Mahlon C. Kennicutt

Dependence of phytoplankton carbon isotopic composition on growth rate and [CO2)aq: Theoretical considerations and experimental results

The carbon isotopic composition of the marine diatom Phaeodactylum tricornutum (δ13Cp) was measured over a series of growth rates (μ) in a continuous culture system in which both δ133CCO2 and [CO2]aq were determined. In accord with theory, a linear relationship was found to exist between μ/ [CO2]aq and ɛp (≡1000(δ13CCO2 − δ13Cp)/1000 + d δ13Cp), the biological fractionation associated with carbon fixation. The range of [CO2]aq in the continuous culture system was 13–31 μmol kg−1. Measurements of δ13CCO2 and [CO2]aq in the mixed layer of the equatorial Pacific and estimates of δ13Cp obtained from the δ13C of chlorophyll a combined with the regression line fit to the P. tricornutum data give phytoplankton growth rates that are in excellent agreement with those estimated via other techniques. Measurement of ɛp and [CO2]aq in the field can provide an estimate of in situ phytoplankton growth rates without the potential artifacts associated with incubation methodologies. These findings also suggest that accurate estimations of ancient CO2(aq) concentrations will require knowledge of both ɛp and phytoplankton growth rate.

Organic geochemistry applied to environmental assessments of Prince William Sound, Alaska, after the Exxon Valdez oil spill—a review

Abstract Organic geochemistry played a major role in the environmental assessments conducted following the Exxon Valdez oil spill, which occurred on March 24, 1989, and released about 258,000 bbls (41 million liters) of Alaska North Slope crude oil into Prince William Sound. Geochemical analyses of more than 15,000 sediment, tar, and biological samples and about 5000 water samples provide the largest database yet collected on oil-spill chemistry, and we review the results here. The marine environment of the Sound has a complex background of petrogenic, pyrogenic, and biogenic hydrocarbons from natural and anthropogenic sources. Geochemical evaluation of the fate and effects of the spilled oil required that this oil and its residues be distinguished from the background. A variety of molecular and isotopic techniques were employed to identify various hydrocarbon sources and to distinguish quantitatively among mixed sources in the samples. Although the specific criteria used to distinguish multiple sources in the region affected by the Exxon Valdez spill are not necessarily applicable to all spill situations, the principles that governed their selection are. Distributions of polycyclic aromatic hydrocarbons (PAH) and dibenzothiophenes distinguish Exxon Valdez oil and its weathered residues from background hydrocarbons in benthic sediments. Ratios of C 2 -dibenzothiophene C 2 -phenanthrene and C 3 -dibenzothiophene C 3 -phenanthrene were particularly useful. Carbon isotopes and terpane distributions distinguished Exxon Valdez residues found on shorelines from tars from other sources. Diesel and diesel soot were identified by the absence of alkylated chrysenes and a narrow distribution of n-alkanes, whereas pyrogenic products were distinguished by the dominance of 4- to 6-ring PAH over 2- to 3-ring PAH and by the dominance of non-alkylated over alkylated homologues of each PAH series. The presence of 18α(H)-oleanane in benthic sediments, coupled with its absence in Exxon Valdez oil and its residues, confirm another petrogenic source. Results of geochemical studies suggest that the petrogenic component in the background of benthic sediments is derived from oil seeps in the eastern Gulf of Alaska. In 1990 and 1991, Exxon Valdez residues, generally forming a small increment to the pre-spill background, were found to be only sporadically distributed in some shallow, near shore sediments adjacent to shorelines that had been heavily oiled in 1989. In 1994, occurrences of Exxon Valdez tars on shoreline surfaces were rare, although residues could be found buried in shoreline sediments at some isolated locations along the spill path where they were protected from wave action. Spilled oil residues collected 16 months after the spill were degraded, on average, by nearly 50%. Shoreline residues from sources other than the spill were also identified and are widespread throughout the Sound. These residues include (1) geochemically distinct tars and oils imported from California oil fields to Alaska for fuel and construction purposes prior to the discovery of the Cook Inlet and North Slope oil fields, (2) diesel and diesel soot, and (3) more highly refined products. Of the more than 2700 chemical analyses of biological samples of higher life forms (fish, birds, and mammals) about 150 (6%) indicate recognizable residues of Exxon Valdez oil, which were identified by their distribution of polycyclic aromatic hydrocarbons (PAH). Most of these samples (138) were collected in 1989 and most were associated with external surfaces or the gastrointestinal tract. Rarely do internal tissues or fluids contain recognizable fingerprints of spilled oil. This observation includes samples from marine mammals that were visibly oiled externally. Other hydrocarbon sources, including diesel and a non-petroleum artifact that occurs when concentrations of individual PAH are at or near their method detection limit, are also identified in biological samples.

