Search

Understanding the response of temperature to elevated atmospheric CO2 during past greenhouse intervals such as the Late Cretaceous can constrain hypotheses of expected future warming tied to the rise of modern atmospheric CO2 levels. Here we present new reconstructions of Gulf and Atlantic Coast coastal marine temperatures through the late Campanian (∼76–72 Ma) and Maastrichtian (72 Ma–66 Ma), as determined by carbonate clumped isotope analysis of marine bivalves and gastropods. We find temperatures in the range of ∼7–25 °C across multiple sites located between 31°N and 36°N paleolatitude, and cooler temperatures of ∼3–14 °C at sites around 39°N paleolatitude. Temperatures agree across a variety of taxa, indicating no appreciable organism-specific vital effects. The calculated paleotemperatures are very similar to modern marine temperatures at the same locations, despite the Late Cretaceous generally being considered a warmer interval. Clumped isotope temperatures are cooler than published temperatures from a nearby site measured using the TEX86 paleotemperature proxy, revealing a potential warm bias in TEX86 temperature estimates. The best agreement between clumped isotope and TEX86 temperatures is achieved when using the TEX 86 L calibration over TEX 86 H or BAYSPAR calibrations.

Use of the clumped isotope thermometer to decipher environments of dolomite formation has been inhibited by a lack of empirical data from dolomites formed at known temperatures. Calibrations for aragonite, calcite, siderite, and apatite indicate that a universal Δ 47 – temperature relationship may exist across all carbonate-bearing mineral phases. However, theoretical modeling and proposed acid fractionation differences have suggested that dolomites may have different Δ 47 values relative to calcites precipitated at the same temperature. To resolve this question, we analyzed five synthetic and four natural dolomites formed at known temperatures. These included synthetic dolomites grown in Mg-Ca-Cl solutions at temperatures of 200–250 °C, and natural samples constrained by fluid inclusion analyses (~ 70 °C), climate (~ 28 and ~ 27 °C), and deep ocean borehole temperature (~ 21 °C). When using calcite Δ 47 acid fractionation values, these data result in a calibration line that is statistically indistinguishable from other clumped isotope calibrations. At least with current precision, we find no evidence for a consistent dolomite Δ 47 offset. These results further support a universal calibration for carbonate clumped isotope thermometry and enable new investigations into conditions of dolomite formation.

Carbonate clumped isotope paleothermometry is a promising method for paleoclimate reconstructions and understanding diagenetic processes, although clumped isotope compositions (measured as Δ47 values) can be altered by recrystallization and thermal resetting. Several studies have investigated the effects of deep burial and exhumation on Δ47 values; we present Δ47 values of samples from an ∼4500-m-long drill core from Andros Island, Bahamas, which represent the spectrum from shallow to deep burial conditions. These limestones and dolomites originally formed under near-surface temperature conditions, which are reflected by ∼27 °C Δ47 temperatures in samples buried to

Meteoric diagenetic cements are ubiquitous throughout geologic history, affecting most carbonate exposures worldwide. They can often be difficult to interpret, as it is usually difficult to separate the influences of water δ18O and temperature on isotopic signals contained within the carbonate rock body. Despite this difficulty in interpretation, meteoric phreatic cements can potentially be a useful proxy material, as they form slowly in equilibrium at mean annual temperature and are not affected by any biogenic effects that can bias other proxy materials. We applied the mass-47 clumped isotope paleothermometer to Pleistocene and Holocene carbonates from Bermuda and Barbados in order to investigate the effects of meteoric diagenesis on Δ47 signals, and to determine their suitability as a paleotemperature proxy. Phreatic calcite cements are found to record the same temperatures as unaltered carbonate sediments, while any sample exhibiting vadose characteristics is biased towards unreasonably hot apparent formation temperatures. Burial heating and re-equilibration are not geologically viable explanations for the anomalously hot temperatures recorded in vadose cements, as neither Bermuda or Barbados has any burial history. Instead, it is likely that precipitation in the vadose zone occurs on timescales quicker than isotopic equilibrium can be achieved, driven by a combination of CO2 degassing and evaporation, which have been previously shown to cause problems in speleothems and pedogenic carbonates. We conclude by suggesting that meteoric phreatic calcites may be an ideal phase for paleotemperature reconstruction, as they accurately record mean annual temperatures and form under equilibrium conditions, while also being resistant to further mineral driven diagenesis. Vadose cements, and any sample likely affected by processes similar to vadose diagenesis, should be avoided for climate reconstructions using the mass-47 clumped isotope thermometer.

Rationale The clumped isotope paleothermometer, a new proxy widely applicable in studies of paleoclimate, tectonics, and paleontology, relates the abundance of doubly substituted isotopologues of carbonate-derived CO2 to the temperature of formation of the carbonate phase. As this technique becomes more widely used, more is discovered about the effects of everyday laboratory procedures on the clumped isotopic composition of CO2 gas. Methods Preparation of CO2 for clumped isotope analysis requires the removal of isobaric contaminants prior to measurement, achieved dynamically by passing the CO2 through a gas chromatography column using a helium carrier gas or cryogenically pumping CO2 through a static trap filled with Porapak™ Q (PPQ) material. The stable and clumped isotopic compositions of carbonate standards prepared at PPQ trap temperatures between −40°C and −10°C were measured by isotope ratio mass spectrometry to evaluate potential artifacts introduced by the static PPQ trap method. Results The stable isotopic composition of carbonates run at temperatures below −20°C was fractionated, despite achieving >99% retrieval of gas at temperatures as cold as −30°C. The δ13C and δ18O values decreased by ~0.01 and ~0.03 ‰/(°C below −20°C). The raw Δ47 values decreased by 0.003–0.005 ‰/(°C below −20°C), but the final reference-frame-corrected values (Δ47-RFAC) were unaffected as long as the carbonate samples and standard gases were prepared identically. Conclusions Preparing carbonate samples for clumped isotope analysis using a PPQ trap that is too cold can result in erroneous stable isotopic compositions. New and existing labs using the static PPQ trap cleaning procedure should determine the ideal PPQ trap temperature for their particular system through monitoring not only yield through the PPQ trap, but also stable isotopic composition at various PPQ trap temperatures. Copyright © 2015 John Wiley & Sons, Ltd.

This study uses stable noble gases' (He, Ne, Ar, Kr, Xe) volume fractions and isotopic ratios from Antrim Shale natural gas to assess compositional variability and vertical fluid migration within this reservoir, in addition to distinguishing between the presence of thermogenic versus biogenic methane. R/Ra values, where R is the measured 3He/4He ratio and Ra is the atmospheric value of 1.384 ± 0.013 × 10− 6, vary from 0.01 to 0.34 suggesting a largely dominant crustal 4He component with minor atmospheric and mantle contributions. Crustal 21Ne, 40Ar and 136Xe contributions are also present but the atmospheric component is largely dominant for these gases. Crustal contributions for 21Ne, 40Ar and 136Xe vary between 1.1% and 12.5%, between 0.7% and 19% and between 0.1% and 2.7%, respectively. A few samples present higher than atmospheric 20Ne/22Ne ratios pointing to the presence of a small mantle Ne component. High horizontal and vertical variability of noble gas signatures in the Antrim Shale are observed. These are mainly due to variable noble gas input from deep brines and, to a smaller extent, variable in-situ production within different layers of the Antrim Shale, in particular, the Lachine and Norwood members. Estimated 4He ages, considering external 4He input for Antrim Shale water, vary between 0.9 ka and 238.2 ka and match well for most samples with the timing of the major Wisconsin glaciation, suggesting that Antrim Shale water was influenced by glaciation-induced recharge. Consistency between measured and predicted 40Ar/36Ar ratios assuming Ar release temperatures ≥ 250 °C supports a thermogenic origin for most of the methane in these samples. This thermogenic methane is likely to originate at greater depths, either from the deeper portion of the Antrim Shale in the central portion of the Michigan Basin or from deeper formations given that the thermal maturity of the Antrim Shale in the study area is rather low.

