 GULLIES WITHOUT ALCOVES: LINKING GULLY MELTWATER TO RECENT ICE AGE DEPOSITS (THE LATITUDE-DEPENDENT MANTLE).  S. C. Schon1 and J. W. Head1 1Department of Geological Sciences, Brown University, Providence, RI 02912 USA Samuel_Schon@Brown.Edu  Introduction: Gullies on Mars were initially hypothesized to be the result of groundwater outbursts [1]. Additional hypotheses proposed alternative sources of water to carve gullies (e.g., ground ice, [2]; "pasted on" terrain, [3]), as well as entirely dry mechanisms for their formation. The global distribution of gullies [4], specific geologic studies [3,5,6,7], terrestrial analog studies (e.g., [8]) and modeling efforts [2,9] favor variations of a meltwater scenario (e.g., [10]) for the formation of gullies. HiRISE data led McEwen et al. [11] to report "evidence of fluvial modification of geologically recent mid-latitude gullies."  With these studies supporting a prominent role for water in forming Martian gullies, ongoing research efforts are focused on 1) constraining the timing of gully formation [12,5,13], 2) investigating specific formation processes [e.g., 14,15,6], 3) exploring linkages between gully processes and inferred climate cycles [16,17,18,19], and 4) determining candidate sources for meltwater (e.g., groundwater, ground ice, perennial ice, snow, and older glacial deposits) that may have been involved in fluvial activity [e.g., 17, 17a]. Here we demonstrate a close association between gully formation processes and meltwater derived from recent (< 5 Ma) ice age deposits [17, 17a].  Latitude-dependent Mantle:  Mid- to highlatitude geomorphology of latest Amazonian age on Mars is characterized by ice-related processes and landforms including a pervasive ice-rich mantling unit first identified in global maps of surface roughness [20] that show topographic smoothing at high latitudes; evidence for this mantle is also seen in visual imaging [16,21,22]. Head et al. [17] synthesized these observations into a theory of recent obliquity-driven "ice ages" on Mars that is supported by global circulation model studies [23,24] and models of ice stability [19]. Additional evidence of the extensive atmospheric deposition of ice is provided by Gamma Ray Spectrometer (GRS) data of ice abundances that far exceed reasonable pore space volumes [25], observations by Phoenix of massive ice just below the surface [26], observations of sublimation-type contraction crack polygons at the Phoenix lander site  [27], observations of layering within the mantling unit [28], and contemporary observations of new mid-latitude impact craters which expose a nearly pure ice substrate that is observed to sublimate upon exposure [29,30]. Could the ice-rich mantling deposits be a potential source of meltwater for gully formation? Surficial Gullies:  Newly identified small surficial gullies occur in degraded mantle on crater walls (Fig. 1). These features lack alcoves, have shallowly incised channels, and have depositional fans that extend onto the cratered floor surface (Fig. 1). In Fig. 1, the uppermost narrow channels (arrows 1-2) are choked with sediment (arrow 3) above a more deeply incised portion of the channel that is also choked with sediment (arrow 4); below this, a channel segment (arrow 5) and subsequent deposition (arrow 6) suggest multiple sediment transport episodes in the development of this gully. The upper reaches (arrows 1-2) of the channel are shallower and appear restricted by the boundary of a mantle layer. Occurrences of small-scale surficial gullies (e.g., Fig. 1) and channels in the degraded mantle, without alcoves that could serve as accumulation zones for snow, provides independent evidence that gullies form through degradation and melting of the ice-rich mantling deposits. This is consistent with interpretations of the correlation between gully activity and obliquity-driven climate cycles [e.g., 17]. Gully activity during the current "interglacial" period: Analysis of Gasa crater gullies by Kolb et al. [31] shows that ten gullies have apex slopes (16.3°20.4°) characteristic of "wet or fluidized emplacement" and eleven gullies have apex slopes (20.7°-26.4°) consistent with dry granular flows. These data are consistent with the Gasa impact occurring prior to the likely waning of meltwater generation for gully formation, which we suggest is contemporaneous with the damping of obliquity-climate cycles, ~400 ka. Kolb et al. [31] also found that the morphologically freshest appearing gullies were more likely to have apex slopes consistent with the movement of dry material, while more degraded gullies were more likely to have slopes consistent with wet flows, leading them to conclude that "gully formation required a time-limited fluidization mechanism, possibly liquid water, that major gully formation is not occurring today, and that activity in gullies today is likely dry mass wasting perhaps aided by CO2 frost" [31]. Dundas et al. [13] detected recent rockfalls and other minor geomorphic changes in two Gasa gullies and suggested a link between the annual CO2 frost cycle and these events, but concluded, "None of these observations contradict the hypothesis that gullies are initiated by H2O snowmelt or that this process drives a significant fraction of gully erosion." In summary, these observations suggest that the role of liquid water in gully activity may have become less important in the transition to the current interglacial period, as environmental conditions became less extreme and ice-rich mantling deposits became depleted.   Figure 1: Eastern Promethei Terra. This crater wall gully is restricted to degraded latitude-dependent mantling deposits (top, above arrosws 1 and 2) and lacks an alcove. Meltwater from degradation of ice-rich mantling (top) is likely to be responsible for its formation, PSP_002293_1450. Conclusions and Implications: New chronostratigraphic data and observations have improved our understanding of the processes and climate context for the formation and evolution of martian gullies. While mid-latitude gullies on Mars are geologically young features dating to the latest Amazonian, their principal formation is likely to have preceded the current (0 - 400 Kyr) epoch of lower and more stable obliquity. In our interpretation, melting of ice-rich deposits related to degradation of a latitude-dependent mantle was responsible for the major fluvial activity forming gully channels and depositional fans (Fig. 1). Erosive gullyforming flows are likely to have been dominated by fluvial sediment transport [e.g., 11, 5, 17a], with a few cases of water-lubricated debris flows [e.g., 6,32]. In contrast, present-day gully activity may favor dry mass movements, which appear to exhibit a seasonal modulation related to the CO2 frost cycle [13,33]. References: [1] Malin M.C. and Edgett K.S. 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