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Over the past eight hundred thousand years, glacial-interglacial cycles oscillated with a period of one hundred thousand years ('100k world'1). Ice core and ocean sediment data have shown that atmospheric carbon dioxide, Antarctic temperature, deep ocean temperature, and global ice volume correlated strongly with each other in the 100k world2-6. Between about 2.8 and 1.2 million years ago, glacial cycles were smaller in magnitude and shorter in duration ('40k world'7). Proxy data from deep-sea sediments suggest that the variability of atmospheric carbon dioxide in the 40k world was also lower than in the 100k world8-10, but we do not have direct observations of atmospheric greenhouse gases from this period. Here we report the recovery of stratigraphically discontinuous ice more than two million years old from the Allan Hills Blue Ice Area, East Antarctica. Concentrations of carbon dioxide and methane in ice core samples older than two million years have been altered by respiration, but some younger samples are pristine. The recovered ice cores extend direct observations of atmospheric carbon dioxide, methane, and Antarctic temperature (based on the deuterium/hydrogen isotope ratio δDice, a proxy for regional temperature) into the 40k world. All climate properties before eight hundred thousand years ago fall within the envelope of observations from continuous deep Antarctic ice cores that characterize the 100k world. However, the lowest measured carbon dioxide and methane concentrations and Antarctic temperature in the 40k world are well above glacial values from the past eight hundred thousand years. Our results confirm that the amplitudes of glacial-interglacial variations in atmospheric greenhouse gases and Antarctic climate were reduced in the 40k world, and that the transition from the 40k to the 100k world was accompanied by a decline in minimum carbon dioxide concentrations during glacial maxima.

18O/16O and 17O/16O ratios of atmospheric and dissolved oceanic O2 are used as biogeochemical tracers of photosynthesis and respiration. Critical to this approach is a quantitative understanding of the isotopic fractionations associated with production, consumption, and transport of O2 in the ocean both at the surface and at depth. We made measurements of isotopic fractionations associated with O2 respiration by E. coli. Our study included wild-type strains and mutants with only a single respiratory O2 reductase in their electron transport chains (either a heme-copper oxygen reductase or a bd oxygen reductase). We tested two common assumptions made in interpretations of O2 isotope variations and in isotope-enabled models of the O2 cycle: (i) laboratory-measured respiratory 18O/16O isotopic fractionation factors (18α) of microorganisms are independent of environmental and experimental conditions including temperature, carbon source, and growth rate; And (ii) the respiratory ‘mass law’ exponent, θ, between 18O/16O and 17O/16O, 17α = (18α)θ, is universal for aerobic respiration. Results demonstrated that experimental temperatures have an effect on both 18α and θ for aerobic respiration. Specifically, lowering temperatures from 37 to 15 °C decreased the absolute magnitude of 18α by 0.0025 (2.5‰), and caused the mass law slope to decrease by 0.005. We propose a possible biochemical basis for these variations using a model of O2 reduction that incorporates two isotopically discriminating steps: the reversible binding and unbinding of O2 to a terminal reductase, and the irreversible reduction of that O2 to water. Finally, we cast our results in a one-dimensional isopycnal reaction-advection-diffusion model, which demonstrates that enigmatic δ18O and Δ17O variations of dissolved O2 in the dark ocean can be understood by invoking the observed temperature dependence of these isotope effects.

Measurements of trace gases in air trapped in polar firn (unconsolidated snow) demonstrate that natural sources of chlorofluorocarbons, halons, persistent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are minimal or non-existent. Atmospheric concentrations of these gases, reconstructed back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric lifetimes. The measurements confirm the predominance of human activity in the atmospheric budget of organic chlorine, and allow the estimation of atmospheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to that at present, while the burden of methyl bromide was probably over half of today's value.

