Fluorescence of dissolved organic matter un marine sediments

The fluorescence of porewaters from marine sediment cores from six different areas was measured. In most cases, fluorescence was affected primarily by the diagenesis of organic carbon first through sulfate reduction and subsequently by methane generation. Typically, fluorescence, dissolved organic carbon (DOC), absorbance, alkalinity, and ammonium ion concentrations correlate quite well, increasing in the upper sections of anoxic sediments and co-varying in deeper sections of these cores. The good correlation of DOC with fluorescence in the three cores in which DOC was measured indicates that fluorescence can be used to make a first order estimate of DOC concentration in anoxic porewaters. Data are consistent with a model in which labile organic matter in the sediments is broken down by sulfur reducing bacteria to low molecular weight monomers. These monomers are either remineralized to CO2 or polymerize to form dissolved, fluorescent, high molecular weight molecules. The few exceptions to this model involve hydrothermally generated hydrocarbons that are formed in situ in the Guaymas Basin or are horizontally advected along the decollement in the Nankai Trench.

Age determination of sediment cores from the Panama Basin

High-resolution percent Corg and delta18Oforam records obtained from Panama Basin core Atlantis II 54-25PC and additional data from nearby core P7 show that enhanced burial of organic carbon has characterized every major glacial period for the last 500 kyr in that area. Both Corg concentration and mass accumulation rate profiles exhibit a sawtooth pattern with maxima occurring typically in the later stages of glacial periods. Comparison with dust records suggests that the carbon accumulation rate profile reflects both the upwelling history and a variable rate of iron input during the late Quaternary. The sawtooth character may derive from increased wind velocities and rates of upwelling during glacials which are indirectly related to ice volume (Sarnthein et al., 1988). The rapid decline in export production at the end of glacials in the equatorial Pacific may be attributed to the retreat of ice sheets (thus reduced wind velocities and upwelling) coupled with a coincident decline in atmospheric dust load and/or delivery rate. The Corg accumulation rate profiles do not correlate well with atmospheric CO2 records. For example, atmospheric CO2 was already at a minimum 40 kyr ago when production in the Panama Basin began increasing dramatically, commensurate with an increase in global dust levels. Using the relationship between the degree of photosynthetic fractionation and the concentration of free CO2 in the surface ocean postulated by Popp et al. (1989), delta13Corg measurements made on core P7 show that Panama Basin surface waters have been supplying CO2 to the atmosphere continually for at least the last 50 kyr. There is no evidence for a flux of CO2 into the surface ocean in this area at any time during this period despite the higher production. If the Panama Basin cores are representative of the eastern and central equatorial Pacific, then these observations weaken the influence on CO2 drawdown postulated for increased glacial productivity at low latitudes.

Results of biogeochemical studies in the Russian Arctic seas: isotope and radioisotope data

Comprehensive biogeochemical studies including determination of isotopic composition of organic carbon in both suspended matter and surface layer (0-1 cm) bottom sediments (more than 260 determinations of d13C-Corg) were carried out for five Arctic shelf seas: White, Barents, Kara, East Siberian, and Chukchi Seas. The aim of this study is to elucidate causes that change isotopic composition of particulate organic carbon at the water-sediment boundary. It is shown that isotopic composition of organic carbon in sediments from seas with high river run-off (White, Kara, and East Siberian Seas) does not inherit isotopic composition of organic carbon in particles precipitating from the water column, but is enriched in 13C. Seas with low river run-off (Barents and Chukchi Seas) show insignificant difference between d13C-Corg values in both suspended load and sediments because of low content of isotopically light allochthonous organic matter in suspended matter. Biogeochemical studies with radioisotope tracers (14CO2, 35S, and 14CH4) revealed existence of specific microbial filter formed from heterotrophic and autotrophic organisms at the water-sediment boundary. This filter prevents mass influx of products of organic matter decomposition into the water column, as well as reduces influx of OM contained in suspended matter from water into sediments.

