Geochemical and Isotopic Evidence for the Escape of Ice Sheet Fluids in an East Antarctic Ice Sheet Outlet Glacier, Taylor Valley, Antarctica
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Geochemical and isotopic evidence for the escape of ice sheet fluids in an East Antarctic Ice Sheet outlet glacier, Taylor Valley, Antarctica 1 1 1 1 1 Michael G. Scudder , Graham H. Edwards , Gavin G. Piccione , Terrence Blackburn , Slawek Tulaczyk 1Earth & Planetary Science Department, University of California Santa Cruz, Santa Cruz, CA, USA 5 Correspondence to: Michael Scudder ([email protected]) Abstract: The isotopic similarities between fluids beneath the East Antarctic Ice Sheet (EAIS) and the subglacial discharge found at the snout of Taylor glacier, one of the outlet glaciers located hundreds of kilometers away, implies that subglacial fluids may transit across this incredible distance. Such a model is consistent with the imaging, by airborne transient electromagnetic (ATE), of high salinity fluids at the 10 base of Taylor glacier to at least 5.75 km up-glacier from the snout, where ATE resolution fails (Mikucki et al., 2015). The transit of subglacial discharge could also be detected through the examination of basal ice, which has been hypothesized to form by subglacial freeze on of fluids at the ice-rock interface. We test these ice formation models as well search for direct evidence for the passage of subglacial discharge beneath Taylor glacier through examination of the elemental and U-Series isotopic compositions of basal 15 ice from Taylor Valley and compare with the compositions for regional lakes, streams, bedrock, and subglacial brines. For example, the basal ice from Suess Glacier, an alpine glacier draining a local accumulation zone in the Asgard Mountains, has 234U/238U and major element compositions nearly identical to surface waters and local silicates indicating mechanical entrainment of surface salts into basal ice by a cold-based formation mechanism. In contrast, the 234U/238U isotopic and major element 20 composition of basal ice from Taylor Glacier is consistent with a mixture of the same surface salts and the englacial brine known as Blood Falls, confirming the passage of EAIS-derived subglacial brines beneath Taylor Glacier. Fluids maturate beneath the EAIS for hundreds of thousands of years and emanate at Taylor Glaciers snout. Our major, minor, and trace element data confirm these fluids come from beneath the ice and link their presence to the movement of ice and the formation of basal ice implying 25 Taylor Valley is not just an outlet for Taylor glacier, but also for EAIS fluids. 1 1. Introduction Basal ice is comprised of sediment-laden layers of ice in the basal portions of glaciers and is distinct from overlying meteoric ice chemically, structurally, and texturally (Souchez & Tison, 1981). The firnification of snowfall creates meteoric ice containing bubbly white ice with <0.05% debris by volume (Samyn et al., 2005), while the underlying basal ice forms from 30 processes at or near the glacier base. The variety of basal ice facies observed include amber ice marked by a uniform bubbly amber-coloured ice layer (Holdsworth, 1974; Samyn et al., 2005), stratified ice characterized by alternation between debris rich and free layers with a small grain size (<0.4 cm) (D. E. Lawson, 1979) (Samyn et al., 2005). Basal ice facies can exhibit differing isotopic compositions indicating different origins within the same facies (Sleewaegen et al., 2003). At the interface between the Earth’s surface and the cryosphere, the basal ice layer and hydrology of glaciers and ice sheets holds implications 35 for their stability, dynamics, and response to climatic forcing (Knight, 1997). Basal conditions such as pressure, temperature, and salinity control whether frozen basal conditions promote friction and limited ice flow or wet basal conditions lubricate the base and facilitate fast ice flow (Clarke, 2005; Pattyn, 2010). Efforts to model the stability of ice masses are improved by illuminating the mechanisms of these basal processes (Marshall & Clark, 2002). 40 The mechanisms of basal ice formation can be grouped into either cold- or warm-based processes. Mechanisms operating beneath warm-based or temperate ice include regelation, congelation, and glacio-hydraulic super cooling (Daniel E. Lawson et al., 1998; Weertman, 1957). Regelation involves ice flow around an obstacle producing a stoss-side pressure excess driving melting, while a lee-side pressure deficit forms basal ice through refreezing (Weertman, 1957). Congelation involves freezing on non-locally derived water to the base of the glacier (Knight, 1997). Glacio-hydraulic super cooling requires subglacial (i.e. 45 at the glacier bed) or englacial water rising at a sufficiently steep bed slope creating a pressure decrease that drives the freeze- on accretion of basal ice (Lawson et al., 1998). Cold-based or polar basal ice formation occurs when sediments are incorporated via a sub-solidus mechanism. One such mechanism