Marine Chemistry最新文献

Humic substances are major components of the dissolved organic carbon pool in aquatic environments that can bind to trace metals like copper and iron as organic ligands. The unique chromophoric and fluorescence properties of humic substances enabled measurements using conventional optical methods but recently a new electrochemical method using cathodic stripping voltammetry has emerged. In this study, we employed both methods to measure humic substances and elucidate their distribution patterns in Otsuchi Bay in Japan. The electroactive humic substances (eHS) detected using the electrochemical method were higher in the rivers (range 0.24 to 4.3 mg L⁻¹) than in the coastal bay (range 0.08 to 0.69 mg L⁻¹). Both methods revealed significantly higher humic concentrations in the surrounding rivers than in Otsuchi Bay, indicating that riverine sources are an important source of humic substances to Otsuchi Bay, consistent with previous observations in riverine systems globally. eHS correlated well with optical indices of dissolved organic matter (DOM) including humic and marine humic-like fluorescent DOM and chromophoric DOM. Furthermore, comparison with data from the literature suggests that about 20% of the weaker Cu-binding organic ligand pool in Otsuchi Bay may consist of eHS. As Cu-binding organic ligands, eHS may be important in buffering temporal or seasonal increases in Cu fluxes in estuarine regions, with possible implications for nutrient bioavailability and toxicity for primary producers.

The simultaneous analysis of B vitamins and biosynthetic precursors in seawater is of great interest as it gives a better insight into the fluctuations of their availability for marine organisms and thus allows us to better understand the nature of interactions between prototrophic and auxotrophic organisms. The analysis of these micronutrients is challenging, as they are usually present in the marine environment in minuscule quantities only. Additionally, some of them are light- and heat-sensitive which complicates their handling. To meet these circumstances, the presented sample work-up method for seawater started with a freeze-drying step for volume reduction of the sample. This shortened the duration of the subsequent solid phase extraction and resulted in an increase in recovery. When comparing three solid phase extraction materials (C₁₈, HLB, PPL), PPL was found to be the most suitable. This method was applied for the quantification of all eleven B vitamins and six precursors in waters from the southern North Sea taken in winter and spring. Quantification was performed by LC-MS. The winter samples showed higher concentrations for nearly all analytes, in contrast to the uptake of most nutrients at the beginning of a potential algal bloom in spring and low analyte concentrations in springtime waters. In addition, the marine bacterium Vibrio campbellii, prototrophic for most B vitamins, was cultivated without the addition of B vitamins in order to measure them intra- and extracellularly during growth. In most cases, intracellular concentrations per cell decreased towards the end of the exponential growth phase, while extracellular concentrations increased. Extracellular B vitamin and precursor concentration measurements show that V. campbellii is in exchange with its environment and thereby possibly enables growth of B vitamin auxotrophs in the environment.

In surface seawater, it is usually very difficult to quantify anthropogenic carbon concentrations. Many processes (such as air-sea exchanges of gases and heat, biological activity, and mixing of water masses), are at play and often on different timescales. Thus, various hypotheses are used to estimate the anthropogenic concentrations in surface waters. Here, using the relatively long (1980s to present) time series data sets from the Bermuda Atlantic Time-series Study site (BATS; 31^°40′N, 64^°10′W) in the North Atlantic Ocean, we evaluate results based upon two different hypotheses. The results clearly confirm that it is very difficult to assess anthropogenic carbon concentrations in surface waters from sole oceanic properties. However, this study further indicates that at this ocean site, they can be appropriately determined from low-frequency variations of atmospheric CO₂ concentrations. Consequently, the impact of anthropogenic carbon penetration in surface waters on their acidification could be predicted.

The eastern James Bay (EJB) coast harbors numerous rivers, but there is a dearth of knowledge concerning dissolved organic matter (DOM) in the downstream coastal water. Here we report a four-year (2018–2021) and multi-seasons field study on the mixing behavior and characteristics of the chromophoric DOM (CDOM) in the nearshore EJB. Freshwater discharged from the extensively regulated La Grande River (LGR) was constantly depleted in DOM compared with the unregulated rivers (URRs), being on average 3.35 times lower in CDOM absorption coefficient at 440 nm (a_CDOM(440)) and 2.50 times lower in dissolved organic carbon (DOC). In contrast, the absorption spectral slope between 275 and 295 nm (S_275–295, a proxy of molecular weight) and the specific absorption coefficient at 254 nm (a*_CDOM(254), an indicator of aromaticity) of the LGR CDOM were only 10.6% lower and 11.7% higher than those of the URRs CDOM, respectively. Riverine input was found to be the dominant source of CDOM in the study area, with little influence from sea ice formation or melting. CDOM distribution fell into two distinct regimes: the LGR-influenced low-CDOM area in the north and the URRs-influenced high-CDOM area in the south. The two areas showed strong conservative but separate a_CDOM(440)–salinity relationships converging at a common marine endmember (salinity ∼25) with little seasonality. The composite data combing both areas and all seasons and years exhibited non-linear relationships between S_275–295, a*_CDOM(254) and a_CDOM(440) and a robust simple linear correlation of DOC to a_CDOM(440). This study suggests a strong impact of river regulation on CDOM input into the EJB, reveals low seasonal variability of CDOM mixing dynamics and characteristics, and demonstrates the feasibility of using remote sensing from space for real-time and synoptical assessment of DOM dynamics and the associated biogeochemical cycles in the EJB.