Composition of organic matter from black shales drilled in the Cape Verde Basin in DSDP Hole 41-367

Results of geochemical studies of organic matter in black shales from the Cape Verde Basin are reported. Based on these results, in combination with data of petrographic analysis, conclusions are made about sapropelic nature of their organic matter and low degree of its coalification. It corresponds to the proto-catagenetic substage of sedimentary rocks. Black shales of the Cape Verde Basin are classified as potential oil source strata.

Inorganic and organic geochemistry and sediment descriptions at DSDP Hole 93-603B

About one hundred samples of sediments and rocks recovered in Hole 603B were analyzed for type, abundance, and isotopic composition of organic matter, using a combination of Rock-Eval pyrolysis, C-H-N-S elemental analysis, and isotope-ratio mass spectrometry. Concentrations of major, minor, and trace inorganic elements were determined with a combination of X-ray fluorescence and induction-coupled plasma spectrometry. The oldest strata recovered in Hole 603B (lithologic Unit V) consist of interbedded light-colored limestones and marlstones, and black calcareous claystones of Neocomian age. The inorganic and organic geochemical results suggest a very terrigenous aspect to the black claystones. The organic geochemical results indicate that the limestones and marlstones contain a mixture of highly degraded marine and terrestrial organic matter. Comparison of the Neocomian carbonates at Site 603 with those on the other side of the North Atlantic, off Northwest Africa at Site 367, shows that the organic matter at Site 367 contains more marine organic matter, as indicated by higher pyrolysis hydrogen indices and lighter values of d13C. Comparison of inorganic geochemical results for the carbonate lithologies at Site 603 with those for carbonate lithologies at Site 367 suggests that the Site 603 carbonates may contain clastic material from both North American and African sources. The black claystones at Site 603, on the other hand, probably were derived almost entirely from North American clastic sources. Lithologic Unit IV overlying the Neocomian carbonates, consists of interbedded red, green, and black claystones. The black claystones at Site 603 contain more than ten times the organic carbon concentration of the interbedded green claystones. The average concentration of organic carbon in the black claystones (2.8%), however, is low relative to most mid-Cretaceous black claystones and shales in the Atlantic, particularly those found off Northwest Africa. The geochemical data all suggest that the organic matter in the black claystones is more abundant but generally more degraded than the organic matter in the green claystones, and that it was derived mainly from terrestrial sources and deposited in oxygenated bottom waters. The increased percentage of black claystone beds in the upper Cenomanian section, and the presence of more hydrogen-rich organic matter in this part of the section, probably resulted from the increased production and accumulation of marine organic matter that is represented worldwide near the Cenomanian/Turonian boundary in deep-sea and land sections. A few upper Cenomanian black claystone samples that have hydrogen indices > 150 also contain particularly high concentrations of V and Zn. Most samples of black claystone, however, are not particularly metal-rich compared with other black claystones and shales. Compared with red claystones from lithologic Unit IV, the green and black claystones are enriched in many trace transition elements, especially V, Zn, Cu, Co, and Pb. The main difference between the "carbonaceous" claystones of lithologic Unit IV and "variegated" or "multicolored" claystones of the overlying Upper Cretaceous to lower Tertiary Unit III is the absence of black claystone beds. As observed at several other sites (105 and 386), the multicolored claystones at Site 603 are somewhat enriched in several trace transition elements-especially Cu, Ni, and Cr-relative to most deep-sea clays. The multicolored claystones are not enriched in Fe and Mn, and therefore are not "metalliferous" sediments in the sense of those found at several locations in the eastern Pacific. The source of the slightly elevated concentrations of transition metals in the multicolored claystones probably is upward advection and diffusion of metals from the black claystones of the underlying Hatteras Formation. The red, orange, and green claystone beds of lithologic Unit II (Eocene), like those of Unit III, really represent a continuation of deposition of multicolored claystone that began after the deposition of the Neocomian carbonates. The color of the few black beds that occur within this unit results from high concentrations of manganese oxide rather than high concentrations of organic matter.