The oxygen ([O.sub.2]) concentration in the surface ocean is influenced by biological and physical processes. With concurrent measurements of argon (Ar), which has similar solubility properties as oxygen, we can remove the physical contribution to [O.sub.2] supersaturation and determine the biological oxygen supersaturation. Biological [O.sub.2] supersaturation in the surface ocean reflects the net metabolic balance between photosynthesis and respiration, i.e., the net community productivity (NCP). We present a new method for continuous shipboard measurements of [O.sub.2]/Ar by equilibrator inlet mass spectrometry (EIMS). From these measurements and an appropriate gas exchange parametrization, NCP can be estimated at high spatial and temporal resolution. In the EIMS configuration, seawater from the ship's continuous intake flows through a cartridge enclosing a gas-permeable microporous membrane contactor. Gases in the headspace of the cartridge equilibrate with dissolved gases in the flowing seawater. A fused-silica capillary continuously samples headspace gases, and the [O.sub.2]/Ar ratio is measured by mass spectrometry. The ion current measurements on the mass spectrometer reflect the partial pressures of dissolved gases in the water flowing through the equilibrator. Calibration of the [O.sub.2]/Ar ion current ratio (32/40) is performed automatically every 2 h by sampling ambient air through a second capillary. A conceptual model demonstrates that the ratio of gases reaching the mass spectrometer is dependent on several parameters, such as the differences in molecular diffusivities and solubilities of the gases. Laboratory experiments and field observations performed by EIMS are discussed. We also present preliminary evidence flint other gas measurements, such as [N.sub.2]/ Ar and pC[O.sub.2] measurements, may potentially be performed with EIMS. Finally, we compare the characteristics of the EIMS with the previously described membrane inlet mass spectrometry (MIMS) approach.

Knowledge of the outgassing history of radiogenic [sup.40]Ar, derived over geologic time from the radioactive decay of [sup.40]K, contributes to our understanding of the geodynamic history of the planet and the origin of volatiles on Earth's surface. The [sup.40]Ar inventory of the atmosphere equals total [sup.40]Ar outgassing during Earth history. Here, we report the current rate of [sup.40]Ar outgassing, accessed by measuring the Ar isotope composition of trapped gases in samples of the Vostok and Dome C deep ice cores dating back to almost 800 ka. The modern outgassing rate (1.1 [+ or -] 0.1 x [10.sup.8] mol/yr) is in the range of values expected by summing outgassing from the continental crust and the upper mantle, as estimated from simple calculations and models. The measured outgassing rate is also of interest because it allows dating of air trapped in ancient ice core samples of unknown age, although uncertainties are large (+ or -] 180 kyr for a single sample or [+ or -] 11% of the calculated age, whichever is greater). geochronology | ice cores | geodynamics | noble gases

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2007.02.007 Byline: Matthew K. Reuer (a), Bruce A. Barnett (a), Michael L. Bender (a), Paul G. Falkowski (b), Melissa B. Hendricks (a) Keywords: Dissolved gases; Oxygen isotopes; Primary production; Southern Ocean Abstract: We report O.sub.2/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved O.sub.2 (a constraint on gross primary production) in samples collected from the surface mixed layer on 23 Southern Ocean transits. Samples were collected at 1-2[degrees] meridional resolution during the austral summer. Methodological limitations notwithstanding, the results constrain the net/gross production ratio, net O.sub.2 production, and gross O.sub.2 production at unprecedented resolution throughout the Southern Ocean mixed layer. Gross O.sub.2 production rates inferred from the oxygen triple isotopes are greater than production rates calculated from a model based on remotely sensed chlorophyll. This result agrees with previous.sup.18O and.sup.14C incubations along 170[degrees]W. O.sub.2/Ar ratios exceeding saturation are consistently observed within the Subantarctic and Polar Frontal Zones south of New Zealand and Australia, showing that a net autotrophic community predominates during austral summer. Lower O.sub.2/Ar values are observed within the Drake Passage and Antarctic Zone, suggesting unresolved influences of low net community production, net heterotrophy, and upwelling of O.sub.2-undersaturated waters. In autotrophic waters of the austral summer mixed layer, ratios of net community production/gross O.sub.2 production scatter about 0.13, corresponding to f ratios of [approximately equal to]0.25. Net community/gross O.sub.2 production ratios show no meridional gradient across the Antarctic Circumpolar Current, suggesting that an approximately constant fraction of gross primary productivity is regenerated or exported. Our calculated net O.sub.2 production rates are in satisfactory agreement with comparable published estimates. Net and gross O.sub.2 production rates are highest in the Subantarctic and decline to the south, paralleling the well-known trend of chlorophyll a concentrations. In an analysis of variance of net O.sub.2 production and gross O.sub.2 production with other environmental variables, the strongest correlations are between net O.sub.2 production and sea surface temperature (SST) (direct correlation), climatological [NO.sub.3.sup.-] (inverse correlation), and estimates of primary productivity derived from a remote sensing (direct correlation). These trends are as expected if aerosol iron input is the most important influence on production. They are unexpected if upwelling-derived SiO.sub.2 and iron are the leading influence or if lower SSTs promote greater export in this region. Author Affiliation: (a) Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544-1003, USA (b) Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901-8521, USA Article History: Received 6 April 2005; Revised 20 February 2007; Accepted 23 February 2007

