Marine Dissolved Organic Carbon Research Articles

Dissolved oxygen fluctuation alters dissolved organic carbon stability and can accumulate resistant fraction

Recurrent dissolved oxygen (DO) depletion is common in many estuaries and coastal areas worldwide. However, its impact on the accumulation of resistant dissolved organic carbon (DOC) remains controversial. To address how recurrent DO depletion affects the degradation and stability of marine DOC, a series of DOC degradation incubations in a particulate-dissolved coexisting system in April 2021 were conducted, followed by a field investigation in the East China Sea (ECS) in October. In the ECS, we found that DO ranged from 4.71 to 7.40 mg/L in the near-bottom waters, while the degradation index (DI), an indicator of amino acids molecules, increased from −0.78–2.84, suggesting an accumulation of labile DOC over DO depletion. This is further supported by the bacteria community composition that showed an increase in anaerobic metabolism families when DO <6.29 mg/L. Laboratory incubation revealed strong labile DOC release (∼ 243 ± 38 μmol/L) from particulate organic carbon (POC) over oxygen depletion, followed by significant DOC loss in either anoxic-oxic or oxic-oxic groups. The degraded original DOC was slightly less in the oxygen depletion-treated group compared to the oxic group, and the resistant proportion was estimated as 1% ∼ 8% after 1–14 days of anoxic treatments. The resistant DOC accumulation after DO alleviation for 30 days was much less in amount when compared to the big release of labile DOC over DO depletion (48 vs. 243 μmol/L) and we believed that the lack of observed resistant DOC accumulation in the field observation was due to the overlap of multiple anoxic-oxic cycling processes and the masking effect of newly labile DOC released from the POC.

Variable aging and storage of dissolved black carbon in the ocean

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 14C ages have been measured (>20,000 14C 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 14C 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" 14C 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.

Mechanisms Underpinning the Net Removal Rates of Dissolved Organic Carbon in the Global Ocean

AbstractWith almost 700 Pg of carbon, marine dissolved organic carbon (DOC) stores more carbon than all living biomass on Earth combined. However, the controls behind the persistence and the spatial patterns of DOC concentrations on the basin scale remain largely unknown, precluding quantitative assessments of the fate of this large carbon pool in a changing climate. Net removal rates of DOC along the overturning circulation suggest lifetimes of millennia. These net removal rates are in stark contrast to the turnover times of days to weeks of heterotrophic microorganisms, which are the main consumers of organic carbon in the ocean. Here, we present a dynamic “MICrobial DOC” model (MICDOC) with an explicit representation of picoheterotrophs to test whether ecological mechanisms may lead to observed decadal to millennial net removal rates. MICDOC is in line with &gt;40,000 DOC observations. Contrary to other global models, the reactivity of DOC fractions is not prescribed, but emerges from a dynamic feedback between microbes and DOC governed by carbon and macronutrient availability. A colimitation of macronutrients and organic carbon on microbial DOC uptake explains &gt;70% of the global variation of DOC concentrations, and governs characteristic features of its distribution. Here, decadal to millennial net removal rates emerge from microbial processes acting on time scales of days to weeks, suggesting that the temporal variability of the marine DOC inventory may be larger than previously thought. With MICDOC, we provide a foundation for assessing global effects on DOC related to changes in heterotrophic microbial communities in a future ocean.

Dynamics of the Marine Dissolved Organic Carbon Reservoir in Glacial Climate Simulations: The Importance of Biological Production

AbstractThe marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation.

Using 224Ra/228Th disequilibrium to quantify carbon transformation and export from intertidal sandy and muddy sediments

The intertidal zone is an important pathway for the transport of dissolved carbon across the land–ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 ± 0.1 to 1084 ± 53 L m−2 h−1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 ± 0.7 L m−2 h−1. Advective porewater exchange was recognized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 ± 0.2 to 206 ± 9 mmol C m−2 d−1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 ± 0.3 to 883 ± 43 mmol C m−2 d−1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h−1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 ± 4 and 206 ± 30 mmol C m−2 d−1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remineralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.

