The last interglacial ocean

The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals.     

Summer algal blooms in a coastal ecosystem: the role of atmospheric deposition versus entrainment fluxes

The nitrogen inputs from atmospheric deposition and bottom water entrainment to the surface layer were modelled in the summer period (May–September) over a 11-year period (1989–1999) and compared to investigate the significance of these fluxes for generating blooms in the Kattegat. In the summer periods the average atmospheric deposition was 2.81 mg N m⁻² d⁻¹ compared to average entrainment fluxes of 5.42 mg N m⁻² d⁻¹, 1.21 mg N m⁻² d⁻¹ and 1.15 mg N m⁻² d⁻¹ for the northern, central and southern part of the Kattegat, respectively. Atmospheric nitrogen deposition alone could not sustain biomass increases associated with observed blooms and entrainment fluxes dominated the high nitrogen inputs to the surface layer. The potential for a bloom through growth was typically obtained after several days of high nitrogen inputs from entrainment in the frontal area of the northern Kattegat and to some extent from atmospheric deposition. The modelled nitrogen input in this area could account directly for 30% of the observed blooms in the Northern sub-basin, and through advective transport 24% and 19% of the observed blooms in the central and southern Kattegat. The direct nitrogen inputs through atmospheric deposition and entrainment to the central and southern sub-basins were small and could not be linked to any bloom observation.     

西藏长青温池波弄贡变质带片麻岩锆石 ELA-ICP-MS U-Pb年龄及地质意义

西藏林芝至通麦一带出露大面积变质岩,变质岩中含长柱状锆石。长柱状锆石晶体形态特征、矿物包裹体、CL图像及Th/U比值在0.1～0.4之间,表明锆石主要来自火山岩,受后期变质流体改造。长柱状锆石ELA-ICP-MSU-Th-Pb年龄为(40.7±0.3)Ma,MSWD=2.35。变质流体改造边缘部分年龄和核部年龄基本一致。长柱状锆石特征及年龄表明长青温池至通麦一带片麻岩原岩为新生代含火山物质的沉积岩而不是元古宙变质基底。     

The origin of shoshonites: new insights from the Tertiary high-potassium intrusions of eastern Tibet

The shoshonitic intrusions of eastern Tibet, which range in age from 33 to 41 Ma and in composition from ultramafic (SiO₂ = 42 %) to felsic (SiO₂ = 74 %), were produced during the collision of India with Eurasia. The mafic and ultramafic members of the suite are characterized by phenocrysts of phlogopite, olivine and clinopyroxene, low SiO₂, high MgO and Mg/Fe ratios, and olivine forsterite contents of Fo₈₇ to Fo₉₃, indicative of equilibrium with mantle olivine and orthopyroxene. Direct melting of the mantle, on the other hand, could not have produced the felsic members. They have a phenocryst assemblage of plagioclase, amphibole and quartz, high SiO₂ and low MgO, with Mg/Fe ratios well below the values expected for a melt in equilibrium with the mantle. Furthermore, the lack of decrease in Cr with increasing SiO₂ and decreasing MgO from ultramafic to felsic rocks precludes the possibility that the felsic members were derived by fractional crystallization from the mafic members. Similarly, magma mixing, crustal contamination and crystal accumulation can be excluded as important processes. Yet all members of the suite share similar incompatible element and radiogenic isotope ratios, which suggests a common origin and source. We propose that melting for all members of the shoshonite suite was initiated in continental crust that was thrust into the upper mantle at various points along the transpressional Red River-Ailao Shan-Batang-Lijiang fault system. The melt formed by high-degree, fluid-absent melting reactions at high-T and high-P and at the expense of biotite and phengite. The melts acquired their high concentrations of incompatible elements as a consequence of the complete dissolution of pre-existing accessory minerals. The melts produced were quartz-saturated and reacted with the overlying mantle to produce garnet and pyroxene during their ascent. The felsic magmas reacted little with the adjacent mantle and preserved the essential features of their original chemistry, including their high SiO₂, low Ni, Cr and MgO contents, and low Mg/Fe ratio, whereas the mafic and ultramafic magmas are the result of extensive reaction with the mantle. Although the mafic magmas preserved the incompatible element and radiogenic isotope ratios of their crustal source, buffering by olivine and orthopyroxene extensively modified their MgO, Ni, Cr, SiO₂ contents and Mg/Fe ratio to values dictated by equilibrium with the mantle.     

First measurement of helium on Mars: implications for the problem of radiogenic gases on the terrestrial planets

108 +/- 11 photons of the martian He 584-angstroms airglow detected by the Extreme Ultraviolet Explorer satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 +/- 10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He+ by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(5), 3 x 10(4), and 7 x 10(3) cm-2 sec-1, respectively. The lifetime of helium on Mars is 5 x 10(4) years. The He outgassing rate, coupled with the 40Ar atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances of K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function delta = delta (0)(t(0)/t)1/2 with delta (0) = 0/1, 0.04, and 0.0125 Byr-1 for Earth, Venus, and Mars, respectively. After a R2 correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassium is depleted by a factor of 2 in Venus and Mars. Mass ratios of helium and argon are close to 5 x 10(-9) and 2 x 10(-8) g/g in the interiors of all three planets. The implications of these results are discussed.     

