Max Coleman

Coupling between sulfur recycling and syndepositional carbonate dissolution: evidence from oxygen and sulfur isotope composition of pore water sulfate, South Florida Platform, U.S.A.

Sulfur cycling in Fe-poor, organic-rich shelf carbonates, known to have rapid rates of SO4−2 reduction, remains poorly studied despite the volumetric significance of shelf deposits in modern and ancient carbon budgets. We investigated sulfur cycling in modern carbonates of the Florida Platform from end-member depositional environments (muddy sands from the Atlantic reef tract and finer-grained mudbank and island flank deposits from Florida Bay). Relations between pore water chemistry (SO4−2, ΣCO2, Ca−2/Cl−) and oxygen and sulfur stable isotope compositions of SO4−2 require direct coupling between sulfur redox cycling and syndepositional carbonate dissolution. Oxygen isotope compositions of pore water sulfate were remarkably shifted away from the established value for marine SO4−2 (+9.5‰), despite near normal SO4−2/Cl− ratios. Chemical evolution was least in reef tract pore waters and greatest in Florida Bay. Relative to overlying seawater, mudbank sediments exhibited sulfate depletion, with δ18OSO4 and δ34SSO4 values both increasing by about 7‰. More bioturbated island flank sediments, colonized by Thalassia grass, had a 5‰ increase in δ18OSO4, variable δ34SSO4 values (+17.7 to +23.3‰) and exceptionally high Ca+2/Cl− ratios. The large excess of Ca+2 (up to 1.7 mM) requires a much larger acid source than the amounts derived from utilization of dissolved O2 (∼0.3 mM) and small degrees of net SO4−2 reduction (<0.5 mM reduced). A conceptual model was constructed using chemical and isotopic data on natural pore waters and on sulfate isotope fractionation factors obtained from sediment incubation experiments. The model outputs show that pore water compositions can be explained by a redox cycle where microbial SO4−2 reduction is followed by very efficient H2S oxidation, thus maintaining virtually invariant SO4−2/Cl− ratios. The enhanced O2 transport may be driven by associated marine grass rhizome systems and microbial communities established in bioturbated sediments. The net result of the cycle is that the rate of sulfide oxidation, which is largely balanced by the rate of microbial sulfate reduction, is stoichiometrically related to the rate of carbonate dissolution. This is consistent with previously reported rates of carbonate dissolution (∼400 μmol/cm2-yr) and average rates of sulfate reduction (∼200 μmol/cm2-yr) from the Florida Platform and a 2:1 stoichiometry.

Controls on development and diversity of Early Archean stromatolites

The ≈3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biology—namely, organic microbial remains or biosedimentary fabrics—has to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans.

Fe-sulphate-rich evaporative mineral precipitates from the Río Tinto, southwest Spain

Abstract The soluble metal sulphate salts melanterite, rozenite, rhomboclase, szomolnokite, copiapite, coquimbite, hexahydrite and halotrichite, together with gypsum, have been identified, some for the first time, on the banks of the Río Tinto, SW Spain. Secondary Fe-sulphate minerals can form directly from evaporating, acid, sulphate-rich solutions as a result of pyrite oxidation. Chemical analyses of mixtures of these salt minerals indicate concentrations of Fe (up to 31 wt.%), Mg (up to 4 wt.%), Cu (up to 2 wt.%) and Zn (up to 1 wt.%). These minerals are shown to act as transient storage for metals and can store on average up to 10% (9.5-11%) and 22% (20-23%), Zn and Cu respectively, of the total discharge of the Río Tinto during the summer period. Melanterite and rozenite precipitates at Río Tinto are only found in association with very acidic drainage waters (pH <1.0) draining directly from pyritic waste piles. Copiapite precipitates abundantly on the banks of the Río Tinto by (1) direct evaporation of the river water; or (2) as part of a paragenetic sequence with the inclusion of minor halotrichite, indicating natural dehydration and decomposition. The natural occurrences are comparable with the process of paragenesis from the evaporation of Río Tinto river water under laboratory experiments resulting in the formation of aluminocopiapite, halotrichite, coquimbite, voltaite and gypsum.

Microbial processes: Controls on the shape and composition of carbonate concretions