Refractory Dissolved Organic Matter has Similar Chemical Characteristics but Different Radiocarbon Signatures With Depth in the Marine Water Column

AbstractThe &gt;5,000‐year radiocarbon age (14C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result, 14C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant 14C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar 14C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ14C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same 14C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ14C gradient for RDOC. The observed Δ14C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean.

Impact of Global Change on Oceanic Dissolved Carbon Chemistry and Acidification: A Review

Increasing atmospheric carbon dioxide and temperature, decrease marine pH and rising dissolved organic carbon (DOC), causing extensive shifts in ocean water carbon chemistry with forecasts of long-term ecosystem impacts. This study aimed to carry out a systematic review and try to find out the actual chemistry, spatial variation at a global scale, future prediction of these natural and human-induced changes, and how this situation impacts the marine ecosystem and green economy. Literature proved that Antarctica and southern shallow polar ocean and any seaside area are particularly vulnerable to marine acidification and disturbed DOC cycle. Based on over a hundred investigations, the study observed that (a) marine acidification and DOC cycle are basically difficult-to-understand phenomena, (b) these two realities are consistent with each other and with climate change, (c) the potency of these threats is very altitudinal, periodic, and stratified (d) the mood of global change stressors on these two facts in the future ocean is unpredictable. It was found that over the past half-century, the acidity of the surface ocean has even now increased by almost 30%, and by 2100 it will increase to 150. Such a major change in ocean chemistry will have and is already having widespread consequences for marine organisms.

Revised microbial and photochemical triple-oxygen isotope effects improve marine gross oxygen production estimates.

The biogeochemical fluxes that cycle oxygen (O2) play a critical role in regulating Earth's climate and habitability. Triple-oxygen isotope (TOI) compositions of marine dissolved O2 are considered a robust tool for tracing oxygen cycling and quantifying gross photosynthetic O2 production. This method assumes that photosynthesis, microbial respiration, and gas exchange with the atmosphere are the primary influences on dissolved O2 content, and that they have predictable, fixed isotope effects. Despite its widespread use, there are major elements of this approach that remain uncharacterized, including the TOI dynamics of respiration by marine heterotrophic bacteria and abiotic O2 sinks such as the photochemical oxidation of dissolved organic carbon (DOC). Here, we report the TOI fractionation for O2 utilization by two model marine heterotrophs and by abiotic photo-oxidation of representative terrestrial and coastal marine DOC. We demonstrate that TOI slopes associated with these processes span a significant range of the mass-dependent domain (λ = 0.499 to 0.521). A sensitivity analysis reveals that even under moderate productivity and photo-oxidation scenarios, true gross oxygen production may deviate from previous estimates by more than 20% in either direction. By considering a broader suite of oxygen cycle reactions, our findings challenge current gross oxygen production estimates and highlight several paths forward to better understanding the marine oxygen and carbon cycles.

Spatial Distributions of Riverine and Marine Dissolved Organic Carbon in the Western Arctic Ocean: Results From the 2018 Korean Expedition

AbstractSeasonal primary production and river discharge increases in the Arctic Ocean exert a significant influence on the dissolved organic carbon (DOC) cycle. To improve our knowledge of the spatial heterogeneity of DOC source and concentration in the rapidly changing Arctic Ocean, we investigated the distributions of riverine and marine DOC in the western Arctic Ocean during the summer of 2018. Although the surface bulk DOC concentration indicated no clear distinction in its distribution between the Chukchi Borderland (CBL)/northern Chukchi Sea (NCS) and East Siberian Sea (ESS)/Mendeleyev Ridge (MR) regions, the estimated riverine DOC concentration (28 ± 4.2 μM C) and its contribution (40 ± 5.7%) in the surface layer of the CBL/NCS region were higher than those (19 ± 5.6 μM C and 26 ± 8.5%) in the ESS/MR region, which was attributed to the accumulation of freshwater, strong stratification, and a longer residence time in the CBL/NCS region. In contrast, although marine DOC was the dominant DOC component in both the CBL/NCS and ESS/MR regions, the higher marine DOC concentration (54 ± 8.1 μM C) and its contribution (73 ± 8.2%) in the East Siberian shelf/slope region were consistent with high bacterial abundance, which was associated with extremely high surface phytoplankton blooms sustained by nutrient supply from the deep layer, suggesting that the supply of bioavailable DOC resulted in active bacterial activities. Overall, the spatial differences in water properties between the two regions had large influences on the regional distributions of riverine and marine DOC.