The naturally occurring isotopes of oxygen are of mass numbers 16,17 and 18, and their ratio in atmospheric O(sub2) relies mainly on the isotopic composition of O(sub2) produced from terrestrial and aquatic plants. An experiment based on estimates of the size of the (super17)O/(super16)O anomaly in tropospheric O(sub2) was undertaken. It is argued that it mainly reflects the influences of biospheric cycling and stratospheric photochemical processes. It is proposed that the magnitude of the tropospheric (super17)O anomaly is useful as a tracer of global biosphere production.

Researchers have developed a new technique for identifying rapid past temperature changes based on the principle of thermal diffusion. In thermal diffusion, gas mixtures are fractionated in a temperature gradient according to their mass. This approach has been used to investigate the timing of sudden climate change at the end of the Younger Dryas interval. It was established that the rise in atmospheric methane concentrations at the end of the Younger Dryas interval started between 0 and 30 years after a sudden temperature rise.

A model developed during the Joint Global Ocean Flux Study's North Atlantic Bloom Experiment to measure the net and gross production of O2 and the assimilation rate of 14C expresses the gross production as a function of irradiance and total chlorophyll concentration. The study shows that the rate of O2 respiration is proportional to the rate of 14C respiration and the rate of O2 production. The relationship between these findings and the North Atlantic spring bloom is discussed.

The S27 ice core, drilled in the Allan Hills Blue Ice Area of East Antarctica, is located in Southern Victoria Land ~80 km away from the present-day northern edge of the Ross Ice Shelf. Here, we utilize the reconstructed accumulation rate of S27 covering the Last Interglacial (LIG) period between 129 and 116 thousand years before present (ka) to infer moisture transport into the region. The accumulation rate is based on the ice age-gas age differences calculated from the ice chronology, which is constrained by the stable water isotopes of the ice, and an improved gas chronology based on measurements of oxygen isotopes of O2 in the trapped gases. The peak accumulation rate in S27 occurred at 128.2 ka, near the peak LIG warming in Antarctica. Even the most conservative estimate yields a six-fold increase in the accumulation rate in the LIG, whereas other Antarctic ice cores are typically characterized by a glacial-interglacial difference of a factor of two to three. While part of the increase in S27 accumulation rates must originate from changes in the large-scale atmospheric circulation, additional mechanisms are needed to explain the large changes. We hypothesize that the exceptionally high snow accumulation recorded in S27 reflects open-ocean conditions in the Ross Sea, created by reduced sea ice extent and increased polynya size, and perhaps by a southward retreat of the Ross Ice Shelf relative to its present-day position near the onset of LIG. The proposed ice shelf retreat would also be compatible with a sea-level high stand around 129 ka significantly sourced from West Antarctica. The peak insS27 accumulation rates is transient, suggesting that if the Ross Ice Shelf had indeed retreated during the early LIG, it would have re-advanced by 125 ka. [ABSTRACT FROM AUTHOR]