Synchronous acceleration of ice loss and glacial erosion, Glaciar Marinelli, Chilean Tierra del Fuego Miche `le KOPPES, 1 Bernard HALLET, 2 John ANDERSON 3 1 Department of Geography, Universityof British Columbia, 1984 West Mall, Vancouver, British Columbia V6T 1Z2, Canada E-mail: koppes@geog.ubc.ca 2 Department of Earth and Space Sciences and Quaternary Research Center, Box 351310, University of Washington, Seattle, Washington 98195-1310, USA 3 Department of Earth Science, Rice University, 6100 Main Street, Houston, Texas 77005, USA ABSTRACT. To contribute to the understanding of the influence of climate on glacial erosion and on orogenic processes, we report contemporary glacial erosion rates from a calving glacier in the Southern Andes and elucidate the influence of ice dynamics on erosion. Using seismic profiles of sediments collected in a proglacial fjord and a documented history of retreat, we determine the time-varying sediment flux of Glaciar Marinelli as a measure of basin-wide erosion rates, and compare these rates with the annual ice budget reconstructed using NCEP–NCAR reanalysis climate data from 1950 to 2005. The rate of erosion of the largest tidewater glacier in Tierra del Fuego averaged 39  16 mm a –1 during the latter half of the 20th century, with an annual maximum approaching 130 mm a –1 following a decade of rapid retreat. A strong correlation emerges between the variable rate of ice delivery to the terminus and the erosion rate, providing quantitative insight into the relationship between ice fluxes and glacial erosion rates. For Glaciar Marinelli, as for other calving glaciers for which suitable data exist, the marked retreat and thinning over the past 50years have resulted in a period of accelerated basal sliding and unusually rapid erosion. 1. INTRODUCTION Several recent studies have documented the close corre- spondence between the peak elevations of an orogen and the perennial snowline elevation (Montgomery and others, 2001; Mitchell and Montgomery, 2006), supporting the hypothesis that higher rates of erosion in glacial and periglacial environments effectively limit the elevation of mountain ranges (e.g. Porter, 1981; Brozovic ´ and others, 1997). Rapid glacial and periglacial erosion creates in effect a ‘snow buzz saw’ whereby only limited crustal material can rise above a certain elevation, often defined as the equilibrium-line altitude (ELA), regardless of the rate of rock uplift. The buzz-saw hypothesis is tantalizing, in particular because it suggests that a lowering of the ELA during cold periods would increase the area subjected to glacial and periglacial processes, thereby accelerating erosion. Our ability to assess the snow buzz-saw hypothesis and other aspects of the interplay between tectonics, climate and topography has been limited to date by a dearth of data both quantifying basin-scale glacial erosion rates and linking such rates to pertinent glacier characteristics. Compiling data from the limited studies that have empirically determined the rate of basin-wide erosion for a number of glaciers (e.g. Powell, 1991; Harbor and Warburton, 1993), Gurnell and others (1996) and Hallet and others (1996) both demonstrated that sediment yields (as a measure of basin- wide erosion) from glaciated basins range from <10 –3 to >10 –2 ma –1 . In general, sediment yields from the rapidly eroding large glaciated basins significantly exceeded those from glacier-free basins of comparable size. However, neither compilation addressed what controls these rapid glacial erosion rates or the significant variations within and between glacier basins, for most of the sediment yield studies were conducted with little attention to the glaciers themselves, or to inter-basin differences in climate or bedrock resistance to erosion. Moreover, many of the studies were focused on a small subset of glaciers in Alaska and northern Europe, chosen for ease of access. Hence, they may not be representative of most glaciers. Much of our current understanding of both the dynamics of tidewater glaciers and the tempo of glacial erosion origi- nates in the coastal mountains of Alaska, USA (e.g. Meier and Post, 1987; Powell, 1991; Humphrey and Raymond, 1994; Van der Veen, 1996; Koppes and Hallet, 2002, 2006; Sheaf and others, 2003). The relationship between glacial erosion and tectonics is also being addressed in this region (e.g. Meigs and Sauber, 2000; Gulick and others, 2004; Spotila and others, 2004; Berger and Spotila, 2008). Koppes and Hallet (2002) suggested that the tidewater glaciers in Alaska have been unusually dynamic and erosive since the end of the Little Ice Age (LIA), when regional warming caused rapid terminus retreat and the drawdown of hundreds of meters of ice over the past century. The acceleration in ice flow required to evacuate such immense volumes of ice from the basins is inferred to result in accelerated basal sliding. Due to unusually fast sliding, recent erosion rates are likely to far exceed those over the long term. Comparing measurements of glacial erosion rates and the response of tidewater glaciers to climate in other regions permits us to assess and improve the empirical relationships between glacier dynamics and erosion developed in Alaska, and to explore whether these relationships are universal. The icefields of Patagonia and Tierra del Fuego are two of the last remaining regions of substantial ice cover aside from the polar ice sheets. Herein, we present one of the first studies of basin-wide rates of contemporary glacial erosion in the Southern Andes, and investigate the relationship between sediment yields over time and the dynamic state of Glaciar Marinelli, Journal of Glaciology, Vol. 55, No. 190, 2009 207 Downloaded from https://www.cambridge.org/core. 13 Dec 2021 at 21:09:35, subject to the Cambridge Core terms of use.