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The oxygen isotope (δ18O) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δ18O offset (Δ18OSIMS-GSMS) of −0.9 ± 0.1‰ (±2 SE). The volume of material analyzed in 10-μm SIMS spots is ~105 times smaller than that analyzed by GSMS; hence, the extent to which these Δ18OSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSMS δ18O difference include sample-standard mismatch by SIMS, differences in standardization of SIMS and GSMS, and non-calcite contaminants in samples. Although the two datasets are consistently offset, SIMS values reproduce inter-shell δ18O variability delineated by shell fragment GSMS values. This strong positive covariance proved useful for bringing the two datasets into agreement (i.e. Δ18OSIMS-GSMS = 0), and confirms that SIMS-based foraminifer δ18O values record changes in calcification temperature and/or δ18O of seawater. Whether shells of foraminifer taxa with differing microcrystalline structures, chemical composition, and/or preservation histories register a similar Δ18OSIMS-GSMS value is a subject of ongoing testing.

Analyses by secondary ion mass spectroscopy (SIMS) of 11 specimens of five taxa of prokaryotic filamentous kerogenous cellular microfossils permineralized in a petrographic thin section of the ∼3,465 Ma Apex chert of northwestern Western Australia, prepared from the same rock sample from which this earliest known assemblage of cellular fossils was described more than two decades ago, show their δ13C compositions to vary systematically taxon to taxon from -31‰ to -39‰. These morphospecies-correlated carbon isotope compositions confirm the biogenicity of the Apex fossils and validate their morphology-based taxonomic assignments. Perhaps most significantly, the δ13C values of each of the five taxa are lower than those of bulk samples of Apex kerogen (-27‰), those of SIMS-measured fossil-associated dispersed particulate kerogen (-27.6‰), and those typical of modern prokaryotic phototrophs (-25 ± 10‰). The SIMS data for the two highest δ13C Apex taxa are consistent with those of extant phototrophic bacteria; those for a somewhat lower δ13C taxon, with nonbacterial methane-producing Archaea; and those for the two lowest δ13C taxa, with methane-metabolizing γ-proteobacteria. Although the existence of both methanogens and methanotrophs has been inferred from bulk analyses of the carbon isotopic compositions of pre-2,500 Ma kerogens, these in situ SIMS analyses of individual microfossils present data interpretable as evidencing the cellular preservation of such microorganisms and are consistent with the near-basal position of the Archaea in rRNA phylogenies.

Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remains cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e. crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U–Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire. Montana sapphires exhibit a large range of δ18O values, from −3‰ to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0‰ to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ18O(Crn) = 4.4‰ to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ18O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2 s = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ18O ranges by up to 7‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ18O(Zrc) when zircon inclusions are surrounded and armored by sapphire. U–Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores—the Boehls Butte anorthosite (~180 km NW of Rock Creek) and the Bitterroot anorthosite (~55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U–Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ~50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth. [ABSTRACT FROM AUTHOR]

Detrital zircons from the Jack Hills are the dominant source of Hadean (pre-4000 Ma) terrestrial material available for study today. Values of δ18O in many of these zircons (6.0 to 7.5‰) are above the mantle-equilibrated value. For two decades, these mildly elevated values have been the primary evidence that protoliths of the zircon-forming magmas interacted at low temperature with liquid water before burial and melting, implying that the surface of Earth cooled quickly after core and Moon formation and that habitable conditions for life existed within 250 Myr of the formation of Earth, over 800 Myr before the oldest generally accepted microfossils. These conclusions are based on oxygen isotope analyses of zircon domains with well-defined growth zoning and nearly concordant U-Pb ages within zircon grains with low magnetic susceptibility, which are further inferred to be unaltered by various tests. However, no studies of Jack Hills zircons have directly correlated oxygen isotope ratios and radiation damage, which facilitates alteration in zircon. Several previous studies have selected zircons that show radiation-damaged, discordant, and/or hydrous domains and have shown that such altered material is not reliable as a record of igneous composition. In contrast, this study targeted zircons that are interpreted as not altered and demonstrates the importance of testing zircons for radiation damage and alteration as part of any geochemical study, regardless of age. This study expands on existing data and presents the first comprehensive evaluation of δ18O, OH/O, CL imaging, U-Pb concordance, and radiation-damage state within Jack Hills zircons. A total of 115 Hadean zircon grains in this study have water contents similar to nominally anhydrous standard reference zircons and are interpreted as pristine. Raman band broadening correlated with δ18O reveals low levels of radiation damage, indicating significant annealing. The present-day effective doses (Deff) are uniformly less than the first percolation point [dose where damage domains, that are isolated at lower damage state, overlap to form a continuous pathway through the crystal, ~2 × 1015 α-decays/mg (Ewing et al. 2003)] and most zircons have Deff <1 × 1015 α-decays/mg. Modeling of representative α-recoil damage and annealing histories indicates that most zircons in this study have remained below the Deff of the first percolation point throughout their history. The δ18O values for these primary zircons include many that are higher than would be equilibrated with the mantle at magmatic temperatures and average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There is no correlation in our suite of unaltered Hadean zircons between δ18O and OH/O, Deff, age, or U-Pb age concordance. These carefully documented Hadean-age zircons have low amounts of radiation damage in water-poor domains sampled by δ18O analysis. The mildly elevated δ18O values are a primary magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ18O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma. [ABSTRACT FROM AUTHOR]

