The major-ion composition of Permian seawater

The major-ion (Mg²⁺, Ca²⁺, Na⁺, K⁺, SO₄²⁻, and Cl⁻) composition of Permian seawater was determined from chemical analyses of fluid inclusions in marine halites. New data from the Upper Permian San Andres Formation of Texas (274-272 Ma) and Salado Formation of New Mexico (251 Ma), analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, along with published chemical compositions of fluid inclusions in Permian marine halites from North America (two formations of different ages) and the Central and Eastern European basins (eight formations of four different ages) show that Permian seawater shares chemical characteristics with modern seawater, including SO₄²⁻ > Ca²⁺ at the point of gypsum precipitation, evolution into Mg²⁺-Na⁺-K⁺-SO₄²⁻-Cl⁻ brines, and Mg²⁺/K⁺ ratios ∼5. Permian seawater, however, is slightly depleted in SO₄²⁻ and enriched in Ca²⁺, although modeling results do not rule out Ca²⁺ concentrations close to those in present-day seawater. Na⁺ and Mg²⁺ in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater are both classified as aragonite seas, with Mg²⁺/Ca²⁺ ratios >2, conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements.The chemistry of Permian seawater was modeled using the chemical composition of brine inclusions for three periods: Lower Permian Asselian-Sakmarian (296-283 Ma), Lower Permian Artinskian-Kungurian (283-274 Ma), and Upper Permian Tatarian (258-251 Ma). Parallel changes in the chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe show that seawater underwent secular variations in chemistry over the 50 million years of the Permian. Modeled SO₄²⁻ concentrations are 20 mmol per kg H₂O (mmolal) and 19 mmolal in the Asselian-Sakmarian and Artinskian-Kungurian, with higher concentrations in the Upper Permian Tatarian (23 mmolal). Modeled Ca²⁺ is at or above its concentration in modern seawater throughout the Permian. Mg²⁺ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal), and Tatarian (52 mmolal), but slightly higher than modern seawater in the Artinskian-Kungurian (60 mmolal). Mg²⁺/Ca²⁺ ratios are 3.5 (total range = 2.7 to 5.5) in the Lower Permian and rose slightly to 3.7 (total range = 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca²⁺. These results are consistent with models that predict oscillations in the major-ion composition of Phanerozoic seawater on the basis of changes in the midocean ridge/river water flux ratio driven by changes in the rate of midocean ridge crust production.The Permian was characterized by low sea levels, icehouse conditions, and southern hemisphere glaciation. Such conditions, analogous to the present ice age, and the similarities between Permian seawater and modern seawater, all suggest that general Phanerozoic supercycles, driven by mantle convection and global volcanicity, also control the major-ion chemistry of seawater.     

Secular variation in the major-ion chemistry of seawater: Evidence from fluid inclusions in Cretaceous halites

The major-ion (Mg²⁺, Ca²⁺, Na⁺, K⁺, , and Cl⁻) chemistry of Cretaceous seawater was determined from analyses of seawater-derived brines preserved as fluid inclusions in marine halites. Fluid inclusions in primary halite from three evaporite deposits were analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy dispersive spectrometry (EDS) technique: the Early Cretaceous (Aptian, 121.0-112.2 Ma) of the Sergipe basin, Brazil and the Congo basin, Republic of the Congo, and the Early to Late Cretaceous (Albian to Cenomanian, 112.2-93.5 Ma) of the Khorat Plateau, Laos, and Thailand. The fluid inclusions in halite indicate that Cretaceous seawater was enriched several fold in Ca²⁺, depleted in , Na⁺, and Mg²⁺, and had lower Na⁺/Cl⁻, Mg²⁺/Ca²⁺, and Mg²⁺/K⁺ ratios compared to modern seawater. Elevated Ca²⁺ concentrations, with Ca²⁺ >  at the point of gypsum saturation, allowed Cretaceous seawater to evolve into Mg²⁺-Ca²⁺-Na⁺-K⁺-Cl⁻ brines lacking measurable .The major-ion composition of Cretaceous seawater was modeled from fluid inclusion chemistries for the Aptian and the Albian-Cenomanian. Aptian seawater was extreme in its Ca²⁺ enrichment, more than three times higher than present day seawater, with a Mg²⁺/Ca²⁺ ratio of 1.1-1.3. Younger, Albian-Cenomanian seawater had lower Ca²⁺ concentrations, and a higher Mg²⁺/Ca²⁺ ratio of 1.2-1.7. Cretaceous (Aptian) seawater has the lowest Mg²⁺/Ca²⁺ ratios so far documented in Phanerozoic seawater from fluid inclusions in halite, and within the range chemically favorable for precipitation of low-Mg calcite ooids and cements. Results from halite fluid inclusions, together with Mg²⁺/Ca²⁺ ratios measured from echinoderm and rudist calcite, all indicate that Early Cretaceous seawater (Hauterivian, Barremian, Aptian, and Albian) had lower Mg²⁺/Ca²⁺ ratios than Late Cretaceous seawater (Coniacian, Santonian, and Campanian). Low Aptian-Albian Mg²⁺/Ca²⁺ seawater ratios coincide with negative excursions of ⁸⁷Sr/⁸⁶Sr ratios and δ³⁴S_SO4, and peak Cretaceous ocean crust production rates, all of which suggests a link between seawater chemistry and midocean ridge hydrothermal brine flux.     

