SIX
From Ice to Fire
INTO THE HOLOCENE
The very words “Great Ice Age” conjure up images of a world petrified in hundreds of thousands of years of profound deep freeze, when our skin-clad forebears hunted mammoth, reindeer, and other arctic animals. Deep-sea borings in the depths of the Pacific Ocean, coral growth series from tropical—and formerly tropical—waters, arctic and Antarctic ice cores, and concentric growth rings from ancient tree trunks tell a different story—of constant and dramatic swings in global climate over the past 730,000 years. Our remote ancestors lived through wild fluctuations from intense glacial cold to much shorter warm interglacials that sometimes brought tropical conditions to Europe and parts of North America. Over these hundreds of millennia, the earth has been in climatic transition for more than three-quarters of the time: At least nine glacial episodes have set a seesaw pattern of slow cooling and then extremely rapid warm-up after millennia of intense cold.
BRIAN FAGAN, Floods, Famines, and Emperors
ICE AGES AND THE WEST
TODAY, WE LIVE IN A RELATIVELY COOL PERIOD that began about 40 million years ago. Over this period, the earth became increasingly cool and icy, culminating in the Quaternary Period, a dance of advancing ice sheets (glacial periods) and retreating ice sheets (interglacials) that started two million years ago. The most recent glacial period reached its maximum extent about 20,000 years ago—a period dubbed the “Last Glacial Maximum.” The average global temperature then was about 18°F lower than it is today.
The earth is currently in a relatively warm interglacial period known as the Holocene epoch that began 11,000 years ago. Whereas one view holds that the Holocene is due to end soon, having equaled (or even exceeded) the span of many previous interglacial periods, some scientists believe that it could last thousands of years longer, perhaps extended by the addition of heat-trapping carbon dioxide and methane into the atmosphere when humans began large-scale agriculture and, most recently, the burning of fossil fuels. In truth, no one knows precisely when the next ice age will return.
The Quaternary has been divided into more than twenty glacial periods, separated by interglacial periods. The different glacial states are marked by changes in oceanic currents, changes in winds and airborne dust, and shifts in storm tracks influencing when and where precipitation falls. Both temperature and carbon dioxide levels have fluctuated between glacial and interglacial states (see figure 16A), based on ice core records discussed in the previous chapter. During glacial periods, sea levels have fallen by up to 120 meters (390 feet) below its present, interglacial level as huge volumes of water were transferred from the oceans to the continents to be stored as ice in massive ice sheets. The lowered sea levels exposed a land bridge between Asia and North America.
When the last glacial period of the Quaternary reached its peak about 20,000 years ago, one of its ice sheets—the Laurentide—was two miles thick and covered most of what is, today, eastern and central Canada, extending southward into Indiana, Illinois, Ohio, and Pennsylvania. On the western half of Canada was the Cordilleran ice sheet that reached from British Columbia as far south as Seattle, Washington. These ice sheets grew so high that scientists believe they altered atmospheric circulation. Mountain glaciers also expanded over the upper reaches of the Cascade, Rocky, and Sierra Nevada ranges.
Our story focuses on the climate history of the American West since the Last Glacial Maximum. Within this relatively short time period, climatic extremes were greater than anything we can imagine from our limited historical experience. Today we live in a time of unusually benign weather, and many “normal” conditions, as viewed over the past 20,000 years, seem inconceivably harsh.
After the peak of the last ice age, the climate began to warm at last, and the ice began to melt, carving new rivers and leaving behind mountainous loads of debris in moraines. Fifteen thousand years ago, enormous lakes formed across the Great Basin of modern-day California, Nevada, Utah, Oregon, and Colorado. Some of these lakes were tens of thousands of miles in area and hundreds of feet deep, covering vast basins between the mountains.
