Anoxic Marine Environments Research Articles

Jellyfish Lake, Palau

Elemental cycling in suboxic to anoxic marine environments has received considerable attention in the past few years. The recent expeditions of the RIV Knorr to the Black Sea, for example, illustrate the interest in deciphering the geochemical processes active in such environments “Murray, 1988”.This interest has been stimulated by the recognition that the type of elemental cycling that occurs in restrictive basins such as the Black Sea also occurs in many other environments. Examples include hydrothermal systems, estuarine and marine sediments, and other water columns with restrictive circulation. In addition, evidence that widespread anoxia has occurred in oceans of the geologic past “Fischer and Arthur, 1977; Jenkyns, 1980” requires us to obtain a better understanding of geochemical processes under such conditions. We report here on preliminary geochemical measurements made in a stratified marine lake—an unusual, yet useful environment as an analog of an anoxic ocean.

The distributions of uranium, radium and thorium isotopes in two anoxic fjords: Framvaren Fjord (Norway) and Saanich Inlet (British Columbia)

Depth profiles of the naturally-occurring radionuclides 238U, 234U, 226Ra, 228Ra and 228Th were obtained in two diverse anoxic marine environments; the permanently anoxic Framvaren Fjord in southern Norway and the intermittently anoxic Saanich Inlet in British Columbia. Concentrations of total H 2S were over three orders of magnitude greater in the anoxic bottom waters of Framvaren Fjord compared to those in Saanich Inlet. In Framvaren Fjord, the O 2/H 2S interface was located at 17 m. While dissolved 238U behaved conservatively throughout the oxic and anoxic water columns, concentrations based on the 238U/salinity ratio in oxic oceanic waters were almost 30% lower. Dissolved 226Ra displayed a sharp maximum just below the O 2/H 2S interface, coinciding with dissolved Mn (II) and Fe (II) maxima in this zone. It is suggested that reductive dissolution of Fe-Mn oxyhydroxides remobilizes 226Ra in this region. In Saanich Inlet, the O 2/H 2S interface was located at 175 m. Dissolved 238U displayed a strongly nonconservative distribution. The depth profiles of dissolved 226Ra and 228Th correlated well with the distribution of dissolved Mn (II) in the suboxic waters above the O 2/H 2S interface, suggesting that reduction of particulate Mn regulates the behavior of 226Ra and 228Th in this region. Removal residence times for dissolved 228Th in the surface oxic waters of both systems are longer than those generally reported for particle-reactive radionuclides in coastal marine environments. In the anoxic waters of Framvaren Fjord and Saanich Inlet, however, the dissolved 228Th removal residence times are quite similar to values reported for dissolved 210Pb in the anoxic waters of the Cariaco Trench and the Orca Basin. This implies that the geochemistries of Th and Pb may be similar in anoxic marine waters.

Radon in Onondaga County, New York: Paleohydrogeology and redistribution of uranium in Paleozoic sedimentary rocks

Elevated indoor (basement) radon levels (average 8.8 pCi/L) define a hot belt that extends across Onondaga County, New York, and includes much of the city of Syracuse. The region is underlain by gently dipping Paleozoic sediments of diverse lithology. Homes located on the Marcellus (black) Shale derive radon from uranium that was incorporated syndepositionally in organic-rich muds formed in an anoxic marine environment. Radon from subjacent limestones and dolostones is attributed to decay of uranium introduced into these carbonate units by ground water that previously had passed through the Marcellus Shale. Uniform distribution of radon within the carbonates suggests that the uraniferous ground water descended steeply through these units toward a basal zone where dissolution of salt has enhanced permeability. The redistribution of uranium has occurred over a period of several tens of millions of years and has been responsible for widening the hot belt from ∼2 km (width of Marcellus outcrop) to ∼12 km.

Lower Cretaceous Organic-Rich Sediments Drilled on Antarctic Continental Margin During ODP Leg 113: ABSTRACT

Lower Cretaceous organic-rich sediments were recovered below a major unconformity at two sites on the eastern Weddell Sea margin during ODP Leg 113. No age overlap exists between the two sites. A more continuous Cretaceous section was previously recovered at DSDP Site 511 on the Falkland plateau. Site 692 (2880 m water depth) is located on a mid-slope bench in Wegener Canyon. Early Cretaceous-age (pre-Albian) sediments extend from 53-98 m below sea floor (mbsf) and are dominated by organic-rich nannofossil claystone. Macrofossils (e.g., belemites and ammonites), thin carbonate lenses (<1 cm), and water escape structures are abundant. Thin beds of devitrified ash, bioturbation, and graded bedding are present. Site 693 (2360 m water depth), 30 km west of Site 692 on the canyon's outer rim, recovered Albian-age organic-rich claystones and mudstones from 416-483 mbsf. Site 693 sediments have lower organic contents than those at Site 692. Glauconite is common in the upper part of the unit. Well-preserved diatoms and diatomite layers suggest high productivity. These sediments differ from Albian-age sediments at Site 511, which consist of open-marine nannofossil ooze. The sediments were deposited in an anoxic or hypoxic upper bathyal (500-1000 mbsl) marine environment. Oxygenation occurred later along the Antarcticmore » margin than the Falkland plateau, possibly as the inflow of Atlantic water proceeded south from an opening near the Falkland plateau.« less

