Carbon Cycling in Santa Barbara Basin Sediments: A Modeling Study

The primary input of organic matter to almost all marine sediments comes from deposition at the sediment surface. However, in many continental margin settings, reduced carbon can also be added to sediments from below—for example, from “deep” geologic hydrocarbon reservoirs derived from ancient source rocks or from the decomposition of deeply buried gas hydrate deposits. To examine the impact of these two differing reduced carbon inputs on sediment biogeochemistry, a modified reaction-transport model for anoxic marine sediments is described here and applied to data from sediment cores in Santa Barbara Basin to a depth of 4.6 m. Excellent model fits yield results consistent with previous studies of Santa Barbara Basin and other continental margin sediments. These results indicate that authigenic carbonate precipitation in these sediments is not centered around the sulfate-methane transition zone (SMTZ), as is seen in many other sedimentary environments but occurs at shallower depths in the sediments and over a relatively broad depth range. Sulfate profiles are linear between the surface sediments (upper ∼20 cm) and the top of the SMTZ (∼105 cm) giving the appearance of refractory particulate organic carbon (POC) burial and conservative sulfate behavior in this intermediate region. However, model results show that linear profiles may also occur when high rates of sulfate reduction occur near the sediment surface (as organoclastic sulfate reduction [oSR]) and in the SMTZ (largely as anaerobic oxidation of methane) with low, but nonzero, rates of oSR in-between. At the same time, linearity in the sulfate profile may also be related to downward pore-water advection by compaction and sedimentation plus a decrease with depth in sulfate diffusivity because of decreasing porosity. These model-determined rates of oSR and methanogenesis also result in a rate of POC loss that declines near-continuously in a logarithmic fashion over the entire sediment column studied. The results presented further here indicate the importance of a deep methane flux from below on sediment biogeochemistry in the shallower sediments, although the exact source of this methane flux is difficult to ascertain with the existing data

Methane dynamics in Santa Barbara Basin (USA) sediments as examined with a reaction-transport model

Here we describe a new reaction-transport model that quantitatively examines δ13C profiles of pore-water methane and dissolved inorganic carbon (DIC) (δ13CCH4 and δ13CDIC) in the anoxic sediments of the Santa Barbara Basin (California Borderland region). Best-fit solutions of the model to these data suggest that CO2 reduction is the predominant form of methanogenesis in these sediments. These solutions also accurately reproduce the isotope depth profiles, including a broad minimum in the δ13CDIC profile and a much sharper (angular) minimum in the δ13CCH4 profile, both of which appear near the base of the transition zone in the sediments between sulfate reduction and methanogenesis (referred to here as the sulfate-methane transition zone, or SMTZ). Such minima in pore-water profiles of δ13CCH4 near the base of the SMTZ have been seen in a number of other marine sediments across a range of depth and timescales. We show here that this minimum in the δ13CCH4 profile in Santa Barbara Basin sediments results from the balance between (1) anaerobic oxidation of methane (AOM), which leads to an increase in δ13CCH4 with decreasing depth in the sediment column through and above the SMTZ; (2) methanogenesis, which produces 13C-depleted methane, both in and below the SMTZ; and (3) an upward flux of CH4 from depth that is relatively enriched in 13C as compared with the methane in these pore waters. Possible sources of this deep methane include the following: geologic hydrocarbon reservoirs derived from ancient source rocks; decomposition of buried gas hydrates; and biogenic (or perhaps thermogenic) methane produced hundreds of meters below the seafloor stimulated by increasing temperatures associated with the sediment geothermal gradient. Although we are unable to resolve these possible sources of deep methane, we believe that the significance of an upward methane flux as an explanation for minima in δ13CCH4 pore-water profiles may not be limited to Santa Barbara Basin sediments but may be common in many continental margin sediments

Separation of realized ecological niche axes among sympatric tilefishes provides insight into potential drivers of co-occurrence in the NW Atlantic

