Lab 10 ESCI

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1 Lab 12 – Sediment Witnesses: Interpreting LakeSediment Records Figure 1. Lake McCarron. Although people often think of lakes as permanent landscape features, from a geological perspective they may be very ephemeral. Minnesota’s numerous lakes are simply low spots in a poorly-drained, recently glaciated landscape. Over time, many of our present lakes will fill with sediment or disappear as streams continue to drain the land. At present, Minnesota has 11,842 lakes larger than 40,000 m 2 (10 acres). However, it had more in the recent past and in the future, as lakes continue to fill in or drain away, our state really will become a ‘Land of 10,000 Lakes’. At some point, our descendants will have to come up with a new state slogan as even more lakes vanish from the landscape. Introduction The still, calm waters of a lake mask a dynamic system of constantly cycling water. Despite some beer and water commercial claims, Minnesota’s lakes are not filled by pure glacial meltwater. Although many lake basins are glacial in origin, the water filling those basins came from recent rains or snowmelt that flowed across the land as streams or moved slowly beneath it as groundwater. Once water reaches a lake, it only stays there a short time (weeks to months) before evaporating into the atmosphere, flowing into downstream channels or entering the groundwater system to be replaced by other water. Because of their dynamic nature, lake systems are extraordinarily sensitive to changes in the surrounding landscape. As prairies give way to forests, droughts occur, or winters lengthen, lake sedimentation changes as well. Human activities, such as agriculture, logging, wetland conversion, or urbanization also dramatically affect lake deposits. Consequently, we can decipher an area’s environmental history from lake sediment records. This lab will concentrate on human alterations to lake systems. Although manicured grass lawns may be attractive, lawn waste and runoff harm lake systems. Road salt on winter roads saves lives but also degrades lake water quality. Fertilizer use led to unprecedented

2 increases in agricultural yields that feed the world’s population, but its runoff threatens the ecological health of many water bodies. So one of the more difficult problems facing modern societies is how to balance the benefits our activities provide against their ecologic costs. ‘Overturn’ in Minnesota Lakes For much of the summer, most Minnesota lakes are ‘thermally stratified’, which means they are subdivided into layers of water with different temperatures. During the summer, the lake surface water is warmer than the underlying water. So on warm summer days, visitors to a lake can submerge beverage containers in deeper water to keep them cool. Once formed, this stratification remains stable since warm water is less dense than cool water. Hence, while summer waves mix the surface water, they seldom disturb the underlying water layers (Figure 2-A). However, during late fall and early spring, surface waters cool which reduces the density differences between water layers (Figures 2-B and 2-D). During winter, the ice- covered surface of a lake is actually colder than the water beneath it (Figure 2-C). As a result, most Minnesota lakes naturally ‘overturn’ in the spring and fall when it is easy for deep water to rise and mix with descending surface water. Figure 2. Natural ‘overturn’ in Minnesota lake systems. This ‘overturning’ alters the distribution of resources necessary for lake organisms, such as dissolved gases (oxygen and carbon dioxide) or nutrients (phosphorus and nitrogen). During summer months, nutrient levels in surface water and oxygen levels in deeper water drop as organisms in each level consume these resources. However, spring and fall overturning restores those resources as it pulls nutrients from the lake bottom up to the surface and moves oxygen-rich water down – at least until summer’s warmth once again stratifies the lake and depletion begins anew. Changes in stratification and overturn also affect the sediment deposited across the lake floor. Yearly overturn results in pairs of light and dark sediment layers known as varves .

3 Each varve pair represents a year’s deposition. In Minnesota lakes, a varve’s dark layer forms during the winter when the lake is ice covered while its light layer forms from late spring through early fall when the lake is ice-free. Human Impacts on Lake Systems Human activities affect lake systems in many different ways. Agriculture & Deforestation – The nutrients in a lake arrived by one of two processes. Water-soluble nutrients (like nitrogen, silica, and sulfur) came in as dissolved material in incoming water. But phosphorus and iron arrived as crystals attached to sediment grains so the movement of the latter nutrients is tied to sediment transport. Since prairie grasses and tree roots once minimized soil erosion, in the past Minnesota lakes were phosphorouslimited. However, as agricultural activities and deforestation led to more soil erosion, phosphorus runoff increased. If farmers raise more livestock or use more fertilizer to increase crop yields, phosphorus runoff becomes high enough to harm lake ecosystems. Wetland Drainage – Draining wetlands or converting them into farm fields dramatically altered lake drainage basins. Natural wetlands trap sediment and filter water which helps protect lake systems. However, as wetlands were drained, flooded, or converted into fields, this protection vanished. Sediment and other materials now flow directly into our lakes with little filtration. Urbanization – High-density housing also transforms lake systems. Yard waste, fertilizer runoff, or household sewage can deliver overwhelming amounts of nutrients to lakes. In addition, urban landscapes dramatically increase surface runoff. In urban areas, roads, pavement, and buildings cover much of the land. Now, instead of seeping into the ground to slowly move into lake systems, rainfall or snowmelt water rushes across impermeable surfaces to flush more sediment and nutrients into lakes. Turbidity (or Water Clarity) – Increasing lake sediment or nutrients decreases water clarity. While this is an aesthetic concern for homeowners along the lakeshore, it is a matter of life or death for many lake organisms. Photosynthesis only occurs in a lake’s clear sunlit shallow water layer – its photic zone. In clear ocean water, the photic zone may extend to 200 meters deep. However, in most Minnesota lakes suspended sediment, nutrients, and algae limit the depth sunlight reaches. If nutrient and sediment levels become too high, sunlight is blocked and photosynthetic generation of oxygen decreases. Organisms living on lake bottoms begin to decline or die out. In severe cases, all life below the sunlit surface layer decreases in abundance, including the many fish cherished by Minnesota anglers. Algal Blooms & Lake Anoxia – When people think of lake organisms, they often imagine fish, birds, turtles, crayfish, or multi-cellular plants. However, folks living along urban lakes are likely to add algae to their list, often with mixed feelings. Photosynthetic algae are necessary for a healthy lake since they produce the dissolved oxygen that other lake organisms depend on. However, increased nutrient levels due to human activities trigger immense algal blooms. These blooms cover the lake, making it less attractive for recreation. More importantly, because algae are very short-lived, these blooms soon die and decay. While this only creates unpleasant smells for people living along the lake, its impacts on aquatic life can be catastrophic. Decay requires oxygen. So large masses of decaying algae can remove all dissolved oxygen from the lake water, turning much of the lake into a barren anoxic (without oxygen) zone devoid of most life.