Tracing the evolution of dissolved organic carbon (DOC) pool in the Ediacaran ocean by Germanium/silica (Ge/Si) ratios of diagenetic chert nodules from the Doushantuo Formation, South China

The Ediacaran Period (ca. 635–538 Ma) was a critical interval in Earth’s history, bridging the Proterozoic and the Phanerozoic Earth systems. It witnessed the diversification of eukaryotic phytoplanktons in the early stage, the evolution of macroscopic multicellular lives in the late stage, and frequent oscillations of carbonate carbon isotopes of large magnitudes, presumably reflecting large perturbations of the global carbon cycle. It has been proposed that a large dissolved organic carbon (DOC) pool, hundreds to thousands times larger than the present one, was developed in the Ediacaran deep ocean. The wax and wane of the marine DOC pool might be responsible for the secular variation of carbon isotopes of marine dissolved inorganic carbon (DIC) pool, which was recorded in the carbonate carbon isotope chemostratigraphy. However, neither the extent nor the evolutionary history of marine DOC pool in the Ediacaran ocean has been well constrained. In this study, we use Germanium/silica (Ge/Si) ratios of chert nodules from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, to trace the evolution of marine DOC pool. These chert nodules yield abundant microfossils, i.e. acanthomorphic acritarchs, and were precipitated in the early diagenetic replacement of calcareous silty/muddy sediments. Most chert nodules show positive correlations between Al contents and Ge/Si ratios, which can be resolved by a binary mixing between a silica (quartz) endmember and an Al-rich silicate (clay) endmember. On the one hand, the Ge/Si ratios of silica component ((Ge/Si)SiO2) are comparable to the modern seawater composition, suggesting the normal seawater rather than hydrothermal fluids was the major source of Si. On the other, the Ge/Si ratios of the clay component ((Ge/Si)clay) are one to two orders of magnitudes higher than that of normal marine clays. The Ge-enrichment of the clay component could be attributed to the extensive chelation of Ge by organic ligands and massive formation of organic-Ge-Clay complex at high DOC concentration. This observation supports the presence of a large DOC pool in the Ediacaran ocean. In addition, there is an increase of (Ge/Si)clay from the Member II to Member III of the Doushantuo Formation, suggesting an increase of DOC concentration. The expansion of marine DOC pool is coincident with the reduction of the sizes of the upper acanthomorphic acritarch assemblage in the Member III. We suggest that the increase of marine DOC pool might be attributed to the reduction of sizes of particulate organic matter, which favored DOC generation and accumulation in the Ediacaran seawater.

Constraining the Ediacaran oceanic dissolved organic carbon reservoir: Insights from carbon isotopic records from a drill core from South China

The evolution of the atmospheric oxygen content through Earth’s history is a key issue in paleoclimatic and paleoenvironmental research. There were at least two oxygenation events in the Precambrian that involved fundamental changes in both biotic innovation and the surface environment. However, a large dissolved organic carbon (DOC) pool maintained in deep oceans during the Neoproterozoic may have extended the time interval between the two oxygenation events. To test the DOC hypothesis, we conducted detailed micro-drilled analyses of carbonate carbon isotopes (δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt;) of a long Ediacaran drill core (the Wangji drill core), for which whole-rock δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt; and organic carbon isotope (δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;org&lt;/sub&gt;) records were available. The micro-drilled δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb &lt;/sub&gt;values obtained in this study are consistent with whole-rock δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt; results, precluding the influence of severe authigenic carbonate incorporation. Importantly, the multiple negative δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt; excursions in the Wangji drill core were likely linked with upwelling events, during which DOC was supplied to the surface water and oxidized. Using box models, we estimate that ~3.6 × 10&lt;sup&gt;19&lt;/sup&gt; mol and ~2.0 × 10&lt;sup&gt;19&lt;/sup&gt; mol DOC were converted to bicarbonate during two negative δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt; excursions spanning millions of years. The estimations are approximately 1000 times the modern marine DOC reservoir. Our results support a relatively high oxidation capacity (elevated atmospheric &lt;i&gt;p&lt;/i&gt;O&lt;sub&gt;2&lt;/sub&gt; and/or oceanic [&lt;inline-formula&gt;&lt;tex-math id="Z-20220120101958"&gt;\begin{document}${\rm{SO}}_4^{2 - }$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="JUST-2021-0226_Z-20220120101958.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="JUST-2021-0226_Z-20220120101958.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;]) of the Earth’s surface during the early Ediacaran Period.

