Abstract The Ross Sea continental shelf plays a key role in the global carbon cycle. However, the sources and cycling of dissolved organic carbon (DOC) in the region are poorly understood. Here, we determined the radiocarbon (14C) and stable carbon isotope (13C) contents of DOC in addition to those of dissolved inorganic carbon (DIC) in the Little America Basin, eastern Ross Sea. Carbon isotope values of DIC indicate rapid vertical mixing across the continental shelf, along with high surface primary productivity near the ice shelf. Despite the supply of freshly produced DOC from high primary productivity, the 14C age of the DOC throughout the water column was unexpectedly old (5,100–7,400 years), with surface DOC being older than that in deeper waters, in contrast to the typical oceanic distribution. A Keeling plot of the relationship between DOC 14C contents and the inverse of the DOC concentrations exhibits a positive linear correlation, which is the opposite trend to that of the global ocean. This demonstrates the presence of DOC with a 14C age of >10,000 years in the surface waters. Although the exact source of this aged DOC remains uncertain, we suggest that DOC from subglacial lake discharge is the likely source.

The Indonesian Throughflow (ITF) plays a critical role in modifying global ocean and climate systems through interactions with Indo‐Pacific climate. The El Niño Southern Oscillation (ENSO) and East Asian Winter Monsoon (EAWM) alter the spatial distribution of surface layer freshwater within the Maritime Continent, which affects heat and freshwater distribution from the Pacific to the Indian Ocean via the ITF. Yet, limited observations prior to the 1980s hinder examination of the role of ENSO and the EAWM on surface layer variability. Here, we use coral Δ 14 C and Ba/Ca records as proxies of ocean circulation from locations along main ITF pathways to investigate behavior in EAWM and ENSO strength and interaction. We focus on biennial variability given that the biennial modulation of the ENSO‐EAWM relationship remains a critical gap in understanding their interaction. From 1953 to 1963, the EAWM influences surface layer pathways at biennial timescales. Following 1963, shifts in EAWM mean state and the frequency of central Pacific El Niño events coincide with lower biennial variance and modulations of EAWM‐ and ENSO‐driven surface layer circulation. These historical ENSO and EAWM influences provide insight into future ITF variability and its associated global impacts.

Black carbon (BC), the most recalcitrant part of the pyrogenic carbon continuum, is formed by the incomplete combustion of biomass and fossil fuels. Methods for detecting BC include the chemical degradation of condensed aromatic compounds into benzenepolycarboxylic acids (BPCA), chemothermal oxidation of organic carbon at 375 degrees C (CTO), 13C nuclear magnetic resonance combined with a molecular mixing model, thermogravimetry-differential scanning calorimetry, and the use of polycyclic aromatic hydrocarbons as tracers. However, there is limited knowledge about the comparability of these methods in marine sediments and their suitability as wildfire proxies. Here, we examined a sediment core from the Congo River outflow using a multi-methodological approach with environmental data and proxies to assess pyrogenic tracers from the Congo River basin over the last 15,000 years and determine commonalities between the methods. Despite differing analytical windows, both dry-weight and total organic carbon concentrations, and delta 13C values for most methods showed a congruous trend. Higher BC concentrations and higher delta 13C values were present during arid periods and lower during humid periods, reflecting changes in vegetation and terrestrial organic matter inputs. For all methods, the sedimentation flux identified significant variations in BC deposition only in the last 1,000 years BP due to anthropogenic land use changes. These findings deepen our understanding of BC in the global carbon cycle and show that BC proxies can reveal distinct transport pathways, with CTO-BC representing atmospheric deposition and BPCA-BC and NMR-BC indicating fluvial inputs to coastal margins, aiding in the reconstruction of past climates and landscapes.