An essential procedure to increase the analytical efficiency of Delta(47) measurements requires raising the temperature of phosphoric acid digestion for carbonate materials. This temperature change introduces a fractionation offset in Delta(47) that must be accounted for prior to calculation of temperatures of carbonate formation and to allow interlaboratory comparison of results. We measured the phosphoric acid fractionation factor relative to reaction at 25 degrees C for calcite, aragonite, and dolomite across a temperature range from 25 to 90 degrees C. Significantly, all three minerals behave similarly during phosphoric acid digestion, allowing for a single temperature dependent acid fractionation relationship:1000 ln infinity CO2(Acid)-Delta(47) = (0.022434 +/- 0.001490) * 10(6)/T-2 - (0. 2524 +/- 0: 0168)where a is the phosphoric acid fractionation factor, and T is in degrees Kelvin. Mineralogical or isotopic compositional effects on the fractionation factorwere not observed, suggesting that this acid fractionation factor may be valid for all carbonate minerals. We also present inorganic temperature calibrations for both calcite and aragonite at low temperatures ( 5-70 degrees C) and find them to agree with prior published data. Using the new acid fractionation factor, published Delta(47)-temperature calibrations are recalculated. This analysis confirms a statistically significant Delta(47)-temperature calibration difference between data analyzed at 25 degrees C versus higher temperatures. The origin of the discrepancy remains unknown, but it appears that the acid fractionation factor is not the cause. (C) 2015 Elsevier B.V. All rights reserved.

Resume Introduction Les statines ou inhibiteurs de la 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) font partie des traitements les plus prescrits en France. Ils sont par ailleurs bien connus pour etre a l’origine d’intolerances musculaires de severite variable. Recemment, ils ont ete impliques dans la survenue de myopathies inflammatoires acquises liees a la presence d’anticorps anti-HMGCR. Cette nouvelle myopathie toxique reste mal connue par des cliniciens. Observation Nous rapportons l’observation d’une patiente de 61 ans, traitee depuis de nombreuses annees par une statine, qui presentait une myopathie invalidante des quatre membres d’evolution rapidement defavorable malgre l’arret du traitement. L’histoire clinique, les investigations paracliniques et notamment le dosage des anticorps anti-HMGCR permettaient de retenir le diagnostic de myopathie necrosante auto-immune a anticorps anti-HMGCR et d’initier un traitement immunosuppresseur efficace. Conclusion Un traitement par une statine peut demasquer ou etre a l’origine d’une authentique myopathie necrosante auto-immune en rapport avec la presence d’anticorps anti-HMGCR. Un traitement immunosuppresseur est necessaire et rendu licite par la demonstration du caractere auto-immun de l’affection depuis l’identification des anticorps anti-HMGCR qui peuvent desormais etre doses en routine.

[1] Quantitative paleoelevation histories can help explain both why and how widespread Cenozoic extension occurred in the Basin and Range Province of western North America. We present new estimates of preextensional paleoelevations for the northern and central Basin and Range using clumped isotope (Δ47) thermometry of lacustrine carbonates collected from each region. Comparison of carbonate Δ47-derived mean annual air temperature (MAAT) estimates (~16°C–20°C) for the Late Cretaceous–Eocene Sheep Pass basin of east central Nevada with published MAAT estimates for the Eocene, coastal northern Sierra Nevada (~20°C–25°C), suggests that the early Paleogene Sheep Pass basin had a paleoelevation of ≤2 km. Such a modest paleoelevation suggests that either (1) the proto–northern Basin and Range did not attain maximum paleoelevations of 3–4 km until the late Eocene–early Oligocene; or (2) the Sheep Pass basin was a local, high-relief (>1 km) setting contained within a >3 km orogenic highland (“Nevadaplano”). Similarity of Δ47-derived MAAT estimates (~17°C–24°C) for carbonates from the central Basin and Range and the near–sea level southern Sierra Nevada Bena basin indicate that middle Miocene paleoelevations in the Death Valley region were ≤1.5 km. These fairly low paleoelevations are incompatible with preextensional crustal thicknesses >52 km and indicate that mean elevation change was minor (≤500 m) and lithospheric mass was not conserved during >100% Neogene extension of the central Basin and Range, but was instead likely compensated by synextensional magmatic additions to the crust.

Calcite Mg/Ca is usually assumed to vary linearly with solution Mg/Ca, that a constant partition coefficient describes the relationship between these two ratios. Numerous published empirical datasets suggests that this relationship is better described by a power function. We provide a compilation of these literature data for biotic and abiotic calcite in the form of Calcite Mg/Ca = F(Solution Mg/Ca)H, where F and H are empirically determined fitting parameters describing the slope and deviation from linearity, respectively, of the function. This is equivalent to Freundlich sorption behavior controlling Mg incorporation in calcite. Using a power function, instead of a partition coefficient, lowers Phanerozoic seawater Mg/Ca estimates based on echinoderm skeletal material by, on average, 0.5 mol/mol from previous estimates. These functions can also be used to model the primary skeletal calcite Mg/Ca of numerous calcite phases through geologic time. Such modeling suggests that the Mg/Ca of all calcite precipitated from seawater has varied through the Phanerozoic in response to changing seawater Mg/Ca and that the overall range in Mg/Ca measured among various calcite phases would be greatest when seawater Mg/Ca was also high (e.g., “aragonite seas”) and lowest when seawater Mg/Ca was low (e.g., “calcite seas”). It follows that, during times of “calcite seas” when the seawater Mg/Ca is presumed to have been lower, deposition of calcite with low Mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and, in turn, lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites.

87 Sr/ 86 Sr measurements were carried out on samples of various macrofossil taxa recovered from the SMS AND-2A core with the main goal of improving age control of the core, and provide insight on marine climate at the time of carbonate precipitation. Shell material was carefully screened using cathodoluminescence (CL) to discern optimum areas for microdrilling. Powders thus obtained were in addition analyzed for Ca, Mg, Sr, Mn, and Fe contents to further evaluate the possibility of diagenetic alteration of the skeletal carbonate before measuring their Sr isotope compositions. Bivalves turned out to be the best candidates for isotopic determinations. Unaltered calcitic bivalves produced reliable 87 Sr/ 86 Sr age ranges. Aragonitic material, on the other hand, produced less radiogenic 87 Sr/ 86 Sr than anticipated, an unexpected result. Preliminary oxygen isotope determinations in calcite samples confirm contrasting marine climate conditions between the late Early Miocene (16.5–16.0 Ma), and the early Late Miocene (~ 11 Ma).

