92 research outputs found
In situ geochemistry of middle Ordovician dolomites of the upper Mississippi valley
The dolomitization and diagenetic history of Ordovician carbonates of southern Wisconsin is complex. Previous studies attributed dolomitization to various diagenetic factors and environments. In this study, high-resolution, in situ laser ablation inductively coupled mass spectrometry analysis of rare earth element patterns of dolomite was used to assess the diagenetic fluids responsible for dolomitization of the Ordovician Decorah Formation. Integrated geochemical data and petrographic evidence suggest that the dolostones are formed in two different diagenetic realms: shallow burial and hydrothermal. Shallow burial dolomites exhibit three distinct rare earth element patterns. Dolomite from the middle portion of the Guttenberg Member exhibits light rare earth element enrichment consistent with early burial dolomitization. Dolomites of the Carimona, Specht\u27s Ferry and Lower Guttenberg members are often burrow associated and exhibit medium rare earth element enrichment associated with Fe-oxide desorption in anoxic porewaters. Leaching of Mg from co-occurring volcanic ash during alteration is a probable source that contributed to the dolomitization. Extensively dolomitized samples in the upper Guttenberg and Ion Member exhibit evidence of hydrothermal dolomitization. The relationship of these heavily dolomitized samples to interbedded limestones provides evidence for a recently proposed hydrothermal dolomitization model invoking pressure solution of calcite and precipitation of dolomite. These early burial and hydrothermal depositional models are consistent with models proposed for overlying and underlying Ordovician dolostones
Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas
© 2016 Elsevier B.V. Molybdenum (Mo) isotope variations recorded in black shales provide important constraints on marine paleoredox conditions. However, suitable shales are not ubiquitous in the geologic record. Moreover, reliable reconstruction of Mo isotope records from shales requires deposition from a water column containing very high concentrations of sulfide-a condition which is both rare and difficult to verify with certainty when examining preserved sediments. The utility of Mo isotopic records could be improved if reconstructions were possible using alternative lithologies, such as marine carbonates, which are more abundant in the geologic record.Here, we focus on the role of early diagenesis in determining the Mo isotopic composition preserved in shallow-water carbonate sediments from four push cores collected in different shallow-water depositional environments in the Bahamas. In contrast with carbonate primary precipitates, which generally contain 10 ppm Mo). The extent of this authigenic enrichment appears to be driven by high concentrations of hydrogen sulfide in the porewaters. In cores with the least authigenic Mo enrichment and lowest pore water sulfide, Mo isotopes are ~1-1.2‰ lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2-2.5‰), even under oxic bottom water conditions. However, the sensitivity of bulk carbonate δ98Mo to syndepositional diagenetic conditions potentially complicates interpretation of a carbonate Mo isotope paleoredox proxy. Robust reconstruction of seawater Mo isotopic composition from carbonates will thus require the ability to place constraints on early diagenetic conditions of pore waters at the time of deposition. We show that in order to record seawater Mo isotope values, carbonate pore waters must contain 50-100 μM H2Saq, which is achieved only in organic- and sulfide-rich carbonate sediments
Biological effects on uranium isotope fractionation (\u3csup\u3e238\u3c/sup\u3eU/\u3csup\u3e235\u3c/sup\u3eU) in primary biogenic carbonates
© 2018 Elsevier Ltd Determining whether U isotopes are fractionated during incorporation into biogenic carbonates could help to refine the application of 238U/235U in CaCO3 as a robust paleoredox proxy. Recent laboratory experiments have demonstrated that heavy uranium (U) isotopes were preferentially incorporated into abiotic aragonite, with an isotope fractionation of ∼0.10‰ (238U/235U). In contrast, no detectable U isotope fractionation has been observed in most natural primary biogenic carbonates, but the typical measurement precision of these studies was ±0.10‰ and so could not resolve a fractionation of the magnitude observed in the laboratory. To resolve this issue, we have developed a high precision 238U/235U method (±0.02‰ 2 SD) and utilized it to investigate 238U/235U in primary biogenic carbonates including scleractinian corals, calcareous green and red algae, echinoderms, and mollusks, as well as ooids from the Bahamas, Gulf of California, and French Polynesia. Our results reveal that many primary biogenic carbonates indeed fractionate U isotopes during U incorporation, and that this fractionation is in the same direction as observed in abiotic CaCO3 coprecipitation experiments. However, the magnitude of isotope fractionation in biogenic carbonates is often smaller than that predicted by abiotic CaCO3 coprecipitation experiments (0.00–0.09‰ vs. 0.11 ± 0.02‰), suggesting that one or more processes suppress U isotope fractionation during U incorporation into biogenic carbonates. We propose that closed-system behavior due to the isolation of the local calcificiation sites from ambient seawater, and/or kinetic/disequilibrium isotope fractionation caused by carbonate growth kinetics, explains this observation. Our results indicate that U isotope fractionation between biogenic carbonates and seawater might help to constrain U partition coefficients, carbonate growth rates, or seawater chemistry during coprecipitation
Uranium isotope fractionation during coprecipitation with aragonite and calcite