The Southern California Bight is an ecologically important region for many local and migratory fauna. We combine bulk and compound-specific amino acid stable isotope measurements in the skeletons of proteinaceous octocorals with new regional ocean modeling system model output to explore biogeochemical changes at two locations within the Bight - Santa Cruz Basin and Santa Barbara Channel. Separated by the Channel Islands, these sites display distinct oceanographic regimes. Corals from the southeastern Santa Cruz Basin display lower bulk δ13C and higher bulk δ15N values than those in the northern Santa Barbara Channel. Amino acid isotope analyses indicate that the higher δ15N values in Santa Cruz Basin reflect both higher δ15N of baseline primary production and nitrate and higher trophic positions of the sinking particles that comprise the coral’s diet. These findings suggest low nitrate concentrations, more complete nitrate utilization, lower productivity, and a longer planktonic food web. A 50-year time series of coral skeleton δ15N bulk values increases with time, consistent with sediment cores that capture an increase in the δ15NNO3 advected into the central Bight. In contrast, the Santa Barbara Channel corals display decadal-scale fluctuations, likely driven by interdecadal fluctuations in upwelling and nitrate supply. These findings agree with physical-biogeochemical model simulations showing greater sensitivity of upwelled surface nitrate concentrations to ocean climate variability in the Santa Barbara Channel. The importance of nutrient availability on ecosystem structure is emphasized using compound specific amino acid analysis, in a way that may be overlooked in bulk isotope palaeoceanographic records.

Abstract 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.

During wildfires and fossil fuel combustion, biomass is converted to black carbon (BC) via incomplete combustion. BC enters the ocean by rivers and atmospheric deposition contributing to the marine dissolved organic carbon (DOC) pool. The fate of BC is considered to reside in the marine DOC pool, where the oldest BC 14 C ages have been measured (>20,000 14 C y), implying long-term storage. DOC is the largest exchangeable pool of organic carbon in the oceans, yet most DOC (>80%) remains molecularly uncharacterized. Here, we report 14 C measurements on size-fractionated dissolved BC (DBC) obtained using benzene polycarboxylic acids as molecular tracers to constrain the sources and cycling of DBC and its contributions to refractory DOC (RDOC) in a site in the North Pacific Ocean. Our results reveal that the cycling of DBC is more dynamic and heterogeneous than previously believed though it does not comprise a single, uniformly “old” 14 C age. Instead, both semilabile and refractory DBC components are distributed among size fractions of DOC. We report that DBC cycles within DOC as a component of RDOC, exhibiting turnover in the ocean on millennia timescales. DBC within the low-molecular-weight DOC pool is large, environmentally persistent and constitutes the size fraction that is responsible for long-term DBC storage. We speculate that sea surface processes, including bacterial remineralization (via the coupling of photooxidation of surface DBC and bacterial co-metabolism), sorption onto sinking particles and surface photochemical oxidation, modify DBC composition and turnover, ultimately controlling the fate of DBC and RDOC in the ocean.

ABSTRACT Dissolved inorganic carbon (DIC) in ocean water is a major sink of fossil fuel derived CO 2 . 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 δ 13 C and Δ 14 C 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 Δ 14 C values have declined by 42‰ and that DIC δ 13 C values have declined by 0.4‰ since 2004. By 2020, DIC Δ 14 C values were within analytical error of nearby clean atmospheric CO 2 Δ 14 C values. These long-term trends are likely the result of significant fossil fuel derived CO 2 in surface DIC from air-sea gas exchange. Seasonally, Δ 14 C values varied by 3.4‰ between 2011 and 2022, where seasonal δ 13 C values varied by 0.7‰. The seasonal variation in Δ 14 C values is likely driven by variations in upwelling, surface eddies, and mixed layer depth. The variation in δ 13 C values appears to be driven by isotopic fractionation from marine primary producers. The DIC δ 13 C and Δ 14 C values record the influence of the drought that began in 2012, and a major upwelling event in 2016.

Abstract We report marine dissolved organic carbon (DOC) concentrations, and DOC Δ14C and δ13C in seawater collected from the West Indian Ocean during the GO‐SHIP I07N cruise in 2018. We find bomb 14C in DOC from the upper 1,000 m of the water column. There is no significant change in ∆14C of DOC in deep water northward, unlike that of dissolved inorganic carbon (DIC), suggesting that transport of deep water northward is not controlling the 14C age of DOC. Variability of DOC ∆14C, including high values in the deep waters, is more pronounced than in other oceans, suggesting that dissolution of surface derived particulate organic carbon is a source of modern carbon to deep DOC in the West Indian Ocean. Low δ13C are present at two of the five stations studied, suggesting a source of low δ13C DOC, or additional microbial utilization of deep DOC.

