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The Katahdin Esker part of Vignettes:Vignette Collection
Introduction Many landforms left by continental ice sheets owe their origin to thermal conditions at the glacier bed. Among these are thrust features1 (Moran and others, 1980); periglacial landforms dating from the last interstadial, such as patterned ground (Kleman and Borgström, 1994) and tors (Kleman and Hättestrand, 1999); ribbed moraines2 formed at boundaries between zones of thawed and frozen bed (Hättestrand and Kleman, 1999); and eskers. Eskers are straight to somewhat sinuous ridges of stratified gravel deposited by water flowing in subglacial conduits. Water in such conduits flows in a direction that is determined only partly by the bed topography. Gradients in the pressure of the ice against the bed play an important role (Shreve, 1972). These pressure gradients explain some otherwise puzzling aspects of observed esker paths such as the tendency for eskers to be up on valley sides and to cross divides at their lowest points (Hooke, 2005, Figs. 8.24, 8.25). In a subglacial conduit, shear in the viscous flowing water generates heat that melts conduit walls. Sediment in the basal ice is thus released into the water. The wall melting is offset by closure of the conduit, driven by pressure in the ice that is slightly higher than that in the water (Röthlisberger, 1972; Shreve, 1972). The resulting continuous supply of sediment quickly overloads the water, so some of it is deposited, forming the esker. To study eskers, with particular reference to the Katahdin esker in Maine, we (Hooke and Fastook, 2007) used Fastook's numerical model of the Laurentide Ice Sheet. The model ice sheet is driven by climate. Past temperatures and winter precipitation are estimated from the GRIP (Greenland) ice core δ18O record (Fig. 1). The model was tuned so that the ice margin at 15.6 ka (15,600 calendar years ago) corresponded reasonably with a well-known massive 15.6 ka moraine in eastern Maine, the Pineo Ridge moraine (Fig. 2). The model was then tested by comparing other predicted margin locations and predicted basal and marginal temperatures with those suggested by field data. Modeled and observed margins are in reasonable agreement, considering the uncertainties in both, especially prior to about 15 ka (Fig. 2). Modeled temperatures suggest arctic to subarctic conditions prior to ~15 ka, consistent with glaciotectonic thrust blocks and permafrost features on Cape Cod, and permafrost features in the Connecticut River Valley. The Pineo Ridge moraine, and also the 16.5 ka Pond Ridge moraine (Kaplan, 1999), appear to have been formed during short cold reversals recorded in the Greenland ice core (Fig. 1). Esker segments Many eskers were formed in segments (e.g. Donner 1965; Ashley and others, 1991). Along the 150 km Katahdin esker, an ideal ~5 km-long segment begins as a relatively small ridge. The ridge increases in size downflow, resulting in a tadpole shape (Fig. 3). The (tadpole) head reflects a period of time during which the margin position was relatively stable. We attribute the increase in size to: (1) melt rates on conduit walls that increase near the margin, thus releasing more sediment, and (2) to melt rates that exceed closure rates beneath the thinner ice near the margin, resulting enlargement of the conduit and thus in a decrease in water velocity. This part of the Laurentide margin terminated in the sea, so material that was flushed past the portal was deposited in submarine fans. As the margin retreated, successive segments were formed. The head of each younger segment, together with its associated fan, laps onto and buries the tail of the previous one. Left unexplained by this conceptual model is why segments link up as nicely as they do. Can eskers be built by subglacial meltwater alone? The basal melt rate predicted by our model rises from 0 at the margin to 5 or 6 mm/yr within a few kilometers (Fig. 4). It then declines gradually. By integrating this melt rate over the drainage area of the Katahdin esker, we estimated that the discharge at the margin from this source would have been only 1.2 m3/s. This flow could have moved the material found in the esker. However, it would have resulted in melting of only ~11 m of ice per year from the conduit walls. Sediment concentrations in basal ice of continental ice sheets are generally less than ~10%. If this was true of the Laurentide Ice Sheet, such melt rates would have been too low to release enough sediment to build esker segments of the observed size in the time available. Water from the surface would have been required. Why do segments line up? Surface meltwater can reach the glacier bed through moulins that form when water-filled crevasses propagate to the bed (Glen, 1952; Weertman, 1973; Zwally and others, 2002). However, new moulins would not necessarily form in precisely the right place to result in a continuous esker. Rather, one would expect to see barbs representing situations in which water reached the bed offset laterally from the esker path. The next segment to develop would then join the earlier one some distance down flow from the base of the moulin that initiated the earlier one. Where ice above a thawed bed is below the pressure melting point, heat is conducted upward into the ice. This loss reduces the heat available for melting of conduit walls and releasing debris, and thus may play a significant role in determining where eskers form. The heat loss is proportional to the temperature gradient in the basal ice, βo. Our model predicts that βo increases rapidly with distance from the margin, especially after the ice sheet begins to retreat, (Fig. 5). On a bed beneath such cold ice, subglacial water is likely distributed in a network of broad low anastomosing channels. If the roof of one of these channels begins to melt upward, deepening the flow, the resulting increase in discharge would result in an increase in energy dissipation in this channel. Under temperate ice (ice that is everywhere at the pressure melting point) this dissipation would result in further melting. Owing to this positive feedback, the resulting conduit would probably become "sharply arched" and an esker could form (Shreve, 1972). Beneath cold ice, however, such a perturbation in conduit height increases βo. This is because the roof of an arched conduit must remain at the pressure melting point, so temperature contours immediately above the conduit are deflected upward while those far above the conduit are not affected (Fig. 6). The resulting increased heat loss will inhibit melting of the conduit roof, and may prevent formation of an esker. Geomorphic implications In summary, eskers may form near the margin because temperature gradients in the basal ice further up glacier prevent development of arched conduits with melting walls (that supply sediment). Water may reach the bed through moulins further from the margin, but it then probably spreads out in a distributed drainage system. 1 Slabs of sediment or bedrock, sometimes 100s of meters in extent, that became frozen to the base of the glacier relatively near the margin and were then sheared 10s to 100s of meters down glacier. 2 Closely-spaced moraines with intervening troughs. If the troughs are closed by sliding the moraines together, they appear to form a continuous sheet. This suggests that the moraines were formed by pull-apart of the sheet.
