Space and time variation of δ¹⁸O and δD in Antarctic precipitation revisited

Abstract

[1] To better understand the variations over time of precipitation water isotopes measured in polar ice cores, we have developed an intermediate complexity model (ICM) of atmospheric water vapor transport and associated isotopic distillation. The model builds directly from earlier work by D. Fisher and M. Hendricks and is calibrated against the measured modern spatial distribution of dD and deuterium excess (d ). Model improvements include a correction to the equation governing advective transport, which solves a major puzzle arising from the earlier work. The new model shows isotopic data to be consistent with dominance of eddy diffusive transport at high latitudes. Model experiments are used to show that a wide variety of climate changes in subtropical source regions can affect Antarctic d and that such d changes are consistently anticorrelated with changes of dD. Magnitudes of DdD/Dd are $À1 to À5 and are much higher at the ice sheet margin than in the interior of the continent. Changes in the relative dominance of diffusive and advective transport are also shown to cause anticorrelated changes of similar magnitude. Isotopic sensitivities to temperature change (DdD/DT ) are shown to also vary spatially, with low values in marginal zones and higher values inland. Effects of ocean surface relative humidity on Antarctic d are shown to be nearly uniform across the continent (in contrast to earlier results given by Petit et al. [1991]) but quantitatively small. The method used by Vimeux et al. [2001] for folding humidity effects into source temperature effects is supported. We draw attention to a pervasive misapplication of marine composition corrections to Antarctic dD records. Arguments are made in support of simple interpretations of calibrated ice core isotopic time series in terms of temperature changes despite the potential complexities in the system.

Deglacial temperature history of West Antarctica

“…Although not perfect thermometers, isotopic data provide a constraint based on temperature-dependent physical processes, and their use avoids a priori assumptions about climate variability that are otherwise required for interpreting borehole temperatures. The temperature (T ) dependence of δD arises from atmospheric distillation processes (17) and, despite potential complexities, is usually treated as a linear relationship: δ = γ T + β. Due to gravitational settling of heavy gases, δ 15 N measures the thickness of firn (18,19), the 50-to 120-m-deep porous layer of consolidated snow that blankets the ice-sheet surface.…”

Magnitude of isotope/temperature scaling for interpretation of central Antarctic ice cores

“…Also, as the spatial slope is defined with respect to T s , any change in the strength of the vertical inversion, for example, between a glacial and an interglacial, will influence R slopes . Changes in cyclonic activity [Holdsworth, 2001] and in the ratio between advection by the mean circulation and eddy transport [Eriksson, 1965;Hendricks et al, 2000, Noone andSimmonds, 2002] can also play a role, suggesting, in particular that the temporal slope can increase inland Antarctica [Hendricks et al, 2000;Kavanaugh and Cuffey, 2003]. The influence of most of these parameters has been systematically investigated for Greenland snow , pointing out the key role of the moisture origin and of the precipitation seasonality.…”

“…Applying this equation for present-day (dD ocean(0) ) and for a certain period in the past, dD ocean(t) = dD ocean(0) + ÁdD ocean , shows that Corr ocean equals ÁdD ocean  (1 + dD ice )/(1 + ÁdD ocean ) and not ÁdD ocean as previously used in the interpretion of Vostok records. Independently, Kavanaugh and Cuffey [2003] have recently pointed out to such an incorrect estimation of the oceanic correction, and have proposed a similar approach. As illustrated in Figure 3, this would have only a minor impact if the temperature record had been inferred from the d 18 O ice record (1 + d 18 O ice is very close to 1).…”

Improved methodologies for continuous-flow analysis of stable water isotopes in ice cores

“…Advances in high-precision laser absorption spectroscopy (LAS) methods are now widely adopted as an alternative to IRMS methods (Kerstel et al, 1999;Lis et al, 2008;Gupta et al, 2009;Brand et al, 2009). There are currently two main LAS methods used: cavity ring-down laser spectroscopy (CRDS; manufactured by Picarro, Inc.) and off-axis integrated cavity output laser spectroscopy (OA-ICOS; manufactured by Los Gatos Research).…”