Grounding-line systems as second-order controls on fluctuations of tidewater termini of temperate glaciers

Abstract

Climate is the first-order control on fluctuations of glacial tidewater termini. However, factors affecting relative water depth such as eustasy, isostasy, and sediment yield are important second-order controls on termini fluctuations. Deposition of grounding-line systems, which include ice-contact deltas, morainal banks, and grounding-line fans, can change relative water depth and consequently alter the stability of tidewater termini independent of climate. Therefore, the rate at which sediment accumulates in these grounding-line systems can control the rate of terminus movement.Modern temperate glaciers at Glacier Bay, Alaska, on average produce about 10 6 m 3 /yr of sediment, most of which accumulates at grounding lines. Sediment volume decreases logarithmically downfjord from a grounding line. In Glacier Bay, readvance rates that average about 10 m/yr are one to two orders of magnitude slower than retreat rates. Rates of glacial advance may be a function of sediment yield (determined by drainage area and rates of glacial erosion, transportation and debris release), total sediment volume of grounding-line systems (determined by fjord width, fjord depth less the maximum water depth for terminus stability, and angle of repose of sediment making the system), and sediment dispersal patterns (determined by process of debris release and dispersion, type of sediment, and marine currents).Consequently, average rates of advance are slower than retreat rates because of the time required for sediment to accumulate as a bathymetric high at a grounding line, which effectively decreases relative water depth and increases terminus stability. Retreat rates are faster because they occur during terminus instability in deep water and cannot be altered by sediment accumulation rates until quasi-stability of the terminus is caused by other means, such as bedrock pinning-points or a decrease in ablation area.As marine-ending glaciers expand, total sediment delivered to a grounding line increases; as they retreat, sediment volume decreases because of the change in size of the drainage basin. Grounding lines are the major depocenters receiving this sediment, and resulting lithofacies packages and geometries are primarily controlled by grounding-line movement over a glacial advance-retreat cycle.During glacial minima, fjords trap sediment while the continental shelf and slope are starved. During glacial advances and retreats, fjords are eroded while the continental shelf receives sediment and the slope may remain starved. At glacial maxima, fjords and commonly the shelf are eroded, and the continental slope receives sediment. Because grounding-line advance and retreat may or may not coincide with eustatic sea-level changes, the sedimentary record may vary depending on environmental settings and the magnitude of glacial cycles.

Observations and modeling of fjord sedimentation during the 30 year retreat of Columbia Glacier, AK

“…While distinct packages of stratified, flat-lying sediments in fjord basins are often interpreted as having formed proglacially after glacial retreat, rather than being remnants from a previous glacial cycle (e.g. Powell, 1991;Hallet and others, 1996;Hallet, 2002, 2006), studies of other Alaskan glaciers and fjords show that substantial packages of unlithified sediments can be overridden by ice (e.g. Motyka and others, 2006;Cowan and others, 2010).…”

Synchronous acceleration of ice loss and glacial erosion, Glaciar Marinelli, Chilean Tierra del Fuego

“…Powell, 1991;Hallet and others, 1996;Hallet, 2002, 2006). The similarity in the correlation between rapid erosion and glacier retreat in both hemispheres suggests that this correlation is universal for retreating tidewater glaciers.…”

“…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 basinwide erosion) from glaciated basins range from <10 -3 to >10 -2 m a -1…”

“…Meier and Post, 1987;Powell, 1991;Humphrey and Raymond, 1994;Van der Veen, 1996;Hallet, 2002, 2006;Sheaf and others, 2003). The relationship between glacial erosion and tectonics is also being addressed in this region (e.g.…”

Deriving mass balance and calving variations from reanalysis data and sparse observations, Glaciar San Rafael, northern Patagonia, 1950–2005

“…This retreat is likely caused in part by regional warming of ∼0.5 • C over the past 40 yr (Rasmussen et al, 2007). Many of these outlet glaciers terminate in fjords or lakes, and their response to climate is complicated by their sensitivity to non-climatic influences such as terminus geometry, sediment delivery to the terminus, ice-front melt rates, and water depth (Meier and Post, 1987;Powell, 1991;van der Veen, 1996;Warren and Aniya, 1999;Motyka et al, 2003;O'Neel et al, 2005;Pfeffer, 2007;Rignot et al, 2010). Hence, retreat histories of such tidewater glaciers contain a record both of past climate and of changing ice-front dynamics.…”