General Scientific Context:
Basal and subglacial studies at the WAIS Divide ice coring site will address several of the top-level scientific goals identified previously by the WAIS Divide science community: (1) determining how the West Antarctic Ice Sheet (WAIS) responded to climate changes in the past, (2) improving models of WAIS contribution to near-future global sea level changes, and (3) understanding microbial life in, and beneath, deep ice. These are long-standing scientific goals with well-established scientific and societal relevance. At the present time the WAIS Divide drilling is planned to cease 25m above the bed, thereby omitting sampling of basal ice and subglacial materials. This document outlines the scientific benefits and technological ramifications of continuing drilling and sampling below the current limit of 25m above the bed at the WAIS divide site.
Location and Setting:
The WAIS Divide drill hole provides an unprecedented opportunity to gain access to deep samples of basal ice, as well as subglacial water and sediments, which will permit unique glaciological, geological, and biogeochemical measurements, including measurements of fundamental transitory geochemical and microbial parameters that have not been analyzed previously in Antarctica. Location of the WAIS Divide drill site in the central part of the ice sheet sets this basal/subglacial sampling site apart from previous similar studies, which focused on settings closer to ice sheet margin with thinner ice cover. Recent radar and seismic investigations performed by teams from Penn State University and CReSIS indicate that: (1) presence of a debris-bearing basal ice layer is expected because basal melting conditions are confined to just a few km upstream of the drill site, (2) the subglacial hydrological system beneath the drill site should have relatively low annual water flux rates, of the order of 1 cubic meter per year per meter width of the ice sheet, (3) the ice base is underlain by a 10-to-15-m-thick layer of sediments, which themselves rest on bedrock.
We have formulated the following three recommendations based on scientific, technological, and operational considerations.
Minimum Plan - Recovery of a continuous basal ice core from 25 meters above the bed to the bed - Acquisition of a basal ice core represents the least complicated extension of the current WAIS Divide drilling plan because it can be accomplished using the existing drill and with limited environmental impact. Analyses of ice, gas, and particulates from a basal core can be used to meet address glaciological, geological, and biological questions identified in this document.
Limited Subglacial Penetration Plan - Recovery of a continuous basal ice core and ~1m of subglacial sediments - Addition of ~1m of subglacial sediments will significantly extend the range of biological, glaciological, and geological investigations which will benefit from analyses of modern subglacial materials. Any extension of the sampling plan into the subglacial environment poses technological, environmental, and operational challenges, which are related to potential contamination, borehole management, and the need for a new sampling tool needed to recover the short subglacial core. Such sampling tool can be a simple push corer attached to the existing drill.
Full Subglacial Sampling Plan - Recovery of a continuous basal ice core, subglacial water sample, full subglacial sediment thickness, and subglacial bedrock - This plan would require development of several new tools for water sampling, 10-to-15m sediment core, and a few meters of bedrock core. Operational considerations would need to take into account the issue of borehole motion during sampling and management of borehole fluid pressure to avoid fluid injection into the subglacial zone.