Ruth E. Blake

Oxygen isotope analysis of phosphates: a comparison of techniques for analysis of Ag3PO4

A comparison has been made of oxygen isotope analyses of natural and synthetic phosphates using three methods in current use and Ag3PO4 as the analyte. Of these methods, conventional fluorination using BrF5 provides the most precise and accurate measurements and these analyses serve as the basis for comparison. Fluorination liberates 100% of the oxygen in Ag3PO4 and the isotopic composition of this oxygen can be readily normalized to accepted oxygen isotope ratios of international reference standards. The widely used method of high-temperature reaction with graphite in isolated silica tubes is also precise but requires calibration for scale compression resulting from a combination of factors including incomplete extraction of oxygen, reaction temperature, possible oxygen exchange with the silica tube and/or differences in the grain size of the graphite used. The recently developed method based on high-temperature carbon reduction and continuous flow mass spectrometric analysis of CO is relatively fast, requires little sample and provides 100% yields for oxygen. At the present time, this method is less precise than the other methods examined and requires calibration against standards on a run to run basis. Five phosphate reference standards with d 18 O values ranging from � 5.2xto 34.0xwere prepared and packaged for distribution to active workers in the field. Analyses of these standards will allow normalization and calibration of results obtained using any available method of oxygen isotope analysis of phosphate. D 2002 Elsevier Science B.V. All rights reserved.

Oxygen isotope systematics of biologically mediated reactions of phosphate: I. Microbial degradation of organophosphorus compounds

Microbial activity has been invoked to explain anomalous oxygen isotope compositions of phosphate mineral deposits as well as fossil biogenic apatite. Results of laboratory experiments on enzyme-mediated reactions of phosphate and microbially mediated degradation of organic matter, an important mechanism for the regeneration of dissolved phosphate in modern porewaters, demonstrate that significant exchange of oxygen isotopes between phosphate and water accompanies the hydrolytic cleavage of organically bound phosphate as well as the metabolism of inorganic orthophosphate. Evaluation of the oxygen isotope systematics of microbially mediated reactions of phosphate suggests that oxygen isotope exchange between phosphate and water mediated by bacteria is governed by equilibrium rather than kinetic factors. Under certain conditions, the microbially mediated exchange appears to result in complete re-equilibration of oxygen isotopes between phosphate and water and in other instances equilibrium exchange may be masked by inheritance of phosphate-oxygen from the organic substrate. Analogous microbial processes in natural sediments may be important in the release of dissolved phosphate to pore fluids, precipitation of authigenic apatite, and in the diagenetic alteration of phosphorite deposits and biogenic apatite. These results have important implications for paleoclimatological and paleoenvironmental studies in which oxygen isotope ratios of biogenic phosphate are used as paleotemperature indicators, as well as for studies employing phosphate oxygen isotopes as a tracer of P transport and cycling in the environment.

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.

Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean

Oxygen and silicon isotope compositions of cherts and studies of protein evolution have been interpreted to reflect ocean temperatures of 55–85 °C during the early Palaeoarchaean era (∼3.5 billion years ago). A recent study combining oxygen and hydrogen isotope compositions of cherts, however, makes a case for Archaean ocean temperatures being no greater than 40 °C (ref. 5). Ocean temperature can also be assessed using the oxygen isotope composition of phosphate. Recent studies show that 18O:16O ratios of dissolved inorganic phosphate (δ18OP) reflect ambient seawater temperature as well as biological processing that dominates marine phosphorus cycling at low temperature. All forms of life require and concentrate phosphorus, and as a result of biological processing, modern marine phosphates have δ18OP values typically between 19–26‰ (VSMOW), highly evolved from presumed source values of ∼6–8‰ that are characteristic of apatite in igneous rocks and meteorites. Here we report oxygen isotope compositions of phosphates in sediments from the 3.2–3.5-billion-year-old Barberton Greenstone Belt in South Africa. We find that δ18OP values range from 9.3‰ to 19.9‰ and include the highest values reported for Archaean rocks. The temperatures calculated from our highest δ18OP values and assuming equilibrium with sea water with δ18O = 0‰ (ref. 12) range from 26 °C to 35 °C. The higher δ18OP values are similar to those of modern marine phosphate and suggest a well-developed phosphorus cycle and evolved biologic activity on the Archaean Earth.