It has been proposed that Phanerozoic variation in the Mg/Ca ratio of seawater has resulted in variation in the primary mineralogies of shallow-water carbonates between aragonite- and calcite-dominated "seas." To test this hypothesis, this study estimates the Mississippian oceanic Mg/Ca and Sr/Ca ratios from the composition of marine carbonate cements and crinoidal carbonate, both of which have been suggested to faithfully record seawater chemistry. The early Mississippian (Tournaisian–Visean) Muleshoe Mound (Lake Valley Fm, Alamogordo, NM, U.S.A.) lends itself well to this endeavor because it contains abundant crinoids and marine carbonate cement. Stable isotope and elemental analyses, as well as cathodoluminescence characteristics, elucidate the diagenetic history of the bioherm and identify the least diagenetically altered materials for reconstruction of paleoseawater chemistry. Isotopic analyses document two diagenetic trends, one meteoric and one burial, diverging from values similar to literature estimates of Mississippian marine calcite (−1.5‰ δ18O, +4.5‰ δ13C). The burial diagenetic trend follows from this origin to more negative oxygen values (−7‰ δ18O, +3‰ δ13C). The meteoric diagenetic trend is defined by a meteoric calcite line at ~ −3‰ δ18O. Similarity of non-luminescent marine cement and non-luminescent crinoid isotopic and elemental compositions suggests that crinoidal carbonate was completely replaced by cement in the marine environment. Average elemental ratios for least altered marine cements are: Mg/Ca = 11 mmol/mol; Sr/Ca = 0.2 mmol/mol; Mn/Ca = 0.2 mmol/mol, and Fe/Ca = 0.4 mmol/mol. Early Mississippian seawater is estimated to have had a Mg/Ca ratio of ~ 0.2–0.3 mol/mol. This Mg/Ca ratio is compatible with the early Mississippian being an interval of "calcite sea" conditions and compares well with the Devonian estimate from marine cements, 0.5, though it is 5–10 times lower than estimates from mass balance modeling. The Sr/Ca for early Mississippian seawater was between 0.5 and 0.9 mmol/mol, which is much lower than previous estimates of ~ 6 mmol/mol from brachiopod calcite and argues for the development of a Sr sink (aragonite?) in the latest Devonian to earliest Mississippian.

A sectioned and polished specimen of the coral Archohelia vicksburgensis from the early Oligocene Byram Formation ( ∼ 30 Ma) near Vicksburg, Mississippi, reveals 12 prominent annual growth bands. Stable oxygen isotopic compositions of 77 growth-band-parallel microsamples of original aragonite exhibit well-constrained fluctuations that range between − 2.0 and − 4.8‰. Variation in δ 18 O of coral carbonate reflects seasonal variation in temperature ranging from 12 to 24 ° C about a mean of 18 ° C. These values are consistent with those derived from a bivalve and a fish otolith from the same unit, each using independently derived palaeotemperature equations. Mg/Ca and Sr/Ca ratios were determined for 40 additional samples spanning five of the 12 annual bands. Palaeotemperatures calculated using elemental-ratio thermometers calibrated on modern corals are consistently lower; mean temperature from Mg/Ca ratios are 12.5 ± 1 ° C while those from Sr/Ca are 5.8 ± 2.2 ° C. Assuming that δ 18 O-derived temperatures are correct, relationships between temperature and elemental ratio for corals growing in today’s ocean can be used to estimate Oligocene palaeoseawater Mg/Ca and Sr/Ca ratios. Calculations indicate that early Oligocene seawater Mg/Ca was ∼ 81% (4.2 mol mol − 1 ) and Sr/Ca ∼ 109% (9.9 mmol mol − 1 ) of modern values. Oligocene seawater with this degree of Mg depletion and Sr enrichment is in good agreement with that expected during the Palaeogene transition from ‘calcite’ to ‘aragonite’ seas. Lower Oligocene Mg/Ca probably reflects a decrease toward the present day in sea-floor hydrothermal activity and concomitant decrease in scavenging of magnesium from seawater. Elevated Sr/Ca ratio may record lesser amounts of Oligocene aragonite precipitation and a correspondingly lower flux of strontium into the sedimentary carbonate reservoir than today.

Stratigraphic relations, detailed petrography, and fine-scale geochemical and isotopic analysis of diagenetic phases formed within Kimmeridgian-age reefal carbonates of the Torrecilla en Cameros Formation of Northern Iberian Ranges in Spain indicate a complex history of alteration in marine, meteoric, burial, and uplift-related settings. The reefal succession is separated from the overlying Tithonian-Berriasian fluvial and lacustrine continental deposits by a single unconformity, which is marked by brecciation and karstification. Nevertheless, a record of three distinct stages of alteration associated with this unconformity is preserved in the succession of calcite cements present within the reefal carbonates. Recognition of temporally distinct episodes of meteoric diagenesis associated with a single unconformity is atypical in carbonate rocks, but this study illustrates how such complex systems can be deconvoluted. The first episode of subaerial exposure and meteoric alteration resulted in neomorphism of marine allochems and precipitation of a first generation of nonferroan clear calcite (NFC) cement. Following local faulting and brecciation, a second NFC precipitated throughout the reefal unit. Although similar in petrographic character, this phase of cementation is distinct in its isotopic composition, reflecting changes in the regional climate of the Iberian Peninsula during the early Tithonian. Another phase of meteoric alteration of the reefal unit is recorded by renewed corrosion, including dissolution of preexisting calcite cements and precipitation of prismatic calcite, prior to the deposition of Tithonian-Berriasian lacustrine and continental sediments. These continental units, in turn, record yet another episode of alteration by meteoric waters as NFC cements formed within intragranular and dissolution porosity in the lacustrine limestones. Notably, these cements are also distinct on the basis of their petrographic and geochemical character. Following subsidence and burial during Aptian to late Cretaceous times, migration of regionally derived fluids (perhaps in response to the onset of tectonic deformation to the north) led to cementation of saddle ankerite and ferroan calcite throughout the sedimentary sequence. Alteration of these burial-related diagenetic phases has subsequently taken place in response to the regional uplift during the Tertiary. This final episode of meteoric alteration is indicated by replacement of the ferroan calcite and ankerite by a cloudy nonferroan calcite.

Prior studies have recently demonstrated a positive linear covariance in the Sr and Mg contents of abiotic marine carbonates such that the slope of this relation is dependent upon the Sr/Mg ratio of seawater at the time of precipitation. Samples of ancient abiotic marine calcite cements that have undergone diagenesis through alteration by meteoric waters exhibit similar linear trends that range from primary marine ratios to low values that converge to zero. Thus, Sr/Mg ratios in diagenetic precipitates are potential proxies for the reconstruction of past variations in the elemental chemistry of ancient seawater. To expand upon limited existing Sr/Mg data, late Silurian and middle Triassic abiotic calcite cements were analyzed for stable isotope and minor element compositions. Silurian cements from the Pipe Creek Junior Quarry, Grant County, Indiana, show a linear covariance of Sr and Mg concentrations expressed by the following relationship: Sr/Ca = 0.0306 (±0.0091) Mg/Ca + 2.349 × 10−6 (±5.812 × 10−5). Triassic cements from the Cozzo di Lupo Formation, Sicily, Italy, exhibit a linear covariance given by: Sr/Ca = 0.0103 (±0.0009) Mg/Ca + 3.818 × 10−5 (±2.800 × 10−5). To evaluate the mechanisms by which Sr/Mg ratios are preserved in diagenetic calcite, a progressive rock-water interaction model was employed that traces the simultaneous evolution of δ13C, δ18O, Sr, and Mg contents of both fluid and solid phases. Results from modeling indicate that the Sr/Mg ratio of diagenetic calcite is primarily controlled by the ratio of distribution coefficients for Sr and Mg in calcite. When DSr/DMg is equal to unity, covariance of Sr/Ca and Mg/Ca in diagenetic calcite is linear, with a slope equivalent to the Sr/Mg value of the original dissolved solid phase. When DSr/DMg deviates from unity, the incorporation of Sr/Ca and Mg/Ca in diagenetic precipitates becomes progressively non-linear. A general agreement of theoretical and empirical results for Pliocene abiotic calcite cements of Frank and Lohmann (1996) indicates that model simulations adequately represent natural diagenetic settings. Additionally, at rock-water ratios where δ13C has converged to and δ18O has shifted toward original rock composition, elemental contents, and thus the Sr/Mg ratio, are effectively preserved. Thus, retention of original Sr/Mg chemistries is possible across a broad range of rock-water ratios, even where primary δ18O values have been significantly modified. Given the correlation between theoretical and empirical Sr/Mg results, we suggest that variation in the Sr/Mg composition of abiotic calcite cements reflect fluctuations in the Sr/Mg chemistry of ancient seawater.