Radium isotopes, other than ²²⁶Ra, and ²²⁷Ac are typically present at low activities in the open ocean. The analysis of these isotopes thus requires the collection of large volumes of seawater and high sensitivity, low background instruments. To obtain the required large volumes (hundreds to thousands of liters), these radionuclides are typically preconcentrated on cartridge-style filters impregnated with MnO₂ (Mn-cartridges) deployed on in-situ pumps. This technique, however, requires the determination of the extraction efficiency of the Mn-cartridges for the radionuclides of interest. For Ra isotopes, we used two methods to estimate the extraction efficiency of these Mn-cartridges at two stations on the South-West Indian Ridge in the Southern Ocean (GEOTRACES GS02). Method (1) compares the ²²⁶Ra activities recovered on the Mn-cartridges versus the activities determined in Mn-fibers, through which seawater was passed at a flow rate < 1 L min⁻¹ to quantitatively sorb Ra (Mn-fiber method) while method (2) combines the ²²⁶Ra activities determined from two Mn-cartridges placed in series on in-situ pumps (A-B method). The second method is also applied to determine the ²²⁷Ac extraction efficiency. We find a relatively wide-range of Ra and ²²⁷Ac extraction efficiencies across the dataset (from 44.8% to 99.6% for Ra, and from 23.7% to 77.5% for ²²⁷Ac). Overall, the yield of ²²⁷Ac extraction is lower than that of Ra (mean value of 49.3 ± 19.0% for ²²⁷Ac, n = 10, mean value of 79.2 ± 10.3% for Ra, n = 13, using the Mn-fiber method; and a mean value of 63.9 ± 12.5%, n = 11 using the A-B method). Our dataset suggests that the Ra extraction efficiencies using either the A-B method or the Mn-fiber method are in relatively good agreement. Consequently, the ²²³Ra_ex, ²²⁴Ra_ex and ²²⁸Ra activities determined from the Mn-cartridges by applying the two Ra extraction yields are similar. We also show that the ²²⁷Ac extraction efficiency can be estimated from the Ra extraction efficiency allowing the use of a single Mn-cartridge. Finally, we recommend to determine the Ra and ²²⁷Ac extraction efficiencies in each individual Mn-cartridge, rather than applying a single extraction efficiency to all the Mn-cartridges, since a significant variability in the extraction efficiencies was observed between the different Mn-cartridges.

The active venting fluids of Milos Island, located within the southern Aegean Sea, belong to a shallow-water hydrothermal system (< 200 m depth) that shows chemical compositions and evolution processes comparable to those of mid-ocean ridges (MOR). In this study, we analyze Li and δ⁷Li in 69 vent water samples, grouped into two types based on their salt content. The low-Cl end-member (EM) Cave fluids show a relatively high Li content (0.39–0.54 mM) with MORB-like δ⁷Li (∼4.5 ‰, MORB = 3.7 ‰) compared to that of seawater, and the high-Cl brine fluids contain remarkably high Li (6.14–10.6 mM) and variable δ⁷Li (1.4–8.7 ‰). The latter fluids may have derived from metamorphic basement modified by seawater interactions at ∼300 °C. A scenario using a steady-state dissolution/precipitation model can generate consistent Li and δ⁷Li patterns, where linear correlations of Cl and Li suggest phase separation occurred after water/rock interaction at depth. On the contrary, no significant δ⁷Li variation in most Milos fluids suggests limited isotopic fractionation occurred during phase separation. More importantly, the detected Li enrichment in the high-Cl fluids implies a large Li flux, ∼3.4 × 10⁷ mol/yr, to the ocean from the Milos system. Assuming that 10% of the world's shallow-water systems discovered to date have similar Li outputs to those of Milos, this Li flux would represent ∼1.8% of MOR hydrothermal fluxes which is on the order of ∼13 × 10⁹ mol/yr. These results emphasize the importance of Li flux derived from shallow-water hydrothermal systems, which should not be excluded from the calculation of the marine Li budget and its impact on the global silicate weathering cycles.

