Marine Chemistry最新文献

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.

The Peruvian upwelling zone is one of the most productive marine ecosystems in the world with a spectacular, pronounced oxygen minimum zone (OMZ). Globally OMZs are increasing in size and intensity with far-reaching consequences for the marine biological carbon pump and carbon export; thus, these zones need to be carefully monitored to be able to understand future climate change impacts. The current study was carried out in 2013 and 2017 to quantify the vertical flux of organic matter exported out of the productive surface layer by measuring ²³⁴Th²³⁸U disequilibria in the water column. Samples were collected in January 2013 and May 2017 along an identical transect located at 12°S off the Peruvian coast near Lima, Peru. Th-234 fluxes ranged from 0 to 2088 ± 95 dpm m⁻² d⁻¹ in 2013 and 698 ± 63 to 3648 ± 113 dpm m⁻² d⁻¹ in 2017. The corresponding POC fluxes varied between 0 and 164.2 ± 7.9 mg C m⁻² d⁻¹ in 2013 and 22.7 ± 2.7 to 133.1 ± 15.2 mg C m⁻² d⁻¹ in 2017, with POC fluxes gradually decreasing with distance from the coast. Despite higher POC fluxes, the export efficiencies were found to be extremely low due to high particle remineralization rates observed within the euphotic zone.

Here we report the seasonal and spatial variation of iodide (I⁻) and total dissolved iodine (TI) along a transect covering the oxygen minimum zone (OMZ) in the Arabian Sea (AS). This data is generated after a gap of more than two decades since the US-Joint Global Ocean Flux Study (JGOFS) reported observations of iodine speciation in the region. Also, this is the first study to report on iodine speciation from the coastal region of the west coast of India. Here the water samples were collected along the meridional transect (68°E) during spring inter-monsoon (SIM), southwest monsoon (SWM), and fall inter-monsoon (FIM), and from the coastal Goa transect during late SWM and FIM. In addition, sediment porewater was also collected from the coastal Goa transect during late SWM. Iodide was found to be dominant in the OMZ, whereas TI was majorly represented by iodate below the OMZ. The I⁻ and TI concentrations were higher around the 15°N stations during all three seasons sampled. Seasonal variability was observed in the study, with high I⁻ and TI during the SWM and FIM as compared to SIM. The coastal region was observed to have high I⁻ values during seasonal anoxia in FIM as compared to the late SWM. This may be due to the release of I⁻ from the sediments which are extremely rich in I⁻. The porewater showed extremely high I⁻ (up to 4545 nM) at the shelf stations, which is 10 times higher than what is measured in the overlying waters. The US-JGOFS study reported iodide values as high as 953 nM in the AS OMZ with excess iodine (difference between the measured TI and the predicted TI) of 400 nM and attributed this to the input from the shelf sediments. In the present study TI values up to 811 nM were observed in the AS. To ascertain the source of excess iodine, porewater extracted from the sediments covering shelf and open ocean stations along the Goa transect was studied for iodine speciation. The diffusive flux of I⁻ calculated from the surface sediments ranged between 14 and 43 μmol I⁻ m⁻² d⁻¹ from the shelf stations off Goa, which is high enough to explain the excess iodine concentrations seen in the OMZ. Apart from the seasonality observed in iodine concentrations, our study infers that the shelf sediments along the west coast of India act as a source of iodine to the AS OMZ.

Most of Japan's coral reefs are distributed in the Ryukyu Islands, in the southwestern part of Japan. Since they support biodiversity in the tropical and subtropical seas and are vulnerable to ocean acidification as well as ocean heat waves and pollution, projecting acidification over multidecadal or longer periods of time is a great interest. Currently, the majority of long-term acidification projections are based on Earth System Models (ESMs), and the validation of these projections relies on intercomparisons among ESMs. This study evaluated the multi-decadal trends in total dissolved inorganic carbon (DIC) concentrations around the Ryukyu Islands over the past 25 years from 1995 to 2019. A multiple linear regression using temperature, salinity and time parameters as explanatory variables was applied to evaluate the salinity-normalized dissolved inorganic carbon (nDIC) concentrations. The coefficient of time (+1.15 ± 0.03 μmol kg⁻¹ yr⁻¹) was not significantly different from the rise of nDIC that was calculated from the growth rate of atmospheric CO₂ concentrations during the same period. Assuming that nDIC in this region will continue to increase at a rate that tracks the expected growth rate of atmospheric CO₂ concentrations, we projected future trends of pH and aragonite saturation state (Ω_A) under scenarios RCP4.5 and RCP8.5. The empirical projection of acidification by the end of the 21st century was generally consistent with projections based on ESMs. At present, global corals are generally distributed in waters with Ω_A > 3.0. According to the empirical projection under the RCP8.5 scenario, Ω_A around Okinawa Island would fall below 3.0 in winter in the 2030s and throughout the year in the 2060s.

