Jean Jouzel

Solar modulation of cosmogenic nuclide production over the last millennium: comparison between 14C and 10Be records

For about the last 30 years it has been recognized that the high frequency component of the tree rings 14C/12C record is dominated by the modulation of the cosmic ray flux by the solar wind. In particular, it has been demonstrated that the three most recent periods of low sunspot occurrence were characterized by high values of atmospheric 14C/12C. During the last millennium other periods of high 14C/12C values were observed but their solar origin is still debatable. In the present work we compare these fluctuations with an independent record of cosmogenic 10Be measured in ice from the South Pole to check the solar origin of the observed 14C/12C variations. In order to compare quantitatively the results obtained on 10Be and 14C, it is necessary to take into account the different behaviour of these two cosmogenic isotopes, and especially the damping effect of the carbon cycle in the case of 14C. As an input to a 12-box numerical model we used the relative fluctuations of the 10Be concentrations record measured in South Pole ice and converted it into a synthetic 14C record. We took into account the fact that 10Be modulation is enhanced in polar regions due to the orientation of the geomagnetic field. As expected, the fluctuations of the modelled 14C record are much smaller (a factor of 20) than those observed for the raw 10Be record. In addition, the variations are smoother and shifted in time by a few decades. The 10Be-based 14C variations closely resemble the 14C measurements obtained on tree rings (R = 0.81). In particular, it is easy to identify periods of maximal 14C/12C which correspond to solar activity minima centred at about 1060, 1320 (Wolf), 1500 (Sporer), 1690 (Maunder) and 1820 (Dalton) yr A.D. Cross-correlation calculations suggest that there is no significant lag between the 10Be-based 14C and the tree-ring 14C records. Our study strongly suggests the dominance of the solar modulation on the cosmonuclide production variations during the last millennium.

Antarctic (Dome C) ice-core dust at 18 k.y. B.P.: Isotopic constraints on origins

We have determined the source area from which dusts from the Last Glacial Maximum (LGM) section of the Dome C ice core were derived, by comparing their strontium and neodymium isotopic ratios with those of samples from potential source areas. The87Sr/86Sr and143Nd/144Nd isotope ratios of the dusts deposited ≈ 18 k.y. B.P. at the East Antarctic Dome C site were compared with potential Antarctic, Australian, southern African and South American sources. The isotope ratios clearly define the Patagonian provenance of the dust, with the other potential source areas being, at most, minor contributors. Contributions by volcanic ash and tephra to the dust sample were also determined to be minimal, based on the patterns of rare earth elements. Knowing the source of the dusts places constraints on the aeolian trajectory by which it was transported to Antarctica, and this serves as a test of the simulation of southern hemispheric circulation by atmospheric global circulation models during the LGM.

Beryllium 10 in the Greenland Ice core Project ice core at Summit, Greenland

Concentrations of the cosmogenic isotope 10Be have been measured in more than 1350 samples from the Greenland Ice Core Project (GRIP) ice core drilled at Summit, Greenland. Although a dust-associated component of 10Be retained by 0.45 μm filters in some of the samples complicates the interpretations, the results confirm that the first-order origin of 10Be concentration variations is changes in precipitation rate associated with different climate regimes. This effect is seen not only between glacial and interglacial periods, but also during the shorter “Dansgaard-Oeschger” interstadials. By contrast, the 10Be data do not support the interpretation of rapidly varying accumulation (i.e., climate) during the last interglacial. They can, however, be used to help place limits on the origin of the ice in these events. After taking into account variable snow accumulation effects, variations in the 10Be flux are observed, probably caused by solar and geomagnetic modulation, but possibly also by primary cosmic ray variations. The most dramatic is a 10Be peak ∼40,000 years ago, similar to that found in the Vostok ice core, thus permitting a very precise correlation between climate records from Arctic and Antarctic ice cores. The 36Cl/10Be ratio (considering either “total” or only ice-associated 10Be) shows significant variability over the whole core depth, thus confirming the difficulty in using this parameter for “dating” ice cores.

Deuterium and oxygen 18 in precipitation: Isotopic model, including mixed cloud processes

Modeling the isotropic ratios of precipitation in cold regions meets the problem of `switching` from the vapor-liquid transition to the vapor-ice transition at the oneset of snow formation. The one-dimensional model (mixed cloud isotopic model (MCIM)) described in this paper focuses on the fractionation of water isotopes in mixed clouds, where both liquid droplets and ice crystals can coexist for a given range of temperatures. This feature is linked to the existence of specific saturation conditions within the cloud, allowing droplets to evaporate while the water vapor condensates onto ice crystals. The isotopic composition of the different airborne phases and the precipitation is calculated throughout the condensation history of an isolated air mass moving over the Antarctic ice sheet. The results of the MCIM are compared to surface snow data both for the isotopic ratios and the deuterium excesses. The sensitivity of the model is compared to previous one-dimensional models. Our main result is that accounting specifically for the microphysics of mixed stratiform clouds (Bergeron-Findesein process) does not invalidate the results of earlier modeling studies.