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4 Road Salt – Putting salt on winter roads to melt ice and improve winter driving conditions is a staple of modern Upper Midwest life. However, this practice also greatly impacts the health of nearby lake systems. Part B will focus on some of those impacts. Lake Sediment Cores Throughout the year, different types of sediment settle to the bottom of Minnesota lakes. Soil, plant debris, toxins, animal waste, and fertilizers make their way into a lake by water flow. Wind also brings pollen grains, dust, and silt to the lake as well as charcoal ash from prairie fires or pollutants from industrial smokestacks. Consequently, lake sediments can reflect any natural or human processes that affects the lake’s surrounding area. Although researchers can easily measure ongoing changes in lake systems, we often have to rely on their accumulated lake bottom sediment to study its past changes. Researchers force coring tools into unconsolidated lake bottom sediment to recover cylindrical sediment cores (Figure 3). Scientists then use radioactive isotopes (like 14 C, 210 Pb, and 137 Cs) to determine the age of sediment layers in those cores. However, in lakes with distinct seasonal sediment changes (such as most Minnesota lakes), they can determine a layer’s age simply by counting the number of annual varve couplets present in the sediment core. Collecting lake sediment cores is relatively easy, but the trick is to select core sites that provide the best records of lake processes. Since lake bottoms vary considerably from one area to another, most research studies take a number of cores from the same lake. However, in the interest of time, this lab can only focus on one core. Lake McCarron researchers chose this core because it includes a representative suite of features that reflect how the lake changed over time (Figure 4). Figure 3. Daniel R. Engstrom and Mark Edlund from the Science Museum of Minnesota’s St. Croix Watershed Research Station of Minnesota, collect lake-sediment cores. ( Photo courtesy of the United States Geological Surv Figure 4. Although Lake McCarron is now an urban lake surrounded by suburban development, as recently as this 1910 photograph it remained a rural lake. However, even in its rural condition, Lake McCarron was still heavily influenced by human use of the surrounding landscape. (Photograph courtesy of Minnesota Historical Society.)

5 Part A – Lake McCarron (1850-1960) - A Lake in Trouble Let’s start our exploration of lake sediment records using a local example. By Twin Cities standards, Lake McCarron is relatively deep with an average depth of 7.5 m (25’) and a maximum depth of 17.4 m (~60’). Because of its depth, winds do not disrupt the lake’s bottom water nor the lake bottom sediment. Lake McCarron covers 330,000 square meters (82 acres) (Figure 5). It is a NW-SE trending lake, roughly a kilometer long and a half kilometer wide. The lake’s main surface outlet is Trout Brook, which flows from its southeast shore through the lake’s only remaining natural wetlands area. Downstream, Trout Brook joins Phalen Creek and their combined waters eventually reach the Mississippi River. The first significant human impacts on the lake system began in 1849 when the McCarron family built a small dairy farm on the lake’s northwest shore. Other families soon joined the McCarrons and their farms not only changed the area’s land use but the landscape itself. Low areas were filled in and ditches dug to improve drainage, converting most of the lake area wetlands into agricultural fields. With those wetlands’ loss, Lake McCarron’s effective drainage area enlarged by a factor of three and increased soil erosion and organic runoff altered the nature of the lake’s sediment (Myrbo, 2008). Figure 5. Index map of Lake McCarron (from Myrbo, 2008) Agriculture dominated the lakeshore and lake drainage basin for the next few decades until the first household subdivision arrived shortly before 1900 (Figure 6). In 1926, the county built one of the metro area’s first public bathing beaches along the lake’s east shore – initiating a decades-long struggle to maintain a sandy beach area (Figure 7). Although Lake McCarron’s waves were too small to create a natural sandy beach, they proved to be large enough to remove all the sand brought in to create a beach. Over the following decades, the Ramsey County Recreation Board trucked in tons of replacement sand and artificially lowered lake levels to try to maintain the recreational beach. This beach also served as an added source of lake nutrients as dirt washed off bodies and children did what children have always done in pools and lakes. Figure 6. 1898 plat map of Lake McCarron area. At the time, family members still owned much of the original McCarron farm (gray areas), but the first housing subdivision lay on the lake’s north shore (black area).

6 Forty years later, homes extended along most of the lakeshore, transforming Lake McCarron into an urban lake system. Due to combined nutrient runoff from agriculture, household, and recreational activities, Lake McCarron’s bottom waters became persistently anoxic (oxygen depleted) and phosphorous released from anoxic sediments accumulated in the lake’s deep water. When the lake seasonally overturned, this excess phosphorus triggered algal blooms. As early as 1928, even before Lake McCarron became a fully urban lake, water quality was so poor that Ramsey County initiated an expensive copper sulfate treatment program to kill its algae (Myrbo, 2008). % Year Male Female Total Increase Increase 1850 1350 877 2227 1860 6260 5890 12150 9923 446% St. Paul growth 1870 12021 11064 23085 10935 90% 1880 24877 21013 45890 22805 99% 1890 73292 66505 139797 93907 205% 1900 88496 82058 170554 30757 22% 1910 116730 106945 223675 53121 31% 1920 122601 121953 244554 20879 9% 1930 139547 147174 286721 42167 17% suburbs growth 1940 149004 160931 309935 23214 8% 1950 171030 184302 355332 45397 15% 1960 202880 219645 422525 67193 19% 1970 227675 248580 476255 53730 13% 1980 219086 240698 459784 -16471 -3% 1990 231984 253781 485765 25981 6% 2000 246288 264747 511035 25270 5% Table 1. Changes in Ramsey County population over time (census data from the Minnesota Historical Society). The first population boom (1860-1910) was primarily in the city of St. Paul. McCarron Lake remained a rural area until the second population boom (1930 to 1970), which reflected the growth of suburban communities. 1 1 Note that Ramsey County’s historic gender balance reflected its frontier territory origin. Not until the 1920s were there equal numbers of women and men – so the area’s Norwegian bachelor farmer tales had a historical basis. Figure 7. Bathers crowd Lake McCarron’s beach a few years after the beach’s 1926 opening. During its heyday, thousands of people visited the beach every year. (Photographs courtesy of Minnesota Historical Society.)