FIGURE 16. (A) Sea level changes (in feet below modern sea level) over the past 500,000 years, showing glacial/interglacial cycles. During glaciations, sea level dropped as seawater was transferred to the poles to build huge ice sheets; during interglacials, the ice sheets melted to release the water back to the oceans, raising their level by 390 feet. (B) Temperature over the past 20,000 years measured in the Greenland ice cores, showing the transition from the Last Glacial Maximum, 20,000 years ago, to the Holocene epoch, which began 11,000 years ago. The climate returned to near glacial conditions during a 1,300-year period called the Younger Dryas. (Drawn by B. Lynn Ingram based on open online sources.)
The first human immigrants made their way to North America from Asia across a land bridge between Siberia and Alaska that would later be swallowed up by the rising sea. They likely entered after the planet warmed sufficiently to melt a narrow corridor between the two enormous ice sheets over Canada, some 13,500 years ago. Archaeologist Brian Fagan reasons that a southward migration through this long corridor (approximately 930 miles) may have taken generations, probably as people followed the seasonal migrations of animals such as caribou and bison.
Upon their arrival in the new world, people would have found plenty of food, as well as predators such as saber-toothed tigers, and rapidly changing habitats. Around 10,300 years ago, the gradual warming trend that had marked the previous several thousand years in the West suddenly ceased and the climate deteriorated. For the next several hundred years, glacial conditions returned, a period known as the Younger Dryas. As suddenly as that period started, it ended, and the warming resumed, marking the beginning of the Holocene (see figure 16B). Melting glaciers once again sent water rushing into the sea, raising the level of the oceans worldwide by nearly an inch a year. The rate of sea level rise was quite high until about 6,000 years ago; by then, the rising seas had flooded what had been coastal river valleys, forming the bays, estuaries, and wetlands that are part of our modern coastline.
VAST ICE AGE LAKES IN THE GREAT BASIN
Climate scientists have learned a great deal about deglaciation (a period of rapid climatic warming and melting ice sheets) by studying enormous lakes in the Great Basin. The Great Basin is a region in the heart of the American West that extends across the states of Nevada, Utah, Oregon, and into Colorado, bracketed on the west by the Sierra Nevada and Cascade ranges, on the north by the Wallowa Mountains, and on the east by the Rocky Mountains. The region comprises a series of linear mountain ranges and basins that formed over millions of years of tectonic movement between the Pacific and North American plates, causing extensional pulling and shearing of the landscape. Today, it is predominantly a high desert region, beautiful and stark with sagebrush-dominated desert valleys alternating with linear mountain ranges. The Great Basin is so named because of its hydrology: the region’s rivers have no outlet to the ocean, instead terminating in the lakes, vast playas, and dry lakebeds of this giant basin.
Climate in this region today is semiarid, with mild winters, warm summers, and little precipitation. Like the West Coast, precipitation is brought to the Great Basin mostly during the winter by westerly winds carrying moisture from the Pacific Ocean. Rain and snow from the mountain peaks feed the rivers that drain into the Great Basin, but freshwater is limited, not in sufficient supply for its growing population.
The Great Basin valley floors contain huge expanses of salty sediments that are the remains of vast ice age lakes. The Great Salt Lake desert, covering some 20,000 square miles of western Utah, is one such region. It was made famous in August 1846, when the Donner pioneer family joined the Reeds and several other families migrating to the West on what was supposed to be a shortcut across this desert. Instead, they encountered a series of setbacks, including a lack of freshwater and thick salty clay sediments that proved heavy going for their ox-drawn wagons, all of which took a toll. The result of using this shortcut (the so-called Hastings Cutoff) was a delay that put the migrants in the Sierra Nevada in October, exactly the wrong time to begin a crossing of this mountain range. The timing was made worse by the onset of an early winter with a series of heavy snowstorms. The misery that ensued is now an ingrained part of California’s history.
