Showing papers by "Jeffrey P. Severinghaus published in 2007"

Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years

Abstract: The Milankovitch theory of climate change proposes that glacial–interglacial cycles are driven by changes in summer insolation at high northern latitudes. The timing of climate change in the Southern Hemisphere at glacial–interglacial transitions (which are known as terminations) relative to variations in summer insolation in the Northern Hemisphere is an important test of this hypothesis. So far, it has only been possible to apply this test to the most recent termination because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined. Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. This ratio is a proxy for local summer insolation5, and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter. The accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation. Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations

Trace gas disequilibria during deep-water formation

Abstract: We present high-precision measurements by a new isotope dilution technique of a suite of inert gases in the North Pacific. Remarkably smooth gradients in Ar, Kr and Xe from near equilibrium in intermediate waters to several percent undersaturated in deep waters were observed. The general pattern in the deepest waters was that Ar, Kr and Xe were undersaturated (Ar least and Xe most), while N2 was close to equilibrium, and Ne was supersaturated. We propose that this pattern was produced by the interaction between the different physical properties of the gases (solubility and the temperature dependence of solubility) with the rapid cooling and high wind speeds that characterize deep-water formation regions. In a simple model of deep-water formation by convection, the saturations of the more temperature-sensitive gases were quickly driven down by rapid cooling and could not reequilibrate with the atmosphere before the end of the winter. In contrast, the gas exchange rate of the more bubble-sensitive gases (Ne and N2) was able to meet or exceed the drawdown by cooling. Our simple convection model demonstrates that the heavier noble gases (Ar, Kr and Xe) are sensitive on seasonal timescales to the competing effects of cooling and air–sea gas exchange that are also important to setting the concentration of CO2 in newly formed waters.

A method to measure Kr/N2 ratios in air bubbles trapped in ice cores and its application in reconstructing past mean ocean temperature

Abstract: [1] We describe a new method for precise measurement of Kr/N2 ratios in air bubbles trapped in ice cores and the first reconstruction of atmospheric Kr/N2 during the last glacial maximum (LGM) ∼20,000 years ago. After gravitational correction, the Kr/N2 record in ice cores should represent the atmospheric ratio, which in turn should reflect past ocean temperature change due to the dependence of gas solubility on temperature. The increase in krypton inventory in the glacial ocean due to higher gas solubility in colder water causes a decrease in the atmospheric inventory of krypton. Assuming Kr and N2 inventories in the ocean-atmosphere system are conserved, we use a mass balance model to estimate a mean ocean temperature change between the LGM and today. We measured Kr/N2 in air bubbles in Greenland (GISP2) ice from the late Holocene and LGM, using the present atmosphere as a standard. The late Holocene δKr/N2 means from two sets of measurements are not different from zero (+0.07 ± 0.30‰ and −0.14 ± 0.93‰), as expected from the relatively constant climate of the last millennium. The mean δKr/N2 in air bubbles from the LGM is −1.34 ± 0.37‰. Using the mass balance model, we estimate that the mean temperature change between the LGM ocean and today's ocean was 2.7 ± 0.6°C. Although this error is large compared to the observed change, this finding is consistent with most previous estimates of LGM deep ocean temperature based on foraminiferal δ18O and sediment pore water δ18O and chlorinity.

Abrupt changes in atmospheric methane at the MIS 5b–5a transition

Abstract: [1] New ice core analyses show that the prominent rise in atmospheric methane concentration at Dansgaard-Oeschger event 21 was interrupted by a century-long 20% decline, which was previously unrecognized. The reversal was found in a new ∼100-year resolution study of methane in the GISP2 ice core, encompassing the beginning of D-O event 21, which also corresponds to the transition from MIS 5b to 5a. Although a corresponding reversal (within age uncertainty) is observed in climate proxies measured in GISP2 ice, including δ18Oice, electrical conductivity, light scattering, and several ions, this feature has not been discussed previously. Abrupt changes in methane are paralleled by changes in δ15N of trapped air, a quantity that reflects local temperature change at Greenland summit. The reversal described here supports the hypothesis that climate can be unstable during major transitions, as was previously described for the last deglaciation.

Firn-air Properties and Influences at the West Antarctic Ice Sheet Divide

Abstract: Samples of firn air were collected at the West Antarctic Ice Divide (WAIS-D) in December 2005. The firn air sampling occurred at 79° 28'S, 112° 7'W at an elevation of ~1800m, approximately 1km from the deep ice coring site presently in use (Fig 1). WAIS-D is a site with moderate accumulation rate (24 cm ice equiv/yr) with a mean annual temperature of -31oC. The site was chosen to be an Antarctic glaciological analogue for Summit, Greenland.