Lignin is one of the more refractory components of dissolved organic matter (DOM) and its concentration and composition are used to trace terrigenous DOM in the ocean. In this study, lignin oxidation products (LOP), and chromophoric and fluorescent DOM were measured in Polar Mixed Layer (PML), Atlantic water (AW), and Canada Basin Deep Water (CBDW) layers in the Canada Basin to elucidate the composition and source of terrestrial DOM. Significant differences in total lignin phenol concentration (Σ11) and cinnamyl to vanillyl ratio (C/V) measured with CuO oxidation in PML, AW, and CBDW samples were found. The fluorescence intensities of humic-like C1 were correlated with Σ11 in PML, congruent with the humic-like terrestrial origin. However, no significant correlation was found when all layers are considered (p > 0.05), suggesting a change in the molecular composition of C1. The protein-like C3 and C5 intensities were strongly correlated with Σ11 (rs = 0.55–0.64) corroborating that lignins can contribute to the protein-like fluorescence in the water column. The principal component analysis (PCA) confirmed the origin and composition of DOM vary between the different water masses, especially in terms of C/V, DOC, and fluorescent components. Significant relationships between humic-like fluorescence intensities and C/V reveal the significant contribution of woody sources to humic material in the Canada Basin. Elevated S/V and humic-like C6 levels were associated with a cold eddy DOM sample but further studies are required to validate the use of these DOM characteristics to trace deep eddy.

Terrestrial organic carbon (OC) could be transported to marginal oceans by large rivers and deposited under different hydrological conditions. A mixture of the fresh & lithospheric OC, superimposed by the OC from different rivers, makes it challenging to distinguish sources and contributions of terrestrial OC. Here, we employed multiple geochemical proxies, including the hydrogen isotopic composition of leaf wax (δD_wax), the carbon isotopic composition of total organic carbon (δ¹³C_org), TOC/TN ratio, and major element distributions, to investigate the variability in sources of terrestrial OC in the Yangtze River Estuary (YRE) since 1270 CE. Our results revealed that the terrestrial OC deposition in the YRE was lower before 1950 CE and considerably increased after 1950 CE. The δD_wax record in core A3 indicates that the OC deposition in the YRE was primarily modulated by the variability of El Niño-Southern Oscillation (ENSO) and East Asia Summer Monsoon. The strengthened El Niño-like state since 1600 CE resulted in increased precipitation in the middle and lower reaches of the Yangtze River basin and in turn, affected the terrestrial OC deposition in the YRE. Several episodes with lower carbon preference index (CPI) values of n-alkanes (CPI < 3) were identified. These episodes were concurrent with the streamflow shift of the ancient Yellow River during the transitions from wet to dry conditions, highlighting the potential contribution of the ancient Yellow River to the aged OC deposition in the YRE.

Unprecedented rates of climate change in the Arctic are causing altered land to ocean transport of dissolved organic matter (DOM) and subsequent processing in the Arctic Ocean. Low salinity waters have been suggested as hotspots for DOM dynamics. Although a wide range of biogeochemical processes have been observed in temperate and tropical estuaries, very little is known about DOM behavior at the Arctic land-ocean interface. Here, we use dissolved organic carbon (DOC) concentration, DOM absorption properties and ultrahigh resolution mass spectrometry to assess DOM mixing behavior at the Yukon and Kolyma land-ocean interface. Mixing behavior varied seasonally in the Yukon River. During freshet, despite high spatial variability, DOC concentration was depleted ~10% compared to conservative mixing, however aromatic DOM was enriched through mid-salinity (≤15). In late summer, DOC concentration was ~20% depleted at mid-salinity, yet DOM composition reflected enhanced in situ production compared to conservative mixing. In the Kolyma, DOC concentration suggested non-conservative loss at salinity <1 (~7%) with concurrent enrichment of aliphatics and heteroatoms before DOC enrichment of ~50% at mid-salinity. This relatively large addition of DOC at mid-salinity in the Kolyma, likely due to in situ primary production or sediment resuspension, deviates from the linear relationship previously observed between colored DOM and DOC concentration in large Arctic rivers. Ultimately, land-ocean transects from the Yukon and Kolyma Rivers represent a variety of DOM mixing behaviors in the near-shore coastal environment and highlight that land-ocean mixing in the Arctic is highly complex, with readily apparent spatial and temporal variability. Furthering our understanding of seasonal and system-specific controls of DOC concentration and DOM composition in Arctic River-Ocean mixing is critical in constraining riverine carbon fluxes on a global scale.

