Nelson M. Frew

Organic matter fluxes from sediment traps in the equatorial Atlantic Ocean

The vertical flux of particulate organic matter to the ocean floor is controlled by complex remineralization and transport processes. Rapidly sinking, large (>32 µm) particles may account for most of the vertical mass flux1–3. Experiments involving collection of sedimenting particles in traps deployed at varying depths in the water column provide a way of assessing these processes directly2–12. Measurement of organic carbon content of trap material may lead to a general understanding of the flux of particulate organic matter to the sea floor3,8–10. However, details of the transport and transformation phenomena involving sinking particulate organic material can be elucidated only by determining distributions of specific organic compounds associated with the particles11,12. We report here direct flux measurements and composition of lipids obtained by organic geochemical studies of particulate material collected in a deep-sea moored sediment trap experiment in the equatorial Atlantic Ocean.

Hydrocarbons in cores of northwestern Atlantic coastal and continental margin sediments

Abstract The concentrations and composition of hydrocarbons in sections from a 210 Pb dated core from Buzzards Bay, Massachusetts are reported. Terrigenous n -alkane concentrations remain constant throughout the 62 cm and 195 years sampled. Gas Chromatographic analysis on SCOT columns revealed an unresolved complex mixture of alkanes and cycloalkanes which decreases in concentration with increasing depth. Phenanthrene and C 1 and C 2 substituted phenanthrene concentrations also decreased with increasing depth and the relative abundance of C 1 and C 2 homologs compared to phenanthrene suggest a pyrolytic origin for these aromatic hydrocarbons. The various sources of hydrocarbons in surface sediments and the processes which could govern their distribution are discussed. The most likely source of the phenanthrenes and the unresolved complex mixture of alkanes and cycloalkanes in the upper core sections circa 1900 to the present seems to be urban air fallout. A depth distribution of alkanes and cycloalkanes similar to the Buzzards Bay core was found for a second area of Buzzards Bay and for cores from sediments of the Gulf of Maine and Hudson Canyon. Two C 25 cycloalkenes were identified as major hydrocarbons in the sediments. Their concentrations decrease with increasing depth in the cores. Mass spectra of these compounds and their hydrogenation products are reported.

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Steroid transformations in Recent marine sediments

Abstract The surface sediments and overlying water column from 3 Recent sedimentary environments—Walvis Bay off southwestern Africa, the Black Sea and the western North Atlantic—have been analyzed for stenols and their transformation products, stanols, sterenes, and steroid ketones. The steroid distributions in surface sediments are controlled by inputs from source organisms and from microbial or chemically mediated reactions occurring at the sediment-water interface or in the overlying water column. A general correlation was found to exist for various steroidal compounds in sediments from each area, suggesting related reaction sequences. Also, the steroid transformation products correlated with photoproduction in the overlying waters as well as with the oxic and anoxic conditions of the sedimentary environment.

Relationship between air-sea gas transfer and short wind waves

Laboratory studies have been conducted in two circular wind wave flumes to investigate the relationship between air-sea transfer velocities of weakly soluble, nonreactive gases and wind-generated surface waves over clean water surfaces and in the presence of surface films. Detailed surface wave measurements have been made using a scanning laser slope gauge. In the circular tanks, longer gravity waves (wavenumber below 12 rad/m) are hardly affected by surfactant, while shorter waves (above 100 rad/m) are significantly reduced. With higher surfactant concentrations, waves above 200–300 rad/m may be completely eliminated. Because of the absence of narrow-banded fetch-limited gravity waves, the wave fields in the circular tanks are significantly different from those in linear wind wave flumes. At a given wind friction velocity, the transfer velocity may decrease by as much as 60% because of surface films. Regardless of the surfactant concentrations, the transfer velocity shows a reasonable correlation with the total mean square slope and with the mean square slope of shorter wind waves (wavenumber above 200 rad/m). However, it shows a poor correlation with the mean square slope of longer wind waves (wavenumber below 50 rad/m). These observations suggest that short wind waves play an important role in air-sea gas exchange.

Glass capillary gas chromatography-mass spectrometry of wax esters, steryl esters and triacylglycerols

Complex mixtures of wax esters, steryl esters and triacylglycerols isolated from representative biological and geochemical samples have been analyzed using combined high resolution gas chromatography and electron impact and chemical ionization quadrupole mass spectrometry. These low volatility neutral lipids containing up to 65 carbons were chromatographed intact on 15–20 m high-temperature (upper limit: 370 C) persilylated SE-52 and SE-30 glass capillary columns. Discrimination effects due to adsorptive losses and degradation were minimized using a nonvaporizing on-column injector and a direct all-glass capillary connection (370 C) to the quadrupole mass spectrometer. Structural information regarding the fatty acid and alcohol moieties was found to be maximal for methane-CI spectra in the case of wax and steryl esters, whereas EI spectra were most useful in interpreting triacylglycerol structures. Principal features of the EI and CI fragmentation patterns are discussed. The molecular composition of complex mixtures of these lipids is reconstructed for selected samples.