Release of methane from large marine reservoirs has been linked to climate change, as a causal mechanism and a consequence of temperature changes, during the Quaternary and the Paleocene. These inferred linkages are based primarily on variations in benthic foraminiferal stable isotope signatures. Few modern analog data exist, however, to assess the influence of methane flux on the geochemistry or faunal characteristics of benthic foraminiferal assemblages. Here we present analyses of the ecology and stable isotopic compositions of living (Rose Bengal stained) and dead (fossil) foraminifera (>150 mm) from cold methane seeps on the slope off of the Eel River, northern California (500‐525 m), and discuss potential applications for reconstructions of methane release in the past and present. Calcareous foraminiferal assemblages associated with Calyptogena clam bed seeps were comprised of species that are also found in organic-rich environments. Cosmopolitan, paleoceanographically important taxa were abundant; these included Uvigerina, Bolivina, Chilostomella, Globobulimina ,a ndNonionella. We speculate that seep foraminifera are attracted to the availability of food at cold seeps, and require no adaptations beyond those needed for life in organic-rich, reducing environments. Oxygen isotopic values of the tests of living foraminiferal assemblages from seeps had a high range (up to 0.69‰) as did carbon isotopic values (up to 1.02‰). Many living foraminiferal isotope values were within the range exhibited by the same or similar species in non-seep environments. Carbon isotopic values of fossil foraminifera found deeper in the sediments (18‐20 cm), however, were 4.10‰ ( U. peregrina) and 3.60‰ ( B. subargentea) more negative than living d 13 C values. These results suggest that d 13 C values of foraminiferal tests reflect methane seepage and species-specific differences in isotopic composition, and can indicate temporal variations in seep activity. A better understanding of foraminiferal ecology and stable isotopic composition will enhance paleo-seep recognition, and improve interpretations of climatic and paleoceanographic change. © 2000 Elsevier Science B.V. All rights reserved.

Using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution, we examined the processes which affect the relationship between foraminiferal Mg/Ca and δ18O. We found that Globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their Mg content. On the seafloor, foraminiferal Mg/Ca is substantially altered by dissolution with the degree of alteration increasing with water depth. Selective dissolution of the chamber calcite, formed in surface waters, shifts the shell's bulk Mg/Ca and δ18O toward the chemistries of the secondary crust acquired in colder thermocline waters. The magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved. In spite of this shift the initial relationship between Mg/Ca and δ18O, determined by their temperature dependence, is maintained. We conclude that paired measurements of δ18O and Mg/Ca can be used for reconstructing δ18Owater, though care must be taken to determine where in the water column the reconstruction applies.

Water temperature, oxygen isotope composition and the δ13C of dissolved inorganic carbon were measured in 2 southern Michigan rivers, the Huron River and Fleming Creek, between late September 1990 and June 1992. The final full year of shell growth in 3 unionids collected in 1992 from these rivers was sampled for stable isotope analysis with a resolution of 30 μm. The δ18O of both shell nacre and the prismatic layer is accurately predicted by a fractionation relationship developed for biogenic aragonite. High resolution sampling of 3 species and bulk sampling of 3 other species suggest that all unionids adhere to this oxygen isotope fractionation relationship. This relationship is used to show that shell growth ceases below approximately 12°C. In these 2 settings the average δ18O value of shell (PDB scale) is within 0.5‰ of the average δ18O of river water (SMOW scale). Unionids can therefore be used in oxygen-isotope-based paleoclimatic and paleohydrologic reconstructions. In contrast, the carbon isotope ratio of shell is not accurately predicted by published fractionation factors between D.I.C. and carbonate. Shell δ13C is more negative than predicted values and the offset is highly variable suggesting a significant and variable incorporation of metabolic carbon into the shell carbonate.

High-resolution isotopic and geochemical data constrain long-debated marine vs. lacustrine depositional models for the Helena Formation, a carbonate unit within the Mesoproterozoic Belt Supergroup, Montana. Paired carbon and oxygen isotope and minor element analyses of microsamples from a diverse array of petrographic components effectively distinguish between primary and secondary isotopic signatures. δ13C values reflecting primary depositional conditions increase progressively from an average of −0.‰ near the base of the Helena Formation to an average of +1.4‰ near the top, over a stratigraphic thickness of 100–200 m. Because the base of this 1.6‰ shift coincides with a basin-wide facies change marked by the appearance of stromatolitic units and an increased occurrence of features indicative of evaporative conditions and subaerial exposure, the shift toward more positive δ13C values is interpreted to reflect an increase in biologic productivity coupled with increasingly evaporative conditions in the eastern part of the Belt basin. δ13C values from the lower part of the Helena Formation are similar to those from other marine carbonates of similar age and suggest that the Belt basin was marine during deposition of the Helena Formation. Taken as a whole, these relations suggest that Helena sediments were deposited in a shallow marine basin that was increasingly isolated from the open Mesoproterozoic ocean. Original δ13O values have been retained only in syndepositional and early diagenetic intergranular cements in oolitic grainstones that occur in the upper part of the Helena Formation. The most positive δ13O values derived from these cements (∼ −6.1‰ PDB) are up to 3‰ higher than previous estimates for the δ13O value of Mesoproterozoic marine calcite. However, because these phases are associated with facies deposited under restricted marine conditions, interpretation of δ13O values from these cements with regard to the composition of the open Mesoproterozoic ocean awaits study of additional coeval marine carbonate successions.

Minor element and isotopic compositions of marine bivalve calcite are frequently used as proxy records of seawater temperature and salinity. Although molluscan calcite is secreted at or near oxygen isotope equilibrium, the influence of metabolic activity (i.e., vital effects) on skeletal SrCa ratios and δ 13C values is not well known. We present measurements of skeletal chemistry from (a) consecutive samples milled in chronological order from the organism's final year of growth and (b) adjacent samples within three separate growth bands of the marine mussel Mytilus trossulus to investigate chemical disequilibrium effects among different parts of the shell. Seawater temperature and salinity were monitored for one year at Squirrel Cove, British Columbia. At the end of the year, a young, rapidly growing mussel (mussel A) and an old, slowly growing mussel (mussel B) were harvested from this site. Growth bands within the shells were sampled to provide a chronological record of shell carbonate chemistry. Results from consecutive samples show (1) a significantly higher average SrCa ratio in mussel A than that in mussel B and (2) δ 13C values that correlate well with salinity in mussel A but not in mussel B. Results from time-equivalent, adjacent samples show (1) higher SrCa and δ 13C values near the ventral margin than at time-equivalent regions on lateral margins of the shell and (2) little or no variation in δ 18O values. These observations suggest that skeletal chemistry (SrCa and δ 13C) is primarily controlled by rate of mantle metabolic activity and secondarily modified by variation in seawater salinity. Because metabolic activity in the mantle at the site of carbonate precipitation varies with shell curvature, the composition of calcite secreted along lateral margins is influenced to a greater extent by metabolic activity than calcite secreted coevally at the central margin of the shell. Hence, the chemistry of calcite secreted at the ventral margin is precipitated in near-equilibrium with seawater and allows accurate estimation of seawater SrCa ratios and carbon isotopic composition. In contrast, shell precipitation along lateral margins is dominantly controlled by metabolic activity, and calcite chemistry is not in equilibrium with ambient seawater. In this context, we present a biomineralization model where variations in skeletal SrCa and δ 13C values are explained in the context of mantle metabolic activity and seawater salinity.