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7 Dramatic local population increases in the 1940s to 1950s transformed Lake McCarron into an urban lake system (Table 1). Housing extended across most of its shoreline and through its drainage basin, along with the commercial and civic development necessary to support an urban population. At present, impermeable surfaces (buildings, parking lots, and roads) cover a fourth of the lake’s drainage basin. Highway 36 runs through the northern part of the lake’s drainage basin (Figure 8). Beginning in the 1950s dramatic increases in the use of road salt contributed to the lake’s woes. Saline runoff sank to the lake bottom forming a dense layer that contributed to the lake’s persistent stratification and anoxic bottom conditions. Recent water surveys show the lake seldom completely overturns so its waters only incompletely mix during spring and fall. Since the presence of anoxic lake bottom sediments increases deep water nutrient levels when the lake does partially overturn, the released nutrients trigger massive algal blooms that harm water quality. In the 1960s, pollution from household waste finally forced the county to construct a sanitary sewer system. Lake McCarron’s water quality once again changed – but this time for the better. Figure 9 illustrates some of the changes in Lake McCarron’s sediment record over time, in terms of both sediment composition and sediment volume. As the graphs show, these two factors did not change in sync. A significant change toward more siliciclastic material (sand, silt, and clay) occurred in the late 1800s (larger graph). However, the volume of annual Figure 8. Boundary of Lake McCarron’s drainage area superimposed on a satellite land image. Hig hway 36 runs across the northern edge of the basin. The present basin is roughly three times larger than it was before the late 1800 s period of wetland draining and ditch digging. Figure 9. The large graph on the left shows the relative abundance of different types of sediment in Lake McCarron deposits over time. Biogenic silica refers to diatom (lake algae) shells while the calcite forms from inorganic precipitation of calcite crystals when the water warms ( as warm water cannot hold as much dissolved calcite as cool water does ). In contrast, the siliciclastic material (sand, silt, and clay) is washed into the lake from the surrounding drainage basin. The small graph above shows changes in the lake’s total mass accumulation . Hence the recent (post 1850) decrease in calcite and biog enic silica proportions reflects dilution by siliciclastic material (Myrbo, 2008). The amount of calcite and bi ogenic silica remained the same but increased runoff transported more siliciclastic material into the lake. High

8 sediment did not significantly increase until around 1925 (smaller graph). When the total volume returned to near original levels beginning in 1970, siliciclastic sediment continued to dominate the record so the shift in sediment type continued. Table 2. Lake McCarron Timeline Date Event Pre-1800’s Dakota hunting and agriculture activities were the dominant human impacts on lake. 1820 Fort Snelling military reservation established. 1841 Pig’s Eye Landing rechristened as St. Paul. 1849 Start of Minnesota Territory – McCarron family moves to area. 1855 McCarron Dairy Farm in full operation on northwest lakeshore. 1858 Minnesota becomes a state. 1860’s-1870’s Dairy, hog, poultry and beef farming dominate lake’s drainage basin. 1880’s-1890’s Ditch digging and wetland draining for agriculture increase lake’s effective drainage basin. 1900 Agriculture still dominates, but first housing subdivision built on north lakeshore. 1926 County develops one of area’s first public bathing beaches on southeast shore. 1930 Housing extends along half of shoreline. Commercial development begins. 1940’s-1950’s Dramatic increase in population occurs within lake’s drainage basin. 1948 Roseville incorporates as village. 1950’s Beginning of dramatic increase in use of salt to de-ice winter roads. 1960’s Sanitary sewer system constructed across lake’s drainage basin. 1985-1986 Wetland catchment basin built along western shore as remediation effort. 2004 Lake treated with alum to sequester phosphorus and improve water clarity. Exercise A – Lake McCarron Sediment Core and Smear Slides You may wish to download the large sediment core image posted on the lab Canvas site. Before starting Exercise A , look at the core image’s overall pattern (Figure 10). One of the more obvious changes occurs between the non-layered sediment in the core’s lower section and the layered sediment towards the top. This change from non-layered to layered deposition reflects a fundamental shift in the lake’s character. Sediments in the core’s laminated upper section formed in a persistently stratified lake system. Because it was stratified, the lake suffered periods of extreme productivity (algal blooms) that damaged its ecosystem. In contrast, the core’s bottom layers formed within a well-mixed lake system seldom stressed by algal blooms. The laminations in the core’s upper section are ‘varves’, paired layers of light and dark sediment. These varves represent annual changes in lake sedimentation. Although, as with tree rings, unusual conditions can result in skipped or added layers. Light-colored layers are composed of authigenic carbonate (carbonate inorganically precipitated from lake water) 2 and diatoms produced in surface waters during the summer months. Diatoms are singlecelled organisms that comprise many of the diverse forms commonly known as ‘lake algae’. Dark-colored layers are composed of organic material and fine-grained mud that settled out during winter months in the still water beneath the lake’s ice-covered surface.

9 2 This authigenic carbonate is a reflection of the close tie between lake and ground water systems. As groundwater moves through the subsurface, it dissolves carbonate rock. The resulting dissolved ions later precipitate in the lake to form carbonate crystals. This authigenic carbonate once comprised the bulk of Lake McCarron’s sediment before the lake’s drainage basin became an agricultural area.

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10 Figure 10. Photograph of the Lake McCarron sediment core used in Exercise A. Note the core’s general subdivision into three main intervals: a lower light-colored non-laminated interval; an intermediate dark-colored non-laminated interval; and an upper dark-colored laminated interval.

11 Exercise A – Lake McCarron Sediment Core and Smear Slides Dating Sediment Core Intervals: Examine the image of a Lake McCarron sediment core (Figure 10 or the larger color image posted on the lab Canvas site). Researchers collected this core in 2000 so its top layer formed that year. Note you can subdivide the core into three sections: a lower light-colored interval; an intermediate dark-colored interval without laminations; and an upper darkcolored, laminated interval of alternating light and dark layers. Each pair of light and dark layers forms a varve and each varve represents a year of lake sediment deposition. Because it is difficult to measure thickness in online images, we measured the depth of every 5 th varve and recorded it in Table 3 (rounding off to the nearest half centimeter). (Note the core’s top begins at 5 cm on the scale, rather than zero.) We then recorded the thickness of every set of five varves. (If a set of five varves is 2.5 cm thick than each varve averaged 5 mm in thickness.) Even though you start with measurements in hand, be sure to look at the core image to understand what the data means before answering the questions. Table 3. Lake Sediment Core’s Varved Interval yellow –compressed thicker varve sets orange – uncompressed varve sets The oldest varve pairs date to around 1915. Before then, lake sediment was not laminated. So what change might have caused varve sedimentation to begin? Question 1 (1 point) Looking back through the lake basin’s history (Table 2), did the start of varve sedimentation coincide with the start of farming in the lake basin? ☐ – Varve deposition started well before the beginning of farming in the basin. ☐ – Varve deposition started about the same time as farming began in the basin. ☐ – Varve deposition started well after farming had begun in the basin. Year Depth Thickness 1930 41 cm 3.5 cm 1925 43 cm 2 cm 1920 45 cm 2 cm 1915 47 cm 2 cm 1910 48 cm bottom of varves Year Depth Thickness Year Depth Thickness 2000 5 cm top of varves 1965 21 cm 2.5 cm 1995 7.5 cm 2.5 cm 1960 24 cm 3 cm 1990 10.5 cm 3 cm 1955 27 cm 3 cm 1985 12.5 cm 2.5 cm 1950 29 cm 2 cm 1980 14.5 cm 2 cm 1945 32.5 cm 3.5 cm 1975 16.5 cm 2 cm 1940 35 cm 2.5 cm 1970 18.5 cm 2 cm 1935 37.5 cm 2.5 cm

