Scott J. Carpenter

Sr/Mg ratios of modern marine calcite: Empirical indicators of ocean chemistry and precipitation rate

Holocene biotic and abiotic marine calcite have a similar range of Mg contents (0 to 22 and 4 to 21 mol% MgCO3, respectively), yet biotic calcite has Sr2+ concentrations that are consistently 1250 ppm higher than those of abiotic calcite. As in laboratory experiments, a positive linear relation is observed between DSr and calcite Mg content. This produces two distinct linear trends on a plot of Sr2+ vs. Mg2+ concentrations. Principal axes of variation for both trends have similar slopes, yet distinctly different Sr2+ concentration intercepts. (Biotic: y = 0.024x + 1298, r2 = 0.70; Abiotic: y = 0.027x + 47, r2 = 0.77). The similar slopes of these trends reflect the constancy of MgCa and SrCa ratios of modern seawater. Equations describing the dependence of DSr on calcite Mg content are derived from both trends (Biotic: DSr = 3.16 x 10t-6 (ppm Mg) + 0.169; Abiotic: DSr = 3.52 x 10−6 (ppm Mg) + 0.0062). Characterization of Sr-Mg trends for Holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater MgCa and SrCa ratios. The relatively high Sr contents of biotic calcite result from rapid precipitation rates associated with shell accretion in marine organisms. Calcites precipitated from seawater in laboratory experiments have Dsr values that are similar to those of biotic marine calcite, suggesting that both precipitate at approximately the same rate. Our estimates of surface area-normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded, pH-stat experiments. We conclude that the DSr values for biotic and experimental marine calcite are kinetically controlled, whereas the lower precipitation rates of abiotic marine calcite yield DSr values that approximate equilibrium conditions. Experimentally derived equations describing the relation between DSr and calcite precipitation rate indicate that the offset in Sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates. However, observations of naturally occurring marine cements suggest that the five-order-of-magnitude offset best represents natural system processes.

δ18O and δ13C values of modern brachiopod shells

Abstract Researchers have not rigorously tested the hypothesis that calcite from modern brachiopod shells is precipitated in oxygen isotope equilibrium with ambient seawater. Isotopic variability at the intraspecimen and intertaxon levels has not been examined. Without such data for modern brachiopods, similar data from ancient brachiopods cannot be accurately interpreted. In this study, a survey is made of δ 18 O and δ 13 C values of Terebratulid, Rhynchonellid, Thecideidine, and Craniacean brachiopods from Antarctica, the Bay of Fundy, Curacao, Japan, New Zealand, Norway, Puget Sound, Palau, Sicily, and South Africa. This suite of samples provides a wide range of taxonomic levels, temperatures, salinities, and depositional environments for evaluating the degree of isotopic equilibrium attained during precipitation of brachiopod calcite. New data indicate that modem brachiopod calcite is not always precipitated in oxygen and carbon isotope equilibrium with ambient seawater. Calcite from the primary layer and specialized shell structures (hinge, brachidium, foramen, interarea, muscle scars) are depleted in both 18 O and 13 C, a characteristic of biological fractionation or “vital” effects often found in other calcerous, marine organisms. Our findings suggest that these portions of the brachiopod shell should be avoided during sampling of ancient brachiopods. Secondary layer calcite, the material most often analyzed in ancient brachiopods, has higher δ 18 O and δ 13 C values which approach and sometimes correspond with predicted equilibrium values. Therefore, secondary layer calcite is the most suitable portion of the brachiopod shell for use as an ancient seawater proxy. Although near equilibrium precipitation in secondary layer calcite is encouraging to those studying the isotopic composition of ancient oceans, these data come with caveats. Large intraspecimen variability in the δ 18 O values of secondary layer calcite (±1‰ in some samples) limits the use of brachiopods as precise indicators of the δ 18 O value or temperature of ancient seawater. The δ 18 O and δ 13 C values of secondary layer calcite deviate from predicted equilibrium values toward the lower values of the primary layer and specialized shell structures, indicating a measurable “vital” effect. In most cases, measured δ 18 O values are lower than the calculated equilibrium values dictated by seasonal variations in the δ 18 O value and temperature of seawater. Ontogenetic variations in the δ 13 C values of secondary layer calcite have also been measured and must be assessed when interpreting ancient data. Modern brachiopods occur in a wide range of seawater salinities and temperatures. Therefore, the relation between salinity and the δ 18 O value of seawater must be known to correctly calculate the temperature of calcite precipitation from secondary layer δ 18 O values. Errors of up to 15°C are found if ambient seawater is assumed to have a constant δ 18 O value (e.g., 0‰ SMOW) and the δ 18 O values of secondary layer calcite are used to calculate temperature. Coupled with “vital” effects and intraspecimen variability, these errors can obscure the resolution of secular variations in the rock record.