If we step further back in time and observe this same region 15,000 years ago, the desert was instead part of a vast, deep body of water, which is today called Lake Bonneville. Had the early pioneers crossed the region during that time, they would not have been contending with empty, endless deserts; rather, they would have been following seemingly endless shorelines that bordered gargantuan lakes. Today, the Great Basin has some sizeable lakes and playas, including Pyramid Lake in Nevada, Owens Lake in California, and the Great Salt Lake in Utah. Drawing on a variety of lines of evidence, both from within the lake-bed sediments and from the geomorphic traces of ancient shorelines that ring the playas along the slopes of nearby mountains, it is clear that, at times in the past, the Great Basin supported vast lakes that were not only hundreds to thousands of square miles in area (for example, the Great Salt Lake’s ancestor, Lake Bonneville, was close to 30,000 square miles at its peak) but also very deep. At one time, Lake Lahontan, in northeastern Nevada, was one of the largest lakes in North America. With over 10,000 square miles in area, this lake once encompassed modern-day Walker Lake in the south, Pyramid Lake on its western edge, and Winnemucca Lake on the northeastern edge. Yet another large lake, Owens, is found in southeast California. Today, this lake is completely dry, but it once covered an area close to 400 square miles. These lakes expanded and contracted through the ages with changing climate.
Enormous lakes were, in fact, a common feature on the glacial landscape. But, in contrast to the colossal glacial lakes that formed farther north along the southern edges of the melting continental ice sheet, it is thought that the lakes of the Great Basin grew to such a great size as the result of both higher precipitation in the West during the ice age and cooler air temperatures, causing less evaporation off their surfaces. During the Pleistocene, eight lakes in the Great Basin covered some 27,800,000 acres, eleven times the surface area covered by lakes today.
One of the largest of these Pleistocene lakes was Lake Bonneville. Mapped by G. K. Gilbert in the late nineteenth century, Lake Bonneville once reached a size equal to Lake Michigan, covering the northwest corner of modern-day Utah and extending into Nevada and Idaho (see figure 18 in the next chapter). At over a thousand feet deep, it was significantly deeper than modern Lake Michigan. The weight of this lake was so great that it depressed the underlying land surface some 240 feet, and, when the lake later went away, the land surface rebounded. Gilbert mapped flood features just to the north of Lake Bonneville and hypothesized that the lake may have burst its dam catastrophically about 15,000 years ago, creating a megaflood that flowed into the Snake River Basin in southern Idaho and finally into the Columbia River. Today, the only remnant of this enormous ice age lake is the Great Salt Lake in Utah, so named because it is one of the saltiest lakes on Earth—seven times saltier than the ocean.
Glacial summers were cooler in the northern hemisphere than they are today, largely as the result of astronomical factors that conspired to reduce, ever so slightly, the total amount of incoming solar radiation and to mitigate the extremes of the seasons. These cooler summers allowed the enormous continental ice sheets, including those that covered Canada and the northernmost United States, to grow to two miles high—a height that was sufficient to influence atmospheric circulation patterns.
Climate scientists have run computer models showing that the jet stream, a high-speed wind current that brings winter storms to the Pacific Northwest and as far south as Northern California today, may have been split into two limbs as it encountered the ice sheet. These models suggest that the southern limb of the jet stream during the late Pleistocene would have brought Pacific storms into the Great Basin, increasing precipitation during the winter. Cooler, cloudier conditions during the summer would have also decreased evaporation in the Great Basin, where today an average of 15 to 30 inches of water evaporates off the lakes every year. The overall effect would have been an increase in moisture in the region—and the expansion of lakes.
Paleoclimatologist Larry Benson and his colleagues at the U.S. Geological Survey have spent their careers studying Great Basin lakes, noting that those in the northern Great Basin—Lahontan and Bonneville lakes—were about ten times larger during the late Pleistocene than today. Searles and Russell lakes in the southern Great Basin, in contrast, expanded by a factor of only four to six. Benson hypothesized that, during the late Pleistocene, the position of the jet stream may have been predominantly situated over the northern Great Basin, bringing many more rain-bearing storms to that region.
THE THAWING OF THE WEST
The world that the earliest human immigrants would have found was quite different from the world we know today. Large regions of North America were still shrouded by ice, and glaciers blanketed the mountain ranges, reaching down to much lower elevations than during the warmer Holocene.