12 Question 2 (1 point) If the start of farming and varve sedimentation coincided, how might farming have triggered that change? If they do not coincide, then what change in the basin’s use might have led to varve sedimentation? ☐ – It coincided with farming because of increased soil erosion. ☐ – It coincided with farming because of increased fertilizer runoff. ☐ – It came after farming began as wetlands were drained. ☐ – It came after farming began as housing subdivisions were built. Note that for the first fifteen years of varve deposition, varve sets were remarkably uniform in thicknesses. From that you can calculate the average yearly sediment thickness in the lake before human activities significantly changed deposition rates. You can then use that yearly average to estimate the amount of time represented by the non-laminated intervals in the lower parts of the core. Of course, lake researchers would use isotope dating to confirm ages, but for our lab exploration rough estimates are good enough. Question 3 (1 point) For the first fifteen years of varve deposition, how much sediment was deposited each year? In other words, how thick is each varve (not a set of five varves)? ☐ – 1 mm ☐ – 2 mm ☐ – 3 mm ☐ – 4 mm ☐ – 5 mm ☐ – 6 mm The dark-colored non-laminated sediment interval is 13 cm thick. The light-colored nonlaminated interval is 72.5 cm thick. As mentioned varves began to form around 1915. Question 4 (1 point) Using your average varve thickness as an estimate of yearly sedimentation rates, when did the shift from light colored to dark colored non-laminated sediment occur? ☐ – ~1870 ☐ – ~1876 ☐ – ~1882 ☐ – ~1888 ☐ – ~1902 ☐ – ~1908 Question 5 (1 point) From Table 2, which change in land use in the lake basin is most likely responsible for this change from light-colored to dark-colored sediment? ☐ – This change occurred as the first farm was built in the area. ☐ – This change occurred as more farms were built in the area. ☐ – This change occurred as ditch digging and wetland draining began. ☐ – This change occurred when the first housing subdivision was built. Question 6 (1 point)

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13 Using your average varve thickness as an estimate of yearly sedimentation rates, how many years are represented by the light-colored non-laminated interval? ☐ – ~140 ☐ – ~150 ☐ – ~160 ☐ – ~170 ☐ – ~180 ☐ – ~190 Question 7 (1 point) To place the core in a historic context, when did the sediment at the core’s bottom form? ☐ – during the Civil War (1860-1865) ☐ – before the Revolutionary War (1775-1783) ☐ – during the War of the Spanish Succession (1701-1714) ☐ – around the start of English and French colonization (1620s-1630s) ☐ – before Spain’s earliest New World colonies (1400s)? The transition from light-colored sediment to dark-colored sediment is not just a matter of color, but changes in sediment composition. Look at the larger graph in Figure 9 to see how the composition of lake sediment changed in the late 1800’s. Question 8 (1 point) Which sediment type began to dominate during the dark, non-laminated interval and why might this change have occurred? ☐ – biogenic silica (diatom shells) ☐ – organic material from land plants ☐ – siliclastic material from runoff ☐ – calcite precipitated from lake water Upper slide is primarily calcite (c) grains (pastel-colored grains in cross-polarized light) and diatoms (d) shells (algae with silica cell walls that disappear in cross-polarized light). Note that many grains in both slides are not identified. Lower slide is primarily brown organic (o) material (amorphous plant remains) and siliciclastic grains (s) such as sand, silt, and clay washed into the lake. The latter appear grey to white in cross-polarized light. Some pyrite (p) also occurs (iron sulfide mineral that forms in organic-rich sediments).

14 Figure 11. Microscopic views of sediment from dark and light layers of a Lake McCarron varve couplet. Both samples are shown in plane-polarized and under cross-polarized light. Under crosspolarized light, many mineral crystal structures create characteristic interference patterns. Compare the microscopic samples from the dark and light layers of a varve couplet (Figure 11). There is a larger version of the image posted on the lab Canvas site. The dark layers formed during winter when the lake was ice covered while light layers formed during the lake’s ice-free months. Question 9 (1 point) Figure 11 lists the different sediment types of the dark and light layers, but what is the primary reason for the change in dominant sediment types? ☐ – most summer sediment forms in the lake water, winter sediment is runoff ☐ – most summer sediment is runoff, winter sediment forms in the lake water ☐ – most summer sediment is biological in origin, winter sediment is detritus ☐ – most summer sediment is detritus, winter sediment is biological in origin Notice how varve thicknesses vary in Table 3. While the thickness of the top 15 varves is due to their lack of compaction, the yellow highlighted intervals were times of unusually thick varves. During those times, more annual deposition occurred in the lake than before or afterwards. Figure 9 confirms this increase in sediment volume. Question 10 (1 point) Reviewing Lake McCarron’s history (pages 6-9 and Tables 1-2), which TWO historic changes were most likely responsible for this increase in varve thickness? ☐ – dairy farming ☐ – recreational beach built ☐ – increased agriculture ☐ – increase in urban population ☐ – wetlands draining ☐ – sewer construction Part B – ‘A Road Runs Through It’ Salt is now widely used to melt ice on Minnesota winter roads to improve winter driving conditions. However, it was not until after World War II that new salt production methods made widespread road salting possible. In Minnesota, winter road salt use exploded from 60,000 metric tons in 1950 to over 900,000 metric tons in 2005. The Twin Cities area alone uses more than a third of Minnesota’s winter salt (Novotny, Murphy and Stefan, 2008). When people consider the possible impacts of salt use on Minnesota’s fresh water systems, they often fear its impact on lake organisms. Few fresh water organisms can tolerate high salinity levels so once dissolved chloride (Cl - ) reaches 230 mg/L, freshwater systems are in danger. Fortunately, few Minnesota lakes have salinity levels that high. However, even at lower concentrations, salinity can still affect lake systems by increasing lake stratification.

