Robert Raiswell

The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales

Abstract A sulfur analysis scheme employing the use of chromium reduction for the determination of reduced inorganic sulfur compounds (pyrite + elemental sulfur + acid volatile monosulfides) in modern sediments and shales is presented. Exhaustive testing shows that chromium reduction does not reduce or liberate either organic sulfur or sulfate sulfur; making the method specific only to reduced inorganic sulfur phases. The high degree of specificity, ease of sample analysis, and excellent analytic precision should make this technique ideally suited for routine analysis of modern and ancient sediments.

Could bacteria have formed the Precambrian banded iron formations

Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.

A comparison of iron extraction methods for the determination of degree of pyritisation and the recognition of iron-limited pyrite formation.

Measurements of degree of pyritisation require an estimate of sediment iron which is capable of reaction with dissolved sulphide to form pyrite, either directly or indirectly via iron monosulphide precursors. Three dissolution techniques (buffered dithionite, cold 1 M HCl, boiling 12 M HCl) were examined for their capacity to extract iron from a variety of iron minerals, and iron-bearing sediments, as a function of different extraction times and different grain sizes. All the iron oxides studied are quantitatively extracted by dithionite and boiling HCl (but not by cold HCl). Both HCl techniques extract more iron from silicates than does dithionite but probably about the same amounts as are potentially capable of sulphidation. Modern sediment studies indicate that most sedimentary pyrite is formed rapidly from iron oxides, with smaller amounts formed more slowly from iron silicates (if sufficient geologic time is available). It is therefore recommended that the degree of pyritisation be defined with respect to the dithionite-extractable (mainly iron oxide) pool and/or the boiling HCl-extractable pool (which includes some silicate iron) for the recognition of iron-limited pyritisation.

Pyrite and organic matter in Phanerozoic normal marine shales

Data have been obtained, from our own chemical analyses and from the literature, for the concentrations of organic carbon (C) and pyrite sulfur (S) in over 600 samples of Cambrian to recent normal marine shales. (Normal marine refers to deposition in oxygenated bottom water as evidenced by the presence of benthic fossils and/or indicators of bioturbation). All samples were selected to minimize 1. (1) loss of C and/or S due to weathering at the outcrop (by emphasizing the use of drill core material) 2. (2) analytical errors and non-diagenetic effects (by avoiding sediments low in C or S content such as sandstones) 3. (3) Fe limitation of pyrite formation (by avoiding limestones, cherts, and euxinic shales) 4. (4) metamorphic loss of C, relative to S (by avoiding rocks obviously subjected to, even low-grade, metamorphism). Our results indicate that (1) there is generally a good positive linear correlation between organic C and pyrite S for normal marine shales of all ages and (2) the mean CS ratio for normal marine shales has varied over time. Devonian to Tertiary shales exhibit mean CS weight ratios (1.8 ± 0.5) somewhat lower than Quaternary sediments (meanCS = 2.8). This we believe is due to preferential C loss (relative to S) during biogenic methanogenesis plus diagenetic heating. Distinctly lower mean C/S values for the Cambrian and Ordovician (0.5 ± 0.1) cannot be explained solely in terms of diagenetic C loss and, instead, must represent low original ratios. The low original ratios, we suggest, were due mainly to the absence of bacterially refractory organic matter added to the marine environment by rivers at that time, because the major source of such material, vascular land plants, had not yet evolved. This, along with a possibly lower degree of bioturbation, contributed to enhanced pyrite formation and preservation and, thus, lower early Paleozoic CS ratios.

Mudrock‐hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition

Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly under‐estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow (<500 m) depths and (iv) cementation can create a framework that prevents compaction and preserves porosity. Cement textures can be used to distinguish two modes of growth; concentric growth, where successive layers of cement are added to the outer surface (radius increases with time), and pervasive growth, where cement crystals grow simultaneously throughout the concretion volume (little or no radius increase with time). Cement textures of siderite concretions are mostly consistent with pervasive growth, but many calcite microsparite concretions show no diagnostic textural features and could grow either concentrically or pervasively. Concretionary cementation, whether concentric or pervasive, occurred such that there was accessible porosity which could be filled by later cements. Pervasive growth in particular is associated with the retention of substantial amounts of porosity which may be filled by chemically and isotopically distinct phases. The resulting chemical gradients across concretions may then reflect variations in the relative proportions of early and later cements more than variations in porewater composition. Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from −30‰ to +15‰, and thus overlaps 13C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).