Tectono-stratigraphic models of foredeep sedimentation have generally presumed a direct link between changing rates of tectonism and concomitant sedimentological response as manifested by change in thickness, composition or depositional environment of sediment accumulating in adjacent basins. Lacustrine limestone units within the early Cretaceous fluviaMacustrine Gannett Group of western Wyoming exhibit systematic variation in several geochemical proxies of relative rates of precipitation and evaporation, indicating that lakewater chemistry was controlled by variation in regional climate. Change in proportion of allochthonous terrigenous clastic vs. autochthonous carbonate deposition, as well as carbonate Mg/Ca ratio and stable isotopic composition, occurs at two scales. Metre-scale alternation of micritic limestone and argillaceous marl is accompanied by mineralogical and isotopic variation within individual beds, indicating preferential carbonate accumulation during intervals of decreased regional meteoric precipitation relative to lake-surface evaporation. Limestone deposition began during intervals of maximum aridity, and decreased as increased meteoric precipitation-driven flux of terrigenous clastic sediment overwhelmed sites of carbonate accumulation. Similar upsection variation in limestone mineralogy and isotopic composition at a scale of tens of metres reflects the multiple processes of long-term increase in meteoric precipitation and lakewater freshening prior to influx of terrigenous sediment, across-basin fluvial-deltaic progradation, and renewed accumulation of riverine terrigenous units. Such trends suggest that formation-scale alternation between fluvial clastic and lacustrine carbonate deposition was controlled by climate change.

Fibrous, high Mg-calcite (HMC) marine cement from the Pliocene Hope Gate Formation, Jamaica occurs as 1–2 mm thick isopachous coatings on skeletal grains in local occurrences of Halimeda grainstone and is commonly syntaxially overgrown by clear, prismatic low Mg-calcite (LMC) interpreted as a mixing zone cement. The presence of inclusions at discrete levels in the fibrous marine cement produces internal growth banding wherein clear, inclusion-free zones alternate with cloudy, inclusion-rich zones. Clear zones have uniform Mg contents (13.7 ± 0.2 mol% MgCO3) and stable isotopic compositions (δ18O = 0.8 ± 0.1%; δ13C = 3.2 ± 0.1%) consistent with a marine origin. In contrast, inclusion-rich zones contain fine-scale intergrowths of LMC, dolomite, and HMC, and have carbon and oxygen isotopic compositions that define a linear mixing trend between compositions of inclusion-free HMC and prismatic LMC cement overgrowths. Petrographic observations indicate that the mixing trend defined by isotopic data from inclusion-rich zones is related to the addition of large volumes of secondary LMC as cement in primary pore space, as intergrowths among fibers of primary HMC marine cement, and/or as replacements of primary HMC crystallites. Dolomite is present in inclusion-rich zones in volumes generally compatible with amounts predicted to result from the closed-system stabilization of HMC to LMC and dolomite. In addition, isotopic and geochemical data from some areas of inclusion-rich zones are consistent with mass balance constraints assuming a closed-system, which require that bulk isotopic and geochemical compositions are retained. These relations suggest that small remnants of HMC in inclusion-rich zones of Hope Gate marine cement have altered to LMC and dolomite while isolated from system pore fluids, such that primary marine isotopic signatures were imparted on diagenetic products. Similar petrographic, geochemical, and isotopic relations have been noted in ancient occurrences, suggesting that δ18O and δ13C values from some former marine cements should be useful for the determination of isotopic compositions of original marine cements and ultimately for the determination of ocean chemistry in the geologic past.

The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have δ13C and δ18O values as high as +16.5‰ and +9.2‰ respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130 000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.

We have applied UPb, trace element, and Sr isotopic analyses to subsamples of a single specimen of the Capitan Formation from the Permian Reef Complex of New Mexico with the goal of calibrating the Late Permian time scale and understanding the factors that set and later alter the initial U/Pb ratio and Pb isotopic composition of the limestone. The data indicate a limestone UPb isochron age of 249.8 ± 4.7 Ma (2σ) and a PbPb age of 249 ± 18 Ma. In addition, with μ values (≜238U/204Pb) ranging up to 4200, among the highest ever recorded for a limestone, we also obtain precise single sample 206Pb*/238U, 207Pb*/235U and 207Pb*/206Pb* ages indicating averages of 250.2 ± 3.2 Ma, 250.0 ± 2.8 Ma, and 256.3 ± 3.5 Ma, respectively. Comparison of the average Pb*/U age of 250 ± 3 Ma for the latest Guadalupian with published radiometric constraints for the Late Permian suggests preservation of essentially the depositional age. Trace element and Sr isotopic analyses highlight four samples that preserve a chemical signature expected for primary marine aragonite, including high Sr (> 3000 μg/g), high U (> 2900 ng/g), low initial Pb ( 26 μg/g), and 87Sr/86Sr (> 0.7073) as well as the lower U and higher Pb, Mg, and Fe expected for a diagenetically altered carbonate. These data clearly delineate the diagenetic alteration of the limestone and allow us to demonstrate the critical importance of diagenesis in controlling the spread in μ values, primarily through the expulsion of U but with some incorporation of Pb. The high μ are simply a function of preservation of original carbonate chemistry and not of U scavenging by Fe oxyhydroxides or phosphates. Although it has long been suspected that limestone UPb and PbPb isochron ages reflect the timing of early diagenesis, as opposed to a strictly depositional age, this is the first study in which this is conclusively demonstrated to be the case. The UPb data also suggest a late-stage (Neogene) alteration that disturbed the system mainly via the redistribution of U. Intriguingly, the four samples that best retain their primary marine aragonitic geochemistry also best maintained a closed UPb system. Most of the subsamples with comparably high μ values but clearly altered trace element compositions showed UPb systematics indicating late disturbance. Unfortunately it is not yet possible to determine which processes are responsible for this late-stage alteration and it is unclear exactly how to avoid such alteration in future work.

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.

Petrological and geochemical examination of Upper Cretaceous chalks from the Abu Roash area and Bahariya Oasis (Egypt) provide insights into factors which control porosity development in fine-grained calcitic carbonates. Petrological studies indicate that primary skeletal fabrics of coccoliths and foraminifera in micrites of Abu Roash are well preserved. In contrast, primary skeletal fabrics of the Bahariya Oasis chalks are pervasively altered. Geochemical analysis of these chalks reveals a striking contrast in composition. The high porosity chalks at Bahariya Oasis have depleted isotopic values ( δ 13 C = −5.0‰, δ 18 O = −8.9‰ PDB ) and low concentrations of trace elements (Sr and Na). In contrast, the low porosity chalks at Abu Roash are less depleted in terms of their isotopic values ( δ 13 C = 1.0‰, δ 18 O = −4.0‰ ) and possess elevated concentrations of Sr, Na, Fe and Zn. Importantly, fracture and collapse veins, present only at Abu Roash, are filled with calcite whose δ18O value is coincident with that of altered micrites at Bahariya Oasis. The lithic fragments cemented by this calcite, however, retain compositions identical to other lithified limestones at Abu Roash. Differences in fabric and chemistry of the pelagic marine micrites observed between Abu Roash and Bahariya Oasis reflect the degree of early lithification and compaction prior to exposure and meteoric diagenetic alteration associated with regional Late Cretaceous/pre-Eocene subaerial unconformity.

