Gifford J. Wong

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.

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.

Core handling and processing for the WAIS Divide ice-core project

Abstract On 1 December 2011 the West Antarctic Ice Sheet (WAIS) Divide ice-core project reached its final depth of 3405 m. The WAIS Divide ice core is not only the longest US ice core to date, but is also the highest-quality deep ice core, including ice from the brittle ice zone, that the US has ever recovered. The methods used at WAIS Divide to handle and log the drilled ice, the procedures used to safely retrograde the ice back to the US National Ice Core Laboratory (NICL) and the methods used to process and sample the ice at the NICL are described and discussed.

Coast-to-interior gradient in recent northwest Greenland precipitation trends (1952–2012)

The spatial and temporal variability of precipitation on the Greenland ice sheet is an essential component of surface mass balance, which has been declining in recent years with rising temperatures. We present an analysis of precipitation trends in northwest (NW) Greenland (1952–2012) using instrumental (coastal meteorological station) and proxy records (snow pits and ice cores) to characterize the precipitation gradient from the coast to the ice sheet interior. Snow-pit-derived precipitation near the coast (1950–2000) has increased (~7% decade−1, p < 0.01) whereas there is no significant change observed in interior snow pits. This trend holds for 1981–2012, where calculated precipitation changes decrease in magnitude with increasing distance from the coast: 13% decade−1 (2.4 mm water equivalent (w.e.) decade−2) at coastal Thule air base (AB), 8.6% decade−1 (4.7 mm w.e. decade−2) at the 2Barrel ice core site 150 km from Thule AB, −5.2% decade−1 (1.7 mm w.e. decade−2) at Camp Century located 205 km from Thule AB, and 4.4% decade−1 (1.0 mm w.e. decade−2) at B26 located 500 km from Thule AB. In general, annually averaged precipitation and annually and seasonally averaged mean air temperatures observed at Thule AB follow trends observed in composite coastal Greenland time series, with both notably indicating winter as the fastest warming season in recent periods (1981–2012). Trends (1961–2012) in seasonal precipitation differ, specifically with NW Greenland summer precipitation increasing (~0.6 mm w.e. decade−2) in contrast with decreasing summer precipitation in the coastal composite time series (3.8 mm w.e. decade−2). Differences in precipitation trends between NW Greenland and coastal composite Greenland underscore the heterogeneity in climate influences affecting precipitation. In particular, recent (1981–2012) changes in NW Greenland annual precipitation are likely a response to a weakening North Atlantic oscillation.

Observations of Pronounced Greenland Ice Sheet Firn Warming and Implications for Runoff Production

Field measurements of shallow borehole temperatures in firn across the northern Greenland ice sheet are collected during May 2013. Sites first measured in 1952–1955 are revisited, showing long-term trends in firn temperature. Results indicate a pattern of substantial firn warming (up to +5.7°C) at midlevel elevations (1400–2500 m) and little temperature change at high elevations (>2500 m). We find that latent heat transport into the firn due to meltwater percolation drives the observed warming. Modeling shows that heat is stored at depth for several years, and energy delivered from consecutive melt events accumulates in the firn. The observed warming is likely not yet in equilibrium with recent melt production rates but captures the progression of sites in the percolation facies toward net runoff production.

Trace-element and physical response to melt percolation in Summit (Greenland) snow

Abstract Surface melt on a glacier can perturb the glaciochemical record beyond the natural variability. While the centre of the Greenland ice sheet is usually devoid of surface melt, many high-Arctic and alpine ice cores document frequent summertime melt events. Current hypotheses interpreting melt-affected ice-core chemistry rely on preferential elution of certain major ions. However, the precise nature of chemistry alteration is unknown because it is difficult to distinguish natural variability from melt effects in a perennially melt-affected site. We use eight trace-element snow chemistry records recovered from Summit, Greenland, to study spatial variability and melt effects on insoluble trace chemistry and physical stratigraphy due to artificially introduced meltwater. Differences between non-melt and melt-affected chemistry were significantly greater than the spatial variability in chemistry represented by nearest-neighbour pairs. Melt-perturbed trace elements, particularly rare earth elements, retained their seasonal stratigraphies, suggesting that trace elements may serve as robust chemical indicators for annual layers even in melt-affected study areas. Results suggest trace-element transport via meltwater percolation will deposit eluted material down-pit in refrozen areas below the nearest-surface chemistry peak. In our experiments, snow chemistry analyses are more sensitive to melt perturbations than density changes or unprocessed near-infrared digital imagery.

Rapid transport of ash and sulfate from the 2011 Puyehue‐Cordón Caulle (Chile) eruption to West Antarctica

The Volcanic Explosivity Index (VEI) 5 eruption of the Puyehue-Cordon Caulle volcanic complex (PCC) in central Chile, which began 4 June 2011, provides a rare opportunity to assess the rapid transport and deposition of sulfate and ash from a mid-latitude volcano to the Antarctic ice sheet. We present sulfate, microparticle concentrations of fine-grained (~5 μm diameter) tephra, and major oxide geochemistry, which document the depositional sequence of volcanic products from the PCC eruption in West Antarctic snow and shallow firn. From the depositional phasing and duration of ash and sulfate peaks, we infer that transport occurred primarily through the troposphere but that ash and sulfate transport were decoupled. We use Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectory modeling to assess atmospheric circulation conditions in the weeks following the eruption, and find that conditions favored southward air parcel transport during 6-14 June and 4-18 July, 2011. We suggest that two discrete pulses of cryptotephra deposition relate to these intervals, and as such, constrain the sulfate transport and deposition lifespan to the ~2-3 weeks following the eruption. Finally, we compare PCC depositional patterns to those of prominent low- and high-latitude eruptions in order to improve multiparameter-based efforts to identify “unknown source” eruptions in the ice core record. Our observations suggest that mid-latitude eruptions such as PCC can be distinguished from explosive tropical eruptions by differences in ash/sulfate phasing and in the duration of sulfate deposition, and from high-latitude eruptions by differences in particle size distribution and in cryptotephra geochemical composition.