Golden and Blueline Tilefish (Lopholatilus chamaeleonticeps and Caulolatilus microps) are keystone taxa in northwest (NW) Atlantic continental shelf-edge environments due to their biotic (trophic-mediated) and abiotic (ecosystem engineering) functional roles combined with high-value fisheries. Despite this importance, the ecological niche dynamics (i.e., those relating to trophic behavior and food-web interactions) of these sympatric species are poorly understood, knowledge of which may be consequential for maintaining both ecosystem function and fishery sustainability. We used stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) to build realized ecological niche hypervolumes to serve as proxies for diet and production use patterns of L. chamaeleonticeps and C. microps. We hypothesized that: (a) species exhibit ontogenetic shifts in diet and use of production sources; (b) species acquire energy from spatially distinct resource pools that reflect a sedentary life-history and differential use of the continental shelf-edge; and (c) species exhibit differentiation in one or more measured niche axes. We found evidence for ontogenetic shifts in diet (δ15N) but not production source (δ13C) in both species, suggesting a subtle expansion of measured ecological niche axes. Spatial interpolation of stable isotope ratios showed distinct latitudinal gradients; for example, individuals were 13C enriched in northern and 15N enriched in southern regions, supporting the assertion that tilefish species acquire energy from regional resource pools. High isotopic overlap was observed among species (≥82%); however, when hypervolumes included depth and region of capture, overlap among species substantially decreased to overlap estimates of 15%–77%. This suggests that spatial segregation could alleviate potential competition for resources among tilefish species inhabiting continental shelf-edge environments. Importantly, our results question the consensus interpretation of isotopic overlap estimates as representative of direct competition among species for shared resources or habitats, instead identifying habitat segregation as a possible mechanism for coexistence of tilefish species in the NW Atlantic

Origin, sedimentation and diagenesis of organic matter in coastal sediments of the southern Beaufort Sea region, Canadian Arctic

The source and fate of organic carbon in coastal sediments of the Beaufort Sea, Canadian Arctic, are examined. Despite the Mackenzie River's discharge to the coastal waters, sediments of the adjacent slope and Amundsen Gulf have a similar organic carbon content and CORG:N ratio to sediments of the continental margin. In contrast, the stable isotope composition of the organic carbon and nitrogen reveal striking differences: low δ13C and δ15N values are found in Beaufort Shelf sediments indicative of a terrigenous origin, whereas higher values, closer to the marine signature, are observed in the Amundsen Gulf sediments. On the Slope, the isotopic signatures are intermediate and may be interpreted as a mixture of terrigenous and marine organic matter. These results indicate that the terrigenous suspended matter carried by the Mackenzie River's plume and discharged to the Beaufort Sea does not spread to the Amundsen Gulf. Organic carbon that settles onto the seafloor fuels most early diagenetic reactions. The nature and amount that reaches the sediment determines the vertical zonation of redox reactions within the sediment. In turn, the vertical distribution of available oxidants can be used to infer the flux of organic carbon to the seafloor. The Mn and Fe oxide content of Gulf and Slope sediments are one order of magnitude larger than in Shelf sediments, indicating that the Mn and Fe cycles are maintained well below the sediment-water interface by low accumulation rates of organic carbon. Conversely, in margin sediments, high organic carbon accumulation rates bring the Mn and Fe cycles closer to the interface and allow their escape to the overlying waters. In strongly seasonal environments, organic carbon is typically delivered to the sediment in pulses. To simulate the influence of episodic organic carbon fluxes on the sediment chemistry, closed-jar incubation experiments were conducted using both Mn and Fe oxide-poor (Mackenzie Shelf) and -rich sediments (AmundLa source et le devenir du carbone organique sont examinés dans les sédiments côtiers de la mer de Beaufort dans l'Arctique Canadien. Malgré l'apport en sédiment de la rivière Mackenzie aux eaux côtières, les sédiments du talus continental et du Golfe d'Amundsen possèdent une concentration en carbone organique et un ratio CORG:N similaires à ceux de la marge continentale. À l'inverse, les compositions en isotope stable du carbone et de l'azote organique révèlent des différences frappantes: des faibles valeurs de δ13C et δ15N indiquant une origine terrigène sont trouvées dans les sédiments de la marge continentale, tandis que des valeurs élevées, plus proches d'une signature marine, sont observées dans les sédiments du Golfe d'Amundsen. Sur le talus continental, les signatures isotopiques sont intermédiaires et pourraient indiquer un mélange de matière organique terrigène et marine. Ces résultats indiquent que l'apport de sédiment de la rivière Mackenzie ne s'étend pas au golfe d'Amundsen. Le carbone organique qui sédimente sur le fond marin alimente la plupart des réactions diagénétiques. Sa nature et sa quantité déterminent la zonation verticale des réactions redox à l'intérieur du sédiment. Par conséquent, la distribution verticale des oxydants peut être utilisée pour déduire le flux de carbone organique sur le fond. Les concentrations en Mn et Fe des sédiments du Golfe et du talus sont un ordre de grandeur supérieure à celles des sédiments de la marge, indiquant que les cycles diagénétiques du Mn et Fe sont maintenus largement au-dessous de l'interface eau-sédiment par des faibles taux d'accumulation du carbone organique. En revanche, dans les sédiments de la marge, de forts taux d'accumulation sont responsables de la migration des cycles du Mn et Fe vers l'interface eau-sédiment et à leur diffusion vers les eaux surnageantes. Dans les environnements fortement saisonniers, le carbone organique est typiqu