As the mountain glaciers flowed downslope, they carved out valleys. Later, when the ice had retreated, these valleys would be left with a characteristic “U shape,” with steep sides and wide, flat valley floors. When the first human immigrants were crossing the land bridge (over what is called the Bering Strait today), the floors of these glacial valleys, like Yosemite in California’s Sierra Nevada, were still buried under hundreds of feet of snow and ice, with only the highest peaks standing above the ice. Cold temperatures during this glacial period displaced the forests in the mountains to lower elevations and lower latitudes. The plant assemblages in the mountains differed as well: firs and certain pines were the dominant trees, and sagebrush communities tolerant of cold, dry conditions were more common at the lower elevations.
After the peak of the last ice age, when the world began to thaw, conditions in the western mountain ranges changed rapidly as the alpine glaciers melted, leaving behind accumulations of rock, gravel, and sand that had been scraped off the canyon bottoms and walls. These deposits were massive enough to block streams swollen with glacial meltwater, forming vast glacial lakes, hundreds of feet deep. Yosemite Lake, for instance, was formed when the ancient Merced River was dammed, forming a lake that covered a region over five miles long. Half Dome and El Capitan towering above the lake would have been reflected in its surface. The lake bottom now forms the flat valley floor of Yosemite Valley. In the higher Sierra Nevada, lakes formed by glacial dams still exist today.
The Great Basin lakes reached their peak in extent about 15,000 years ago. Lake Lahontan reached a high stand during the late Pleistocene, 15,500 years ago, then began to shrink, reaching an area of only one-seventh the size by the early Holocene, 10,000 years ago. Other lakes showed similar declines, such as the Great Basin’s Owens Lake in California and Lake Bonneville in Utah, and, outside the Great Basin, Tulare Lake in California’s Central Valley, all of which reached low stands coinciding with the end of the Pleistocene and the start of the Holocene.
CLIMATE AND THE FIRST WESTERNERS
Humans arrived in the western landscape around 13,500 years ago. These stone-age hunter-gatherers, the so-called Clovis people who left behind pointed spears fashioned from stone, migrated south from Alaska into the Plains and Great Basin and continued into the Southwest and California. They settled close to sources of reliable water: lakes, springs, and perennial streams. When the climate deteriorated and water sources dried up, they moved on. Their low population numbers gave them flexibility and mobility, which were key to their survival. From the coast to inland valleys, these earliest westerners could choose from a wide diversity of foods: fish, including salmon during their spring and fall migrations upstream to spawn; small and large game; wild plants; and migratory birds that stopped over in wetlands like California’s Central Valley en route to more southerly or northerly feeding and breeding grounds. Strange ice age animals—the Pleistocene “megafauna”—also roamed the region, including giant sloths, herds of mastodons and mammoths, native camels, horses, and elk, as well as fearsome predators like the saber-toothed tiger and the grizzly bear.
Along the coast, food would have been plentiful—particularly mollusks, fish, and sea mammals—but the early inhabitants would have had to contend with rapidly changing environments during the late Pleistocene and early Holocene. During this time of transition, the coast was perhaps one of the most rapidly changing environments in the West, its configuration changing dramatically with the meltwaters of glacial ice flowing into the oceans. Between about 18,000 and 6,000 years ago, sea level rose an average of three-quarters of an inch per year, transforming coastal river valleys into shallow bays and estuaries.
THE YOUNGER DRYAS COOLING EVENT
Just before the Pleistocene ended, about 11,000 years ago, the steady shrinking of lakes in the Great Basin briefly halted and reversed for several hundred years. Lake Bonneville expanded during this period, creating the so-called “Gilbert Shoreline.” This short-lived return to greater moisture in the American West coincided with a brief advance of glaciers in Europe, during what has come to be known as the Younger Dryas event (see figure 16B). Evidence from Greenland ice cores suggests it lasted about 1,300 years in the North Atlantic region, though its duration appears significantly shorter in the American West. In the broad timescales of climate, this return to glacial conditions was so brief that paleoclimatologists think of it as an “event,” though one that evidence suggests may have been hemispheric, if not global.
