Initial results from geophysical surveys and shallow coring of the Northeast Greenland Ice Stream (NEGIS)

Abstract

Abstract. The Northeast Greenland Ice Stream (NEGIS) is the sole interior Greenlandic ice stream. Fast flow initiates near the summit dome, and the ice stream terminates approximately 1000 km downstream in three large outlet glaciers that calve into the Greenland Sea. To better understand this important system, in the summer of 2012 we drilled a 67 m firn core and conducted ground-based radio-echo sounding (RES) and active-source seismic surveys at a site approximately 150 km downstream from the onset of streaming flow (NEGIS firn core, 75° 37.61' N, 35°56.49' W). The site is representative of the upper part of the ice stream, while also being in a crevasse-free area for safe surface operations. Annual cycles were observed for insoluble dust, sodium and ammonium concentrations and for electrolytic conductivity, allowing a seasonally resolved chronology covering the past 400 yr. Annual layer thicknesses averaged 0.11 m ice equivalent (i.e.) for the period 1607–2011, although accumulation varied between 0.08 and 0.14 m i.e., likely due to flow-related changes in surface topography. Tracing of RES layers from the NGRIP ice core site shows that the ice at NEGIS preserves a climatic record of at least the past 51 kyr. We demonstrate that a deep ice core drilling in this location can provide a reliable Holocene and late-glacial climate record, as well as helping to constrain the past dynamics and ice-lithosphere interactions of the Greenland Ice Sheet.

Enhanced Firn Densification in High‐Accumulation Shear Margins of the NE Greenland Ice Stream

“…Even if our shots were at the surface, we would only detect three to four raypaths that turn through the upper 20 m of the firn. However, the five detailed seismic refraction profiles that were acquired through the firn along our seismic line (Christianson et al, ; Vallelonga et al, ) all yield similar seismic velocity profiles for the upper ~25 m of the firn (Figure S3). We are thus confident that our limited observations of the upper 20 m of the firn are consistent with a relatively homogeneous firn structure for the upper ~20 m of the firn across our entire seismic profile.…”

“…Because firn is a mixture of ice and air only, velocity can be approximated well solely as a function of density (e.g., Kohnen, ; Robin, ). We develop this calibration using density measured on the firn core that was collected in the center of the geophysical survey (Vallelonga et al, ), ~28 m from the seismic survey line. The spatial offset of the core relative to our profile introduces negligible error, in part because our seismic technique smooths firn properties across ~1 km.…”

“…Figure d shows the location of the summer 2012 field campaign where the geophysical data discussed here were collected. Previous work detailed the results of the associated ground‐based ice‐penetrating radar, shallow ice coring, and active‐seismic efforts at this site (Christianson et al, ; Keisling et al, ; Vallelonga et al, ).…”

“…Several previous studies have detailed subglacial conditions at our field site (Christianson et al, ; Keisling et al, ; Vallelonga et al, ). The results of those studies are summarized in Figure .…”

“…These enhanced accumulation rates should favor the development of a thicker firn layer (Cuffey & Paterson, ) in the shear‐margin depressions. However, seismic data from five sites across the upper reaches of NEGIS (one in the ice stream, two in the shear margins, and two outside) showed that the firn layer was thinner in the margin sites than in the others (Christianson et al, ; Vallelonga et al, ). Here we hypothesize that these surface troughs and variable firn conditions play a role in stabilizing the location of the NEGIS shear margins.…”