A late archean sulfidic sea stimulated by early oxidative weathering of the continents

Of Ancient Iron and Oxygen Finding clues to understand the early evolution of ocean and atmospheric chemistry and its links to the evolution of life remains a daunting task. Often just a few rock samples provide our only evidence of what conditions on Earth were like long ago. Reinhard et al. (p. 713) combined iron speciation data from a 2.5-billion-year-old shale from Australia with sulfur isotope data from this and nearby formations to conclude that oxygen chemistry predominanty consisted of an anoxic sulfide–rich water column, instead of iron-rich oceans, as previously speculated. Thus, brief pulses of reduced iron from hydrothermal vents may have been responsible for the formation of nearby banded iron formations and may have provided enough buffering to prolong the appearance of atmospheric oxygen generated by the expansion of newly evolved cyanobacteria. Before Earth’s atmosphere became oxidizing, the oceans may have been sulfide-rich while receiving periodic pulses of iron. Iron speciation data for the late Archean Mount McRae Shale provide evidence for a euxinic (anoxic and sulfidic) water column 2.5 billion years ago. Sulfur isotope data compiled from the same stratigraphic section suggest that euxinic conditions were stimulated by an increase in oceanic sulfate concentrations resulting from weathering of continental sulfide minerals exposed to an atmosphere with trace amounts of photosynthetically produced oxygen. Variability in local organic matter flux likely confined euxinic conditions to midportions of the water column on the basin margin. These findings indicate that euxinic conditions may have been common on a variety of spatial and temporal scales both before and immediately after the Paleoproterozoic rise in atmospheric oxygen, hinting at previously unexplored texture and variability in deep ocean chemistry during Earth’s early history.

Bioavailable iron in the Southern Ocean: the significance of the iceberg conveyor belt

Productivity in the Southern Oceans is iron-limited, and the supply of iron dissolved from aeolian dust is believed to be the main source from outside the marine reservoir. Glacial sediment sources of iron have rarely been considered, as the iron has been assumed to be inert and non-bioavailable. This study demonstrates the presence of potentially bioavailable Fe as ferrihydrite and goethite in nanoparticulate clusters, in sediments collected from icebergs in the Southern Ocean and glaciers on the Antarctic landmass. Nanoparticles in ice can be transported by icebergs away from coastal regions in the Southern Ocean, enabling melting to release bioavailable Fe to the open ocean. The abundance of nanoparticulate iron has been measured by an ascorbate extraction. This data indicates that the fluxes of bioavailable iron supplied to the Southern Ocean from aeolian dust (0.01–0.13 Tg yr-1) and icebergs (0.06–0.12 Tg yr-1) are comparable. Increases in iceberg production thus have the capacity to increase productivity and this newly identified negative feedback may help to mitigate fossil fuel emissions.

Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England

Carbon, oxygen and sulphur isotope data for transects across two pyrite-bearmg carbonate concretions, and their host sediments, from the Upper Lias of N.E. England show symmetrical zonation. δ13CPDB values of the calcite cement (−12.9 to −15.4%.) indicate that most of it originated from organic matter by bacterial reduction of sulphate, augmented with marine and, to a lesser extent, fermentation derived carbonate. Organic carbon (δ13CPDB = −26.1 to −37.0%.). reflects the admixture of allochtho-nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation. Two phases of pyrite are present in each concretion. The earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has δ34SCD values of −22 to −26%. indicating formation by open system sulphate reduction. The later euhedral phase is more abundant and reaches values of − 2.5 to − 5.5%. at concretion margins. This phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system. The δ13C and δ34S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ18O values of the calcite in one concretion (δ18OPDB = 2.3 to −4.8%.) indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater. The other concretion is isotopically much lighter (δ18OPDB−8.9 to −9.9%.) and large δ18O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.

The microbiological formation of carbonate concretions in the Upper Lias of NE England

Abstract Carbonate concretions from the Jet Rock (Upper Lias, Lower Jurassic) of NE England grew in uncompacted sediment, close to the sediment surface. Microbiological activity created isolated microenvironments in which dissolved carbonate and sulphide species were produced more rapidly than they could be dispersed by diffusion, so establishing the localised supersaturation of calcite and metastable iron sulphides. Precipitation of these minerals in the microenvironment formed a single concretion. Mass-balance calculations demonstrate that at least two different microbiological processes participated in concretionary growth. The early growth stages had an unidentifiable microbiological source of carbonate which declined in importance relative to sulphate reduction as growth proceeded. It is suggested that the diffusion of dissolved organic material was important in sustaining microbiological activity. Mineralogical zonations in the concretions result from changes in the chemistry of the microenvironment due to variations in the rates of addition/removal Ca 2+ , Fe 2+ , HCO − 3 and HS − by microbiological activity, the crystallization of authigenic minerals and diffusion between the microenvironment and surrounding pore waters. Such changes are of only local significance and the resulting mineralogical zonations in a concretion cannot be used to deduce successive stages of diagenesis in the whole sediment.

Chemical model for the origin of minor limestone-shale cycles by anaerobic methane oxidation

Chemical and isotopic studies of Jurassic carbonate concretions indicate an origin by anaerobic methane oxidation. Concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late, rejuvenated phase of sulfate reduction. Similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone-shale sequences, and an origin by anaerobic methane oxidation is proposed for these sequences also. The evolution of isolated concretions into diagenetic limestones depends on the extent of, and depth variations in, carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus. Diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive /sup 34/S values than the adjacent shales.