One hypothesis explaining the cause of the Younger Dryas relates to another great Pleistocene lake. At its peak, the area of Lake Agassiz was larger than all the Great Lakes combined. Named after Louis Agassiz—the father of glacial geology—this lake owed its massive size to an ice dam that backed up the glacial meltwater as the Pleistocene ice age was ending. This enormous volume of water was abruptly and catastrophically released after the ice dam collapsed, with the resulting wave flowing into Hudson Bay, through the St. Lawrence Seaway, and into the North Atlantic, a critical location for deep ocean circulation. In this region, salty Gulf Stream waters, which have been transported northward, are cooled, becoming dense enough to sink to great depths, forming deepwater that flows slowly southward. But when the rush of fresh lake waters burst out into the North Atlantic, they would have floated on the surface (freshwater being less dense than saltwater, therefore more buoyant), preventing the sinking of cold, salty water that forms the deepwater. This, in turn, slowed the normal northward flow of warm, salty Gulf Stream water to the region.
Today, this warm current heats the atmosphere above it along the coastlines of the continents surrounding the North Atlantic. When the current was slowed, or even stopped altogether, the result would have been a dramatic cooling. These changes in atmospheric and oceanic circulation that occurred during the Younger Dryas appear to have affected regions as far away as California and the American West, where the Great Basin lakes once again grew large.
IMPACTS ALONG THE WEST COAST
The Younger Dryas as well as other major global climate changes influenced conditions along the Pacific Coast. Evidence for these impacts is found in sediment cores from the eastern Pacific Ocean, including those accumulating in the Santa Barbara Basin off the Southern California coast. As described in the preceding chapter, sediments are laid down there, year after year, leaving behind distinct layers. During warm interglacial periods, the deepwater entering the basin bottom is very old and depleted in oxygen, so that bottom-dwelling marine animals cannot survive to mix and churn the sediments. The basin sediments form in undisturbed seasonal layers, with light-colored layers resulting from the siliceous diatoms raining down the water column during the spring and summer, and darker layers resulting from the silts and clays transported with runoff from the land during the winter. But was this also the case during the ice ages and the Younger Dryas event?
This was one of the questions that James Kennett, a founding father of the field of paleoceanography—the study of ocean and climate conditions of the past—set out to answer. Kennett was the co–chief scientist for the Ocean Drilling Program’s coring efforts in the Santa Barbara Basin in the early 1990s. There, he and his team retrieved the longest cores from the eastern Pacific to date, spanning the past 160,000 years. These cores reveal that, during the last ice age, Santa Barbara Basin sediments lacked their unique annual layers. In a collaborative effort between Kennett and one of this book’s authors, B. Lynn Ingram, we discovered that, during the last ice age, ocean circulation appeared to have been altered, with the flow of younger, more oxygenated waters to the eastern Pacific Ocean and into the basin. These more oxygenated waters would have supported bottom-dwelling organisms—worms, clams, and so on—that churn through the bottom sediments as they dig burrows and search for food, destroying the fine laminations. Kennett had observed that the dark brown and finely layered sediments typical of the late Pleistocene and Holocene sequences in the Santa Barbara Basin were abruptly replaced by a three-foot-thick layer of light-colored, homogeneous sediment during the Younger Dryas, a period when younger, more oxygenated waters entered the basin.
Kennett separated out the fossil shells of tiny, single-celled protists called foraminifera (or “forams”) from the sediments to extract from their chemical composition information about the water conditions in which these organisms grew. From these shells, Kennett has been able to reconstruct the temperature of surface waters surrounding the forams using the relative proportions of the heavier isotope of oxygen (oxygen-18) compared to the lighter isotope (oxygen-16). The oxygen isotopic ratios (oxygen-18/oxygen-16) preserved in the calcium carbonate shells of these forams showed that coastal sea surface temperatures dropped by 3°C (5.4°F) during the Younger Dryas event.