15 Earlier, we noted that temperature differences could separate warm less-dense shallow water from underlying cool more-dense water. In a similar manner, density differences due to salinity also lead to lake stratification. Figure 12. Impacts of winter road salt use on lake overturn. Without road salt runoff, Minnesota lakes tend to completely overturn during the spring when the lake is cool enough that temperature stratification is reduced (A). However, saline runoff sinks to the lake bottom creating a salinity stratification that limits lake overturn (B). In addition, phosphorous from older lake sediments leach into this anoxic water layer – so when the lake does more completely overturn immense algal blooms are triggered by the excess phosphorous. As saline meltwater from winter roads flows into a lake, it sinks to form a dense bottom water layer. Even when surface waters cool in the fall or spring, this salinity-generated stratification may be great enough to delay or prevent the lake from completely turning over and mixing (Figure 12). Consequently, a lake’s bottom layer may remain persistently anoxic (without oxygen) for much of the year. This harms bottom-dwelling organisms but also alters water composition. Phosphorus escapes from anoxic sediment faster than from aerated (oxygenated) sediment. So during periods of bottom water anoxia, phosphorous builds up in deeper water layers. When the lake finally overturns, this excess phosphorous triggers massive algal blooms. As a result, even if people manage to eliminate current phosphorus runoff into salinitystratified lakes, phosphorus from older lake bottom sediments may continue to trigger algal blooms for decades afterwards. While road salt impacts tend to create local problems that affect adjacent lakes, other changes in nutrient supply can have more extensive impacts. See Part D for examples of how Minnesota land use affects downstream water bodies all the way to the Gulf of Mexico. Exercise B – ‘A Road Runs Through It’ Road Salt Use and Lake McCarron Highway 36 runs across the northern part of Lake McCarron’s drainage basin (Figure 8) so winter road salt has washed into the lake. As a result, Lake McCarron now exhibits a strong seasonal cyclicity in salinity (salt) levels, with higher salinity during the winter and spring thaw and lower salinity in the summer and fall. Table 4 shows chloride (Cl - ) concentrations measured over a four-year interval as reported by Novotny, Murphy and Stefan, 2008 . Table 4. Chloride levels in Lake McCarron Min Max Year Summer-Fall Winter-Spring % seasonal Cumulative Increase in

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16 (mg/L) (mg/L) increase Min. Chloride Levels (mg/L) from 2004 level Chloride levels in Lake McCarron are higher than rainfall values (0.2 to 2 mg/L) or the average for Minnesota’s non-urban lakes (4 mg/L), but they still fall well below the level that would pose a significant threat for freshwater systems (230 mg/L). Question 11 (1 point) Roughly how many times greater is Lake McCarron’s 2007 minimum chloride level than the average for rural Minnesota lakes (~4 mg/L)? ☐ – 3x ☐ – 10x ☐ – 30x ☐ – 100x ☐ – 300x ☐ – 1000x If we assumed the county continued to use as much road salt in the future as it did in 20042007, then how long will it be before Lake McCarron’s salinity reaches a dangerous level? Question 12 (1 point) If it took three years for minimum chloride levels in Lake McCarron to rise by 19 mg/L, then in which year would Lake McCarron’s minimum salinity reach 230 mg/L (Fortunately, the county has taken steps to reduce salt use since 2007.) ☐ – ~2020 ☐ – ~2024 ☐ – ~2028 ☐ – ~2032 ☐ – ~2036 ☐ – ~2040 While present chloride levels are not high enough to harm freshwater organisms directly, they can still negatively affect Lake McCarron’s ecology. Question 13 (2 points) How might salinity continue to negatively affect the lake system in terms of algal blooms? (Be sure to explain how or why changes occur, do not just list them.) Part C – McCarron Lake (1960-Present) – A Lake on the Mend Determining lake remediation goals is not always as intuitive as one might hope. For sports enthusiasts the quality of the lake’s fishing may be paramount, but homeowners living along the shore might consider water clarity more highly as it affects property values and lakeshore lifestyles. In contrast, biologists and wildlife enthusiasts may consider a lake’s wetland areas of more value than its open water. These perspectives are all valid but may require different, sometimes conflicting, approaches to ‘improve’ the lake. Consequently, 2004 102 123 21% 2006 113 132 17% 2007 121 139 15% 11 19

17 communities need to consider what they hope to achieve before embarking on expensive remediation projects. The first significant improvement in Lake McCarron water quality occurred in the 1960s after the city of St. Paul discovered nitrates and waste in its water supply. In response, a sanitary sewer system was installed, part of which extended across Lake McCarron’s drainage basin. Construction of this sewer system significantly affected Lake McCarron’s sedimentation. Figure 13 plots the ratio of organic carbon to nitrogen (C:N) in the lake’s sediment against the carbon isotope value of the sediment’s organic component (δ 13 C org ). While that might sound complicated, the implications are relatively simple. High C:N ratios (>10) suggest most organic material present in a lake came from land plant decay while low C:N values (<10) suggest organic matter instead came from lake algae. This difference reflects land plants’ greater need for tough tissues to withstand gravity and prevent drying out. In addition, high δ 13 C org values suggest periods of CO 2 stress. Since CO 2 stress is most often caused by high algal production, high δ 13 C org values suggest a lake troubled by algal blooms. In Figure 13, the white circles are data from the core’s lower light-colored, non-laminated section which formed before the lake basin became highly agricultural. White stars are data from the intermediate dark-colored, non-laminated interval and the white squares are from the core’s upper section of dark-colored, laminated sediments BEFORE the sewer system’s construction. After the sanitary sewer system was installed across the lake’s drainage basin, measurements began to shift back towards pre-agricultural levels (Figure 14). Figure 13. Cross plot of C:N against δ 13 C org Figure 14. The same graph as Figure 13, but values in Lake McCarron sediment. Arrow with the addition of data (black circles) from highlights changes over time (from Myrbo, sediments deposited AFTER the construction 2008). of a sanitary sewer. Arrow shows the change in values after the sewer system’s construction

(from Myrbo, 2008). As welcome as that improvement was, the sewer system did not fully restore Lake McCarron’s ecology. Continued high nutrient levels and algal blooms led to construction of the McCarrons Wetland Treatment System (MWTS) northwest of the lake, along the main stream that flows into the lake (Figure 15). In the 1800’s, farmers had drained the original wetlands that were here and converted them to farmland. A century later, Ramsey County began transforming them back to wetlands. In 1985, the county built two sediment basins to trap sediment before it reached the lake and developed six wetland areas (called Villa Park Ponds) – planting vegetation designed to reduce phosphorus levels in water flowing into the lake (Figure 16). The Capital Region Watershed District reengineered the wetland system in 2004 to improve its efficiency. Figure 15. Location of the MWTS (after Myrbo, 2008) Figure 16 – Photographs of the sediment containment basin wall (left) and a restored wetland area (right). Both are part of the MWTS designed to reduce the lake’s sediment and nutrient influx. While remediation efforts helped Lake McCarron, restoring lake systems’ health may take a long time. In a study of a Swiss alpine lake that became eutrophic (subject to algal blooms) because of 14 th and 15 th century animal pasturing, it took 85 years after people abandoned the area before the lake returned to its original condition (Hausmann et al. , 2002). A lake that remains vulnerable to urban pressures, like Lake McCarron, may take even longer to recover. While that is discouraging, Lake McCarron is responding to remediation rather than getting worse. Remediation efforts, even in urban settings, can improve lake quality. We just need to be patient. Eventually, as local lake systems improve, we should see changes in larger water systems. Exercise C– Lake Remediation Efforts Sanitary Sewer System (~1960)