Paleomagnetic study of subsurface samples of the Upper Ordovician Trenton and Black River Limestones collected from six industrial wells in the southern part of the Michigan Basin yields a well-clustered characteristic direction (mean Dec/Inc of 170.6°/ - 10.9°, $\alpha_{95} = 7.6^\circ$) after rotation of random declinations by aligning a present day field overprint with geographic north. The corresponding paleomagnetic pole at 52.4°N 111.0°E is similar to those of the remagnetized carbonates in Ontario and New York State, suggesting that the Michigan formations were also remagnetized in the late Paleozoic. Rock magnetic characterizations suggest that the characteristic magnetization is carried by single and pseudosingle domain magnetite grains. Very fine rounded single crystal magnetite grains, believed to be products of chemical precipitation, observed by scanning and scanning transmission electron microscope (SEM/STEM) are inferred to be the carrier of the remagnetization. Paired carbon isotope analys...

Textural and compositional attributes of dolomite and calcite within Lower Cretaceous Peterson Limestone micrite indicate that these phases coprecipitated during stabilization of precursor magnesian calcite in a largely closed diagenetic system. Uniform difference in the isotopic compositions of contained carbonate phases over a range of whole rock compositions reflect equilibrium partitioning of18O and13C between dolomite and calcite. Diagenetic dolomite is enriched in18O by 4.4 o/oo and in13C by 1.8 o/oo relative to co-existing calcite, values which are discemibly different from expected open system mixing trends, and which are in general agreement with high temperature fractionation data extrapolated to sedimentary temperatures as well as with calculated δ18O dolomite-calcite from Holocene dolomites.

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.

Sr isotope data from two boreholes within the lagoon at Enewetak Atoll have been used to evaluate the use of such data to correlate, date, and monitor sea level change in shallow water carbonate sequences. Correlative stratigraphic intervals of relatively invariant δ87Sr, separated by abrupt transitions to lower δ87Sr with increasing depth, are recognized in both boreholes. Conversion of δ87Sr values to age via calibration with the seawater δ87Sr trend with age indicates that correlative and synchronous deposition of atoll sediments occurred at ∼ 0.4, 1.2, and 2.1 Ma. In contrast, a ∼5 m.y. hiatus is recognized in one borehole but not the other. Sr isotope stratigraphy (SIS) is a powerful stratigraphic and chronostratigraphic tool in shallow water carbonate sequences only when significant secular variation of δ87Sr occurs and retention of depositional δ87Sr values is demonstrated. The latter is best demonstrated when δ87Sr data, are integrated with δ18O, δ13C, Sr content data and petrographic observations. Several diagenetically altered intervals have greater δ87Sr values, low δ13C values, and low Sr/Ca ratios relative to adjacent intervals, a combination that is consistent with open-system meteoric diagenesis. Calcite cements from these intervals have early Pleistocene (∼1.2 Ma) δ87Sr values despite their occurrence well within the late Pliocene (∼2.1 Ma) sequence. Thus local sedimentological and diagenetic processes have produced intralagoon variability in the SIS of the two boreholes, complicating subsurface stratigraphic correlations. The occurrence of anomalously young calcite cement relative to adjacent limestone is a direct response of the interaction of sea level change and meteoric phreatic diagenesis whereby overlying metastable carbonates, with greater δ87Sr values, are dissolved during periods of atoll emergence and sea level lowstand liberating Sr and soil-gas CO2 to the pore fluid, which is then incorporated into downflow meteoric phreatic diagenetic calcite. The stratigraphic relation between the sites of carbonate dissolution and diagenetic calcite precipitation suggest an early Pleistocene (∼1.2 Ma) sea level fall of between 34 and 64 m. Moreover, the stratigraphic position and inferred age of paleophreatic lens calcite when backtracked for subsidence suggest that early Pleistocene (∼1.2 Ma) sea level was ∼72–81 m (range of 60–100 m) below present sea level. The Enewetak record of early Pleistocene sea level lowstand elevation agrees reasonably well with proxy sea level records inferred from sequence stratigraphy and deep-sea foraminiferal δ18O data. However, the Enewetak record of the amplitude of early Pleistocene sea level change is more consistent with the deep-sea foraminiferal δ18O proxy than the sequence stratigraphy proxy.

Alpers, C.N., Dettman, D.L., Lohmann, K.C. and Brabec, D., 1990. Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin. In: S.E. Kesler (Editor), Soil and Rock Gas Geochemistry. J. Geochem. Explor., 38: 69-86. Stable isotope ratios of oxygen and carbon were determined for CO2 in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO2 concentrations detected previously by McCarthy et al. (1986). Values of tSl3C in soil gas CO2 from depths between 0.5 and 1.0 m were found to range from - 12.68%0 to -20.03%0 (PDB). Organic carbon from the uppermost meter of soil has ~ t3C between -24.1 and -25.8%0 (PDB), indicating derivation from plant species with the Ca (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C3 and C4 (HatchSlack) plants, together with atmospheric CO2 are the likely sources of carbon in soil gas CO2. Values of t~tsO in soil-gas CO2 range from 32 to 38%0 (SMOW). These ~sO values are intermediate between that calculated for CO2 gas in isotopic equilibrium with local groundwaters and that for atmospheric CO2. The 6IS o data indicate that atmospheric CO2 has been incorporated by mixing or diffusion. Any CO2 generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters. The isotopic composition of soil-gas CO2 taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the t~3C value of soil-gas CO2 was observed, contrary to what would be expected if the anomalous CO2 were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C3 plant material were responsible for most CO2 generation both in the vicinity of the massive sulfide and in the "background" area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO2 anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. ( 1983 ) and McCarthy et al. ( 1986 ). Further work is needed on seasonal and daily variations of CO2 concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.

Burial diagenesis of the Upper Jurassic Torrecilla Reef Complex is recorded by a complex paragenetic sequence initiated with emplacement of ferroan calcite cements and followed by the precipitation of ferroan saddle dolomite. Ferroan calcite cements contain micro-sized dolomite inclusions (MDIs). Elemental and isotopic compositions of MDIs are virtually indistinguishable from those of ferroan saddle dolomite cements. In contrast to other scenarios for the formation of microdolomite inclusions that invoke either a magnesian calcite precursor or incomplete dedolomitization, the paragenetic relations of MDIs to their host ferroan calcite and their geochemical composition implies formation by fine-scale burial replacement of ferroan calcite by fluids associated with the emplacement of ferroan saddle dolomite. Petrographic observations and mass-balance considerations suggest that it is unlikely that Fe and Mg incorporated into ferroan dolomite could have been derived from the cannibalization of the reefal host rock. Rather, dolomitizing fluids were likely associated with the hydrothermal, low-grade metamorphic event that affected the Cameros Basin during the middle to Late Cretaceous. Interaction of these fluids with preexisting burial cements produced MDIs in the ferroan calcites.

