Ice and firn heterogeneity within Larsen C Ice Shelf from borehole optical televiewing

Abstract

We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light‐emitting diode‐illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding‐based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density‐luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV‐derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E‐W gradient in föhn‐driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice.

Centuries of intense surface melt on Larsen C Ice Shelf

“…The five borehole images are described in detail in Hubbard et al (2016) and Ashmore et al (2017); in summary, across the sites, four different ice types, or units, were identified on the basis of visual appearance, density, and refrozen ice content. Image thresholding was used to determine the proportion of ice within each unit that is composed of refrozen infiltration ice.…”

Observationally constrained surface mass balance of Larsen C ice shelf, Antarctica

“…We are effectively lumping all changes in the FAC into the surface density, which do not necessarily represent real surface processes. In reality, surface melt is seasonal, and meltwater infiltration and refreezing do not occur right at the surface, resulting in complex ice structures that can be interspersed with firn pockets throughout the vertical firn column (Ashmore et al, 2017). Therefore, in areas of high melt, the scheme will yield unrealistically high surface density estimates in order to accommodate for refreezing within the firn column that is not accounted for in our dry snow densification model.…”

“…The advection of warm, dry air masses over the ice shelf during föhn winds is the likely cause of this surface melt distribution . A notable expression of this surface melt gradient is seen in the composition of the firn layer over the LCIS: the smallest amounts of firn air are found in the inlets in the western part of the LCIS (Holland et al, 2011;Ashmore et al, 2017). In Cainet Inlet (Fig.…”

“…1. In 2015, a 90 m-long borehole was drilled with hot water and surveyed with an optical televiewer (Hubbard et al, 2008) at the site of AWS14 (referred to as CI-120 in Ashmore et al, 2017). Using the vertical profile of luminosity as a proxy for density (Hubbard et al, 2013), an estimate of firn density is available for the entire length of the borehole.…”

“…To convert the estimated reflector depth to mass, we assume that the reflector is below the pore close-off depth (estimated at 3-9 m within 120 km from the grounding line (Ashmore et al, 2017) and likely around 10-15 m further downstream), and then subtract the firn air thickness of Holland et al (2011) from the reflector depth, yielding an iceequivalent thickness, which is converted to mass using an ice density of 910 kg m −3 . Column mass is corrected for differences in acquisition dates of the various radar lines, assuming a mean accumulation of 0.45 m w.e.…”