Total organic carbon content and maceral composition of sediment at DSDP Holes 72-515B and 72-516F

The amount, type, and thermal maturation of organic matter in sediments from two DSDP holes in the South Atlantic (Leg 72) were investigated. Isolated kerogens were studied by microscopy, and nonaromatic hydrocarbons were characterized by capillary gas chromatography. Organic carbon values are low in all samples and range between 0.05 and 0.21% in Hole 515B (Brazil Basin) and only between 0.02 and 0.10% in Hole 516F (Rio Grande Rise). The organic matter is predominantly terrigenous, mixed with some unicellular marine algae; it is severely oxidized in most samples. N-alkane distributions are usually dominated by long-chain wax alkanes with odd-over-even carbon number predominance; when the marine organic matter is relatively more abundant, however, significant amounts of n-alkanes are centered upon n-C17. The organic matter is not mature enough in any sample to generate appreciable amounts of hydrocarbons.

Epoch, organic carbon and compounds at DSDP Holes 93-605 and 95-613

Twenty-four sediment samples from DSDP Holes 605 (Leg 93) and 613 (Leg 95) on the New Jersey continental rise were analyzed by pyrolysis-gas chromatography. Twelve of these samples were also analyzed by pyrolysis-gas chromatography/mass spectrometry. The degree of preservation of sediment organic matter, as determined by these techniques, helped to distinguish slumped sediments from sediments that have not moved from their original place of deposition. Total levels of pyrolyzable organic material, as determined from pyrolysis-gas chromatography, were low in sediments that were not slumped, indicating that the organic material is highly degraded. Nitrogen- and oxygen-containing compounds were the primary compounds detected by gas chromatography/mass spectrometry (GCMS) analysis of the pyrolyzate of non-slumped sediments. Smaller amounts of aromatic compounds and branched alkanes were also present in some of these samples. In contrast, slumped sediments showed larger amounts of pyrolyzable organic matter, as determined from pyrolysis-gas chromatography, and better preservation of alkyl chains in the sediment organic matter, as suggested by the presence of n-alkanes in GCMS analysis of the pyrolyzate. Better preservation of the organic matter in slumped sediments can be attributed to more moderate bioturbation by bottom-dwelling organisms at the original deposition site.

Pliocene to Pleistocene calcium carbonate and organic carbon record of ODP Hole 117-722B

Site 722 provides high resolution records of percent CaCO3, magnetic susceptibility, d18O, organic carbon, and coarse fraction for the past 3.4 m.y. from the crest of the Owen Ridge, northwestern Arabian Sea. Within this time interval, most of the carbonate percent variations can be attributed to terrigenous dilution and do not reflect changes in the carbonate system. From the late Pliocene to Present, the average rate of calcium carbonate accumulation increases from 1 to 3 g/cm**2/k.y. and the average accumulation of organic carbon decreases from 75 to 30 mg/cm**2/k.y. The carbonate component is more dissolved in the older interval. The long-term variations in carbonate accumulation may reflect a greater input of organic matter in the late Pliocene, which decomposes to produce CO2 and dissolve carbonate. Magnetic susceptibility and % noncarbonate (100 - CaCO3%) reflect changes in the amount of the lithogenic component in the sediments. The period of variation of lithogenic material is the same period as the original forcing of the regional summer monsoon, however, the timing matches global aridity patterns and global ice volume (sea level) changes. This preliminary analysis suggests that the high frequency variation of lithogenic material persists for at least the last 3.4 m.y. Within the last million years, calcium carbonate accumulation has a large amplitude signal that covaries with major changes in ice volume. Both calcium carbonate and noncarbonate (mostly terrigenous) accumulation are greatest during glacial stages. Interglacial intervals are characterized by low mass accumulation rates, increased foraminifer fragmentation, and increased opal concentration. The accumulation of organic carbon matches the high frequency changes in sedimentation rates. We attribute this high correlation to enhanced preservation of organic carbon by increased sedimentation rate. Of the three major biological components studied, only opal exhibits the variations expected for a biological productivity system forced by monsoonal upwelling driven by changes in northern hemisphere summer radiation.