Decadal and multidecadal changes in the meridional overturning circulation may originate from either the subpolar North Atlantic or the Southern Hemisphere. New records of carbon and oxygen isotopes from an eastern Martinique Island (Lesser Antilles) coral reveal irregular, decadal, double-step events of low ∆ 14 C and enhanced vertical mixing, high δ 18 O and high δ 13 C values starting in 1885. Comparison of the new and published ∆ 14 C records indicates that the last event (1956–1969) coincides with a widespread, double-step ∆ 14 C low of South Atlantic origin from 32°N to 18°S, associated with a major slowdown of the Caribbean Current transport between 1963 and 1969. This event and the past Martinique ∆ 14 C lows are attributed to pulses of northward advection of low ∆ 14 C Sub-Antarctic Mode Waters into the tropical Atlantic. They are coeval with changes of the tropical freshwater budget and likely driven by meridional overturning circulation changes since ~1880.

Abstract The East Sea (also known as the Japan Sea) is connected to the Northwest Pacific via shallow straits and has independent deep water circulation, as a model miniature ocean. The radiocarbon age of dissolved organic carbon (DOC) in the East Sea ranged from 2,000 to 3,700 years, exceeding the water turnover time (∼100 years). The oldest DOC was found in the subsurface layer characterized by the Tsushima Warm Water. Comparison of the radiocarbon content and concentration of DOC in the East Sea to those in the ocean suggests that aged DOC was transported conservatively from the Northwest Pacific to the East Sea via the shallow Tsushima Warm Current. The fractions of DOC released by serial‐oxidation of the oldest DOC sample had identical radiocarbon ages, implying that refractory DOC was produced in situ and added to the DOC pool in the East Sea.

Rivers discharge significant quantities of dissolved organic carbon (DOC) to the ocean, yet biomarker and isotope studies suggest that terrigenous DOC makes up only a small amount DOC in the ocean. One of the removal pathways proposed for riverine DOC is sorption to marine sediments. This process is chemically selective, but whether sorption alters the isotopic composition of riverine DOC is unknown. Because there is isotopic variability across different organic compound classes, sorptive removal of DOC could also alter the isotopic composition of DOC. As a first step in addressing this question, we examined phase partitioning and isotopic composition of a riverine DOC standard in the presence of marine sediment particles. In a series of controlled experiments, the standard was mixed with marine sediment in 35‰ NaCl solution, then separated into particulate and dissolved phases for analyses of mass, δ13C, and ∆14C of organic carbon (OC). Across a range of sediment OC to DOC mass ratios (from < 0.1 to ~ 3), we found that: (1) sediment sorbed 0.8 μg OC per mg of sediment; and (2) DOC compounds with higher ∆14C and lower δ13C values relative to the bulk DOC was preferentially removed from solution. In effect, mixing a riverine DOC standard with marine sediment resulted in increased ∆14C and decreased δ13C of the DOC that remained in solution. These results show that sorption of DOC to sediment can alter the isotopic content of riverine DOC.

Organic matter degradation and sequestration in marine sediments are important processes involved in carbon cycling in the ocean. Here, we present the results of carbon isotope ( 14 C and 13 C) and concentration measurements of sedimentary organic carbon (SOC), pore‐water dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in sediments collected from the East China Sea, Yellow/Bohai Sea, and South China Sea. Our results indicated that selective degradation and preservation of organic matter occurred in these sediments, and marine‐derived young organic carbon degraded preferentially and rapidly, resulting in high concentrations of pore‐water DOC and DIC with distinct carbon isotopic signatures. The average 14 C age of pore‐water DOC was thousands of years younger than that of SOC in the sediment, suggesting that DOC was newly produced and cycled much faster than SOC. Aged SOC was refractory and preserved in sediments. Using a dual‐isotope three‐end‐member model, the contributions of potential sources to SOC, DOC, and DIC were estimated. Marine‐derived biomass organic carbon contributed the most to DOC, and dissolution of biogenic carbonate contributed the most to DIC. Riverine inputs of pre‐aged soil and fossil organic carbon dominated the SOC pool. Our study demonstrated that marginal sea sediments are important sites of young DOC and DIC fluxes into the water column, thus acting as a major pathway for carbon and nutrient cycling in the ocean.

