Juske Horita

Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites

The chemical evolution of seawater during the Phanerozoic is still a matter of debate. We have assembled and critically analyzed the available data for the composition of fluid inclusions in marine halite and for the mineralogy of marine evaporites. The composition of fluid inclusions in primary marine halite reveals two major long-term cycles in the chemistry of seawater during the past 600 myr. The concentration of Mg2+, Ca2+, and SO42− has varied quite dramatically. The Mg2+ concentration in seawater during most of the early Paleozoic and Jurassic to Cretaceous was as low as 30 to 40 mmol/kg H2O; it reached maximum values ≥50 mmol/kg H2O during the Late Neoproterozoic and Permian. The Ca2+ concentration in seawater during the Phanerozoic has reached maximum values two to three times greater than the concentration in seawater today (10.6 mmol/kg H2O), whereas SO42− concentrations may have been as low as 5 to 10 mmol/kg H2O (a third to a fifth of the modern value) during the Jurassic and Early Paleozoic. The Mg2+/Ca2+ ratio in seawater ranged from 1 to 1.5 during the early to middle Paleozoic and Jurassic-Cretaceous to a near-modern value of 5.2 during the Late Neoproterozoic and Permian. This change in seawater Mg2+/Ca2+ ratio is consistent with the notion of alternating “calcite-aragonite seas” recorded in oolites and marine carbonate cements. Several models have been proposed to explain the chemical evolution of seawater. These have invoked significant changes in one or more of the major geochemical processes that control the composition of seawater. The pattern and magnitude of the variations in the composition of seawater proposed in this study are similar to those proposed elsewhere that suggest that seawater fluxes through midocean ridges have played a major role in the evolution of seawater during the past 600 myr. Two Phanerozoic supercycles of the Earth’s exogenic processes were recognized in the literature that are caused by mantle convection and plate activity. The composition of seawater has apparently undergone dramatic secular changes in phase with these supercycles and as a consequence of biological evolution. Analyses of fluid inclusions containing unevaporated seawater and a better understanding of the processes that affect the composition of seawater are needed to refine our understanding of the history of Phanerozoic seawater.

LIQUID-VAPOR FRACTIONATION OF OXYGEN AND HYDROGEN ISOTOPES OF WATER FROM THE FREEZING TO THE CRITICAL TEMPERATURE

The equilibrium fractionation factors of oxygen and hydrogen isotopes between liquid water and water vapor have been precisely determined from 25 to 350°C on the VSMOW-SLAP scale, using three different types of apparatus with static or dynamic techniques for the sampling of water vapor. Our results for both oxygen and hydrogen isotope fractionation factors between 25 and 100°C are in excellent agreement with the literature (e.g., Majoube, 1971). Our results for the hydrogen isotope fractionation factor above 100°C also agree quantitatively with the literature values of Merlivat et al. (1963) and Bottinga (1968). The results for the hydrogen isotope fractionation factor obtained in this study and from most of the literature were regressed to the equation, 103Inα1−v(D) = 1158.8(T3109) −1620.1 (T2106) + 794.84(T103) −161.04 + 2.9992(109T3), from 0°C to the critical temperature of water (374.1°C) within ± 1.2(1σ) (n = 157); T(K). The cross- over temperature is 229 ± 13°C (1σ). Our values for the oxygen isotope fractionation factor between liquid water and water vapor are, however, at notable variance with the only dataset available above 100°C in the literature (Bottinga, 1968), which is systematically higher (av. + 0.15 in 103 In α1−v(18O)) with large errors (± 0.23 in 1σ). Our results and most of the literature data below 100°C were regressed to the equation, 103 In α1−v(18O) = −7.685 + 6.7123(103T) − 1.6664(106T2) + 0.35041 (109T3), from 0 to 374.1°C within ± 0.11 (1σ)(n = 112); T(K). A third water-steam isotope geothermometer, using the ratio of ΔδD/Δδ18O given by water and steam samples, is readily obtained from the above equations. This geothermometer is less affected by incomplete separation of water and steam, and partial condensation of steam than those employing the oxygen and hydrogen isotopic compositions alone.