Kinetics of feldspar and quartz dissolution at 70–80°C and near-neutral pH: effects of organic acids and NaCl

Abstract Effects of the organic acid (OA) anions, oxalate and citrate, on the solubility and dissolution kinetics of feldspars (labradorite, orthoclase, and albite) at 80°C and of quartz at 70°C were investigated at pH 6 in separate batch experiments and in media with different ionic strength (0.02–2.2 M NaCl). Although it has been shown that OAs can increase rates of feldspar dissolution, prior experiments have focused primarily on dilute, highly undersaturated and acidic conditions where feldspar dissolution kinetics are dominated by H + adsorption and exchange reactions. Many natural waters, however, are only weakly acidic and have variable ionic strength and composition which would be expected to influence mineral surface properties and mechanisms of organic ligand-promoted reactions. Oxalate and citrate (2–20 mM) increased the rate of quartz dissolution by up to a factor of 2.5. Quartz solubility, however, was not increased appreciably by these OAs, suggesting that Si–OA complexation is not significant under these conditions. The lack of significant OA–SiO 2 interaction is important to understanding the effects of OAs on the release of both Si and Al from feldspars. In contrast to quartz, both the rates of dissolution and amounts of Si and Al released from the three feldspars studied increased regularly with increasing OA concentration. Feldspar dissolution was congruent at all but the lowest OA concentrations. Total dissolved Al concentrations increased by 1–2 orders of magnitude in the presence of oxalate and citrate, and reached values as high as 43 mg/l (1.6 mM). Si concentrations reached values up to 65 mg/l (2.3 mM) in feldspar–OA experiments. Precipitation of authigenic clays was observed only in experiments without or at very low concentrations of OAs. The high concentrations of dissolved Si attained during dissolution of feldspars in OA solutions, relative to Si concentrations in quartz–OA experiments, is attributed to concomitant release of Si driven by strong Al–OA interactions. Modeling of the dependence of feldspar dissolution rates on OA concentration in natural diagenetic environments is complicated by the competing effects of overall solution chemistry and ionic strength on the dissolution mechanism. Results of experiments using labradorite (An 70 ) indicate that in OA-free solutions, dissolution is progressively slower at increasing NaCl concentrations (up to 2.2 M), in agreement with prior experiments on the effects of alkali metals on feldspar dissolution. The combined effects of oxalate and NaCl on labradorite dissolution rates are such that the rate increase due to oxalate is suppressed by the addition of NaCl. Thus, feldspar dissolution kinetics should be most significantly affected by a given concentration of OAs in low ionic strength solutions.

Sites of anomalous organic remineralization in the carbonate sediments of South Florida, USA: The sulfur cycle and carbonate-associated sulfate

The modern shallow-platform, calcium-carbonate–dominated sediments of the Florida Keys (Florida Bay and Atlantic reef tract) are diverse in their biological, sedimentological, and geochemical properties. Sites of intense bioturbation and thick seagrass cover are pervasive within Florida Bay and are often characterized by appreciable early diagenetic aragonite dissolution. Additional, less common sites show atypical diagenetic profi les that suggest strong reworking and/or very rapid deposition of the upper sediment layer extending to a depth of at least 20 cm. Diagenesis in these seagrass-free areas is dominated by rapid burial of labile organic matter that would otherwise be degraded aerobically under conditions of slower burial. Correspondingly, these oozy, water-rich sediments display anomalously high rates of microbial decomposition as recorded in S-sulfate reduction rates and patterns of sulfate depletion, high dissolved sulfi de concentrations in excess of several millimolar (mM), and elevated alkalinities. Unlike many sites in Florida Bay where solute concentrations suggest volumetrically signifi cant net dissolution of metastable carbonate phases, dramatic increases in carbonate alkalinity from organic matter oxidation during bacterial sulfate reduction support net precipitation of CaCO 3 in the highly reactive surface layer. This early carbonate mineralization is indicated by measured depletions in Ca approaching 4 mM relative to overlying seawater. Geochemical signatures of sediment reworking or rapid sedimentation are corroborated by porosity 162 T.W. Lyons et al.

Oxygen isotope ratios of PO4: An inorganic indicator of enzymatic activity and P metabolism and a new biomarker in the search for life

The distinctive relations between biological activity and isotopic effect recorded in biomarkers (e.g., carbon and sulfur isotope ratios) have allowed scientists to suggest that life originated on this planet nearly 3.8 billion years ago. The existence of life on other planets may be similarly identified by geochemical biomarkers, including the oxygen isotope ratio of phosphate (δ18Op) presented here. At low near-surface temperatures, the exchange of oxygen isotopes between phosphate and water requires enzymatic catalysis. Because enzymes are indicative of cellular activity, the demonstration of enzyme-catalyzed PO4–H2O exchange is indicative of the presence of life. Results of laboratory experiments are presented that clearly show that δ18OP values of inorganic phosphate can be used to detect enzymatic activity and microbial metabolism of phosphate. Applications of δ18Op as a biomarker are presented for two Earth environments relevant to the search for extraterrestrial life: a shallow groundwater reservoir and a marine hydrothermal vent system. With the development of in situ analytical techniques and future planned sample return strategies, δ18Op may provide an important biosignature of the presence of life in extraterrestrial systems such as that on Mars.

