Second, the indirect method quantifies Cex as the difference between processed and unprocessed (bulk) Δ14C values in a variety of modern and 14C-free or “dead” BC standards. Considering a suite of hypothetical marine sedimentary samples of various sizes and Δ14C values and BC Ring Trial standards, we compare both methods of corrections and find agreement between samples that are >50 μg C. Because Cex can profoundly influence the measured Δ14C value of compound specific samples, we strongly advocate the use of multiple types of process standards that match the sample size to assess Cex and investigate corrections throughout extensive sample processing.

Marine radiocarbon (14C) is a widely used tracer of past ocean circulation, but very few high-resolution records have been obtained. Here, we report a time series of carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from the Newport Beach pier in Orange County, within the Southern California Bight, from 2005 to 2010. Surface seawater was collected bimonthly and analyzed for Δ14C, δ13C, and salinity. Results from May 2005 to November 2010 show no long-term changes in δ13C DIC values and no consistent variability that can be attributed to upwelling. Δ14C DIC values have lowered from ∼34‰ to about ∼16‰, an 18‰ decrease from the beginning of this project in 2005, and is consistent with the overall 14C depletion from the atmospheric thermonuclear bomb pulse at the end of the 1950s. Δ14C DIC values, paired with salinity, do appear to be suitable indicators of upwelling strength with periods of upwelling characterized by more saline and lower DIC Δ14C values. However, a similar signal was not observed during the strong upwelling event of 2010. These results were obtained in the Southern California Bight where upwelling is fairly weak and there is a complex occanographic circulation in comparison with the remaining western USA coastline. It is therefore likely that the link between DIC Δ14C, salinity, and upwelling would be even stronger at other sites. These data represent the longest time series of Δ14C data from a coastal Southern California site performed to date.

We report carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from a time-series site off the Newport Beach Pier in Orange County, California. These data represent the first time series of Δ14C data for a coastal southern California site. From a suite of samples collected daily from 16 October to 11 November 2004, Δ14C values averaged 32.1 ± 4.4‰. Freshwater input from the Santa Ana River to our site caused Δ14C and δ13C values to decrease. Since this initial set of measurements, a time-series site has been maintained from November 2004 to the present. Surface seawater has been collected bimonthly and analyzed for Δ14C, δ13C, salinity, and ΣCO2 concentrations. Water samples from the Santa Ana River were collected during the wet season. California sea mussels and barnacle shells, ranging from 4 to 6 months old, were also collected and analyzed. Results from May 2005 to January 2008 show no long-term changes in δ13C DIC values. Δ14C DIC values over the 2005–2006 period averaged 33.7‰; high Δ14C values were observed sporadically (every 6–7 months), suggesting the presence of open water eddies at our site. Finally, in 2007, a stronger upwelling signal was apparent as indicated by correlations between Δ14C, salinity, and the Bakun index, suggesting that the Δ14C record is an indicator of upwelling in the Southern California Bight.

We report compound-specific radiocarbon analyses of organic matter in ocean sediments from the northeast Pacific Ocean. Chemical extractions and a preparative capillary gas chromatograph (PCGC) were used to isolate phospholipid fatty acids (PLFA) and n-alkanes from 3 cores collected off the coast of California, USA. Mass of samples for accelerator mass spectrometry (AMS) 14C analysis ranged from 13–100 μg C. PLFA extracted from anaerobic sediments in the Santa Barbara Basin (595 m depth) had modern Δ14C values (–20 to +54‰), indicating bacterial utilization of surface-produced, post-bomb organic matter. Lower Δ14C values were obtained for n-alkanes and PLFA from coast (92 m depth) and continental slope (1866 m) sediments, which reflect sources of old organic matter and bioturbation. We present a brief analysis of the blank carbon introduced to samples during chemical processing and PCGC isolation.

We present a time-saving modification to the ultraviolet (UV) oxidation method for analyzing dissolved organic carbon (DOC) concentration, Δ14C, and δ13C measurements in seawater and standard materials. A low background (∼0.2 ± 0.2 μM) was reported for pre-irradiated Milli-Q (MQ) water that was used to dilute samples for DOC 14C analysis (Beaupré et al. 2007). We use MQ water without pre-irradiation (background ∼0.9 ± 0.2 μM) to dilute the sample. This method is suitable for small-volume, high-concentration samples (mass of sample DOC overwhelms mass of MQ water DOC). An acceptable precision of Δ14C measurements (5–9‰) is maintained. This revised method reduces the preparation time for diluted DOC Δ14C samples from 2 days to 1 day.

The Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI) has developed protocols for analyzing radiocarbon in samples as small as ∼0.001 mg of carbon (C). Mass-balance background corrections for modern and 14C-dead carbon contamination (MC and DC, respectively) can be assessed by measuring 14C-free and modern standards, respectively, using the same sample processing techniques that are applied to unknown samples. This approach can be validated by measuring secondary standards of similar size and 14C composition to the unknown samples. Ordinary sample processing (such as ABA or leaching pretreatment, combustion/graphitization, and handling) introduces MC contamination of ∼0.6 ± 0.3 μg C, while DC is ∼0.3 ± 0.15 μg C. Today, the laboratory routinely analyzes graphite samples as small as 0.015 mg C for external submissions and ≅0.001 mg C for internal research activities with a precision of ∼1% for ∼0.010 mg C. However, when analyzing ultra-small samples isolated by a series of complex chemical and chromatographic methods (such as individual compounds), integrated procedural blanks may be far larger and more variable than those associated with combustion/graphitization alone. In some instances, the mass ratio of these blanks to the compounds of interest may be so high that the reported 14C results are meaningless. Thus, the abundance and variability of both MC and DC contamination encountered during ultra-small sample analysis must be carefully and thoroughly evaluated. Four case studies are presented to illustrate how extraction chemistry blanks are determined.

We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during 12 cruises to Station M in the northeast Pacific off central California. Individual surface Δ14C values ranged from 22‰ to 70‰ over 10 yr. Variability of average cruise values is highest during winter likely due to increased mixing. A general decrease of Δ14C values was observed at a rate of about 3‰ per year between 1994 and 2004, about half of that in atmospheric CO2 during this period (Levin and Kromer 2004). The Δ14C results ranged from 2–18‰ during individual cruises and were often significantly larger than the total uncertainty for individual measurements (±3.9‰). This indicates that a single Δ14C result from a surface site is not sufficient to capture the true variability of Δ14C in the surface ocean.

Contaminant carbon (blank carbon) was studied for its impact on the carbon isotope measurements (Δ14C and δ13C) of 3 organic compound classes of oceanic particulate organic matter. Two methods of blank correction and associated uncertainties were studied. First, the carbon blanks were quantified manometrically and the isotope ratios of the blank carbon were measured directly. Second, the isotope ratios of the blank carbon were estimated using the standard dilution method from the difference in Δ14C values between unprocessed and processed standards. The 2 methods agreed within the uncertainties. The standard deviations of numerous Δ14C measurements made on processed standard compounds were comparable to those of real samples. Blank correction using the standard dilution method is much less sensitive to the error in determination of blank carbon mass than is correction using the directly measured mass and Δ14C values of the blank carbon. The standard dilution method is recommended for correcting Δ14C analyses of small samples that involve incorporation of a significant amount of blank carbon.

Radiocarbon (Δ14C) measurements of monthly samples from a Galapagos surface coral are among the first data sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine. An average Δ14C value of −62 is obtained for 144 measurements of samples from monthly coral bands that lived from about AD 1760–1771 (±6 yr). High Δ14C values were found during January through March, when upwelling was weak or absent at the Galapagos Islands. Low Δ14C values were obtained mid-year during strong upwelling. The average seasonal variability of Δ14C was 15–25, which is greater than that at other tropical and subtropical locations in the Pacific Ocean because of intense seasonal upwelling at this site. Periods of sustained high Δ14C values were found during 1762–1763 and 1766. A spectral analysis revealed that the spectral density for the Δ14C data displays most of its variance at the 5-yr cycle, which is reflective of El Niño periodicity during the 20th century.

We show that high-precision radiocarbon (Δ14C) measurements from annual bands of a Hawaiian surface coral decreased by 7‰ from AD 1893 to 1952. This decrease is coincident with the Suess Effect, which is mostly due to the dilution of natural levels of 14C by 14C-free fossil fuel CO2. This decrease is equal to that expected in surface waters of the subtropical gyres, and indicates that the surface waters of the North Pacific were in steady state with respect to long term mixing of CO2 during the past century. Correlation between Δ14C and North Pacific gyre sea surface temperatures indicates that vertical mixing local to Hawaii and the North Pacific gyre as a whole is the likely physical mechanism to result in variable Δ14C. Prior to 1920, this correlation starts to break down; this may be related to the non-correlation between biennial Δ14C values in corals from the southwest Pacific and El Niño events observed during this period as well.