Summary of Results from the WAIS Divide Ice Core Project

Ice cores are highly valued in paleoclimate research because they record environmental parameters that range on spatial scales from individual snowflakes to the earth’s atmosphere, and on time scales from hours to hundreds of millennia. Ice cores are our only source of samples of the paleoatmosphere. They are especially valuable for investigating climate forcing and response, because they record many aspects of the climate system in a common, well-dated archive.

The main objective of the WAIS (West Antarctic Ice Sheet) Divide ice core project (drilling operations from 2006-2013) was to investigate climate from the last glacial period to modern conditions, with greater time resolution than previous Antarctic ice cores.  In addition, the project investigated the dynamics of the West Antarctic ice sheet and cryobiology. Funding was provided by the United States National Science Foundation (NSF) through the United States Antarctic Program (USAP).  The distinguishing characteristic of the project was the development of environmental records of the last glacial period and early Holocene, with greater time resolution and dating precision than previous Antarctic ice cores. This is particularly true for the records of atmospheric gases, water isotopes and chemistry. This special issue of Paleoceanography includes all papers that AGU has published on the project as of May 2016.

A site in Antarctica was required to provide a southern hemisphere equivalent to the deep Greenland ice cores. An Antarctic site also allowed development of a more detailed record of atmospheric carbon dioxide, which cannot be obtained in Greenland due to in situ reactions associated with the higher levels of impurities in Greenlandic ice. The selected site was near an ice-flow divide, and characterized by a combination of moderate ice accumulation rate, thick ice, and other characteristics that preserved environmental records with the desired time resolution and duration (Morse et al. 2002 and Neumann et al. 2008).

Logistics support for field operations was initially provided by Raytheon Polar Services Company and later by NSF’s Antarctic Support Contractor (Lockheed Martin).  Air transport was provided by the USAF 109th Air National Guard. Drilling support was provided by the Ice Drilling Program Office and the Ice Drilling Design and Operations group at the University of Wisconsin, Madison. Drilling was halted 50 m above the bed at a depth of 3,405 m to leave an environmental barrier between the drilling fluid and the pristine basal aqueous environment.  Additional core was collected from five zones of special interest by drilling through the side of the borehole and collecting a total of 285 meters of core nearly parallel to the main core. This major advance in ice coring technology left the entire main hole intact for borehole logging.

The core was sampled at the U.S. National Ice Core Laboratory in Denver, Colorado and subsamples where sent to 17 individually funded institutions for analysis. The Science Coordination Office, at the Desert Research Institute (Nevada System of Higher Education) and University of New Hampshire, coordinated the science, logistics, drilling and sampling.

The high accuracy of dating was possible due to many factors. A drill site with relatively thick annual layers and other favorable characteristics was selected (Morse et al. 2002). Improvements in drilling (Shturmakov et al. 2007) and core handling (Souney et al. 2014) resulted in exceptional core quality.  New core analysis methods with high time resolution were utilized (Ahn et al. 2009, Rhodes et al. 2013 and 2015, McConnell et al. 2002 and 2007, Fudge et al. 2016). The WAIS Divide ice core provides a new reference chronology for Antarctica ice cores, enabling detailed reconstructions of paleotemperatures, volcanic forcing and anthropogenic pollution histories (PAGES 2k Consortium 2013, Steig et al. 2013, 2014, and 2015; McConnell et al. 2014).

The most recent 31 ka was dated by continuously counting annual layers identified in records of electrical conductivity, multi-parameter aerosols and trace elements (Sigl et al. 2016 and McGwire et al. 2011). Continuous identification of annual layers to this age was a major advance for Antarctic ice cores, enabling synchronization of ice cores from Greenland and Antarctica. This clarified the influence of volcanic eruptions on climate, societal disruptions, famines and pandemics during the last 2,500 years (Sigl et al. 2015), and demonstrated that cooling from volcanic sulfate aerosols is the primary driver of short term climate variability (Sigl et al. 2013 and 2015). High resolution, multi-parameter measurements coupled with exact synchronization between Greenland and Antarctic ice cores underpinned detailed investigation of extreme cosmic ray events during the first millennium (Mekhaldi et al. 2015), as well as evaluation of biomass burning and other aerosols in Earth System Model simulations (Bisiaux et al. 2012, Bauer et al. 2013, Lee et al. 2013, and Lamarque et al. 2013). 

The annual layer timescale and simple ice flow at the site resulted in the first record of ice accumulation rate during the last deglaciation that is independent of an assumed relationship to water stable isotopes. The ice accumulation rate did not consistently correlate with water isotopes, particularly at times of abrupt climate change in the Northern Hemisphere (Fudge et al. 2016).  This calls into question the common practice of using water isotopes as a surrogate for the ice accumulation rate (WAIS Project Members 2013 and Buizert et al. 2015).

The time period from 31 ka to 68 ka was dated using stratigraphic techniques to tie the record to records of other ice cores and radiometrically dated speleothems (Buizert, et al., 2015).  In ice cores, the age of the ice is older than the age of the atmospheric gases that are trapped in the ice.  At WAIS Divide this delta-age was a half to a tenth smaller than in most other deep Antarctic cores. The low uncertainty in delta-age made it possible to more accurately determine the relative timing of changes in atmospheric carbon dioxide and Antarctic climate during the last deglaciation, conclusively showing the close coupling between the concentration of carbon dioxide in the atmosphere and climate (Marcott et al. 2014).  