Kennett’s research team hypothesized that this homogeneous sediment layer was formed when younger, oxygen-rich waters flowed into the basin to replace the older, oxygen-depleted bottom waters. But how did these oxygen-rich waters suddenly appear in the basin and why only during this geologically brief period? Just as during the ice age, the oceanic and atmospheric changes during the Younger Dryas may have caused an increase in deep and intermediate water formation in the North Pacific. These waters would have reached the Santa Barbara Basin more quickly, with less time for the oxygen to be removed by bacterial decay of organic matter. Our radiocarbon analyses of the foraminifera support this hypothesis, showing us that major changes in the circulation patterns of the ocean and atmosphere can occur quite rapidly. Studies of ice cores reveal that such changes can occur within decades.
Evidence of this major climatic cooling during the Younger Dryas has not been found in all paleoclimatic records throughout the American West. In some regions, only subtle impacts were found, with small shifts in vegetation—such as increases in pine and decreases in alder and oak in some of the mountain areas. Furthermore, as we saw above, many of the Great Basin lakes expanded, as did California’s Tulare Lake. Some scientists suggest (though not all agree) that the West’s climate briefly returned to colder and wetter conditions during the Younger Dryas.
THE GREAT WARMING: INTO THE HOLOCENE
The Younger Dryas ended almost as abruptly as it began, ushering in the Holocene, our current period of climatic warmth. As the climate began to warm and dry, the lakes again shrank. The seasons were also more pronounced at that time in the West, because the amount of solar radiation hitting the Northern Hemisphere during the summer was about 8 percent higher than today, so summers were warmer and winters were cooler. The increased summer temperatures were caused by slight changes in the earth’s orbit around the sun that changed the amount of solar radiation received by the earth. In the early Holocene, the earth’s axis was tilted slightly more toward the sun, and, during the Northern Hemisphere summer, the earth was slightly closer to the sun, resulting in an increase in solar radiation during that season. These changes in the earth’s orbit over time are known as the Milankovitch Cycles. We describe these cycles and their influence on the earth’s climate in more depth in chapter 11.
Pollen evidence of vegetation change suggests that the transition from the late Pleistocene to the early Holocene brought warming and drying to the American West, concurring with the Great Basin lake-level evidence. Just as the massive lakes of the Great Basin were shrinking, Sierra lakes were forming in California: the massive alpine glaciers had scoured out the bedrock, and their moraines later dammed these carved valleys, forming chains of mountain lakes. Freshwater, supplied by glaciers melting in the higher elevations, fed the mountain lakes even as overall conditions were drying.
Forests usually colonize the mountainsides in the wake of retreating glaciers. Natural vegetation zones exist along the steep mountain slopes, tied to elevation and climate. In the higher elevations within the forested slopes, pine trees dominate, even throughout major climate shifts. Paleoclimate researchers look for particularly sensitive locations at mid and low elevations, where meadows, marshes, and lakes are found. These sensitive environments contain sedimentary archives that record the regional environmental history. Relatively small changes in climate—such as a slight warming or a change in effective moisture availability—affect the local plant assemblages as the vegetation shifts in response to the new conditions. The forests surrounding small mountain lakes or meadows produce pollen and other macrofossils that are blown or washed into the lakes, to be incorporated into the slowly accumulating sediments on the lake bottom. Researchers can derive a series of snapshots from the sediments that reflect changes in the local flora over time.
One such lake, called Lake Moran, has provided a wealth of paleoclimatic information about the transition from the late Pleistocene to the Holocene. Moran is a small lake set in a mid-elevation forest, a bit over 6,600 feet, on the western slopes of the Sierra Nevada. It is the type of setting desired by paleoclimate researchers: set well into the forest away from any main roads, the lake is like a jewel surrounded by sugar pines, ponderosa pines, and fir trees. The lake itself drains a relatively small area (about thirty acres) with no inlet stream, so that climate researchers infer that the pollen, charcoal, and other fossil evidence found in the sediments are derived locally. Since the late 1980s, Roger Byrne, a paleoclimatologist and pollen expert in the Geography Department at the University of California, Berkeley, has used fossilized pollen and other macrofossil evidence to decipher past changes in climate around Lake Moran. Together with Eric Edlund, then a graduate student, Byrne collected several cores from the lake.