The control of Phanerozoic atmosphere and seawater composition by basalt–seawater exchange reactions

We present results from a long term geochemical cycling model, with a focus on the sensitivity of atmospheric carbon dioxide, oxygen, and the major element composition of seawater to seafloor spreading rates. This model incorporates rock weathering, basalt–seawater exchange reactions, and the formation and destruction of chemical sediments and organic matter. Hydrothermal reactions between seafloor and seawater involving calcium, magnesium, sodium, potassium, sulfate and carbon are the high temperature counterparts to low temperature redox, weathering, precipitation and diagenetic reactions. A major source of uncertainty is the extent to which these exchange fluxes are controlled by seafloor spreading rate. In addition, the return fluxes of these components to the atmospheric and primary silicate reservoirs reflect not only the overall rates of subduction and metamorphism, but the distribution of the overlying sedimentary burden and authigenic minerals formed during basalt alteration as well. In particular, we show how the stoichiometry of exchange fluxes (Mg/Ca and SO₄/Ca) may buffer atmospheric CO₂ and O₂ concentrations.     

Cation hydrolysis and the regulation of trace metal composition in seawater

Thermodynamic calculations have been performed for cation hydrolysis, including temperatures from 2°C to the high values of significance near Mid-Oceanic Ridge Systems (MORS). Eighteen elements with wide range of residence times (t) in seawater (Mn, Th, Al, Bi, Ce, Co, Cr(III), Fe, Nd, Pb, Sc, Sm, Ag, Cd, Cu, Hg, Ni and Zn) have been considered. A model for the regulation of trace metal composition in seawater by cation hydrolytic processes, including those at MORS, is presented. Results show an increase in the abundance of neutral metal hydroxyl species with increase in temperature. During hydrothermal mixing, as the temperature increases, transformation from lower positive hydroxyl complexes to higher or neutral complexes would occur for Cd, Ce, Co, Cr(III), Cu, Mn, Nd, Ni, Pb, Sm and Zn. pH values for adsorption of the metal ion onto solid surfaces have direct relation with pH values of hydrolysis. Co, Mn and Pb could be oxidized to higher states (at Mn-oxide surfaces) that would occur even at MORS. Ce can also be oxidized at 25°C. Solubility calculations show that Al, Bi, Cr(III), Sc, Fe and Th are saturated while Ce, Nd and Sm are not with respect to their oxyhydroxide solids at their concentrations in seawater at 25°C. Cu, Hg, Ni and Zn reach saturation equilibrium at 250°C, whereas Co, Mn and Pb exhibit unsaturation. The results suggest an increase in scavenging capacity of a cation with rise in temperature.     

The isotopic composition of strontium and sulfur in seawater of Late Permian (Zechstein) age

The Paleozoic minima of ⁸⁷Sr/⁸⁶Sr ratios and δ³⁴S values in seawater are observed in anhydrite layers of the Stassfurt (A-2) and Aller (A-4) cycles of evaporation of Late Permian age in Germany. These minima were probably caused by extremely low rates of continental runoff and bacterial sulfate reduction. Thereafter conditions changed suddenly, with a steeply increasing supply of radiogenic Sr and of nutrients into seawater. The latter induced a large growth in the production of organic material, in bacterial activity in sediments, and in the flow of isotopically heavy sulfur back into the oceans. This large change in the cycles of S and Sr (as well as of C and O) within the relatively short Zechstein period (2–5 Ma) reflects a considerable modification of the plate tectonic and environmental conditions shortly before the beginning of the Mesozoic era. The Ochoan deposition in the USA is probably of the same age as the Stassfurt-Leine evaporation in W Europe.     

