Mid-latitude glacial erosion hotspot related to equatorial shifts in southern Westerlies

Glaciation has affected the shape of mountain ranges and has induced a global increase in erosion rates during the past 2 m.y. The observed increase in erosion rates appears to vary with latitude, reaching a maximum at mid-latitudes that is particularly well defined in the Southern Hemisphere. Although it is likely that climate played an important role, the processes responsible for such latitudinal distribution of erosion are unclear. Here we exploit the meridional extent of the Patagonian Andes and identify an erosion hotspot at ~44°S. Using a glacial erosion model and formally inverting the available thermochronometric and geobarometric data, we show that this hotspot coincides with the location of maximum precipitation that follows the Southern Hemisphere Westerlies during glacial periods. We propose that the increased precipitation rates at ~44°S led to greater ice sliding velocities and faster glacial erosion. Our results imply that the migration of the westerly wind belt toward the equator since 2–3 Ma may have played an important role in determining the distribution of mountain erosion in the Southern Hemisphere. INTRODUCTION During the late Cenozoic, climate cooled and led to the development of glaciers and ice caps in most midto high-latitude mountains on Earth (Shackleton and Opdyke, 1977). Although this is still subject to discussion (Willenbring and von Blanckenburg, 2010), global climate changes related to the onset of Northern Hemisphere glaciation at ca. 2.7 Ma are thought to have led to the modification of mountainous landforms through a global acceleration of glacial erosion (Zhang et al., 2001; Molnar, 2004; Herman et al., 2013). Furthermore, the global wind and precipitation patterns also changed through a progressive equatorward migration of the westerly wind belts during the Quaternary (e.g., Brierley et al., 2009; Lawrence et al., 2013) that led to latitudinal changes in storm-track location in the Southern Hemisphere (Heusser, 1989; Hulton et al., 1994; Lamy et al., 1999; Moreno and León, 2003; Kaplan et al., 2008). The modern glacial coverage in the Patagonian Andes coincides with the modern storm track and precipitation, centered on 50°S (Garreaud et al., 2013). If the storm track and precipitation were to be offset, then we might anticipate that the pattern of glacial extent and ice flux would be offset as well. It has been proposed that glacial and periglacial erosion may have also set a limit on the height of mountains globally, an idea that is corroborated by an observed correlation between the location of the equilibrium line altitude (ELA, i.e., where ice accumulation rate equals ice ablation rate) and the mean elevation of mountain ranges (e.g., Egholm et al., 2009). This idea relies implicitly on the postulate that glaciers and ice caps are sufficiently efficient to reduce summits within a few million years. However, the correspondence between the ELA and mountain height breaks down toward highlatitude regions, where mountains have attained elevations above the snowline of the Last Glacial Maximum (LGM, ca. 19 ka) or the present day (Broecker and Denton, 1989; Egholm et al., 2009). This observation is supported by patterns of Quaternary erosion that is fast at mid-latitudes (~45°S) but significantly slower toward high latitudes (Herman et al., 2013; Champagnac et al., 2014), at least in the Southern Hemisphere. Yet the specific processes responsible for such patterns remain elusive. Here we quantify how glacial erosion rates have varied with latitude during the Pliocene and Pleistocene Epochs, using the glaciated landscape of the Patagonian Andes as a natural laboratory (Fig. 1). We propose that equatorward shifts of the southern Westerlies have played a significant role in setting erosion patterns in the southern Andes. MODERN AND PALEO-CLIMATE SETTING The Patagonian Andes are particularly appropriate to study glacial erosion because their nearly 2000 km meridional extent exhibits large gradients in temperature and precipitation rates, and they have experienced mountain glaciation for ~6 m.y. (Mercer and Sutter, 1982). The southern Westerlies currently show a distinct maximum in precipitation and storms at ~50°S, while paleoclimate studies have also revealed a well-defined maximum in precipitation at ~44°S during the LGM (Heusser, 1989; Hulton et al., 1994; Lamy et al., 1999; Moreno and León, 2003). These observations indicate that the polar jet and its associated precipitation maximum were shifted by ~600 km toward the equator during the LGM relative to the present day. Furthermore, we observe a strong correlation between snowline and mountain height between 40° and 46°S that becomes less clear with increasing latitude (Fig. 1B) (Montgomery et al., 2001). This pattern coincides with the distribution of erosion, with higher erosion rates in the northern parts compared to the south (Thomson et al., 2010; Herman et al., 2013). GLACIAL EROSION MODEL Most physical models for glacial erosion assume that erosion is dominated by abrasion and plucking, which are both a function of the ice sliding velocity (e.g., Hallet, 1979, 1996; Iverson, 2012). The sliding velocity is itself nonlinearly proportional to ice thickness and ice surface slope, which are set by the balance between the divergence of the ice flux and the surface mass balance (i.e., the balance between ice accumulation and ablation rate). It is also well established that the surface mass balance of ice sheets and glaciers depends on precipitation and tempera*E-mail: [email protected] GEOLOGY, November 2015; v. 43; no. 11; p. 987–990 | Data Repository item 2015333 | doi:10.1130/G37008.1 | Published online 7 October 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. Modern snowline 52°S 50°S 48°S 46°S 44°S 42°S 40°S 38°S 36°S