The high temporal resolution of the atmospheric carbon dioxide record revealed three abrupt increases in carbon dioxide at the times of major climate events (Heinrich Stadial 1, the Bølling warming, and the termination of the Younger Dryas).  These enigmatic features of the deglaciation have spurred a discussion of the possible sources of carbon capable of responding to climate on the centennial-scale (Bauska et al. 2016). The high temporal resolution also allowed detailed records of late Holocene variability in the concentration (Ahn et al. 2012) and isotopic composition (Bauska et al. 2015) of atmospheric carbon dioxide, revealing multi-decadal changes in land carbon storage.  

The detailed atmospheric methane record defined new modes of variability, including sharp increases during some of the cold Greenland “Heinrich” stadials. This led to a hypothesis that extreme southward migration of the ITCZ during the Greenland stadials activated southern hemisphere methane sources (Rhodes et al. 2015).  During Heinrich Stadial 1, this possible southern hemisphere source of methane is directly associated with an abrupt rise in atmospheric carbon dioxide. Additional work constrained the anthropogenic contribution to atmospheric methane during the late Holocene (Mitchell et al. 2013), possibly including preindustrial human influence (Mischler et al. 2009), and other details of methane during the Holocene period (Sowers 2010).

The large amounts of high quality ice facilitated progress in ice core trace gas research. New advancements include measurements of ultra-trace level gases such as carbonyl sulfide and ethane that provide new constraints on the global carbon cycle and the atmospheric methane budget (Aydin et al. 2014 and 2016 and Nicewonger et al. 2015).

The improved dating showed that the abrupt temperature changes in Greenland associated with the Dansgaard-Oeschger events were followed by opposing temperature changes in the Antarctic, ~200 years later. This was observed for both warming and cooling, and for larger and smaller events (WAIS Divide Project Members 2015). This northern-lead pattern is consistent with the hypothesis that abrupt reduction of heat transport between the hemispheres by the Atlantic Meridional Overturning Circulation (AMOC), driven by changes in deep water sinking in the North Atlantic, links the climate of the Northern and Southern Hemispheres during these events (WAIS Divide Project Members 2015).  The deuterium excess record from the same time shows that abrupt latitudinal shifts in the southern westerlies occurred in phase with the Dansgaard-Oeschger events, with no discernable lag (Markle et al. 2017). This clarifies the role of both the ocean and the atmosphere in propagating abrupt climate change between the hemispheres.

The record of paleo-surface temperature was determined using a combination of borehole paleothermometry (Orsi et al. 2012), water isotopes (Steig et al. 2013, WAIS Divide Project Members 2013 and 2015 and Jones et al. 2016), isotopes of atmospheric nitrogen which are linked to the thickness of the firn (Buizert et al., 2015), bubble characteristics which are linked to processes in the firn (Fegyveresi et al. 2016), and the ice accumulation rate (Fudge et al. 2016). This integrated approach provided an exceptionally well-constrained record of past surface temperature (Cuffey et al. 2016). The result confirmed that global temperature changes were amplified in the Antarctic, and that Antarctica warmed substantially by 15 ka, coincident with glacier retreat in Southern Hemisphere mountain ranges. The combination of ice core and borehole temperature data also provides independent confirmation (Orsi et al. 2012 and Steig and Orsi 2013) of the recent, rapid warming of West Antarctica, previously inferred from the sparse instrumental record (Steig et al. 2009 and Bromwich et al. 2013).

The water stable isotope record shows distinct differences from records in central East Antarctica cores. In particular, the West Antarctic isotopic warming began ~2 ka prior to the East Antarctic warming at ~18 ka, indicating an influence of orbital forcing (WAIS Divide Project Members 2013). Climate model simulations, coupled with measurements of the novel water-isotope parameter 17Oexcess, show that the early warming was amplified by a reduction in sea ice, forced by the local insolation change (Schoenemann et al. 2014).

The physical properties of the core provided a window into deformational processes in the ice sheet as recorded by the layering and characteristics of the c-axis and ice grains. The ice flow did not disrupt the climate record, except for cm scale features. (Fitzpatrick et al. 2014 and Fudge et al. 2016). The numerous crusts in the ice core did not affect the trapped-gas records (Mitchell et al. 2015) because they do not incorporate refrozen meltwater in large volumes (Orsi et al. 2015), and they typically were broken into polygonal patterns by contraction from nighttime cooling in the days after they formed on the surface  (Fegyveresi et al. 2016).  Snow accumulation rates do impact gas trapping in firn through snow microstructure characteristics (Gregory et al 2014).

Analysis of the depth-age and temperature profiles with thermo-mechanical ice-flow models suggests that the ice thickness at WAIS Divide has not changed by more than a few hundred meters during the last 70 ka  (Koutnik et al. 2016). The divide is currently migrating toward the Ross Sea at a rate of ~10 m/yr (Conway and Rasmussen 2009 and Koutnik et al. 2016). They hypothesized that the migration is a result of dynamical thinning that is presently stronger in the Amundsen Sea sector than in the Ross Sea sector. The site does not provide information on the status of WAIS during MIS-5e because basal melting would have melted any ice from that time (WAIS Divide Project Members 2013).

Biology was an integral part of the project. Research on the core showed that prokaryotic cells respond to large-scale environmental and climatic processes on millennial time scales. Higher cell concentration occurred during the LGM and the early-Holocene than during the last deglaciation. Collectively, the data reveal that variability in prokaryotic cell concentration may reflect changes in marine/sea-ice regional environments related to sea-ice extent, sea-level rise and ice sheet retreat. (Santibanez et al. 2016)

In addition to these and many more science results, the project left an archive of ice for future investigations, and developed a new generation of researchers who will build on these results for decades to come.

Kendrick Taylor
Research Professor, Desert Research Institute, Nevada System of Higher Education

WAIS Divide Project Science Team
Figure 1. The WAIS Divide Project Science Team

Citations

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