December 7, 2000

History and Process of Climatic Change during the Holocene Epoch:

Warm Periods and Drought Indicators

Abstract: A general overview of Quaternary climate includes causes of both long-term and short-term climatological change and physical evidence for that change. Discussions of two warm intervals in Holocene climatology are presented. Altithermal interval in the Southern High Plains, Californian Sierra Nevada, and Great Plains is investigated through varved lake sediment data, isotopic evidence, and submerged tree stump data. The Late Holocene Warm Interval is explained using data obtained from Mono Lake sediments. Stine’s articles about the Medieval Climatic Anomaly are considered along with examples of drought indicators from Coastal Southern California, Pacific Island cultures, and the Colorado Plateau. Evidence from varied lakes located near the Sierra Nevada range are discussed due to their extensive data on submerged drowned tree stumps and compared with similar evidence in Patagonia. Data is presented to summarize indicative geologic, archaeological, and ecologic factors in Holocene drought research.

Approximately 5000 years ago, in the Mid-Holocene, the Sierra Nevada region (see Fig. 1), along with the rest of North America, experienced an extremely dry, warm period. This period has many names such as Altithermal, Megathermal, Hypsithermal, Xerothermic interval, Climatic Optimum, and others (Wright, 1976). Again, around 1100 CE and 1350 CE other similar events took place. This warm period, the Medieval Climatic Anomaly (Stine, 1990), was short term and spanned only centuries. This paper seeks to answer the question: What is the history and process of climatic change during the Holocene Epoch in relation to warm intervals and drought, and what are the present day geologic, archaeological, and ecological indicators of these events?

To understand the geologic and regional setting of these events, I first researched the Holocene generally. I studied numerous scientific journal articles about vegetation, forest development and migration, pollen analysis, lake sedimentation trends, and the climatic history. I also read a book that dealt with Quaternary climate and its effects on people and resources. I, then, continued to collect data and focused on cause and effect of these drought intervals. Glacial ice cores, varved lake core samples, isotopic records, treeline migration at high altitudes, pollen samples that demonstrated species migration based on climate, and submerged tree stumps were all important topics in my research. This research introduced me to various factors that affect accuracy of drought indicators. Dating methods and calibration issues were introduced, along with the concept of closed lake basins as more reliable records of climatic change than other water features. Many journal articles talked about the poor preservation of geologic and archaeological drought indicators they were studying. Susan Lindstrom (1990) talks about Lake Tahoe tree stumps from the Mid-Holocene Maximum that are obscured by the water level and milling features that have been destroyed by erosion and the effects of water level fluctuation. Another issue that was brought to my attention was calibration of radiocarbon dating with dendrochronologic sources. Pollen analysis from samples within stratigraphic columns and tree ring samples from submerged stumps need to be accurately dated for these studies, for example.

In this paper, I will discuss the various causes and effects of two warm periods in Holocene climatic history. One is a long term event spanning several thousand years; the other is a short term event lasting only about 400 years at the outward boundaries of evidence.

To write this paper I designed a rough outline while I collected data and refined my topic focus. I collected data for about a month, reading articles, finding new issues and researching those, before finalizing a second draft outline. After I finish collecting data, I write my introduction using my outline. When I finished the body of research portion, I wrote my conclusion and abstract.

A general overview of Quaternary climatic events, what caused them, and the evidence to support those claims is presented here to frame the events of interest. A Greenland ice core, a reliable source of climatic data, provided this data (Yu, 1998). The Bolling-Alerod was a time of general warming from the last glacial interval. This was followed by the Younger Dryas, which lasted from ~11,000 to 10,000 c14 BP and was extremely cold. There was a 3 degree Celsius decrease in the mean annual temperature from the preceding warm stage. Then came another warming trend, during the Holocene, which saw a 6 degree Celsius increase in the mean annual temperature from the Younger Dryas temperature.