δ18O values, 87Sr86Sr and Sr/Mg ratios of Late Devonian abiotic marine calcite: Implications for the composition of ancient seawater

Abstract Late Devonian (Frasnian) abiotic marine calcite has been microsampled and analyzed for 87 Sr 86 Sr ratios, δ18O and δ13C values, and minor element concentrations. Portions of marine cement crystals from the Alberta and Canning Basins have escaped diagenetic alteration and preserve original marine δ18O values (−4.8%. ± 0.5, PDB), δ13C values (+2.0 to +3.0%., PDB), 87 Sr 86 Sr ratios (0.70805 ± 3), and Sr/Mg weight ratios (0.04 to 0.05). Marine 87 Sr 86 Sr ratios are globally consistent and can be correlated within the Alberta Basin, and among the Alberta, Canning, and Williston Basins. Correlation of isotopic and chemical data strengthen the conclusion that marine cements from the Leduc Formation preserve original marine δ18O values which are 3 to 4%. lower than those of modern marine cements. These low δ18O values are best explained by precipitation from 18O-depleted seawater and not by elevated seawater temperature or diagenetic alteration. For comparison with Devonian data, analogous data were collected from Holocene Mg-calcite and aragonite marine cements from Enewetak Atoll, Marshall Islands. Mg-calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater, and Mg-calcite cements are homogeneous with respect to Sr and Mg contents. Empirically derived homogeneous distribution coefficients for Mg and Sr in modern, abiotic Mg-calcite from Enewetak Atoll are 0.034 and 0.15, respectively. An equation describing the dependence of DSr on Mg content was based on a compilation of Sr and Mg data from Holocene abiotic marine calcite (DSr = 3.52 × 10−6 (ppm Mg) + 6.20 × 10−3). Unlike that derived from experimental data, this Sr-Mg relation is consistent over a range of 4 to 20 mol% MgCO3 and may represent precipitation phenomena which are minimally controlled by kinetic effects. Comparison of Sr and Mg contents of analogous Devonian and Holocene marine cements suggests that the Mg/Ca ratio of Late Devonian seawater was significantly lower and that the Sr/Ca ratio was significantly higher than that of modern seawater.

Stalagmite stable isotope record of recent tropical cyclone events

We present a 23 yr stalagmite record (1977-2000) of oxygen isotope variation, associated with 11 tropical cyclones (TCs), from Actun Tunichil Muknal cave in central Belize. High-resolution microsampling yielded a record of monthly to weekly temporal resolution that contains abrupt decreases (negative excursions) in calcite δ 18 O values that correspond with recent TC rain events nearby. A logistic discriminant model reliably identifi ed TC proxy signals using the measurable parameters δ 18 O and δ 13 C values, and single point changes in δ 18 O value. The logistic model cor- rectly identifi ed 80% of excursions as TC events and incorrectly classifi ed only 1 of nearly 1200 nonstorm sampling points. In addition to enabling high-resolution TC frequency reconstruction, this geologic proxy also provides information about the intensity of individual TCs. A multiple regression predicted TC intensity (R 2 = 0.465, p = 0.034) using sampling frequency and excursion amplitude. Consistent with previous low-resolution studies, we found that the decadal average δ 18 O value was lower during the 1990s when several TCs produced rainfall in the area, but higher during the 1980s when only one TC struck. Longer, accurately dated, high-resolution speleo- them stable isotope records may be a useful new tool for paleotempestology, to clarify associa- tions between highly variable TC activity and the dynamic range of Quaternary climate.

Inorganic calcite morphology; roles of fluid chemistry and fluid flow

ABSTRACT Evaluation of calcite crystal habits and fabric and their relation to precipitating fluids in spelean environments indicates that habit variations are controlled primarily by degree of supersaturation. At less than 6 times supersaturation, crystal habits are limited to those with shallow to steep rhombohedral forms (i.e., {1011}, {4041}). With increasing supersaturation, scalenohedrons, extremely steep rhombohedrons, pinacoids and hexagonal prisms are developed. Slight elongation due to differences in crystal dimensions of these forms may arise (i.e., scalenohedrons versus shallow rhombohedrons). At extreme supersaturation (> 12 times), curvature of crystal faces or curved surfaces appear. The development of fibrous versus equant fabrics is the result of changes or modification of reactant supply rates and the number of crystallites (nuclei) present. Under rapid flow conditions, growth rates are accelerated and greater numbers of nuclei are present; thus, compromise boundaries parallel to c-axis and perpendicular to flow (and substrate) are rapidly attained. In the absence of flow, fewer crystallites are present, and larger, well developed crystals or branching forms dictated by saturation conditions arise. In the absence of flow at low supersaturation, elongate composite crystals, and hence fibrous fabrics, may result from migration of the fluid-crystal interface as a result of changing or oscillating water levels. A flow control mechanism can be applied to numerous geological conditions in which transitions from fibrous to equant crystal morphologies are observed.