Distribution and Sources of Carbon in Serpentinized Mantle Peridotites at the Atlantis Massif (IODP Expedition 357)

AbstractThe carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered during IODP Expedition 357 on the Atlantis Massif (AM) was examined to characterize carbon sources and the fate of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in seawater during long‐lived hydrothermal circulation and serpentinization. Carbon isotopes reveal three stages of carbonate formation, starting at least 38,000 yr ago: (a) Early dispersed carbonate precipitation, with low water/rock ratios and high temperatures (50°C–190°C); (b) carbonate vein formation related to high and focused fluid fluxes still at high temperatures (30°C–200°C); and (c) seawater circulation leading to cold carbonate precipitation controlled by late, brittle fractures during uplift, and unroofing of the oceanic core complex. Our study reveals three main DIC sources in the system: (a) DIC from abiotic hydrothermal degradation of dissolved organic matter (OM); (b) DIC from seawater; and (c) DIC from mantle‐derived volatiles. Basement rocks containing dispersed carbonates are characterized by high concentrations (∼800 ppm) of total non‐carbonate carbon (NCC) and 13C‐depleted carbonates. We propose that high seawater fluxes in the southern part of the AM likely favor the transport and incorporation of marine DOC in serpentinites and that carbonates record isotopic signals of OM decay. Our study indicates that organic carbon accounts for a significant proportion of the total carbon stored in the AM and suggests that serpentinites may be an important sink of DOC from seawater.

A largely invariant marine dissolved organic carbon reservoir across Earth's history

Marine dissolved organic carbon (DOC), the largest pool of reduced carbon in the oceans, plays an important role in the global carbon cycle and contributes to the regulation of atmospheric oxygen and carbon dioxide abundances. Despite its importance in global biogeochemical cycles, the long-term history of the marine DOC reservoir is poorly constrained. Nonetheless, significant changes to the size of the oceanic DOC reservoir through Earth's history have been commonly invoked to explain changes to ocean chemistry, carbon cycling, and marine ecology. Here, we present a revised view of the evolution of marine DOC concentrations using a mechanistic carbon cycle model that can reproduce DOC concentrations in both oxic and anoxic modern environments. We use this model to demonstrate that the overall size of the marine DOC reservoir has likely undergone very little variation through Earth's history, despite major changes in the redox state of the ocean-atmosphere system and the nature and efficiency of the biological carbon pump. A relatively static marine DOC reservoir across Earth's history renders it unlikely that major changes in marine DOC concentrations have been responsible for driving massive repartitioning of surface carbon or the large carbon isotope excursions observed in Earth's stratigraphic record and casts doubt on previously hypothesized links between marine DOC levels and the emergence and radiation of early animals.

Evaluation of the moderate DI13C isotope enrichment method for measuring photochemical mineralization of marine dissolved organic carbon

AbstractThe moderate DI13C isotope enrichment (MoDIE) method by Powers et al. (2017) is a promising method to precisely measure the photochemical mineralization of dissolved organic carbon (DOC) in water samples without dramatically altering a sample's pH or organic carbon pool. Here, we evaluated the analytical uncertainties of the MoDIE method and used Monte Carlo simulations to optimize the experimental design for the most precise measurements of dissolved inorganic carbon (DIC) that is produced photochemically (DIChν). Analytically, we recommend calculating yields of DIChv with an exact expression of conservation of mass that intrinsically reduces error and uncertainty. Methodologically, the overall uncertainty and detection limit of the MoDIE method can be significantly reduced by partially stripping away the original DIC pool, enriching the residual DIC with more DI13C, and increasing the yields of DIChv via longer irradiation. Instrumentally, more precise measurements of enriched δ13C values before and after irradiation are needed to further improve the precision of DIChν concentration determinations. Higher precision DIChv measurements via the optimized MoDIE method can improve our understanding of the photochemical mineralization of DOC and thus the budget of marine DOC. The optimizations and detection limits reported here will become more refined as measurements and associated uncertainties from future MoDIE experiments become available.

