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Mongolia, a landlocked country in central Asia, is located in a region of the highest degree of continentality (seasonal contrast) on Earth (see Fig. 1). The extreme continental climate of Mongolia is reflected in the annual temperature range of about 45。, with limited precipitation (generally less than 250 mm per year) occurring largely in July and August. The climate in the Mongolian region represents an important transition from the subarctic in the north to the Gobi Desert in the south. The lakes we are studying are located in the semiarid, north-central portion of Mongolia.
      One quarter of Mongolia掇 approximately two million people live in the capital city of Ulaanbaatar and three quarters live in the countryside in small towns or in gers, felt and canvas tents (see Fig. 2). The steppe grasses are intensely grazed by approximately 20 million domesticated horses, sheep, goats, cows, camels, and yaks. Droughts have had major impacts on livestock and people in Mongolia. It is critical to have a better understanding of the frequency and magnitude of past climate change in Mongolia in order to effectively manage its natural resources. Previous studies of meteorological instrument records have been used to identify the spatial pattern and timing of drought in Mongolia. Since 1940, at least half of Mongolia掇 provinces have experienced a severe summer drought once every three to five years. An important goal of our study is to use lake sediments to extend the shorter Mongolian instrument and tree ring climate records back in time to assess the range of natural climatic variability in Mongolia. Understanding the range of past natural climate variability provides an indication of the types of future climate change that are possible in Mongolia.
      Lake/watershed systems have the potential to yield important insights on past climatic conditions for a variety of reasons. A lake, particularly if it has a deep central basin, can accumulate sediment, which yields a continuous geologic record. Continuous geologic records are often difficult to obtain on continents because much of the land surface is subjected to erosion or episodic deposition. Some lakes rapidly accumulate sediment rich in organic matter that is suitable for radiocarbon dating. These lakes can yield a well-dated, high-resolution record. The sediment particles that accumulate within a lake have origins that are both autochthonous (e.g., diatoms produced within the lake itself) and alloch-thonous (e.g., pollen from terrestrial plants, soil erosion, and wind blown dust from outside the lake). Therefore, lake sediments can provide a record of both the lake and the watershed ecosystem responses to climate change. Lakes that are located near major ecosystem boundaries are particularly sensitive recorders of climate. As climate changes, ecosystem boundaries shift, and new vegetation, soil conditions, and lake levels are established at the site. These changes, in turn, influence the type of sediment that accumulates in the lake. Several of the lakes we are studying are closed lakes that have no river flowing from them and represent a sensitive balance between climate parameters such as evaporation, precipitation, and temperature. Small changes in these parameters can result in large sedimentary and geochemical limnological changes.
      Changes to the Mongolian Constitution in 1992 and a democratically elected government in 1996 brought opportunities for scientists from the United States to collaborate with Mongolian scientists. A group of scientists from the Mongolian Academy of Sciences led by Dr. Khosbayar has worked with U.S. scientists including John King (GSO), Sarah Fowell (University of Alaska), Doug Williams (University of South Carolina), and me to study the climate signal recorded in Mongolian lake sediments. This group is conducting an interdisciplinary study of sediment cores recovered from five lakes during two field trips (see Fig. 3). Lake Telmen, one of the five lakes sampled, illustrates well how lake/watershed systems provide insight to past climate change.
      In the Lake Telmen region today, the mean temperature in January is -32。 (-26；) and the mean temperature in July is 12。 (54；). The mean annual precipitation is 250mm. The lake lies near the boundary between the forest-steppe and steppe ecosystems. Lake Telmen, a closed lake basin, is slightly salty (about 4g per L salinity). In July, a well-developed thermocline at 10m depth corresponds with a large increase in turbidity generated by dense patches of plankton found at this depth. As this organic matter decomposes and descends, it lowers dissolved oxygen to hypoxic levels (2.36 mg per L) in the bottom water. Bottom-dwelling organisms, such as worms, typically inhabit lake floors and mix the sediment as they feed. However, in the low-oxygen conditions of the deep water in Lake Telmen, benthic organisms cannot survive to mix the sediment. Sediment cores from the central basin of the lake show fine laminations less than 1mm thick. Two laminae are deposited per year and the resulting couplet is referred to as a varve. During the warm productive summer months, photosynthesis by plants in the lake depletes the lake waters of CO2, induces supersaturation with respect to calcium, and results in a sediment lamina comprising abundant calcium aggregates and very small (less than 8 microns) calcite crystals. When the lake surface is frozen, from the fall to the spring, the second lamina of the varve couplet accumulates. This lamina is composed of amorphous organic matter and clay particles that settle from the water column. The absence of sediment mixing by benthic organisms preserves the varves, and yields a high-resolution geologic record from Lake Telmen sediments.
      From 6,210 to 3,960 years ago, as determined by radiocarbon dating, Lake Telmen was between 15 and 20m shallower than it is at present. Coarser-grained sediment containing pollen from vegetation indicative of arid conditions and poor unstable soils accumulated in a small lake that covered about 40 percent of the present lake area. Without extensive vegetation cover, numerous sand dunes in the Telmen vicinity were created by the wind. With an increase in moisture balance (precipitation minus evaporation), the level of Lake Telmen rose. Sediment cores collected in 12.8m of water contain an ancient soil at their base that was inundated by the rising level of the lake about 3,570 radiocarbon years ago. The lake level had risen to the extent that mixing generated by the wind did not reach the deeper water; thus hypoxic water conditions became established and varved sediment accumulated. By 2,250 radiocarbon years ago, the lake level had risen to 9.95 m above the present lake surface, and wave action cut a high-stand terrace into the hillsides around the lake. Varved sediment accumulated over a much larger area that was now in deeper water. Consistent with the sedimentological evidence, pollen from specific land plants suggests that the climate was more humid between 3,960 and 1,500 radiocarbon years ago. Lake levels declined beginning 1,500 years ago. Three periods of stable lake level are recorded by wave-cut terraces on headlands and relict beaches in embayments at 5.27m (dated at 1,400 years ago), 3.25m, and 0.70m above the present lake surface. Through an interdisciplinary study of lake systems, we are able to reconstruct the history of moisture balance in central Mongolia. Ongoing studies of the fossils (diatom, pollen, and others), the sedimentology, the mineralogy, and the sediment geochemistry will give us more insight on the nature of lake and watershed ecosystem response to past climate change. By studying five lake systems, we will be able to document regionally coherent climate signals preserved in the sediment record. In addition to defining the long-term trends in climate change as we have done through lake level histories, it is important to look at how rapidly these changes can occur. For example, due to unusually heavy snow fall and spring rains in 1993, Lake Telmen rose approximately 1.5m with the addition of 55km3 of water from the surrounding mountains. Understanding the rate at which changes impact ecosystems has great importance because it determines how quickly Mongolian society will need to adjust to new environmental conditions.