ABSTRACTWe 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.

RationaleThe isotopic measurement of environmental sample CO2 via isotope ratio mass spectrometry (IRMS) can present many analytical challenges. In many offline applications, exceedingly few samples can be prepared per day. In such applications, long‐term storage (months) of sample CO2 is desirable, in order to accumulate enough samples to warrant a day of isotopic measurements. Conversely, traditional sample tube cracker systems for dual‐inlet IRMS offer a capacity for only 6–8 tubes and thus limit throughput. Here we present a simple method to alleviate these concerns using a Gas Bench II gas handling device coupled with continuous‐flow IRMS.MethodsSample preparation entails the cryogenic purification and quantification of CO2 on a vacuum line. Sample CO2 splits are expanded from a known volume to several sample ports and allowed to isotopically equilibrate (homogenize). Equilibrated CO2 splits are frozen into 3 mm outer diameter Pyrex break‐seals and sealed under vacuum with a torch to a length of 5.5 cm. Sample break‐seals are scored, placed into 12 mL Labco Exetainer® vials, purged with ultrahigh‐purity helium, cracked inside the capped helium‐flushed vials and subsequently measured via a Gas Bench equipped IRMS instrument using a CTC Analytics PAL autosampler.ResultsOur δ13C results from NIST and internal isotopic standards, measured over a time period of several years, indicate that the sealed‐tube method produces accurate δ13C values to a precision of ±0.1‰ for samples containing 10–35 μgC. The tube cracking technique within Exetainer vials has been optimized over a period of 10 years, resulting in decreased sample failure rates from 5–10% to <1%.ConclusionsThis technique offers an alternative method for δ13C analyses of CO2 where offline isolation and long‐term storage are desired. The method features a much higher sample throughput than traditional dual‐inlet IRMS cracker setups at similar precision (±0.1‰).

Radiocarbon (∆ 14 C) measurements suggest the deep ocean stores marine dissolved organic carbon (DOC) on millennial timescales. The mechanisms that mediate this residence time remain unconstrained. Solid‐phase extraction (SPE) has emerged as a widely used technique to isolate DOC for subsequent analyses. We present SPE‐DOC concentrations and ∆ 14 C values for three GO‐SHIP Repeat Hydrography transects, spanning the Pacific, Southern and Indian Oceans. Comparisons of SPE‐DOC with total DOC ∆ 14 C values are used with an isotopic mass‐balance to estimate the size of the refractory DOC (RDOC) reservoir and changes in RDOC relative abundance in the global ocean. Estimated RDOC abundance is similar across the deep Pacific and Indian Oceans (average = 93 ± 5%, 35 ± 6 μM), whereas RDOC in the surface ocean varies as a function of total DOC concentration. Our results fill in spatial SPE‐DOC ∆ 14 C sampling gaps for the global ocean, and our mass‐balance RDOC estimates are consistent with previous observations.

In a zonally averaged and simplified sense, there exists an upper oceanic Hadley Cell in the Pacific: during the winter season subduction occurs in the subtropics and extra-tropics and this water ventilates the tropical thermocline where it upwells and returns to the subducting regions through surface flow (e.g., Wyrtki & Kilonsky, 1984). Significant interior pycnocline exchange occurs between the subtropics and tropical thermocline (Johnson & McPhaden, 1999). Building on the observational evidence of Deser et al., (1996), it has been hypothesized that temperature anomalies originating at the sea-surface in the subtropics can be propagated via this subsurface pathway and interact with the equatorial thermocline, changing the character and sensitivity of the El NiñoSouthern Oscillation (ENSO) (Gu & Philander, 1997; Zhang et al., 1998). Tritium and He tracer data indicate that the ventilation time-scale of the tropical thermocline is on the order of decades (Fine et al., 2001; Jenkins, 1996). It is therefore a logical extension to hypothesize that the intergyre exchange between the extra-tropical subduction zones and the tropical thermocline could determine the decadal-scale climate character of the tropical Pacific (Gu & Philander, 1997), as well as other important processes.