Deep-sea coral aragonite as a recorder for the neodymium isotopic composition of seawater

Deep-sea corals have been shown to be useful archives of rapid changes in ocean chemistry during the last glacial cycle. Their aragonitic skeleton can be absolutely dated by U-Th data, freeing radiocarbon to be used as a water-mass proxy. For certain species of deep-sea corals, the growth rate allows time resolution that is comparable to ice cores. An additional proxy is needed to exploit this opportunity and turn radiocarbon data into rates of ocean overturning in the past.Neodymium isotopes in seawater can serve as a quasi-conservative water-mass tracer and initial results indicate that deep-sea corals may be reliable archives of seawater Nd isotopes. Here we present a systematic study exploring Nd isotopes as a water-mass proxy in deep-sea coral aragonite. We investigated five different genera of modern deep-sea corals (Caryophyllia, Desmophyllum, Enallopsamia, Flabellum, Lophelia), from global locations covering a large potential range of Nd isotopic compositions. Comparison with ambient seawater measurements yields excellent agreement and suggests that deep-sea corals are reliable archives for seawater Nd isotopes.A parallel study of Nd concentrations in these corals yields distribution coefficients for Nd between seawater and coral aragonite of 1-10, omitting one particular genus (Enallopsamia). The corals and seawater did however not come from exactly the same location, and further investigations are needed to reach robust conclusions on the incorporation of Nd into deep-sea coral aragonite.Lastly, we studied the viability of extracting the Nd isotope signal from fossil deep-sea corals by carrying out stepwise cleaning experiments. Our results show that physical removal of the ferromanganese coating and chemical pre-cleaning have the highest impact on Nd concentrations, but that oxidative/reductive cleaning is also needed to acquire a seawater Nd isotope signal.     

A silurian porphyry copper prospect near Yeoval,New South Wales

The Yeoval porphyry copper prospect lies in a complex of dioritic rocks which form part of the eastern margin of the Yeoval Batholith in central‐western New South Wales. Rocks of the batholith are mainly granite and adamellite whose age is about 370 m.y. The diorite complex, (411 m.y.) is composed of rocks ranging from granodiorite to gabbro and pyroxenite.

Hydrothermal alteration of granodiorite in the Yeoval Mine area, 3.5 km north of Yeoval, is associated with disseminated and stockwork‐veinlet copper‐sulphide‐bearing zones. Alteration assemblages are similar to those described from some disseminated or porphyry copper/molybdenum deposits of southwestern USA.

The ubiquity of potassic zones in veinlet alteration envelopes and the poor development of sericitic and argillic zones suggest that the Yeoval prospect formed at or below the level of the Ajo deposit, Arizona, and the Los Loros deposit, Chile, which formed some 5 km below surface near the base of the ‘porphyry system’.

High Rb and Ba contents in the Yeoval diorites and their associated andesitic volcanics, and the presence of garnet‐bearing rhyodacite of similar age, imply that the Yeoval area was part of an Andean type of continental margin in the middle Palaeozoic.     

Diurnal variations in the isotopic composition of dissolved inorganic carbon in seawater from coral reef environments

The isotopic composition of total dissolved inorganic carbon in seawater was determined as a function of time of day in coral reef environments at Saipan, Abaiang Atoll, Tahiti, Florida and Heron Island on the Great Barrier Reef of Australia. At each locality water collected during the day was enriched in ¹³C with respect to water collected at night. The form and magnitude of the diurnal cycles is dependent upon the ratio of local biomass to local water mass and on the degree of exchange between local water mass and the open ocean reservoir. Comparison of open and closed system models is made using computer generated δ¹³C vs. time curves to illustrate modulation by the tidal cycle of the simple diurnal variation in a closed system. The results are of geochemical significance in that the δ¹³C of CaCO₃ precipitated in certain environments is dependent on the time of crystallization. This must be taken into account in isotope ratio studies of both plant and animal secreted carbonates which are preferentially precipitated during certain periods of the day.     

