David A. Peel

Eight glacial cycles from an Antarctic ice core

The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.

Validity of the temperature reconstruction from water isotopes in ice cores

Well-documented present-day distributions of stable water isotopes (HDO and H218O) show the existence, in middle and high latitudes, of a linear relationship between the mean annual isotope content of precipitation (δD and δ18O) and the mean annual temperature at the precipitation site. Paleoclimatologists have used this relationship, which is particularly well obeyed over Greenland and Antarctica, to infer paleotemperatures from ice core data. There is, however, growing evidence that spatial and temporal isotope/surface temperature slopes differ, thus complicating the use of stable water isotopes as paleothermometers. In this paper we review empirical estimates of temporal slopes in polar regions and relevant information that can be inferred from isotope models: simple, Rayleigh-type distillation models and (particularly over Greenland) general circulation models (GCMs) fitted with isotope tracer diagnostics. Empirical estimates of temporal slopes appear consistently lower than present-day spatial slopes and are dependent on the timescale considered. This difference is most probably due to changes in the evaporative origins of moisture, changes in the seasonality of the precipitation, changes in the strength of the inversion layer, or some combination of these changes. Isotope models have not yet been used to evaluate the relative influences of these different factors. The apparent disagreement in the temporal and spatial slopes clearly makes calibrating the isotope paleothermometer difficult. Nevertheless, the use of a (calibrated) isotope paleothermometer appears justified; empirical estimates and most (though not all) GCM results support the practice of interpreting ice core isotope records in terms of local temperature changes.

The ratio of MSA to non‐sea‐salt sulphate in Antarctic Peninsula ice cores

Methane sulphonic acid (MSA) in an ice core from Dolleman Island (70°35′S, 60°56′W) shows significantly high concentrations (typically 1-2 µm, but up to 5 µm) compared to values recorded in ice cores and in snowfall from elsewhere in Antarctica. MSA data from two other higher altitude Antarctic Peninsula ice cores, Dyer Plateau (70°31′S, 65°01′W) and Gomez Nunatak (74°01′S, 70°38′W), show that the high concentrations measured at Dolleman Island are not representative of the Peninsula region as a whole. However the mean molar MSA/nss-SO4 2- ratios at the three sites are similar (Dolleman Is, 0.46; Gomez, 0.37; Dyer, 0.32). Exceptionally high concentrations observed at Dolleman Island may be related to its proximity to the biologically productive Weddell Sea, an important source of dimethyl sulphide (DMS), the precursor of MSA. The MSA data from this site are further unusual in that in deeper sections of this core they demonstrate a well defined seasonal maximum in winter rather than in summer and are out of phase with non sea-salt sulphate, another product of the decomposition of DMS. In contrast, in a near-surface section, MSA variations are in phase with non sea-salt sulphate, with a maximum concentration in the summer layer. A change in the season of deposition of MSA from winter to summer in the recent past is not considered likely. An alternative explanation is that there has been a relocation of the MSA from summer to winter layers during burial.

Antarctic snow record of cadmium, copper, and zinc content during the twentieth century

A snowpit in Coats Land, Antarctica, has been sampled in order to obtain a record of Cd, Cu and Zn covering the period 1923–1986. The snowpit record gives an indication of southern hemisphere (SH) pollution reaching Antarctica. For Zn, concentrations (averaging 1.5 ng kg-1) can be explained as arising from natural crustal dust (based on Zn/Al ratios). No increase is observed over the period of the record here, despite a large increase in emissions from smelting operations. The main emitters are near the equator, and this may explain the lack of response in the Antarctic record. For Cd, concentrations (averaging 0.1 ng kg-1) cannot easily be explained in terms of natural emissions, unless the volcanic input is dominant. No significant increase is seen in the snow for this metal also. For Cu, the natural input can explain only a small part of the concentration (averaging 3.5 ng kg-1) measured, and increased concentrations (factor 2) are seen in the 1970s and 1980s compared to earlier decades. This is consistent with increased emissions from Cu smelting activities, particularly in Chile, where emissions are relatively far south compared to the main part of SH landmasses. Cu thus joins Pb as a metal whose natural cycle has been significantly perturbed even in the Antarctic atmosphere.

Dimethyl sulfide and its oxidation products in the atmosphere of the Atlantic and Southern Oceans

Dimethyl sulfide, methane sulfonate, non-sea-salt sulfate and sulfur dioxide concentrations in air were obtained during a cruise between the U.K. and the Antarctic during the period October 1992–January 1993. In equatorial regions (30°N to 30°S) the atmospheric DMS concentration ranged from 3 to 46 ng (S)m−3 with an average of 18 ng(S)m−3. In the polar waters and regions south of the Falkland Islands concentrations from 3 to 714ng(S)m−3 were observed with a mean concentration of 73ng(S)m−3. Methane sulfonate concentrations were also enhanced in the vicinity of the Antarctic Peninsula and in the Weddell Sea. A simple model of DMS oxidation was used to estimate the ocean to atmosphere flux rate, and this was found to be within the range of previous estimates, with a mean value of 1011 ng(S) m−2 h−1.