Carbon isotope ratios of Phanerozoic marine cements: Re-evaluating the global carbon and sulfur systems

Original δ13C values of abiotically precipitated marine cements from a variety of stratigraphic intervals have been used to document secular variations in the δ13C values of Phanerozoic oceans. These, together with the °34S values of coeval marine sulfates, are used to examine the global cycling of carbon and sulfur. It is generally accepted that secular variation in δ13C and δ34S values of marine carbonates and sulfates is controlled by balanced oxidation-reduction reactions and that their long-term, steady-state variation can be predicted from the present-day isotopic fractionation ratio (Δc/Δs) the ratio of the riverine flux of sulfur and carbon (Fs/Fc). The predicted slope of the linear relation between δ13Ccarb and δ34Ssulfate values is approximately −0.10 to −0.14. However, temporal variation observed in marine cement δ13C values and the 6345 values of coeval marine sulfates produces a highly significant linear relation (r2 = 0.80; α > 95%) with a slope of −0.24; approximately twice the predicted value. This discordance suggests that either the Phanerozoic average riverine Fs/Fc was 1.6–3.3 times greater than todays estimates or that an additional source of 34S-depleted sulfur or 13C-enriched carbon, other than continental reservoirs, was active during the Phanerozoic. This new relation between marine δ13C and δ34S values suggests that the flux of reduced sulfur, iron, and manganese from seafloor hydrothermal systems affects oceanic O2 levels which, in turn, control the oxidation or burial of organic matter, and thus the δ13C value of marine DIC. Therefore, the sulfur system (driven by seafloor hydrothermal systems) controls the carbon system rather than organic carbon burial controlling the response of δ34S values (via formation of sedimentary pyrite). Secular variation of marine 87Sr86Sr ratios and δ13C values argues for a coupling of δ34S and δ34S values to variation in the relative contribution of seafloor hydrothermal and continental weathering fluxes. These trends indicate that the early Paleozoic was dominated by low temperature silicate weathering, whereas the Late Paleozoic to Modern was dominated by high temperature seawater-basalt interactions. Variation in Proterozoic δ13Ccarb and δ34Ssulfate values produces a slope that is greater than that of the Phanerozoic ( −0.50 vs. −0.24). This steeper slope is consistent with other geochernical data that indicate relatively high seafloor hydrothermal fluxes during the late Precambrian. We speculate that the dramatic evolutionary changes of the Neoproterozoic-Paleozoic transition occur during a waning of seafloor hydrothermal fluxes and a concomitant decrease in O2 consumption that permitted the oxygenation of seawater thought to be critical in metazoan evolution.

Way out of Africa: Early Pleistocene paleoenvironments inhabited by Homo erectus in Sangiran, Java

A sequence of paleosols in the Solo Basin, Central Java, Indonesia, documents the local and regional environments present when Homo erectus spread through Southeast Asia during the early Pleistocene. The earliest human immigrants encountered a low-relief lake-margin landscape dominated by moist grasslands with open woodlands in the driest landscape positions. By 1.5 Ma, large streams filled the lake and the landscape became more riverine in nature, with riparian forests, savanna, and open woodland. Paleosol morphology and carbon isotope values of soil organic matter and pedogenic carbonates indicate a long-term shift toward regional drying or increased duration of the annual dry season through the early Pleistocene. This suggests that an annual dry season associated with monsoon conditions was an important aspect of the paleoclimate in which early humans spread from Africa to Southeast Asia.

Migration of a Late Cretaceous fish.

Late Cretaceous sediments from the Western Interior of North America yield exceptionally well preserved fossils that serve as proxies for the rapidly changing climate preceding the Cretaceous/Tertiary boundary (about 67–65 Myr ago). Here we reconstruct the ontogenetic history of a Maastrichtian-age fish, Vorhisia vulpes, by using the carbon, oxygen and strontium isotope ratios of four aragonite otoliths collected from the Fox Hills Formation of South Dakota. Individuals of V. vulpes spawned in brackish water (about 70–80% seawater) and during their first year migrated to open marine waters of the Western Interior Seaway, where they remained for 3 years before returning to the estuary, presumably to spawn and die. The mean δ18O from the marine growth phase of V. vulpes yields a seawater temperature of 18 °C, which is consistent with leaf physiognomy and general-circulation-model temperature estimates for the Western Interior during the latest Maastrichtian.