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Prior to the lake basin becoming an agricultural area (white dots of Figure 13), low C:N values suggest most of its organic material came from lake algae rather than from land plants in the surrounding drainage basin. Question 14 (2 points) After the lake basin became agricultural, but before sewer system construction, how did the sources of the lake’s organic material change? (Use Figure 13 for this question.) ☐ – most became terrestrial sourced ☐ – most came from the lake itself How did sanitary sewer construction (Figure 14) affect the lake’s organic matter? After construction, did land plants contribute more of the lake’s organic material or less? ☐ – land plants contributed more ☐ – land plants contributed less What was the most likely reason for these changes? ☐ – a decrease in surface runoff as sewer system diverted water downstream ☐ – an increase in surface runoff as sewer system brought water into lake ☐ – an increase in land plants as wetland systems recovered and spread ☐ – a decrease in land plants as wetland systems were reduced in range Look at the smaller graph on the right side of Figure 9, which shows changes in the total volume of sediment (mass accumulation rate) deposited in the lake over time. Question 15 (1 point) How did construction of the sanitary sewer system affect the volume of sediment that reached the lake? (Consider whether this makes sense in light of your answers above.) ☐ – volume increased ☐ –volume stayed the same ☐ – volume decreased After sewer construction, δ 13 C org values from lake sediments tend to cluster in two groups (Figure 14). Many samples show low values that reflect normal productivity levels, but some still exhibit higher δ 13 C org values, suggesting the lake still suffers from some algal blooms. Hence, although the sewer improved the lake system, it did not eliminate all algal blooms. Question 16 (2 points) What is the most likely phosphorous source for the algae blooms that occurred after the sewer system was in place? So far, this lab has explored human impacts on a single lake basin. In parts D and E, we will expand scales to examine our broader impacts. Part D will explore how lake cores illuminate how human management of the Minnesota and Mississippi Rivers (Lab 8) has affected the downstream Lake Pepin basin. Part E will consider insights lake sediment cores provide on the cause of the mass extinction of megafauna that recently dominated the North American continent, such as giant ground sloths, mammoths, and mastodons.

Part D: Regional Impacts – Close to Home (Lake Pepin) Lake Pepin, an enlargement of the Mississippi River on the Minnesota-Wisconsin border, is over 250 times larger than Lake McCarron (Figure 17). While it is easy to understand how human impacts might affect a small lake like McCarron, could agricultural processes alter a water system the size of Lake Pepin? Unfortunately, the answer is a resounding ‘yes’. Figure 17. Lake Pepin map with the locations of five lake sediment cores (A-E). Inset map shows the lake’s location relative to upstream drainage basins (after Engstrom et al. , 2009) At present, Lake Pepin faces many environmental threats, but one of the most important arose in the upstream Minnesota River Valley. Agricultural fields along the Minnesota River now rely on tile drainage systems to increase productivity. Tile drainage systems consist of kilometers of perforated plastic tubing buried beneath fields to drain excess water into neighboring streams. Tile drainage use has significantly increased agricultural productivity. However, extensive use of tile drainage systems also has significant environmental costs. Tile drainage systems eliminate wetland areas and with fewer wetlands there is less water evaporation. Hence, tile drainage systems greatly increased runoff. Increased runoff in turn erodes riverbanks and bluffs to increase the amount of sediment moving through streams. People can underestimate this erosion as it comes at the expense of little used ravines and bluffs rather than valuable cropland. However, sediment eroded from ravines and bluffs is now filling downstream lakes. Those lakes also suffer from increased nutrient levels as tile drainage systems transfer fertilizer from farm fields into streams. Lake Pepin faces the brunt of these multiple assaults. Phosphorus levels have risen dramatically, creating algal blooms that cause extensive fish kills and threaten the lake’s ecological health. However, increased sediment deposition is the greatest threat to Lake Pepin’s continued existence. 12,000 years ago, Lake Pepin extended north to downtown St. Paul. Holman Field Airport, across the river from downtown St. Paul, rests on lake sediment fill. For the past 12,000 years, river sediments have slowly filled much of the lake’s northern basin. If deposition rates remained at pre-agricultural levels, sediment would completely fill Lake Pepin in a few thousand years. However, within the past 150 years, agricultural activities have greatly increased lake sedimentation, reducing the lake’s expected lifespan from a few millennium to a few centuries. Sediment accumulation (kg/m2/a) by location Whole Lake Interval A B C D E (kg/m2/a) (metric tons/a) (m3/a)

1990-1996 5.15 9.20 12.23 14.79 20.31 8.64 875576 1,607,538 1980-1990 4.10 6.48 9.89 12.24 17.98 7.35 744661 1,368,774 1970-1980 3.63 6.26 9.91 12.23 15.67 6.06 613840 1,157,650 1960-1970 3.22 5.34 9.45 12.74 14.56 7.09 718628 1,302,040 1950-1960 2.23 3.81 9.57 10.38 12.32 6.04 611792 1,093,263 1940-1950 2.04 3.19 9.56 7.78 11.07 4.62 467922 847,897 1930-1940 1.56 3.15 7.72 5.52 6.29 3.13 316680 601,994 1910-1930 1.22 2.16 4.58 4.22 6.42 2.63 266210 497,136 1890-1910 1.25 1.54 2.86 2.84 4.73 1.96 198210 373,843 1860-1890 1.21 1.63 2.65 2.97 3.82 1.86 188684 361,494 1830-1860 0.87 1.16 2.65 2.10 3.82 1.50 151625 287,346 Before 1830 0.54 0.70 1.18 1.24 2.14 0.78 79304 151,654 Table 5. Sediment accumulation rates in Lake Pepin from the five cores shown in Figure 17 and for the lake as a whole (box on the right). Data compiled by Dan Engstrom (St. Croix Watershed Research Station, Science Museum of Minnesota) as research reported in Engstrom et al. (2009). a = abbreviation for year (annum) Figure 18. Graphic representation of data from Table 5, showing rates of sediment accumulation in Lake Pepin. Table 5 and Figure 18 show changes in the lake’s sediment accumulation rate over time. Not only did the volume of sediment change but also its composition. Figure 19 shows the relative abundance of different biologic components in the lake sediments. Diatoms are the most abundant algae present. These single-celled organisms include forms that live on the lake sediment surface (benthic diatoms) and organisms that float in the lake’s shallow water (planktonic). Both groups flourish when the lake water is clear and sunlight reaches the lake bottom floor. However, if muddy water or algal blooms prevent sunlight from reaching the lake bottom, planktonic forms will dominate the lake community. As shown in Figure 19, as Lake Pepin’s sedimentation rate increased, a pronounced shift in dominance occurred from benthic forms to planktonic forms. This occurred as increased erosion from agricultural activities raised phosphorous levels to trigger algal blooms that decreased water clarity. Regional Impacts – Further Afield (Gulf of Mexico) Figure 1 9 . Graph showing relative proportions of planktonic diatoms, benthic diatoms, and chrysophyte cysts in Lake Pepin’s basin sediment. Higher levels of cysts and benthic diatoms reflect periods of clear water conditions with sunlit lake bottom (from Engstrom et al. , 2009). 0 200000 400000 600000 800000 1000000 1200000 1400000 1600000 1800000 >1830 1830-1860 1860-1890 1890-1910 1910-1930 1930-1940 1940-1950 1950-1960 1960-1970 1970-1980 1980-1990 1990-1996 Sediment Deposition in Lake Pepin over Time ) m3/a (