Association of gas hydrates and oil seepage in the Gulf of Mexico

Abstract Gas hydrates were recovered from eight sites on the Louisiana slope of the Gulf of Mexico. The gas hydrate discoveries ranged in water depths from 530 to 2400 m occurring as small to medium sized (0.5–50 mm) nodules, interspersed layers (1–10 mm thick) or as solid masses (> 150 mm thick). The hydrates have gas:fluid ratios as high as 170:1 at STP, C1/(C2 + C3) ratios ranging from 1.9 to > 1000 and δ13C ratios from −43 to −71‰. Thermogenic gas hydrates are associated with oil-stained cores containing up to 7% extractable oil exhibiting moderate to severe biodegradation. Biogenic gas hydrates are also associated with elevated bitumen levels (10–700 ppm). All gas hydrate associated cores contain high percentages (up to 65%) of authigenic, isotopically light carbonate. The hydrate-containing cores are associated with seismic “wipeout” zones indicative of gassy sediments. Collapsed structures, diapiric crests, or deep faults on the flanks of diapirs appear to be the sites of the shallow hydrates.

NOAA gulf of Mexico status and trends program: Trace organic contaminant distribution in sediments and oysters

Polynuclear aromatic hydrocarbons (PAH), chlorinated pesticides, and polychlorinated biphenyls (PCB) concentrations were determined in sediment and oysters to provide information on the current status of the concentration of these contaminants in Gulf of Mexico coastal areas removed from point sources of input. Coprostanol analyses of sediments showed that anthropogenic materials are associated with the sediments at all 153 stations sampled. The levels of contaminants encountered are low compared with areas of known contamination. Average PAH concentrations are nearly the same in oysters and sediments, although the molecular weight distribution is different. Average DDT and PCB concentrations are higher by a factor of 10 to 130 in oysters as compared to sediments. Continued sampling and analyses will allow for long-term trends in the concentrations of these contaminants to be determined.

Sediment contaminants in Casco Bay, Maine. Inventories, sources, and potential for biological impact

An inventory-based approach to environmental assessment that determines concentrations of sedimentary contaminants, defines their origins, and assesses the potential for biological impact is illustrated in Casco Bay, ME. The most widespread contaminants in Casco Bay are petroleum and petroleum byproducts. The highest concentrations of contaminants are associated with population centers, effluent outfalls, and spills. The majority of PAH in sediments are the product of high-temperature combustion processes. PAH concentrations at sites in close proximity to Portland exceed values believed to produce toxic responses in marine benthic organisms. In contrast, PCB, DDTs, and chlordane concentrations in the sediments are below concentrations thought to produce toxic effects in marine organisms. Metal concentrations in sediments are also below those that elicit biological responses. The geographic distribution of contaminants is initially controlled by the proximity to sources, and the regional differences in concentrations are the result of sediment accumulation patterns. Detrital (terrestrial), autochthonous marine, pyrogenic, and petroleum sources for PAH, alkanes, and trace metals are defined. 21 refs., 10 figs., 6 tabs.

Leakage of deep, reservoired petroleum to the near surface on the gulf of Mexico Continental slope

Reservoired oils, shallow sediment cores (2m), sea slicks and tar balls were collected in the Green Canyon Lease area of the northern Gulf of Mexico continental slope. The gaseous and liquid hydrocarbons associated with near surface sediments and water have migrated from deep (2000–3000 m) subsurface reservoirs and/or source rocks. This conclusion is based on molecular (GC/FID, GC/FPD, GC/MS) and carbon isotopic evidence. Visual observations at two locations on the continental slope confirm the presence of massive amounts of active liquid as well as gas seepage. Hydrate gas recovered in sediment cores originates from deep, oil-associated gas. This gas has migrated to shallow sediments with little or no isotopic fractionation. In contrast, near surface hydrocarbon liquids (shallow bitumens and sea slicks) are depleted in aliphatics, 4-ring or larger aromatics, naphthalene, C1-naphthalenes and C2-naphthalenes as compared to the reservoired fluids. These near-surface fluids are extensively altered by the concurrent processes of migration, dissolution and microbial degradation. However, the distributions of highly alkylated (> C2) naphthalenes, phenanthrenes and dibenzothiophenes, triterpanes, steranes and triaromatized steranes are similar to the precursor reservoired oil. This study documents, for the first time, a direct link between natural seepage in a deep water marine setting and sea slick and tar ball formation. This and other studies suggest that the natural seepage of oil and gas can be a significant process in the deep ocean.