Effects of petroleum contamination on soil microbial numbers, metabolic activity and urease activity.

The influence of petroleum contamination on soil microbial activities was investigated in 13 soil samples from sites around an injection water well (Iw-1, 2, 3, 4) (total petroleum hydrocarbons (TPH): 7.5-78 mg kg(-1)), an oil production well (Op-1, 2, 3, 4, 5) (TPH: 149-1110 mg kg(-1)), and an oil spill accident well (Os-1, 2, 3, 4) (TPH: 4500-34600 mg kg(-1)). The growth rate constant (μ) of glucose stimulated organisms, determined by microcalorimetry, was higher in Iw soil samples than in Op and Os samples. Total cultivable bacteria and fungi and urease activity also decreased with increasing concentration of TPH. Total heat produced demonstrated that TPH at concentrations less than about 1 g kg(-1) soil stimulated anaerobic respiration. A positive correlation between TPH and soil organic matter (OM) and stimulation of fungi-bacteria-urease at low TPH doses suggested that TPH is bound to soil OM and slowly metabolized in Iw soils during OM consumption. These methods can be used to evaluate the potential of polluted soils to carry out self-bioremediation by metabolizing TPH.

Biotic and Abiotic Pathways of Phosphorus Cycling in Minerals and Sediments: Insights from Oxygen Isotope Ratios in Phosphate

A key question to address in the development of oxygen isotope ratios in phosphate (δ(18)O(p)) as a tracer of biogeochemical cycling of phosphorus in ancient and modern environments is the nature of isotopic signatures associated with uptake and cycling of mineral-bound phosphate by microorganisms. Here, we present experimental results aimed at understanding the biotic and abiotic pathways of P cycling during biological uptake of phosphate sorbed to ferrihydrite and the selective uptake of sedimentary phosphate phases by Escherichia coli and Marinobacter aquaeolei. Results indicate that a significant fraction of ferrihydrite-bound phosphate is biologically available. The fraction of phosphate taken up by E. coli attained an equilibrium isotopic composition in a short time (<50 h) due to efficient O-isotope exchange (between O in PO(4) and O in water; that is, actual breaking and reforming of P-O bonds) (biotic pathway). The difference in isotopic composition between newly equilibrated aqueous and residual sorbed phosphate groups promoted the ion exchange (analogous to isotopic mixing) of intact phosphate ions (abiotic pathway) so that this difference gradually became negligible. In sediment containing different P phases, E. coli extracted loosely sorbed phosphate first, whereas M. aquaeolei preferred Fe-oxide-bound phosphate. The presence of bacteria always imprinted a biotic isotopic signature on the P phase that was taken up and cycled. For example, the δ(18)O(p) value of loosely sorbed phosphate shifted gradually toward equilibrium isotopic composition. The δ(18)O(p) value of Fe-oxide-bound phosphate, however, showed only slight changes initially but, when new Fe-oxides were formed, coprecipitated/occluded phosphate retained δ(18)O values of the aqueous phosphate at the time of formation of new Fe oxides. Concentrations and isotopic compositions of authigenic and detrital phosphates did not change, suggesting that these phosphate phases were not utilized by bacteria. These findings support burgeoning applications of δ(18)O(p) as a tracer of phosphorus cycling in sediments, soils, and aquatic environments and as an indicator of paleo- environmental conditions.

Ion microprobe measurements of 18O/16O ratios of phosphate minerals in the Martian meteorites ALH84001 and Los Angeles

Abstract Oxygen isotope ratios of merrillite and chlorapatite in the Martian meteorites ALH84001 and Los Angeles have been measured by ion microprobe in multicollector mode. δ18O values of phosphate minerals measured in situ range from ∼3 to 6‰, and are similar to Martian meteorite whole-rock values, as well as the δ18O of igneous phosphate on Earth. These results suggest that the primary, abiotic, igneous phosphate reservoir on Mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on Earth. This is an important first step in the characterization of Martian phosphate reservoirs for the use of δ18O of phosphate minerals as a biomarker for life on Mars. Cumulative textural, major-element, and isotopic evidence presented here suggest a primary, igneous origin for the phosphates in Los Angeles and ALH84001; textural and chemical evidence suggests that phosphates in ALH84001 were subsequently shock-melted in a later event.