Human Impacts on the Landscape part of Vignettes:Vignette Collection
Humans move tremendous amounts of earth every year. They are arguably the premier geomorphic agent sculpting the surface of Earth today. In the early 1990s, people in the United States were moving about 0.8 Gt (Gt = gigatons, or 1 billion metric tons) of earth in house construction, 3.2 Gt in mineral production, and 3 Gt in road construction every year (Hooke, 1994). If this much earth were dumped into the Grand Canyon annually, it would fill the canyon in about 400 years, or about 0.01 percent of the time it has taken the Colorado River to carve it! Such a comparison is not so far-fetched as one might think. A common method of coal mining in West Virginia today is mountain top removal (Fig. 1). To gain access to coal seams at depth, entire mountain tops may be unceremoniously shoved into adjacent valleys, filling the valleys. Scaling this earth moving to the world as a whole, I estimated that humans were moving 30 to 35 Gt of earth every year (Hooke, 1994). This estimate is likely to have been conservative, as it did not include earth moved in construction of commercial buildings, dams, levees along rivers, and in many other endeavors. Nor did it include earth moved in agriculture. For comparison, meandering rivers, worldwide, may shift 25 to 40 Gt of sediment from the outside bank of one meander bend to the point bar in the next during a year, and other geomorphic processes move much less material (Table 1). Table 1 (Acrobat (PDF) 31kB Oct29 09) Earth moving in agriculture About 1500 Gt of soil are moved annually, worldwide, in plowing (Hooke, 1994). Most of this is simply transferred from furrows to ridges, and is then washed back to fill the furrows; no permanent landform is created. On the other hand, about 75 Gt/y is actually eroded from the plowed fields by wind and water (Wilkinson and McElroy, 2007). A lot of this is deposited only a short distance from the field on slopes and in floodplains, but 10 to 15 Gt/y may be transported all the way to the oceans (Judson, 1968; Hooke, 1994). Soil is being removed from farm fields at about 10 times the rate at which it is being formed by weathering processes (Montgomery, 2007), an estimate that does not bode well for the long term viability of the world's food supply, especially in view of the alarming rate of population growth. Montgomery (2007) has noted that civilizations tend to expand rapidly while agriculture in fertile valley bottoms allows populations to grow. To feed the growing populations, farmers begin to use sloping land on valley sides. Erosion of the hillslope soils follows. Once no new land is available on valley sides, nutrient depletion and soil loss encourage increasingly intensive farming, which results in further soil loss. In some cases, agricultural capacity eventually becomes inadequate to support the burgeoning population, triggering societal decline, commonly accompanied by warfare, and finally collapse. This was a key thread in the collapse of the Mayan civilization in Central America, and also contributed to the collapse of the civilization on Easter Island in the Pacific, and of the Viking settlements in Greenland (Diamond, 2005). There is no inherent reason why a similar scenario could not play out, worldwide, in the near future, especially considering the current global food shortage, the overdependence that most of us have on oil exporting countries, and the vastly increased military prowess available. History of human earth moving Of course humans have not always been such prolific earth movers. The earliest archeological record of human earth moving is from 1.5 million years ago when people, probably of the species Homo erectus, pressed pebbles into muddy soil on the banks of the Jordan River near present day Ubediyeh in Israel to make a firmer living surface (Nur, 2008, p. 52). Subsequent milestones in the motivation and ability of humans to move earth occurred in the Mesolithic, about 9,000 B.P. (B.P. = before present), when the hunter-gatherer way of life gave way to farming and village life; about 5,000 B.P. and 3,500 B.P. at the beginning of the Bronze and the Iron ages, respectively, when the desire for minerals led to expanded mining, and metal tools facilitated earth-moving activities; about 200 B.P. when steam power and the Industrial Revolution led to a need for coal and at the same time provided machinery for mining coal and other earth-moving endeavors, and finally a century ago when the internal combustion engine eventually led to the humongous excavators of today, some of which can pick up 30 tons at a swipe (Figs. 2 and 3). When the curve in Figure 3 is multiplied by the population at various times in the past, the explosion in human earth moving in the last century is staggering (Fig. 4). Deep time To examine sediment fluxes over a still longer time span, Wilkinson (2005) has estimated sediment movement to the oceans over the past 500 million years by looking at the volume of sedimentary rock preserved in the geologic record (Fig. 5). The Pliocene and the present are unusual in Earth's history. The reasons for the large volumes of sediment delivered to the oceans during this time period are not known: glaciation may have increased erosion rates, but there were periods of glaciation in the Paleozoic. Perhaps continents are more emergent (higher above sea level) now than at any time in the past. Concluding statement Regardless of the reasons for the increase in sediment flux to the oceans in the past few million years, it is clear that humans have become exceptionally proficient at moving earth, and that the consequences of this, particularly the resulting loss of agricultural soil, are cause for concern. Coupled with an impending shortage of the raw ingredients for fertilizer and active consumption of groundwater at rates faster than it is being replenished in many agricultural areas, it seems unlikely that Earth's present human population can be sustained for many more decades, let alone the projected increased population.