Kurt M. Cuffey

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.

Large arctic temperature change at the Wisconsin-Holocene glacial transition

Analysis of borehole temperature and Greenland Ice Sheet Project II ice-core isotopic composition reveals that the warming from average glacial conditions to the Holocene in central Greenland was large, approximately 15°C. This is at least three times the coincident temperature change in the tropics and mid-latitudes. The coldest periods of the last glacial were probably 21°C colder than at present over the Greenland ice sheet.

Temperature, accumulation, and ice sheet elevation in central Greenland through the last deglacial transition

We present a combined heat- and ice-flow model, constrained by measurements of temperature in the Greenland Ice Sheet Project 2 (GISP2) borehole and by the GISP2 δ18O record and depth-age scale, which determines a history of temperature, accumulation rate, and ice sheet elevation for the past 50,000 years in central Greenland. Important results are: that the temperature increase from average glacial to Holocene conditions was large, approximately 15°C, with a 20°C warming from late glacial to Holocene; that the average accumulation rate during the last glacial maximum (between 15 and 30 kyr B. P.) was 5.5 to 7 cm yr−1, approximately 25% of the modern accumulation rate; that long-term (500–1000 years) averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the Holocene; and that the Greenland Ice Sheet probably thickened during the deglacial transition. The inverse correlation of accumulation rate and temperature in the mid and late Holocene suggests that the Greenland Ice Sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature. The elevation history of the ice sheet is poorly constrained by the model, and independent evidence is needed. We also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins. This was approximately 1900 years for central Greenland during deglaciation.

How glaciers entrain and transport basal sediment: Physical constraints

Abstract Simple insights from the physics of ice, water and sediment place constraints on the possible sediment-transport behavior of glaciers and ice sheets. Because glaciers concentrate runoff, streams generated by glaciers transport much sediment and may erode bedrock rapidly. Deforming glacier beds also can transport much sediment, particularly in marginal regions. Rapid sediment entrainment producing thick debris-rich basal zones may occur by regelation into subglacial materials, and by freeze-on from rising supercooled waters. Numerous other mechanisms may be important but primarily near ice margins, especially those of advancing or fluctuating glaciers. Several sediment-entrainment mechanisms may be active beneath a single glacier, but one process is likely to be dominant at any place and time.

Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet

During the last interglacial period (the Eemian), global sea level was at least three metres, and probably more than five metres, higher than at present. Complete melting of either the West Antarctic ice sheet or the Greenland ice sheet would today raise sea levels by 6–7 metres. But the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the West Antarctic ice sheet, with model studies attributing only 1–2 m of sea-level rise to meltwater from Greenland. This result was considered consistent with ice core evidence, although earlier work had suggested a much reduced Greenland ice sheet during the last interglacial period. Here we reconsider the Eemian evolution of the Greenland ice sheet by combining numerical modelling with insights obtained from recent central Greenland ice-core analyses. Our results suggest that the Greenland ice sheet was considerably smaller and steeper during the Eemian, and plausibly contributed 4–5.5 m to the sea-level highstand during that period. We conclude that the high sea level during the last interglacial period most probably included a large contribution from Greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the West Antarctic ice sheet.

Onset of deglacial warming in West Antarctica driven by local orbital forcing

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

Covariation of carbon dioxide and temperature from the Vostok ice core after deuterium-excess correction

Ice-core measurements of carbon dioxide and the deuterium palaeothermometer reveal significant covariation of temperature and atmospheric CO2 concentrations throughout the climate cycles of the past ice ages. This covariation provides compelling evidence that CO2 is an important forcing factor for climate. But this interpretation is challenged by some substantial mismatches of the CO2 and deuterium records, especially during the onset of the last glaciation, about 120 kyr ago. Here we incorporate measurements of deuterium excess from Vostok in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of climate in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO2 and temperature, of r2 = 0.89 for the past 150 kyr and r2 = 0.84 for the period 350–150 kyr ago. Given the complexity of the biogeochemical systems involved, this close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO2 may be more responsive to temperature than previously thought.

Entrainment at cold glacier beds

Here we present measurements of the gas content and isotopic composition of debris-rich basal layers of a polar glacier, Meserve Glacier, Antarctica, which has a basal temperature of −17 °C. These measurements show that debris entrainment has occurred without alteration of the glacial ice, and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers, without bulk freezing of water. Entrainment at subfreezing temperatures may have formed the U-shaped trough containing Meserve Glacier. In addition to possibly allowing some cold-based glaciers to be important geomorphic agents, entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets, which are rheologically distinct and can limit the time span of ice-core analyses. Furthermore, accumulating evidence suggests that geomorphologists should abandon the assumption that cold-based glaciers do not slide and abrade their beds.

New insights into Southern Hemisphere temperature changes from Vostok ice cores using deuterium excess correction

Abstract The combination of both Vostok ice core deuterium and deuterium excess histories over the last four climate cycles back to 420 ka BP offers a unique opportunity for reconstructions of vapor source region and local temperature variations (ΔTsource and ΔTsite respectively) relative to their modern values. Our study is based on an inversion of ice isotopic composition using a Rayleigh-based isotopic model which estimates the dependence of Vostok precipitation isotopic composition on different climate controls (moisture source temperature, local deposition temperature and ocean isotopic composition). Both reconstructed local and source temperatures show no substantial effects from this correction, in the sense that none of the rapid or large variations in raw deuterium and deuterium excess profiles can be entirely attributed to a second order climate control. Our discussion is focused on moisture source temperature and meridional temperature gradient history (ΔTsource−ΔTsite). First, using sea surface temperature time series, we confirm that moisture source temperature signal recorded in Vostok deuterium excess over the last 150 ka fully reflects the obliquity time-varying relative contribution of low and high ocean latitudes to Vostok precipitation. Second, comparisons of source-to-site meridional temperature gradient with the Vostok sodium record over 420 ka suggest that moisture transport and sea salt transport to the ice sheet are controlled by processes in different latitudes (the sea salt being more of a high-latitude signal).

Diffusion of stable isotopes in polar firn and ice : the isotope effect in firn diffusion

Ice core records are often affected by post-depositional processes that need to be better understood to prevent wrong interpretation of the data. Records of stable isotopes are affected by diffusion both in the firn and in the deeper ice. We present a quantitative theory for diffusion in firn that applies the measured tortuosity factors for O2 and CO2 in fim to the diffusing water vapor. Because of different fractionation factors, the theory predicts stronger smoothing for 8 0 than for bD, in excellent agreement with our data. This effect opens up the possibility for using detailed isotope records to estimate paleotemperatures in deeper strata. We show that this differential smoothing can create an artificial annual cycle in deuterium excess, which was not present at the time of deposition. It also increases the slope observed in high resolution data series between bD and 8 0 variations. For the annual cycles, we observe that this slope can increase from 8 at the surface up to II in deeper firn. In the Holocene ice for the GRIP core, we observe much stronger smoothing than predicted from diffusion in solid ice; this suggests an anomalous diffusion process in glacier ice. Possible models for this excess diffusion are discussed i.a., in terms of the thickness of water films on grain boundaries and in veins.