The effect of marine dissolved organic carbon on nickel accumulation in early life-stages of the sea urchin, Strongylocentrotus purpuratus

Dissolved organic carbon (DOC) is known to ameliorate the toxicity of the trace metal nickel (Ni) to aquatic animals. In theory, this effect is mediated by the capacity of DOC to bind Ni, rendering it less bioavailable, with the resulting reduction in accumulation limiting toxicological effects. However, there is a lack of experimental data examining Ni accumulation in marine settings with natural sources of DOC. In the current study, radiolabelled Ni was used to examine the time- and concentration-dependence of Ni accumulation, using naturally sourced DOC, on developing larvae of the sea urchin Strongylocentrotus purpuratus. Contrary to prediction, the two tested natural DOC samples (collected from the eastern United States, DOC 2 (Seaview park, Rhode Island (SVP)) and DOC 7 (Aubudon Coastal Center, Connecticut)) which had previously been shown to protect against Ni toxicity, did not limit accumulation. The control (artificial seawater with no added DOC), and the DOC 2 sample could mostly be described as having saturable Ni uptake, whereas Ni uptake in the presence of DOC 7 was mostly linear. These data provide evidence that DOC modifies the bioavailability of Ni, through either indirect effects (e.g. membrane permeability) or by the absorption of DOC-Ni complexes. There was some evidence for regulation of Ni accumulation in later-stage embryos (96-h) where the bioconcentration factor for Ni declined with increasing Ni exposure concentration. These data have implications for predictive modelling approaches that rely on known relationships between Ni speciation, bioavailability and bioreactivity, by suggesting that these relationships may not hold for natural marine DOC samples in the developing sea urchin model system.

Global Model for Depth‐Dependent Carbonyl Photochemical Production Rates in Seawater

AbstractA photochemical model was used to quantify the global contribution of carbonyl photoproduction in the photodegradation of marine dissolved organic carbon (DOC). As model input, wavelength‐ and temperature‐dependent apparent quantum yields (AQYs) for the photochemical production of carbonyl compounds were determined in seawater collected from the Northwest Atlantic Ocean. These AQY data and published AQY data from the North Pacific were used with remotely sensed seawater optical properties and solar irradiance data in a global model to calculate depth‐resolved, mixed‐layer photochemical fluxes of acetaldehyde and glyoxal in seawater. Based on this model, the annual global surface mixed‐layer photochemical production is 89.7 ± 36 Tg year−1 for acetaldehyde and 20.0 ± 8.0 Tg year−1 for glyoxal. This work significantly improves our understanding of the impact of photochemistry on the cycling of DOC in the surface oceans. Low‐molecular‐weight carbonyl compounds represent the second largest carbon flux among all known carbon products that are produced during the photolysis of DOC. The annual photoproduction of carbonyl‐compound carbon is ~110 ± 23 Tg C year−1, comprising approximately 9.6% of the total carbon and 22% of the biologically labile carbon that are produced globally from the photolysis of marine DOC.

Sargassum sp. Act as a Large Regional Source of Marine Dissolved Organic Carbon and Polyphenols

AbstractDissolved organic carbon (DOC) plays critical roles in marine carbon cycling, but its sources and sinks remain uncertain. In this study, we monitored DOC exudation rates of Sargassum natans under visible light (λ &gt; 390 nm) and solar radiation. DOC release rates ranged from 7 to 10 μg C g−1biomass hr−1 (wet weight) under visible light, but increased to 23 to 41 μg C g−1biomass hr−1 when exposed to natural sunlight. Results indicate that DOC released by Sargassum could amount to 0.3 to 1.2 Tg C/year, potentially contributing significantly to the marine DOC pool in the Gulf of Mexico and Western North Atlantic. We employed the Folin‐Ciocalteu phenolic content method, nuclear magnetic resonance (NMR) spectroscopy, and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) to characterize the diverse pool of organic compounds exuded from Sargassum. Results from these complementary methods showed that Sargassum release large quantities of phlorotannins, a class of polyphenols that have very similar properties to terrestrial DOC. These phlorotannins and their oxygenated phenolic derivatives exhibit a high hydrogen deficiency and functionalization (i.e., 4 to 6 oxygen atoms per aromatic ring), representing 5 to 18% of the released DOC isolated by solid phase extraction. Thus, Sargassum is the largest biological source of open ocean polyphenols recorded to date. The amount of polyphenolic DOC released by Sargassum challenges previous beliefs that all polyphenols found within the oceans are remnants of terrestrial organic matter, although the stability of phlorotannins and their derivatives needs to be further evaluated.