NN detectability in Pluto’s atmosphere

Based on the vapor pressure behavior of Pluto’s surface ices, Pluto’s atmosphere is expected to be predominantly composed of N₂ gas. Measurement of the N₂ isotopologue ¹⁵N/¹⁴N ratio within Pluto’s atmosphere would provide important clues to the evolution of Pluto’s atmosphere from the time of formation to its present state. The most straightforward way of determining the N₂ isotopologue ¹⁵N/¹⁴N ratio in Pluto’s atmosphere is via spectroscopic observation of the ¹⁴N¹⁵N gas species. Recent calculations of the 80–100 nm absorption behavior of the ¹⁴N₂ and ¹⁴N¹⁵N isotopologues by Heays et al. (Heays, A.N. et al. [2011]. J. Chem. Phys. 135, 244301), Lewis et al. (Lewis, B.R., Heays, A.N., Gibson, S.T., Lefebvre-Brion, H., Lefebvre, R. [2008]. J. Chem. Phys. 129, 164306); Lewis et al. (Lewis, B.R., Gibson, S.T., Zhang, W., Lefebvre-Brion, H., Robbe, J.-M. [2005]. J. Chem. Phys. 122, 144302), and Haverd et al. (Haverd, V.E., Lewis, B.R., Gibson, S.T., Stark, G. [2005]. J. Chem. Phys. 123, 214304) show that the peak magnitudes of the ¹⁴N₂ and ¹⁴N¹⁵N absorption bandhead cross-sections are similar, but the locations of the bandhead peaks are offset in wavelength by ∼0.05–0.1 nm. These offsets make the segregation of the ¹⁴N₂ and ¹⁴N¹⁵N absorption signatures possible. We use the most recent N₂ isotopologue absorption cross-section calculations and the atmospheric density profiles resulting from photochemical models developed by Krasnopolsky and Cruickshank (Krasnopolsky, V.A., Cruickshank, D.P. [1999]. J. Geophys. Res. 104, 21979–21996) to predict the level of solar light that will be transmitted through Pluto’s atmosphere as a function of altitude during a Pluto solar occultation. We characterize the detectability of the isotopic absorption signature per altitude assuming ¹⁴N¹⁵N concentrations ranging from 0.1% to 2% of the ¹⁴N₂ density and instrumental spectral resolutions ranging from 0.01 to 0.3 nm. Our simulations indicate that optical depth of unity is attained in the key ¹⁴N¹⁵N absorption bands located between 85 and 90 nm at altitudes ∼1100–1600 km above Pluto’s surface. Additionally, an ¹⁴N¹⁵N isotope absorption depth ∼4–15% is predicted for observations obtained at these altitudes at a spectral resolution of ∼0.2–0.3 nm, if the N₂ isotopologue ¹⁵N/¹⁴N percent ratio is comparable to the 0.37–0.6% ratio observed at Earth, Titan and Mars. If we presume that the predicted absorption depth must be at least 25% greater than the expected observational uncertainty, then it follows that a statistically significant detection of these signatures and constraint of the N₂ isotopologue ¹⁴N/¹⁵N ratio within Pluto’s atmosphere will be possible if the attainable observational signal-to noise (S/N) ratio is ?9. The New Horizons (NH) Mission will be able to obtain high S/N, 0.27–0.35 nm full-width half-max 80–100 nm spectral observations of Pluto using the Alice spectrograph. Based on the NH/Alice specifications we have simulated 0.3 nm spectral resolution solar occultation spectra for the 1100–1600 km altitude range, assuming 30 s integration times. These simulations indicate that NH/Alice will obtain spectral observations within this altitude range with a S/N ratio ∼25–50, and should be able to reliably detect the ¹⁴N¹⁵N gas absorption signature between 85 and 90 nm if the ¹⁴N¹⁵N concentration is ∼0.3% or greater. This, additionally, implies that the non-detection of the ¹⁴N¹⁵N species in the 1100–1600 km range by NH/Alice may be used to reliably establish an upper limit to the N₂ isotopologue ¹⁵N/¹⁴N ratio within Pluto’s atmosphere. Similar results may be derived from 0.2 to 0.3 nm spectral resolution observations of any other N₂-rich Solar System or exoplanet atmosphere, provided the observations are attained with similar S/N levels.     

Measurements of the helium 584 Å airglow during the Cassini flyby of Venus

The helium resonance line at 584 Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme Ultraviolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ∼320 R, reaching ∼700 R at the bright limb. These values are slightly higher than those determined from previous observations. The sensitivity of the 584 Å intensity to the helium abundance is analyzed using recent cross-sections and solar irradiance measurements at 584 Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux model and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8×10⁶ cm⁻³ at the level of where the CO₂ is 2×10¹⁰ cm⁻³.     

Using Q-mode factor analysis in biostratigraphic ordination

Although traditionally restricted to the analysis of closed compositional data, the Imbrie Q-mode method appears also to be of utility when applied to binary data sets common in biostratigraphy. Its advantages include its utilization of the tally concept, and its ability to analyze very large numbers of species simultaneously. When the structure of the data set is simple, the method appears to be capable of ranking samples along a temporal gradient, or coenocline, defined by the second unroated factor. Varimax and oblique rotations do not appear to increase the efficacy of this ranking. Even when facies effects are important, a temporal ranking of samples can be achieved, providing that the variation due to time is greater than that due to facies.