General descriptions, organic contents and ratios of geolipid components at DSDP Leg 74 Holes

Two samples of Miocene sediments from Site 525 and four samples of sediments ranging in age from Pleistocene to Miocene from Site 528 have been analyzed for concentrations of organic and carbonate carbon, carbon/nitrogen ratios of organic matter, and extractable hydrocarbons and fatty acids. Organic carbon concentrations average 0.32% and show a diagenetic decrease with greater sediment age. Distributions of n-alkanes and n-alkanoic acids give evidence of considerable microbial reworking and of eolian contributions of terrigenous components. Organic contents of these sediments reflect a history of low marine productivity and poor preservation of organic matter in the eastern South Atlantic since middle Miocene times.

Zooplankton distribution and migration in low-oxygen modewater eddies

The eastern tropical North Atlantic (ETNA) features a mesopelagic oxygen minimum zone (OMZ) at approximately 300-600 m depth. Here, oxygen concentrations rarely fall below 40 µmol O2 kg-1, but are expected to decline under future projections of global warming. The recent discovery of mesoscale eddies that harbour a shallow suboxic (<5 µmol O2 kg-1) OMZ just below the mixed layer could serve to identify zooplankton groups that may be negatively or positively affected by on-going ocean deoxygenation. In spring 2014, a detailed survey of a suboxic anticyclonic modewater eddy (ACME) was carried out near the Cape Verde Ocean Observatory (CVOO), combining acoustic and optical profiling methods with stratified multinet hauls and hydrography. The multinet data revealed that the eddy was characterized by an approximately 1.5-fold increase in total area-integrated zooplankton abundance. At nighttime, when a large proportion of acoustic scatterers is ascending into the upper 150 m, a drastic reduction in mean volume backscattering (Sv, shipboard ADCP, 75kHz) within the shallow OMZ of the eddy was evident compared to the nighttime distribution outside the eddy. Acoustic scatterers were avoiding the depth range between about 85 to 120 m, where oxygen concentrations were lower than approximately 20 µmol O2 kg-1, indicating habitat compression to the oxygenated surface layer. This observation is confirmed by time-series observations of a moored ADCP (upward looking, 300kHz) during an ACME transit at the CVOO mooring in 2010. Nevertheless, part of the diurnal vertical migration (DVM) from the surface layer to the mesopelagic continued through the shallow OMZ. Based upon vertically stratified multinet hauls, Underwater Vision Profiler (UVP5) and ADCP data, four strategies have been identified to be followed by zooplankton in response to the eddy OMZ: i) shallow OMZ avoidance and compression at the surface (e.g. most calanoid copepods, euphausiids), ii) migration to the shallow OMZ core during daytime, but paying O2 debt at the surface at nighttime (e.g. siphonophores, Oncaea spp., eucalanoid copepods), iii) residing in the shallow OMZ day and night (e.g. ostracods, polychaetes), and iv) DVM through the shallow OMZ from deeper oxygenated depths to the surface and back. For strategy i), ii) and iv), compression of the habitable volume in the surface may increase prey-predator encounter rates, rendering zooplankton and micronekton more vulnerable to predation and potentially making the eddy surface a foraging hotspot for higher trophic levels. With respect to long-term effects of ocean deoxygenation, we expect avoidance of the mesopelagic OMZ to set in if oxygen levels decline below approximately 20 µmol O2 kg-1. This may result in a positive feedback on the OMZ oxygen consumption rates, since zooplankton and micronekton respiration within the OMZ as well as active flux of dissolved and particulate organic matter into the OMZ will decline.