Freshwater habitats have to cope with dynamic changes in nutrient availability, temperature and light. Most freshwater habitats become increasingly eutrophicated with the result of high primary production. High „standing crop” of pigmented phytoplankton cells absorb PAR efficiently in the upper surface layer leading to a steep light gradient characterised by dynamic changes in light quality and quantitiy. In addition, freshwaters are charged with organic carbon from waste waters or from natural sources. In many cases these compounds themselves absorb visible light („gilvin“) leading to strong attenuation of light, especially when the waters are shallow and turbid [1]. The light climate together with changes in nutrient availabily have selective influence on the growth rates of the different phytoplankton taxa leading to very fast changes in phytoplankton population structure during the year.

Pre-Pennsylvanian low-magnesium calcite cements from the Mississippian Lake Valley Formation of south central New Mexico comprise three major cathodoluminescent zones. Previous work by Meyers and Lohmann (1985) suggested that interzonal isotopic and minor-element variation resulted from spatial and temporal exchange between meteoric water and marine carbonate within a single meteoric system, that the carbonate for cementation was derived from dissolution of metastable carbonate grains, and that any contribution from marine water was insignificant. Isotopic analysis of the first two cathodoluminescent zones, using detailed micron-scale sampling methods, revealed a record of simultaneous variation in 18O and 13C values at the intrazonal scale that is incompatible with compositional change anticipated from the evolution of meteoric fluids in response to progressive water-rock interaction. Rather, isotopic data, in conjunction with stratigraphic and petrographic constraints, suggest that regionally extensive mixing between marine and meteoric water along the basal margin of the phreatic lens is the process responsible for the cementation of much of this platform succession. Although trends of progressive 18O and 13C depletion in cement composition are compatible with precipitation from a fluid that evolved from a marine- to a meteoric-dominated composition, low Mg and Sr concentrations are problematic in the context of experimentally derived partition coefficients for calcite. Because of compelling evidence indicating a mixing-zone origin for these cements, however, we attribute low Mg and Sr contents to temperature and/or PCO2 effects, or to differences in conditions under which calcite is precipitated.

Vertical tension fractures that crosscut proximal flank beds exposed in the Pipe Creek Junior Quarry in north-central Indiana formed during compaction of underlying interreefal carbonate mud, and are commonly filled with well-sorted pisolitic grainstone made up of coated grains ranging up to 1 cm in diameter. Pisolith cortices are petrologically indistinguishable from marine cement between pisoliths, from marine cement lining tension fractures, and from marine cement lining rugs in surrounding flank beds. Isotopic signatures for these components are also consistent (13C = -0.6 to 2.3 PDB; 18O = -8.7 to - .9 PDB). In addition, pisolites are present exclusively within tension fractures, and commonly show lateral size grading, with grain size increasing toward fracture interiors. These features show that pisoliths are in fact mobile marine cements that precipitated from Silurian seawater in synsedimentary tension cracks that formed soon after deposition of flank beds. Requisite conditions for the formation of such large coated grains include high-energy, shallow-water settings, where sea level is generally coincident with the depositional surface of the reefal complex. This origin supports an interpretation that allochthonous material forming flank beds was predominantly generated immediately upslope on wave-swept platforms, that in situ accumulation of biotic debris was insignificant in the formation of the Pipe Creek Junior complex, and that many of the larger Silurian "reefal" buildups like Pipe Creek Junior throughout the midwestern United States pr bably had a similar origin.

Stable isotopic compositions of cyclic Middle Pennsylvanian Gobbler Formation limestones record two stages of alteration. Early open-system meteoric diagenesis during episodic subaerial exposure stabilized cycle-top carbonate in the presence of meteoric fluid at high water/rock ratios (400-4000). Subsequent closed-system burial diagenesis stabilized meteoric and residual marine carbonate in the presence of connate pore fluids at elevated temperatures (to 150°C) and low water/rock ratios (1.0-10). Present 18O distributions record normal to high geothermal gradients (27-86°C/km) during burial stabilization. High basin-center geothermal gradients probably resulted from elevated heat flow due to rapid crustal extens on and subsidence of the Orogrande Basin during the Late Pennsylvanian. A dike-injection lithospheric extension model with 40% "oceanization" best accounts for subsidence and geothermal patterns of the Orogrande Basin.

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.

Coeval crystallographic growth sectors were microsampled from a fracture-filling calcite cement crystal with well-developed crystal faces and concentric zoning to test empirically for isotopic sector zoning. Isotopic and minor element homogeneity among coeval parts of the {0112} and {2131} sectors suggests that isotopic sector zoning does not occur in calcite. Empirical results are compatible with theoretical considerations based on crystal surface geometry, growth mechanisms, and a thermodynamic growth model.

Stable oxygen isotopic compositions of deep-marine calcite spars contained in Early and middle Cretaceous, bank-interior limestones exposed erosionally on the Bahama escarpment suggest that these spars are Late Cretaceous to Miocene in age, two-thirds of the spars being middle Eocene or older in age. The Eocene and older spars are identical to pelagic and fore-reef strata unconformably overlying truncated bank-interior carbonates exposed on the Bahama escarpment. Spars contained in limestones taken from the northwest Bahama escarpment are nearly all (20 of 22 analyses) Late Cretaceous (Campanian and Maastrichtian) to middle Eocene in age. Most (12 of 20 analyses) spars from southeast Bahama escarpment limestones are late Eocene or younger in age. Thus, isotopic data are consistent with the initiation of erosion of the Bahama escarpment in the Late Cretaceous and its continuation until the middle of the Eocene. In addition, these data suggest that erosion of the southeast Bahama escarpment continued into, or was renewed during, the late Eocene and possibly the Oligocene-Miocene. Continued, or renewed, erosion of the southeast Bahama escarpment probably reflects tectonic interaction with the Caribbean-North American plate boundary along the Puerto Rico trench, or the flow of Antarctic Bottom Water only at the base of the southeast escarpment.

Monomineralic, abiotic marine cements formed in low-latitude Phanerozoic reefs provide the direction and amplitude of secular variation of {delta}{sup 13}C and {delta}{sup 18}O in marine calcite and defines two end member compositions - 580 to 360 my ({minus}7 to {minus}5{per thousand}{delta}{sup 18}O{sub PDB}) and 360 to present ({minus}3 to 0{per thousand}{delta}{sup 18}O{sub PDB}). Sampling of the Devono-Carboniferous transition (375-320 my) at several global sites reveals a rapid change in carbonate isotopic compositions. Bracketed within Fammenian to Early Visean-aged strata, a 7 to 15 my time interval, this shift corresponds to a 2% offset in mean {delta}{sup 13}C and 3-4% offset in {delta}{sup 18}O. The abruptness of such change, and its overall correlation with variations in {sup 87}Sr/{sup 86}Sr, {delta}{sup 34}S, {delta}{sup 13}C, and Li/Al ratios in marine sediments suggests a primary offset in marine water composition.