Remineralization and preservation of sedimentary organic carbon, and authigenic mineral formation in Alian Bay and its adjacent areas, China: Implication for the influence of abalone (Haliotis discus hannai Ino) and kelp (Saccharina japonica) mariculture

Although mariculture activities play a significant role in coastal carbon cycling, there is a lack of research on sedimentary organic carbon (SOC) in mariculture areas.In this study, remineralization of SOC in Ailian Bay, China, and an adjacent offshore control area was analyzed by combining sediment and pore water analyses, to investigate the effects of mariculture on the preservation of SOC and the related mechanisms. The significant higher total organic carbon (TOC) to sediment surface area in abalone culture area (0.74 ± 0.10 mg·m−2) than those in control area (0.59 ± 0.06 mg·m−2) suggested the maricultural activities may increase the preservation efficiency of SOC. During anaerobic incubations, higher vertical profiles and production rates of ΣCO2 in pore water of sediments in the abalone culture area than those in the control area indicated that maricultural activities may increase the rate of organic carbon remineralization. Furthermore, similar findings in dissolved organic carbon (DOC) suggested maricultural activities may increase the release of DOC in the sedimentary environment, which is an important supplement to the marine DOC pool. Additionally, the higher vertical concentrations and fluxes of the three major cations (i.e., K+, Ca2+, and Mg2+) coupled with the increased rate of SOC remineralization in the abalone culture area demonstrated that the maricultural activities strongly strengthened the formation of authigenic mineral. In conclusion, these findings suggested that maricultural activities play an important role in the coastal carbon cycle, significantly affecting the remineralization, preservation of SOC and the biogeochemical cycling of other important biogenic elements.

UV Photochemical Oxidation and Extraction of Marine Dissolved Organic Carbon at UC Irvine: Status, Surprises, and Methodological Recommendations

ABSTRACTThe 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.

Dissolved organic carbon in basalt-hosted deep subseafloor fluids of the Juan de Fuca Ridge flank

Marine dissolved organic carbon (DOC) is highly depleted in radiocarbon and thus inferred to be largely refractory to removal processes that operate on less than millennial timescales. However, a growing number of reports have shown that a large fraction of marine DOC can be effectively removed during circulation through submarine hydrothermal systems. What is not clear, however, is whether the DOC that remains in hydrothermal fluids is remnant non-reactive DOC from recharged seawater, or DOC that has been largely produced in the subsurface. We collected and characterized warm (∼65°C) hydrothermal fluids from deep (18, 40, 73, 200 m) within the basalt-hosted basement of the Juan de Fuca Ridge flank in the Northeast Pacific Ocean. DOC concentrations in hydrothermal fluids were 9 to 18 μM, much lower than measured in local deep seawater (37.5 μM). DOC Δ14C values of −683‰ to −856‰ were much lower than the Δ14C-dissolved inorganic carbon (DIC) values of −880‰ to −918‰, while DOC δ13C values of −23.6‰ to −27.0‰ were much heavier than that of the particulate organic carbon (POC) pool (∼−34‰), suggesting that biological production in the subsurface is not a primary source of DOC. Rather, our data suggest that isotopically enriched DOC are selectively removed from recharged seawater, leaving DOC that is very isotopically depleted in the basaltic basement fluids. Despite the removal of 50–75% of DOC in the subsurface, nuclear magnetic resonance (NMR) functional group analyses indicate that aromatic compounds were added to basaltic basement fluids during passage through the deep subseafloor and may partly contribute to the depleted 14C DOC in the ridge-flank basement fluids.