Gully formation in the McMurdo Dry Valleys, Antarctica: multiple sources of water, temporal sequence and relative importance in gully erosion and deposition processes

We report on a decade of fieldwork designed to determine the conditions required for erosion of Mars-like gully channels in the McMurdo Dry Valleys (MDV) of Antarctica. We have outlined the major factors in the morphological evolution of gullies in the Inland Mixed Zone of the MDV: (1) the distribution of ice sources; (2) the temporal aspects of ice melting; and (3) the relative significance of melting events in gullies. We show that significant erosion of gully channels can be achieved if geometrical and environmental conditions combine to concentrate ice where it can rapidly melt. In contrast, annual melting of surface ice and snow deposits during late-season discharge events contribute to transport of water, but flux rarely surpasses the infiltration capacity of the active layer. These small discharge events do not erode channels of significant width. Even when the flux is sufficient to carve a c. 10–20 cm deep channel during late summer (January– February) runoff, these small channels seldom persist through multiple seasons, because they are seasonally muted and filled with aeolian deposits. We briefly discuss the application of these results to the study of gully systems on Mars. Supplementary material: Eight videos showing activity and events are available at https://doi. org/10.6084/m9.figshare.c.3935992 Martian gullies are important tools for understanding the fate of volatiles on the surface in the late Amazonian. Since their discovery (Malin & Edgett 2000), it has been clear that gullies are climate-related and involve phase changes of volatiles at the surface or within the shallow subsurface. A decade of changedetection efforts has shown that modification of existing gullies is temporally associated with the loss of CO2 frost at the surface (Dundas et al. 2015), with rare instances being more consistent with the removal of H2O frost (Vincendon 2015). This has promoted a fundamental question: is the erosion and deposition observed today sufficient to gradually form gullies over thousands of years, or does contemporary activity represent only the ephemeral modification of larger gullies that formed from higherenergy events under different climate conditions? On Earth, the same question can be asked about gullies that form in the frozen polar desert of Antarctica and the Transantarctic Mountains (Marchant & Head 2007; Hauber et al. this volume, in review). The combination of extremely lowmean annual temperatures (c. −18°C: Riordan 1973; Doran et al. 2002), hyper-arid conditions (3–50 mmwater equivalent snowfall: Thompson et al. 1971; Thompson 1973; Fountain et al. 1999), an absence of pluvial activity of any significance and a lack of vascular plants makes the McMurdo Dry Valleys (MDV) a more appropriate geomorphological analogue for Mars than any other location on Earth (Marchant & Head 2007). Despite cold and dry conditions, sinuous channels (McKnight et al. 1999) have formed and are currently evolving in this desiccated landscape. Unlike larger glacier-fed streams, however, which From: CONWAY, S. J., CARRIVICK, J. L., CARLING, P. A., DE HAAS, T. & HARRISON, T. N. (eds) Martian Gullies and their Earth Analogues. Geological Society, London, Special Publications, 467, https://doi.org/10.1144/SP467.4 © 2017 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics experience near-continuous flow during austral summer (December–January), ephemeral gullies permit the documentation of flow initiation, a signal generally obscured in higher-flux streams. Further, while we are currently unable to study gullies on Mars in situ, deconstructing the behaviour of MDV gullies in detail provides hypotheses and constraints that can be used to make more detailed testable predictions about their counterparts onMars. In this contribution, we utilize a variety of environmental monitoring techniques to document the presence of water snow/ice reservoirs, their volumes and distribution in relation to the gullies, the conditions under which they undergo melting, and their relative contributions to erosion and deposition in MDV gully systems. Despite their small size (c. 4500 km of ice-free area, slightly more than the size of the State of Rhode Island) (Fig. 1b) (Levy 2013), the MDV exhibit a wide spectrum of glacial, periglacial and fluvial landforms, the distribution of which is controlled by the range in climate conditions that occur in the Trans-Antarctic Mountains at this location (Marchant & Denton 1996; Marchant & Head 2007; Head & Marchant 2014). Based on elevation, distance from the coast, temperature and meltwater availability, a hydrological continuum exists in the MDV (Head & Marchant 2014; Levy 2015). This range of processes and landforms is bracketed by: (1) coastal environments where peak summer (January) conditions produce enough surface melting of sub-aerial cold-based glaciers to produce stream channels tens of kilometres long (Conovitz et al. 1998; McKnight et al. 1999); and (2) high-elevation inland environments devoid of significant geomorphological evidence for surface or near-surface water flow, potentially since the Miocene (Sugden et al. 1995; Marchant et al. 2002). The transition between these two end members, referred to as the Inland Mixed Zone (IMZ) in Marchant & Head (2007) and Zone 2 in Marchant & Denton (1996), hosts features indicative of fluvial transport, but not as widespread nor at the scale observed at the coast, with fluvial activity concentrated primarily on warmer equator-facing slopes (Marchant & Denton 1996; Fountain et al. 1999). It follows, then, that the upper extent of this zone represents the coldest and driest region of the MDV where concentrated fluvial erosion and transport can occur. Our study site, the South Fork of Upper Wright Valley, is found at this upper extent of the IMZ (Fig. 1b, c) (Marchant & Head 2007). Two narrow gullies incise the southern equator-facing wall of the valley (Fig. 2). These channels are prominent evidence for fluvial activity that is hydrologically distinct from streams in the neighbouring Taylor Valley, in that these gullies lack a large glacial meltwater source. Like gullies on Mars, it is unknown whether these channels form through iterative annual erosion from small amounts of summer (December– February) melting or through punctuated and anomalous high-energy flow events. Determining how these gullies form under conditions of limited and episodic liquid-water availability will provide the minimum requirements for what is necessary to create a fluvial transport system on Earth. Here, we use a decade of field data and observations in South Fork to test the hypothesis that previously documented peak-season melting of surface snow deposits (Levy et al. 2009) is sufficient, over long timescales, to erode gully channels gradually in the McMurdo Dry Valleys. This terrestrial investigation allows us to assess the conditions required in polar deserts on Earth to produce net erosion in gully channels and accumulation on alluvial fans. South Fork geographical and hydrological