For geologists and geochemists concerned about the chemical and thermal evolution of oceans, deciphering primary marine 13C and 18O values from ancient marine carbonates can provide useful data for the physical and chemical modelling of ancient oceans. Although abiotic aragonite and magnesian calcite marine cements have commonly been overlooked because of their susceptibility to diagenetic alteration, petrographic and chemical analysis of radiaxial fibrous calcite from two Upper Devonian (Frasnian) pinnacle reefs (Golden Spike and Nevis) of central Alberta indicates that these magnesium-rich cements preserve original marine 18O and 13C values. In the pursuit of primary marine isotopic compositions it is necessary to characterize the isotopic compositions of diagenetic cement phases. Subaerial exposure and meteoric diagenesis of the interior of the Golden Spike reef is recorded by vadose cements, caliche, and clear, non-ferroan, meteoric calcite spar with distinctive black and yellow cathodoluminescence (CL) zones. 18O-13C crossplots of caliche and meteoric phreatic cement data form an "inverted J-curve" typical of rock-water reactions in meteoric systems. A late phase of calcite spar with variable inclusion density and Fe2+ content and diffusely-zoned red-orange CL is associated with late fractures and stylolites, and it precipitated during subsequent burial of the reef. Unaltered centers of radiaxial fibrous cement crystals are nonluminescent, inclusion-free, and enriched in magnesium, while diagenetically altered crystal terminations, inter-crystalline boundaries, and crystal substrates are brightly luminescent, inclusion-rich (microdolomite and fluid inclusions), low-Mg calcite. Altered cloudy marine cements have variable 18O and invariant 13C values which define a trend that diverges from isotopically heavier unaltered marine cements. The isotopic compositions of inclusionrich marine cement are coincident with those of meteoric phreatic spars. Although burial spars are present in all samples, petrographic and chemical data suggest that portions of the marine cement crystals were altered by meteoric fluids. The 18O values of unaltered radiaxial fibrous calcite are similar (invariant) throughout both reefs, although marine cements from older (deep) parts of both reefs are enriched in 13C relative to younger (shallow) parts. This records temporal changes in the 13C value of marine bicarbonate within the Alberta Basin. Variability of 18O and 13C values of unaltered marine cements is very small (± 0.5 for a given suite of samples), making these cements ideal for h gh time resolution studies of the isotopic evolution of ancient oceans. The presence of well preserved magnesium-rich calcite from Late Devonian reefal sequences suggests that marine cements throughout the Phanerozoic should be examined more carefully, as they may preserve primary isotopic signatures. Independent confirmation of primary isotopic compositions is found in similar values for marine cements from Middle Frasnian reefs of the Canning Basin (Australia) and the Dinant Synelinorium (Belgium). The 18O values for the well preserved marine cements of this study are considerably lower than modern marine carbonate values (by 3 to 4). Detailed chemical and petrographic analysis indicates that diagenetic alteration is not the cause of these low 18O values. In addition, such a difference cannot be fully explained by warmer Late Devonian oceans. These data suggest that Late Devonian seawater had a 18O value lower than modern oceans. This in turn, suggests that the balance of low temperature silicate weathering and high temperature seawater-basalt exchange reactions must have been different from today's.

The oxygen isotopic composition of oxygen in minerals and oxygen in water. This sea water is determined by interactions with equilibrium fractionation is a strong function of fresh, silicate rocks. Interactions at high temperature varying, for many minerals, from about

Two distinct varieties of Mississippi Valley-type ore occur in the northern Arkansas zinc-lead district: (1) open-space fillings in solution collapse breccia, and (2) strata-bound replacement deposits generally conformable with their carbonate host rocks. The relationship between these two types of ore is unusually well exposed in the Monte Cristo mine, as is the relationship between these ores and their host rocks. Primary pore spaces in the micritic and stromatolitic limestones and dolostones (Ordovician Everton Formation) were largely occluded by cementation during diagenesis. Dissolution of limestone, sometime after the Early Mississippian period, locally produced solution collapse breccias composed chiefly of dolostone fragments in a quartz sand and dolomite matrix. After lithification of this matrix, further fracturing opened channels later used by the ore fluid.Mineralization (deposition of sphalerite and pink sparry dolomite) was preceded and/or accompanied by silicification of limestone in the bedded orebodies and the outer margin of the breccias. Fluid inclusion and stable isotope (C and O) data suggest a mixing of two brines during mineralization, both of probable deep burial origin. The mixtures were hot (90 degrees - 150 degrees C) and highly saline (19-25 equiv. wt % NaCl) and were displaced by cool, dilute, presumably meteoric waters during deposition of late gangue quartz and calcite. It is noteworthy that, in the Monte Cristo mine, the bedded and breccia mineralizations are physically and mineralogically continuous and yield indistinguishable fluid inclusion, stable isotope, and rare earth element signatures. Hence, the bedded ores, which possess many characteristics of Mississippi Valley-type deposits attributed to a syngenetic or diagenetic origin elsewhere in the world, formed simultaneously with the discordant breccia ore and are, at least in northern Arkansas, epigenetic in origin.

Inclusions in synsedimentary prismatic calcites of the Mississippi Lake Valley Formation bioherms, New Mexico, are 1-10-micron subhedral to euhedral "microdolomites" which occur in crystallographic continuity with the host calcites. These microdolomite inclusions occur exclusively in echinoderm grains, in the cloudy prismatic calcites, and in rare cloudy syntaxial overgrowths on echinoderms; they do not occur in adjacent mud nor in other skeletal grains. Cathodoluminescence of the cloudy dolomite-rich prismatic calcites reveals a nonluminescing, radial fibrous component, and an interfiber luminescing calcite. The radial fibrous mosaic, whose fibers are length slow, reflects the fabric of the cement precursor to the cloudy radiaxial prismatic calcites; whereas the luminescing calcite i interpreted as younger syntaxial interfiber pore filling. The microdolomite inclusions are restricted dominantly to the nonluminescing portions. This restricted distribution of the microdolomite inclusions argues for an autochthonous source for the Mg on a scale of single grains or crystals, and thus indicates a high-Mg calcite precursor to the cloudy calcites. Infilling of the interfiber pores of the radial fibrous mosaic with the luminescent, low-Mg calcite effectively isolated the metastable high-Mg calcites from conventional diagenetic stabilization. The Mg was redistributed within individual grains and fibers into microdolomite inclusions and low-Mg calcites by either an incongruent-dissolution or solid-state exsolution process. The presence of the microdolomite inclusions in cloudy calcites is prime evidence for a high-Mg calcite precursor, and, therefore, for a marine precipitational environment. We suggest this criterion is widely applicable to coarse prismatic fabrics throughout the geologic column, and is of particular importance considering that crystal fabric alone is generally inadequate in distinguishing coarse prismatic cements precipitated in marine waters from those precipitated in fresh meteoric waters.

Inclusion-rich echinoderm-syntaxial cements comprise up to 38% of the intergranular cements in Mississippian skeletal calcarenites of the Lake Valley Fm., N.M. These cements have crystal morphology, substrate selectivity, and luminescence characteristics identical to previously interpreted fresh water phreatic cements, yet contain microdolomite inclusions, implying that they were originally Mg calcite. These petrographic characteristics, plus the restriction of the microdolomite-rich syntaxial cements to the lower part of the Lake Valley Fm., argue for their having precipitated in the zone of mixing between meteoric and marine phreatic groundwaters. Petrography of these cements, their association with interpreted fresh water phreatic cements, and their regional restriction to the south, suggest that the mixing zone cements were formed mainly daring the slowing down stages of a late Mississippian global sea level drop. These cements plus other microdolomite-rich calcites reported on elsewhere indicate that coarse crystals of Mg calcite commonly act as closed or semi-closed systems during re-equilibration to low-Mg calcite, rather than undergoing complete loss of Mg through conventional incongruent dissolution.

Carbonate speleothems and precipitating fluids from Carlsbad Caverns, New Mexico, have been analyzed for their major and minor element and stable isotopic compositions in order to evaluate processes controlling the chemical evolution of cave water and factors determining the mineralogy and composition of cave carbonates. Chemistry and isotopic composition of fluids are determined by rates of CO2 degassing, evaporation, and carbonate precipitation. Evaporation and calcium carbonate precipitation cause changes in the Mg/Ca ratio of fluids which, coupled with changes in CO3 = content, control the minor element chemistry and mineralogy of the precipitating phase.