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The Minnesota River Valley tile drainage systems not only affect Lake Pepin but the Gulf of Mexico. However, Lake Pepin is an effective sediment trap so the tile drainage systems’ impacts to the south are tied to increased nutrient runoff rather than sediment deposition. Phosphorous and nitrogen are two of the most important nutrients in aquatic systems and the scarcity of one or the other is typically the main limiting factor on aquatic productivity. However, phosphorous and nitrogen arrive in aquatic systems by different paths. Phosphorous typically moves as salt crystals attached to sediment particles while nitrogen is water-soluble and moves as dissolved ions in flowing water. Consequently, the increased sedimentation in Lake Pepin also increased phosphorous in the lake, which triggers algal blooms and subsequent fish kills. Phosphorous is also the main nutrient responsible for Lake McCarron’s algae production. However, because phosphorous moves as crystals attached to sediment, it is less mobile than dissolved nitrogen. While Lake Pepin traps phosphorous-laden sediment, dissolved nitrogen can flow downstream to enter the Gulf of Mexico. There, increased nitrogen levels trigger immense algal blooms along the Gulf coast especially during summer months. As these short-lived algal blooms die and decay, their decay consumes the water’s dissolved oxygen, making it hypoxic (oxygen depletion). This results in large areas of the Gulf Coast becoming ‘dead zones’ devoid of multicellular life. The nitrogen responsible for the Gulf Coast ‘dead zone’ comes from waters across the Mississippi River drainage basin but the greatest contributors are Minnesota River Valley tile drainage systems. Figure 20. Map of a typical Gulf of Mexico ‘dead zone’ overlaid across the Upper Midwest area to illustrate the dead zone’s scale. The core of the ‘dead zone’ (red) ranges from 7000 to 8000 square miles, an area larger than all of Minnesota’s lakes combined.

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Figure 20 overlays the Gulf’s summer ‘dead zone’ on a map of the Upper Midwest. Although algae and bacteria flourish in this dead zone, all commercial fisheries have vanished. Hence, agricultural practices that increase crop yields in the river’s headwaters are destroying the fishing industry at its mouth. Balancing these intertwined processes is a formidable legal and environmental challenge. Exercise D – Regional Impacts Deposition rates are greater upstream (near cores E & D of Figure 17) and decrease downstream (towards cores B & A). This is because most sediment drops out when the river flow slows down as it enters Lake Pepin. However, to answer the questions below, you should use the whole lake figures in the far right column of Table 5 (shown in m 3 /a). Question 17 (1 point) Roughly how many times greater is the recent sediment deposition rate (1990-1996) than the deposition rate before Euro-American agriculture began in the region (~1830)? ☐ – 2x ☐ – 5x ☐ – 10x ☐ – 20x ☐ – 50x ☐ – 100x The initial increase in sediment deposition occurred as farmers converted prairies to farm fields. However, by 1940 farmers had created most of our present agricultural areas. Consequently, changes in sediment loads after 1940 are primarily due to the use of tile drainage systems to drain wetland areas rather than to an increase in cultivated acreage. Question 18 (1 point) From Figure 18, which factor played the greater role in increasing sediment deposition: increased cultivated acreage (pre-1940) or tile drainage system use (1940 on)? ☐ – increased acreage under cultivation ☐ – tile drainage system use Let’s play with some numbers. Before 1830, Lake Pepin had an estimated volume of 653,000,000 m 3 and each year upstream streams brought ~151,650 m 3 of sediment into the lake. Presently, the remaining volume of Lake Pepin is 553,000,000 m 3 and upstream rivers now annually deliver ~1,607,500 m 3 of new sediment (Engstrom et al. , 2009). Question 19 (1½ points) How long would it have taken to fill Lake Pepin if sedimentation levels had remained at their pre-1830 level? _______ years If sedimentation rates stay at the 1990-1996 level, how many years will it take to fill Lake Pepin’s remaining volume? (Note that this is a maximum estimated lifespan, as sedimentation rates are still climbing.) _______ years Your answers above are for the whole lake basin. In contrast, the shallow, northern part of the lake (shaded areas around cores E & D of Figure 17) is estimated to fill in less than a fourth of that time as deposition rates are higher where the river first begins to slow. If this ¼ estimate is correct, if you have children or grandchildren are they likely to see the northern part of Lake Pepin significantly fill in?

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☐ – yes, it could be in their lifetime ☐ – no, it shouldn’t happen in their lifetime Question 20 (2 points) Why is phosphorous the main fertilizer that threatens the environmental health of Lake Pepin yet nitrogen is the primary cause of the Gulf’s ‘dead zone’. Part E: Continental Impacts – A Recent Megafauna Mass Extinction Figure 21. Mammoth (left) and Mastodon (right) by Dantheman9758 Wikipedia Well, let’s finish by exploring the megafauna extinction that transformed the world’s terrestrial ecosystems. Until relatively recently, mammoths and mastodons roamed North America (Figure 21). Then they abruptly died out. As early as the 1840s, when scientists first realized there had been a past ice age, two ideas have been forwarded for the demise of these beasts and other recent megafauna: climate change and human hunting. More recently, some have suggested a meteorite impact around 12,900 years ago was responsible for their extinction. One of the difficulties in settling this question is timing. The megafauna fossil record is not continuous through time and even when fossils are found, we have no way of knowing how many others were present. Surprisingly, lake sediment records provide a way around this dilemma. Obviously, we do not find megafauna in lake sediment cores, but we do find Sporormiella spores (Figure 22). This fungus lived in megafauna digestive tracts and its spores were expelled in their dung. So the presence and abundance of Sporormiella spores in lake sediment records reflects the abundance of megafauna living in the lake basin. Figure 22. Sporormiella spores courtesy of Alain Brissard