The Holocene was categorized as an unstable period of long-term climatic change (see Fig, 2). The Early Holocene Amelioration is recorded through pollen data as a stage of abrupt temperature increase starting 10,000 years BP (Bell, 1953). Further analysis suggested a 3 to 4 degree increase very 500 years. Fossil evidence suggested even more rapid rates.

Next, the Climatic Optimum or Altithermal is recorded. This period lasted from 9ka BP. to 4ka BP. Greenland ice cores indicate a thermal maximum at 6ka to 5ka BP. In Antarctica, it is recorded at 9ka BP. Data from various U.S. sites indicate a thermal maximum from 10ka to 5ka BP. through pollen analysis (Bell, 1953). A variety of proxy data sources show evidence for these conclusions. In the Northwest United States, migration of mixed woodland occurred. High altitude tree lines rose, indicating a rise in temperatures. Indicator species such as holly (Ilex), ivy (Hedera), and mistletoe (Viscum) were used to establish this. Reduced lake levels and water tables also occurred.

A general cooling trend called the Late Holocene Deterioration followed this warm period. This event is difficult to study because indicators used previously have been affected in this period by early farming practices.

The Little Climatic Optimum (see Fig. 3) occurred from 700 CE to 1300 CE. Glacial retreat and reduced sea ice characterize this event. Warm conditions in the Arctic allowed Viking expansion to Greenland at this time (Bell, 1953). Data from a Greenland ice core, in fact, indicates a 2 degree Celsius average temperature from present day during this time (Bell, 1953). Increased viticulture in England is another good indicator of this event, as this would be due to increased summer and winter temperatures (Bell,1953).

After this warm period, approximately 5000 years ago, another cold interval called the Little Ice Age took place. Uncommon cold, increased storms, glacial advance, and colder, wetter, winter months all occurred. The temperature dropped 3 degrees Celsius from the Climatic Optimum (Bell, 1953).

Causes for long-term changes on a millennial scale like the Younger Dryas are glacial movement and atmospheric changes. However, short-term changes like the Little Climatic Optimum, or Medieval Climate Anomaly (Stine, 1994), are different. Changes over 10 to 10,000 years are caused by solar output, volcanic activity, and atmospheric CO2 fluctuations (Bell, 1953). The Sun alternates between active and inactive times that are indicated through the growth and disappearance of sunspots. Solar output cycles are recorded by tree ring proxy records, ice core isotopic data, and varved lake sediments (Bell, 1953).

Wright (1976) states, “Holocene glacial and vegetational progressions provide a good record of climatic change”. Other reliable records of climatic change are provided by proxy data such as pollen analysis, dendrochronology, and lacustrine sediments. Alpine tree lines respond to climatic change (Scuderi, 1994), which can be studied through analysis of tree rings and species migration through palynological research. Varved lake sediments are created by seasonal change, biogenic production, water chemistry, and the inflow of mineral matter (Bell, 1953). These sediments are used to interpret a chronological sequence of climatic change. To study tree migration and tree line limits, plant remains preserved in peat bogs and lake sediments are used. Lake sediments are also useful in preserving microscopic life that has specific habitat preferences, like plant indicator species. Diatoms, microscopic unicellular algae, and Forminifera, marine protozoan, are two such indicators of temperature or other climatic data (Bell, 1953).

The two periods of focus are warm, dry periods: the Altithermal, previously referred to as the Mid-Holocene Amelioration, and the Medieval Climatic Anomaly