Abstract Shape, size and mineralogical/chemical compositions of carbonate concretions vary considerably. The questions to be answered are—Why do they form at all? What processes are involved? Why do they so often form as regular spheroids? Why do they sometimes form laterally-extensive bands? What controls whether the mineral composition is calcite, dolomite, rhodochrosite or siderite (or mixtures of them)? The questions are addressed and the answers illustrated by selected examples. Carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents (organic matter). Microbes take advantage of the energy source and are believed to be involved in most of the various processes. Pore waters in steadily-accumulated sediments show a depth-related succession of chemical and carbon stable-isotope compositions. This results from degradation of organic matter proceeding via an orderly succession of processes during burial, each mediated by a specific microbial group. Aerobic oxidation, using dissolved oxygen occurs first, followed by Mn(IV), reduced to Mn(II), as the oxidant. Fe(III) reduction and sulphate reduction are probably more significant. Methanogenesis is the final, deepest microbial process. The common product of all these reactions is CO 2 or dissolved HCO 3 − which can result in carbonate supersaturation. The specific process controls the chemical and isotopic compositions of pore-water and leads to a characteristic mineralogy of any carbonate precipitated. However, the processes may not proceed successively and localised micro-environments occur. Recent work on siderite concretions in very young salt-marsh sediments (Norfolk, U.K.) has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth. Concretions in ancient rocks can be described in terms of the same processes observed in modern sediments. Laterally-extensive siderite bands in Bullhouse Quarry (Westphalian, U.K.) resulted from interaction of two sets of complementary processes: shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation. The model is supported by a mass and charge balance calculation. Similar, complementary processes formed spheroidal concretions (Gammon Shale, USA) but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction. The spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes. Calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model. Kimmeridge Bay (UK) dolomite ledges are marine analogues to the Bullhouse examples. For the pyrite-rimmed calcite Jet Rock concretions (Early Jurassic, Yorkshire, U.K.) the rate of production of sulphide (from sulphate reduction) locally exceeded availability of Fe(II), to precipitate pyrite. Outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion. Because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore-water evolution. However, all represent an expression in ancient rocks of microbiological processes similar to those operating today.

Geochemistry of inorganic and organic sulphur in organic-rich sediments from the Peru Margin

Abstract We have determined (1) the abundance and isotopic composition of pyrite, monosulphide, elemental sulphur, organically bound sulphur, and dissolved sulphide; (2) the partition of ferric and ferrous iron; (3) the organic carbon contents of sediments recovered at two sites drilled on the Peru Margin during Leg 112 of the Ocean Drilling Program. Sediments at both sites are characterised by high levels of organically bound sulphur (OBS). OBS comprises up to 50% of total sedimentary sulphur and up to 1% of bulk sediment. The weight ratio of S to C in organic matter varies from 0.03 to 0.15 (mean = 0.10). Such ratios are like those measured in lithologically similar, but more deeply buried petroleum source rocks of the Monterey and Sisquoc formations in California. The sulphur content of organic matter is not limited by the availability of porewater sulphide. Isotopic data suggest that sulphur is incorporated into organic matter within a metre of the sediment surface, at least partly by reaction with polysulphides. Most inorganic Sulphur occurs as pyrite. Pyrite formation occurred within surface sediments and was limited by the availability of reactive iron. But despite highly reducing sulphidic conditions, only 35–65% of the total iron was converted to sulphide; 10–30% of the total iron still occurs as Fe (III). In surface sediments, the isotopic composition of pyrite is similar to that of both iron monosulphide and dissolved sulphide. Either pyrite, like monosulphide, formed by direct reaction between dissolved sulphide and detrital iron, and/or the Sulphur species responsible for converting FeS to FeS2 is isotopically similar to dissolved sulphide. Likely stoichiometries for the reaction between ferric iron and excess sulphide imply a maximum resulting FeS2 :FeS ratio of 1:1. Where pyrite dominates the pool of iron sulphides, at least some pyrite must have formed by reaction between monosulphide and elemental sulphur and/or polysulphide. Elemental sulphur (S0) is most abundant in surface sediments and probably formed by oxidation of sulphide diffusing across the sediment-water interface. In surface sediments, S0 is isotopically heavier than dissolved sulphide, FeS and FeS2 and is unlikely to have been involved in the conversion of FeS to FeS2. Polysulphides are thus implicated as the link between FeS and FeS2.

Diverse styles of submarine venting on the ultraslow spreading Mid-Cayman Rise

Thirty years after the first discovery of high-temperature submarine venting, the vast majority of the global mid-ocean ridge remains unexplored for hydrothermal activity. Of particular interest are the world’s ultraslow spreading ridges that were the last to be demonstrated to host high-temperature venting but may host systems particularly relevant to prebiotic chemistry and the origins of life. Here we report evidence for previously unknown, diverse, and very deep hydrothermal vents along the ∼110 km long, ultraslow spreading Mid-Cayman Rise (MCR). Our data indicate that the MCR hosts at least three discrete hydrothermal sites, each representing a different type of water-rock interaction, including both mafic and ultramafic systems and, at ∼5,000 m, the deepest known hydrothermal vent. Although submarine hydrothermal circulation, in which seawater percolates through and reacts with host lithologies, occurs on all mid-ocean ridges, the diversity of vent types identified here and their relative geographic isolation make the MCR unique in the oceans. These new sites offer prospects for an expanded range of vent-fluid compositions, varieties of abiotic organic chemical synthesis and extremophile microorganisms, and unparalleled faunal biodiversity—all in close proximity.