A diverse ichnofauna from silurian flysch of the aberystwyth grits formation,Wales

The most extensive ichnofauna yet recorded from a deep water Lower Palaeozoic sequence occurs within the distal turbidites of the Lower Silurian Aberystwyth Grits Formation of Central Wales. The strata contain an abundant assemblage comprising 25 ichnogenera: Asteriacites, Bergaueria, Chondrites, Cochlichnus, Cosmorhaphe, Glockerichnus, Gordia, Helicolithus, Helminthopsis, Helminthoida, Hormosiroidea, Lorenzinia, Megagrapton, Monomorphichnus, Neonereites, Nereites, Palaeophycus, Paleodictyon, Planolites, Protopaleodictyon, Spirorhaphe, Spirophycus, Squamodictyon, Subphyllochorda, Taphrhelminthopsis; 36 ichnospecies are described, three of which (Asteriacites aberensis, Helminthopsis regularis, Cosmorhaphe elongata) are new. The inorganic sedimentary structures and trace fossils of some 418 sandstone beds were examined in detail; 16 per cent of the beds commence with Divisions A or B and 84 per cent with Division C of the turbidite sequence. This indicates a relatively distal environment, mainly receiving low velocity turbidity currents, and favouring trace fossil preservation. The most common traces were Helminthopsis, Paleodictyon, and Squamodictyon which were found on 46 per cent, 34 per cent, and 19 per cent of the beds examined. Data from this, and other recently described sequences, confirms that there was a gradual increase in trace fossil diversity in the deep oceans throughout the Lower Palaeozoic, in contrast to the situation in shallow water shelf seas where a peak was reached as early as the Lower Cambrian.     

Petrology and provenance of late silurian fluviatile sandstones from the ringerike group of Norway

The Ringerike Group, a late Silurian fluviatile sequence in the Oslo Graben of Norway was deposited in two distinct sedimentary provinces. In the northern part of the area the Sundvollen Formation (meandering-stream sediments) and the overlying Stubdal Formation (braided) contain antiperthitic feldspars and drop perthites characteristics of the jotunites of the Jotunheim area of central Norway. Other textural, mineralogical and palaeocurrent evidence supports the conclusion that these two formations were derived from a relatively distant source area, which included the Jotunheim, part of the newly raised Caledonian orogen.To the south, the sandstones of the Holmestrand area are much coarser grained and show a distinctive mineralogy, composition and texture. This evidence, supported by palaeocurrent studies, is consistent with a proximal source area on the eastern flank of the Oslo Graben. Source rocks consisted predominantly of high-grade metamorphic and igneous rocks of the Precambrian basement and the overlying Eocambrian sedimentary rocks.     

The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice

The composition of carbonate minerals formed in past and present oceans is assumed to be significantly controlled by temperature and seawater composition. To determine if and how temperature is kinetically responsible for the amount of Mg incorporated in calcite, we quantified the influence of temperature and specific dissolved components on the complex mechanism of calcite precipitation in seawater. A kinetic study was carried out in artificial seawater and NaCl-CaCl₂ solutions, each having a total ionic strength of 0.7 M. The constant addition technique was used to maintain [Ca²⁺] at 10.5 mmol kg⁻¹ while [] was varied to isolate the role of this variable on the precipitation rate of calcite.Our results show that the overall reaction of calcite precipitation in both seawater and NaCl-CaCl₂ solutions is dominated by the following reaction:

     