Carbon isotope exchange in the system CO2-CH4 at elevated temperatures

Abstract Carbon isotope exchange was investigated for the system CO 2 -CH 4 at 150 to 600°C in the presence of several potential catalysts by use of isotopically normal or 13 C-enriched gases. Silica gel, graphite, molecular sieve Linde 4A, magnetite, and hematite oxidized small amounts of CH 4 in starting CO 2 -CH 4 mixtures to CO and CO 2 but failed to enhance the net rate of carbon isotope exchange between CO 2 and CH 4 , even after 169 to 1833 h at 400 to 500°C. In contrast, several commercial transition-metal catalysts (Ni, Pd, Rh, and Pt) promoted reactions significantly toward chemical and isotopic equilibrium. With the Ni catalyst, the attainment of carbon isotopic equilibrium between CO 2 and CH 4 was demonstrated for the first time at temperatures from 200 to 600°C by complete isotopic reversal from opposite directions. The experimentally determined carbon isotope fractionation factors between CO 2 and CH 4 (10 3 lnα) were similar to, but slightly greater than (0.7–1.1‰, 0.89‰ on average), those of statistical-mechanical calculations by Richet et al. (1977) . The experimental results can be described by the following equation between 200 and 600°C only: 10 3 lnα(CO 2 -CH 4 ) = 26.70 − 49.137(10 3 /T) + 40.828(10 6 /T 2 ) − 7.512(10 9 /T 3 ) (T = 473.15–873.15 K, 1σ = ±0.14‰, n = 44). Alternatively, an equation generated by fitting Richet et al. (1977) data in the temperature range from 0 to 1300°C can be modified by adding +0.89‰ to its constant; 10 3 lnα(CO 2 -CH 4 ) = 0.16 + 11.754(10 6 /T 2 ) − 2.3655(10 9 /T 3 ) + 0.2054(10 12 /T 4 ) (T = 273–1573 K, 1σ = ±0.21‰, n = 44). This and other recent experimental studies in the literature demonstrate that transition metals, which are widespread in many natural materials, can catalyze reactions among natural gases at relatively low temperatures (≤200°C). The role of natural catalysts, “geocatalysts,” in the abiogenic formation of methane, hydrocarbons, and simple organic compounds has important implications, ranging from the exploration of hydrocarbon resources to prebiotic organic synthesis.

The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: III. Vapor-liquid water equilibration of NaCl solutions to 350°C

Abstract The effect of dissolved NaCl on equilibrium oxygen and hydrogen isotope fractionation factors between liquid water and water vapor was precisely determined in the temperature range from 130–350°C, using two different types of apparatus with static or dynamic sampling techniques of the vapor phase. The magnitude of the oxygen and hydrogen isotope effects of NaCl is proportional to the molality of liquid NaCl solutions at a given temperature. Dissolved NaCl lowers appreciably the hydrogen isotope fractionation factor between liquid water and water vapor over the entire temperature range. NaCl has little effect on the oxygen isotope fractionation factor at temperatures below about 200°C, with the magnitude of the salt effect gradually increasing from 200–350°C. Our results are at notable variance with those of Truesdell (1974) and Kazahaya (1986), who reported large oxygen and hydrogen isotope effects of NaCl with very complex dependencies on temperature and NaCl molality. Our high-temperature results have been regressed along with our previous results between 50 and 100°C (Horita et al., 1993a) and the low-temperature literature data to simple equations which are valid for NaCl solutions from 0 to at least 5 molal NaCl in the temperature range from 10–350°C. Our preliminary results of oxygen isotope fractionation in the system CaCO3-water ± NaCl at 300°C and 1 kbar are consistent with those obtained from the liquid-vapor equilibration experiments, suggesting that the isotope salt effects are common to systems involving brines and any other coexisting phases or species (gases, minerals, dissolved species, etc.). The observed NaCl isotope effects at elevated temperatures should be taken into account in the interpretation of isotopic data of brine-dominated natural systems.

The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: I. Vapor-liquid water equilibration of single salt solutions from 50 to 100°C