The Altithermal lasted from approximately 8000 BP to 4000 BP at its most outward records. The first evidence of this event was found in early bog studies in Denmark (Wright, 1976). “In the early Holocene Epoch, greater-than-present summer insolation increased summer temperatures, decreased effective precipitation, and indirectly strengthened the eastern Pacific subtropical high-pressure system, which intensified drought (Whitlock, 1997)”. Changing climatic factors were beginning to affect temperature, which would eventually lead to the Mid-Holocene aridity and water loss of serious drought. In the Southern High Plains of Texas and the surrounding region a large body of data regarding Holocene drought has been reported by Holliday (1989). His stratigraphic, geomorphic, and pedologic research has yielded indications of widespread aeolian erosion and sedimentation. These processes were common during the interval of 6000 years to 4500 years before present. This warm dry period reduced vegetation cover, which caused aeolian dune development (Holliday, 1989). Great Plains grasses declined. Steppe shrubs such as sagebrush grew more commonly, as evidenced from pollen records (Wright, 1976). A low water table causes people living in the region at the time to dig well to reach water sources. Archaeological research has revealed evidence of these wells. A temperature increase indicated by the presence of calcerous marsh deposits contributed to the arid climate. Lakebed stratigraphy containing strata of aridity-adapted fauna have been found contemporary with mid-Holocene drought as well (Holliday, 1989).

Lake sediments from another region also contributed data that further proves the existence of the Altithermal. Owens Lake has been cored to study climatic data from its stratigraphy of bed sediments. During the warm interglacial period, fine grain sediment was deposited at a high rate. Alternately, during glacial periods, fine grain sediments were deposited at a low rate (Litwin et al, 1999). Aeolian forces moved more material into the dry lake bed during the warm dry period, while cold climate caused only fine silt to settle on the lake bottom after filtering down through the water. Other lake sediments provide different information through similar geologic processes. Varved, or seasonal, deposition of sediments provides a proxy record of lake activity for the Altithermal. Evidence suggests the Mid-Holocene dry interval was actually two intervals separated by a cool wet period. This sequence would further explain the variation in sediment layers from coarse to fine by changing depositional climate. A dry environment is more effected by the sudden introduction of water than a wet climate. Materials were transported at a more rapid rate. During the next interval, a wet environment going into a drying trend would preserve the sedimentation and deposition of the past. At Elk Lake, Minnesota, the Altithermal is recoded in varved lake sediments from 8500 BP to 4000 BP. The lake was shallower and more saline due to a concentration of minerals in less water volume (Dean et al, 1984). The diatom Melosira, which prefers shallow water found in deeper areas of the lake also suggest lower lake levels and prairie conditions (Dean et al, 1984).

In the Great Basin during the Altithermal, arid conditions and economic crisis caused by drought forced the inhabitants to abandon the basin (Jones et al, 1999). The hunter-gatherers and agriculturalists could not manage to survive in the harsh drought conditions.

Mid latitude lakes supply evidence of a climatic change from cold to warm in a transition from minerogenic to organic deposits (Scuderi, 1994). “Reworked minerogenic material in lake sediments and changes in the sediment limit around lake margins provide evidence of fluctuating water levels during the course of the Holocene which have, in turn, been interpreted in terms of temporal variations in precipitation. (Bell, 1953)”

Isotopic evidence is a complex, but useful tool in climatic research. The ratios between oxygen and hydrogen isotopes will change over time as water components change due to evaporation and water level fluctuation. Submerged aquatic plants use carbon dioxide and produce a carbonate byproduct known as marl. The oxygen isotope content of marl reflects the isotopic composition of the lake at the time.

Another tool of climatic reconstruction are indicator species. The presence of such remains indicates a certain climatic environment. Also, treeline and glacial evidence has a response lag, which can be calibrated using indicator species. Treeline evidence, like glacial movement evidence, is affected by geography and climate, whether it is deteriorating or ameliorating. The use of cross checks and more than one kind of evidence assures more accurate conclusions.

Submerged drowned tree stumps are an excellent means to gain timeframe data on Holocene drought and climate change. It can be radiocarbon dated and the tree rings can provide data on yearly changes in growth reflecting climate, precipitation, and environment. Also, their position below lake surfaces also adds information about past climate through lake levels, which respond to both temperature and precipitation.