Isotopic disequilibrium in marine calcareous algae

A survey of the δ13C and δ18O values of CaCO3 precipitated by marine calcareous algae was conducted to examine ‘vital effects’ produced by the various styles of calcification of the major algal subdivisions (Codiacea, Dasycladacea, Coralline algae and Calcareous red algae). Algae are categorized on the basis of stable isotope ratios and styles of calcification. Styles of calcification and associated processes in these plants are diverse and produce a wide range of δ13C values that are both lower and higher than predicted equilibrium values (from approximately −6‰ to +8‰). In general, δ13C values of algal carbonate reflect phylogenetic and ontogenetic changes in photosynthesis that produce changes in the δ13C value of the calcifying fluid due to modification of the photosynthesis/respiration ratio. δ18O values are less variable, and in many cases, approximate predicted equilibrium values. Kinetic fractionation of carbon and oxygen isotopes associated with hydroxylation of CO2 is observed in the carbonates of Neogoniolithon sp. and Bossiella sp., where δ13C and δ18O values are positively correlated. This positive correlation is similar to trends observed in many carbonate-secreting organisms. Stable isotope data from green algae (Codiaceans and Dasycladacean) are related to the style of each species calcification (i.e., microenvironment of calcification site). Intercellular calcification in Halimeda sp. and Udotea sp. is characterized by significant metabolic effects in carbonate δ13C values (up to 7‰ variation within a specimen). Extracellular calcification in Acetabularia sp. and sheath calcification in Penicillus sp. have carbonate δ13C and δ18O values that are near predicted equilibrium values (similar to inorganic precipitates). The δ13C values of coralline algae are lower than Codiacean green algae by as much as 14‰. We suggest that this difference is due largely to the unique ontogeny of coralline algae. Calcification in the younger portions of coralline algae occurs under strong influence of respiration. Carbonate from Amphiroa sp. and Galaxaura sp. has relatively invariant δ18O values that may be useful proxies of ambient conditions.

Diagenesis of Fossiliferous Concretions from the Upper Cretaceous Fox Hills Formation, North Dakota

ABSTRACT Three fossiliferous concretions of the Fox Hills Formation (Upper Cretaceous) of North Dakota display similar diagenetic histories. Petrographic and geochemical data from each concretion have yielded the following diagenetic sequence: 1) bioturbation by marine benthos; 2) partial phosphatization of glauconite, fecal pellets, and shell material; 3) sulfate reduction and pyrite formation; 4) siderite precipitation; 5) radial-fibrous calcite precipitation, and 6) equant calcite precipitation. Although the occurrence of open-marine faunas and glauconite indicates that concretion sediments were deposited under marine conditions, geochemical data suggest that marine pore fluids were replaced by brackish and meteoric water. As the Fox Hills Formation is part of a regressive, marginal marine equence, it is likely that the transition from marine to freshwater is recorded in concretion paragenesis. Early diagenetic minerals indicate oxidation of organic matter at shallow burial depths under normal marine conditions. Locally reducing conditions, produced initially by degradation of molluscan soft parts and later by degradation of organic material in crustacean and annelid fecal pellets, occurred at shallow burial depths. Alteration of iron silicates (glauconite) produced pore fluids enriched in Fe2+. Due to the abundance of reactive organic matter, subsequent reduction of marine sulfate yielded a small amount of disseminated pyrite. Following the removal of H2S, elevated Fe2+, pH, and bicarbonate concentrations soon exceeded siderite saturation and siderite was precipitated. Elevated Ca and Mg compositions and marine 18O values of siderite cements indicate that pore fluids were essentially reduced marine fluids. Depleted 13C values suggest that siderite precipitated in an environment dominated by the oxidation of organic matter. Isotopic compositions from the earliest and latest carbonate phases constrain intermediate phases and allow comparison of environments of precipitation. Carbon and oxygen isotopic data from skeletal aragonite provide an estimate of the isotopic composition of marine carbonate. The latest phase of equant calcite cement is characterized by invariant 18O, coupled with highly variable and light 13C. This characteristic isotopic signature and occurrence in overlying fluvial sediments suggests a meteoricphreatic origin for equant spars. Isotopic values of radial-fibrous calcite lie between those of marine (aragonite, siderite) and freshwater (equant calcite) ph ses and represent a complex mixing of marine and meteoric pore fluids. The paragenetic sequence of carbonate cements records a gradual decrease in ambient fluid Mg, Fe, and Mn content during shallow burial. Such decreases are consistent with the influx of oxidizing meteoric fluids.