The effect of biodegradation on crude oil bulk and molecular composition

Abstract A laboratory simulation of an oil spill was used to monitor the effect of microbial alteration on crude oil molecular, bulk and carbon isotopic compositions. The rate of microbial alteration of alkanes decreased with increasing carbon number. Straight-chain alkanes were more rapidly removed than branched (isoprenoid) hydrocarbons though ultimately even the isoprenoids were degraded. Aromatic compounds were also altered. Isomer specific degradation was observed within a given aromatic alkylation (i.e. methylphenanthrenes). The most stable properties, under the given conditions, were carbon isotopic composition, Ni/V ratios, total scanning fluorescence spectra, and various molecular distributions. Aromatics with two or more rings and more than a two carbon substitution, triterpanes and steranes were relatively stable. Mono- and tri-aromatized steranes were substantially altered. These stable chemical properties or fingerprints are suggested as unique and sensitive indicators that can be used to determine the source of microbially altered hydrocarbons in the environment.

Observations of gas hydrates in marine sediments, offshore northern California

Biogenic gas hydrates were recovered in shallow cores (< 6 m deep) from the Eel River basin in offshore northern California between 40°38′ and 40°56′N. The gas hydrates contained primarily methane (δ13C = −57.6 to −69.1‰) and occurred as dispersed crystals, small (2–20 mm) nodules, and layered bands within the sediment. These hydrates, recovered in sediment at water depths between 510 and 642 m, coincide nearly, but not exactly, with areas showing bottom-simulating reflectors (BSRs) on seismic-reflection records. This study confirms indirect geophysical and geologic observations that gas hydrates are present north of the Mendocino Fracture Zone in sediment of the Eel River basin but probably are absent to the south in the Point Arena basin. This discovery extends the confirmed sites of gas hydrates in the eastern Pacific region beyond the Peruvian and Central American margins to the northern California margin.

Human Contamination of the Marine Environment-Arthur Harbor and McMurdo Sound, Antarctica.

Polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCBs), pesticides, and trace metal concentrations in McMurdo Sound and Arthur Harbor, Antarctica, sediments and marine biota are reported. Biomarkers of contaminant exposure, biliary metabolites and EROD assays, were also measured. Hydrocarbon and trace metal contamination are generally limited to within hundreds of meters of human settlements. Local releases of fossil fuels, disposal of waste materials, and aging of ship and station structures contribute to contamination. High concentrations of PCBs were detected in sediments (250-4200 ng g{sup -1}) and organisms (up to 420 ng g{sup -1}) from Winter Quarters Bay (WQB). Trace metal and PAH sediment concentrations rarely exceed levels known to cause toxic effects in marine organisms, whereas PCBs in WQB often do. Biological responses to exposure include the formation of PAH metabolites and the inducement of the P4501A detoxification system in fish. Induction of EROD activity in in vitro rat hepatoma H4IIE cell bioassays by tissue extracts correlated with known levels of PCB contamination in invertebrate tissues. Local sources of contaminants greatly exceed those attributable to long-distance atmospheric transport. 31 refs., 4 figs.

Hydrocarbon contamination on the Antarctic Peninsula: I. Arthur harbor—Subtidal sediments☆

Abstract Near-field contamination in Arthur Harbor can be traced to spills, ship and boating activities, and run-off. Soil samples from Palmer Station and Old Palmer Station contain hydrocarbons derived from diesel fuel, lubrication oil, and hydraulic fluid. The majority of contamination in subtidal sediments around Palmer Station is due to diesel fuel spills. Subtidal sediments below an abandoned open incineration site also contain combustion-derived polynuclear aromatic hydrocarbons (PAH). Soils collected at Old Palmer Station were also contaminated with diesel fuel residues and combustion-derived PAH. High concentrations of these contaminants were detected in nearby subtidal sediments. Small amounts of diesel fuel contamination are detectable throughout Arthur Harbor. Despite being abandoned for years, soils in the vicinity of Old Palmer Station and Base N represent the most concentrated source of contaminants in Arthur Harbor. Environmentally sound practices at Palmer Station have helped to minimize localized contamination.