The Altithermal period caused water levels to drop in the Lake Tahoe Basin (see Fig. 4). In fact, Lake Tahoe and Pyramid Lake are the only two lakes that in the ancient Lahontan system that did not dry up during the Altithermal (Lindstrom, 1990). The Tahoe basin is fed by perennial streams like the Truckee River, which provide good record of climatic events. Lakes in basins like the Tahoe respond to change by rising and falling. The present surface elevation of Lake Tahoe is 6228 feet above sea level. Six submerged stumps lay below the surface at an elevation of 6220 ft. Tahoe had a stable low stand at this elevation for approximately 100 to 350 years. A tree ring analysis shows that these trees grew for about that amount of time before the water rose, killing the trees by drowning. To date, 21 stumps of drowned trees have been found and studied. They lie between the elevations of 6223.25 ft and 6210.8 ft. The deepest stump lies 12 feet below the natural sill of Lake Tahoe. Radiocarbon analysis for these stumps reveals dates of 5510+_90 BP to 4250+_200 BP. The lake level rose after 5510 years BP. Lake Tahoe reached its natural sill about 1300 years later. These trees could not grow if their bases were submerged; therefore, once the water level reached the bases of the trees, the tree died, and stopped adding rings to its stump. This gives us a clear dating methods to establish the time of the water rise, and the time period the water stood below the level of the trees and allowed them to grow is established through tree ring analysis.

The Late Holocene Warm Interval discussed in Stine’s article about Mono Lake (1990) has been include to augment the body of data regarding Holocene droughts and their rate of occurrence within this period of time. Mono Lake is a closed lake which declined to a low stand ~1807 cal BP. This Marina Low Stand dropped water levels to 1940.9-meter elevation. That is about 40 meters below its high stand at 3770 BP. Sedimentary and geomorphic records are obscured at this site by submergence beneath the water, erosion of features by aeolian and fluvial process during the low stands, and wave and shoreline currents during fluctuations (Stine, 1990). However, Mono Lake lies in one of the continents most active volcanic areas and tephra dating has been utilized. Other means to establish evidence of drought are several generations of tree stumps, shrubs and vegetation in the shore area that were drowned when the lake rose from low levels that persisted for a long period of time. In addition to data from Mono Lake, Pyramid lake evidence also points to it falling to its lowest level contemporaneously with Mono Lake. The main evidence used to reconstruct Mono Lake data was sedimentary sequence of the deltaic sediments exposed by newly cut channels of feeder streams (Stine, 1990). Also correlating with Stine, Bristlecone Pine tree ring data confirms a dry, warm period at this time.

A period of warm, dry interval occurring between 700 CE and 1300 CE is defined as the Medieval Climatic Anomaly (Stine, 1990). Other scientists have also studied this recent drought period, assigning various nomenclatures to the event. Douglass first discovered the Medieval Climatic Anomaly in tree ring records of the 13th century in 1929 (Jones et al, 1999). This climatic event occurred globally and has been identified through numerous methods and geologic and archaeological features. During this warm period, sea levels rose along with the sea temperature.

This Medieval Climatic Anomaly greatly affected islanders of the Pacific. Island economies were affected by the declining water and food sources due to changes in species habitats in lower sea levels.