We report quantitative carbon isotope evidence showing that porewater dissolved organic carbon (DOC) is produced from different sources in Arctic Ocean sediments, resulting in dramatic changes in both delta C-13 and Delta C-14 values of DOC. The very low delta C-13 (-42.7%) and Delta C-14 (-947%) values measured for porewater DOC indicate that bacteria consume methane hydrate-derived OC and convert a significant fraction of this delta C-13 depleted and old OC (44-97%) into DOC in the sediments of the Arctic Ocean. We suggest that the production of extremely old, methane-derived porewater DOC (5,430-23,600 years) in Arctic Ocean sediment is an important pathway for carbon cycling that provides not only a necessary energy source for microbial communities in this extreme environment but also an ancient DOC source that could flux and contribute to the old and biologically recalcitrant DOC found in the deep ocean. Plain Language Summary Dissolved organic carbon (DOC) produced in sediment porewater is a crucial intermediary during the early diagenesis of organic matter in marine sediments, and DOC production is also an important pathway of carbon cycling in the ocean. In this study, we present radiocarbon and stable carbon isotope results measured for porewater DOC and sedimentary solid phase total organic carbon in sediment cores collected from the Arctic Ocean. We find that the carbon isotopic signatures of porewater DOC differed dramatically between samples produced from mostly marine-derived organic matter (delta C-13 = -22.9%, Delta C-14 = -80%) in the Chukchi Sea shelf and samples produced from methane hydrate-derived organic matter (delta C-13=-42.7%, Delta C-14=-947%) on the Arctic slope. We estimate that in the slope region, 44-97% of the extremely old DOC preserved in the sediment was produced from methane hydrate. The production of this ancient, methane-bearing porewater DOC could be an important but insufficiently studied pathway for carbon cycling that provides not only a necessary energy source for microbial communities in the Arctic sediment but also an ancient DOC source that could contribute to the old and biologically recalcitrant DOC found in the deep ocean.

The removal mechanism of refractory deep-ocean dissolved organic carbon (deep-DOC) is poorly understood. The Amundsen Sea Polynya (ASP) serves as a natural test basin for assessing the fate of deep-DOC when it is supplied with a large amount of fresh-DOC and exposed to strong solar radiation during the polynya opening in austral summer. We measured the radiocarbon content of DOC in the water column on the western Amundsen shelf. The radiocarbon content of DOC in the surface water of the ASP reflected higher primary production than in the region covered by sea ice. The radiocarbon measurements of DOC, taken two years apart in the ASP, were different, suggesting rapid cycling of DOC. The increase in DOC concentration was less than expected from the observed increase in radiocarbon content from those at the greatest depths. Based on a radiocarbon mass balance, we show that deep-DOC is consumed along with fresh-DOC in the ASP. Our observations imply that water circulation through the surface layer, where fresh-DOC is produced, may play an important role in global DOC cycling.

Despite the potential importance in the oceanic carbon cycle and benthic ecosystem, global feature of lateral supply of aged organic matter hosted on lithogenic particles derived from sediment resuspension has not been systematically examined. We compiled concentrations and fluxes of lithogenic material in the ocean in a global-scale by using literature data of sediment trap studies to understand the contribution of resuspended sediment to sinking particulate matter. We find that these contributions are significant in various oceanic settings, particularly over continental margins. Lithogenic material flux decreased with increasing distance from the margins and above the seafloor. Examination of Δ14C values of sinking POC revealed strong relationships with parameters that represent contribution of resuspended sediment. We then derive estimates for the contribution of aged POC from sediment resuspension to sinking POC based on these relationships and global lithogenic material flux data.