J. Jouzel and G. Delaygue
420,000 years BP-present
Because isotopic fractions of the heavier oxygen-18 (18O) and deuterium (D) in snowfall are temperature-dependent and a strong spatial correlation exists between the annual mean temperature and the mean isotopic ratio (18O or δD) of precipitation, it is possible to derive ice-core climate records. The record presented by Jouzel et al. (1987) was the first ice core record to span a full glacial-interglacial cycle. That record was based on an ice core drilled at the Russian Vostok station in central east Antarctica. The 2083-m ice core was obtained during a series of drillings in the early 1970s and 1980s and was the result of collaboration between French and former-Soviet scientists. Drilling continued at Vostok and was completed in January 1998, reaching a depth of 3623 m, the deepest ice core ever recovered (Petit et al. 1997, 1999). The resulting core allows the ice core record of climate properties at Vostok to be extended to ~420 kyr BP.
The first isotopic analysis of the Vostok ice core was described in Lorius et al. (1985). Sampling of ice for 18O and deuterium was done in the field during the 1982-83 austral summer by cutting a continuous slice from the length of ice after careful cleaning. Sampling was performed on 1.5- to 2-m increments of ice. Samples were sent in solid form to Grenoble, France, and then melted before isotopic analysis in Saclay, France. Two independent series of samples were obtained. For the discontinuous series, duplicated to check reproducibility, one sample was taken at each 25-m interval from the surface down to the bottom of the core. For the continuous series, samples were collected between 1406 and 2083 m. Oxygen-18 and deuterium determinations were simultaneously performed on all the samples, and the δ18O results were discussed in Lorius et al. (1985).
The 420-kyr Vostok temperature record presented here was reconstructed from the continuous deuterium profile measured along the core. The new measurements were taken along ice in increments between 0.5 and 2 m in length to a depth of 2080 m and then every 1 m for the remainder of the upper 3310-m of the ice core. Isotopic analysis was again performed by the Geochemistry team at LSCE at Saclay. A sudden decrease from interglacial-like δDice values to glacial-like-values, followed by an abrupt return to interglacial-like values, occurs between 3320 and 3330 m (Petit et al. 1999). This occurrence plus the presence of volcanic ash layers at 3311 m suggests that the Vostok climate records may be disturbed below 3311 m. Thus, discussion of the new data set is limited to the upper 3310 m of the ice core. Petit et al. (1999) reported an ice recovery rate of 85% or higher and a measurement accuracy of ± 0.5°/°° Surface Mean Ocean Water (SMOW). The temperature estimates are based on both experimental and theoretical arguments. One of the fundamental arguments used in deriving this temperature record is that the deuterium content distribution is well documented over East Antarctica and over a large range of temperatures (-20° to -55° C); thus, there is a linear relationship between the average annual surface temperature and the snow deuterium content. The slope of this δD/surface temperature relationship was found by Jouzel et al. (1993, 1996) and Petit et al. (1999) to be 9°/°° per °C. Further details on the methodology are presented in Jouzel et al. (1987), Lorius et al. (1985), and Petit et al. (1999).
The strong correlation between atmospheric greenhouse-gas concentrations and Antarctic temperature, previously described by Barnola et al. (1987), is confirmed by the extension of the Vostok ice-core record (Petit et al. 1999). From the extended Vostok record, Petit et al. (1999) concluded that present-day atmospheric burdens of carbon dioxide and methane seem to have been unprecedented during the past 420,000 years. Temperature variations estimated from deuterium were similar for the last two glacial periods (Jouzel et al. 1996), and the detailed δDice record confirms the main features of the third and fourth climate cycles described by Petit et al. (1997). The records also indicate both similarities and differences between successive interglacial periods. Although the third and fourth climate cycles are of shorter duration than the first two cycles in the Vostok record, all four climate cycles show a similar sequence of a warm interglacial, followed by colder glacial events, and ending with a rapid return to an interglacial period. Minimum temperatures are within 1°C for the four climate cycles. The overall amplitude of the glacial-interglacial temperature change is ~8°C for the temperature above the inversion level and ~12°C for surface temperatures. Climate cycles deduced from the Vostok ice core appear to be more uniform than those in deep-sea core records (Petit et al. 1999).
Date revised January 2000