In Coastal Southern California and its islands, the drought was severe. Elevated sea temperatures from 1150 CE to around 1300 CE disrupted island subsistence economies. Bristlecone Pine in the eastern Sierra Nevada show less moisture in their tree rings combined with a sharp increase in temperature at high altitude. Across the continent, scientists find evidence for this pandemic drought. Pack rat middens in the Mojave region further corroborate the events in the area through analysis of the types of vegetational materials found within the midden. Human populations had to adapt their settlement strategies to accommodate lessened water resources. Occupation sites during the Holocene tended to be close to perennial water sources. People living in these settlement sites experienced higher levels of malnutrition, disease, and interpersonal violence brought on by stress and competition for resources (Raab et al, 1997). Osteological records from Santa Cruz Island provide data for these conclusions (Raab et al, 1997). Increased dependence and use of shrinking water sources caused water contamination. Animals, people, and agriculture all exploiting the same water resources led to a spread of Cribra orbitalia, which is caused by nutritional deficiencies, and linked to childhood anemia and diarrhea (Raab et al, 1997). Osteological analyses of remains from this period show evidence of increased occurrence of this condition (Raab, 1997). Settlement was greatly affected by this change in the climatic regime because prior to 1000 BP it was stable. From 1000 BP on it was unstable. Sites occupied earlier than 1200 CE are abandoned around this period and sites inhabited from 1200 – 1400 CE show no signs of earlier habitation. Populations were on the move, trying to adapt and find water sources.

Besides archaeological evidence of the drought’s effect on human population, forests also reacted to a changing climate. The Colorado Plateau underwent many changes in the climate between 1050 CE and 1300 CE. Drought became the main climatic theme, average temperature increased, hydrologic instability ensued, the water table dropped, and aeolian erosion increased. Changing environment impacted other aspects of the natural surroundings, as well. Fires increased. In the Sierra Nevada, the highest fire rates of any interval of a 2000-year record of fire scars in Giant Sequoias are found during the late Holocene centering around 1300 CE (Jones et al, 1999).

One of the most widely studied and published drought indicators are submerged tree stumps. Sites containing old stumps of submerged trees are found in countless lakes in California, the Sierra Nevada, Patagonia, the Great Basin region, and worldwide. These stumps are evidence of drought because they are located below the present water level of the lakes where they are located. At one time, the lake was lower, caused by drought, and remained so for a long enough time for trees to grow for decades, and in some cases even hundreds of years.

In California, drowned tree stumps in lakes are found bordering in submerged in lakes, swamps, rivers, and streams (Street-Perrott, 1994). The fact that the cause of death of these trees was drowning indicates that low-level lakes were flooded. Archaeological data supporting this theory are found throughout the Americas. Tiwanaku, the highest elevation city center during Medieval times in South America, collapsed ~1100 CE (Street-Perrott, 1994). Anasazi cliff dwelling culture declines and lake desiccation occurs in eastern China around this period (Street-Perrott, 1994). Five causes of these decadal climate changes in these areas as posited by Street-Perrott are: random atmospheric variability, variations in the production of North Atlantic Deep Water, solar variability, volcanic activity, and atmospheric gas variability.

In California, extreme and persistent drought was experienced in medieval time (Stine, 1990). Besides the specific cases discussed here, stump evidence has been found in Independence Lake, where ten stumps with radiocarbon dates of 690+_50 BP were found. Also, Donner Lake and Fallen Leaf Lake contain stumps from Holocene drought. Four sites are discussed in detail that all contain extensive evidence of the arid period. Mono Lake, Tenaya Lake, the West Walker River, and Osgood Swamp are all located in the Sierra Nevada region at high elevations. Their differing characteristics give each location distinct features of drought indication.

Mono Lake is a saline alkaline body of water that receives inflow from the Sierra Nevada runoff. It lacks an outlet and therefore responds to climatic changes (see Fig. 5). In 1940, the city of Los Angeles diverted its flow to provide drinking water for the municipality. This dropped the lake surface elevation 14 meters. Its natural elevation today would be 1957 meters above sea level. Two generations of drowned stumps are found in the lake that represent medieval drought. The tree species found include Jeffrey Pine, Populus trichocarpa, and Artemesia tridentata. The first generation of trees has been dated to a death of around 1112 CE (Stine, 1994). These stumps are located 15m below natural water level. The second generation of stumps is 11 meters below natural water level. They died around 1350 CE. Indicated by tree ring analysis, the first low stand lasted 50 years. The second lasted 50 years, also. Low precipitation resulting in low Sierra runoff caused the lake to recede that low stand. Conditions persisted and large trees grew in the fertile lake bottom. A stratigraphic sequence of the lake was studied and it was concluded that the tree deaths coincided with water level rising after periods of drought. The trees drowned as the lake regained its pre-drought surface elevation.