The differences between oxygen and hydrogen isotope activity and composition ratios of water in single salt solutions (NaCl, KCl, MgCl2, CaCl2, Na2SO4, and MgSO4) were determined by means of a vapor-liquid water equilibration method over the temperature range of 50 to 100°C. A parallel equilibration technique of pure water and salt solutions with the same isotopic composition at the same experimental conditions enabled the precise determination of the isotope salt effects. Hydrogen isotope activity ratios of all of the salt solutions studied were appreciably higher than composition ratios. That is, DH ratio of water vapor in isotope equilibrium with a solution increases as salt is added to the solution. Magnitudes of the hydrogen isotope effects are in the order CaCl2 ≥ MgCl2 > MgSO4 > KCl ≈ NaCl > Na2SO4 at the same molality. Except for KCl solutions at 50°C, oxygen isotope activity ratios in the solutions were lower than, or very close to, the composition ratios. The isotope effects observed are all linear with the molalities of the salt solutions, and either decrease with temperature or are almost constant over the temperature range. Salt solutions of divalent cations (Ca and Mg) exhibited oxygen isotope effects much larger than those of monovalent cations (Na and K). Magnitudes of the oxygen isotope effects in NaCl solutions, and of the hydrogen isotope effects in Na2SO4 and MgSO4 solutions, may increase from 50 to 100°C. Our results agree with most of those from the literature near room temperature, but are at notable variance with those by Truesdell (1974) around 100°C. The results in this study and the literature data near room temperature were satisfactorily fitted to simple equations as a function of concentration of the salt solutions and temperature.

Isotope Effects in the Evaporation of Water: A Status report of the Craig - Gordon Model

The Craig–Gordon model (C–G model) [H. Craig, L.I. Gordon. Deuterium and oxygen 18 variations in the ocean and the marine atmosphere. In Stable Isotopes in Oceanographic Studies and Paleotemperatures, E. Tongiorgi (Ed.), pp. 9–130, Laboratorio di Geologia Nucleare, Pisa (1965).] has been synonymous with the isotope effects associated with the evaporation of water from surface waters, soils, and vegetations, which in turn constitutes a critical component of the global water cycle. On the occasion of the four decades of its successful applications to isotope geochemistry and hydrology, an attempt is made to: (a) examine its physical background within the framework of modern evaporation models, (b) evaluate our current knowledge of the environmental parameters of the C–G model, and (c) comment on a general strategy for the use of these parameters in field applications. Despite its simplistic representation of evaporation processes at the water–air interface, the C–G model appears to be adequate to provide the isotopic composition of the evaporation flux. This is largely due to its nature for representing isotopic compositions (a ratio of two fluxes of different isotopic water molecules) under the same environmental conditions. Among many environmental parameters that are included in the C–G model, accurate description and calculations are still problematic of the kinetic isotope effects that occur in a diffusion-dominated thin layer of air next to the water–air interface. In field applications, it is of importance to accurately evaluate several environmental parameters, particularly the relative humidity and isotopic compositions of the ‘free-atmosphere’, for a system under investigation over a given time-scale of interest (e.g., hourly to daily to seasonally). With a growing interest in the studies of water cycles of different spatial and temporal scales, including paleoclimate and water resource studies, the importance and utility of the C–G model is also likely to grow in the future.

Lipid and carbon isotopic evidence of methane-oxidizing and sulfate-reducing bacteria in association with gas hydrates from the Gulf of Mexico

An integrated lipid biomarker–carbon isotope approach reveals new insight to microbial methane oxidation in the Gulf of Mexico gas-hydrate system. Hydrate-bearing and hydrate-free sediments were collected from the Gulf of Mexico slope using a research submersible. Phospholipid fatty acids consist mainly of C16–C18 compounds, which are largely derived from bacteria. The phospholipid fatty acids suggest that total biomass is enhanced 11–30-fold in gas-hydrate–bearing sediment compared to hydrate-free sediment. Lipid biomarkers indicative of sulfate-reducing bacteria are strongly depleted in 13C (δ13C = −48‰ to −70‰) in the hydrate-bearing samples, suggesting that they are involved in the oxidation of methane (δ13C = −47‰ for thermogenic methane and −70‰ for biogenic methane). Isotopic properties of other biomarkers suggest that sulfur-oxidizing bacteria ( Beggiatoa ) may also contribute to the lipid pool in hydrate-bearing samples, which are characterized by less negative δ13C values (to −11.2‰). In the hydrate-free sample, fatty acid biomarkers have δ13C values of −27.6‰ to −39.6‰, indicating that crude oil (average ∼−27‰) or terrestrial organic carbon (average ∼−20‰) are the likely carbon sources. Our results provide the first lipid biomarker–stable isotope evidence that sulfate- reducing bacteria play an important role in anaerobic methane oxidation in the Gulf of Mexico gas hydrates. The coupled activities of methane-oxidizing and sulfate-reducing organisms contribute to the development of ecosystems in deep-sea environments and result in sequestration of carbon as buried organic carbon and authigenic carbonates. These have implications for studying climate change based on carbon budgets.