Determining the mechanisms by which the earliest continental crust was generated and reworked is important for constraining the evolution of Earth's geodynamic, surface, and atmospheric conditions. However, the details of early plate tectonic settings often remain obscured by the intervening ~4 Ga of crustal recycling. Covariations of U, Nb, Sc, and Yb in zircon have been shown to faithfully reflect Phanerozoic whole-rock-based plate-tectonic discriminators and are therefore useful in distinguishing zircons crystallized in ridge, plume, and arc-like environments, both in the present and in deep time. However, application of these proxies to deciphering tectonic settings on the early Earth has thus far been limited to select portions of the detrital zircon record. Here, we present in situ trace-element and oxygen isotope compositions for magmatic zircons from crystalline crustal rocks of the Acasta Gneiss Complex and the Saglek-Hebron Complex, Canada. Integrated with information from whole-rock geochemistry and zircon U-Pb, Hf, and O isotopes, our zircon U-Nb-Sc-Yb results reveal that melting of hydrated basalt was not restricted to a single tectonomagmatic process during the Archean but was operative during the reworking of Hadean protocrust and the generation of juvenile crust within two cratons, as early as 3.9 Ga. We observe zircon trace-element compositions indicative of hydrous melting in settings that otherwise host seemingly differing whole-rock geochemistry, zircon Hf, and zircon O isotopes, suggesting contemporaneous operation of stagnant-lid (oceanic plateau) and mobile-lid (arc-like) regimes in the early Archean. [ABSTRACT FROM AUTHOR]

Our understanding of climatic conditions, and therefore forcing factors, in North America during the past two glacial cycles is limited in part by the scarcity of long, well-dated, continuous paleoclimate records. Here, we present the first, to our knowledge, continuous, millennial-resolution paleoclimate proxy record derived from millimeter-thick pedogenic carbonate clast coatings (pedothems), which are widely distributed in semiarid to arid regions worldwide. Our new multiisotope pedothem record from the Wind River Basin in Wyoming confirms a previously hypothesized period of increased transport of Gulf of Mexico moisture northward into the continental interior from 70,000 to 55,000 years ago based on oxygen and carbon isotopes determined by ion microprobe and uranium isotopes and U-Th dating by laser ablation inductively coupled plasma mass spectrometry. This pronounced meridional moisture transport, which contrasts with the dominant zonal transport of Pacific moisture into the North American interior by westerly winds before and after 70,000-55,000 years ago, may have resulted from a persistent anticyclone developed above the North American ice sheet during Marine Isotope Stage 4. We conclude that pedothems, when analyzed using microanalytical techniques, can provide high-resolution paleoclimate records that may open new avenues into understanding past terrestrial climates in regions where paleoclimate records are not otherwise available. When pedothem paleoclimate records are combined with existing records they will add complimentary soil-based perspectives on paleoclimate conditions.

The recent discovery of a deep-water sulfur-cycling microbial biota in the ∼ 2.3-Ga Western Australian Turee Creek Group opened a new window to life's early history. We now report a second such subseafloor-inhabiting community from the Western Australian ∼ 1.8-Ga Duck Creek Formation. Permineralized in cherts formed during and soon after the 2.4- to 2.2-Ga "Great Oxidation Event," these two biotas may evidence an opportunistic response to the mid-Precambrian increase of environmental oxygen that resulted in increased production of metabolically useable sulfate and nitrate. The marked similarity of microbial morphology, habitat, and organization of these fossil communities to their modern counterparts documents exceptionally slow (hypobradytelic) change that, if paralleled by their molecular biology, would evidence extreme evolutionary stasis.

In this study, we present experimental results from the planktic foraminifer Orbulina universa, cultured in the laboratory. We demonstrate that the δ13C of shell calcite precipitated in 13C-labeled seawater for 24h can be resolved and accurately measured using Secondary Ion Mass Spectrometry (SIMS). Specimens maintained at 20°C were transferred from ambient seawater (δ13CDIC=+1.3‰) into seawater with δ13CDIC=+51.5‰ and enriched [Ba2+] for 24h. Specimens were then transferred into ambient seawater with elevated [87Sr] for 6-9h of calcification, followed by a transfer back into unlabeled ambient seawater until gametogenesis. This technique produced O. universa shells with calcite layers of distinct geochemical signatures. We quantify the spatial positions of trace element labels in the shells using laser ablation ICP-MS depth profiling. Using fragments from the same shells, we quantify intrashell δ13Ccalcite using SIMS with a 6 or 8μm spot (2 SD range ±0.5‰ to 1.7‰). Measured δ13Ccalcite values in O. universa shell layers precipitated in ambient seawater are within 2‰ of predicted δ13Ccalcite values. In 13C-labeled bands of calcite, 6μm SIMS spot measurements are within 2‰ of predicted δ13Ccalcite values, whereas 8μm SIMS spots yield intermediate, mixed values. The spatial agreement between trace element and carbon isotope data suggests that 13C and cation tracers are synchronously incorporated into shell calcite. These results demonstrate the ability of SIMS δ13C measurements to resolve ~10μm features in foraminifer shell calcite using a 6μm spot, and highlight the potential of this technique for addressing questions about ecology, biomineralization, and paleoceanography. © 2014 .