1000 year ice-core records from Berkner Island, Antarctica

Abstract Two medium-depth ice cores were retrieved from Berkner Island by a joint project between the Alfred-Wegener-Institut and the British Antarctic Survey in the 1994/95 field season. A 151m deep core from the northern dome (Reinwarthhöhe) of Berkner Island spans 700 years, while a 181 m deep core from the southern dome (Thyssenhöhe) spans approximately 1200 years. Both cores display clear seasonal cycles in electrical conductivity measurements, allowing dating by annual-layer counting and the calculation of accumulation profiles. Stable-isotope measurements (both δ18O and δD), together with the accumulation data, allow us to estimate changes in climate for most of the past millennium: the data show multi-decadal variability around a generally stable long-termmean. In addition, a full suite of major chemistry measurements is available to define the history of aerosol deposition at these sites: again, there is little evidence that the chemistry of the sites has changed over the past six centuries. Finally, we suggest that the southern dome, with an ice thickness of 950 m, is an ideal site from which to gain a climate history of the late stages of the last glacial and the deglaciation for comparison with the records from the deep Antarctic ice cores, and with other intermediate-depth cores such as Taylor Dome and Siple Dome.

Air Temperature and Snow Accumulation in the Antarctic Peninsula During the Past 50 Years (Abstract)

Trends in climate affecting the West Antarctic ice sheet may be detected first in the Although the area contains the records for any part of accumulation data are lacking. Antarctic Peninsula region . most comprehensive weather Antarctica, reliable snowMainly as a result of the large snow-accumulation rate in the region (typically in the range 4.0-10.0 kg m2 ai), stratigraphic evidence of climate derived from ice cores can be resolved in much greater detail than is possible over most of the continent. Ice cores have been drilled at two sites, representing the extremes of climate type encountered in the region. A 133 m core has been obtained from Dolleman Island (70 °35.2, S, 60 °55.5, W) to represent the continental-type climate of the Weddell coast region, and an 87 m core has been obtained from the Palmer Land plateau (74 °01 , S, 70 °38 , W) to represent the more maritime regime of the west coast and central areas. Replicated cores were obtained at both sites in order to assess the contribution of local noise factors to the climatic signal preserved in the cores. Climatic trends during the period 1938-86 have been assessed on the basis of stable-isotope analysis of the top 47 m of the Palmer Land core and of the top 32 m of the Dolleman Island core . A statistical analysis of derived profiles of mean annual 60 and accumulation rate indicates that the local noise factors at these sites are sufficiently small that data averaged over periods as short as 5 years should reveal climatic shifts at the level of 0.29X> and 5% respectively. These changes are much smaller than trends that have actually occurred during the past 50 years. The most notable trend over the past 30 years is an increase of more than 30% in the snow-accumulation rate that has occurred in parallel with an overall temperature increase of 0.06 ° Ci a during the same period . Increases of similar magnitude can be inferred from studies in East Antarctica, and may be related to a significant increase in precipitation rate that has been documented recently at midto high-latitude stations in the Northern Hemisphere. The finding may have relevance to studies of the possible consequences of a CO2-induced climate change. More extensive accumulation time series are now required from Antarctica, if satisfactory models of the long-term balance of the ice sheet are to be derived.

The Chemistry and Climatic Role of Biogenic Sulfur: Group Discussion

Discussions centred on the behaviour of dimethyl sulfide (DMS), the most abundant volatile sulfur compound detectable in sea-water, and its primary end oxidation products methyl sulphonic acid (MSA) and sulfate. MSA is believed to be exclusively derived from MSA and therefore is potentially an important tracer for biogenic sulfate productivity. The ratio of MSA/nss sulfate in particular has been proposed [Saltzman, this volume] as a tracer for the origin of sulfate in marine air masses and in subsequent snowfall deposits, MSA has therefore become a target for ice-core studies aimed to detect either a response in the sulfur cycle to a climate shift, or evidence for possible climate forcing by sulfate aerosol. However, due to the inhomogeneous global distribution of DMS producing phytoplankton, combined with uncertainty about their likely climate sensitivity, the spatial pattern of emissions is still poorly known. Signals detected in ice cores are therefore a complex integral of a variety of processes affecting not only the emission rates at source, but also the pathways and efficiencies involved during transport and deposition onto the ice sheets, In addition, sub-annual changes recorded in the ice may be affected by limited post-depositional migration within the snowpack which has been reported at several near-coastal Antarctic sites.