Tenaya Lake had a similar sequence of events. It also received runoff from the mountains. Its high water mark is ringed by Lodgepole Pines. Nine large Lodgepole stumps are found in the lake, sticking out of the water. These stumps stand in 8 to 19 meters of water. Dating has established them as contemporary with Mono Lake first generation stumps. Dendrochronology found that Lake Tenaya stood below its natural sill for about 70 years allowing these massive trees to grow to their size (Stine, 1994).

The West Walker River originated east of the Sierra Nevada and flows through a narrow floored gorge. Dozens of Jeffrey Pine stumps are located on the gorge floor. Jeffrey Pine cannot survive if its base is submerged for more than a few weeks during active growth, which coincides with the high runoff period of the Sierra (Stine, 1994). Therefore, these pines could not have grown while the river flowed through the gorge, since it is too narrow to allow banks on its sides within the canyon. These stumps, some with 220 annual rings, are radiocarbon dated to 1044 to 1213 CE as a date of death (Stine, 1994). These are also contemporary with the Tenaya and Mono G1 stumps. This data indicates that the stream flow did not run through the entire gorge for more than 200 years, allowing tree growth.

At Osgood swamp Lodgepole Pine rings the borders. They do not grow within the swamp due to the ground’s saturation year-round. However, stumps within the swamp have been counted with 100 or more annual growth rings with an age at death dated to 1024 to 1228 CE. Low Sierra runoff for more than 100 years allowed the wet ground to dry out and for tree establishment and growth to begin.

Evidence from these 4 sites offers the conclusion that flooding followed 2 periods of extreme dry conditions. The first dry period spanned ~892 CE to 1112 CE. A very wet period followed. Mono Lake reached its second highest stand at this time and rose to an elevation of 1961 m. The second drought lasted from 1209 CE to 1350 CE. This unstable climatic history is not restricted to California. Similar evidence of submerged stumps from Patagonia between 1051 CE and 1226 CE have been documented at Lago Argentino. The Medieval Climatic Anomaly was experienced globally (see Fig. 6).

Archaeological evidence to complement the stump data is also located in the Sierra Nevada region. Lake Tahoe contains milling features with grinding cups in rocks found nearby. These archaeological features are located between 6229 m and 6222.5 m. More evidence of this kind may be located beneath the surface or buried by deposits.

The mid-Holocene up to about only 600 years ago was an unstable climatic period. Processes such as aeolian erosion and deposition, unstable hydrologic responses, and ecologic change caused by increased temperatures and aridity are recorded in various ways. Geologic formations such as stream cuts, dunes, lakebed stratigraphy, and aeolian deposits, archaeological evidence such as middens, settlement remains, and osteological finds, and ecologic indicators such as ice cores, tree ring studies, drowned stumps, and palynological analyses are all used to isolate and study short and long term climatic change in history. Issues affecting the research include availability of evidentiary data and preservation of geologic features. The various methods used to find and study drought indicators in the geologic and archaeological record are very effective. However, more emphasis should be placed on geographically expansive results of drought on both the natural surroundings such as vegetation, geology, and flora and fauna, but also the human factor. What are the sociocultural repercussions of severe drought? How does the changing landscape affect people’s worldview? Does religion reflect long-term climate changes? To answer this question, more cultural data needs to be researched. From a geoarchaeological perspective, the current studies and conclusions are both detailed and comprehensive. Many scientists have devoted long term research to specific Holocene drought issues. Holocene drought as a general climatic concept lacks focus. To fully understand these events, one must first realize the variety of sources and issues involved. Holocene drought is a complex, fascinating issue encompassing many scientific disciplines.

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