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Roughly the same time the megafauna declined, there was a pronounced shift in plant pollen records to plant communities that have no modern analogs. There was also a significant change in the number of wildfires. Hence, this was a time of change, but how can we sort out the timing of each change relative to one another? As you have probably already guessed, we will turn to lake sediment cores. Figure 24 shows the distribution and abundance of sediment types through time in cores taken from Appleman Lake, Indiana (modified from J.L. Gill, et al. 2009) . At first the figure may appear daunting but the first five columns simply show the distribution and abundance of five tree pollen types through time while column G shows the abundance of Sporormiella spores through time. The number of charcoal pieces found in the sediment (column I) reflects the fire or burn pattern of the area over time. Depth is recorded on the right and the time before present is indicated in thousand-year increments (ka) on the left. Figure 24. Distribution through time of various tree pollen types (A, B, C, E, F), Sporormiella (G), and charcoal counts (I). Picea – spruce, Pinus - pine, Quercus – oak, Fraxinus Nigra – black ash, and Ostrya/Carpinus - ironwood. From a plant community perspective, the most unusual part of this diagram is the interval from 12,000 to 13,700 years ago (12 to 13.7 ka) when the surrounding forests consisted of a

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mixture of spruce, ash, and ironwood trees. In the present, that particular mix of trees is rare. Spruce forests now tend to be boreal, distinct from the oak, ash, and ironwood trees that form the region’s modern deciduous forest cover. Megafauna Extinction The North American megafauna extinction did not occur at the same time in every area so we will only concentrate on their disappearance from this one lake basin. Several factors have been proposed for their extinction. Climate Shifts A warming period began about 14.7 ka, but was abruptly interrupted by a rapid shift back to glacial conditions known as the Younger Dryas from 12.9ka to 11.7 ka. Because climate shifted so abruptly, the start of the Younger Dryas climate change has been proposed as a possible cause for the megafauna extinction. Plant Community Shifts Although changes in plant communities might have driven megafauna extinction, realize the reverse may also be true. In Africa, elephants and other large animals strongly impact plant communities, keeping savannah grasslands open by preventing forests’ spread. When North American bison herds were exterminated, the loss of bison meant that woodlands were no longer held in check and many prairie grasslands were lost to woodland expansion. Meteorite Impact Although the idea remains controversial, some researchers claim a meteorite impact occurred about 12.9 ka that was responsible for both the Younger Dryas cooling event and the megafauna extinction as well as the decline of the Clovis culture. Human Impacts Humans may have played a role both in intensifying fire regimes (the number and scale of fires) and in contributing to the megafauna extinction. In southeastern Wisconsin, butchered mammoth bones found at one site date from 14.8 to 14.1 ka. Exercise E – Megafauna Extinction Well, with the increased temporal resolution lake sediment cores provide, let’s examine how the timing of these changes relate to one another in order to figure out which proposed extinction cause makes the most sense. So when did the megafauna die out, fires become abundant, and plant communities change? Question 21 (1 point) Based on spore and pollen patterns, is it more likely that a changing plant community led to megafauna extinction or did the loss of megafauna lead to changes in the plants? ☐ – changes in plant communities led to megafauna extinction ☐ – megafauna extinction led to changes in plant communities Question 22 (1 point) From the pollen patterns, which type of plants do you think the megafauna fed on (and kept in check)?

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☐ – needle trees (spruce & pine) ☐ – broadleaf trees (oak, ash, and ironwood) An area’s fire patterns depend on which plants are present, fuel loads (dead plants), and human activities. The types of plants present can be affected by the feeding patterns of megafauna while the loss of megafauna could lead to the buildup of fuel loads across an area. Question 23 (1½ points) Based on the spore, pollen, and charcoal patterns, when did the first significant fires begin? ☐ – ~9.3 ka ☐ – ~10.8 ka ☐ – ~11.3 ka ☐ – ~12.8 ka ☐ – ~14.3 ka When did fires become common? ☐ – ~15.8 ka ☐ – ~9.3 ka ☐ – ~10.8 ka ☐ – ~11.3 ka ☐ – ~12.8 ka ☐ – ~14.3 ka ☐ – ~15.8 ka Which factor is most closely associated with this change to fires becoming common? (Note that we do not know when humans first became a significant part of this regional ecosystem.) ☐ – the loss of megafauna ☐ – a switch in plant communities Question 24 (2 points) From the patterns you see, which of the following hypotheses are possible or unlikely? A meteorite impact at 12.9 ka played a significant role in the megafauna extinction. ☐ – possible ☐ – unlikely Climate warming beginning ~14.7 ka played caused the megafauna extinction. ☐ – possible ☐ – unlikely Human hunting played a significant role in the megafauna extinction. ☐ – possible ☐ – unlikely An abrupt shift to colder conditions (12.9-11.7 ka) caused the megafauna extinction ☐ – possible ☐ – unlikely

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Lab Conclusion The question of to what extent humans are responsible for the megafauna extinction seems like a good ending point. Humans are now one of the most potent geologic forces affecting the Earth’s surface systems. We now move more sediment than rivers or glaciers, transform ecosystems on an unprecedented scale, and are even altering Earth’s climate. Environmentally, one of our greatest challenges going forward is to determine how to best manage our many impacts on Earth Systems. This laboratory exercise is based upon three primary references: Myrbo, A., 2008, Sedimentary and historical context of eutrophication and remediation in urban Lake McCarrons (Roseville, Minnesota), Lake and Reservoir Management, v. 24, p. 349-360 Engstrom, D.R., Almendinger, JJ.E. and Wolin, J.A., 2009, Historical changes in sediment and phosphorous loading to the upper Mississippi River: mass-balance reconstructions from the sediments of Lake Pepin, Journal of Paleolimnology, vol. 41, p. 563-588 Gill, J.A., Williams, J.W., Jackson, S.T., Lininger, K.B., Robinson, G.S., 2009, Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America, Science, Vol. 326, Issue 5956, p. 1100-1103 Additional References: Hausmann, S., Lotter, A.F., van Leeuwen, J.F.N., Ohlendorf, C., Lencke, G., Grönlund, E., and Sturm, M., 2002, Interactions of climate and land use documented in the varved sediments of Seebergsee in the Swiss Alps, The Holocene, vol. 12, p. 279-289 Novotny, E.V., Murphy, D., and Stefan, H.G., 2008, Increase of urban lake salinity by road deicing salt, Science of the total environment, 406(1-2), 131.

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Lab 10 ESCI

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