Surfactant effects on air-sea gas exchange under turbulent conditions

Abstract In a series of laboratory gas exchange studies we found that under turbulent conditions additions of two synthetic surfactants (polyethylene oxide and oleyl alcohol) to distilled water and seawater led to reductions in oxygen evasion at the air-liquid interface. The oxygen exchange coefficient relative to that of a distilled water control asymptotically reached a lower limit of ≈50% as surfactant concentration was increased. For natural seawater samples, an asymptotic reduction in relative gas exchange rate was demonstrated for increasing amounts of surface-active material as determined from surface pressure-area isotherms. Possibly surfactants act to reduce gas exchange by creating surface pressure forces that oppose and reduce turbulent eddy velocities and, correspondingly, reduce surface renewal. However, the greatest reductions in the oxygen exchange coefficient occurred at initial surface pressure ( π i ) less than 0.5 mN m −1 . This result may have been due to the presence of soluble surfactants, which are known to be very effective in reducing gas exchange but which do not display concentration-dependent surface pressures. Based on results from an opportunistic sampling survey of marine waters and a cruise to the Sargasso Sea, a gradient in gas exchange reductions from 5–15% in oceanic waters to 50% in nearshore waters was found. However, reductions of 50% occured for surface film material obtained from the Sargasso Sea. Our results provide some measure of the potential for reduction of gas exchange at high turbulence in marine waters in the presence of natural surfactants.

Water‐air flux of dimethylsulfide

The water-air exchange of dimethylsulfide (DMS) has been measured in a laboratory wind-wave tank in fresh and seawater. To understand the transport behavior of DMS, its exchange was measured simultaneously with that of O2, SF6, Ne, CH4, and He under varying wind speeds and hydrodynamic conditions. No unpredictable differences between fresh and seawater were found, indicating that DMS surface affinities do not exist in seawater. Results also indicate that Schmidt number corrections can be applied to DMS gas exchange. While the rate of transfer of relatively insoluble or sparingly soluble permanent gases between the ocean and the atmosphere is primarily controlled by the rate of flux through the aqueous boundary layer, interfacial mass balances indicate that increased DMS solubility increases the significance of the airside control of flux. The hypothesis that DMS transport across the water-air interface is subject to waterside control for moderate environmental conditions is supported. However, for ocean-atmospheric DMS exchange with low sea surface temperature or moderate wind speeds, there may be a significant influence by the atmospheric mass boundary layer. An atmospheric gradient fraction γa applied to the waterside air-sea gas transfer velocity will correct for these effects. Estimates of ocean-atmospheric DMS transfer velocities for these conditions are provided.

No. 2 fuel oil compounds in Mytilus edulis

Mytilus edulis contaminated by a brief 2-d exposure to a slick from a No. 2 fuel oil spill in the Cape Cod Canal, Massachusetts, USA were sampled six times during an 86-d post-spill period to study the rate of release of fuel oil compounds under field conditions. Detailed measurements of compounds by high resolution glass capillary gas chromatography and quantitative glass capillary gas chromatography-mass spectrometry-computer systems analyses provided a more comprehensive examination of release rates of different types of compounds. Biological half-lives were calculated for selected compounds for the first 21 d during which the release rates were exponential. Typical half-lives were n-alkanes, 0.2–0.8 d; pristane, 1.5 d; C-2 (dimethyl or ethyl) napthalenes, 0.9 d; methyl phenanthrenes, 1.7 d. Changes in relative ratios of C-2 phenanthrenes during the release period were observed. The evidence available to date strongly supports the role of molecular weight and accompanying properties of water solubility as the main controlling factors in the rate of release of fuel oil compounds by M. edulis. However, the data for the rapid release of n-alkanes and C-2 phenanthrenes also indicate molecular type and molecular configuration as additional key factors. The data from this study are compared and contrasted to data from short term experimental studies in the laboratory, longer term studies from chronic exposure conditions, and data from two other oil spills with longer term exposure.

Isolation of marine microlayer film surfactants for ex situ study of their surface physical and chemical properties

Surface-active organic matter from natural marine microlayers has been isolated by solid phase adsorption using reversed phase (C 18) cartridges. Surfactants collected from a suite of microlayer films in the California Bight and the Gulf of Maine and isolated by this technique were respread on clean seawater to reconstitute surface films which exhibited surface pressure-area (,r-A) isotherms and static (Gibbs) surface elasticities closely approximating those measured for films formed by diffusion in the original fresh microlayer samples. Microlayer-subsurface (10 cm depth) carbon enrichment factors were estimated to range from 3.9 to 16. The estimated mean recovery of microlayer dissolved organic carbon (DOC) was 20 -+ 5%. The select hydrophobic DOC fraction isolated was the dominant surfactant class responsible for microlayer film characteristics as measured quasi-statically, even though other relatively hydrophilic DOC fractions which may have been enriched in the microlayer were not absorbed and passed through the isolation procedure. This technique allows collection of marine microlayer films for systematic ex situ study and correlation of their surface physical and chemical properties. For the first time, the ,r-A isotherms for microlayer films can be expressed on a specific area basis and, in cases where molecular weights can be estimated, according to mean molecular area.