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

Ashmore, David W.; Hubbard, Bryn; Luckman, Adrian; Kulessa, Bernd; Bevan, Suzanne; Booth, Adam; Munneke, Peter Kuipers; O’Leary, Martin; Sevestre, Heïdi; Holland, Paul R.

doi:10.1002/2016jf004047

Cited by 23 publications

(36 citation statements)

References 57 publications

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“…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.…”

Section: Introductionmentioning

confidence: 99%

Centuries of intense surface melt on Larsen C Ice Shelf

et al. 2017

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Abstract. Following a southward progression of ice-shelf disintegration along the Antarctic Peninsula (AP), Larsen C Ice Shelf (LCIS) has become the focus of ongoing investigation regarding its future stability. The ice shelf experiences surface melt and commonly features surface meltwater ponds. Here, we use a flow-line model and a firn density model (FDM) to date and interpret observations of meltaffected ice layers found within five 90 m boreholes distributed across the ice shelf. We find that units of ice within the boreholes, which have densities exceeding those expected under normal dry compaction metamorphism, correspond to two climatic warm periods within the last 300 years on the Antarctic Peninsula. The more recent warm period, from the 1960s onwards, has generated distinct sections of dense ice measured in two boreholes in Cabinet Inlet, which is close to the Antarctic Peninsula mountains -a region affected by föhn winds. Previous work has classified these layers as refrozen pond ice, requiring large quantities of mobile liquid water to form. Our flow-line model shows that, whilst preconditioning of the snow began in the late 1960s, it was probably not until the early 1990s that the modern period of ponding began. The earlier warm period occurred during the 18th century and resulted in two additional sections of anomalously dense ice deep within the boreholes. The first, at 61 m in one of our Cabinet Inlet boreholes, consists of ice characteristic of refrozen ponds and must have formed in an area currently featuring ponding. The second, at 69 m in a mid-shelf borehole, formed at the same time on the edge of the pond area. Further south, the boreholes sample ice that is of an equivalent age but which does not exhibit the same degree of melt influence. This west-east and north-south gradient in the past melt distribution resembles current spatial patterns of surface melt intensity.

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Section: Introductionmentioning

confidence: 99%

Centuries of intense surface melt on Larsen C Ice Shelf

et al. 2017

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“…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.…”

Section: Ground-penetrating Radarmentioning

confidence: 95%

“…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.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

Section: Snow Pits and Firn Coresmentioning

confidence: 99%

“…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.…”

Section: Ground-penetrating Radarmentioning

confidence: 99%

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

et al. 2017

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Abstract. The surface mass balance (SMB) of the Larsen C ice shelf (LCIS), Antarctica, is poorly constrained due to a dearth of in situ observations. Combining several geophysical techniques, we reconstruct spatial and temporal patterns of SMB over the LCIS. Continuous time series of snow height (2.5-6 years) at five locations allow for multi-year estimates of seasonal and annual SMB over the LCIS. There is high interannual variability in SMB as well as spatial variability: in the north, SMB is 0.40 ± 0.06 to 0.41 ± 0.04 m w.e. year −1 , while farther south, SMB is up to 0.50 ± 0.05 m w.e. year −1 . This difference between north and south is corroborated by winter snow accumulation derived from an airborne radar survey from 2009, which showed an average snow thickness of 0.34 m w.e. north of 66 • S, and 0.40 m w.e. south of 68 • S. Analysis of ground-penetrating radar from several field campaigns allows for a longer-term perspective of spatial variations in SMB: a particularly strong and coherent reflection horizon below 25-44 m of waterequivalent ice and firn is observed in radargrams collected across the shelf. We propose that this horizon was formed synchronously across the ice shelf. Combining snow height observations, ground and airborne radar, and SMB output from a regional climate model yields a gridded estimate of SMB over the LCIS. It confirms that SMB increases from north to south, overprinted by a gradient of increasing SMB to the west, modulated in the west by föhn-induced sublimation. Previous observations show a strong decrease in firn air content toward the west, which we attribute to spatial patterns of melt, refreezing, and densification rather than SMB.

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The Impact of Föhn Winds on Surface Energy Balance During the 2010–2011 Melt Season Over Larsen C Ice Shelf, Antarctica

et al. 2017

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We use model data from the Antarctic Mesoscale Prediction System (AMPS), measurements from automatic weather stations and satellite observations to investigate the association between surface energy balance (SEB), surface melt, and the occurrence of föhn winds over Larsen C Ice Shelf (Antarctic Peninsula) over the period November 2010 to March 2011. Föhn conditions occurred for over 20% of the time during this period and are associated with increased air temperatures and decreased relative humidity (relative to nonföhn conditions) over the western part of the ice shelf. During föhn conditions, the downward turbulent flux of sensible heat and the downwelling shortwave radiation both increase. However, in AMPS, these warming tendencies are largely balanced by an increase in upward latent heat flux and a decrease in downwelling longwave radiation so the impact of föhn on the modeled net SEB is small. This balance is highly sensitive to the representation of surface energy fluxes in the model, and limited validation data suggest that AMPS may underestimate the sensitivity of SEB and melt to föhn. There is broad agreement on the spatial pattern of melt between the model and satellite observations but disagreement in the frequency with which melt occurs. Satellite observations indicate localized regions of persistent melt along the foot of the Antarctic Peninsula mountains which are not simulated by the model. Furthermore, melt is observed to persist in these regions during extended periods when föhn does not occur, suggesting that other factors may be important in controlling melt in these regions.

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

Cited by 23 publications

References 57 publications

Centuries of intense surface melt on Larsen C Ice Shelf

Centuries of intense surface melt on Larsen C Ice Shelf

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

The Impact of Föhn Winds on Surface Energy Balance During the 2010–2011 Melt Season Over Larsen C Ice Shelf, Antarctica

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