Analysis and distribution of iron sulfide minerals in recent anoxic marine sediments

Acid-volatile sulfide (AVS), pyrite-sulfur, iron and organic carbon distributions were examined in sediments from a variety of oxic and anoxic marine environments. Multiple determinations of AVS using different acid conditions showed that most extractants give similar AVS concentrations, with the exception of stannous chloride in hot HCl which digested between 10 and 81% of pyrite-sulfur. An extensive examination of iron sulfide minerals in sediments, using scanning electron microscopy combined with simultaneous elemental analysis, showed that identifiable iron sulfides were almost always pyrite. Occurrence of greigite observable by SEM was limited to one site and mackinawite was not found in any samples. Thus, if these minerals are precursors to pyrite formation, as has been frequently hypothesized, they must be primarily present as coatings on other mineral grains or submicron particles.

Base metal mineralization in the Kanmantoo Group, South Australia; a regional sulfur isotope study

A number of base metal sulfide deposits are hosted by multiply deformed metagraywackes of the Kanmantoo Group, South Australia. Sulfide mineralization is of three main types: (1) copper deposits which include the Kanmantoo and Bremer mines, South Hill prospect, and several minor occurrences; (2) lead-zinc deposits at the Aclare, Wheal Ellen, and Strathalbyn mines; (3) pyrite-pyrrhotite mineralization of the Nairne pyrite deposit and other pyritic schists within the Kanmantoo Group.Copper deposits are discordant and pipelike and contain chalcopyrite, pyrrhotite, magnetite, and pyrite as the major opaque minerals. Lead-zinc deposits lack magnetite and are apparently concordant. Beds of pyritic schist reach an overall thickness of 100 m and persist for as much as 32 km.Sulfur isotope ratios for ore shoots from the Kanmantoo mine, excluding one sample, range from 3.5 to 12.4 per mil and average 8.0 + or - 2.3 per mil. Average values for individual ore shoots differ by as much as 4 per mil. 34 S enrichment is noted at Kanmantoo among some low-grade sulfide mineralization (delta 34 S = 16ppm) and supergene pyrite from the main orebody (delta 34 S = 39.6ppm). The minor deposits of copper and lead-zinc all show a similarity in sulfur isotope composition. The delta 34 S values for sulfides range from -4.7 to 6.5 per mil and averages for these deposits lie close to zero per mil. The pyritic schists are strongly depleted in 34 S and contain biogenic sulfur formed in an anoxic marine environment; individual sulfides range from -19.9 to -11.9 per mil. Isotopic disequilibrium is a characteristic feature of coexisting sulfide pairs from many of the deposits, despite subsequent amphibolite facies metamorphism (3.8 kb and 500 degrees -600 degrees C). Sphalerite-galena pairs at the Wheal Ellen mine yield consistent calculated temperatures (379 degrees C) and may have reequilibrated below the metamorphic peak.Base metal mineralization is accounted for by subsurface (copper) and sea-floor (lead-zinc) accumulation from hydrothermal fluids circulating to or expelled from depths of at least 3 to 4 km within the sedimentary basin. This minimum depth estimate for fluid extraction corresponds to the stratigraphic interval occupied by pyritic schists, which underlies the horizon of base metal mineralization. Sulfur isotope evidence suggests that sulfur was extracted by hydrothermal leaching from the pyritic schists. Sulfur within the base metal mineralization shows a mixed Cambrian seawater and biogenic (pyritic schist) source. Variations in mixing proportions of the two forms of sulfur are responsible for differences among ore lenses at the Kanmantoo mine and individual deposits within the region.

Organic matter diagenesis in the anoxic sediments of saanich inlet, British Columbia, Canada: a case for highly evolved community interactions

Cytochrome c and ferricyanide reducing activities are measured in a Desulfovibrio desulfuricans (strain ATCC No. 17990) that indicate that the APS reductase activity is constitutive. This enzymatic activity is also measured in a Saanich Inlet isolate identified as Desulfovibrio salexigens (Saanich). Successful measurement of the Cytochrome c reducing activity was also made in the sediment extracts from Saanich Inlet, and this activity was found to be present in sediments as deep as 50 cm. Bacterial biomass distributions were estimated using measurements of ATP in the Saanich Inlet sediments. ETS activity measurements correlated well with the in situ [ 35S]-SO 4 2− reduction rate measurements; both of the methods showing a subsurface maximum in activity at a depth of about 10 cm. However, some differences between the results obtained from these two procedures were also found, e.g., the in situ [ 35S]-SO 4 2− reduction activity approximated zero values at or near 25 cm depth, while 5–10% of the surface ETS activity was present at a depth of as much as 50 cm. The hypothesis presented is that the subsurface maximum may be due to the anaerobic oxidation of methane by [ 35S]-SO 4 2− ions as mediated by the sulfate-reducing bacteria. A discussion of the strong interactive relationships between a variety of organisms in an ecosystems context, so necessary in bringing about organic matter diagenesis in anoxic marine environments, is also presented.