© 2016 Elsevier Ltd. Natural variations in 238U/235U of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~8.5, with heavier U isotopes preferentially enriched in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007 + 0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3(aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, PCO2, Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (\u3c0.13‰), but are nevertheless potentially significant because of the small natural range of variation of 238U/235U
Testing the early Late Ordovician cool-water hypothesis with oxygen isotopes from conodont apatite
© 2017 Cambridge University Press. Latest Sandbian to early Katian sequences across Laurentia\u27s epicontinental sea exhibit a transition from lithologies characterized as \u27warm-water\u27 carbonates to those characterized as \u27cool-water\u27carbonates. This shift occurs across the regionally recognized M4/M5 sequence stratigraphic boundary and has been attributed to climatic cooling and glaciation, basin reorganization and upwelling of open ocean water, and/or increased water turbidity and terrigenous input associated with the Taconic tectophase. Documentation of oxygen isotopic trends across the M4/M5 and through bracketing strata provides a potential means of distinguishing among these alternative scenarios; however, oxygen isotopic records generated to date have failed to settle the debate. This lack of resolution is because δ18O records are open to multiple interpretations and potentially confounding factors related to local environmental conditions have not been tested by examining the critical interval in multiple areas and different depositional settings. To begin to address this shortcoming, we present new species-specific and mixed assemblage conodont δ18O values in samples spanning the M4/M5 boundary from the Upper Mississippi Valley, Alabama, and Virginia. The new results are combined with previous studies, providing a record of δ18O variability across SE Laurentia. The combined dataset allows us to test for regional trends at a resolution not previously available. Our results document a ~1.5‰ decrease in values across Laurentia instead of increasing δ18O values across the M4/M5 as predicted in various \u27cool-water\u27 scenarios. In short, these results do not support a shift to \u27cool-water\u27 conditions as an explanation for changes in early Katian carbonates across the M4/M5
Modern Michelson-Morley experiment using cryogenic optical resonators
We report on a new test of Lorentz invariance performed by comparing the
resonance frequencies of two orthogonal cryogenic optical resonators subject to
Earth's rotation over 1 year. For a possible anisotropy of the speed of light
c, we obtain 2.6 +/- 1.7 parts in 10^15. Within the Robertson-Mansouri-Sexl
test theory, this implies an isotropy violation parameter beta - delta - 1/2 of
-2.2 +/- 1.5 parts in 10^9, about three times lower than the best previous
result. Within the general extension of the standard model of particle physics,
we extract limits on 7 parameters at accuracies down to a part in 10^15,
improving the best previous result by about two orders of magnitude
Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction
Copyright © 2018 The Authors. Explaining the ∼5-million-year delay in marine biotic recovery following the latest Permian mass extinction, the largest biotic crisis of the Phanerozoic, is a fundamental challenge for both geological and biological sciences. Ocean redox perturbations may have played a critical role in this delayed recovery. However, the lack of quantitative constraints on the details of Early Triassic oceanic anoxia (for example, time, duration, and extent) leaves the links between oceanic conditions and the delayed biotic recovery ambiguous. We report high-resolution U-isotope (δ238U) data from carbonates of the uppermost Permian to lowermost Middle Triassic Zal section (Iran) to characterize the timing and global extent of ocean redox variation during the Early Triassic. Our δ238U record reveals multiple negative shifts during the Early Triassic. Isotope mass-balance modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, peaking during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. Comparisons of the U-, C-, and Sr-isotope records with a modeled seawater PO43- concentration curve for the Early Triassic suggest that elevated marine productivity and enhanced oceanic stratification were likely the immediate causes of expanded oceanic anoxia. The patterns of redox variation documented by the U-isotope record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic. Our results indicate that multiple oscillations in oceanic anoxia modulated the recovery of marine ecosystems following the latest Permian mass extinction
Relativity tests by complementary rotating Michelson-Morley experiments
We report Relativity tests based on data from two simultaneous
Michelson-Morley experiments, spanning a period of more than one year. Both
were actively rotated on turntables. One (in Berlin, Germany) uses optical
Fabry-Perot resonators made of fused silica; the other (in Perth, Australia)
uses microwave whispering-gallery sapphire resonators. Within the standard
model extension, we obtain simultaneous limits on Lorentz violation for
electrons (5 coefficients) and photons (8) at levels down to ,
improved by factors between 3 and 50 compared to previous work.Comment: 5 pages revtex, 2 figure
Optical cavity tests of Lorentz invariance for the electron
A hypothetical violation of Lorentz invariance in the electrons' equation of
motion (expressed within the Lorentz-violating extension of the standard model)
leads to a change of the geometry of crystals and thus shifts the resonance
frequency of an electromagnetic cavity. This allows experimental tests of
Lorentz invariance of the electron sector of the standard model. The material
dependence of the effect allows to separate it from an additional shift caused
by Lorentz violation in electrodynamics, and to place independent limits on
both effects. From present experiments, upper limits on Lorentz violation in
the electrons' kinetic energy term are deduced.Comment: 17 pages revte
- …