Radiocarbon dating of these cores showed that Lake Moran formed 15,000 years ago, after glacial ice began retreating in the Sierra Nevada. The sediment cores were also dated by matching layers of volcanic ash that settled into the lake with eruptions from nearby Mono Craters. An ash layer provides a basal date of the lake as 14,500 to 15,000 years old. Over the millennia, the lake has been slowly filling with sediments from its small watershed. These sediments contain a record of environmental change that spans the critical transition from the late Pleistocene into the Holocene.
Lake Moran contains seasonal layers: dark, organic rich layers deposited during the winter, and lighter, denser layers during the summer. In total, about ten feet of sediment have accumulated in the lake during its 15,000-year history, each foot of sediment containing about 1,500 years of environmental and climatic history.
The record of environmental change that Byrne and Edlund reconstructed from these sediments contains insights about how the region experienced the climatic transition at the end of the relatively moist and cool Pleistocene and the subsequent climate fluctuations of the Holocene. They found some surprises in this record. The notion so commonly held by ecologists of “plant communities,” with specific species living in close association in specific habitats, appears to be a function of particular climatic conditions. Edlund and Byrne found some unlikely combinations of plants growing in close proximity to each other around Lake Moran at times in the past that would never be found today. For example, about 12,000 years ago, a dense coniferous forest surrounded the lake for a period lasting between one thousand and two thousand years. Sugar pine, which prefers warm summers, grew with mountain hemlock, which prefers wetter conditions. Today, the sugar pine is generally restricted to the lower elevation forests in the Sierra, and the mountain hemlock is found at higher, moister locations. The fact that these were found together suggests a very different climate regime in the Sierra in latest Pleistocene, one with warm summers but abundant moisture. These plant assemblages have no analog in modern vegetation communities.
We can infer certain things about regional climate conditions from these unusual combinations of plants. Although the region was generally warming rapidly, the spring season was apparently cooler in the Sierra Nevada than today, with more snowstorms than rain, and the snowpack persisted later into the summers. Edlund and Byrne concluded that the Sierras must have lacked the pronounced summer drought that is so characteristic of the region today.
The pollen preserved in Lake Moran sediments also shows that the early Holocene was a time of rapid change. Over the course of several thousand years, the pine forests thinned, transforming the region into a more open landscape occupied by drought-adapted oaks as the climate continued to warm and dry. This warming reached a maximum about 7,000 years ago.
Pollen evidence from other lakes throughout the West also suggests a period of warm and dry climate during the early Holocene. Closed-basin lakes grew smaller as the climate grew drier. These lakes include Pyramid Lake in western Nevada and Owens Lake in eastern California, both of which decreased in size between 8,000 and 6,500 years ago.
CHARCOAL AND WILDFIRES
The climatic trends indicated by vegetation change are also reflected in the frequency and intensity of wildfires in the West. For instance, the abundance and size of charcoal fragments that reached Lake Moran increased during times of large forest fires. These charcoal fragments settled to the bottom of the lake and became part of the bottom sediments. Charcoal can range in size from fine dust particles to large chunks up to an inch or more across. The finer particles are carried longer distances in the wind. Furthermore, larger, more intense fires produce higher plumes of air that rise above them, carrying charcoal farther from the site of a fire. Because large forest fires often occur during periods of warmer and drier climate (particularly if they follow a wetter period, which increases the amount of vegetation that later becomes fuel), the size and amount of charcoal fragments in buried sediments provide additional evidence for past climate conditions.
At Lake Moran, the charcoal record suggests that large fires were uncommon during the late Pleistocene. This comes as no surprise, given the wetter and cooler climate that prevailed. But during the transition into the early Holocene, charcoal abundances increased as summers became warmer and drier. This increased amount of charcoal from forest fires remained high until about 7,000 years ago. In fact, the early Holocene appears to have been a period of more extensive and intense forest fires than any time since. Byrne and Edlund speculated that the large number of forest fires may have significantly shaped forest communities, and therefore fire would have been the means by which the early Holocene climate change dramatically shifted the vegetation from a dense forest regime to a more open forest dominated by oak trees.