Antarctic sea ice plays an important role in Southern Ocean biogeochemistry and mediating Earth's climate system. Yet our understanding of biogeochemical cycling in sea ice is limited by the availability of relevant data over sufficient temporal and spatial scales. Here we present a new publicly available compilation of macronutrient concentration data from Antarctic land-fast sea ice, covering the full seasonal cycle using datasets from around Antarctica, as well as a smaller dataset of macronutrient concentrations in adjacent seawater. We show a strong seasonal cycle whereby nutrient concentrations are high during autumn and winter, due to supply from underlying surface waters, and then are utilised in spring and summer by mixed ice algal communities consisting of diatoms and non-siliceous species. Our data indicate some degree of nutrient limitation of ice algal primary production, with silicon limitation likely being most prevalent, although uncertainties remain around the affinities of sea-ice algae for each nutrient. Remineralisation of organic matter and nutrient recycling drive substantial accumulations of inorganic nitrogen, phosphate and to a lesser extent silicic acid in some ice cores to concentrations far in excess of those in surface waters. Nutrient supply to fast ice is enhanced by brine convection, platelet ice accumulation and incorporation into the ice matrix, and under-ice tidal currents, whilst nutrient adsorption to sea-ice surfaces, formation of biofilms, and abiotic mineral precipitation and dissolution can also influence fast-ice nutrient cycling. Concentrations of nitrate, ammonium and silicic acid were generally higher in fast ice than reported for Antarctic pack ice, and this may support the typically observed higher algal biomass in fast-ice environments.

Continental shelves host 90% of annual organic carbon (OC) deposition in the global ocean and are regarded as “hot spots” of carbon burial and decomposition. Numerous studies have thus investigated OC sources, recent accumulation, long term preservation and key processes involved. Nonetheless, OC reactivity or lability, as a key property governing the fate of OC in the long term, received less attention, primarily due to a lack of proper technique of investigation. In this study, we conducted thermochemical decomposition analysis of OC using ramped-temperature pyrolysis/oxidation technique to investigate the reactivity of sedimentary OC along the Yangtze River estuary-shelf continuum. Our results reveal that sedimentary OC in the Yangtze River estuary-shelf region is relatively more stable than global average level, which is attributed to the winnowing of sediments due to frequent sedimentation-resuspension cycles. In general, OC reactivity increases gradually from the estuary to the inner shelf, which is governed by organo-mineral interactions and the progressive absorption of marine OC. Based on our results, we propose that OC reactivity is a key OC property to be considered in future organic carbon cycle frameworks.

The sources and behaviors of particulate organic carbon (POC) and particulate iron and manganese (pFe and pMn, respectively) in the bottom nepheloid layer (or benthic nepheloid layer, BNL) of the southwestern East Sea, also known as the Japan Sea, along a transect from the shelf to the central Ulleung Basin were investigated. The fluxes of POC, pFe, and pMn from the BNL to the seafloor on the shelf and in the basin were determined based on thorium-234 (²³⁴Th). The influence of resuspended sediment on POC was quantified using radiocarbon isotope ratio of POC (Δ¹⁴C). Sources and behaviors of pFe and pMn in the BNL were investigated by comparison to those of particulate aluminum (pAl). The stable carbon isotope ratios mainly indicated the marine origin of the POC, and the Δ¹⁴C values indicated that the majority of the POC in the BNL (68% ± 22%) was supplied by sediment resuspension. pAl and pFe were lithogenic in origin, whereas pMn was mainly authigenic (89–100%). The deficiency of ²³⁴Th activity relative to that of ²³⁸U increased toward the seafloor in the BNL, implying the efficient removal of ²³⁴Th by adsorption to the resuspended sediment particles. The ²³⁴Th-based settling fluxes of POC, pAl, pFe, and excess Mn (pMn_xs) to the seafloor in the central basin agreed with the results previously obtained from a sediment trap study in the Ulleung Basin. The settling flux of pAl and pFe in the central basin was 2–8% of the lateral transport from the Korea Strait, implying that the transported lithogenic particles mostly settled on the slope before reaching the central basin. In contrast, the settling flux of pMn in the central basin was much larger than that of pMn in the shelf, implying that pMn is further transported toward the central basin or supplied from the local sediments.