The two-step shape and timing of the last deglaciation in Antarctica

The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record.

Past Accumulation rates derived from observed annual layers in the GRIP ice core from Summit, Central Greenland

Based on dated reference horizons down to 1623 m an ice flow model has been developed. The model is used to reconstruct past accumulation rates from the sequences of detected annual layers in the upper 2321 m of the 3029 m deep GRIP ice core. Comparison of these past time accumulation rates with the corresponding δ 18O values show a strong correlation. This relationship can be used in a non steady state flow model, in which past accumulation rates, deduced from the continuous δ 18O record, are used to model a time scale. The hereby determined time scale and the modelled annual layers compare well with the observed data.

A comparison of the Vostok ice deuterium record and series from Southern Ocean core MD 88-770 over the last two glacial-interglacial cycles

Taking advantage of the fact that the Vostok deuterium (δD) record now covers almost two entire climatic cycles, we have applied the orbital tuning approach to derive an age-depth relation for the Vostok ice core, which is consistent with the SPECMAP marine time scale. A second age-depth relation for Vostok was obtained by correlating the ice isotope content with estimates of sea surface temperature from Southern Ocean core MD 88-770. Both methods lead to a close correspondence between Vostok and MD 88-770 time series. However, the coherence between the correlated δD and insolation is much lower than between the orbitally tuned 8D and insolation. This reflects the lower accuracy of the correlation method with respect to direct orbital tuning. We compared the ice and marine records, set in a common temporal framework, in the time and frequency domains. Our results indicate that changes in the Antarctic air temperature quite clearly lead variations in global ice volume in the obliquity and precession frequency bands. Moreover, the average phase we estimated between the filtered δD and insolation signals at precessional frequencies indicates that variations in the southern high latitude surface temperature could be induced by changes in insolation taking place during a large period of the summer in northern low latitudes or winter in southern low latitudes. The relatively large lag found between Vostok δD variations and obliquity-driven changes in insolation suggests that variations in the local radiative balance are not the only mechanism responsible for the variability in surface temperature at those frequencies. Finally, in contrast to the cross-spectral analysis method used in previous studies, the method we use here to estimate the phases can reveal errors in cross-correlations with orbitally tuned chronologies.

Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record

In the interpretation of the Antarctic deep ice-core data, little attention has been given to the Holocene part of the records. As far as translation of the stable isotope content in terms of temperature is concerned, this can be understood because expected temperature changes may be obscured by isotopic noise of various origins and because no 14C dating has yet been available for this type of sequence. In this article, we focus on the Dome C and Vostok cores and on a new 850-m long ice core drilled out at Komsomolskaïa by the Soviet Antarctic Expeditions. These three sites are located in East Antarctica, on the Antarctic plateau, in a region essentially undisturbed by ice-flow conditions, so that their detailed intercomparison may allow us to identify the climatically significant isotopic signal. Our results compare well with the proximal records of Southern Hemisphere high latitudes and support the existence of a warmer “climatic optimum” between 10 and 6 ka y BP. Maximum temperatures are reached just at the end of the last deglaciation, which confirms previous observations at high latitudes, in contrast with later dates for the Atlantic and hypsithermal optima in Europe and North America.

Origin of July Antarctic precipitation and its influence on deuterium content : a GCM analysis

The NASA/GISS GCM is used to estimate the evaporative contributions of several oceanic regions (defined by temperature) to Antarcticas July precipitation. Tracer diagnostics in the GCM suggest that the weighted average evaporative source temperature for Antarctic precipitation as a whole is about 12°C. The average source temperature for local precipitation there varies from 9° C to 14° C. To examine the effect of evaporative source on water isotope concentration, the GCM also follows a global deuterium (HDO) tracer and deuterium tracers evaporating from each oceanic region. The results suggest that although evaporative source temperature does affect the concentrations of the individual HDO tracers, differences in evaporative source do not explain the scatter in the roughly linear relationship between condensation temperature and isotope concentration.

Nonlinear variability of the climatic system from singular and power spectra of Late Quaternary records

Stable-isotope records from seven marine cores and one ice core provide invaluable information on the intricate behavior of the climatic system over time scales of 104 to 105 years. These records, in conjunction with a simple coupled climate model, help us understand major mechanisms of paleoclimatic variability. The time intervals covered by the records include the last glacial-interglacial cycle. In spite of the difference in the nature of the records, common features are revealed by advanced spectral-analysis tools. The dominant features are the presence of orbital frequencies, on the one hand, and a low number of internal degrees of freedom, on the other. The climatic system appears therefore to act on the Quaternary time scales considered as a forced nonlinear oscillator. The internal mechanisms giving rise to the aperiodic oscillations include ice-albedo feedback, precipitation-temperature feedback, and interactions between the ice sheets and the bedrock.