HUMANS AND FIRE
Although fires and climate largely controlled vegetation shifts during the Holocene, some archaeologists argue that early humans may have also played an important role in shaping the types of plant communities in some parts of the West. These researchers suggest that native populations managed the land over the millennia with controlled burning, pruning, weeding, irrigation, and tillage. In other words, the early humans were not merely passive hunter-gatherers who collected their food from a wild landscape, as was once thought.
According to Kent Lightfoot and Otis Parrish at the University of California, Berkeley, hunter-gatherer populations may have used controlled burning to clear underbrush and areas of dead timber, eradicate pests and diseased plants, and recycle nutrients back into the soil. Fires may also have been used to clear water holes of vegetation, drive wildlife into traps during group hunting, and mark territories or claim resources. The geographic extent and intensity of these controlled burns over time is a subject of Lightfoot’s current research, and, in his work with Parrish, he has hypothesized that some of the landscape patterns we see over the past 13,500 years or so after humans arrived in the West were related to intentionally set fires, not just those ignited naturally by lightning strikes.
EARLY HOLOCENE CLIMATE CYCLES
Broad cycles of climate change during the Holocene have been apparent from the early part of the epoch. The evidence is found both on land and in the oceans. The bristlecone tree line migrated downslope as climate cooled and upslope again as climate warmed, approximately every 2,500 years. These broad cycles in temperature are also apparent in the waxing and waning of mountain glaciers.
Off the West Coast in the Pacific, there is also evidence that seawater temperatures shifted with similar broad cycles—temperature shifts that affected the assemblages and oxygen isotopic compositions of coastal marine organisms. James Kennett and his research group analyzed the fossil remains from sediment cores taken from the Santa Barbara Basin. They used two species of foraminifera: Globigerina bulloides, a surface dweller, and Neogloboquadrina pachyderma, a species that lives at about 60 meters’ (195 feet) depth, where ocean temperatures become much cooler. Their results show that coastal waters were unusually warm (by 2–3°C, or 3.5–5.5°F) from 11,000 to 10,300 years ago. After that time, during the earliest Holocene, temperatures began to drop, cooling by 4–5°C (7–9°F), and remained low for the next 1,500 years. Temperatures then increased again between 8,200 and 6,700 years ago, and they fell once more between 6,700 and 5,200 years ago.
These millennial-scale climate cycles during the Holocene were initially identified about a decade ago in climate records from Greenland ice cores and North Atlantic sediment cores. Since then, researchers have found evidence for them in pollen records showing vegetation change throughout North America as well as marine records from the Pacific Ocean. We will return to these broad cycles of climate in the next few chapters and will discuss their possible causes in chapter 11.
The transition from the peak of the last ice age to the Holocene encompassed some of the most rapid and monumental climate shifts in the geologic record. Global cooling at the peak of the last ice age is important because it represents a period when the climate was quite different from the warm conditions we experience today and are likely to experience in the future. The evidence shows that the West was significantly wetter during this cooler period, resulting in the formation of the enormous Great Basin lakes, which were ten times larger than they are today. These lakes have since largely dried up, and they are predicted to dry even more as the climate continues to warm. As these lakes gradually desiccate into salt flats, they serve as a warning of how rapidly water can vanish in a region that is highly sensitive to climate change.
Compared to the monumental environmental and climatic upheaval of the previous ice age, the Holocene epoch was a more tranquil period. There were no wild expansions and contractions of great lakes; no large, abrupt shifts in temperature. Even so, the climate fluctuations and extremes of the past 11,000 years have been large enough to dramatically alter the landscape, disrupt human societies, and, in some cases, lead to mass migration and societal collapse. As we shall explore in the next chapter, the most extreme of these fluctuations was one of the longest and most severe droughts of the Holocene in the West.