Sinking particles play a crucial role in transferring carbon from the atmosphere to the deep ocean. However, due to intensive particle transformations such as aggregation, disaggregation, and remineralization, only a small portion of the organic carbon produced in the euphotic zone ends up being sequestered in the deep ocean or sediment. Mesoscale eddies can significantly impact the surface ocean nutrient budget, primary production, and carbon export. Despite this, there is still a lack of research on how particle dynamics in eddy-impacted regions affect the efficiency of carbon export. In this study, we used observations of thorium isotopes (²³⁴Th and ²²⁸Th) and particulate organic carbon (POC) at two stations in the South China Sea (TS1: a decaying-eddy-impacted station and SEATS: an oligotrophic station) and an inverse model to investigate the impact of particle dynamics on particle export efficiency. Our findings indicate that particle remineralization/fragmentation was enhanced inside the eddy, which counteracted the nutrient pumping effect that promotes surface ocean productivity and eventually led to even lower carbon flux compared to the oligotrophic station.

Heterotrophic bacteria process much of the organic matter produced by phytoplankton in the ocean. A large proportion of this organic matter is in the form of polysaccharides, structurally complex, high molecular weight sugars. To consume this complex organic matter, microbes must initially produce enzymes of the correct structural specificity to transform it to smaller pieces that can be brought into the cell. The extent to which bacteria can hydrolyze organic matter determines how much carbon is transformed by a given microbial community. Because microbial community composition differs with location and depth in the ocean, quantifying their enzymatic potential at different stations and depths is essential to reveal patterns in microbial functional capabilities. To investigate links between patterns of bacterial composition and function, we assessed the bacterial community composition and measured glucosidase, peptidase, and polysaccharide hydrolase activities throughout the water column at 16 stations in the western North Atlantic. We found that bacterial community composition and polysaccharide hydrolase activities were depth stratified and showed regional variability, while glucosidase and peptidase activities were more similar among locations and depths. These findings suggest that polysaccharide hydrolase activities are expressed by a narrower range of organisms within bacterial communities, while the abilities to degrade peptides occurs more widely among community members, likely due to the broader substrate specificity for proteins compared to polysaccharides. The data and findings presented here highlight the extent to which patterns of microbial community composition and function and the physical oceanography of the western North Atlantic are interwoven and contribute to the overall transformation of carbon in the ocean.

Black carbon (BC) is ubiquitous in pelagic sediments and presumed to have an older radiocarbon age due to long ocean residence times and pre-aging in terrestrial soils. Here, we analyzed sediments from five regions in the subtropical Atlantic Ocean to quantify the black carbon fraction of the total organic carbon pool. Black carbon, derived from the chemothermal oxidation method, comprised between 17 ± 6% of the sedimentary organic carbon in the Northwest Argentina Basin and 65 ± 18% in the Amazon Delta. Black carbon sediment accumulation rates were six times greater in the Sierra Leone Rise (8.4 ± 4.1 mg cm⁻² kyr⁻¹) compared to the remote Northwest Argentina Basin (1.3 ± 0.4 mg cm⁻² kyr⁻¹), possibly due to enhanced regional atmospheric deposition from annual African grassland fires. The radiocarbon age for BC from subtropical Atlantic sediments were more modern compared to the bulk total organic carbon, and BC source was apportioned as biomass burning byproducts from their stable carbon isotopic signatures and characteristic ratios of polycyclic aromatic hydrocarbons. This study demonstrated that subtropical Atlantic Ocean sediments serve as an important sink for young BC.