Michael L. Bender

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica

The recent completion of drilling at Vostok station in East Antarctica has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial–interglacial cycles. The succession of changes through each climate cycle and termination was similar, and atmospheric and climate properties oscillated between stable bounds. Interglacial periods differed in temporal evolution and duration. Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Abstract Pore water profiles of total-CO 2 , pH, PO 3− 4 , NO − 3 plus NO − 2 , SO 2− 4 , S 2− , Fe 2+ and Mn 2+ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized ( O 2 > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO 2 concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.

Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice

Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 ± 3 °C colder than today.

Evolution of the Ratio of Strontium-87 to Strontium-86 in Seawater from Cretaceous to Present

A detailed record of the strontium-87 to strontium-86 ratio in seawater during the last 100 million years was determined by measuring this ratio in 137 well-preserved and well-dated fossil foraminifera samples. Sample preservation was evaluated from scanning electron microscopy studies, measured strontium-calcium ratios, and pore water strontium isotope ratios. The evolution of the strontium isotopic ratio in seawater offers a means to evaluate long-term changes in the global strontium isotope mass balance. Results show that the marine strontium isotope composition can be used for correlating and dating well-preserved authigenic marine sediments throughout much of the Cenozoic to a precision of �1 million years. The strontium-87 to strontium-86 ratio in seawater increased sharply across the Cretaceous/Tertiary boundary, but this feature is not readily explained as strontium input from a bolide impact on land.

Carbon and oxygen isotopic disequilibria of recent deep-sea benthic foraminifera

Abstract The carbon and oxygen isotopic compositions of 149 samples of benthic foraminifera from deep-sea core tops indicate that none of the nine species studied secrete calcium carbonate in isotopic equilibrium with ambient bottom water. Uvigerina, Pyrgo murrhina, and Oridorsalis tener are the closest to 18O equilibrium (with average deviations about −0.4‰), while Planulina wuellerstorfi and P. murrhina are the closest to 13C equilibrium (with average deviations about −1‰). P. wuellerstorfi shows the most systematic relationship between δ 13C and bottom water apparent oxygen utilization. The intraspecific variabilities in δ 18O and δ 13C suggest that estimates of bottom water paleotemperatures can be made to a precision of ± 0.7°C, while estimates of past apparent oxygen utilization (AOU) can be made to ± 35 μmol/kg. Based on intraspecific comparisons of the Recent samples with fossils, no temporal changes in the degree of either 18O or 13C disequilibrium have been detected for Planulina wuellerstorfi, Uvigerina, Oridorsalis tener and Globocassidulina subglobosa.

Temperature Dependence of Carbon Isotope Composition in Marine Plankton and Sediments.

Samples of marine plankton collected in high-latitude areas of the South Atlantic where surface water temperatures are near 0�C show a carbon-12 enrichment of 6 per mill relative to samples collected where temperatures are about 25�C. The organic carbon in sediments in the Drake Passage and Argentine Basin also shows a carbon-12 enrichment relative to warmer areas.

Climate Records Covering the Last Deglaciation

The oxygen-18/oxygen-16 ratio of molecular oxygen trapped in ice cores provides a time-stratigraphic marker for transferring the absolute chronology for the Greenland Ice Sheet Project (GISP) II ice core to the Vostok and Byrd ice cores in Antarctica. Comparison of the climate records from these cores suggests that, near the beginning of the last deglaciation, warming in Antarctica began approximately 3000 years before the onset of the warm B�lling period in Greenland. Atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in Greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the Northern Hemisphere.

The distribution of manganese in the Pacific Ocean

Abstract This report presents the vertical distributions of “total dissolvable manganese” at 13 stations in the Pacific. Manganese concentrations vary with depth in a unique way. There is commonly a maximum concentration at the surface and a minimum at the top of the thermocline. At stations where the oxygen in the oxygen minimum falls below 100 μmol/kg, there is a manganese maximum in the oxygen minimum. At stations where the oxygen minimum is so intense that nitrate reduction occurs, especially high manganese concentrations are observed in the zone of nitrate reduction. This maximum in the O 2 minimum is believed to originate either as reduction of Mn oxides in nearshore sediments and subsequent advection or by seawater equilibrium with a metastable oxide similar to hausmannite (Mn 3 O 4 ). Surface water values range from about 3 to about 0.3 nmol/kg. The surface concentrations show a general correlation with 210 Pb, suggesting that the major source of manganese to the surface waters of the Pacific is desorption from atmospheric particulates. The residence time of Mn in surface waters is 5 to 25 years.

Triple-isotope composition of atmospheric oxygen as a tracer of biosphere productivity

Oxygen has three naturally occurring isotopes, of mass numbers 16, 17 and 18. Their ratio in atmospheric O2 depends primarily on the isotopic composition of photosynthetically produced O2 from terrestrial and aquatic plants, and on isotopic fractionation due to respiration. These processes fractionate isotopes in a mass-dependent way, such that 17O enrichment would be approximately half of the 18O enrichment relative to 16O. But some photochemical reactions in the stratosphere give rise to a mass-independent isotope fractionation, producing approximately equal 17O and 18O enrichments in stratospheric ozone and carbon dioxide,, and consequently driving an atmospheric O2 isotope anomaly. Here we present an experimentally based estimate of the size of the 17O/16O anomaly in tropospheric O2, and argue that it largely reflects the influences of biospheric cycling and stratospheric photochemical processes. We propose that because the biosphere removes the isotopically anomalous stratosphere-derived O2 by respiration, and replaces it with isotopically ‘normal’ oxygen by photosynthesis, the magnitude of the tropospheric 17O anomaly can be used as a tracer of global biosphere production. We use measurements of the triple-isotope composition of O2 trapped in bubbles in polar ice to estimate global biosphere productivity at various times over the past 82,000 years. In a second application, we use the isotopic signature of oxygen dissolved in aquatic systems to estimate gross primary production on broad time and space scales.

A record of atmospheric halocarbons during the twentieth century from polar firn air

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 todays value.