termed the apron model, proposes permafrost or calved ice contaminated with sediment is mechanically incorporated into the glacier base as it advances (Holdsworth & Bull, 1968; Hooke, 1973; Shaw, 1977). Another mechanism of polar basal ice formation invokes entrainment of dry sediments into sub-solidus ice by melt- 50 refreeze occurring across nm-scale interfacial water films at the ice-sediment interface leaving bulk ice unaltered (Cuffey et al., 2000). Despite the presence interfacial water films, there is no bulk refreezing of liquid water. While not necessarily mutually exclusive, these processes operate in different pressure-temperature regimes within glaciers. Those glaciers where both temperate and polar processes occur at separate locations are termed polythermal. One such polythermal system, Taylor Glacier (Hubbard et al., 2004; Robinson, 1984; Samyn, et al., 2008), is predicted to reflect both warm- and cold-based basal 55 conditions and offers a case study of the interactions between its glacial and subglacial environments via the mechanisms of basal ice formation. 2 t n e t Commonwealth x E Glacier n Canada r Seuss Glacier u Glacier Lake Hoare b Lake Fryxell h s a W e k Taylor Glacier la Blood Falls o Lake Bonney le a P McMurdo Sound N 0 km 10 km 20 km Figure 1: Regional Map Sample locations and main geologic features shown. Shaded area traces the 336 m contour which is the approximate extent of paleolake Washburn. Taylor Glacier is an outlet glacier of the East Antarctic Ice Sheet (EAIS) that occupies upper Taylor Valley, one of the 60 McMurdo Dry Valleys (MDVs) and one of few places in Antarctica where basal ice is exposed above sea level. Taylor Valley is a hyper arid polar desert with mean annual temperature and precipitation of -15 to -30 °C and <100 mm respectively, with glacier melt as the primary source of liquid water in the valley lake system (Doran et al., 2002). Taylor Valley is flanked by the Asgard Range in the north and the Kukri Hills in the south, out of which flow alpine glaciers, and minor melt from these glaciers form streams that feed into three perennially frozen lakes (Fig. 1) (Welch et al., 2010). The ice-free lower valley 65 terminates in the Ross Sea (Fig. 1). Thick layers of stratified and englacial basal ice facies crop out near Taylor Glacier’s terminus (Appendix B3)(Samyn et al., 2005). The regional climate predicts a frozen glacier bed for Taylor Glacier with basal ice temperatures < -7°C (Hubbard et al., 2004). Yet, a hypersaline iron-rich brine, known as Blood Falls, periodically emanating from the terminus of Taylor Glacier (Lyons et al., 2019) and extensive basal debris imply an active subglacial hydrology and at least localized warm-based processes of basal ice formation, such as congelation. This is further evidenced 70 by remote sensing measurements that detect a subglacial brine reservoir feeding Blood Falls and thermal conditions at the pressure melting point ~6 km upstream of Taylor Glacier’s terminus (Badgeley et al., 2017; Foley et al., 2016; Hubbard et al., 2004; Mikucki et al., 2015; Robinson, 1984). Stable isotopes are consistent with this interpretation, showing Taylor Glacier basal ice has undergone melting and refreezing of meteoric ice evidenced by an enrichment in heavy isotopes (Mager, Fitzsimons, Frew, & Samyn, 2007). The alpine glaciers of the valley hold no evidence for sub-solidus basal conditions like 75 Taylor Glacier, although thin bands of massive to stratified basal ice do occur. Locally sourced Suess Glacier contains amber 3 basal ice and is predicted to be frozen at its bed evidenced by shallow ice thickness and no subglacial brine, and to interact with the regional lakes (Sleewaegen et al., 2003). Taylor Glacier and Taylor Valley alpine glaciers advance out of phase with the nearby Ross Ice Shelf, such that Taylor Valley 80 glaciers have advanced during interglacial periods and receded during glacial periods (Brook et al., 1995; Hendy et al., 1979). Taylor Valley is covered with a mix of glacial, lacustrine, and fluvial deposits primarily derived from local granitoids, sandstones, Ferrar sill tholeiites, and McMurdo Volcanic Group rocks (Toner et al., 2013). Taylor Glacier has left a geomorphologic record of at least five Quaternary advances into Taylor Valley (Denton et al., 1970; Hendy et al., 1979), advancing with the expansion of the EAIS as far as present-day Canada glacier (Bockheim et al., 2008; Brook et al., 1993). 85 Taylor Glacier terminates into Lake Bonney, which is comprised of eastern and western lobes. Bonney Narrows (Fig. 1) limits mixing between East and West Lake Bonney (Spigel & Priscu, 1998). The composition of waters found at the bottom of Lake Bonney mirror that of Blood Falls, while waters from Lake Hoare and Fryxell resemble that of McMurdo Sound seawater and have not interacted with Taylor Glacier. No longer present today, Paleolake Washburn (Fig. 1) formed when the Ross Sea Ice 90 dammed the mouth of Taylor Valley at the Last Glacial Maximum and was ~336 m deep (Hall et al., 2000).