The thermodynamics of the carbonate system in seawater

The apparent constants (K'_i) for the ionization of carbonic acid in seawater at various salinities (S,%.) have been fit to equations of the form ln $\text{K'}{}_{\mathrm{i}}\text{= ln K}{}_{\mathrm{i}}\text{+ A}{}_{\mathrm{i}}\text{S}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+ B}{}_{\mathrm{i}}\text{S}$whereK_i is the thermodynamic ionization constant in water, A_i, and B_i are adjustable parameters. The temperature dependence (TK) of K_i, A_i and B_i were of the form, a₀ + a₁/T + a₃ ln T. Equations of similar forms have been used to analyze the ionization constants for water and boric acid and the solubility product of calcite in seawater. The effect of pressure on the apparent constants ($\text{K}{}^{\mathrm{p}}{}_{\mathrm{i}}\text{K}{}^{\mathrm{o}}{}_{\mathrm{i}}$) have been fit to equations of the form ln $\text{(}\text{K}{}^{\mathrm{p}}{}_{\mathrm{i}}\text{K}{}^{\mathrm{o}}{}_{\mathrm{i}}\text{) = ? (ΔVP + 0.5 ΔK P}{}^2\text{)/RT}$ where the volume (ΔV) and compressibility (ΔK) changes are polynomial functions of temperature. The equations generated for various açids in seawater have been used to examine the carbonate system in seawater. Equations relating the NBS and Tris pH scales have been derived as well as equations of pH as a function of temperature and pressure. The equations from Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Mehrbachet al. (1973, Limnol. Oceanogr.18, 897–907) have been used to examine the components of the carbonate system. At a fixed total alkalinity and total carbon dioxide, differences of ±0.01 m-equiv kg^?1 in HCO^?₃ and CO^2?₃ were found; however, the [CO₂] and P_co2 are nearly the same. The contribution of borate ion, B(OH)^?₄ determined from the equations of Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Lyman (1957, Ph.D. Thesis, University of California, Los Angeles) differ by ±0.01 m-equiv kg^?1 for waters with the same salinity and temperature.     

The biostratigraphy and formational relationships of the upper aeronian and lower telychian (llandovery,silurian) formations of western mid-wales

The stratigraphical relationships of the upper Aeronian-lower Telychian formations in the Aberystwyth area are examined in the light of a new biostratigraphical subdivision of this interval based on graptolites, in which part of the Monograptus turriculatus Biozone is divided into six subzones and the Stimulograptus halli Biozone is resurrected. In the Aberystwyth area (B.G.S. 1:50 000 Sheet 163 (1984)) there is shown to be greater diachroneity of formational —boundaries than previously recognized. Notably, the Cwmsymlog Formation facies persisted in the northeast of the area far longer than anywhere else, due to the persistence of a bathymetric high into the lower Telychian. Also, the basal beds of the Aberystwyth Grits in the west of the area are shown to be of the same age as the uppermost beds of the Devil's Bridge Formation in the east. Thus for a short period the area was subject to turbiditic incursions from two sources. The degree of diachroneity of the base of the Aberystwyth Grits Formation as a whole is less than previously thought — the oldest beds are approximately one subzone older in the south than in the north. The core of the in the clinal pericline responsible for the crescentic outcrop pattern of the Aberystwyth Grits is demonstrated to lie in the vicinity of Llanrhystud, rather than at Aberarth as previously suggested. Four new graptolite species are described and one new genus is erected.     

Joints and other structures in the silurian rocks of the southern Shap Fells,Westmorland

The Coniston Grits and Bannisdale Slates south of Shap are folded into the major Bannisdale Syncline, an asymmetrical fold with a longer northwest limb, a half wave length exceeding 5 miles, and numerous minor parasitic folds with half wave lengths ranging from 200 yds to 2 or 3 yds. These minor folds generally plunge from 5° to 10° to the northeast and occur in belts of tight folds which alternate with belts of steeply, uniformly dipping strata. It is notable that the axial planes of the folds are not parallel to the cleavage and in fact the trend of the former (005°) is about 5° oblique to the latter (060°). There are also complementary dextral and sinistral wrench faults and numerous sets of joints related both to the folds and the faults. The joints present distinctly different patterns on opposing limbs of the minor folds and it is suggested that they formed at different stages of the orogeny. The earliest phase, shortly after initiation of folding, is thought to have been “wrench type” joints, subsequently folded and succeeded by post folding dextral and sinistral “wrench” joints accompanied by Reidel shear and kink bands, and then finally by high and low angle tension joints at a late stage in the orogeny.     

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of ²⁴Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of ²⁶Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ²⁶Mg_sw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ²⁶Mg_sw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ²⁶Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in ²⁴Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ²⁶Mg_sw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ²⁶Mg_sw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ²⁶Mg of the Huangjin limestone makes it difficult to reconstruct δ²⁶Mg_sw using bulk carbonate/calcareous sediments. Finally, δ²⁶Mg_sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.     

87Sr/86Sr composition of seawater during the Phanerozoic

${}^{87}\text{Sr}{}^{86}\text{Sr}$ measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest ⁸⁷Sr/⁸⁶Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate.