Edwin D. Waddington

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.

Numerical simulations of glacial-valley longitudinal profile evolution

Glaciers shape alpine landscapes. They broaden valley bottoms, enhance local valley relief, generate multiple steps, overdeepen valley floors, and cause tributary valleys to hang. These distinctive glacial signatures result from 10 4 –10 5 yr of erosion, during which swings in climate drive advances and retreats of alpine glaciers. We use a numerical model of glacial erosion to explore the development of the longitudinal profiles of glaciated valleys. The model is driven by the past 400 k.y. of variable climate. Because both sliding speed, which dictates abrasion rate, and water-pressure fluctuations, which strongly modulate quarrying rate, should peak at the equilibrium-line altitude (ELA), we expect the locus of most rapid erosion to follow the transient ELA. Simulations of a single glacial valley show rapid flattening of the longitudinal profile. Inclusion of a tributary glacier creates a step immediately downvalley of the tributary junction that persists over multiple glaciations and commonly leaves the tributary valley hanging. Steps and overdeepenings result from an increase in ice discharge immediately below the tributary junction, which is accommodated primarily by increased ice thickness and hence sliding rate. The size of the step increases with the ratio of tributary to trunk ice discharge, while the height of a hanging valley reflects the difference in the time-integrated ice discharge in tributary and trunk valleys and therefore increases as the discharge ratio decreases.

Wisconsinan and Holocene Climate History from an Ice Core at Taylor Dome, Western Ross Embayment, Antarctica

Geochemical data and geophysical measurements from a 554‐m ice‐core from Taylor Dome, East Antarctica, provide the basis for climate reconstruction in the western Ross Embayment through the entire Wisconsinan and Holocene. In comparison with ice cores from central East and West Antarctica, Taylor Dome shows greater variance of temperature, snow accumulation, and aerosol concentrations, reflecting significant variability in atmospheric circulation and air mass moisture content. Extreme aridity during the last glacial maximum at Taylor Dome reflects both colder temperatures and a shift in atmospheric circulation patterns associated with the advance of the Ross Sea ice sheet and accounts for regional alpine glacier retreats and high lake levels in the Dry Valleys. Inferred relationships between spatial accumulation gradients and ice sheet configuration indicate that advance of the Ross Sea ice sheet began in late marine isotope stage 5 or early stage 4. Precise dating of the Taylor Dome core achieved by trace‐gas correlation with central Greenland ice cores shows that abrupt deglacial warming at Taylor Dome was near‐synchronous with the ∼14.6 ka warming in central Greenland and lags the general warming trend in other Antarctic ice cores by at least 3000 years. Deglacial warming was following by a warm interval and transient cooling between 14.6 and 11.7 ka, synchronous with the Bølling/Allerød warming and Younger Dryas cooling events in central Greenland, and out of phase with the Antarctic Cold Reversal recorded in the Byrd (West Antarctica) ice core. Rapid climate changes during marine isotope stages 4 and 3 at Taylor Dome are similar in character to, and may be in phase with, the Northern Hemisphere stadial–interstadial (Dansgaard–Oeschger) events. Results from Taylor Dome illustrate the importance of obtaining ice cores from multiple Antarctic sites, to provide wide spatial coverage of past climate and ice dynamics.

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.

Climate Change During the Last Deglaciation in Antarctica

Greenland ice core records provide clear evidence of rapid changes in climate in a variety of climate indicators. In this work, rapid climate change events in the Northern and Southern hemispheres are compared on the basis of an examination of changes in atmospheric circulation developed from two ice cores. High-resolution glaciochemical series, covering the period 10,000 to 16,000 years ago, from a central Greenland ice core and a new site in east Antarctica display similar variability. These findings suggest that rapid climate change events occur more frequently in Antarctica than previously demonstrated.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

The accumulation pattern across Siple Dome, West Antarctica, inferred from radar-detected internal layers

The spatial distribution of accumulation across Siple Dome,West Antarc-tica, is determined from analysis of the shapes of internal layers detected by radio-echo sounding (RES) measurements. A range of assumed accumulation patterns is used in an ice-flow model to calculate a set of internal layer patterns. Inverse techniques are used to determine which assumed accumulation pattern produces a calculated internal layer pattern that best matches the shape of internal layers from RES measurements. All of the observed internal layer shapes at Siple Dome can be matched using a spatially asymmetric accumulation pattern which has been steady over time. Relative to the divide, the best-fitting accumulation pattern predicts 40% less accumulation 30 km from the divide on the south flank of Siple Dome and 15^40% more accumulation 30 km from the divide on the north flank. The data also allow the possibility for a small time variation of the pattern north of the divide. The mismatch between the calculated and the observed layer shapes is slightly reduced when the accumulation rate north of the divide is higher in the past (>5 kyr BP) than at present. Sensitivity tests show that the predicted change in the spatial accumulation pattern required to cause the slight Siple Dome divide migration (inferred from previous studies) would be detectable in the internal layer pattern if it persisted for 42 kyr. Our analysis reveals no evidence that such a change has occurred, and the possible change in accumulation distribution allowed by the data is in the opposite sense. Therefore, it is unlikely that the Siple Dome divide migration has been caused by a temporal change in the spatial pattern of accumulation. This conclusion suggests the migration may be caused by elevation changes in Ice Streams C and D at the boundaries of Siple Dome.

Possible displacement of the climate signal in ancient ice by premelting and anomalous diffusion

The best high-resolution records of climate over the past few hundred millennia are derived from ice cores retrieved from Greenland and Antarctica. The interpretation of these records relies on the assumption that the trace constituents used as proxies for past climate have undergone only modest post-depositional migration. Many of the constituents are soluble impurities found principally in unfrozen liquid that separates the grain boundaries in ice sheets. This phase behaviour, termed premelting, is characteristic of polycrystalline material. Here we show that premelting influences compositional diffusion in a manner that causes the advection of impurity anomalies towards warmer regions while maintaining their spatial integrity. Notwithstanding chemical reactions that might fix certain species against this prevailing transport, we find that—under conditions that resemble those encountered in the Eemian interglacial ice of central Greenland (from about 125,000 to 115,000 years ago)—impurity fluctuations may be separated from ice of the same age by as much as 50 cm. This distance is comparable to the ice thickness of the contested sudden cooling events in Eemian ice from the GRIP core.

A search in north Greenland for a new ice-core drill site

A new deep ice-core drilling site has been identified in north Greenland at 75.12°N, 42.30°W, 316 km north-northwest (NNW) of the GRIP drill site on the summit of the ice sheet. The ice thickness here is 3085 m; the surface elevation is 2919 m. The North GRIP (NGRIP) site is identified so that ice of Eemian age (115-130 ka BP, calendar years before present) is located as far above bedrock as possible and so the thickness of the Eemian layer is as great as possible. An ice-flow model, similar to the one used to date the GRIP ice core, is used to simulate the flow along the NNW-trending ice ridge. Surface and bedrock elevations, surface accumulation-rate distribution and ratio-echo sounding along the ridge have been used as model input. The surface accumulation rate drops from 0.23 m ice equivalent year -1 at GRIP to 0.19 m ice equivalent year -1 50 km from GRIP. Over the following 300 km the accumulation is relatively constant, before is starts decreasing again further north. Ice thickness up to 3250 m bring the temperature of the basal ice up to the pressure-melting point 100-250 km from GRIP. The NGRIP site is located 316 km from GRIP in a region where the bedrock is smooth and the accumulation rate is 0.19 m ice equivalent year -1 . The modeled basal ice here has always been a few degrees below the pressure-melting point. Internal radio-echo sounding horizons can be traced between the GRIP and NGRIP sites, allowing us to date the ice down to 2300 m depth (52 ka BP). An ice-flow model predicts that the Eemian-age ice will be located in the depth range 2710-2800 m, which is 285 m above the bedrock. This is 120 m further above the bedrock, and the thickness of the Eemian layer of ice is 20 m thicker, than at the GRIP ice-core site.

Migration of the Siple Dome ice divide, West Antarctica

The non-linearity of the ice-flow law or a local accumulation low over an ice divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a divide bump in the internal layer pattern. This divide signature is analyzed using ice-flow models and inverse techniques to detect and quantify motion of the Siple Dome ice divide, West Antarctica. The principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a -1 toward Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calculations also suggest a strong south-north accumulation gradient of 5-10 x 10 -6 a -1 in a narrow zone north of the divide. A consequence of divide migration is that pre-Holocene ice is thickest about 0.5 km south of the present divide position. Divide motion indicates that non-steady processes, possibly associated with activity of the bounding ice streams, are affecting the geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 10 3 -10 4 years.