Biogeochemical processes affecting total arsenic and arsenic species distributions in an intermittently anoxic fjord

Total arsenic, arsenate and arsenite concentration profiles for the water column of Saanich Inlet, an intermittently anoxic fjord located on Vancouver Island, B.C., Canada, were measured using independent analytical techniques for total arsenic and arsenic speciation to evaluate the accuracy of the speciation technique in both oxic and anoxic marine environments. Total arsenic profiles indicate a mid-depth minimum of about 1.0 ppb above the oxic—anoxic interface and an enrichment in the anoxic zone to about 2.0 ppb. This minimum may be due to either advection of arsenic-poor water into Saanich Inlet at mid-depth or arsenic incorporation onto solid phases within a bacteria- and manganese-rich particulate layer located immediately above the oxic—anoxic interface and subsequent removal via sinking particulate material. Ratios of total arsenic to phosphorus in the deep, anoxic waters of the basin are similar to those reported for marine algae, suggesting that the enrichment of total arsenic within the anoxic bottom layer may be due to its release upon organic matter decomposition. Arsenate and arsenite concentration versus depth profiles indicate a rapid (but incomplete in a thermodynamic sense) response to the oxic—anoxic interface. The arsenate/ arsenite concentration ratio is 15/1 in the oxic region of the water column and 1/12 in the anoxic zone. Arsenate—arsenite interconversion occurs at a depth shallower than ferric-ferrous but deeper than MnO 2 —Mn 2+ interconversions. Measurements of arsenite oxidation rates at near-ambient arsenite concentrations and temperatures using an 74As 3+ radioactive tracer technique indicate that arsenite oxidation is initially ten times faster in seawater taken from the manganese-rich particulate layer at 165 m depth than in seawater collected near the surface at 50 m depth. Addition of antibiotics to seawater from 165 m depth initially suppressed the rate of arsenite oxidation, indicating that it may be partially microbially mediated.

Volatile C 1C 7 organic compounds in an anoxic sediment core from the Pettaquamscutt River (Rhode Island, U.S.A.)

Surface sediments from an anoxic marine environment, the Upper Basin of the Pettaquamscutt River, Rhode Island, were analyzed for volatile organic compounds in the C 1C 7 range. The compounds identified included methane, ethane, alkenes (C 2C 5), carbon disulfide, cyclopentane, 3-methylpentane, methylfuran, aldehydes and ketones. Ethane, methylfuran, and most of the aldehydes and ketones showed maxima at the sediment water interface. Methane levels were very high-10–100 times greater than observed in most other surface sediments examined in this laboratory.

Chlorite and its Relationship to Pyritization in Anoxic Marine Environments

ABSTRACT Pyritiferous basal marine shales of the early mid-Devonian Hamilton Group of New York State display an inverse relationship between pyrite abundance, as a proportion of the chlorite present, and the iron content of the chlorite. This relationship suggests that chlorite was a significant source of iron during pyritization. The data also suggest that chlorite was broadly segregated into an iron-rich fraction, occuring nearer the source, and one of lower iron content, found farther west.

Consumption of dissolved nitrous oxide in an anoxic basin, Saanich Inlet, British Columbia

DATA on dissolved nitrous oxide (N2O) in the ocean indicate that its distribution is strongly affected by oxygen-controlled biochemical reactions1–4. N2O has been measured in oxygenated, open ocean waters of the Atlantic and Pacific Oceans1–3, and in the oxygen deficient waters of the eastern tropical North Pacific4 but no measurements of N2O in anoxic marine environments have yet been made. This report presents measurements of N2O from Saanich Inlet, an intermittently anoxic basin on the southeastern side of Vancouver Island, British Columbia, and evidence for its consumption during denitrification.

ANOXIC WATER IN THE PETTAQUAMSCUTT RIVER1

Deep basins in the Pettaquamscutt River (Rhode Island), a narrow estuary, are highly stratified. Biogenic H2S accumulates in bottom water to a concentration of 4.5 mm, several times the concentration reported in other anoxic marine or estuarine environments. The alkalinity, which reaches a concentration of 12.66 meq/liter, as well as the observed pH, can be explained by considering the water a solution of several alkaline substances and one acid, H2CO3. Knull and Richards’ model of alkalinity in anoxic water is improved by taking NH3 into account. Richards’ model of sulfate reduction by organic matter accurately predicts the concentrations of those major products influencing alkalinity and pH.