The composition of Permian seawater

Forty-nine brine inclusions in marine halite from the Ochoan Salado Formation in the Delaware Basin and fifteen inclusions in halite from the Leonardian Wellington Formation in the Kansas Basin were extracted, and their chemical compositions were determined. The brines are of the Na-K-Mg-Cl-SO4 type; their compositions resemble those of evaporated modern seawater. The values of (mCl(-) - mNa+)/mBr- and (mMg(2+) + mCa(2+) - mSO4(2-) - 1/2mHCO3-)/mBr- of the inclusion brine from the two formations are equal to or slightly higher than those of modern seawater. The original mNa+/mBr- and mCl-/mBr- ratios of the inclusion brines were probably equal to or slightly larger than those of modern seawater. The values of mMg2+/mBr- of the inclusion brines from the Salado Formation are very close to that of modern seawater; the ratios of inclusion brines from the Wellington Formation are slightly lower, probably due to the formation of dolomite/magnesite. The mMg2+/mBr- ratio in the initial seawater was probably close to the parent seawater of the Salado brines. The values of (mSO4(2-) - mCa(2+) + 1/2mHCO3-)/mBr- of the inclusion brines appear to be reduced by the formation of dolomite/magnesite, and the value of this ratio in Permian seawater was probably similar to that of modern seawater. The mK+/mBr- ratios of the inclusion brines are variable, but the original ratios are probably close to or slightly larger than that of modern seawater. If the Br- concentration of Permian seawater was equal to that of modern seawater, the composition of Permian seawater can be narrowly constrained; in mmol/kg H2O, 460 < or = mNa+ < 630, 550 < or = mCl- < 730, mMg2+ = 54 +/- 6, mK+ approximately equal to 11, (mSO4(2-) - mCa(2+) + 1/2mHCO3-) > or = 17, 20 < mSO4(2-) < 45, 5 < mCa2+ < 20, and 0.15 < mHCO3- < 5. The composition of Permian seawater was therefore quite similar to that of modern seawater.

Automatic δD and δ18O analyses of multi-water samples using H2- and CO2-water equilibration methods with a common equilibration set-up

An automatic CO2-water equilibration set-up originally designed for δ18O measurement of water samples was employed for δD analysis using the lately established H2-water equilibration method without any modification of the instrument. No special skill is needed and the time required for both δD and δ18O analyses of 24 waters was only 15 h. Long-term reproducibilities were ± 1.5 and ±0.16% for δD and δ18O analyses, respectively. This method is very suitable for multi-water analyses.

Temperature-dependent oxygen and carbon isotope fractionations of biogenic siderite

Isotopic compositions of biogenic iron minerals may be used to infer environmental conditions under which bacterial iron reduction occurs. The major goal of this study is to examine temperature-dependent isotope fractionations associated with biogenic siderite (FeCO 3). Experiments were performed by using both mesophilic (,35°C) and thermophilic (.45°C) iron-reducing bacteria. In addition, control experiments were performed to examine fractionations under nonbiologic conditions. Temperature-dependent oxygen isotope fractionation occurred between biogenic siderite and water from which the mineral was precipitated. Samples in thermophilic cultures (45-75°C) gave the best linear correlation, which can be described as 10 3 lnasid-wt 5 2.56 3 10 6 T 22 (K) 1 1.69. This empirical equation agrees with that derived from inorganically precipitated siderite by Carothers et al. (1988) and may be used to approximate equilibrium fractionation. Carbon isotope fractionation between biogenic siderite and CO 2, based on limited data, also varied with temperature and was consistent with the inorganically precipitated siderite of Carothers et al. (1988). These results indicate that temperature is a controlling factor for isotopic variations in biogenic minerals examined in this study. The temperature-dependent fractionations under laboratory conditions, however, could be complicated by other factors including incubation time and concentration of bicarbonate. Early precipitated siderite at 120-mM initial bicarbonate tended to be enriched in 18 O. Siderite formed at ,30 mM of bicarbonate tended to be depleted in 18 O. Other variables, such as isotopic compositions of water, types of bacterial species, or bacterial growth rates, had little effect on the fractionation. In addition, siderite formed in abiotic controls had similar oxygen isotopic compositions as those of biogenic siderite at the same temperature, suggesting that microbial fractionations cannot be distinguished from abiotic fractionations under conditions examined here. Copyright