Preface to Bjørn Sundby’s Special Issue of Aquatic Geochemistry

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Female Pacific walruses (Odobenus rosmarus divergens) show greater partitioning of sea ice organic carbon than males: Evidence from ice algae trophic markers

The expected reduction of ice algae with declining sea ice may prove to be detrimental to the Pacific Arctic ecosystem. Benthic organisms that rely on sea ice organic carbon (iPOC) sustain benthic predators such as the Pacific walrus (Odobenus rosmarus divergens). The ability to track the trophic transfer of iPOC is critical to understanding its value in the food web, but prior methods have lacked the required source specificity. We analyzed the H-Print index, based on biomarkers of ice algae versus phytoplankton contributions to organic carbon in marine predators, in Pacific walrus livers collected in 2012, 2014 and 2016 from the Northern Bering Sea (NBS) and Chukchi Sea. We paired these measurements with stable nitrogen isotopes (δ15N) to estimate trophic position. We observed differences in the contribution of iPOC in Pacific walrus diet between regions, sexes, and age classes. Specifically, the contribution of iPOC to the diet of Pacific walruses was higher in the Chukchi Sea (52%) compared to the NBS (30%). This regional difference is consistent with longer annual sea ice persistence in the Chukchi Sea. Within the NBS, the contribution of iPOC to walrus spring diet was higher in females (~45%) compared to males (~30%) for each year (p \u3c 0.001), likely due to specific foraging behavior of females to support energetic demands associated with pregnancy and lactation. Within the Chukchi Sea, the iPOC contribution was similar between males and females, yet higher in juveniles than in adults. Despite differences in the origin of organic carbon fueling the system (sea ice versus pelagic derived carbon), the trophic position of adult female Pacific walruses was similar between the NBS and Chukchi Sea (3.2 and 3.5, respectively), supporting similar diets (i.e. clams). Given the higher quality of organic carbon from ice algae, the retreat of seasonal sea ice in recent decades may create an additional vulnerability for female and juvenile Pacific walruses and should be considered in management of the species

Photoferrotrophs thrive in an Archean Ocean analogue

Considerable discussion surrounds the potential role of anoxygenic phototrophic Fe(II)-oxidizing bacteria in both the genesis of Banded Iron Formations (BIFs) and early marine productivity. However, anoxygenic phototrophs have yet to be identified in modern environments with comparable chemistry and physical structure to the ancient Fe(II)-rich (ferruginous) oceans from which BIFs deposited. Lake Matano, Indonesia, the eighth deepest lake in the world, is such an environment. Here, sulfate is scarce (<20 μmol·liter−1), and it is completely removed by sulfate reduction within the deep, Fe(II)-rich chemocline. The sulfide produced is efficiently scavenged by the formation and precipitation of FeS, thereby maintaining very low sulfide concentrations within the chemocline and the deep ferruginous bottom waters. Low productivity in the surface water allows sunlight to penetrate to the >100-m-deep chemocline. Within this sulfide-poor, Fe(II)-rich, illuminated chemocline, we find a populous assemblage of anoxygenic phototrophic green sulfur bacteria (GSB). These GSB represent a large component of the Lake Matano phototrophic community, and bacteriochlorophyll e, a pigment produced by low-light-adapted GSB, is nearly as abundant as chlorophyll a in the lake's euphotic surface waters. The dearth of sulfide in the chemocline requires that the GSB are sustained by phototrophic oxidation of Fe(II), which is in abundant supply. By analogy, we propose that similar microbial communities, including populations of sulfate reducers and photoferrotrophic GSB, likely populated the chemoclines of ancient ferruginous oceans, driving the genesis of BIFs and fueling early marine productivity

Thorough structural and optical analyses of GaP-based heterostructures monolithically grown on silicon substrates for photonics on Si applications: toward the laser on silicon and high efficiency photovoltaics on silicon

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