Microbial influence on the oxygen isotopic composition of diagenetic siderite

Abstract Numerous early diagenetic siderite concretions previously described have been interpreted as the results of microbially mediated reactions. Interpretation of oxygen isotope data for such material requires an understanding of the effect of temperature on fractionation processes. However, whilst equilibrium fractionation of oxygen isotopes between siderite and water has been measured down to 33°C, extrapolation to lower temperatures may be invalid. Furthermore, inorganic measurements may not be applicable to microbial systems. The specific iron-reducing microorganism, Geobacter metallireducens, has been cultured anaerobically in the laboratory using acetate as an organic substrate and amorphous FeOOH as an electron acceptor. The acetate is oxidised to CO2, with concurrent iron reduction and extracellular siderite precipitation. Rhombohedral siderite crystals up to 25 μm in size have been precipitated over a range of temperatures (18–40°C). Stable isotopic analysis of these crystals and the solutions from which they precipitate shows that this microbial precipitation of siderite has an associated isotopic fractionation different from the published equilibrium, and which is not simply a function of temperature. In all cases, δ18O values of siderite are lower than predicted by inorganic equilibrium fractionation data. This may explain the numerous anomalously-low δ18O values reported for early diagenetic marine siderites and previously attributed to mechanisms such as mixing with meteoric water, sediment-water interaction, recrystallisation, or variable isotopic fractionation, despite a lack of supporting evidence.

Repeat intercontinental dispersal and Pleistocene speciation in disjunct Mediterranean and desert Senecio (Asteraceae)

To explore the biogeographic history of Mediterranean/arid plant disjunctions, Old and New World Senecio sect. Senecio were analyzed phylogenetically using nuclear ribosomal DNA sequences (ITS). A clade corresponding to sect. Senecio was strongly supported. Area optimization indicated this clade to be of southern African origin. The Mediterranean and southern African floras were not distinguishable as sources of the main New World lineage, estimated to have become established during the middle Pliocene. Another previously suspected recent dispersal to the New World from the Mediterranean was confirmed for the recently recognized disjunction in S. mohavensis. The loss of suitable land connections by the Miocene means that both New World lineages must represent long-distance dispersal, providing the first evidence of repeat intercontinental dispersal in a Mediterranean group. In contrast, migration within Africa may have utilized an East African arid corridor. Recent dispersal to northern Africa is supported for S. flavus, which formed part of a distinct southern African lineage. Novel pappus modifications in both disjunct species may have enabled dispersal by birds. An estimated early Pliocene origin of sect. Senecio coincides with the appearance of summer-dry climate. However, diversification from 1.6 BP highlights the importance of Pleistocene climate fluctuations for speciation.

Characterisation of chlorinated hydrocarbons from chlorine and carbon isotopic compositions : scope of application to environmental problems

Abstract A set of chlorinated hydrocarbons (TCE, PCE, DCM, 1,1,1-TCA, chloroform) provided by four manufacturers has been isotopically characterised for both C and Cl, using a new sensitive method. A very large range of δ 13 C (from −51.66 to −24.07‰/PDB) associated with a very large range of δ 37 Cl (from −2.7 to +3.4‰/SMOC) was obtained. This range of δ 37 Cl is much larger than that of inorganic Cl (±1‰ SMOC) and most individual solvents show a very distinct δ 37 Cl compared to inorganic Cl isotopic signatures. Moreover, δ 37 Cl/ δ 13 C pairs are distinct from one solvent/manufacturer to another. In a δ 13 C versus δ 37 Cl diagram, δ 37 Cl / δ 13 C pairs show different trends for the products of a single manufacturer compared to another. This suggests that Cl isotopic compositions are probably highly fractionated during organic synthesis. The δ 37 Cl values can be interpreted in terms of the probable manufacturing processes. Unlike the data published previously, with one exception, all the new results for samples reported here have positive δ 37 Cl values which might differentiate natural Cl from that derived from degradation. This method has significant potential as a tool for investigating environmental pollution problems; in particular, it offers the possibility for validating models of transport and fate of pollutants.

A simple three-dimensional model of diffusion-with-precipitation applied to localised pyrite formation in framboids, fossils and detrital iron minerals

A three-dimensional diffusion-with-precipitation model is constructed to estimate radial variations in the amounts of pyrite which precipitate where a spherical mass of organic matter, producing H2S by sulphate reduction, is enveloped in a dissolved-iron bearing porewater. The model indicates that higher rates of sulphate reduction (more readily metabolisable organic matter), and larger organic masses, require increasingly high dissolved iron concentrations in order to confine pyrite (or iron sulphide) precipitation to the decay site. The maximum size sphere of exceedingly metabolisable organic matter (equivalent to fresh planktonic material) which can be pyritised is about 50 μm radius, where decay occurs in porewaters with typical dissolved iron levels. This radius is close to the maximum radius of framboidal pyrite, the formation of which could involve model-type processes. Fossil carcases, although mainly composed of less readily metabolisable organic matter, may be orders of magnitude larger and the model demonstrates that their pyritisation requires unusually high porewater dissolved iron concentrations. These inferred chemical conditions are consistent with sedimentological observations of pyritisation in Beechers Trilobite Bed (New York State). At greater depths within the sediment, pyritisation is controlled by the kinetics of iron mineral reactivity towards H2S. Sediments vary widely in their exposure times to H2S which can range at least from 50 to 106 years. At low exposure times only iron oxides are pyritised, whereas at high exposure times even the most refractory iron silicates can become pyritised.